EAST AFRICAN STANDARD - EAC QUALITY · 2010-07-17 · Draft for comments only — Not to be cited as East African Standard CD/K/002:2009 ... EN 476, General requirements for components

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CD/K/002:2009ICS 93.030

© EAC 2010 First Edition 2010

EAST AFRICAN STANDARD Drain and sewer systems outside buildings

EAST AFRICAN COMMUNITY

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CD/K/002:2009

ii © EAC 2010 — All rights reserved

Foreword Development of the East African Standards has been necessitated by the need for harmonizing requirements governing quality of products and services in East Africa. It is envisaged that through harmonized standardization, trade barriers which are encountered when goods and services are exchanged within the Community will be removed. In order to meet the above objectives, the EAC Partner States have enacted an East African Standardization, Quality Assurance, Metrology and Test Act, 2006 (EAC SQMT Act, 2006) to make provisions for ensuring standardization, quality assurance, metrology and testing of products produced or originating in a third country and traded in the Community in order to facilitate industrial development and trade as well as helping to protect the health and safety of society and the environment in the Community. East African Standards are formulated in accordance with the procedures established by the East African Standards Committee. The East African Standards Committee is established under the provisions of Article 4 of the EAC SQMT Act, 2006. The Committee is composed of representatives of the National Standards Bodies in Partner States, together with the representatives from the private sectors and consumer organizations. Draft East African Standards are circulated to stakeholders through the National Standards Bodies in the Partner States. The comments received are discussed and incorporated before finalization of standards, in accordance with the procedures of the Community. Article 15(1) of the EAC SQMT Act, 2006 provides that “Within six months of the declaration of an East African Standard, the Partner States shall adopt, without deviation from the approved text of the standard, the East African Standard as a national standard and withdraw any existing national standard with similar scope and purpose”.

East African Standards are subject to review, to keep pace with technological advances. Users of the East African Standards are therefore expected to ensure that they always have the latest versions of the standards they are implementing.

© East African Community 2010 — All rights reserved*

East African Community

P O Box 1096

Arusha

Tanzania

Tel: 255 27 2504253/8

Fax: 255-27-2504481/2504255

E-Mail: [email protected]

Web: www.each.int

*

© 2010 EAC — All rights of exploitation in any form and by any means reserved worldwide for EAC Partner States’ NSBs.

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© EAC 2010 — All rights reserved iii

Contents 1 Scope ...................................................................................................................................... 1 2 Normative references .............................................................................................................. 1 3 Terms and definitions .............................................................................................................. 2 4 Objectives ................................................................................................................................ 9 4.1 General .................................................................................................................................... 9 4.2 Public health and safety ........................................................................................................... 9 4.3 Occupational health and safety.............................................................................................. 10 4.4 Environmental protection ....................................................................................................... 10 4.5 Sustainable development ...................................................................................................... 10 5 Requirements ........................................................................................................................ 10 5.1 Functional requirements ........................................................................................................ 10 5.2 Determination of performance requirements for the drain and sewer system ........................ 13 6 Integrated sewer system management .................................................................................. 15 6.1 Introduction ............................................................................................................................ 15 6.2 Investigation .......................................................................................................................... 16 6.3 Assessment ........................................................................................................................... 21 6.4 Developing the plan ............................................................................................................... 23 6.5 Implementation ...................................................................................................................... 29 7 Health and safety principles ................................................................................................... 30 8 Design principles ................................................................................................................... 30 8.1 General .................................................................................................................................. 30 8.2 Types of systems ................................................................................................................... 31 8.3 Layout and profile .................................................................................................................. 32 8.4 Hydraulic design .................................................................................................................... 33 8.5 Environmental considerations ................................................................................................ 36 8.6 Structural design .................................................................................................................... 39 8.7 Operational considerations .................................................................................................... 40 9 Detailed Design ..................................................................................................................... 41 9.1 Introduction ............................................................................................................................ 41 9.3 Preliminary investigations ...................................................................................................... 44 9.4 Hydraulic design .................................................................................................................... 45 9.5 Environmental considerations ................................................................................................ 48 9.6 Operational considerations .................................................................................................... 49 10 Construction Principles .......................................................................................................... 51 10.1 General .................................................................................................................................. 51 10.2 Pipelines ................................................................................................................................ 51 10.3 Ancillaries .............................................................................................................................. 52 11 Operations and Maintenance ................................................................................................. 52 11.1 Introduction ............................................................................................................................ 52 11.2 Objectives .............................................................................................................................. 53 11.3 Data requirements ................................................................................................................. 53 11.4 Investigation and analysis of operational problems ................................................................ 54 12 Performance testing .............................................................................................................. 55 13 Qualifications and training ..................................................................................................... 55 14 Sources of additional information ........................................................................................... 56 Annex A (informative) Relevant EAC Directives ................................................................................ 57 Annex B (informative) Sources of additional information ................................................................... 58 Annex C (normative) Operations and maintenance ........................................................................... 59 Annex D (normative) Health and safety ............................................................................................. 66 Annex E (normative) Hydraulic design .............................................................................................. 68 Annex F (normative) Pumping Installations ....................................................................................... 75 Bibliography ....................................................................................................................................... 85

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iv © EAC 2010 — All rights reserved

Introduction Drain and sewer systems are part of the overall wastewater system that provides a service to the community. This can be briefly described as:

⎯ removal of wastewater from premises for public health and hygienic reasons;

⎯ prevention of flooding in urbanized areas;

⎯ protection of the environment. The overall wastewater system has four successive functions:

⎯ Collection;

⎯ Transport;

⎯ Treatment;

⎯ Discharge. Drain and sewer systems provide for the collection and transport of wastewater. Historically, drain and sewer systems were installed because there was a need to remove the polluted water, to prevent diseases. Traditionally, drain and sewer systems were constructed to collect and transport all types of wastewater together irrespective of the initial source. This led to difficulties in handling the peak flows in times of heavy rainfall and to the introduction of combined sewer overflows, which discharged polluted water to surface receiving waters. It was later recognized that separate systems, where foul wastewater was kept separate from runoff derived from surface water, would be an improvement over such combined systems. Although many drain and sewer systems started out as combined systems there are strong arguments for considering the separation of foul wastewater and surface water. The pollutant effects are not the same and the separation of effluents allows for the different treatment for each element of wastewater, providing more environmentally friendly solutions. This concept is included in the approach of integrated sewer management. This East African Standard provides a framework for the design, construction, rehabilitation, maintenance and operation of drain and sewer systems outside buildings. It is supported by more detailed standards for the investigation, design, construction, organization and control of drain and sewer systems such as those listed in the lower part of the diagram. In the preparation of this East African Standard, the following source was consulted extensively: BS EN 752:2008, Drain and sewer systems outside buildings Assistance derived from this source and others inadvertently not mentioned is hereby acknowledged.

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© EAC 2010 — All rights reserved 1

Drain and sewer systems outside buildings 1 Scope This East African Standard sets out the objectives for drain and sewer systems outside buildings. It specifies the functional requirements for achieving these objectives and the principles for strategic and policy activities relating to planning, design, installation, operation, maintenance and rehabilitation. It is applicable to drain and sewer systems, which operate essentially under gravity, from the point where wastewater leaves a building, roof drainage system, or paved area, to the point where it is discharged into a wastewater treatment plant or receiving water. Drains and sewers below buildings are included provided that they do not form part of the drainage system for the building. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. EN 476, General requirements for components used in discharge pipes, drains and sewers for gravity systems EN 858-1, Separator systems for light liquids (e.g. oil and petrol) — Part 1: Principles of product design, performance and testing, marking and quality control EN 858-2, Separator systems for light liquids (e.g. oil and petrol) — Part 2: Selection of nominal size, installation, operation and maintenance EN 1295-1, Structural design of buried pipelines under various conditions of loading — Part 1: General requirements EN 1610, Construction and testing of drains and sewers EN 1990, Eurocode — Basis of structural design EN 1991-1-1, Eurocode 1 — Actions on structures — Part 1-1: General actions Densities — self-weight, imposed loads for buildings EN 1991-1-2, Eurocode 1 — Actions on structures — Part 1-2: General actions — Actions on structures exposed to fire EN 1991-1-3, Eurocode 1 — Actions on structures — Part 1-3: General actions — Snow loads EN 1991-1-5, Eurocode 1 — Actions on structures — Part 1-5: General actions — Thermal actions EN 1991-2, Eurocode 1 — Actions on structures — Part 2: Traffic loads on bridges EN 1991-4, Eurocode 1 — Actions on structures — Part 4: Silos and tanks EN 1992-1-1, Eurocode 2 — Design of concrete structures — Part 1-1: General rules and rules for buildings

EAST AFRICAN STANDARD

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2 © EAC 2010 — All rights reserved

EN 1992-1-2, Eurocode 2 — Design of concrete structures — Part 1-2: General rules - Structural fire design EN 1992-3, Eurocode 2 — Design of concrete structures — Part 3: Liquid retaining and containment structures ENV 1993-1-1, Eurocode 3 — Design of steel structures — Part 1-1: General rules and rules for buildings EN 1994-1-1, Eurocode 4 — Design of composite steel and concrete structures — Part 1-1: General rules and rules for buildings EN 1996-1-1, Eurocode 6: Design of masonry structures — Part 1-1: General rules for reinforced and unreinforced masonry structures EN 1997-1, Eurocode 7: Geotechnical design — Part 1: General rules EN 1998-1, Eurocode 8: Design of structures for earthquake resistance — Part 1: General rules, seismic actions and rules for buildings EN 1998-3, Eurocode 8: Design of structures for earthquake resistance — Part 3: Assessment and retrofitting of buildings EN 1998-1, Eurocode 8: Design of structures for earthquake resistance — Part 1: General rules, seismic actions and rules for buildings EN 1999-1-1, Eurocode 9: Design of aluminium structures — Part 1-1: General structural rules EN 12889, Trenchless construction and testing of drains and sewers EN 13508-2, Condition of drain and sewer systems outside buildings — Part 2: Visual inspection coding system EN 14654-1, Management and control of cleaning operations in drains and sewers — Part 1: Sewer cleaning 3 Terms and definitions For the purposes of this document, the following terms and definitions apply. 3.1 aerobic dissolved oxygen is present 3.2 aesthetic <of pollution> aspects sensed by sight or smell, e.g. floating solids, oil films or bank-side litter 3.3 air valve valve used to allow air to escape from or enter into a rising main 3.4 anaerobic dissolved oxygen, nitrate, nitrite and sulfate is absent 3.5 backdrop manhole manhole with a connection, by means of a vertical pipe, at or just above invert, from a drain or sewer at a higher level

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3.6 backwater level elevation of the surface of the wastewater predicted or occurring in a drain or sewer system due to the hydraulic conditions downstream 3.7 biochemical oxygen demand (BOD) concentration of dissolved oxygen consumed under specific conditions (/ days at 20 °C with or without nitrification inhibition) by the biological oxidation of organic and/or inorganic matter in water 3.8 catchment area area draining to a drain, sewer or watercourse 3.9 cleaning ball spherical device, having an indented surface, designed to be carried through a drain or sewer by the flow to facilitate removal of sediments 3.10 confined space space in which the ventilation is restricted to the extent that special safety precautions need to be taken 3.11 combined sewer overflow device, on a combined system that relieves the system of excess flow 3.12 combined system drain and sewer system designed to carry both foul wastewater and surface water in the same pipeline(s) 3.13 common trench trench in which more than one pipe is located 3.14 dam board removable plank or section placed across a sewer or drain to divert or hold back the flow 3.15 depression storage precipitation retained in surface hollows that does not contribute to runoff 3.16 design life notional lifetime of an asset used for the purposes of design 3.17 detention tank tank or reservoir for the temporary storage of wastewater 3.18 domestic wastewater water discharged from kitchens, laundry rooms, lavatories, bathrooms, toilets and similar facilities 3.19 drain

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pipeline, usually underground, designed to carry wastewater from a source to a sewer 3.20 drainage service natural or artificial system for the draining of a catchment area 3.21 dry weather flow flow not affected by rainfall or snow melt 3.22 dry well dry chamber forming part of a pumping station and containing pumping equipment, normally used in conjunction with a wet well 3.23 duty point rate of flow and the corresponding total head for which a pump is designed or selected 3.24 exfiltration escape of wastewater from a drain or sewer system into surrounding ground 3.25 extraneous water unwanted flow in a drain or sewer system 3.26 explosion proof protected from causing ignition of flammable gases 3.27 flooding condition where wastewater and/or surface water escapes from or cannot enter a drain or sewer system and either lies on the surface or enters buildings (see also surface flooding) 3.28 flow balancing reduction in peak discharge by means of temporary storage of flow 3.29 flushing use of a temporary substantially increased flow to facilitate removal of obstructions or sediments from drains or sewers 3.30 gravity system drain or sewer system where flow is caused by the force of gravity and where the pipeline is designed usually to operate partially full 3.31 hydro-biological stress detrimental impact on aquatic flora and fauna, caused by high flow velocity and scour 3.32 infiltration (into the ground) the movement of surface water or treated effluent into the ground 3.33 infiltration

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(into the drain or sewer system) unwanted flow resulting from an ingress of groundwater into a drain or sewer system (see Figure 2) 3.34 inspection chamber chamber with a removable cover constructed on a drain or sewer that permits the introduction of cleaning and inspection equipment from surface level, but does not provide access for personnel 3.35 integrated sewer system management co-ordinated management of the planning, design, construction, rehabilitation, operation and maintenance of all drain and sewer systems in a catchment area taking into account all aspects of their performance 3.36 integrated urban drainage management co-ordinated management of the planning, design, construction, rehabilitation, operation and maintenance of all urban drainage systems in a catchment area taking into account all aspects of their performance 3.37 integrated water policies co-ordinated policies for the management of all bodies of water within a river basin including urban drainage systems within it 3.38 inverted siphon length of gravity drain or sewer that is lower than upstream or downstream lengths to allow the pipeline to pass below an obstacle, and which consequently operates under pressure 3.39 jetting use of high-pressure water jetting equipment to facilitate removal of obstructions or sediments from drains or sewers 3.40 maintenance routine work undertaken to ensure the continuing performance of drain and sewer systems 3.41 manhole chamber with a removable cover constructed on a drain or sewer to permit entry by personnel 3.42 outfall structure or point from which wastewater is discharged to a wastewater treatment plant or receiving water 3.43 operations actions taken in the course of normal functioning of drain and sewer systems (e.g. monitoring and regulation or diversion of wastewater) 3.44 pumping installation pumping station together with any associated rising main(s) 3.45 pumping station

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building, structures and equipment used to transfer wastewater through a rising main or otherwise to raise the wastewater 3.46 rainfall intensity depth of rain falling in unit time, or volume of rain falling in unit time per unit area 3.47 ramp manhole manhole with a steeply inclined pipe or channel from a drain or sewer at a higher level 3.48 rain water water arising from atmospheric precipitation, which has not yet collected matter from the surface (see Figure 2) 3.49 receiving water any type of water body where water or wastewater is discharged 3.50 rehabilitation measures for restoring or upgrading the performance of existing drain and sewer systems 3.51 relevant authority organisation with appropriate statutory powers of control 3.52 renovation work incorporating all or part of the original fabric of the drain or sewer by means of which its current performance is improved 3.53 repair rectification of local damage 3.54 replacement construction of a new drain or sewer, on or off the line of an existing drain or sewer, the function of the new drain or sewer incorporating that of the old 3.55 retention time time during which wastewater is held within the pumping installation 3.56 rising main pipe through which wastewater is pumped 3.57 rodding use of appropriate device on the end of flexible rods to facilitate removal of obstructions (or sediments) from or sewers 3.58 rodding point small diameter non-man access connection to a drain or sewer that facilitates cleaning or inspection 3.59

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runoff water from precipitation which flows off a surface to reach a drain, sewer or receiving water (see Figure 2) 3.60 runoff coefficient factor dependent on the ground catchment, and by which the rain water quantity per unit of time must be multiplied in order to indicate the flow expected to be carried to the drain or sewer system 3.61 self-cleansing ability of the flow in a drain or sewer to carry away solid particles, which would otherwise be deposited in the pipe 3.62 self-purifying capacity ability of receiving waters to recover from pollution by natural processes 3.63 separate system drain and sewer system, usually of two pipelines, one carrying foul wastewater and the other surface water 3.64 septic wastewater anaerobic wastewater which usually contains hydrogen sulphide 3.65 sewer pipeline or other construction, usually underground, designed to carry wastewater from more than one source 3.66 sewer system network of pipelines and ancillary works which conveys wastewater from drains to a treatment plant or other place of disposal 3.67 structural condition state of a drain or sewer in matters relating to the integrity of its fabric 3.68 sub-critical flow state of flow when the water velocity is less than the velocity of the small surface wave with water levels tending to be stable 3.69 super-critical flow state of flow when the water velocity is greater than the velocity of the small surface wave with violent fluctuations in water level being possible 3.70 surcharge condition in which wastewater and/or surface water is held under pressure within a gravity drain or sewer system, but does not escape to the surface to cause flooding 3.71 surface flooding condition where wastewater and/or surface water escapes from, or cannot enter, a drain or sewer system and either lies on the surface or enters buildings from the surface (see also flooding)

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3.72 surface receiving water receiving water body that is on the surface of the ground (e.g. river, lake or sea) (see Figure 2) 3.73 surface water water from precipitation, which has not seeped into the ground and which is discharged to the drain or sewer system directly from the ground or from exterior building surfaces (see Figure 2) 3.74 time of concentration time taken for runoff to travel from the hydraulically most distant point of the catchment area to a defined point in the drain or sewer 3.75 tank sewer section of sewer which acts as a detention tank 3.76 trade effluent wastewater discharge resulting from any industrial or commercial activity 3.77 urban drainage system systems used for the collection and transport of wastewater and other rain water runoff in an urban area

Key 1 Rain Water (see 3.48) 2 Runoff (see 3.59) 3 Surface Water (see 3.73) 4 Infiltration (see 3.32) 5 Surface Receiving Water (see 3.72)

Figure 2 — Terminology for flows derived from rain water

3.78 utility services

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services provided to customers and industry such as gas, electricity, telephone, cable TV and water 3.79 vortex manhole circular manhole within which a large difference in level is accommodated by the wastewater entering tangentially and descending helically 3.80 wastewater water composed of any combination of water discharged from domestic, industrial or commercial premises, surface run-off and accidentally any sewer infiltration water 3.81 wastewater treatment plant facility for the physical, biological and/or chemical treatment of wastewater 3.82 wet well chamber forming a part of a wastewater pumping station into which wastewater discharges prior to pumping. It can include submersible pumping equipment and pipework 3.83 whole life cost aggregate cost of a scheme over its design life, being the sum of the construction, operating and maintenance costs all calculated at the same time base 3.84 winching use of a bucket or other device pulled through a drain or sewer to facilitate removal of sediments (or obstructions) 4 Objectives 4.1 General The four objectives of drain and sewer systems are: ⎯ Public health and safety; ⎯ Occupational health and safety; ⎯ Environmental protection; ⎯ Sustainable development. Drain and sewer systems are part of the urban drainage system (see Figure 3). Urban drainage systems comprise all infrastructures for the management of wastewater and rain water in the built environment. The extent and role of the drain and sewer system within the urban drainage system will depend on local circumstances for each system. Urban drainage systems are part of a wider system of water management (see Figure 3) and form part of an integrated management of the whole water management system through the river basin management plan. Integrated sewer system management includes a consideration of the interactions of the drain and sewer system with the urban drainage system as a whole, and the wider water environment. 4.2 Public health and safety Drain and Sewer systems are provided in order to:

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⎯ prevent spread of disease by contact with faecal and other waterborne waste; ⎯ protect drinking water sources from contamination by waterborne waste; ⎯ carry runoff and surface water away while minimizing hazards to the public. Poorly designed, constructed or maintained systems can cause health or safety hazards to the public. The objective is to design, construct, operate, maintain and rehabilitate the system in order to minimize the health and safety risks associated with the conveyance of wastewater. 4.3 Occupational health and safety All work associated with the installation, operation, maintenance and rehabilitation of drain and sewer systems presents a range of occupational health and safety hazards. The objective is to minimize the occupational health and safety risks likely to arise during installation, operation, maintenance, and rehabilitation. 4.4 Environmental protection The objective is to design, construct, operate and maintain the system to minimize the impact on the environment. The impact of drain and sewer systems on the receiving waters shall meet the requirements of any national or local regulations or the relevant authority. Other environmental requirements specified by any national or local regulations or the relevant authority shall also be met. 4.5 Sustainable development The objective is to design, construct, operate, maintain and rehabilitate the system at the best environmental, social and economical costs so that it: a) uses materials that minimize the depletion of finite resources; b) can be operated with the minimum practicable use of energy; and, c) can be constructed, operated and, at the end of their life, decommissioned with the minimum

practicable impact on the environment. 5 Requirements 5.1 Functional requirements 5.1.1 Introduction Functional requirements cover the drain and sewer systems, together with combined sewer overflows, pumping installations and other components, including the effects of their discharges on receiving waters and the receiving wastewater treatment plant. The requirements shall be considered in respect of the whole system to ensure that additions or modifications to the system do not result in failure to meet the target standards. Requirements shall be established that, whilst taking into account sustainable development and whole life costs including indirect costs (e.g. cost of social disruption), ensure that drain and sewer systems convey and discharge their contents without causing unacceptable environmental nuisance, risk to public health, or risk to personnel working therein.

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Each functional requirement can relate to more than one objective. An indication of the relevance of each of the functional requirements to achieving the objectives is shown in Table 1.

Table 1 — Relationship between objectives and functional requirements

Clause No Public health and safety

Occupational health and

safety

Environmental protection

Sustainable development

5.1.2 Protection from flooding XXX XX XXX — 5.1.3 Maintainability XX XXX XX XX 5.1.4 Protection of surface receiving waters

XXX X XXX XX

5.1.5 Protection of groundwater XXX — XXX XXX 5.1.6 Prevention of odours and toxic, explosive and corrosive gases

XXX XXX XXX XXX

5.1.7 Prevention of noise and vibration XX XXX X X 5.1.8 Sustainable use of products and materials

— — XX XXX

5.1.9 Sustainable use of energy — — XX XXX 5.1.10 Structural integrity and design life XXX XXX XXX XXX 5.1.11 Maintaining the flow XXX — XXX X 5.1.12 Watertightness XXX X XXX XX 5.1.13 Not endangering adjacent structures and utility services

XXX XXX X XX

5.1.14 Inputs quality XX XXX XXX XX NOTE XXX is High;

X is Low and; — is not related.

5.1.2 Protection from flooding Flooding from drains and sewers can have a major impact on the health of people affected. The economic impact can be high and depends on the type of location flooded. Flooding shall be limited to nationally or locally prescribed frequencies taking into account the: ⎯ health and safety effects of the flooding; ⎯ costs of the flooding; ⎯ extent to which any surface flooding can be controlled without causing damage; ⎯ whether surcharge is likely to lead to flooding of basements. NOTE In some jurisdictions it is the responsibility of the property owner to provide protection to prevent flooding of basements due to surcharge. The hydraulic capacity shall limit flooding to nationally or locally prescribed levels and frequencies taking into account backwater levels. The hydraulic capacity shall allow for foreseeable increases in flow over the design life of the system. The effects of flows discharged into downstream sewers or receiving waters shall be considered. Further details are included in Clause 8. Where there are components in the system, which have a high risk of failure, measures should be taken to avoid or minimise the risk of flooding in the event of failure of those components. 5.1.3 Maintainability The system shall be planned, designed, constructed and rehabilitated to allow appropriate maintenance activities to be carried out safely and without risks to the health of personnel. Adequate access and working space shall be provided for maintenance purposes. 5.1.4 Protection of surface receiving waters

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Surface receiving waters shall be protected from pollution within nationally or locally prescribed limits. The impact of drain and sewer systems on the surface receiving waters shall meet the requirements of any national or local regulations or the relevant authority. Other environmental requirements specified by any national or local regulations or the relevant authority shall also be met. 5.1.5 Protection of groundwater Groundwater shall be protected from pollution within nationally or locally prescribed limits. The effect of the drain and sewer system on the local recharge of aquifers shall be considered. The impact of drain and sewer systems on the receiving groundwater shall meet the requirements of any national or local regulations or the relevant authority. Other environmental requirements specified by any national or local regulations or the relevant authority shall also be met. 5.1.6 Prevention of odours and toxic, explosive and corrosive gases Sewer systems shall be designed, constructed, maintained and operated to avoid odour nuisance, or toxic, explosive or corrosive gases. 5.1.7 Prevention of noise and vibration The system shall be designed, constructed, maintained and operated so that noise and vibration are minimised. 5.1.8 Sustainable use of products and materials Products, materials, and their construction methods shall be selected that minimise depletion of finite resources having regard to the design life of the component and the potential for re-use or recycling, for example minimising the volume of excavated material and the reuse of excavated material. 5.1.9 Sustainable use of energy The design and operation of the drain and sewer system shall, so far as is practical, minimise the use of energy over the life of the system. 5.1.10 Structural integrity and design life Drains, sewers and other components shall be designed, constructed, maintained and operated to ensure structural integrity over the design life. 5.1.11 Maintaining the flow The system shall be designed, constructed, maintained and operated to reliably convey all design flows that can legally be discharged into the system to the point of discharge, ensuring that the operation of the system is safe, environmentally acceptable and economically efficient. 5.1.12 Watertightness New drains, sewers and ancillary structures shall be watertight. Existing drains, sewers and ancillary structures shall be watertight in accordance with national or local testing requirements. 5.1.13 Not endangering adjacent structures and utility services The design, construction, maintenance and operation of drains and sewers shall not endanger existing adjacent structures and utility services. 5.1.14 Inputs quality

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The drain and sewer system can be designed to receive both domestic and non-domestic wastewater inputs. The quality of the non-domestic inputs shall be controlled so that they do not compromise the integrity of the fabric of the system or its function or constitute a danger for the environment. National or local regulations can give requirements for inputs quality. 5.2 Determination of performance requirements for the drain and sewer system In order to evaluate the performance of the system and to allow development of design standards, measurable performance requirements shall be determined from each functional requirement. The process for determining performance requirements is illustrated in Figure 4. For each functional requirement there can be legal requirements, public expectations and financial constraints which will influence the performance requirements. For each aspect of performance different levels could be required for example: ⎯ trigger levels which justify early upgrading action according to priority; ⎯ target levels to aim for in upgrading, which shall be equal to the requirements for new

construction, but which sometimes can only be achievable or necessary in the longer term. Examples of performance requirements in use in different countries can be obtained from the organisations listed in Annex B. Performance requirements shall be reviewed periodically and updated if necessary. The performance requirements for the system should be updated after major extension, maintenance or rehabilitation. In principle the performance requirements for a rehabilitated system shall be the same as those for a new system.

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Figure 3 — Drain and sewer systems in the river basin

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Figure 4 — Process for determining performance requirements 6 Integrated sewer system management 6.1 Introduction Integrated sewer system management is the process of achieving an understanding of existing and proposed drain and sewer systems, and using this information to develop strategies to ensure that the hydraulic, environmental, structural and operational performance meets the specified performance requirements taking into account future conditions and economic efficiency. The integrated sewer system management process is illustrated in Figure 5. The integrated sewer system management process has four principal activities. ⎯ Appropriate level of investigation of all aspects of the performance of the drain and sewers

system; ⎯ Assessment of the performance by comparison with the performance requirements including

identification of the reasons for the performance failures; ⎯ Developing the plan of measures to be taken; ⎯ Implementation of the plan.

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Figure 5 — Integrated sewer system management process The need for further investigation can become apparent either during the performance assessment or the development of the plan. Integrated sewer system management forms the basis for the operation and rehabilitation of the drain and sewer system. The information is regularly updated for the future management of the drain and sewer system. The role of the drain and sewer system should be determined within the context of the whole river basin catchment and the other elements of the urban drainage system. To determine this role account should be taken of integrated water policies set by any national or local regulations or the relevant authority together with any requirements of the integrated river basin management plan. Account should also be taken of any policies resulting from integrated urban drainage management. The boundary conditions should also be considered. 6.2 Investigation 6.2.1 Introduction The investigation is the first stage in the Integrated Sewer System Management as described in 6.1 (see Figure 5). The process for investigation is outlined in Figure 6. Damaged, defective or hydraulically overloaded drains and sewers represent a potential hazard through flooding and collapses, and through pollution of surface receiving waters, groundwater and soil. The problems found in existing drain and sewer systems are frequently interrelated and upgrading works will often be designed to overcome a number of problems at the same time. The investigation and planning of rehabilitation work should be carried out on complete catchment areas so that all problems and their causes can be considered together. In large sewer systems it could be

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necessary to start by investigating appropriate parts of the system. The procedures described in this Standard can be applied in any drain and sewer system, but detailed application should take account of the age, location and type of system, the materials used in its construction, together with functional and climatic factors. 6.2.2 Purpose of investigation The investigation is carried out in order to make an assessment of the performance of the drain and sewer system and its components. This can include: ⎯ investigation aimed at strategic planning; ⎯ investigation aimed at operational planning. The purpose of the investigation influences the way in which it will be carried out (e.g. choice of method, degree of detail, desired accuracy) and the way in which the results will be assessed. The components of the drain and sewer system included in the investigation shall be those that are necessary to fulfil the purpose of the investigation. Examples include; drains, surface water and foul sewers, combined sewers, gravity sewers, pressure/vacuum sewers, manholes, inspection chambers and other access facilities, pumping stations, rising mains, storage and retention tanks, combined sewer overflows, monitoring facilities, control facilities, outfalls, gravel and sand traps, flushing facilities, ventilation, sedimentation tanks, light liquid/grease separators. 6.2.3 Review of performance information An indication of the type of performance problems, if any, on existing systems is likely to be known through reports of incidents such as sewer collapses, flooding or polluted watercourses and from previous investigations. Records of past incidents and any other relevant information should be brought together and a detailed review should be carried out to establish the scope of the investigations. Examples are; records of flooding incidents, pipe blockage incidents, sewer collapse incidents, rising mains failures, disease, injury or fatal incidents to operators, disease, injury or fatal incidents to members of the public, sewer damage incidents, compliance with discharge consents into and out of the system, closed circuit television (CCTV) survey and visual inspection data, wastewater related odour complaint incidents, hydraulic performance analysis, performance of mechanical/electrical equipment, results of monitoring, performance and condition of flow control structures, sewer surcharge incidents. The relevant authorities will be the source of many of the records listed above. All appropriate records should be retained. Where large numbers of complete or partial catchments are in need of investigation, the existing information collected may also be used to assign priorities to the investigation of the perceived problems in each catchment (for example by comparing the cost of the investigation with the benefit that might be achieved). These can then be used to draw up a comprehensive programme so that the catchments with the most serious problems are investigated first.

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Figure 6 — Process for investigation 6.2.4 Determine the scope of the investigation Following the review of the current performance information it will be possible to decide whether to carry out an investigation and whether the extent of the problems justifies an investigation of the entire catchment area. The extent and detail of the subsequent investigation of the hydraulic, environmental, structural and operational aspects shall be determined..

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6.2.5 Review existing information The collection and review of all available relevant information about the sewer system shall be carried out and is the basis from which all other activities are subsequently planned. This information should include historical records. In addition to the performance information listed in 6.2.3, examples are: ⎯ inventory including:

⎯ location, dimensions, shape and type of material of all drains and sewers;

⎯ position depth and levels of manholes and the levels of connections to the manholes;

⎯ positions of connections to drains and sewers;

⎯ layout of ancillary structures such as combined sewer overflows, outfalls and pumping installations, including details of any special plant (e.g. pumps or screens).

⎯ relevant permits and legal requirements; ⎯ previous operational, maintenance, structural and safety measures to overcome the problems; ⎯ nature and quantities of trade effluent; ⎯ previous inspections; ⎯ previous hydraulic calculations or hydraulic models; ⎯ previous assessments of environmental impact; ⎯ existing drain and sewer condition data; ⎯ receiving water quality and use; ⎯ groundwater levels and velocities; ⎯ ground conditions including infiltration capacity; ⎯ groundwater protection zones; ⎯ previous test information; ⎯ characterisation of wastewater; ⎯ information on proposed new development or redevelopment within the catchment area. Some of this information can be available from as-constructed drawings. This information should be assessed to determine what further information is required in order to carry out the investigation. 6.2.6 Inventory update Where the inventory is incomplete it shall be updated so that a sufficient record of the sewer system is available to carry out the investigation. NOTE The update of the other information is included in the hydraulic, environmental, structural and operational investigations.

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6.2.7 Hydraulic investigation Testing and inspection procedures can be required in order to ensure an adequate evaluation of flows (wet and dry weather, infiltration, flow through gaps in manhole tops (between the cover and frame), exfiltration and wrong connections). Surveys can include precipitation and flow measurements, identification of wrong connections and groundwater measurements. In some cases it is not possible to understand the hydraulics of the system without using a hydraulic model. This sewer flow simulation model should be based on an as-built report updated after onsite investigation of the main works. However a model is not usually recommended where: ⎯ there are no known hydraulic problems (particularly where the sewer system takes only foul

wastewater flows); and, ⎯ there are no combined sewer overflows; and, ⎯ structural problems are to be solved using techniques which do not reduce the hydraulic capacity

of the sewer. Information on the use of computer based sewer flow simulation programs is given in 8.4.3. Calibration and/or verification of the models shall be carried out whenever sufficient information is available. The procedures used depend on the sewer flow simulation program. If suitable agreement is not obtained, the model input data should be checked and then the sewer records. Having identified possible causes of error it will often be necessary to confirm these by site inspection and then adjust the model accordingly. Data shall not be modified without justification based on an inspection of the system. 6.2.8 Environmental investigation The environmental impact will depend on the nature of the wastewater and its potential to escape from the system. In particular the location of trade effluent sources and contaminated surface water sources shall be identified and the nature, quality, quantity and the potential environmental hazards reviewed. Where necessary, surveys shall be carried out to provide any data not available from records. Investigations can be required to determine where leakage from drains and sewers is affecting groundwater quality, giving priority to drains or sewers which pass through aquifer protection zones or where they carry particularly hazardous substances. The quality of surface receiving waters shall be ascertained to see whether they meet the requirements and if not, whether the sewer system is a significant factor. Consideration should be given to other environmental factors such as noise, odour, visual intrusion and potential soil contamination. 6.2.9 Structural investigation It is important to ensure that investigation of the system is selective in order to avoid duplication of previous work. The structural investigations may include either a complete survey of the drain and sewer system or a more selective approach. Consideration should be given to the age and location of existing infrastructure, geotechnical data including the pipe bedding and surround, and the vulnerability of existing buildings and other utility services. Wherever practicable the recording of the structural condition of drain and sewer systems should be carried out by an indirect system (e.g. closed circuit television (CCTV) system) in order to avoid personnel entering the system (see Clause 7). Where it is not possible to obtain sufficient information from indirect inspection then direct inspection (e.g. by walking through the pipeline) may be used. The

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drain and sewer system shall be cleaned as necessary to make it possible to record and assess the actual condition. The nature and quantity of any material removed can be relevant to the structural investigation. During the survey the system shall be kept free from wastewater as far as necessary. The condition of the system shall be observed and recorded as accurately and comprehensively as practicable. A uniform coding system shall be used to ensure that the results can be compared. The observations recorded shall include all those that could affect the structural integrity of the system. Examples include: ⎯ unacceptable fissures; ⎯ deformation; ⎯ displaced joints; ⎯ defective connections; ⎯ roots, infiltration, settled deposits, attached deposits, other obstacles; ⎯ subsidence; ⎯ defects in manholes and inspection chambers; ⎯ mechanical damage or chemical attack. Where appropriate, other qualitative and quantitative investigation techniques may be used. These include sonar (for pipes that are filled with water) and ground probing radar or other geophysical techniques (e.g. for detecting voids behind the wall of the sewer pipe) or mechanical techniques (e.g. internal jacking to measure the stiffness of the side wall support). Investigation of the chemical composition of the groundwater and the soil should be carried out where this could affect the structural integrity. The results of the structural investigations can also be relevant to the assessments of the hydraulic performance and environmental impact. 6.2.10 Operational investigation Existing operational procedures, inspection schedules and maintenance plans shall be identified and documented. The frequency and location of recorded operational incidents (e.g. blockages, pumping station failures, sewer collapses etc.) shall be reviewed. The impact of operational problems on the hydraulic, environmental and structural performance of the system should be determined from incident records. The causes of significant recurrent operational incidents shall be investigated. To deal with operational problems in the most cost effective way, it is necessary to investigate and understand the causes. Further information can be found in 11.4. 6.3 Assessment 6.3.1 Introduction The performance of the system shall be assessed against the performance requirements.

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Figure 7 — Process for assessment 6.3.2 Assessment of the hydraulic performance The results of the hydraulic surveys and/or the verified flow simulation model shall be used to assess the hydraulic performance of the system for a range of rainfall conditions related to the performance requirements (see 8.4.3). 6.3.3 Assessment of environmental impact The results of the investigations shall be considered together with information on the frequency, duration and volume of discharges to receiving waters, determined using a verified flow simulation model (see 6.2.7) where this is available or from site measurements. This information shall then be used to assess the environmental impact (including impact on soil and groundwater) of the drain and sewer system (see 8.5). The results of the structural investigation (see 6.2.9), the trade effluent survey and other relevant investigations shall be examined to identify: ⎯ sources of hazardous effluents; ⎯ exceedence of permissible concentrations and discharges; ⎯ other deviations from permits.

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6.3.4 Assess structural condition Once the system has been inspected, the next stage is to examine the results to identify those areas requiring action. A number of methods have been developed to assist in this process. Details of these can be obtained from the organisations listed in Annex B. 6.3.5 Assess operational performance The operational performance of the system as measured by the number of operational incidents or failures should be assessed. 6.3.6 Compare with performance requirements The results of the assessment of the hydraulic, environmental, structural and operational performance should be brought together so that the overall performance of the system and its components can be compared to the performance requirements (see 5.2). Performance indicators are one method of comparing the overall performance of a system with performance requirements. Any performance indicators used should be: ⎯ clearly defined, concise and unambiguous; ⎯ verifiable; ⎯ simple and easy to use. 6.3.7 Identify unacceptable impacts Details of those parts of the system where the hydraulic, environmental, structural or operational performance of the system or its components does not meet the performance requirements should be recorded. 6.3.8 Identify causes of performance deficiencies Based upon the results of the hydraulic, environmental, structural and operational investigations, the causes of performance deficiencies shall be determined. The relative impact of each cause should be assessed in order to develop appropriate solutions and to set the priority for action. 6.4 Developing the plan 6.4.1 Introduction The process of producing the plan to fulfil the performance requirements is outlined in Figure 8. 6.4.2 Develop integrated solutions 6.4.2.1 Introduction Integrated solutions shall be developed that fulfil the performance requirements, taking into account future conditions.

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Figure 8 — Process for developing the plan 6.4.2.2 Hydraulic solutions Hydraulic options include: a) Maximise use of existing flow capacity by:

⎯ removal of constrictions;

⎯ cleansing. b) Source control — Reducing the hydraulic input to the sewer system by:

⎯ diversion of surface water flows to infiltration drainage systems or pervious areas;

⎯ use of porous pavements;

⎯ diversion of flows to another system;

⎯ reduction of infiltration and inflow of extraneous water. c) Attenuate peak flows by:

⎯ mobilisation of existing storage potential within the system (strategically placed flow controls);

⎯ mobilisation of surface storage (including storage within the property boundary);

⎯ provision of additional storage (tank sewer or detention tank).

d) Increase sewer system flow capacity by:

⎯ replacement with larger pipe,

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⎯ construction of additional pipeline;

⎯ renovation of the existing drain or sewer. 6.4.2.3 Environmental solutions Environmental options include: a) Reduce pollutant inputs to system by:

⎯ sediment basins and grit separators;

⎯ use of vegetation to absorb pollutants from runoff before entering the system;

⎯ controlling inputs (e.g. trade effluents). b) Decrease planned pollutant discharges to receiving waters by:

⎯ increase of flows to treatment (see hydraulic solutions above);

⎯ treatment of surface water discharges (e.g. by separators, retention ponds etc.)

⎯ improve solids retention and hydraulic performance of combined sewer overflows;

⎯ real time control. c) Decrease impact by relocation of points of discharge. d) Reduce exfiltration by rehabilitation measures such as:

⎯ repair techniques (e.g. sealing leaks);

⎯ renovation techniques (e.g. provision of watertight lining);

⎯ replacement of pipeline using open-cut or trench-less techniques.

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Figure 9 — Decision process for selection of structural solutions

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6.4.2.4 Structural solutions Structural options include: a) Protect fabric of sewer by provision of appropriate linings or internal coatings. b) Rehabilitate fabric of sewer by:

⎯ repair;

⎯ renovation;

⎯ replacement. The decision process necessary to select the appropriate structural solution is given in Figure 9.

6.4.2.5 Operational solutions In some cases an operational solution can be appropriate. Examples of possible operational solutions include: ⎯ planned inspection and cleaning of a drain or sewer; ⎯ increased frequency of maintenance of pumps or pumping stations. 6.4.3 Assess solutions Solutions shall be assessed and the optimal solution selected having regard to the basic performance requirements (see Clause 5) and factors such as: a) Safety in construction and operation — The minimisation of risks to health and safety during

construction and subsequent operation of the system. b) Social disruption — The disruption to local residents and other members of the public due to

traffic delays, dust, noise and other social factors shall be considered. c) Sustainable use of resources — The use of energy and other finite resources in the

construction and operation of the system shall be taken into account. The ability to recycle materials used in the upgrading works and any waste produced shall be considered.

d) Phasing of the works — The possibility of integrating the solution into a staged programme of

works shall be considered. This shall take into account the priorities of the works and the benefits in terms of improved performance associated with each identified phase of the works, and the cost savings associated with deferral of the later stages.

e) Relationship to other infrastructure works — The benefits of phasing the works with other

infrastructure works shall be considered. f) Capacity and resource constraints — Account should be taken of the resource constraints

(e.g. personnel, supply chain and financial) in the selection and phasing of the options. g) Future maintenance liabilities — The cost of future maintenance works and other operational

costs of the system should be taken into account. The environmental impact of disposal of maintenance residues (see 8.5.1) shall also be considered.

h) Economic appraisal — The costs and benefits shall be considered to determine whether the

additional benefits of one solution over another, for example increased asset life, are justified. i) Whole life cost — The whole life cost of the solutions including temporary works, diversion of

other utility services and all design, investigation and operational costs shall be taken into account as well as the indirect costs (e.g. cost of social disruption).

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6.4.4 Prepare action plan The selected integrated solution shall be documented to give a single plan for the drain and sewer system. The documentation should include: ⎯ detailed objectives; ⎯ legal requirements and permits, including any timescales for improvement; ⎯ performance criteria; ⎯ priorities; ⎯ proposed works including costs and phasing; ⎯ relationship to other construction or planned development; ⎯ consequences for operations and maintenance. Four types of plan can be prepared: a) New development — Information on drainage of new developments proposed. Where

significant new development or redevelopment is proposed in the catchment, a plan should be produced showing:

1) whether the foul and surface water from the new development should be drained by

extension to an existing drain or sewer system or by an independent system or, for surface water, by an infiltration system;

2) if the system is to be an extension of an existing system, the upgrading works to the

existing system to accommodate the additional flows should be described in the rehabilitation plan for that system;

3) outline of the main sewer systems to serve the development.

b) Rehabilitation plan — Information on proposed rehabilitation works. The options to be

considered will fall into one or more of the four categories: Hydraulic, Environmental, Structural and Operational performance. The works necessary to upgrade an existing drain and sewer system to meet the performance requirements should be incorporated into a rehabilitation plan. This should include:

1) Details of the necessary upgrading works; 2) Other options for upgrading the system; 3) Any anticipated phasing of the work; 4) Whether any of the items are conditional on any planned developments.

c) Operational plan — Inspection schedules, operational procedures and contingency plans. The

operations plan shall indicate the approach to be taken in a particular drain and sewer system. The plan shall include:

1) inspection routines (see C.2.1); 2) procedures used in the operation of the elements of the system (see C.2.2); 3) contingency and emergency plans (see C.2.3).

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d) Maintenance plan — Details of maintenance policies and schedules for each component of the system. The maintenance plan shall include:

1) type of maintenance strategy to be used in each component of the system and the

monitoring requirements and frequencies. The strategies for maintaining drain and sewer systems are planned or reactive maintenance, or a combination of both.

i) Planned maintenance includes a programme of work to remedy the defects and

problems identified during inspection. It is particularly required to reduce the incidence of failure where the consequences are severe.

ii) Reactive (or crisis) maintenance involves responding to failures and problems as they

are identified. It is appropriate for those parts of the system that can function with little or no maintenance.

2) risk assessment, taking into account the probability of failures and their consequences.

6.5 Implementation 6.5.1 Introduction

Figure 10 — Process for implementation 6.5.2 Carry out work Where it is necessary to extend, reduce or rehabilitate the drain or sewer system these works should be designed in accordance with clause 8 and clause 9 and constructed in accordance with Clause 10. The operations and maintenance plans should be implemented in accordance with Clause 11. 6.5.3 Monitoring performance It is important to monitor the effectiveness of solutions and to update the plan, including the records (inventory) and the hydraulic model (see Clause 12). 6.5.4 Review performance requirements and update plan

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The performance requirements should be reviewed periodically and the whole planning process repeated so that all the plans remain up to date. 7 Health and safety principles The fundamental strategy for occupational health and safety requires employers to avoid risks to health and safety, to assess the risks which cannot be avoided and to use the results of that assessment to mitigate the residual risks through combating these risks at source in preference to the application of protective measures and to ensure that preference is given to collective protective measures over those protecting the individual only. These are the principles of prevention. The strategy also requires that employees are appropriately instructed in the risks to their health and safety. With regard to design, due account shall be taken of the principles of prevention, in the design and preparation stages of the project, that specific documentation regarding heath and safety risk is drawn up and made available to parties to the project and that workers are consulted over and informed of the risks to their health and safety. Accordingly drain and sewer systems shall be designed, constructed and operated so that the occupational health and safety risks to personnel undertaking work associated with the drain and sewer system are minimised. In addition welfare facilities shall be provided where appropriate. Those who are responsible for work in drains and sewers, including the operator of the drain or sewer system shall ensure that the work does not present a risk to the health or safety of any person carrying out the work or any person who can be affected by their actions. In addition it is the responsibility of employers to: ⎯ provide safe systems of work including arrangements for safe access to and egress from the

sewer system, and sufficient working space while in the sewer system; ⎯ ensure that their employees are properly instructed, trained and supervised in the work being

carried out and in the safe systems of work in use. National or local regulations or the relevant authority can lay down requirements regarding the health, safety and welfare of the public and/or personnel. They can include more requirements than indicated in this standard. The employer should define all tasks, competences and the ensuing responsibilities relating to the health and safety activities. Furthermore, the employer should provide a well-structured documentation of its hierarchy and organization of workflow [see ISO 9000 (all parts)]. Managers and supervisors responsible for safety should-check all relevant regulations for their proper application. If they detect flaws in the hierarchical and workflow organization and/or the documented regulations, they should initiate immediate remedial action. Further information on Health and Safety is found in Annex D. 8 Design principles 8.1 General The basis of the design stage should be the action plan (see Clause 6). Design is the process of defining the project in sufficient detail so that instructions can be given to others for the system to be constructed or maintained. The design process includes the following stages: ⎯ conception;

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⎯ preliminary topographical, geotechnical and other investigations (see 9.3); ⎯ preliminary calculations to check the feasibility of the proposed approach; ⎯ refinement of the general concept; ⎯ more detailed calculations; ⎯ production of detailed drawings or specifications. The design process is typically iterative. Designers should take into account the practicability of safely constructing, operating and maintaining the v system (see Clause 7). The design shall fulfil the objectives (see Clause 4) and meet the functional requirements (see Clause 5) and the action plan (see Clause 6). Specific requirements shall be considered in case of rehabilitation. Together with the functional requirements (see clause 5) the financial and economic aspects of the various options shall also be considered before reaching a decision as to the preferred solution. The above matters shall be considered in terms of their implications on the whole life cost. 8.2 Types of systems There are two types of wastewater to be collected and transported by the system; foul wastewater and surface water. Two options are available as follows: ⎯ Combined system — where both types of wastewater are mixed; ⎯ Separate system — where each type of wastewater is collected and transported in a dedicated

sewer (surface water in a surface water sewer and foul wastewater in a foul sewer). Variants of these basic systems are also possible. The selection of a system will mainly depend upon: ⎯ national or local water management policies; ⎯ type of system which presently exists and how it is expected to evolve; ⎯ possible future changes in the catchment; ⎯ capacity and quality of receiving waters; ⎯ nature of influents to the system; ⎯ need for prior treatment; ⎯ topography; ⎯ ground characteristics; ⎯ treatment plant; ⎯ economic considerations; ⎯ other local conditions.

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Adopting an integrated approach towards surface water drainage benefits both the quality and quantity impacts on receiving waters. Where a new system is being proposed, surface water should be kept separate from other wastewater. The discharge of surface water should be in accordance with the following hierarchy. a) To an adequate infiltration drainage system. b) Directly to a surface receiving water. c) To a drain or sewer system. Storage should be provided to limit the peak discharges to acceptable flow rates. National or local regulations or the relevant authority can specify the type of system to be used. Permissible influents to the system are: ⎯ Domestic wastewater. ⎯ Authorised trade effluents. (In some cases pre-treatment of such trade effluents will be

necessary before discharge to the system is permitted in order to achieve the quality required by national or local regulations or the relevant authority).

⎯ Surface water and, where exceptionally permitted, groundwater. The nature of the expected

influents shall be assessed. The design shall take into account the conveyance of wastewater, including trade effluents, which will neither damage the system and/or the wastewater treatment plant, nor impair their operation. 8.3 Layout and profile The design of the system shall ensure that the layout and profile meets all relevant functional requirements including: ⎯ Maintainability (see 5.1.3); ⎯ Protection of occupational health and safety (see 4.3); ⎯ Not endangering adjacent structures and utility services (see 5.1.13). The layout and profile will be influenced by topography, the character of developments served, existing and future flows from the catchment, the suitability of receiving waters or receiving wastewater treatment plant and the adequacy of any existing system to accept the design flow. Economical design is usually achieved when drains and sewers follow the natural falls of the ground. Where practicable they should be laid at such gradients as will prevent excessive accumulation of solid matter in the invert. The route shall be selected so as not to impair the stability of structures. Access chambers should be sited in locations where they can be reached by operator personnel and equipment. Access should also be provided for excavation to repair a sewer, if this were to be necessary. Circumstances can make the pumping of wastewater either necessary or advisable and should be considered alongside the long-term energy commitments and the whole life costs involved. Detailed design of pumping installations is considered in 9.2.6. Positive and negative pressure systems are relatively independent of gradient and the depth of cover. In certain circumstances they are alternatives to, or can form part of, systems operating essentially under gravity. Long-term energy commitments and whole life costs including maintenance shall be

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considered for each option. Positive pressure systems should be designed in accordance with EN 1671. Vacuum sewer systems should be designed in accordance with EN 1091. Where an appropriate sewer is not available and cannot immediately be provided, provision for local treatment of wastewater shall be provided. The levels of the receiving waters at the outfalls shall be taken into account in the planning of the sewer system. Detailed design of layout and profile is described in 9.2. 8.4 Hydraulic design 8.4.1 General The Hydraulic design of the system shall ensure that the design meets all relevant functional requirements including: ⎯ Protection from flooding (see 5.1.2); ⎯ Maintainability (see 5.1.3); ⎯ Maintaining the flow (see 5.1.11). 8.4.2 Foul drains and sewers The design flows for drains and sewers comprise: ⎯ Domestic wastewater flows; ⎯ Authorised trade effluent flows. Extraneous water flows may be included in the calculation where these flows cannot be avoided. Design flows should be calculated in accordance with 9.4.2. The hydraulic capacity of the pipelines shall be calculated in accordance with Annex E. Surcharging is undesirable in foul gravity drain and sewer systems. Foul drains and sewers should therefore be designed to run at less than pipe full conditions. Rising mains shall be designed to carry the required design flows in self-cleansing conditions without using excessive energy (see Annex F). The retention time should also be limited so that septicity does not occur (see 9.5.3). Detailed design of the hydraulic design of foul drains and sewers is described in 9.4.2. 8.4.3 Surface water drains and sewers 8.4.3.1 General Surface water drains and sewers collect and transport runoff generated within a catchment area during rainfall, for safe discharge into a receiving water or treatment plant. The magnitude of peak flows depends on the intensity and duration of rainfall, the size and configuration of impermeable areas and measures taken to reduce surface water. The topography, soil type and its permeability have also to be considered when estimating the flows emanating from other areas. Surface water drains and sewers are dimensioned in order to limit flooding. It is usually impracticable to avoid flooding from very severe storms. A balance therefore has to be drawn between cost and the political choice of the level of protection provided.

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The level of protection should be based on a risk assessment of the impact of flooding to persons and property. The level of protection should be specified in performance criteria for flooding frequencies or design storm events used in calculation. The design criteria shall be based on the performance criteria. The hydraulic capacity of surface water drains and sewers shall limit surcharge to nationally or locally prescribed levels and frequencies taking into account backwater levels. Surface water drains and sewers shall be designed so that the effect of any flooding caused by storms in excess of the nationally or locally prescribed flooding frequencies causes the minimum of impact to persons and property. Storage should be provided (e.g. by use of detention tanks and ponds) to minimise the hydraulic impact on receiving waters. Other techniques can be used to reduce the runoff entering the drain and sewer system either in addition to or as a substitute to the use of drains and sewers. These techniques are based on one or more of the following principles: ⎯ infiltration systems; ⎯ minimising the area of impermeable surfaces connected to the drain and sewer system; ⎯ time lag and attenuation of the flow. In setting hydraulic design performance criteria for surface water sewers, allowance shall be made for the design methods that are likely to be used. In all cases the scale of the consequences of flooding should be taken into account. For surface water drains and sewers, design flows for the surface water pipelines will be runoff. No allowance shall be made for any other wastewater component. 8.4.3.2 Surface water inlets Surface water inlets shall be designed in order to ensure an adequate transfer of runoff from impermeable areas into the surface water drains and sewers. 8.4.3.3 Design criteria Design criteria shall take into account any changes in flows expected over the design life of the drain or sewer system if these changes are not otherwise taken into account in the design. The potential effects of climate change should be considered. This is to ensure that the sewer continues to meet the performance criteria over the design life of the system. The frequency of an event may be expressed either as a return period or a probability of occurrence in any one-year period. Surcharge frequencies and depths in surface water drains and sewers shall be limited to any nationally or locally prescribed values having regard to: ⎯ Whether there are any connected basements not protected by anti-flooding valves, effluent lifting

stations or pumping stations; ⎯ Whether the surcharge is likely to lead to flooding of basements. Design flooding frequencies should be set in order to manage the risk of flooding, having regard to both the frequency and consequences of flooding. National or local regulations or the relevant authority can specify design storm frequencies or design flooding frequencies or both. Different design criteria may be set for combined and separate systems.

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The designer shall assess risk of flooding in events that exceed the design flood frequency, taking into account both the consequences of the flooding and the frequency. Flow routes for excess flows should be investigated to determine the consequences and where possible, the design should be changed to minimise the impact. Where the risk of flooding cannot be reduced by these means the design frequency should be decreased. The nature of design criteria will depend on the type of design methods used. a) Design criteria for use with simple design methods

In simple design methods the pipes are usually designed to run full, without surcharge, for relatively frequent storms in the knowledge that this provides protection against flooding from much larger storms. In the absence of any design criteria specified in national or local regulations or by the relevant authority the "design storm frequency" for no surcharge criteria in Table 2 should be used for small schemes. These approaches should be applied when an existing system is being considered for upgrading. Both the storm frequency and the flood frequency may be expressed as a return period, which is the average period in years between events, or a probability that an event will occur in any year.

Table 2 — Recommended design frequencies for use with simple design methods Location

Design storm frequency a

Return period (1 in "n" years)

Probability of exceeding in any 1 year

Rural areas 1 in 1 100% Residential areas 1 in 2 50% City centres/industrial/commercial areas 1 in 5 20% Underground railway/underpasses 1 in 10 10% a For those design storms no surcharge shall occur.

b) More complex methods For larger developments and for schemes particularly where risks to public health or the environment are significant, time-varying design rainfall and computer based flow simulation models shall be used. Any model used shall be chosen in cooperation with the relevant authority. For any application it is necessary to select a method where the appropriate balance between cost complexity and required accuracy is achieved. Guidance on when they should be used and the type of method to select is given in normative Annex E. In these cases design should be undertaken to limit frequency of surcharge following which the design should be checked to ensure that the design meets the design flood frequency criteria at specific sensitive locations. These design checks are particularly important on steeply sloping catchment areas. In the absence of any design criteria specified in national or local regulations or by the relevant authority the design flooding frequency values given in Table 3 may be used.

Table 3 — Recommended design frequencies for use with complex design methods

Location

Design flooding frequency Return period (1 in

"n" years) Probability of exceeding in

any 1 year Rural areas 1 in 10 10 % Residential areas 1 in 20 5 %

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City centres/industrial/commercial areas 1 in 30 3 % Underground railway/underpasses 1 in 50 2 % Detailed design of the hydraulic design of surface water drains and sewers is described in 9.4.3.

8.4.4 Combined drains and sewers The hydraulic design of combined drain and sewers shall be undertaken in the same way as surface water drains and sewers with the design flows increased to include the dry weather flow. Since the dry weather flow is generally only a small proportion of the capacity of the sewer, less allowance is necessary for the peak dry weather flow rate. Detailed design of the hydraulic design of combined drains and sewers is described in 9.4.4. For combined drains and sewers, the design flowrate is made up of runoff, which is by far the predominant component, plus an allowance for foul wastewater flows. The runoff component should therefore be estimated using the methods outlined in 9.4.3. The foul wastewater component is estimated as described in 9.4.2. As the foul wastewater flows are usually considerably lower than the design flowrates, particular consideration should be given to self-cleansing velocities during dry weather conditions. 8.5 Environmental considerations 8.5.1 General The environmental design of the system shall ensure that the design meets all relevant functional requirements including: ⎯ Protection of surface receiving waters (see 5.1.4); ⎯ Protection groundwater (see 5.1.5); ⎯ Prevention of odours, toxic, explosive and corrosive gases (see 5.1.6); ⎯ Prevention of noise and vibration (see 5.1.7); ⎯ Minimising the use of finite resources (see 5.1.8 and 5.1.9). Control of the sources of pollution from drain and sewer systems shall be considered as it can limit the environmental impact to levels acceptable to the relevant authority. Consideration of impacts shall pay due regard both to short-term effects and to cumulative long-term effects. Short-term effects can include fall in the concentration of dissolved oxygen, acute toxicity and hydro-biological stress. Cumulative long-term effects can include the build up, in aquatic biota and sediments, of persistent pollutants such as heavy metals and certain organic compounds. Sources of environmental impact include: ⎯ outfalls; ⎯ combined sewer overflows; ⎯ emergency overflows, for example from pumping installations or detention tanks; ⎯ exfiltration to groundwater; ⎯ infiltration of groundwater;

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⎯ untreatable non-domestic wastewater; ⎯ disposal of residues produced during sewer cleaning (see EN 14654-1). Any untreatable non-domestic wastewater can have an impact when subsequently discharged from a combined sewer overflow or the wastewater treatment plant. Infiltration can also affect the frequency and volume of discharges from combined sewer overflows and the quality of discharges from wastewater treatment plants. Consideration shall be given to the risk of spills of noxious substances within the catchment area, particularly on surface water systems. Where there is significant risk of spillage or discharge of significant quantities of fire fighting water containing harmful substances, appropriate measures shall be taken to avoid entry into or exit from the sewer system of these substances by, for example, the provision of separators (see 8.7.2) or retention tanks. Pipelines shall be designed to avoid leakage that might cause pollution of groundwater. To minimise odour nuisance gravity drains and sewers shall be sufficiently ventilated to atmosphere by allowing a free passage of air through the system. In very cold areas special precautions can be required. Disused drains and sewers shall be removed or, where this is impracticable, they shall be filled with suitable material to prevent for example, structural deterioration, unauthorised use, ingress of groundwater and infestation by rodents. Care should be taken in the design of systems to minimise the accumulation of deposits as residues from sewer cleaning activities can cause pollution. 8.5.2 Protection of surface receiving waters The quality, quantity and frequency of any discharge to receiving water from any sewer including a surface water sewer, combined sewer overflow, pumping installation or treatment works shall meet the requirements of any national or local regulations or the relevant authority. Design shall be such that the receiving water will be protected against overloading of its self-purifying capacity. It shall take account of physical, chemical, biochemical, bacteriological, aesthetic and any other relevant considerations. There are two approaches to the control of pollution from drain and sewer systems: ⎯ uniform emission limits can be set by national or local regulations or the relevant authority for

general use with each of the different types of discharge; ⎯ site-specific emission limits can be set by any national or local regulations or the relevant

authority for individual points of discharge, to satisfy requirements for the quality and characteristics of the receiving water taking into account any emission limits and the specific needs of the receiving water.

In many cases a combination of the two approaches should be considered. Uniform emission limits are generally set in relation to what is technically feasible for the different types of discharge. They form a baseline standard prior to the determination of site-specific limits which will not put the self-purifying capacity of the receiving water at risk. They are unlikely to be applicable where discharge is to sensitive waters such as recreational areas, sources for water supply or lakes. Generally in such cases, more stringent site-specific emission limits will be necessary to satisfy the receiving water quality requirements. The site-specific emission limit approach is sensitive not just to the effects of an individual discharge, but also to the combined effects of the whole range of discharges to receiving waters. These

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discharges, including those from industry, treatment works and non-point sources can demand an integrated approach to the identification of solutions. The relevant authority for environmental regulation can classify receiving waters according to current or projected uses or interests, for example: ⎯ abstraction for potable supply; ⎯ fishery; ⎯ bathing or other water contact activities; ⎯ special ecosystem. The emission limits can then be set by any national or local regulations or the relevant authority using, where appropriate, water quality simulation models. 8.5.3 Protection of groundwater In order to protect groundwater, the national or local regulations or the relevant authority can require stringent performance and testing in high-risk areas such as drinking water abstraction or aquifer protection zones and areas with high groundwater levels. In such areas a number of zones with different levels of protection can be specified by any national or local regulations or the relevant authority, depending on risk. 8.5.4 Prevention of septicity Septicity caused by the prolonged retention of wastewater under anaerobic conditions is undesirable and should be avoided by limiting the time of retention in rising mains, sewers/detention tanks and siphons and by the provision of self-cleansing conditions in sewers to achieve aerobic conditions within the liquid. Where this is not possible or effective, intervention can be necessary by using, for example, chemical oxidation and/or precipitation. The choice of chemicals shall take account of their potential environmental impact. Septic wastewater can produce lethal or explosive gases, particularly hydrogen sulfide (H2S) and methane (CH4) and lead to offensive odours, chemical attack, difficulties in wastewater treatment processes, safety hazards and danger to life. 8.5.5 Combined sewer overflows and surface water treatment Combined sewer overflow and outfall structures shall be designed to minimise the impact of any discharges on the environment. Overflows are normally necessary on combined sewer systems. Close to the entrance of the wastewater treatment plant overflows are installed on separate sewer systems for use under exceptional circumstances. The site and discharges, from these and other outfalls, to receiving waters shall be controlled to limit pollution. Various methods such as flow detention and sedimentation can be used to assist in limiting pollution inputs to receiving waters. Factors to be considered include: ⎯ flow rates; ⎯ volume, duration and frequency of discharges; ⎯ pollution concentrations and loads; ⎯ hydro-biological stress; ⎯ aesthetic impacts.

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The pollution discharges to receiving waters from overflows and treatment works shall be considered together. Detailed consideration of the environmental impact of combined sewer overflows and surface water treatment is described in 9.4.7. 8.5.6 Emergency overflows Emergency overflows can be installed immediately upstream of wastewater treatment plants, at pumping stations and at other critical parts of the system. The environmental impact of such overflows shall be considered. 8.6 Structural design 8.6.1 Introduction The structural design of the system shall ensure that the design meets all relevant functional requirements including: ⎯ Prevention of noise and vibration (see 5.1.7); ⎯ Sustainable use of products and materials (see 5.1.8); ⎯ Structural integrity and design life (see 5.1.10); ⎯ Not endangering adjacent structures and utility services (see 5.1.13). The structural design of drain and sewer systems shall take account of: ⎯ structural integrity taking into account the loads; ⎯ watertightness of the drain and sewer system (see EN 1610); ⎯ prevention of floatation; ⎯ bearing capacity of the soil; ⎯ chemical nature of the soil will have an effect on the materials used; ⎯ effect of aggressive, corrosive and/or erosive wastewater on the materials used; ⎯ possible differential settlement between structures and all drains and sewers and outgoing rising

mains and other services; ⎯ any requirements of national or local regulations or the relevant authority. Where buildings are near to drains or sewers or where a building is proposed near a drain or sewer the design should consider: ⎯ effect on the sewer of the building; and, ⎯ effect of failure of the sewer on the building. 8.6.2 Structural design of pipelines The structural design of buried pipelines shall be carried out in accordance with one of the methods described in EN 1295-1 or otherwise designed in accordance with EN 1990 to EN 1999 if applicable.

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In the case of renovation, a partial structural contribution of the existing pipe may be taken into account only if the soil and pipe fabric is stable and when this contribution can be quantified. Where common trenches for drains or sewers and other utility services are used care shall be taken to ensure the stability of the pipes. Where a pipeline is to be laid close to foundations of a structure, the potential effects of the structure on a pipeline shall be considered. Care shall be taken to ensure that the foundations are not undermined or damaged. 8.6.3 Structural design of other components The structural design of other components of drain and sewers systems shall be designed in accordance with EN 1990 to EN 1999 if applicable, or otherwise in accordance with relevant product standards. 8.6.4 Materials selection The selection of an appropriate material is an important part of the design of the structure. The durability of materials used in the construction of components of drain and sewer systems can be affected by the chemical action of groundwater and the wastewater. In some conditions the material can also be affected by the physical and chemical action of sediments contained in the wastewater. In selecting materials designers shall consider: ⎯ chemical content of the wastewater, ⎯ possible presence of hydrogen sulfide (see 9.5.3), ⎯ abrasive nature of any sediments carried in the wastewater, ⎯ corrosive properties of any sediments and the effect of chemicals they could generate, ⎯ chemical content of the ground and groundwater, ⎯ physical properties of the soil, ⎯ environmental impact of chemicals released during installation. 8.7 Operational considerations 8.7.1 General The operational considerations of the design shall ensure that the design meets all relevant functional requirements including: ⎯ Protection from flooding (see 5.1.2); ⎯ Maintainability (see 5.1.3); ⎯ Prevention of odours, toxic, explosive and corrosive gases (see 5.1.6); ⎯ Prevention of noise and vibration (see 5.1.7). Planning, design, construction and rehabilitation shall take into account the operation and maintenance j requirements. The system shall be designed to minimise the risks to the health and safety of operator personnel.

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The cost of maintenance of the system shall also be considered. When balancing the construction costs with future maintenance costs, the whole life cost should be minimised. 8.7.2 Separators Where appropriate, separators shall be provided on or near inlets to exclude or minimise the entry of solids or other materials that could impair the operation of the drain or sewer system. These can include: ⎯ Grit separators to limit entry of sediments which could accumulate in the system restricting the

flow; ⎯ Grease separators to limit the entry of fats and grease which could be deposited in the system

restricting the flow; ⎯ Light liquid separators to limit the entry of flammable liquids which could cause a hazard in the

drain and sewer system. National or local regulations can require provision of separators. Detailed design of separators is described in 9.6.2. 8.7.3 Self-cleansing conditions Wastewater is likely to contain a variety of materials that could accumulate in drains and sewers or cause blockage. Drains and sewers shall be designed to minimise the risk of blockage from any material permitted in wastewater. The build up of permanent deposits of solids in drains or sewers can significantly increase the risk of flooding and pollution. Drains and sewers shall be designed to provide sufficient shear stress to limit the build up of solids to levels which do not significantly increase this risk. Special maintenance provisions can be required to ensure frequent sewer cleaning on sewers where it had not been possible to provide self-cleansing conditions. 8.7.4 Access to drains and sewers Safe access shall be provided at reasonable intervals to allow for inspection and maintenance. Manholes shall be designed to facilitate safe entry and egress by operator personnel and to provide sufficient working space. Wherever possible, provisions should be made for work to be carried out from surface level. National or local regulations or the relevant authority can specify requirements for the access. Detailed design of the operational considerations relating to access is described in 9.6.4. 9 Detailed Design 9.1 Introduction Unless explicitly specified the requirements in this clause apply both for new construction and rehabilitation. 9.2 Layout and profile 9.2.1 Introduction The principles for the physical design of drain and sewer systems are specified in 8.1.

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9.2.2 Layout The layout will be influenced by factors such as: a) site conditions, environmental considerations, retained features and existing utility services; b) protection of water abstraction areas; c) availability of suitable sewers or outfalls; d) layout of buildings, disposition of drainage systems inside buildings, appliances located on levels

necessitating direct connections to drains, e) use of buildings served; f) planning and coordination of utility services; g) social disruption during construction and its cost implications; h) practical aspects of construction methods, working space, adequate protection and support; i) stability of building during and following construction of the drains and sewers; j) existing, planned and future development; k) connections to or from existing drains and sewers which are to be retained; I) provision for phased construction and occupation; m) available gradients and depths of construction; n) possibility of real-time control; o) levels of receiving waters; p) effects of tides, waves and currents; q) groundwater levels; r) access for inspection and maintenance; s) overland flood flow paths; t) obstacles from other infrastructure (e.g. utility services, railways, waterways, major roads); u) land ownership; v) proximity of trees and other vegetation. The routing of drains and sewers to take into account such factors can have major consequences: e.g. excessive depths or length, need of pumping or inverted siphons. 9.2.3 Accessibility Manholes and inspection chambers should be in locations where they can be accessed by all those who could have need to use them, having regard to the need for access in an emergency. Where frequent access is required (e.g. pumping stations or CSOs with screens) or access by special plant may be required (e.g. tanks, pumping stations or washout valve chambers) this should be taken into account (see also 9.6.4).

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Sewers serving more than one property should be at locations where the access for repair is possible. 9.2.4 Depth The approximate depth is determined during the conceptual design and finalised during the detailed design. Depth will have a significant effect on the cost of construction and maintenance. In deciding the method of construction, the depth of drains and sewers shall be considered, in conjunction with other factors such as: ⎯ protection against flooding; ⎯ nature of the ground; ⎯ presence of groundwater; ⎯ proximity of foundations; ⎯ proximity of utility services; ⎯ proximity of trees or heavy root growth; ⎯ protection against frost; ⎯ minimum cover. 9.2.5 Need for pumping Circumstances which can make the pumping of wastewater either necessary or advisable include the following: a) avoidance of excessive depths of sewer; b) drainage of low lying or other parts of the catchment area susceptible to flooding; c) development of areas not capable of gravitational discharge to an adjoining drain or sewer

system, a wastewater treatment plant or an outfall; d) overcoming an obstacle, e.g. a ridge, a watercourse, a railway or for avoiding the use of an

inverted siphon; e) correction of difficulties in a drain or sewer system resulting from mining subsidence; f) provision of sufficient head for operation at a wastewater treatment plant; g) centralisation of wastewater treatment; h) raising wastewater to detention tanks. Where part of a system cannot be effectively drained using a gravity system then consideration should be given to the use of one or more pumping installations. The optimum number of installations shall first be determined having regard to the whole life cost. 9.2.6 Pumping installations Where pumping installations are provided the design shall take into account: a) whole life cost;

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b) energy usage; c) operations and maintenance requirements; d) risk and consequences of failure; e) health and safety of public and operating staff; f) environmental impact; g) nature of wastewater which can:

⎯ be aggressive, corrosive and/or erosive;

⎯ have a high solid content increasing the potential for blockage;

⎯ be toxic;

⎯ lead to potentially explosive conditions. The design of the pumping station and the rising main shall take into account the interaction between them. Where the wastewater is pumped consideration shall be given to the effects of the pump discharge rates on the downstream parts of the system. Detailed design of pumping installations shall be designed in accordance with Annex F. 9.3 Preliminary investigations 9.3.1 General Attention needs to be paid to both the topographical features present in the localities concerned and to the geological nature of the underlying strata. 9.3.2 Topography Surface reconnaissance and examination of contour maps and aerial photographs will enable preliminary lines for drains, sewers and rising mains to be established so that the general feasibility of the proposals can be determined before detailed layouts and longitudinal sections are prepared. It is important to use any available geological survey data in conjunction with contour maps when deep open-cut and trench-less options have to be considered. 9.3.3 Geotechnical survey At the conception stage of the design an understanding of the ground conditions to be encountered during the construction of the scheme is essential in order to be able to evaluate fully all the route and construction options. The aim in this initial geotechnical survey will be to gain broad information in the most cost effective manner. As the project develops, more intensive investigations will be necessary. Geological maps are, even with their limitations, a source of general information. Where these are inadequate, a preliminary ground investigation should be undertaken. The data gathered in a geotechnical survey should, as appropriate, be sufficient to be able to assess: a) ground loadings on the pipes/structures; b) landslide conditions;

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c) subsidence conditions; d) fine particle movement; e) any likely swelling of clay strata; f) groundwater levels and movement; g) aquifer recharge potential; h) loadings from adjacent structures and highways; i) previous land use (including mining); j) alternative construction methods; k) options in pipe type choice; I) pipe bedding options; m) aggressive soil or groundwater conditions. Soil and rock samples should be retained especially where tunnelling or other trench-less methods are contemplated. 9.3.4 Groundwater Where appropriate, groundwater levels including seasonal variations shall be determined during representative periods of time. Investigations shall be carried out to identify conditions which can be detrimental to the integrity of the pipeline. 9.3.5 Existing drainage services The lines, levels, hydraulic adequacy and structural condition of all relevant existing features (e.g. drains, sewers, ditches, land drains and watercourses) shall be ascertained. When designing a rehabilitation project the existing pipe shall be assessed in accordance with 6.4. 9.3.6 Other existing utility services The positions of other existing relevant utility services shall be ascertained as accurately as possible. 9.3.7 Extraneous water If the risk of extraneous water (e.g. infiltration) entering drains and sewers, is considered to be unacceptable, investigations shall be carried out to determine the extent of this risk. 9.4 Hydraulic design 9.4.1 Introduction The principles for the hydraulic design of drain and sewer systems are specified in 8.4. Drains and sewers shall be designed to provide sufficient capacity for the design flows. In selecting the diameter and/or gradient of the pipe consideration shall also be taken of the need to minimise build up of sediments and to minimise the risk of blockages (see 9.6.3) 9.4.2 Foul drains and sewers

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Drain and sewer systems shall be designed to collect and transport wastewater influents from domestic, trade premises to the point of treatment without prejudice to health and safety. Such design should also include allowances for future growth and for extraneous discharges up to such flow that will justify rehabilitation. a) Drain systems

The design of drains (and sewers) to serve individual or small groups of buildings where discharges from individual appliances will give relatively high flows of an intermittent and irregular nature shall use a peak rate of flow derived from the number and type of appliances connected. The rates of flow in the drains from the buildings or premises, calculated using EN 12056-2, should be used in the design of downstream drain systems. Flow rates for individual appliances and factors to be applied can be specified by national or local regulations or the relevant authority. Trade effluent flows shall be calculated separately. Having completed the design of the drain system, the interaction between the drain and the sewer system shall be checked.

b) Sewer systems

For domestic wastewater sewers, flow rates are usually based on either by population and a rate of flow per head or, for new developments where such data is not available, on the planning criteria for the population or the type and number of dwellings. For a new development and for an upgrading scheme on an existing development, the estimates used shall be appropriate for the specified planning horizon. Existing water supply statistics may be helpful to derive future water supply consumption and hence domestic wastewater flows. Flow patterns for daily consumption and anticipated variations between different types of development can also be established. Consumer water usage that does not enter the drain and sewer system and distribution leakage are of particular importance in assessing domestic wastewater flows. The rate of flow per head may be based on local water supply statistics allowing for consumption that does not result in discharge to the sewers and, where appropriate meters are not available, distribution losses. Typical discharge figures for developments similar to those under consideration may also be used. The flow per head, in the range from 120 I/day to 400 I/day, commonly used in various countries is shown in Annex E, Table E.4. The peak design flow takes account of the diurnal variation in domestic wastewater flow. The domestic peak design flow rates commonly used in various countries are shown in Annex E, Table E.5. To these peak design flows shall be added trade peak flows and, where unavoidable, extraneous flows. Where a scheme is to be developed in phases, consideration should be given to the likely flows following the initial stages of construction so that either self-cleansing conditions are attained at least at times of daily peak flow or other cleansing arrangements are made.

9.4.3 Surface water drains and sewers The hydraulic design of drains and sewers serving impermeable surfaces, such as roads and car parks, is dependent on the hydraulic performance of the interface between the impermeable surface and the drain or sewer system. The flow at this interface shall be adequately considered in order to minimise the impact of flooding. The hydraulic capacity of the pipes shall be calculated in accordance with Annex E.

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It can be necessary to provide a means of flow detention to intercept and hold back temporary peak storm discharges in order to avoid flooding. The effects of flow balancing within the drain and sewer system on the performance of the wastewater treatment plant shall be taken into consideration. Arrangements for maintenance and safety of these structures will be required. Runoff shall be calculated taking into account a number of factors including: ⎯ design rainfall; ⎯ area that could drain to the inlets connected to the system: ⎯ extent of impermeable area; ⎯ extent of permeable area; ⎯ likely losses of runoff due to infiltration of rainfall into the ground; ⎯ likely increases in connected area. The possible impact of climate change should also be considered. A simple method of calculating the runoff from small areas is included in Annex E. 9.4.4 Combined drains and sewers For combined drains and sewers, the design flow rate is made up of runoff, which is by far the predominant component, plus an allowance for foul wastewater flows. The runoff component should therefore be estimated using the methods outlined in 9.4.3. The foul wastewater component is estimated as described in 9.4.2. As the foul wastewater flows are usually considerably lower than the design flow rates, particular consideration should be given to self-cleansing velocities during dry weather conditions. 9.4.5 Capacity of pipelines Pipes shall be selected to: ⎯ transport the required design flows; ⎯ limit sediment build up (see 9.6.3); ⎯ ensure that the risk of blockage is reduced (see 9.6.3); and, ⎯ ensure that effective maintenance can be reasonably achieved (see 9.6.4). Two equations are recommended for use in calculating turbulent flows in drains and sewers: Colebrook-White and Manning, taking into account headlosses of the pipeline (see Annex E, E.2). Two methods of calculating total headlosses are: ⎯ adding local headlosses (see Annex E, E.2.1.4.2) to the pipeline headlosses (see Annex E,

E.2.1.4.1); ⎯ accounting for local headlosses by assuming a higher value of hydraulic pipeline roughness in

the calculation of pipeline headless. When using recommended hydraulic pipeline roughness values it is necessary to establish whether allowance has been included for local headlosses. Values currently in use range from 0.03 mm to 3.0 mm for k (Colebrook-White formula) and 70 m1/3s-1 to 90 m1/3s-1 for K (Manning formula).

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In cases where deposits in the invert cannot be avoided, the reduced cross-section of the pipe shall be taken into account when calculating headlosses. 9.4.6 Sewers with steep gradients Where sewers with steep gradients are required, consideration shall be given to consequences of high velocities such as: ⎯ air entrainment and its effects; ⎯ release of hydrogen sulfide; ⎯ erosion; ⎯ need for energy conservation measures on changes from super-critical flow to sub-critical flow; ⎯ special safety measures for operatives. Backdrop manholes, ramp manholes or vortex manholes may be installed in a sewer system to dissipate excessive static head in a controlled manner, thereby avoiding the installation of sewers with steep gradients and meeting any imposed velocity limitation. 9.4.7 Outfall design requirements Where surface water is to be discharged to a surface receiving water, the invert level of the outfall should be above the peak design water level of the surface receiving water so as to provide free discharge conditions. Where periodic backflooding cannot be avoided, a non-return valve should be considered. 9.5 Environmental considerations 9.5.1 Introduction The principles of the environmental considerations for drain and sewer systems are specified in 8.5. 9.5.2 Outfall design requirements Outfalls shall be so formed as to avoid, or provide protection against, local erosion. It can be necessary to provide additional protection to the outfall opening to prevent damage, interference or entry. The visual impact of the outfall shall also be taken into account. 9.5.3 Prevention of septicity Septicity within a drain or sewer system is undesirable and therefore shall be minimised. It will affect the wastewater treatment process and can lead to the production of hydrogen sulfide (H2S) and mercaptans. Hydrogen sulfide is malodorous, toxic and potentially lethal even in small concentrations. Depending on its concentration and local conditions it can be oxidized to sulfuric acid, it will tend to attack some materials in pipelines, treatment works and pumping installations. Parameters on which the concentration of hydrogen sulfide depend, and which shall be taken into account include: wastewater temperature, biochemical oxygen demand (BOD), sulfate availability, retention time, flow velocity, turbulence, pH, ventilation, existence of rising mains or particular trade effluent discharges upstream of the gravity sewer. Predictive equations can be applied in order to quantify sulfide formation both in pressure and gravity sewers. Other gases can also be produced from normal wastewater in anaerobic conditions as follows: a) Methane has little odour and whilst it is a simple asphixiant by displacing oxygen, its main

property is that it produces an explosive mixture with air over a wide range of concentrations. A

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particular vehicle for methane production is leachate from landfill sites which has been allowed to enter the drain or sewer system. Such leachates could need to be treated at source.

b) Ammonia has a distinct and strong odour which gives good warning characteristics before

reaching toxic levels, which are themselves unlikely to be generated from normal wastewater. c) Carbon dioxide has no odour and will act as an asphixiant by displacing oxygen. d) Carbon monoxide is also odourless and is highly toxic and lethal after only limited exposure.

Production of all these gases can be limited by the application of strategies to reduce septicity. Other gases can be produced under anaerobic conditions where particular trade effluents have been allowed to enter the drain or sewer system. Methods to control the effects of anaerobic conditions can be used, considering the potential distribution of gases and their odours, including: ⎯ Natural or forced ventilation; ⎯ Natural or forced entrainment of air in the flow; ⎯ Addition of reagents to the flow. 9.5.4 Drains and sewers near water abstraction areas Installation of drains and sewers can be restricted in areas where water is to be abstracted for human consumption. If in such areas drain and sewer systems are unavoidable, the designer shall take measures to ensure permanent control and to prevent pollution of the ground and/or groundwater. Such measures can include: ⎯ installation of an additional equally watertight surrounding pipe; ⎯ installation of alarm systems for leakage and breakage; ⎯ installation of house connections directly to manholes and not to the sewer; ⎯ special requirements for system components and method of construction. 9.6 Operational considerations 9.6.1 Introduction The principles of the operational considerations for drain and sewer systems are specified in 8.7. 9.6.2 Separators Separators shall be used where appropriate for example to minimise the entry of materials that can accumulate in drains and sewers or otherwise cause a blockage. a) Grit separators shall be provided on or near inlets where the wastewater is likely to contain

significant sources of grits or other sediments that could accumulate in drains or sewers. b) Grease separators shall be provided where the wastewater is likely to contain significant volumes

of grease or fats. c) Where wastewater is likely to contain significant volumes of light liquids (e.g. oil or petrol)

separator systems for light liquids shall be provided on or near inlets. In considering whether to provide a separator system for light liquids, account shall also be taken of the likely environmental impact of oils that could be discharged into surface receiving waters or groundwaters. Separators systems for light liquids shall comply with the requirements of EN 858-

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1. The nominal size of separator systems for light liquids shall be selected in accordance with EN 858-2.

9.6.3 Design for self-cleansing 9.6.3.1 Sediment transport For small diameter drains and sewers (less than DN 300) self-cleansing can generally be achieved by ensuring either that a velocity of at least 0.7 m/s occurs daily, or that a gradient of at least 1:DN is specified. Steeper gradients or higher velocities can be required by national or local regulations or the relevant authority. To achieve self-cleansing conditions in sewers with low gradients there should be strict CH27 requirements for the bedding and accurate laying of the pipes. For larger diameter drains and sewers, higher velocities can be necessary particularly if relatively coarse sediment is expected to be present. Local guidance, in the form of tables or equations can be available in national reference documents and may be used. Where self-cleansing conditions cannot be achieved provision should be made for adequate maintenance activities. 9.6.3.2 Minimisation of blockages To minimise the risk of blockage, drains and sewers should be smooth and laid to self-cleansing conditions. However where the flows in the drain or sewer are low, steeper gradients (up to 1:DN/2.5) can be required. 9.6.4 Access to drains and sewers The principles for provision of access are described in paragraph 8.7.4. The maximum spacing between access points should take account of the maintenance equipment and practices likely to be used. In the case of drains, access shall be provided where practicable, at every change of alignment or gradient by means of manholes, inspection chambers and rodding points. Where this is not practicable, provision for access shall ensure that changes of alignment or gradient can be reached. In the case of non man-entry sewers, access shall be provided, where practicable, at every change of alignment or gradient, at the head of all sewers, at every junction of two or more sewers, wherever there is a change in the size of a sewer and in addition at reasonable intervals for inspection and maintenance. In general, access should be provided through manholes. Inspection chambers may be used within the system. In the case of man-entry sewers, access shall be provided at reasonable intervals to allow for inspection and maintenance, having regard to the nature of the work to be undertaken and the proposed safe systems of working. Manholes and inspection chambers should be designed and installed so as to avoid any acute changes in direction of flow from branch drains. National or local regulations can specify requirements for the: a) location and spacing of access points on drains and sewers; b) positioning or the maximum spacing of manholes or inspections chambers; c) dimensions of manholes or inspection chambers.

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The dimensions for manholes and inspection chambers shall not be less than the minimum dimensions given in EN 476. EN 476 contains requirements for three types of chamber as follows. d) Manhole with access for cleaning and inspection by personnel — EN 476:1997, 6.1.1 requires

that manholes for all maintenance works with access for personnel have a DN/ID 1 000 mm or greater, or a nominal size for rectangular sections of 750 mm × 1 200 mm, or greater, or a nominal size for elliptical sections of 900 mm × 1 100 mm, or greater.

e) Manhole with access for cleaning and inspection — EN 476:1997, 6.1.2 requires that manholes

for the introduction of cleaning equipment, inspection, and test equipment, with occasional possibility of access for a man equipped with a harness, have a DN/ID of 800 mm or greater but less than 1 000 mm.

f) Inspection or connection chamber — EN 476:1997, 6.1.3 states that inspection chambers having

DN/ID's less than 800 mm permit the introduction of cleaning, inspection, and test equipment, but do not provide access for personnel.

National or local regulations or the relevant authority can prohibit entry of personnel into chambers below a certain size. 10 Construction Principles 10.1 General Drain and sewer systems shall be constructed in accordance with the design. Unless explicitly specified the requirements in this clause apply both for new construction and rehabilitation. During construction the following issues should be taken into account: a) Health, safety and welfare of construction personnel and other people; b) Optimum sequence of construction particularly with regard to maintaining the operation of

existing drain and sewer systems which could be affected by the work; c) Methods of dealing with existing flows in part completed systems or when carrying out

rehabilitation of existing flows; d) Protection of the environment. 10.2 Pipelines In the case of new construction, pipelines shall be constructed in accordance with EN 1610 or EN 12889 as appropriate. In case of rehabilitation, pipelines shall be constructed according to the relevant installation manual (referring to the corresponding system standards when existing). In any case the following points should notably be accounted for: ⎯ pipelines geometry; ⎯ flow performance; ⎯ watertightness; ⎯ proper load transfer from soil to pipeline through embedment; ⎯ proper structural embedment.

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Where common trenches for sewers or drains and other utility services are used, care shall be taken to ensure the stability of the pipes. Testing shall be carried out to verify that the work has been constructed in accordance with the design. 10.3 Ancillaries Ancillaries, such as inspection chambers and manholes, should be constructed in order to ensure correct junctions with the pipes (pipelines). NOTE Special care should be granted to differential displacements. In any case the following points should notably be accounted for: ⎯ geometry; ⎯ flow performance; ⎯ water tightness; ⎯ proper structural embedment. For ancillaries the performance requirements specified in relevant European product standards should be taken into account. Grease separators shall be installed in accordance with EN 1825-2. Separator systems for light liquids shall be installed in accordance with EN 858-2. 11 Operations and Maintenance 11.1 Introduction The purpose of operations and maintenance is to ensure that the drain and sewer system perform in accordance with the functional requirements defined in clause 5 and in accordance with any operations and maintenance plan (see 6.4). Operations include: ⎯ starting or stopping pumps; ⎯ inserting dam boards; ⎯ regulating valves and weirs; ⎯ using detention tanks; ⎯ acting in accordance with contingency and emergency plans; ⎯ measuring wastewater quality; ⎯ inspecting periodically; ⎯ pest control (see C.10); ⎯ making connections to existing drains and sewers (see C.11); ⎯ control of disused drains and sewers (see C.12); ⎯ control of building activities over or adjacent to sewers (see C.13).

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Urgent interventions that are generally intended to be temporary are included in operations. Maintenance includes: ⎯ local repair or local replacement of damaged pipes or other structures in order to maintain the

functioning; ⎯ cleaning and removal of sediments, obstructions etc. to restore hydraulic capacity; ⎯ maintenance of mechanical plant (e.g. pumps). Effective operation and maintenance of the drain and sewer system will depend on, for example: ⎯ planning; ⎯ rights of access; ⎯ sufficient number of competent personnel; ⎯ clear assignment of responsibilities; ⎯ suitable equipment; ⎯ knowledge of the system, its operational components and the users connected; ⎯ adequate records and analysis. There can also be requirements relating to the resolution of performance deficiencies, for example to remedy failures and problems within acceptable timescales. 11.2 Objectives Operations and maintenance has the following objectives to: ⎯ ensure that the entire system is operationally ready at all times and functions within the

performance requirements; ⎯ ensure that the operation of the system is safe, environmentally acceptable, and economically

efficient; ⎯ ensure that as far as possible the failure of one section of a sewer system will not adversely

affect the performance of the other parts. 11.3 Data requirements Data shall be collected: ⎯ for management purposes; ⎯ for regulatory reporting purposes (e.g. properties at risk of flooding); ⎯ meet statutory requirements (e.g. maintaining plans showing the location of the public sewers). It is possible to store a wide range of data on drain and sewer systems. However, collection, validation, storage and updating the data can be expensive. The amount of data collected depends on the reasons listed above. The information can include:

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⎯ inventory of the system including records of drains, sewers, manholes, pumping installations,

combined sewer overflows, detention tanks etc.; ⎯ details of permits for influents into the system (trade effluents, hazardous materials etc.); ⎯ details of permits to discharge from the system into receiving waters (combined sewer overflows,

pumping installations etc.); ⎯ records of inspections of the system (e.g. closed circuit television (CCTV) survey reports); ⎯ records of incidents such as blockages, collapses, pumping station failures, rising main failures

and flooding incidents; ⎯ information on rainfall; ⎯ records of planned maintenance work carried out; ⎯ actual response times for dealing with emergencies; ⎯ information on the cost of incidents and maintenance activities to allow budgetary control and

performance review; ⎯ information about the hydraulic capacity; and ⎯ records of system performance (see 6.2.3). Computer based geographical information systems (GIS) are a powerful tool for storage retrieval and analysis of information on sewer systems. 11.4 Investigation and analysis of operational problems To deal with operational problems in the most cost effective way, it is necessary to investigate and understand their causes and effects. Investigations can be required to determine: ⎯ route of a pipeline; ⎯ cause and location of the sediment, blockage or collapse; ⎯ cause and location of a surface depression; ⎯ location, source, quality of making of a connection; ⎯ quality of a repair; ⎯ condition of a pipe; ⎯ extent of scale or grease build up; ⎯ effectiveness of sewer cleaning work; ⎯ origin, quality and composition of influent; ⎯ quantity and composition of the wastewater; ⎯ presence of hazardous gasses; ⎯ watertightness.

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Operational investigation techniques available include: ⎯ dye tracing; ⎯ electronic location (using radio transmitters and a directional receiver); ⎯ closed circuit television (CCTV); ⎯ walking through sewers; ⎯ mirrors; ⎯ flow measurement; ⎯ sampling and analysis; ⎯ insitu measurement of the composition of influent; ⎯ watertightness tests (see EN 1610). Operational problems concern the various components of the drain and sewer system. The techniques available to resolve them are described in Annex C. 12 Performance testing It is necessary to test and assess the performance of the drain and sewer systems during construction, at the completion of the construction stage and also during the operational life of the system. Examples of tests and assessments are: a) watertightness test with water; b) watertightness test with air; c) infiltration test; d) visual inspection; e) dry weather flow assessment; f) monitoring of inputs to the system; g) monitoring effluent quality, quantity and frequency at point of discharge to receiving water; h) monitoring within the system for toxic and/or explosive gas mixtures; i) monitoring of discharge from system to treatment works. The tests to be undertaken to determine the performance being achieved by the drain or sewer system will depend on whether it is a new system, a rehabilitated system, or an existing system being tested. The effectiveness of maintenance should be assessed by comparing the performance of the drain or sewer system with the requirements (see 5.1). In addition, for reactive maintenance, target response times can be used as an assessment. 13 Qualifications and training

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Personnel at all levels shall have appropriate training to allow them to carry out their work safely and competently. This on-going training shall introduce and explain relevant legislation and techniques. Training shall be repeated periodically when required and should cover safety, technical and legal topics where appropriate. The owners of construction works (procuring entity for public work or private agent for a private contract, or their designer) shall request a proof that the enterprise carrying out the work is sufficiently qualified for the specific work. 14 Sources of additional information Various national organisations provide supplementary detail and guidance on the planning, design, construction and maintenance of drain and sewer systems outside buildings. Examples of the sources of relevant information include: ⎯ Complementary National Standards; ⎯ European, national or local regulations; ⎯ Guidance issued by professional or trade associations; ⎯ Guidance documents issued by national or local government organisations; ⎯ Suppliers of technical software. A list of organisations that produce relevant supplementary guidance is given in Annex B.

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Annex A

(informative)

Relevant EAC Directives

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Annex B

(informative)

Sources of additional information B.1 National Standards Bodies B.2 National Regulatory Authorities

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Annex C

(normative)

Operations and maintenance C.1 Introduction The purpose of operations and maintenance is to ensure that the drain and sewer system performs in accordance with the functional requirements defined in Clause 5. C.2 Operations planning C.2.1 Inspection routines Inspection routines, including frequencies, shall be established for the system, taking into consideration the requirements and importance of each component. Routines shall include the inspection of: ⎯ pipelines including inspection chambers, manholes and outfalls, taking into account the gradient

and/or velocity; ⎯ pumping installations, according to potential risk and type of equipment; ⎯ overflows and detention tanks, taking into account storm frequency; ⎯ inverted siphons, depending on risk of blockage and potential consequences; ⎯ separators, according to technical requirements; ⎯ grit chambers, gullies etc., taking into account storm frequency, capacity and land use. C.2.2 Operations procedures Procedures for the operation of the components of the system should include plans for: ⎯ operation of pumping stations; ⎯ operation of any special components (e.g. vacuum or pressure installations within the system); ⎯ setting dam boards, valves and weirs; ⎯ operation of detention tanks; ⎯ showing the assignment of responsibilities for carrying out procedures. C.2.3 Contingency planning Contingency planning is the process of setting out procedures to be used in case of breakdown of a part of the system. It should also include procedures for dealing with major failures and other emergencies. Procedures could be required for a range of possible incidents including: ⎯ accidental spillages of toxic, noxious or explosive substances; ⎯ discharge of special substances used in fire fighting; ⎯ failure of pumping stations or pre-treatment facilities; ⎯ flooding due to an exceptional rainfall event;

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⎯ major sewer collapse. Contingency plans shall include: ⎯ details of emergency services; ⎯ estimated times for response (in general terms); ⎯ lists of those to be notified; ⎯ location of available resources; ⎯ procedures to be followed (including protection of receiving waters and wastewater treatment

plant). The resource requirements will need to be determined, including: ⎯ personnel; ⎯ vehicles, ⎯ equipment; ⎯ materials. These resources will sometimes need to be available at short notice. This can influence resourcing decisions for normal operations and maintenance work. C.3 Pipelines C.3.1 General The common problems associated with drains and sewers (man entry and non man-entry) can be divided into two types, functional problems and structural problems. C.3.2 Functional problems Functional problems can include: ⎯ blockage — this usually occurs when sediments/debris are deposited within the sewer system,

forming obstructions and a reduced pipe capacity; ⎯ sedimentation — this can also lead to blockages; ⎯ encrustation — build up of mineral deposits on the wall of the pipeline; ⎯ grease — deposited on the wall of the pipeline; ⎯ intrusion of tree roots; ⎯ infiltration or exfiltration caused by structural problems (see C.3.3); ⎯ failure of air valves and other protection systems (for rising mains). Examples of available methods are: ⎯ jetting;

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⎯ winching; ⎯ rodding; ⎯ cleaning balls; ⎯ remote controlled equipment; ⎯ flushing; ⎯ manual methods. When carrying out cleaning activities consideration shall be given to the potential impact of the work on the receiving wastewater treatment plant. Measures shall also be taken to avoid discharges of heavily polluted matter to receiving waters through combined sewer overflows. Residues from maintenance activities on drain and sewer systems shall be disposed of in accordance with the requirements of national or local regulations or the relevant authority in such a way as not to cause pollution. In severe cases, rehabilitation can be necessary. Cleaning activities in drains and sewers shall be carried out in accordance with EN 14654-1. C.3.3 Structural problems Structural problems can include: ⎯ collapse; ⎯ cracking or fracturing of the pipe; ⎯ chemical attack or corrosion; ⎯ ground erosion outside the wall of the pipe - usually caused by infiltration of soil into the pipe; ⎯ defective connections; ⎯ pipe deformation; ⎯ open or displaced joints between pipes. The following methods can be used to deal with the problems described above: ⎯ repair; ⎯ renovation; ⎯ replacement. Where problems are widespread or a significant length of sewer is involved a drainage area study (see clause 6) of the whole catchment area or part of it should be considered. C.4 Manholes and inspection chambers Manholes and inspection chamber are needed for access to sewers and drains for maintenance and operations. The problems include:

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⎯ defective covers — these include covers which are broken, cracked, ill-fitting or are not flush with the ground level;

⎯ problems with access — e.g. inadequately sized access shaft, or defective steps or ladders; ⎯ structural problems with the fabric of the chamber including chemical attack and infiltration; ⎯ sediment in the invert; ⎯ odours or gas/oxygen deficiency. These can be solved by works such as: ⎯ cleaning; ⎯ replacement and resetting of the covers; ⎯ repair, renovation or renewal of the fabric of the chamber; ⎯ reconstruction of access; ⎯ replacement of steps or ladders; ⎯ efficient ventilation. C.5 Combined sewer overflows The purpose of combined sewer overflows is to spill excess flows from a system to receiving water (see 8.5 and 9.4.7). Problems associated with combined sewer overflows include: ⎯ blockages; ⎯ siltation of the chamber; ⎯ fouling of screens; ⎯ structural problems. Blockages can be caused by: ⎯ restriction in size of the downstream sewer resulting in low flow velocity upstream leading to

silting; and, ⎯ general build-up of silt/debris in the chamber. Siltation can be minimised by: ⎯ high pressure water jets to clean the chamber; ⎯ high volume suction units to remove debris; ⎯ flushing of the chamber. Planned maintenance can be necessary in order to limit the environmental impact to satisfy the requirements of national or local regulations or the relevant authority. The problem can be solved using planned maintenance procedures, which includes inspection and reporting. From the report, work can be given a priority and inspection frequencies determined.

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Screens, where fitted, can require cleaning following heavy storms. Structural problems arising may be dealt with in a similar fashion to structural problems in manholes. C.6 Detention tanks The role of the detention tank is to reduce peak flows by the temporary storage of wastewater within the system. They are often used to reduce flooding and to reduce discharge and pollution load from combined sewer overflows. The problems include: ⎯ blockage of flow control devices; ⎯ removal of sediment. Methods of optimising the removal of sediments are: ⎯ modifications to the structure of the tank e.g. by use of low friction coatings (these shall not be

used on areas required for access as it can be a hazard to operatives); ⎯ modification of inlet design to increase scour; ⎯ modification to the benching or installation of dry weather flow channels; ⎯ use of mechanical plant in the tank to periodically remove sediments. Where a blockage has occurred and wastewater has been detained for some time, clearing the blockage suddenly can have an unacceptable impact on the wastewater treatment plant. Consideration shall be given to the gradual emptying or removal of effluents from the tanks. C.7 Separators, settling chambers and gullies Separators are used to intercept light liquids e.g. oil, petrol, etc., grease or solids. Planned maintenance of separators is required if they are to function efficiently. Grit separators, settling chambers and gullies are often used to prevent sand and gravel from entering the system. Separators, settling chambers and gullies shall be emptied periodically to prevent blockage, especially after spillages and, where appropriate, severe storms. Separator systems for light liquids shall be operated and maintained in accordance with EN 858-2. Grease separators shall be operated and maintained in accordance with EN 1825-2. C.8 Pumping installations The main problems associated with pumping installations are as follows: ⎯ blockage of pumps, valves, screens etc. by debris; ⎯ power failure; ⎯ failure of rising main; ⎯ electrical or mechanical failure of a component of the pump, its control equipment, or telemetry

unit; ⎯ crust formation inhibiting the operation of control devices;

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⎯ noise and/or vibration; ⎯ odour complaints; ⎯ excessive power consumption; ⎯ vandalism. To minimise maintenance and operations requirements and costs, careful attention needs to be given to the design of the pumping station and its equipment. Where the composition or volume of flows have changed substantially or where equipment is coming to the end of its life a reconsideration of the design is necessary (see Annex F). Solutions to some of these problems include the following: ⎯ repair or replace the pumping equipment; ⎯ reduce extraneous water; ⎯ installation of warning or telemetry systems; ⎯ installation of septicity prevention plant or ventilation of wet well; ⎯ review of the control system; ⎯ installation of standby power supplies. In addition the installation of warning or telemetry systems can help reduce the impact of failure by allowing early correction of actual or incipient failures. C.9 Inverted siphons The main problem associated with inverted siphons is sedimentation and blockage of the pipe. Planned inspection and maintenance should be carried out to ensure that inverted siphons continue to operate efficiently. Inspection can include: ⎯ checking that washout valves and pumps can be operated; ⎯ checking for surcharging at the upstream end of each pipe, which can be a sign of partial

blockage; ⎯ visual inspection of pipelines. Cleaning methods can include:

⎯ high pressure water jets;

⎯ high volume suction units to remove debris;

⎯ flushing of the inverted siphon;

⎯ use of cleaning balls. C.10 Pest control The principal pest problem in sewers is related to rats, though in some areas especially where there is insufficient ventilation, or deposits of faecal sediments, insects such as cockroaches or mosquitoes can also be a problem.

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Since the sewer network can be a refuge for rats, control of rodents is needed to minimise health risks (including Leptospirosis and Salmonella) and to prevent structural damage caused by burrowing. Treatment programmes should be carried out in accordance with the requirements of national or local regulations or the relevant authority, to control the infestation. To ensure maximum effectiveness treatment programmes for sewers and drains should be carried out on a catchment wide basis and should be coordinated with the treatment of surface infestations. Areas for treatment should be identified, in collaboration with national or local regulations or the relevant authority and by reference to records of sightings of rodents. Areas may also be categorised according to the risks to public health. The treatment programmes should be recorded and the effectiveness measured so that records can be used to plan future programmes. C.11 Making connections to existing drains and sewers A large proportion of structural problems on drains and sewers are associated with poorly made lateral connections. Problems are particularly common where manholes, inspection chambers or pre-formed junctions are not used. The control of new connections shall be undertaken to ensure that: ⎯ fabric of the drain or sewer is not weakened or damaged by the connection; ⎯ no operational problems are caused by the connection; ⎯ sewer is inspected at the point of connection before and after construction; ⎯ system is watertight at the point of connection; ⎯ connections are made to the correct sewer, where there are separate sewer systems. Connections other than at manholes or inspection chambers should be made using pre-formed junctions. New connections to brick sewers should be avoided, however if one is necessary, a thorough inspection of the sewer shall be made beforehand. C.12 Control of disused drains and sewers Disused drains and sewers shall be removed or, where this is impracticable, they shall be filled with suitable material to prevent, for example, structural deterioration, unauthorised use, ingress of groundwater or infestation by rodents. C.13 Control of building over or adjacent to sewers The construction of buildings in close proximity to drains and sewers should be controlled in order that the operation and maintenance of the sewer system is not impaired by:

⎯ excessive loading leading to structural failure of part of the drain or sewer system;

⎯ prevention of access by maintenance personnel or equipment to manholes or inspection chambers, wastewater pumping stations, or other ancillary structures;

⎯ prevention of access by maintenance personnel or equipment for excavation to a repair a defect on a pipeline;

⎯ creating an undue risk of failure of the building in the event of a structural failure of the drain or sewer;

⎯ obstructing an overland flow path leading to an excessive risk of flooding in the building.

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Annex D

(normative)

Health and safety D.1 Safe systems of work Employers shall, so far as is reasonably practical, provide and maintain systems of work that are safe and without risks to health. The systems of work shall cover all aspects of the works including above-ground operations (for example manhole location and traffic control), access to the sewer system and all operations in the confined space of the sewer system. There shall be a written plan detailing systems of work for rescue and emergency evacuation procedures. Employers shall also set out procedures for detection and prevention of sudden inflows of toxic, flammable or potentially explosive substances, hot liquids or flood water discharged into the sewer system. Special precautions shall be taken when entering inverted siphons. The team size shall be sufficient to ensure that suitably trained personnel are: ⎯ on the surface to summon assistance and/or effect a rescue should it become necessary; ⎯ on the surface and in manholes to ensure that there is communication between personnel in the

sewer and both the entry and exit manholes. D.2 Training and supervision All personnel shall have appropriate training to enable them to carry out their work safely. In particular, all personnel involved in sewer work shall have appropriate training in safety procedures for work in confined spaces. Supervisors shall be competent in the management of work in the confined space of drains and sewers. D.3 Hazardous atmospheres D.3.1 Oxygen deficient and toxic atmospheres A range of oxygen deficient or toxic atmospheric conditions can occur in sewer systems. Appropriate atmospheric monitoring equipment must be used continuously whilst any worker is in the system. Forced ventilation should be used to maintain an atmosphere fit for respiration. D.3.2 Potentially explosive atmospheres A potentially explosive atmosphere can occur at any time during the operation of a sewer system, and this should be addressed as part of the design of the system. Appropriately protected plant and equipment along with their power and control systems should be specified and installed. The build up of potentially explosive atmospheric contaminants should be avoided by the use of forced ventilation coupled with adequate atmospheric monitoring. Whilst concentrations of potentially explosive atmospheric contaminants can build up when workers are already in the sewer system, and evacuation should then take place in accordance with the nationally recognized action limits, entry to the system should not take place when hazardous concentrations of potentially explosive contaminants exceed 10 % of their lower explosive limits or the permissible national limits. Additional national or local requirements can apply. D.4 Traffic control

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Measures shall be taken to warn and control traffic. These shall comply with the requirements of national or local regulations or the relevant authority and can include the provision of warning road signs and flashing beacons. D.5 Protective equipment and welfare facilities All necessary ventilation, lighting, communication and lifting equipment and, rescue equipment shall be provided and shall be appropriate to the task undertaken. Personal protective equipment including appropriate protective clothing and warning clothing shall be provided. All persons employed in work which involves entry into manholes or sewers or contact with raw wastewater shall have access to washing and showering facilities. Self rescuers and first aid equipment shall also be provided. D.6 Emergency procedures

Breathing apparatus shall be available on site and the team shall be sufficiently trained in its use to escape or be able to affect rescue in the event of oxygen deficiency or operatives inhaling toxic or asphyxiating gases. In the event of a collapse of a person in a confined space, no-one shall attempt to enter the confined space to attempt a rescue without breathing apparatus.

When the working area could be flooded a warning and evacuation procedure shall be foreseen, and the organization of the working area shall take into account these constraints. D.7 Temporary works Temporary works and arrangements for dealing with flow shall be designed with safety in mind. Care shall be taken to ensure that exhaust fumes from pumps or other machinery are kept away from manholes and that any dams or stoppers are sufficiently robust to withstand any hydraulic pressure likely to be applied while in use. D.8 Excavation work

When carrying out excavation work precautions shall be taken to avoid any danger to persons caused by collapse of the sides of the excavation, and to avoid damage to other utility services in the proximity of the excavations. Due regard shall also be taken to the need for safe operation of machinery and in particular the need for adequate working space. NOTE EN 1610 provides guidance on trenching work. D.9 Hazardous materials

The use of materials and chemicals in sewer renovation work, which can be toxic, flammable, or irritate human skin or internal organs, or are otherwise hazardous, should be minimised. When handling, storing, or using hazardous materials, the system of work shall deal specifically with the precautions necessary, including particular reference to their use in confined spaces. Processes can also generate dusts and fumes. Careful checks shall be made on the levels of harmful atmospheric contaminants and appropriate remedial measures taken where necessary. D.10 Vaccinations

National or local regulations or the relevant authority can require vaccination (e.g. against polio, tetanus) for personnel working in contact with foul wastewater.

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Annex E

(normative)

Hydraulic design E.1 General E.1.1 Introduction For smaller schemes a relatively simple, but safe, approach is recommended, though use of simulation models is not excluded. Sewers are usually designed to run full, without surcharge, for relatively frequent storms in the knowledge that this provides protection against flooding from much larger storms. For these schemes the "design storm frequency" criteria in Table 2 should be used in the absence of any specified by national or local regulations or the relevant authority. The designer shall use rainfall intensity and duration figures applicable to that particular area. For larger schemes and for smaller schemes to be designed using a simulation model and for larger schemes, particularly where damage or public health risks are significant, it is recommended that the level of flooding protection be directly assessed. The sewer system may be initially designed, as above, to give no surcharge with an appropriate "design storm frequency". A sewer flow simulation model should then be used to check the level of flood protection against the "design flooding frequency" and the design adjusted where the required flooding protection is not achieved. There will be cases, however, where adjustments are appropriate to avoid unnecessary over-design. Any requirements from national or local regulations or the relevant authority shall be followed, but in their absence the design flooding frequency values given in Table 3 should be used. E.1.2 Selection of flow simulation method E.1.2.1 General A variety of methods have been developed to assist in the design of drain and sewer systems. In all cases the runoff process has been simplified to enable the design parameters to be estimated cost effectively. This annex reviews the range of methods available and gives guidance where they should be used. E.1.2.2 Flow simulation methods Three levels of sophistication for the hydrodynamics of flow in pipes are recognised: ⎯ Simple/empirical methods

In these methods the flow is regarded as uniform and steady. The velocity at full flow conditions may be used to compute a travel time (time of concentration). They are used primarily for design of small development schemes (see E.3).

⎯ Kinematic wave methods

In these methods uniform unsteady flow can also be simulated. Lag time and in-pipe storage are taken into account, but the methods cannot simulate unsteady flows. They are effective for the initial design of large schemes, for the checking of existing systems, or for the simulation of network performance under long series of storm rainfall.

⎯ Dynamic wave methods

In these methods non-uniform, unsteady flow can also be simulated even under conditions of surcharge and backwater. They can be used to check the performance of systems under conditions of flooding.

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For each level the aboveground hydrological processes can be treated in either a simple or detailed manner (S or D in Table E.1 below). Table E.1 gives guidance on the applicability of the methods. Methods may be combined for sub-catchments.

Table E.1 — Applicability of flow simulation methods

Application

MethodSimple

empirical methods

Kinematic wave methods

Dynamic wave methods

Design of small development schemes S S b

Design of large schemes a S S or D Hydraulically simple road drainage systems

S — —

Checking performance against flooding a a S or D Checking existing systems a S or D S or D Planning of outfalls//overflows a S or D S or D Impact on receiving water quality a S S or D Impact on receiving water quantity a S S or D Real-time control of a system a S or D S or D NOTE S Hydrological processes treated in simple manner. D Hydrological processes treated in detailed manner. a Not applicable. b Generally not recommended.

E.2 Hydraulic calculations E.2.1 Velocity equations E.2.1.1 General The basis for design is that flows in drains and sewers are turbulent. Two equations are recommended for use in calculating turbulent flows in drains and sewers: Colebrook-White and Manning. E.2.1.2 The Colebrook-White equation For circular pipes flowing full, the velocity of flow is given by the equation:

( )( ) ⎟

⎟

⎠

⎞

⎜⎜

⎝

⎛+−=

E10E

251.2

71.3log22

gDJDV

DkgDJv (E.1)

where v is the velocity averaged across the flow cross-section, expressed in metres per second (m/s); g is the gravitational constant, expressed in metres per second squared (m/s2); D is the internal pipe diameter, expressed in metres (m); JE is the hydraulic gradient (energy loss per unit length), dimensionless; k is the hydraulic pipeline roughness, expressed in metres (m); V is the kinematic viscosity of fluid, expressed in metres squared per second (m2/s).

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For partially full pipes or pipes with non-circular cross-sections the velocity of flow is given by equation (E.1) by replacing D by 4Rh where Rh is the hydraulic radius (flow cross-sectional area divided by the wetted perimeter). E.2.1.3 The Manning equation For both circular and non-circular cross-sections whether running full or partially full, the velocity of flow is given by the equation:

2/1E

3/2h JKRv = (E.2)

where K is the Manning coefficient, expressed in metres raised to the power one third per second (m1/3/s); Rh is the hydraulic radius, expressed in metres (m); JE is the hydraulic gradient (energy loss per unit length), dimensionless. E.2.1.4 Headlosses E.2.1.4.1 Pipeline headlosses The hydraulic pipeline roughness (k) or the flow coefficient (K) allows for headlosses due to pipe material, discontinuities at the joints and slime growth on the pipe surface below the water level. E.2.1.4.2 Local headlosses Headlosses, in addition to those mentioned in E.2.1.4.1, occur at junctions, changes of cross-section, manholes, bends and other fittings. If direct calculations are to be made, the following equation shall be used:

gvk

h2

2L

L = (E.3)

where hL is the local headloss, expressed in metres (m); kL is the headloss coefficient dimensionless; v is the velocity of the liquid, expressed in metres per second (m/s); g is the gravitational constant, expressed in metres per second squared (m/s). E.2.1.4.3 Total headlosses Two methods of calculating total headlosses are: ⎯ adding local headlosses (see E.2.1.4.2) to the pipeline headlosses (see E.2.1.4.1); ⎯ accounting for local headlosses by assuming a higher value of hydraulic pipeline roughness in

the calculation of pipeline headloss. When using recommended hydraulic pipeline roughness values it is necessary to establish whether allowance has been included for local headlosses. Values currently in use range from 0.03 mm to 3.0 mm for k and 70 m1/3s-1 to 90 m1/3s-1 for K. Approximate comparisons of velocity estimates using equations (E.1) and (E.2) above may be made using the following equation:

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⎟⎠⎞

⎜⎝⎛

⎟⎠⎞

⎜⎝⎛=

kD

DgK 7.3log324 10

6/1

(E.4)

where K is the Manning coefficient, expressed in metres raised to the power of one third per second

(m1/3/s); g is the gravitational constant, expressed in metres per second squared (m/s2); D is the internal pipe diameter, expressed in metres (m); k is the hydraulic pipeline roughness, expressed in metres (m). E.2.2 The Saint Venant equations The flow conditions may be calculated by application of the Saint Vennant equations. These partial differential equations describe the gradually varied unsteady, non-uniform flow in open and closed channels. There are two equations; the dynamic equation, and the continuity equation. Depending on the application and the flow conditions different levels of simplification may be applied. These equations are shown in Table E.2 at various levels of simplification.

Table E.2 — The Saint Venant equations

Type of flow Dynamic equation Nr Continuity equation

Nr

gradually varied non-uniform discontinuous

+gAcvq

+×tv

g δδ1

+×xv

gv

δδ

=xh

δδ

JS – JF (E.5a)

qtA

xQ

=×δδ

δδ

(E.5a)

gradually varied non-uniform

+×

tv

g δδ1

+×xv

gv

δδ

=xh

δδ

JS – JF (E.5b)

0=×tA

xQ

δδ

δδ

(E.5b)

gradually varied simplified non-uniform

+×

tv

g δδ1

=xh

δδ

JS – JF (E.5c)

0=×tA

xQ

δδ

δδ

(E.5b)

simplified gradually varied non-uniform

+×

xv

gv

δδ

=xh

δδ

JS – JF (E.5d)

0=×tA

xQ

δδ

δδ

(E.5b)

simplified gradually varied simplified non-uniform

=

xh

δδ

JS – JF (E.5e)

0=×tA

xQ

δδ

δδ

(E.5b)

steady-state non-uniform

+×

xv

gv

δδ

=xh

δδ

JS – JF (E.5d)

0=xQ

δδ

(E.5c)

steady-state simplified non-uniform

=

xh

δδ

JS – JF (E.5e)

0=xQ

δδ

(E.5c)

steady-state uniform(normal discharge)

0 = JS – JF (E.5f) 0=

xQ

δδ

(E.5c)

Term ref (see below)

5 4 3 2 1 — — —

where Q is the flow, expressed in metres raised to the power of three per second (m3/s); q is the lateral inflow per unit of length in the direction of the flow (assumed steady state),

expressed in metres cubed per second metre (m3/(s.m)); A is the flow cross-section perpendicular to the sole, expressed in metres squared (m2);

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JS is the sole gradient (with open channel possibly not constant), dimensionless; JF is the friction gradient, dimensionless; NOTE For most practical purposes the friction gradient (JF) can be considered as equal to the hydraulic gradient (JE). x is the path co-ordinate in direction of flow, expressed in metres (m); t is the time co-ordinate, expressed in seconds (s); h is filling height in profile or depth of water (perpendicular to sole) or the pressure head in

completely filled drains at the sole of the pipe or profile, expressed in metres (m); v is the mean velocity, expressed in metres per second (m/s), in a cross-section in the direction of

flow; g is the acceleration due to gravity, expressed in metres per seconds squared (m/s2); c is the factor with inclusion of additional losses, dimensionless. The different terms in the dynamic equation can be described as follows: ⎯ Term 1 of the dynamic equation is the difference between the gradient of the invert of the pipe

and the friction gradient. ⎯ Term 2 of the dynamic equation is the kinematic wave term. ⎯ Term 3 of the dynamic equation is the diffusive wave term and takes into account backwater and

wave attenuation. ⎯ Term 4 of the dynamic equation is the local flow acceleration term. ⎯ Term 5 of the dynamic equation is the lateral flow inflow term. E.3 Methods of calculating runoff from small development schemes In the absence of a method specified by national or local regulations or the relevant authority, a simple method of estimating the peak rate of discharge of surface water, applicable for areas of up to 200 ha or times of concentration up to 15 min and assuming a uniform rate of rainfall intensity, may be used. The rainfall intensity to be adopted will depend on factors such as time of concentration of the contributing area and the analysis of local rainfall data. Peak flow rate is given by:

Q = C.i.A (E.6) where Q is the peak flow rate, expressed in litres per second (l/s); C is the runoff coefficient (between 0.0 and 1.0), dimensionless; i is the rainfall intensity, expressed in litres per second hectare (l/s/ha); A is the area receiving rainfall (measured horizontally), expressed in hectares (ha). Appropriate values for (C) are given in the Table E.3.

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Table E.3 — Runoff coefficients for calculating runoff from small development schemes

Nature of connected area Runoff coefficient C CommentsImpermeable areas and steeply sloping roofs a

0.9 to 1.0 Depending on depression storage

Large flat roofs 0.5 Over 10000 m2 Small flat roofs 1.0 Less than 100 m2 Permeable areas 0.0 to 0.3 Depending on ground slope and cover a Impermeable areas may be increased by 30 % of large vertical surfaces.

E.4 Calculation of foul wastewater flows for drain systems Wastewater flows for drain systems should be calculated in accordance with EN 12056-2. E.5 Calculation of foul wastewater flows for sewer system Table E.4 and Table E.5 give indicative values for average flow rate and peak design flow in common use for the design of surface water and combined sewers.

Table E.4 — Domestic flow rate

Country Flow rate Range (l/head.day) CommentCzech Republic 100 to 150 Ordinary daily flow; does not contain

provision on infiltration. Denmark 120 to 150 50 % to 100 % should be added to make

allowance for infiltration. France 150 to 200 — Germany 150 to 300 Depending on level and age of sanitary

system. No infiltration allowance included. Netherlands 100 to 120 — Portugal 120 to 350 — Switzerland 170 to 200 — United Kingdom 150 to 300 —

Table E.5 — Domestic peak design flow

Country Peak Design Flow CommentDenmark 4 l/s to 6 l/s per 1000 inhabitants Depending on the size of the catchment

area, excluding 50 % to 100 % infiltration allowance.

France (1.5 to 4.0) × domestic flow rate 1.5 to 4.0 is the peak coefficient, it depends on the location of the sewer, its gradient, its size and the size of the town.

Germany 4 l/s per 1000 inhabitants For design of sewers. Additional allowance made for infiltration, non-designed flows.

4 l/s per 1000 inhabitants or 200 I/inhabitant per day

For design of treatment works and for stormwater treatment.

Netherlands — 10% of daily flow. Portugal (2,0 to 5,0) x domestic flow rate — Switzerland 6 l/s to 7 l/s per 1 000

inhabitants 8 l/s to 10 l/s per 1 000 inhabitants often used to include allowance for commercial flows.

United Kingdom Up to 6 × domestic flow rate Dependent on catchment area. Additional allowance made for infiltration.

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E.6 Combined sewer overflows The allowable discharges and impact of combined sewer overflows on receiving waters depend on local conditions. Requirements are generally specified by national or local regulations or the relevant authority. The location of combined sewer overflows, pollution loads, duration and frequency of discharges, pollution concentrations and hydro-biological stress are factors to be considered. The impacts of combined sewer overflows on receiving waters occur only for short time periods. However, they can be many times higher than the impact and environmental loads from wastewater treatment plant. The main objective of combined sewer overflow design, therefore, is to protect the receiving water without causing hydraulic overload of the sewer or reduced treatment efficiency of downstream wastewater treatment plant. Sewer flow simulation models (see 8.4) shall be needed to assess compliance with many of the specified emission limits (see 8.5.2). Two relatively simple approaches are available. A combined sewer overflow may be designed to begin overflow discharge only after reaching a critical rainfall intensity, generally in a range of rates of 10 l/s ha (impermeable area) to 30 l/s ha (impermeable area), depending on the degree of protection required. Alternatively, when the self-purifying capacity is not at risk, a single criterion (commonly a dilution of 5 to 8 times dry weather flow before spill) may be used as an emission standard. Associated storage in, for example, a detention tank, or length of tank sewer, can greatly reduce the environmental impact of combined sewer overflows. Further reductions in environmental impact can be achieved by partial treatment (e.g. settling). If the retained flow in a combined system exceeds the capacity of the treatment works, it will be necessary to incorporate storage or partial treatment of the retained flow. This may be sited at the treatment works or within the sewer system. In the design of a combined sewer overflow, steps shall be taken to keep the discharge of floating solids and other unsightly material to acceptable levels. This can require the provision of baffles, screens or other means of control.

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Annex F

(normative)

Pumping Installations F.1 General Pumping installations are occasionally required in gravity drain and sewer systems in order to avoid excessive depths, or to drain low lying areas. They can also be necessary at combined sewer overflows or outfalls to discharge flows into treatment works or receiving waters. Pumping installations shall be designed in accordance with the principles described in clause 8. NOTE For layout, operation and maintenance requirements for lifting plants for wastewater within buildings and sites requirements apply according to EN 12056-4. Installations shall be planned and designed taking into account: a) whole life cost; b) energy usage; c) operations and maintenance requirements; d) risk and consequences of failure; e) health and safety of public and operating staff; f) environmental impact; g) nature of wastewater which can:

⎯ be aggressive, corrosive and/or erosive;

⎯ have a high solid content increasing the potential for blockage;

⎯ be toxic;

⎯ lead to potentially explosive conditions. F.3, F.4 and F.5 deal separately with the design of pumping stations, rising mains and components, however these shall not be considered in isolation as there is interaction between them. F.2 Planning of pumping installations F.2.1 Preliminary Considerations The preliminary considerations for each pumping installation shall include: ⎯ general location in relation to such features as flood plains, rivers, railways, major roads and

overall topography; ⎯ relation to existing sewer systems; ⎯ environmental considerations including the potential impact on any environmentally sensitive

areas and the siting of combined sewer overflows; ⎯ access requirements;

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⎯ land ownership; ⎯ availability of power supply, water supply and telecommunications; ⎯ risk of explosion; ⎯ risk of flooding; ⎯ risk of floatation; ⎯ risk of saline infiltration; ⎯ suitability of geotechnical conditions. Once these have been established consideration can be given to more detailed planning. F.2.2 Planning The requirements for the pumping installation shall be determined in accordance with the functional requirements in clause 5, with particular reference to: ⎯ nature and quantity of flows including the range of flow rates (diurnal, dry/wet weather, etc.) and

the range of heads to be pumped; ⎯ effect of the flows on the downstream sewer system and treatment works; ⎯ measures to limit the impact of failure, which can include the use of standby pumps, generators,

duplicate rising mains, emergency overflows, screens, over-pumping arrangements, detention tanks, all including the requirements of national or local regulations or the relevant authority;

⎯ requirements of national or local regulations or the relevant authority with regard to combined

sewer overflows and emergency overflows; ⎯ limiting noise and odour; ⎯ limiting retention time in order to avoid septicity and/or sedimentation; ⎯ provision of facilities and equipment for operation and maintenance; ⎯ requirements for future expansion; ⎯ consideration of special conditions (e.g. aquifer protection zones). Once the requirements for the pumping installation have been determined, consideration can be given to the requirements for the site and the location taking into account: ⎯ estimated size of the pumping station, relating to the number, size and type (e.g. centrifugal,

screw or ejector) of duty and stand-by pumps, whether there is to be a wet well/dry well, wet well only arrangement, or duplicated wet wells;

⎯ space for detention tanks, screens or grit chambers if required; ⎯ space for future expansion of the pumping station; ⎯ access to the site in all weather conditions; ⎯ space for maintenance vehicles and ancillary equipment;

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⎯ route and levels of the incoming and outgoing sewers; ⎯ environmental impact including odour, noise, visual impact, impact of discharge to receiving

waters; ⎯ location of receiving waters for overflows if required; ⎯ risk of vandalism, site security and the need for fencing. F.3 Design of pumping stations F.3.1 Layout Design requirements shall be determined for: ⎯ pumps; ⎯ drive units; ⎯ controls and electrical equipment; ⎯ instrumentation and telemetry; ⎯ alarms; ⎯ pipework and valves. These shall take account of the basic requirements (see F.2). Consideration shall be given to: ⎯ maximum and minimum predicted flow rates to determine the duty points of the pumps and the

size of mechanical and electrical equipment; ⎯ type and number of pumps being used (At least two pumps should be installed to provide

standby in the event of failure of one pump); ⎯ fixed speed, multi-speed or variable speed drive units; ⎯ provision of screens, grit chambers or, where permitted, macerators at the inlet to minimise the

risk of clogging of, or damage to the pump impellers and downstream components; ⎯ removal of screenings and grit; ⎯ odour control; ⎯ physical size of the various items of plant such as pumps; ⎯ provision of access to, and sufficient working space around, all components which could require

maintenance or replacement; ⎯ means of lifting for removal or dismantling of equipment; i ⎯ size of the wet well(s) and, where applicable, the dry well; ⎯ inlet configuration; ⎯ welfare facilities for staff where required; ⎯ power source for drive units (e.g. electricity or diesel) and if necessary, standby power source;

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⎯ fuel storage capacity, where appropriate; ⎯ over-pumping facilities; ⎯ susceptibility to vandalism. The layout shall also: ⎯ allow pumps to be installed so they can be primed; ⎯ keep the suction pipelines substantially horizontal, as short as possible and with no areas for air

to become trapped; ⎯ ensure that non-immersible electrical and mechanical equipment is protected from flooding. Where possible control equipment should be in the same location. The hydraulic design of the pumping station and rising main shall be considered together. Buildings and chambers shall be adequately ventilated to avoid build-up of toxic or explosive gases. Wet wells shall be provided with forced ventilation where necessary. Gas testing facilities shall be made available (either portable or permanently installed). F.3.2 Wet well design The wet well should be designed so that: ⎯ sump extends below the level of incoming sewers; ⎯ it is possible to isolate, empty and clean the wet well (e.g. by partitioning or duplicating the wet

well); ⎯ "dead zones" where sedimentation can build up are avoided (in some cases model testing could

be useful); ⎯ intake configuration ensures stable flow conditions to the pump, particularly avoiding air

entrainment (in some cases model testing could be useful); ⎯ there is adequate clearance between the base and sides of the wet well and the pump inlet; ⎯ it is protected against septicity (see 9.5.3); ⎯ any necessary measures to guard against explosion are taken. The size of the wet well and its detailed design shall be determined from the maximum and minimum flow rates. The capacity between start and cut out shall be set to limit the frequency of switching to within the drive unit manufacturer's recommendations. Start levels shall give adequate wastewater levels to allow pumps to prime. F.3.3 External layout and access Access and appropriate parking shall be provided at all times for emergency vehicles, maintenance vehicles and ancillary equipment. Adverse weather conditions shall be considered. The site shall be designed to deter unauthorised access. Adequate protection against lightning shall be provided. F.3.4 Environmental impact

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The design of the pumping station shall take into account its effect on the environment including: ⎯ consequences of discharges from combined sewer overflows; ⎯ noise, vibration and odour inside and outside the pumping station; ⎯ consequences of failure; ⎯ visual impact. The national or local regulations or relevant authority can lay down requirements pertaining to the quality, quantity and frequency of discharges to receiving waters (see 8.5.2 and 9.5.2). Where emergency overflows are provided they shall be designed to ensure maximum retention of solids. F.3.5 Structural design The structural design of chambers and buildings shall be designed in accordance with EN 1990 to EN 1999 if applicable or otherwise in accordance with relevant product standards and shall take account of: ⎯ structural integrity (taking into account loads from lifting equipment and seismic loadings

where appropriate); ⎯ watertightness; ⎯ prevention of floatation; ⎯ bearing capacity and chemical nature of the soil; ⎯ aggressive, corrosive and/or erosive effluents; ⎯ possible differential settlement between the structure and all incoming sewers and outgoing

rising mains and other services; ⎯ requirements of the national or local regulations or relevant authority. F.3.6 Maintenance considerations Mechanical and electrical equipment shall be selected which is robust and reliable and shall require minimal maintenance. Consideration should also be given to the availability of spare parts. The provision of appropriate lifting hoists and beams, and of lifting eyes or similar features on heavy equipment, shall be considered. Complete sets of current general arrangement and sectional drawings, operational, maintenance and service manuals, circuit diagrams and parts lists shall be supplied and be available at all times. F.4 Design of rising mains F.4.1 Principal considerations The principal design considerations for rising mains for sewers include: ⎯ choice of a route; ⎯ choice of diameter;

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⎯ positive and negative pressures and external loads; ⎯ choice of materials; ⎯ thrust; ⎯ discharge points; ⎯ control of septicity; ⎯ valve chambers; ⎯ whole life cost. The hydraulic design of the rising main and pumping station shall be considered together. Methods for calculation of head losses and flows in pipes are described in E.2. F.4.2 Choice of route Where possible the route should avoid pipeline summits and valleys. The location of rising mains shall take account of the requirements for access for maintenance and operations. Flushing or rodding connections may be incorporated. Appropriate vehicular access shall be provided to valve chambers for operations and maintenance purposes (see F.4.9). F.4.3 Choice of diameter The diameter of the rising mains shall be selected by considering: ⎯ design flow rates and associated velocities and pumping costs; ⎯ capital cost; ⎯ minimum velocities to limit sedimentation, ⎯ minimum diameter to limit clogging; ⎯ septicity implications of retention time. F.4.4 Pressures and external loads Pipelines shall be designed for pressure resulting from maximum flow, no flow and transient pressures (positive or negative), also taking account of external loads. In the case of transient conditions, the amplitude and frequency shall be estimated. Surge analysis shall be carried out taking into account all possible operating conditions. Numerous methods to reduce or suppress surge are available. F.4.5 Choice of materials The material for the rising main shall be selected in accordance with the principles in 8.6.4. Particular care shall be taken: ⎯ where wastewater contains aggressive substances; ⎯ in contaminated or aggressive soil conditions;

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⎯ in adverse ground conditions; ⎯ in difficult terrain; ⎯ in aquifer protection zones. F.4.6 Thrust Thrust forces occur at valves, changes in direction and diameter, branches and blank ends and shall be contained. The methods available include: ⎯ restrained joints over an adequate length of pipeline; ⎯ thrust or anchor blocks; ⎯ cradles and clamps, generally for non-buried pipelines. Anchorages should be designed to avoid transmitting vibration. Where thrust or anchor blocks are to bear against the soil, the safe bearing pressures shall be determined. The possibility of shear failure, sliding, and potential disturbance of the block by subsequent excavation shall be considered. F.4.7 Discharge points Discharge points shall be designed to minimise splashing and noise. Manholes into which rising mains discharge shall be well ventilated having regard to the need for odour control. Receiving manholes shall be protected against chemical attack and erosion where appropriate. F.4.8 Control of Septicity Septicity should be limited (see 9.5.3). F.4.9 Valve chambers Valve chambers shall be provided where necessary to allow maintenance of valves. The design of valve chambers shall include for: ⎯ removal and replacement of valves; ⎯ safe access for personnel into the chamber; ⎯ vehicle access to the site of the chamber. Arrangements shall be made for removing standing water from the valve chambers. Air valve chambers shall be adequately ventilated. F.5 Components and appliances F.5.1 Pumps Each pump and its drive unit shall be suitable for the nature and composition of the wastewater to be pumped and for duty throughout the specified range of station requirements such as flow rates, heads and duty points. In some cases it could be necessary to modify the pumping station design to find an acceptable combination of pump and pumping station to avoid: ⎯ overloading of pumps, leading to abnormal increase in power consumption;

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⎯ cavitation throughout the permissible range of operating speeds, flows and available suction

level; ⎯ negative suction head. In addition to any testing carried out before delivery, pumps shall be tested after installation for compliance with user requirements. Performance tests for acceptance shall be agreed with the pump supplier. Further factors to be considered shall include: ⎯ optimisation of efficiency; ⎯ anticipated future flows taking into consideration the design life of the pump; ⎯ pump speed (fixed speed, multi-speed or variable speed); ⎯ materials used in pump construction, including susceptibility to corrosion and erosion; ⎯ ability to pass permitted solids without clogging. F.5.2 Prime movers and drives Prime movers and drives shall be suitable for the types of pump selected and rated for all the operational conditions. They shall be designed to be energy efficient. Where electric motors are to be in contact with potentially explosive atmospheres, they shall be explosion proof. All non-submersible plant shall be located in a machinery room which is protected from flooding. Types of prime movers which may be used include: ⎯ electric motors; ⎯ internal combustion engines. These may be multiple or variable speed prime movers. Types of drive which may be used are: ⎯ direct; ⎯ geared; ⎯ belt; ⎯ close coupled; ⎯ intermediate shafting. Vibration shall be minimised taking into account any requirements of national or local regulations or the relevant authority. F.5.3 Valves Valves of varying types may be required as follows:

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⎯ isolating valves to allow sections of pipework, pumps, valves etc. to be removed without emptying the whole rising main,

⎯ washout valves at low or intermediate points to allow sections of the rising main to be emptied; ⎯ non-return valves at pumps to prevent backflow from the rising main; ⎯ air valves at summits and other points indicated by the surge analysis. When a single valve is

used this shall be double acting. When fully open valves should not disturb the flow distribution. Consideration shall be given to the surge effects of valve operation. To minimise surge pressures in the rising mains, valves on rising mains may be arranged to close before pumps are stopped, and to open after they have reached full speed, both at controlled rates. All valves shall be suitable for use with wastewater and shall be designed to prevent retention of solids. All valves shall be identified by durable tags. F.5.4 Controls and electrical equipment All electrical installations shall meet the requirements of national or local regulations or the relevant authority and, where appropriate, shall be protected by suitable enclosures (e.g. drip proof, explosion proof). High voltage equipment shall be secure from access by unauthorised personnel. All electrical equipment shall be properly earthed and protected from lightning damage. Switchboards and motor control centres should be of modular construction. Each circuit should be totally segregated. Each pump set shall be provided with a separate starter. Safeguards shall be incorporated in pump controls to stop units in the event of loss of suction pressure or unacceptable flow conditions. Control systems shall ensure that unnecessary repeated stopping and starting or speed changes are avoided. Controls may use various devices to activate the closing of the electrical circuit e.g. floats, electrodes, ultrasonic's, pressure transducers, time controls. Control systems should allow for the switching sequences to be varied, where two or more pumps are used in parallel or to change from a normal duty pump to the standby pump. A separate connection point for a temporary power generator, with switching arrangements, should be considered. F.5.5 Instrumentation Suitable instrumentation shall be provided. This can include: ⎯ monitoring equipment (e.g. level, flow, pressure, speed, voltage, current, power factor, gas

content, hours run etc.); ⎯ indication of operation of duty/standby pumps. Information, alarms, and instructions can be relayed by telemetry to or from a remote location. The design of telemetry systems shall consider present and future data requirements and the means of data transmission. F.5.6 Alarms Provision of alarms shall be considered, these can include: ⎯ flammable gas; ⎯ fire;

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⎯ high water level; ⎯ bearing temperature; ⎯ motor temperature; ⎯ pump failure; ⎯ power failure; ⎯ vandalism. An alarm system should have an emergency power source capable of operating for at least 24 h in the event of failure of the main power supply. Alarms should be relayed by telemetry to a central location. F.6 Health and safety The principles for occupational health and safety set out in clause 7 should be applied to the design, construction and operation of pumping stations. Adequate welfare and first aid facilities should be provided. National or local requirements can also apply. The principal risks to be addressed arise from: ⎯ work in a confined space; ⎯ work in a wet and poorly illuminated location; ⎯ falls from height; ⎯ potentially explosive, oxygen deficient or toxic atmosphere, ⎯ fire; ⎯ moving machinery; ⎯ noise and vibration; ⎯ electrical installations; ⎯ biological contaminants; ⎯ liquid filled tanks; ⎯ remote working; ⎯ alarm systems (see F.5.6).

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Bibliography

[1] EN 773:1999, General requirements for components used in hydraulically pressurized discharge

pipes, drains and sewers [2] EN 1293:1999, General requirements for components used in pneumatically pressurized

discharge pipes, drains and sewers [3] EN 13380:2001, General requirements for components used for renovation and repair of drain

and sewer systems outside buildings [4] EN 13689:2002, Guidance on the classification and design of plastics piping systems used for

renovation [5] ISO 9000:2005, Quality management systems — Fundamentals and vocabulary [6] ISO 9001:2008, Quality management systems — Requirements [7] ISO 9004:2000, Quality management systems — Guidelines for performance improvements [8] EN 14457:2004, General requirements for components specifically designed for use in

trenchless construction of drains and sewers [9] EN 1085:2007, Wastewater treatment — Vocabulary [10] EN 12056-4, Gravity drainage systems inside buildings — Part 4: Wastewater lifting plants,

Layout and calculation [11] EN 1091, Vacuum sewerage systems outside buildings [12] EN 1671, Pressure sewerage systems outside buildings [13] EN 12056-2, Gravity drainage systems inside buildings — Part 2: Sanitary pipework, layout and

calculation [14] EN 13508-1, Condition of drain and sewer systems outside buildings — Part 1: General

Requirements

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© EAC 2010 — All rights reserved

Documents

EAST AFRICAN STANDARD - EAC QUALITY · 2010-07-17 · Draft for comments only — Not to be cited as East African Standard CD/K/002:2009 ... EN 476, General requirements for components