Tahuna
Wastewater from the greater Dunedin area is treated at Tahuna. The Tahuna plant has been upgraded and a new 1.1km outfall off the St Kilda coast has replaced the former outfall at Lawyers Head. The plant treats around 30 million litres of wastewater a day from around 80,000 residents and industry.
Screened wastewater is pumped from the Musselburgh station to Tahuna, where it is screened again. From this point, waste follows two paths – one for liquids (wastewater) and one for solids (sludge).
The wastewater passes over two filters containing millions of plastic discs. These encourage biological growth, which removes more pollutants. From here, the wastewater is disinfected with ultra-violet light, then discharged through the outfall.
Solids collect at the bottom of the settling tanks before being thickened and dehydrated. The dehydrated sludge is incinerated at around 825˚C.
The incineration gases are washed to remove ash and for cooling. The gases are filtered through bark to remove odours before being discharged.
Green Island
The Green Island plant treats wastewater from Brighton through to Burnside, parts of Kaikorai Valley and Wingatui.
The screened wastewater is put in aeration tanks, where bacteria consume organic material. The wastewater enters a clarifier tank, then is UV disinfected and discharged to sea at Waldronville.
Solids are collected in a sludge at the bottom of the settling tanks before being transferred to a thickening tank. The thickened sludge is transferred to a digester that produces methane, which generates electricity to help run the plant. The sludge is then dehydrated and disposed of at the Green Island landfill.
Mosgiel
The Mosgiel plant at the end of Carlyle Road services approximately 10,000 people. Wastewater is screened before being clarified and filtered. The treated effluent is pumped to the Green Island plant, where it is UV treated before being discharged. The sludge is digested to produce methane, which is used as an energy source at the plant. The sludge is then dehydrated and disposed of at the Green Island landfill.
Warrington
Warrington’s wastewater is pumped to an oxidation pond in the Warrington reserve. It is treated and then sprayed onto a designated part of the sand spit reserve.
Seacliff
Seacliff has a small gravity-fed sewage scheme at the end of Kilgour Road, which services 29 properties. The sewage settles in two tanks before being transferred to a sand bed filter. After filtering, the treated wastewater is irrigated over land.
Waikouaiti/Karitane
Wastewater from Waikouaiti and Karitane is piped through a series of pump stations to the Waikouaiti oxidation ponds. The wastewater is screened and discharged into the Waikouaiti pond, which then flows into the Karitane pond. The land disposal site is a pine plantation on a sandy foreshore.
Middlemarch
The Middlemarch oxidation ponds are about 800 metres east of the township along Garthmyl Road. Treated effluent passes through a wetland before being discharged into the bed of a dry oxbow of the Taieri River, downstream of the confluence with March Creek.