wastewater treatment there are four wastewater treatment plants in the wellington metropolitan area, one for each of the catchments. catchmenttreatment plant location wellington citymoa point poriruatitahi bay karoriwestern karori hutt valleyseaview when wastewater networks were first installed, they generally discharged untreated wastewater to the coastal marine environment. treatment of wastewater in the wellington metropolitan area began with the installation of fine screening at seaview and moa point in the 1980’s. a secondary treatment plant for the porirua catchment was constructed at titahi bay in the late 1980’s. secondary treatment was installed at moa point, seaview and the western plant between 1996 and 2002. together, the four treatment plants receive about 150 million litres of wastewater on a typical day, using biological and ultraviolet treatment processes. the treated water is discharged to the sea. the sludge resulting from filtration and treatment is de-watered (with this water further treated) and the solid content is sent to landfills. the four wastewater catchment areas for the metropolitan area are: there are also wastewater treatment plants at featherston, greytown, martinborough and lake ferry. wastewater pipeline age and condition the 2018/19 national performance review published by water new zealand compares the average wastewater pipeline age and condition across networks. the data for the wellington water network shows that 33% of the pipelines are in poor or very poor condition. the wellington wastewater network ranks the worst on this measure out of the six large networks nationwide. (the next worst is christchurch, with only 12% in poor or very poor condition). the review also includes average pipeline age. the wellington wastewater network has an average age of 53 years, and ranks as the second oldest of the six large networks. the interceptor as part of the early development of the wastewater network, a large pipe was constructed to take wastewater from manners st through mt victoria and out to an ocean outfall at moa point on the south coast. this main trunk wastewater line was known as "the interceptor". in the 1930s the interceptor was extended from manners st through to pipitea st, as the city's population topped 100,000. as suburbs spread further north to johnsonville, another extension of the interceptor was made from ngauranga gorge and through ngaio gorge to connect at thorndon. moa point treatment plant the moa point treatment plant handles all the wastewater from most of wellington city, other than karori and some northern suburbs. treated wastewater is discharged via a 1.8 km ocean outfall into cook strait. sludge is separated from the wastewater at the plant, and pumped at high pressure over a 9 kilometres (5.6 mi) route from moa point to the landfill at carey’s gully, where it is dewatered and placed into a landfill. in november 2021, wellington city council announced that it was considering installing a new technology thermal drying plant at moa point to treat sewage sludge. the benefits of this project would be to eliminate the current process of pumping sewage sludge from moa point to the landfill at carey’s gully, and reduce the amount of sludge going into the landfill by 80%. the estimated project cost is in the range $160m – $220m. the funding of the project is proposed to come from additional levies on residential and commercial properties. performance - wellington catchment in march 2020, wellington water reported the discovery of severe corrosion in the large main trunk sewer (the interceptor) over a length of 250 m in a section close to the treatment plant near moa point. work commenced on re-lining the affected section of the interceptor in april. ōwhiro bay sewage pollution ōwhiro bay is located on the wellington south coast. the shoreline is within the taputeranga marine reserve. there have been persistent problems with sewage pollution of the ōwhiro stream and beach for at least ten years, and the beach has been closed for swimming for long periods. in march 2020, wellington water announced that it had found and resolved incorrect connections of wastewater into stormwater systems at five locations in a suburb in the stormwater catchment area of the ōwhiro stream. wellington water said that the mistake occurred when the subdivision was built about eight years ago, and there were likely to be more cross-connections that it had yet to identify. pipeline collapse in dixon st on 20 december 2019, a wastewater pipe built in the 1930s collapsed beneath the intersection of willis and dixon streets in the central business district, leading to the diversion of untreated wastewater into wellington harbour at an initial rate of up to 100 litres a second. the broken pipe serves much of the central city, taking wastewater to the interceptor (or main trunk sewer), that leads to the moa point wastewater treatment plant. the public were warned not to swim in the inner harbour, but local iwi placed a rāhui on the entire wellington harbour. an emergency above-ground pipe was placed along upper willis street—closing it to all traffic—to bypass the failed underground pipe while permanent repairs were made. the swimming restriction on the whole of wellington harbour was lifted on 26 december. however, the area of the inner harbour from the whairepo lagoon entrance past the dive platform to the clyde quay wharf remained off-limits because of a “separate network issue”. willis street was eventually re-opened at the end of march 2020, after the permanent replacement had been installed beneath the road, and the temporary above-ground pipe removed. failure of sludge pipeline beneath mt albert in january 2020, there was a failure of pipelines carrying sludge pumped at high pressure from the moa point treatment plant to the landfill at carey’s gully. the pipelines are 9 km long and are about 25 years old. there are two in parallel allowing one to operate while maintenance can occur on the other pipe. in this incident both pipes failed at the same time. to avoid discharging the sludge into the ocean, a fleet of sewage trucks was mobilized to carry about one million litres of sludge a day to the dewatering plant at carey’s gully. up to 150 round trips each day were required, with trucks sometimes operating around the clock to keep up with the volume. some local residents were impacted by the increased heavy traffic and smells from the trucks. the break in the pipelines was located 200 m inside a sewage tunnel beneath mt albert, making repairs a significant challenge. the repair solution required the manufacture in germany of a custom-made polyester woven liner. this liner was winched from one end of each pipe to the other, and then expanded to essentially act as a new pipeline within the old one. specialists from the manufacturer flew to new zealand to assist in the installation, but were required to spend 14 days in isolation because of the border restrictions imposed in response to the covid-19 pandemic. in april, the wellington city council agreed to borrow $16 million to fund repairs to the sludge pipelines, together with the on-going costs of transporting sludge via truck from the moa point treatment facility to the landfill site while the repairs were carried out. on 24 may, wellington water announced that the first of the two sludge pipelines had been repaired and put into service, allowing the sludge trucking operations to end. porirua catchment wastewater from the northern suburbs of wellington and from porirua city as far north as pukerua bay is treated at a wastewater treatment plant (wwtp) located at rukutane point to the south-west of titahi bay beach. there is an ocean outfall adjacent to the plant. the wwtp was officially opened in september 1989, ending the continuous discharge of untreated wastewater that had occurred at rukutane point since the sewage network was constructed in 1951. the catchment area includes pipe networks owned by wellington city council in northern johnsonville, paparangi, northern newlands, woodridge, grenada, churton park and tawa. sewage mains from the boundary between wellington and porirua city council areas to the treatment plant are jointly owned by the two councils. performance - porirua catchment as part of a submission to a porirua city council committee in 2018 in relation to renewing resource consents, wellington water provided a background document about the issues in the porirua wastewater system. the issues reported included: poor existing freshwater quality in the porirua catchment frequent overflows from wastewater networks into freshwater and coastal water during periods of wet weather, and overflows from the wastewater treatment plant into coastal water inflow and infiltration from stormwater into the wastewater network ageing network prone to failures and with insufficient capacity to accommodate future growth insufficient capacity of the treatment plant to accommodate future growth wastewater network overflows during storms discharges from the wastewater network can occur from multiple overflow points in the network. there are 20 overflow points built into the network. discharge of untreated sewage from these overflow points occurs when the total flow exceeds the capacity of pipes and pumping stations, typically during periods of heavy rain or where there is large amounts of infiltration from groundwater. wastewater overflows also occur from manholes in some cases. these discharges cause pollution of waterways, harbour, and beaches, and create public health hazards. in february 2020, the mayor of porirua announced the establishment of a roving team to search for issues such as cross connections in stormwater and sewage pipes that contribute to overflows and pollution of waterways. in august 2021, the porirua city council approved a process to designate land adjacent to state highway 1 and the north island main trunk railway for the construction of a $42.9 million wastewater tank. the proposed facility will intercept high flows during storms and temporarily store wastewater until it can be pumped to the treatment plant. this will reduce the risk of overflows from pipes, pumping stations and the wastewater treatment plant. the storage tank is part of the long term strategy for reduction in wastewater overflows published in november 2019. the strategy includes additional but lower priority storage tanks at north plimmerton, paremata, whitby and north wellington. wastewater pipe failures in july 2021, a wastewater main on state highway 1 failed twice, and another failure occurred when a cross-harbour pipeline was put into service. these failures led to sewage pollution of porirua harbour. the initial failure occurred during heavy rain. wellington water said that the pipeline on state highway 1 was in a fragile condition and a long term fix would take months to complete. ngāti toa placed a two-week rāhui on te awarua o porirua. illegal discharge from porirua wwtp in october 2018, a series of errors and omissions in the management of the porirua wwtp led to a spill of approximately 5000 cubic metres of wastewater and solids from the outfall at rukutane point adjacent to titahi bay beach. in a subsequent hearing in the environment court in september 2019, wellington water was fined $67,500 for the illegal discharge. titahi bay beach pollution titahi bay viewed from whitireia park the maximum hydraulic capacity of the wwtp as at 2020 is 1,000 litres per second. however, peak flows into the plant during storm events in 2018 were recorded at 1,275 litres per second, leading to overflows where some wastewater bypasses some of the treatment stages. there were 12 such bypass events in 2020. each bypass event results in the discharge of some untreated wastewater at the outfall adjacent to titahi bay. planned upgrades to the wwtp would boost capacity to 1,500 litres per second. warning signs were posted at titahi bay beach in late february 2020, warning against swimming because water quality monitoring had revealed high levels of faecal coliforms. the water at the beach was deemed unsafe for recreational use, including for swimming, fishing or collecting seafood. a further beach closure notice was posted in mid-march 2020. wellington water stated that likely causes such as cross-connections from wastewater to stormwater pipes or damaged pipes provide intermittent flows, so contamination is not constant. as a consequence, some issues can lie undetected for years, while tracking faults can take weeks and sometimes has to be delayed as more urgent repairs take priority. in a statement acknowledging public concern, wellington water stated that the recent water quality warning for titahi bay beach was not linked in any way to the nearby wastewater