HASTINGS TRUNK SEWERS PIPE REHABILITATION

Saadia Ali and Gary Schofield MWH Global (now part of Stantec), Hastings

Abstract Hastings District Council (HDC) operates three large diameter trunk sewers between the urban area of Hastings and the wastewater treatment plant (WWTP) at East Clive, each is approximately 10km long. The flat topography and long residence time lends itself to the generation of hydrogen sulphide (H2S) which through biogenic action which is corrosive to the concrete pipes conveying the sewage. With increasing demands the trunk sewers were constructed between the 1930’s and early 1980’s and range in size from 900mm to 1800mm diameter. To optimise the WWTP requirements HDC operates domestic and industrial waste streams with a separated sewerage system through part of Hastings. This paper describes the investigation and monitoring of the damage caused by H2S to the concrete pipes, programming of works and rehabilitation works undertaken. Investigation included odour monitoring, CCTV, laser profiling and pipe samples. Trenchless rehabilitation works were chosen over open cut to reduce disruption to the public, road users, land owners and overall cost. Monitoring of the sewers allows programming and prioritising of works and planning HDC’s forward budgets. Keywords Hydrogen sulphide, concrete pipe corrosion, sewer rehabilitation, spiral wound, biogenic hydrogen sulphide corrosion

Introduction Hastings District Council (HDC) engaged MWH (now part of Stantec) to undertake the monitoring, investigation, design, contract and construction monitoring for the rehabilitation of the trunk sewers in Hastings. This paper describes the works undertaken to ascertain the condition of the trunk sewers, the options assessment and the rehabilitation works undertaken with the goal to avoid collapsed trunk sewers due to biogenic H2S corrosion and the reduced structural integrity of the pipelines. This work has allowed HDC to prioritise the rehabilitation of their trunk sewer network and manage their forward works and budgeting. History of the Hastings sewers The first sewer main was laid shortly after 1885 and conveyed sewage from what is now the Hastings CBD to the nearby Ngaruroro River. In 1938 a new gravity sewer was laid from the Hastings CBD to a short marine outfall at East Clive, part of this sewer is what is now known as the No. 1 Trunk Sewer. The 1938 sewer was designed to serve a population of 27,000 people plus factories within the Hastings District (Watties and Unilever) and factories outside of the Hastings urban area (Tomoana Freezing Works, Whakatu Freezing Works, and W Tucker Woolscours). In 1958 the main sewer was duplicated, creating the No. 2 Trunk Sewer, along with a new short marine outfall. Havelock North Borough and Whakatu (Hawke’s Bay County) connected to the system in 1964 and Clive (Hawke’s Bay County) connected in 1967. In 1974 the third inland sewer, No. 3 Trunk Sewer, was laid along with the 2,750m long offshore ocean outfall.

Figure 1: Construction of the long ocean outfall in the 1980's Following construction of the long ocean outfall wastewater was split into two different wastewater streams. The “clean” wastewater which comprised mainly of fruit and vegetable processing wastewater was discharged through a new short outfall. The “coloured” wastewater which comprised the human sewage (domestic sewage) plus wastes from freezing works and woolscours was discharged through the long ocean outfall. The first Trade Waste Bylaw was instigated at the same time. The “coloured” wastewater was treated by communition (chopped up through ¼ inch slots) prior to discharge. All industrial discharges were also treated on-site at industrial premises to comply with the Trade Waste Bylaw. From 1992 to 2009 the wastewater network underwent various changes which included decommissioning of the short ocean outfall, combining the two waste streams and then later separation of them again and construction of the Hastings Waste Water Treatment Plant at East Clive. The two waste streams and three trunk sewers as they have been operating since 2009 are as follows: Domestic and Non-separable Industry

Stream (DNSI). The wastewater is treated at the WWTP by screening through a 3mm diameter hole screening unit and then treated through the two biological tricking filters (BTF’s) and passed through the rock passage before joining the industrial wastewater stream.

Industrial Wastewater Stream – this is pre-treated on-site at individual industrial

premises and then conveyed to the WWTP and screened through a 1mm slotted screen at the WWTP prior to mixing with the treated DNSI wastewater stream. The single stream is then pumped to the outfall.

No. 1 Trunk Sewer

o Constructed in 1938s o Varying diameter up to 1050mm ø

RC pipe o Grade 1:2200 o Conveys industrial waste

predominantly from food processing industries such as Watties, McCains and Silver Fern Farms

No.2 Trunk Sewer o Constructed in 1958 o Varying diameter up to 1200mm ø

RC pipe o Grade 1:2700 – 1:1850 o Operates as a spare providing

redundancy in the system and conveys industrial waste in peak summer processing season

No.3 Trunk sewer o Constructed in 1974 o Varying diameter up to 1800mm ø

RC pipe o Grade 1:2600 o Conveys domestic sewage

Condition Assessment Recent anecdotal evidence had suggested that the trunk sewers may be deteriorating due to H2S corrosion, it was also widely accepted that there are high concentrations of organics in the waste streams. Given the long residence time of wastewater in the pipeline it is not unexpected that the waste is generating H2S gas. This is more problematic in the No. 1 and No. 2 trunk sewers (industrial) as they receive a higher organic loading from the industrial wastes with an associated increase in sulphide production. Condition assessment by CCTV inspection was undertaken to identify areas of high corrosion, areas to further investigate and prioritise rehabilitation. Initial assessment was undertaken in accordance with the New Zealand Pipe Inspection Manual however this does not make enough distinction between ‘surface

damage’ to the inside walls of concrete pipes, particularly over long lengths. Under the New Zealand Pipe Inspection Manual, in most cases the pipe would be considered ‘in failure’. MWH developed an alternative visual scoring method to quantify the level of damage by pipe section based on visual inspection. This scoring ranged from no obvious damage being the best score to exposed reinforcement being the worst score, exposed aggregate and reinforcement staining were ranked in between. Once the reinforcement is exposed the structural integrity of the pipeline is considered minimal. CCTV inspections were undertaken along the lengths of sewer in 2009, 2013 and 2015. The CCTV inspection from 2009 was initially used to assess the relative condition between the three sewers and assisted HDC to make a decision whether to maintain and operate two or three of the trunk sewers. Given that the No. 2 Trunk Sewer was in significantly better condition than the No. 1 and No. 3. It was decided to operate and retain all three trunk sewers. The more recent inspections used high definition cameras with laser profiling and synchronised sonar. The 2009 CCTV was reassessed using the alternative scoring method to provide consistency in scoring and comparison between pipe lengths for prioritising of rehabilitation works.

Figure 2: Boat unit: with HD CCTV, laser profiling and sonar, battery, data storage and wifi communication

Score 1 – No significant pipe wall deterioration visible

Score 2 – Pipe material eroded and aggregate exposed

Score 3 – Rebar staining visible, but, rebar not exposed and/or severe aggregate exposed

Score 4 – Rebar just visible, generally less than 25 % diameter

Score 5 – Rebar significantly exposed, generally between 25-50 % diameter

Table 1: Visual scoring reference

Laser profiling provides data above the water level and sonar below the water level. Although data was captured from the laser and sonar along the full length of the pipeline under investigation, due to the relatively high cost of analysing the data only 20 to 30% of the pipe length was analysed using this method. These were areas considered high risk based on the earlier visual scoring. The laser/sonar profiling allowed the analysis to be quantitative.

Figure 3: Typical analysis from the laser profiling The typical analysis from the laser profiling as shown above shows outside pipe wall, current pipe wall and original internal pipe wall.

Figure 4: Flat graph analysis of the sewer from the laser profiling The loss of pipe wall as identified from the laser profiling was then compared against physical pipe samples from around the pipe circumference at selected locations. These pipe samples were taken at the soffit, 12 o’clock, 3 o’clock and 4-5 o’clock depending on the flow level. This allowed verification of the laser profiling/flat graphs with regards to loss of pipe wall and also the level of steel reinforcement within the pipe wall.

Figure 5: Cut-out for chimney manhole from the No. 1 Trunk Sewer

Figure 6: Cut-out for chimney manhole from the No. 3 Trunk Sewer The calculation of the loss of pipe wall for the pipe samples and flat graphs was highly dependent on the original pipe diameter used. Significant work was undertaken to provide some level of assurance that the correct original pipe diameter was used in the calculations. This included historical investigations with the pipe supplier, review of as built drawings and concrete pipe standards at the time of construction. Pipes closest to the WWTP tended to be in the worst condition, with the No. 1 trunk sewer being the worst of the three sewers. From the condition scoring the remaining life of each pipe was assessed and entered into HDC’s asset management system. Pipes with similar renewal dates were clustered together to create viable packages of work up to 30 years into the future. This created a profile of HDC’s future trunk sewer renewal budgets.

A programme of works effectively dropped out, that is the lengths of pipe in the worst condition were the highest priority. The programme was adjusted to allow for other risks such as trafficked sections of trunk sewer pipe under roads and intersections. Sections under roads were considered higher risk from impact loadings, than pipes within the berm. The full length of the DN1050 (No. trunk Sewer) under SH2 was lined in 2014. Options Assessment and Procurement Options Assessment The five options that were considered for renewal / rehabilitation of the trunk sewers are as follows; Open Cut Pipe Renewal Spot Repairs CIPP (Cured in place pipe) Spray coatings Spiral PVC linings An open cut pipe renewal would require a new pipe laid to flat grade in areas of high ground water adjacent to or within high speed trafficked areas and within a corridor with three existing trunk sewers. This was not a desirable option given time, sewer capacity, spatial and financial constraints. Spot repairs which are generally considered acceptable on sewer mains were deemed unpractical due to the extent of H2S corrosion in the trunk sewer network. CIPP (cured in place pipe) is a felt sock pressed against the internal pipe wall and impregnated with resin and hardened with hot water or UV light within the host pipe. This was considered an appropriate solution however there were reservations due to the often large amounts of hot water required to cure the resin and that it would require complete isolation of the sewer network during curing. There are various spray coatings available all of which have their benefits and drawbacks. Spray coatings such as magnesium oxide and calcium aluminate composition were considered for their corrosion resistance properties. While these coatings were deemed unlikely to meet the performance specification and unpractical on the pipeline itself due to

physical constraints and application method, they are suitable for structures within the network such as manholes and chambers. Spiral PVC linings which is the installation, via manual or mechanical means, of a plastic liner against the internal pipe wall of the existing pipe were considered appropriate especially where installation was considered and that the spiral linings can be installed with low sewer flows. Recent sewer rehabilitation contracts suggested that the nature of the trunk sewer lended itself to spiral wound lining rehabilitation, while CIPP is a known and trusted rehabilitation method curing usually requires large amounts of hot water, which made this method less viable for the length and diameter of the trunk sewers. The CIPP method also requires complete isolation of the sewer which while there is redundancy in the network HDC did not wish to guarantee. The large diameters of the sewers means that most of the manholes are ‘chimney type’ manholes. Manholes within the lined sections were coated with a calcium (CCA) product sewper coat. New chimney manholes were installed to suit the access requirements of the contractor. Procurement Previous sewer rehabilitation works suggested that the nature of the trunk sewers and extent of rehabilitation favoured spiral wound lining however this was not a restriction put in the tender document. The most recent (2016) sewer rehabilitation tender document did not specify the rehabilitation method to be used, i.e. CIPP or spiral wound linings, instead performance based requirements were specified that a successful tenderer would have to meet. The most significant requirement was to provide a structural pipe independent of the host pipe. The pipe was to have a minimum design life of 50 years. While there were limitations on the reduction in capacity, the capacity of the sewer was not critical, so the lining method was less restrictive. The redundancy in the trunk sewer system meant that HDC could offer contractors largely isolated sewers lengths,

with only small flows, giving some flexibility around methodology. In the most recent rehabilitation contract in 2016, three tenders were received with two rehabilitation methods CIPP (cured with hot water) and spiral wound lining. The tender evaluation team evaluated the proposed methods against the required performance requirements. The rehabilitation method’s performance had to be supported by design calculations and producer statements. Spiral wound lining was chosen for this rehabilitation as it was likely to meet the performance requirements (being a relatively proven technology) while providing cost savings compared to other methods. Contract Works Contract Works 120m DN1800 under road intersections 360m DN1050 in road berm 42m DN1800 within the WWTP Installation of chimney manholes to suit

the Contractor’s required access Sewper coat of internal manhole

surfaces Cleaning and debris removal Methodology Debris removal and host pipe preparation Installation of PVC liner Grouting of annulus Sewper coating of manholes Debris removal required the Contractor to undertake high pressure jetting and in some places use chain flails to prepare the pipe for lining. Installation of the PVC liner was via manual winding of the liner into the trunk mains. The PVC liner was then hammered into place with rubber mallets.

Figure 7: PVC lining coils

Figure 8: Feeding the PVC liner into a manhole Grouting of the annulus between the existing pipe and new liner was completed in stages with high strength grout. Grouting ports were drilled into the lining along its length and around its diameter to allow injection of grout and monitoring during the grout filling. The grouting was completed in three stages to minimise pressure pushing the liner away from the host pipe wall. The liner was sealed at the ends to hold the grout behind the liner.

Figure 9: Lining in place, pre-grouting Calcium aluminate composition ‘Sewper coat’ was applied to all new and existing manholes along the length of the rehabilitated pipe. Challenges Sediment debris: given that the internal walls of the sewer are corroding, the loose aggregate finds its way to the invert of the sewer. As well as this the waste discharged into the industrial sewer is often high in sediment and the flat grades mean that it partially accumulates in the pipe invert. A measure and value rate was included in the schedule of prices for sediment removal. The depth of sediment was measured and agreed with the Contractor prior to the lining works. This minimised the risk to the Principal while allowing time to fund any significant related variations should they eventuate.

Figure 10: Sewer condition at the limit of cleaning No. 1 Trunk Sewer

Figure 11: Debris removed from the pipe invert at the WWTP Ground water infiltration: the Contractor encountered groundwater infiltration into the bottom of pipe, particularly at the WWTP. The Contractor had previous experience with this and had a method to manage and ultimately seal the infiltration as part of the lining works. Rainfall Events: one high rainfall event meant the sewer isolation had to be lifted mid-contract works due to the level of stormwater infiltration into the sewer network. Fortunately there was sufficient notice of the potentially high rainfall event and the Contractor was prepared and available to lift the sewer valve isolation. Sewer Alignment: a small unidentified bend in the sewer at a road intersection meant that a continuous reline through this section could not be achieved and extra joining pieces were required. This also changed the position of a manhole to be installed which was not realised until excavation had already started. Grout volumes: the grout volumes were greater than expected by the Contractor however this was a Contractor risk as the investigation data had been provided during tender showing the condition of pipe and extent of corrosion, based on the results from the CCTV and laser profiling. Health and safety The methodology undertaken to line the sewer provided both benefits and drawbacks from a health and safety perspective. Drawbacks: confined space entry into live sewer pipelines with high levels of H2S gas. The Contractor had a robust health and safety

plan which implemented a lock out procedure requiring individual tags on lock boxes at the sewer isolation valves during man entry and confined space procedures in accordance with best practice guidelines. Benefits: the ability to line the sewer reduced the hazards at ground level particularly those around traffic management in high speed areas and large excavations when compared to an open cut methodology. Future works Investigate the causes and mitigations of

biogenic H2S corrosion - “The Odour Model”. Results from the H2S odour monitoring and modelling are not part of this paper, but have described HDC’s trunk sewer network as a ‘perfect storm’ with regard to H2S. H2S measurements at the WWTP have peaked above 1000ppm, measurements at manholes can often be in excess of 100 ppm.

Re-assess and re-prioritise renewal plan based on–going pipe samples/condition assessments.

Investigate sources of sediment in the industrial sewer

Estimate rate of corrosion of concrete pipes

It is proposed that the future works be undertaken in three year cycles, one year of investigation followed by two years of rehabilitation. Conclusions Although there is still significant works to be undertaken this work has allowed HDC to understand the condition of the trunk sewer network, prioritise rehabilitation, and plan future expenditure and budgets in accordance with their long term planning. This process will not only minimise the costs of future works through appropriate planning and timing of rehabilitation intervention it also allows the investigation and implementation of emerging technologies and methodologies that cannot always be utilised for ad hoc reactive works. Monitoring will continue in the future to verify and adjust the remaining life of the trunk sewer network and planned rehabilitation expenditure.

Acknowledegments - Hastings District Council - Drain Surgeons Ltd. - Monadelphous Engineering NZ Pty

Ltd.

Authors Biography Gary Schofield Senior Water and Waste Engineer gary.schofield@stantec.com Gary is a ‘3 waters’ (sewer, water and stormwater) engineer with 16 years’ experience in both UK and New Zealand. He has seen numerous projects through from concept, investigation, design and procurement to completion. He spent 7 years with MWH UK in roles that have included planning and design of wastewater treatment plant upgrades, improving combined sewer over flows and long term asset optimisation as part of a seconded role with Scottish Water and Southern Water. Gary’s current role as Senior Water & Waste Engineer with MWH, is supporting Hastings District Council with condition investigations, asset renewals, asset management planning, design and contract administration of pump station and pipeline projects. Saadia Ali Civil Engineer saadia.ali@stantec.com Saadia is a graduate engineer working within the 3 waters sector. She has been with MWH for the past year and previously worked for the Hastings District Council in the Asset Management department for two years. Her role includes investigation, design and construction contracts procurement and administration for Hastings District Council.