Watson Hawksley

Yuen Long andKam Tin Sewerage

Masferplan StudyAUTHOR MO

UNIVERSITY OF HONG KONGLIBRARY

Hong Kong Collectiongift from

Hong Kong. Environmental Protection Dept.

CONTENTS

Introduction

Background to the Study

Study Area

Objectives

Approach

Study Programme

Existing Conditions

Existing Facilities

Yuen Long Sewerage System

NWNT Sewerage Scheme

Other Sewered Areas

Yuen Long Sewage Treatment Plant

San Wai Sewage Treatment Plant

Other Sewage Treatment Plants

On-site Treatment and Disposal Systems

Future Situation

Trends and Forecasts

Sewage Flows and Loads

Master Plan Development

Approach

Constraints and Opportunities

Key Criteria

Option Selection

Basic Strategies

Effluent System Capacity

Master Plan Selection

The Recommended Master Plan

Recommendations

Principal Components

Estimated Costs

Trunk Sewerage

Village Sewerage

Operation and Maintenance Requirements

Implementation

Priorities

Packages

Programme

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INTRODUCTION

BACKGROUND TO THE STUDY

The last twenty years has seen the transformation of theNorth West New Territories from a quiet self containedrural area into a complex mixture of urban, semi urban andrural development.

Historically Yuen Long and the rural townships haveplayed a significant role in the life of the New Territories.Although natural focal points for development they wereinitially overshadowed by the rapid build up of the NewTowns of Tsuen Wan, Sha Tin and Tuen Mun.

However with the drawing up of further developmentplans, including the North Western New TerritoriesDevelopment Strategy, the scope and pace of developmentin Yuen Long New Town, Tin Shui Wai New Town andthe surrounding areas has greatly increased.

STUDY AREA

The Yuen Long and Kam Tin Study Area is a region ofabout 14,000 ha of which approximately half is gentlygraded or inter tidal land and the remainder is steeplysloping hillside. The largest community within the area isYuen Long New Town which also accommodates much ofthe commercial and industrial development within the area.Other important population centres exist at Lam Tei, HungShui Kiu and Ping Shan, in the Tuen Mun/Yuen LongCorridor, and at Kam Tin, Fairview Park and San Tin.Tin Shui Wai New Town is also developing rapidly.

Fishfarming, livestock rearing and vegetable growing arestill important but there is an increasing tendency towardsthe use of agricultural land for small scale manufacturingand open storage of containers, scrap metal andconstruction plant and materials.

The Country Parks and adjacent areas provide importantrecreational facilities within the area.

The Mai Po Marshes, which cover some 400 ha on theedge of Deep Bay, are of international importance as afeeding, nesting and resting habitat for hundreds ofthousands of migratory birds.

Vegetable growing is still important

The Yuen Long and Kam Tin area is undergoing significantand rapid expansion and as a consequence pollutionproblems are increasing. Already the watercourses withinit are amongst the most polluted in the Territory and theDeep Bay ecosystem is showing signs of stress as a resultof the amount of pollution being discharged into its waters.It is important that these issues are addressed immediatelyto prevent further deterioration of the environment.

The largest community is Yuen Long

There is an increasing tendency towards open storage

OBJECTIVES

The emphasis of the Study Brief is on the provision ofsewerage to all parts of the Study Area with specialemphasis on rural areas.

The primary objectives of the Study were:

• To determine the requirements of a Sewerage MasterPlan based on investigation of the inadequacies inexisting sewerage schemes and the level of pollutiongenerated from all sources;

• To identify schemes which provide effective reduction ofpollution entering watercourses and Deep Bay;

• To define an implementation programme whichoptimises benefits with respect to costs;

• To take full account of management issues to ensureefficient operation of facilities constructed as a result ofthe Master Plan.

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APPROACH

The development of the Sewerage Master Plan was basedon a systematic consideration of the following issues andconstraints:

development trends and pressures;• pollution loadings;

• watercourse quality;

• existing and committed sewerage and sewage treatmentfacilities;

• water quality objectives and standards;

• land ownership, use and development control;

• the existing administrative structure and regulationsgoverning the provision of sewerage;

• existing environmental conditions.

Phase 1 of the Study concentrated on the collection andanalysis of data, surveys of existing facilities and theidentification of possible master plan options and interimsolutions to achieve early environmental improvements.

Phase 2 developed and evaluated master plan options andpresented details of the recommended Master Plan includingestimated costs and an implementation programme.

STUDY PROGRAMME

The Study commenced in June 1990 and was completed inDecember 1991.

Seven Working Papers were produced on specific aspectsof the Study. The Phase 1 work was summarised in anInterim Report issued in April 1991. Phase 2 wassubstantially completed in September 1991 and the findingsof the whole Study are presented in the Final Report.

Common faults being, siltation blockages and greaseaccumulation

Stream pollution from industrial wastes

EXISTING CONDITIONS

Pollution entering the watercourses within the Study Areaoriginates principally from livestock farming, industrialwastes and domestic sources. Levels of organic, heavymetal, nutrient and faecal pollution are substantial with allbut the upper reaches of most watercourses beinganaerobic.

Because of the low baseline flow in most watercourses thereis little opportunity for dilution, dispersion, re-aeration orself purification resulting in further deterioration in waterquality where flows enter Deep Bay.

EXISTING FACILITIES

YUEN LONG SEWERAGE SYSTEM

The Yuen Long sewerage system serves Yuen Long NewTown and adjacent areas including Yuen Long IndustrialEstate, Tung Tau Industrial Area, Long Ping Estate, partof the Tong Yan San Tsuen Industrial Area and severalsmall outlying institutional and residential areas. Thesystem consists of some 30km of sewers and two majorpumping stations.

Field investigations undertaken as part of the systemanalysis included a closed circuit television (CCTV)inspection of the sewerage system, manhole inspections anda programme of in-sewer flow gauging.

The surveys showed that more than 50% of the sewersystem is in an unacceptable service condition, commonfaults being siltation, blockages and grease accumulation.The system exhibits signs of a lack of regular or plannedmaintenance and cleansing is recommended to ensure thehydraulic capacity of the system is restored. This work canbe undertaken by conventional water jetting but the use ofwinched devices or rotators may be necessary where siltdeposits are excessive.

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Fairview Park

In general the sewers are in sound structural condition withfew major structural problems. The system does howeverdisplay signs of some deterioration as indicated bycracking, displaced and open joints and, more seriously,broken joints, fractures and collapses. Repair of the brokenjoints, fractures and collapses is recommended to preventfurther deterioration of the system.

Hydraulic modelling of the system showed that it exhibitedsome response to rainfall, caused by the expedientconnection of surface water drains to the system. Analysisshowed that in the short term the system is capable ofcoping with existing flows except for the Tong Yan SanTsuen area where the existing system is inadequate.

In the longer term the existing system may not be able tocope with projected flows; potential short falls in capacitybeing identified in:• the eastern part of the New Town;

• Tung Tau Industrial Estate;

• Yuen Long Industrial Estate;

• Ping Shan Street pumping station.

Alternative strategies for these parts of the existing systemwere considered as part of the Master Plan development.

NWNT SEWERAGE SCHEME

A major sewerage and sewage disposal scheme (NWNTSS)is currently being constructed to serve Au Tau, Long Ping,Tin Shui Wai, the Tuen Mun/Yuen Long Corridor, theperipheral areas around Yuen Long and North Tuen Mun.Following preliminary treatment at the San Wai treatmentplant sewage will be conveyed through a tunnel to UrmstonRoad where it will be discharged to a submarine outfall.The NWNTSS is currently under construction and Stage 1is expected to be commissioned by the end of 1992.

The hydraulic capacity of the NWNTSS was reviewed toidentify any potential shortfalls in capacity. The reviewshowed that in general the proposed system has adequatecapacity for the projected flows.

OTHER SEWERED AREAS

Conventional sewerage exists in a few other areas namely:

• the military camps and quarters at Shek Kong, Tarn Meiand Cassino Lines;

• private developments such as Fairview Park, Wo ShanWai and Maple Gardens.

Each of these systems collects sewage for treatment at alocal sewage treatment plant.

YUEN LONG SEWAGE TREATMENT PLANT

Stage 1 of the Yuen Long sewage treatment plant,commissioned in 1984, provides screening and gritremoval, primary sedimentation and secondary treatment(activated sludge) to achieve a 20:30 standard (BOD:SS)with full nitrification. Treated effluent is discharged to theShan Pui River. Sludge is anaerobically digested,dewatered and disposed of to landfill.

Stage 2 of the treatment plant is currently underconstruction and when it is commissioned in 1992 thetreatment plant will have treatment capacity for anequivalent population of approximately 250,000 persons.

The crude sewage strength is much as expected at designstage but peak flows to the plant are significantly lowerthan designed for. Problems have been experienced as aresult of shock loads of industrial waste arriving at thetreatment plant.

Overall the plant is performing reasonably well and isgenerally meeting its design criteria.

The Yuen Long Sewage Treatment Plant

SAN WAI SEWAGE TREATMENT PLANT

When commissioned in late 1992 the San Wai sewagetreatment plant will have capacity to give preliminarytreatment (grit removal and fine screening) to flows froman equivalent population of some 580,000 persons. Treatedsewage will be discharged via the NWNTSS effluent tunneland submarine outfall to the waters of Urmston Road.

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X

Legend

Area Served „ Pumping Main/Effluent Tunnel — Submarine Outfall

The North West New Territories Sewerage Scheme

OTHER SEWAGE TREATMENT PLANTS

Domestic

Eleven private domestic sewage treatment plants wereinspected during the Study ranging from the extendedaeration plant at Fairview Park, which is designed to serve30,000 persons, to much smaller plants designed to serveas few as 200 persons.

Overall plant reliability would be increased at FairviewPark by the provision of fine screening upstream of theaeration plant and discharge standards will not beconsistently met unless the jet aeration equipment installedin one of the two aeration lanes is replaced with morereliable and efficient equipment.

A number of common problems were identified amongstthe smaller treatment plants namely:

• oversizing of process units;

• poor sludge disposal;

• improper chlorination;

• lack of skilled operation.

Military

Five military sewage treatment plants were inspected duringthe Study and all were found to be producing anunacceptable effluent quality. The Malaya/Borneo Linestreatment plant at Shek Kong suffers from a high level ofinfiltration in the sewerage system, shock loading from therefugee detention centre and inadequate desludging. Thefour remaining plants at Shek Kong Village (RAF andArmy), Tarn Mei Camp and Cassino Lines need to beoperated on a continuous basis, improved desludgingprocedures and, in the case of the Shek Kong and CassinoLines plants, the provision of final settlement tanks.

Industrial

A total of 20 industrial treatment plants were identifiedwithin the Study Area of which approximately halfprovided only simple pretreatment in the form of flowequalisation, pH correction and sedimentation prior todischarge to the foul sewerage system.

Of the eleven plants designed to provided treatment otherthan simple settlement or pH/temperature control:

• two plants were no longer required;

• one plant was no longer used;

• three plants produced effluent which complied with theWPCO Technical Memorandum;

• four plants produced effluent which did not comply withthe WPCO Technical Memorandum.

Given the very significant increases in industrial effluentswhich are expected within the Study Area strictenforcement of the provisions of the WPCO TechnicalMemorandum will be required.

ON-SITE TREATMENT AND DISPOSAL SYSTEMS

Existing Practice

Outside of the sewered areas the wastewater fromhouseholds and institutions is permitted to be disposed ofon-site. Within the Study Area just under half of thepopulation live outside sewered areas and therefore rely onon-site disposal systems.

Surveys carried out during the Study concluded that:

• sullage (the washing water component of domesticsewage) is almost universally discharged directly to

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surface water drains from which it is discharged to theenvironment, either on to land or into watercourses;

• toilet wastes from between 15% and 40% of thepopulation are discharged to septic tanks;

• public toilets are depended upon by a majority of thepopulation;

• a significant proportion of the population use toiletfacilities which dispose of toilet wastes directly to theenvironment.

A septic tank provides pretreatment of raw sewage prior toits discharge by a combination of settlement and digestion.A 20% reduction in organic pollution load, nominalreduction in nutrient pollution load and a reduction infaecal pollution load can be expected.

Soakaways receive effluent from the septic tank anddisperse it to the surrounding soil.

Providing the soil is reasonably permeable and the watertable is sufficiently below the bottom of the soakawayalmost complete BOD removal and nitrification may beexpected within a metre or two of the soakaway as a resultof filtration and absorption by the biological slimes whichwill form on the exposed face and within the pores of theadjacent soil.

Residential development in the Study Area is generallylocated on alluvial soils. The composition of these soils ishighly variable with gravels, sands, silts, and clays forminglayers of different character. The hydraulic permeability ofthese soil is therefore also highly variable. Where there aresandy soils, soil soakage rates may be sufficiently high forsoakaways to be operable. Where silts and clays arepresent in significant amounts, soakage rates will be lowerand soakaway s are likely to fail.

Design and Construction Standards

At present neither the design nor the construction of thesesystems is satisfactory. Generally septic tanks andsoakaway s are constructed according to standard guidelines.Common and significant departures include:

• incorrect sizing, particularly where a single tank is toserve a single village type house constructed as threeliving quarters;

• omission of details such as inlet and outlet baffles;

• soakaway s not of the specified dimensions;

• overflows from soakaways connecting directly to surfacewater drainage systems;

• insufficient soakaway capacity to accommodate septictank effluent where both sullage and toilets wastes aredischarged to the septic tank;

• insufficient soakaway capacity to accommodate septictank effluent where toilet wastes from a house with threeliving quarters are discharged to the septic tank.

Controls and Legislation

The Building Ordinance provides legislative means forrequiring the provision of sewage treatment and disposalsystems. Two broad categories of development arerecognised in this respect:

• New Territories Exempted Housing which is exemptedfrom the drainage provisions of the Building Ordinanceprovided a septic tank and soakaway is installed;

• all other development for which either on-site sewagetreatment with off-site effluent disposal or connection toa sewerage system is required.

Effluents from household septic tanks which are dischargedto ground are not required to comply with the provisions ofthe Technical Memorandum on Effluent Standards issuedunder Section 21 of the Water Pollution Control Ordinance.With the creation of the Deep Bay Water Control Zoneseptic tanks are however required to be licensed and theEnvironmental Protection Department is empowered tocontrol septic tanks by setting design and maintenancestandards.

Selection of Systems

theThe following approach is proposed to improveselection of on-site treatment and disposal systems:

• establish sewage flows and loads on the basis of housingtype, number of occupants and facilities to be provides;

• establish site characteristics including topography,groundwater level, soil type and permeability, landavailability and access;

• identify constraints on the use of on-site treatment anddisposal systems e.g. groundwater level, absorptivecapacity of the soil, land requirements;

• test on-site disposal system requirements against siteconstraints;

• test on-site treatment system requirements against sitecharacteristics and for compatibility with selecteddisposal system;

• decide on appropriateness of on-site treatment anddisposal for the site under consideration and selectpreferred system or off-site disposal.

Operation and Maintenance

The most common problems with septic tanks are pipeworkblockages, leading to overflow of sewage, and loss ofbiological activity within the septic tank. These problemscan be avoided by simple preventative measures.

As septic tanks fill with settled solids the retention time ofthe tank reduces, efficiency will drop and solids will becarried over into the soakaway system. To avoid thisregular desludging is necessary on either a fixed time basisor when "full11.

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Septic tanks should not be washed out or disinfected afterthey have been desludged and the sludge, or septage, mustbe disposed of at an approved site. Soakaway performancecan deteriorate significantly if:

• deteriorating septic tank effluents leads to excessivesolids carryover;

• toxic substances kill or inhibit the growth of biologicalslimes;

• groundwater rises into the soakaway system.

Management

Control needs to be exercised by defining standards for:

• lot size requirements;

• sanitary, plumbing and drainage fittings;

• site investigation requirements;

• design parameters and construction requirements;

• operation and maintenance.

Code of Practice for On-site Treatment and DisposalSystems

A comprehensive Code of Practice covering on-sitetreatment and disposal systems is being prepared and thefindings and recommendations of this Study, together withthose from other sewerage master plans, will beincorporated into the Code of Practice. The Code ofPractice will be used as the basis for the control of septictank effluents through design and maintenance standards asset out in the Technical Memorandum on EffluentStandards issued under Section 21 of the Water PollutionControl (Amendment) Ordinance.

FUTURE SITUATION

TRENDS AND FORECASTS

Population

Forecasts of future population and levels of employmentform the basis of the estimates of sewage flows andpollution loads used in the Study.

Two forecasting techniques were used to assess populationsfor the year 2005.

Supply led or top-down forecasting; which assumes thatsupply of population and employment is unconstrained andthat development will follow very closely the availability ofhousing stock.

Demand led or bottom-up forecasting; which implies aconstraint on housing led development due to competitionfrom other areas for a finite supply of population andemployees.

The ultimate population capacity of the Study Area wasalso assessed.

Tin Shui Wai New Town

The most significant urban development planned for theperiod 1990 to 2005 is the Tin Shui Wai New Town withother development taking place in Yuen Long, Hung ShuiKiu, Ping Shan and Lau Fau Shan.

Recent Territory wide trends indicate a decline in thepopulations of rural areas and more recent projectionsprepared by TDD take account of this. Whilst this trend istrue on a Territory wide basis it is not true in all areas andevidence suggests that the Yuen Long area is one of theexceptions.

If rural growth is a significant feature in the future it isunlikely that Tin Shui Wai will also be fully developed by2005, however to ensure maximum flexibility in the MasterPlan the current projections for Tin Shui Wai have beenconsidered as an upper limit on population growth.

The final factor taken into account was "strategic growth"which assumed a further 100,000 people would be locatedin the Study Area.

For master planning purposes three scenarios were thereforeconsidered each reflecting different assumptions in respectof population growth distribution.

The first scenario, Urban Concentration, was based onsupply led forecasting and assumed that growth would onlytake place in the major urban centres of Yuen Long and TinShui Wai. Overall growth from 1990 to 2005 was 2.6%per annum to give a population of 400,000 in 2005.

As an alternative the second scenario, Rural Growth, wasbased on demand led forecasting and assumed that whilstoverall growth would be the same as for Scenario 1 thespatial distribution would be different with growth alsotaking place outside of the urban centres of Yuen Long andTin Shui Wai.

The third scenario, Rural Growth and Strategic Growth,combined strategic growth with rural growth assuming thatan additional 75,500 persons would be accommodated,primarily in the Tin Shui Wai area, by 2005. In all otherrespects this scenario was identical to Scenario 2. The totalpopulation estimated for this scenario was 475,500.

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1990 2005(1) 2005(2) 2005(3) Ultimate

Development Scenarios

Ngau Tarn Mei/San Tin CD Kam Tin CD The Corridor/Yuen Long

Population Projections

The assessment of the ultimate population capacity of theStudy Area was based on an assessment of maximumdevelopment potential, taking into account existing plansand constraints and included for a strategic growth elementof 100,000 persons within the major urban areas. The totalpopulation estimated for this scenario was 620,000.

Planning Department are currently carrying out the NorthWest New Territories Strategy Review and populationprojections have been produced. Comparison of theseprojections with those prepared for this Study showed theNWNT Strategy Review projections to be lower.

Employment

Within the NWNT there is a nett migration of workers toother parts of the Territory because employmentopportunities do not yet exist within the region. Over thelast ten years there has been a 7% per annum growth inemployment compared to a Territory wide figure of justover 2 % per annum, with a high proportion of this in themanufacturing and service sectors.

The long term objective of New Town development is abalanced community where the nett migration of employeesis zero. This is considered to be an optimistic goal by theStudy but has been considered as a long term possibility.

In the shorter term it has been assumed that:

• the present ratio of workforce to population (1:2) willbe maintained;

• agricultural sector employment will decline;

• commercial sector employment will increase with YuenLong remaining as the primary commercial centre;

• service sector employment will increase;

• employment in the manufacturing sector will continue toincrease.

Effluent Producing Industry

The recent growth of the manufacturing sector within theStudy Area has been primarily in the major effluentproducing industries, particularly the bleaching and dyeing,chemical products, and printed circuit board manufacturingsectors. There has been a marked decline in non effluentproducing industries and also in the food and leatherproducts sector.

Projections of effluent producing industrial growth werebased on the following assumptions:

• continued growth in bleaching and dyeing, papermanufacturing, chemical products, electroplating andprinted circuit board manufacture;

• a decline in food processing and leather products;

• effluent generating industries increasing their share ofthe employment market from 25% to approximately40%;

• infrastructure and land availability not constrainingindustrial development;

• no significant industrial development taking placeoutside the principal urban centres of Yuen Long, TinShui Wai and The Corridor.

Livestock

Livestock, primarily pigs and chickens, are locatedthroughout the more rural areas. Under the LivestockWaste Control Scheme programme there are three ControlAreas within the Study Area and a further five will becreated. Experience to date suggests that a high percentageof farmers are prepared to accept ex-gratia payments andstop raising livestock, the others will be given assistance toinstall the necessary waste treatment facilities.

Given these trends it was estimated that in the long termthe introduction of further Control Areas would result in a90% reduction in pollution from this source.

Livestock is located throughout the more rural areas

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SEWAGE FLOWS AND LOADS

Basis of Assessment

For the estimation of sewage flows and loads the factorsproposed in the Territory wide Sewage Strategy Study wereadopted. This approach allowed major daily populationmovements to be taken account of and was based on thefollowing flow categories:

• Residential; per capita flows and loads applied toresident population to obtain the residential componentof sewage. The per capita flows vary according tohousing type and are "averages" which represent typicalhousehold structure for each housing type. Account wastaken of the fact that a percentage of household membersare employed or are students and therefore spend asignificant period of the day outside the home. Theproportion of sewage generated at places of work orstudy was taken account of separately.

• Total employment and students; per capita flows andloads for this group generated at the place of work orstudy were accounted for separately. In this way theland use patterns of various catchments were accountedfor e.g. the concentration of employment in Yuen LongNew Town and the nearby industrial area.

• Commercial activities; a wide range of commercialactivities were accounted for including restaurants,bakeries, and laundries. The per capita value usedrepresents commercially generated sewage and notemployee contribution which was accounted forseparately.

• Industrial; flows were assessed on the basis of theRapid Assessment Method, which relates pollutionlevels to employee numbers by category of industry, andwere correlated with local flow monitoring data.

Future Flows and Loads

Sewage flows are expected to rise from the present 93,000mVday to some 375,000 mVday in 2005 and 490,000mVday ultimately; pollution loads will rise in a similarmanner.

Although population growth is expected to be just under4 % per annum sewage flows and loads are expected to riseby just under 10% per annum because:

• employment levels will rise significantly;

• there will be a relatively greater increase in the growthof effluent generating industries within themanufacturing sector as a whole;

• upgrading and rebuilding of traditional village housingand temporary squatter housing will increase domesticflows;

• natural growth in water consumption and per capitapollution loads will take place in the future and areestimated as 5% to 10%.

At present approximately 50 % of the sewage flow producedin the Study Area is from residential areas but the majorsource of pollution loadings is industry. By 2005 it isestimated that industry will account for approximately twothirds of all sewage flows and pollution loads. The urbanareas of Yuen Long, Tin Shui Wai and The Corridor willgenerate some 85% of the total sewage from all sourceswith Yuen Long itself accounting for at least half.

Residential

Others

Industry

1990Residential

Others

Industry

2005

Sources and Growth of Pollution

MASTER PLAN DEVELOPMENT

APPROACH

Background

Previous Studies have concluded that domestic andindustrial sewage should not be discharged to Deep Bay butshould be transported southwards away from the StudyArea. The North West New Territories Sewerage Schemeis being constructed with this objective in mind and thiswas taken as the starting point for developing master planproposals.

Areas to be Sewered

In developing master plan options the following approachwas used:

• identify where in the Study Area:- sewerage is appropriate- sewerage may be appropriate- sewage disposal is prohibited or strictly controlled- on-site treatment and disposal may be appropriate

• consider the options available to meet the requirementsof each area.

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The Study Area was divided into four zones on the basis ofthe above criteria:

• Sewerage Areas (Zone A); areas where sewerage isalready provided, is planned for or is required given thelevel and nature of development. Included within thiscategory are existing sewered areas such as Yuen Longand Fairview Park, areas for which sewerage will beprovided under the NWNT Sewerage Scheme, villagedevelopment/squatter areas and areas zoned fordevelopment which are inappropriate for on-site disposalsystems. Areas zoned for residential or industrial useunder the NWNT Development Strategy Review werealso included.

Sewerage proposals were prepared for these areas.

• Transition Areas (Zone B); areas where sewerage maybe required but is not planned. These areas eithercontain polluting activities which should be broughtunder control in the future e.g. livestock farming or areareas in which polluting industries might develop unlesscontrols are exercised. Examples are livestock areas incountryside conservation/coastal protection/recreationpriority areas, open storage areas which already generatesome sewage flows but might be turned into industrialareas in the future and areas identified for possiblePADS industrial use.

Sewerage proposals were not prepared for these areas.

• Prohibition Areas (Zone C); areas where the dischargeof sewage is either prohibited or required to meetparticularly high standards. Areas such as WaterGathering Grounds, Country Parks and Sites of SpecialScientific Interest are included in this category.

Sewerage proposals were not prepared for these areas.

• On-site System Areas (Zone D); all areas not includedin Zones A, B and C. Within these areas on-site sewagetreatment and disposal systems are deemed to beacceptable unless site conditions or the size of aproposed development dictates otherwise.

Sewerage proposals were not prepared for these areasbut the problems of on-site sewage treatment anddisposal were addressed.

Sewerage Drainage Areas

The areas to be sewered (Zone A areas) were split intothree Sewerage Drainage Areas (SDAs) for developingmaster plan proposals:

• The Corridor/Yuen Long which is the most intensivelydeveloped part of the Study Area. This SDA includesthe existing sewered areas of Yuen Long, the areas to besewered under the NWNT Sewerage Scheme, the areasouth of the Yuen Long Southern Bypass, the approvedNam Sang Wai development, Lau Fau Shan, Sha KongWai and Mong King Tsuen.

• Kam Tin Valley which includes Kam Tin town, ShekKong Military Camp and surrounding areas.

• Ngau Tarn Mei/San Tin.

CONSTRAINTS AND OPPORTUNITIES

In preparing master plan proposals account has been takenof a range of factors which could constrain master plandevelopment or provide opportunities for implementation.

Legislation, standards and guidelines exist whichcollectively define the minimum standards which masterplan proposals must meet. Of particular importance is theWater Pollution Control Ordinance and the water qualityobjectives (WQOs) for the Deep Bay Water Control Zonewhich were formally gazetted in November 1990.

Planning requirements are steadily increasing and willprovide a more certain picture regarding futuredevelopment. The NWNT Development Strategy Reviewscheduled for completion during 1992 should provide aclear guideline for future development. The two majorcomponents of development control, the Block CrownLease and the New Territories Exempted Housing are nothowever conducive to the orderly planning of sewerage.

Land is a major issue in relation to the provision of villagesewerage. Many of the village pathways are undercommunity or "tso" ownership which, if current policywere continued, would require resumption of the narrowpathways at considerable cost; effectively doubling the costof village sewerage. An alternative is the use of wayleaveswhich allow for right of access but leave the pathwayownership unchanged.

Transport and Drainage infrastructure within the StudyArea is being significantly expanded. Account was takenof these changes in terms of development impact and theopportunity for the provision of trunk sewer or pumpingmain alignments to make use of newly created routes.

The Rural Planning and Improvement Strategy (RPIS) willinclude improved access to villages, improved sewerage,water supply and waste collection and formation andservicing of land for village expansion. This strategy couldbe a significant factor in the implementation of master planproposals.

Village pathway

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KEY CRITERIA

Important criteria for reducing public health risk andpollution within the Study Area which need to be borne inmind when preparing and evaluating master plan optionsinclude:

• reduction of opportunities for direct or indirect contactwith polluted waters;

• provision of improved sewage disposal where groundconditions, development density or proximity towatercourses are unsuitable for septic tank systems;

• reversal cf the decline of dissolved oxygen levels inwatercourses and the trends of increasing toxic metalpollution;

• reversal of the potential for the accumulation of toxicsediments in major rivers and inner Deep Bay;

• reversal of the potential for bioaccumulation of toxins inestuarine and marine life.

The significant increases in sewage flows and load call forflexibility in the recommended sewerage master plan. Tocope with these uncertainties, the master plan proposalsmust include a set of firmly recommended packages forinitial implementation and a series of further packages forstaged implementation in the future as the pace ofdevelopment become clearer. Engineering solutions alsoneed to be inherently flexible e.g. dual transfer systems andprovision for future uprating.

Potential impacts of sewerage options during constructionand operation must be evaluated in order to ensure thatboth are minimised and the costs and benefits of eachscheme taken into account when comparing options.

Many schemes may be capable of providing satisfactorysolutions under normal operating conditions, but therelative chances of partial or complete system failure andthe magnitude of the resulting impacts are important inpreparing and evaluating of options. It is probable thatsignificant portions of the sewerage within each option willrely on pumping and the risk and consequences of failuresmust be taken into account.

Two strategies were identified

The objectives of the Study must be achieved at a moderatelong-term cost. Unnecessarily high long-term maintenanceburdens should be avoided, and the various options shouldbe compared on the basis of net present values to take fullaccount of both capital and operation and maintenancecosts.

OPTION SELECTION

BASIC STRATEGIES

Two basic strategies have been considered, the full exportstrategy and the partial export strategy. In each case thesewage arising in The Corridor/Yuen Long SDA isexported out of the Study Area via the NWNT SewerageScheme. For the full export strategy the sewage arising inthe Kam Tin and Ngau Tarn Mei/San Tin SDAs is alsoexported through new sewerage delivering to the NWNTSewerage Scheme system. For the partial export strategysewage arising in the Kam Tin and Ngau Tarn Mei/San TinSDAs is treated at local treatment plants, one within eachSDA.

EFFLUENT SYSTEM CAPACITY

The master plan options have been developed on the basisof some or all of the sewage being exported out of theStudy Area and disposed of in Urmston Road through theNWNT Sewerage Scheme effluent tunnel and outfall.

A comparison of the effluent tunnel capacity with predictedflows shows that up to the year 2005 there is adequatecapacity for all options. For the Ultimate Developmentscenario there may be a shortfall in capacity if flowsdevelop as forecast and peaking factors are greater thantwo.

The effluent outfall was designed for a dry weather flow ofjust over 70% of that of the effluent tunnel but allowancewas made in the design for outfall duplication if required.In terms of predicted flow the outfall has adequate capacityup to the year 2005 for Scenario 2 but would requireduplication to deal with flows from Scenario 3 andUltimate.

In practice it is probable that there will be sufficient longterm capacity in the effluent tunnel, and for the foreseeablefuture also the outfall, because:

• the most recent population projections by PlanningDepartment suggest that even by 2011 the Study Areapopulation will still only be some 85% of UltimateDevelopment;

• industrial flows which represent the most significantelement of total flow may not develop as rapidly aspredicted;

• the significant storage capacity within the seweragesystem, when fully developed, will lead to attenuation ofpeak flows which will result in a lower than predictedpeaking factor.

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Watson Hawksley

MASTER PLAN SELECTION Comparison of Options

Possible Solutions

Four potential solutions were evaluated in detail; thesewere:

• Option la; which collects sewage from all of the SDAsand transfers it to the San Wai sewage treatment plantfor preliminary treatment prior to discharge to UrmstonRoad. Under this option the Yuen Long sewagetreatment plant would not be used.

• Option Ib; which collects sewage from of all the SDAsand transfers it to either the San Wai sewage treatmentplant or the Yuen Long sewage treatment plant forpreliminary treatment prior to discharge to UrmstonRoad. The Yuen Long sewage treatment plant wouldreceive sewage from the Ngau Tarn Mei/San Tin SDAand a portion of The Corridor/ Yuen Long SDA.

• Option 2a; which collects sewage from TheCorridor/Yuen Long SDA and transfers it to either theSan Wai sewage treatment plant or Yuen Long sewagetreatment plant for preliminary treatment prior todischarge to Urmston Road. The catchments within TheCorridor/ Yuen Long SDA served by the two majortreatment plants would be the same as for Option Ib.The Ngau Tarn Mei/San Tin and Kam Tin SDAs wouldbe served by two local treatment plants providingtertiary treatment and discharging effluent to adjoiningwatercourses.

• Option 2b; is the same as Option 2a except that the twolocal treatment plants serving Ngau Tam Mei/San TinSDA and Kam Tin SDA are replaced by continued useof the Yuen Long sewage treatment plant. For thisoption the Yuen Long sewage treatment plant wouldrequire upgrading to provide tertiary treatment for theseflows. As for Option 2a, the Yuen Long sewagetreatment plant would also be used for preliminarytreatment of a proportion of the sewage generated in TheCorridor/Yuen Long SDA.

The essential difference between Options la and Ib is theretention of the Yuen Long sewage treatment plant inOption Ib to provide preliminary treatment to a proportionof the flow.

Option Ib is inherently more flexible than Option la interms of phasing and sewerage routing because of thegreater use of pumping stations and pumping mains.

In terms of capital cost Option Ib is almost HK$140million cheaper than Option la and in terms of overallcosts, capital and operating costs expressed as a Net PresentValue, it is also cheaper. Option Ib was therefore selectedas the preferred export option.

The most significant difference between Options 1 and 2 isthe level of sewage treatment. For Option 1 onlypreliminary treatment is required for all of the sewagegenerated in the Study Area, whereas for Option 2 "full"treatment, including a tertiary stage, is required for theKam Tin and Ngau Tam Mei/San Tin sewage. Althoughthis high level of treatment is only applied to less than 10%of the total Study Area flow the cost of this treatmentrepresents a significant element of the total cost of Option2. Provision of full treatment plants within each of theSDAs, Option 2a, means that the cost of this option issome HK$680 million more than Option Ib.

The capital cost of treatment for Option 2 can be reducedsignificantly if the Yuen Long sewage treatment plant isutilised to serve the Kam Tin and Ngau Tam Mei/San TinSDAs instead of constructing two new regional treatmentplants. The cost of upgrading the Yuen Long sewagetreatment plant to provide the necessary level of treatmentis estimated to be HK$86 million, a further HK$95 millionis needed for trunk sewerage. The capital cost of Option2b is estimated to be HK$617 million less than that ofOption 2a but still HK$63 million more than Option Ib.

Option la

To Urmston Road

Sai WaiSTP

Option Ib

Sai WaiSTP

Ngau Tam Mu SaB|

The Corridor/Yuen LongSDA

Kam TinSDA

Legend

Preliminary Treatment & Discharge to Urmston Road

Tertiary Treatment & Discharge to Watercourse

Option 2a

Option 2b

Ngau Tam Mei/San TinSDA

The Corridor/Yuen LongSDA

Kam TinSDA

Ngau Tam Mei/San TinSDA

Kam TinSDA

The Corridor/Yuen LongSDA

Summary of Master Plan Options

Page 11

Watson Hawksley

In terms of operating costs Option Ib is approximately 60%cheaper than either Option 2a or 2b because of the highcost of providing tertiary level treatment to the flows fromthe Kam Tin and Ngau Tarn Mei/San Tin SDAs.

In terms of overall costs Option Ib is approximately 60%cheaper than Option 2a and 30% cheaper than Option 2b.Option Ib is therefore preferred in terms of overall cost.

Extensive construction work is an ongoing feature of theStudy Area and the environmental impacts of theconstruction of the sewers, pumping stations etc. associatedwith the proposed master plan must be viewed against thisbackground.

In terms of overall impact the construction of Option Ib isseen as having more potential for environmental nuisance,because of the longer lengths of sewer/pumping main beingconstructed and the proximity of much of the works towatercourses and the intertidal zone, than the constructionof either of the Option 2 alternatives.

This is balanced however, to some extent, by reducedimpact from construction traffic in terms of noise and dust.Traffic disruption however will inevitably be moresignificant for Option 1.

If long term operation is considered the situation changeswith the local treatment plants causing more impact interms of noise, air quality and visual intrusion than thesewers and pumping stations required for Option Ib.

The long term impact associated with operation areconsidered to be of more concern than the transient impactsduring construction, particularly in view of the overalldisruption which already exists within the Study Area fromongoing projects to improve drainage and transportationand to develop the Tin Shui Wai New Town.

From an environmental standpoint Option Ib is thereforepreferred.

Recommended Option

In summary Master Plan Strategy 1 (Full Export) ispreferred to Strategy 2 (Partial Export) on environmentalgrounds and is also estimated to be more economic in termsof capital and operating costs; Master Plan Option Ib istherefore recommended.

THE RECOMMENDED MASTER PLAN

RECOMMENDATIONS

The basic master plan recommendations are that:

• Effluent disposal should be via the Urmston Roadoutfall currently under construction as part of theNWNT Sewerage Scheme;

• Preliminary treatment be given prior to discharge toUrmston Road and sufficient land be reserved to permitupgrading to primary treatment in the future;

• The NWNT Sewerage Scheme be extended to serve thewhole of the Study Area;

• Sewage treatment be carried out at the San Wai andYuen Long sewage treatment plants;

• Sewerage be provided to the majority of the unseweredvillage areas within the Study Area;

• Maximum use be made of existing facilities by ensuringthe earliest possible utilisation of sewerage constructedas part of the NWNT Sewerage Scheme.

PRINCIPAL COMPONENTS

The principal components of the recommended master planare:

• San Wai sewage treatment plant; currently underconstruction as part of the NWNT Sewerage Schemeretained as a preliminary treatment plant;

• Yuen Long sewage treatment plant; retained but downgraded to a preliminary treatment plant:

• The NWNT Sewerage Scheme; retained as presentlyproposed but extended to serve the whole Study Area asdetailed below;

• Trunk sewerage; comprising gravity sewerage, pumpingstations and pumping mains to transfer sewage from theKam Tin area to the San Wai sewage treatment plant viathe Ha Tsuen Pumping Station and from the SanTin/Ngau Tarn Mei area to the Yuen Long sewagetreatment plant;

• Village sewerage; to serve approximately 125,000people living in the existing unsewered areas;

• Interim and Priority Works; to enable early utilisationof existing trunk sewerage facilities, to serve industrialand village areas within the Corridor, and modificationsto existing watercourse interceptors to ensure theireffective utilisation.

ESTIMATED COSTS

The estimated costs of the various elements of therecommended master plan are summarised below.

Village Sewerage

Trunk Sewerage

Interim & Priority Works

YL STP Modification

i 100 200 300 400 500 600 700HK$ Million

Estimated Costs

Page 12

Watson Hawksley

TRUNK SEWERAGE

The trunk sewerage system will comprise four trunksewerage routes:

• Route A; to serve part of The Corridor, the southernand eastern fringe areas of Yuen Long and the wholeKam Tin SDA. The initial sections of this route fromHa Tsuen pumping station and those serving Tin ShuiWai and Hung Shui Kiu have been constructed;

• Route B; to serve Yuen Long New Town and the NgauTarn Mei/San Tin SDA. This route will have two mainbranches. The first branch will serve Yuen Long NewTown with transfer from the existing Ping Shun Streetpumping station to Ha Tsuen pumping station and fromthere to the San Wai STP. The second branch servingthe Yuen Long Industrial Estate, Long Ping Estate andthe Ngau Tarn Mei/San Tin SDA will transfer flows tothe Yuen Long STP;

• Route C; to serve Lau Fau Shan and village groups onthe western fringe of Tin Shui Wai;

• Route D; to serve the balance of The Corridor. Themost downstream sections of this route have alreadybeen constructed but it will be extended to serve TheCorridor South.

VILLAGE SEWERAGE

Key Elements

The successful implementation of sewerage to serve thevillages is an essential element of the Master Plan.Without it much of the considerable investment in trunksewerage will be wasted and the environmentalimprovement sought will not be achieved.

Many indigenous villages have, over the last ten years,undergone significant redevelopment and expansion. As aresult many villages have been transformed from orderlywell planned traditional villages into congested and built upurban environments whilst infrastructure provisions haveremained virtually unchanged.

The provision of village sewerage therefore requires aninnovative approach to a range of technical and institutionalproblems.

Key elements in any successful village sewerage policy willbe:

• requirements for sewerage to be provided asdevelopment proceeds;

• consideration of "way leaves" beneath village pathwaysto reduce the costs of land resumption within villages;

• adoption of a flexible approach to sewerage design andconstruction to take advantage of modern materials andto overcome the existing problems of restricted accessand pathways congested with existing services.

Village Sewerage

Proposed System

The sewerage system proposed for villages is a ModifiedConventional Gravity System (MCGS). The system is aconventional gravity sewerage system modified to suit theparticular requirements of existing villages and to ensurethat the costs of sewerage are kept to a minimum. The keyfeatures of the system are:

• the use of plastic pipes for small diameter gravity sewersand pumping mains;

• increased spacing between manholes;

• the use of "rodding eyes" rather than manholes atchanges of horizontal and vertical direction;

• the use of excavated soil for backfill, but with tightercompaction standards;

• reduced minimum cover requirements.

The MCGS has the following distinct advantages overconventional sewerage using concrete or clay ware pipes:

• sewers can be laid more easily in difficult locations;

• the superior flow characteristics of plastic pipes are morefully utilised;

• advantage can be taken of the large range of standardfittings;

• plastic pipes and fittings are light and easy to handle,trim to length, assemble, repair, and transport;

• plastic materials are not vulnerable to corrosion byseptic sewage.

The estimated cost of the MCGS system is some 50% lessthan conventional sewerage but, more importantly, it is apractical solution in an area where conventional sewerageis not feasible.

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Watson Hawksley

W A N

\/

s.^x

g mmj .$*11

\-

Legend

I/*

Existing Trunk Sewerage

Existing Effluent Tunnel

Stage 1

-X Stage 2

.U— Stage 3

Stage 4

Stage 5

Recommended Sewerage Master Plan; Proposed Phasing of Trunk Sewers and Connections

Page 14

Watson Hawksley

OPERATION AND MAINTENANCEREQUIREMENTS

The Master Plan proposes major extensions to the trunksewerage system but envisages, in the long term, areduction in the level of treatment at the Yuen Long sewagetreatment plant. The operation and maintenance of theproposed master plan components will increase existingcosts by some HK$22 million per year. Significantresources will be required for the successful operation ofthe proposed system and particular attention will need to bepaid to:

• The trunk sewerage system; which will contain 18pumping stations and some 60 kilometres of sewerageand pumping mains;

• Village sewerage; which will require a differentapproach to conventional sewerage and will involve theoperation and maintenance of numerous small pjumpingstations.

IMPLEMENTATION

PRIORITIES

To ensure that the maximum environmental benefits areachieved as soon as possible cost and environmental criteriawere used to set implementation priorities. It was foundthat the highest benefit to cost ratios were in TheCorridor/Yuen Long area and the lowest were in the NgauTarn Mei/San Tin Area.

As a result of this analysis the following implementationpriorities were set on an area basis:

• Stage 1 - Immediate Improvement Works; within TheCorridor which provide immediate pollution reductionbenefit and allow early utilisation of the NWNTSewerage Scheme trunk sewerage system;

• Stage 2 - Further Works within The Corridor; whichextend the trunk sewerage system to serve the rest ofThe Corridor together with associated village sewerage;

• Stage 3 - Works in Yuen Long and the Yuen LongFringe; to extend the trunk sewerage system to serve theYuen Long area. There are two elements of this; firstlyextensions to the southern fringe of Yuen Long to servethe Yuen Long valley and secondly diversion of theexisting Yuen Long New Town flows from the YuenLong sewage treatment plant to the Ha Tsuen pumpingstation and hence to San Wai;

• Stage 4 - Extensions to Kam Tin SDA; to serve theKam Tin town centre and the Shek Kong military base.Provision of part of sewerage to Lau Fau Shan is alsoincluded in this stage;

• Stage 5 - Extensions to Ngau Tarn Mei/San Tin SDAand Completion of the Master Plan; involves theprovision of sewerage to serve Ngau Tarn Mei/San WaiSDA and conversion of the Yuen Long sewage treatmentplant to a preliminary treatment plant and provision ofa transfer system to the head of the effluent tunnel atSan Wai.

Cumulative PRB/Capital Cost Ratio

Note :PRB isCapital

in terms of kg BOD/YearCost is in HK$

Master Plan Staging, Cumulative Benefit/Cost Ratio

Page 15

Watson Hawksley

PACKAGES

The Works to be carried out under each of the five stageshave been split into a number of packages.

Stage 1 Works are split into eleven separate packagescovering trunk sewerage (3 No.); rehabilitation of theexisting Yuen Long sewerage system; village sewerage inChung Uk Tsuen, Shek Po Tsuen, Hang Mei Tsuen,connection of Tan Kwai Tsuen residential area and theTong Yan San Tsuen and Kiu Tau Wai industrial areas,Tuen Mun Interceptor upgrading; improvement of low flowinterceptors at Hung Shui Kiu and the Tai River andmodifications to the Yuen Long Nullah interceptor.

Stage 2 Works comprise three further trunk seweragepackages and three village sewerage packages.

Stage 3 Works include two trunk sewerage packages,significant extension to the trunk sewerage network to thesouth of Yuen Long together with the new link from Yuen

Long to the Ha Tsuen pumping station, together with fourvillage sewerage packages.

Stage 4 Works comprise two trunk sewerage packageswhich extend the system to Kam Tin Village and Lau FauShan together with two further village sewerage schemes.

Stage 5 Works complete the Master Plan proposals byextending the trunk sewerage system to Ngau Tarn Mei/SanTin completing trunk sewerage in the Kam Tin andCorridor/Yuen Long areas (3 packages). Six further villagesewerage packages complete the Stage 5 Works.

PROGRAMME

It is recommended that implementation of the Stage 1Works be put in hand immediately and that Stages 2 to 5follow on as shown to give overall completion by the year2005.

A simplified version of the overall programme is givenbelow.

Package Year

Stage 1

Priority Works

Stage 2

The Corridor

Stage 3

Yuen Long

Stage 4

Kam Tin

Stage 5

Ngau Tarn MeiSan Tin/Kam Tin

1992

•1

1993 1994

«

1995

=-

1996

mm

1997 1998 1999 2000

•

2001

"

2002

"

2003

"

2004

-

2005

—Legend

•§••••• Design/Tender/Evaluation/Award••̂ ^^^" Construction Stage

Implementation Programme

Page 16

[HKP] 628.3 Y94 s

XOElE'nEb

NO

HKP 628. c

Date Di

- 9 NOV 199?

I Y94 s

je