Wellington Water is advising residents that there may be more frequent discharges of partially treated, diluted wastewater into Cook Strait from the Moa Point Wastewater Treatment Plant this winter due to one of the plant’s three clarifiers needing repair.
A bearing in the drive assembly for this clarifier has failed and Wellington Water has ordered a replacement assembly (and two spares) from the manufacturer in the United States.
In the wastewater treatment process at Moa Point, liquid is first screened to remove solids and then goes through settling tanks, then biological treatment with a clarifier separating bacteria from the liquid. The water is then treated with ultraviolet light to reduce remaining bacteria and viruses to safe levels before the fully treated water is sent out to sea.
The Moa Point plant is designed to fully treat a peak flow of 3,000 litres of wastewater per second (L/s) when all three of its clarifiers are in operation. The required repairs to the clarifier will reduce the plant’s full treatment capacity to 2,200 L/s.
Any flows over2,200 L/s will still be screened to remove solids and grit. This partially treated wastewater will then be significantly diluted by mixing it with the fully treated wastewater before flowing 1.8km out to sea through the long ocean outfall.
During the summer months we didn’t exceed the capacity of the plant, even with one clarifier out of action. However, as we head into winter the number of discharges is likely to increase as we see more incidents of heavy rainfall.
“It is important to be clear that no untreated wastewater will be discharged when the plant is operating at reduced capacity,” says Wellington Water’s Group Manager, Network Management Jeremy McKibbin.
Wellington City Council’s consent obligations allow partially treated and diluted wastewater to be discharged during heavy rain when the plant reaches its capacity (of 3,000 L/s) to deal with incoming wastewater flows.
“We can’t be certain when the clarifier will be returned to service. We ordered the replacement in late April after considering repair or replace options. The manufacturer in the United States has advised they expect the new drive assembly will be ready to ship in mid-September with an expected arrival date of mid-October. Those dates are subject to any supply chain delays. Once it arrives it will take a month to install, so our current planning is for the clarifier to be returned to service in mid-November,” says Jeremy.
The new drive assembly and two spares will cost $560,000.
Wellington Water is working with mana whenua, local stakeholders, and recreational water users to ensure everyone has a clear understanding of the issue and the time expected to fix it. Any discharges during the repair period will be notified on the Wellington Water website and signage will be erected in the area warning of the discharge.
As always, Wellington Water recommends recreational water users refer to advice provided by Land, Air and Water Aotearoa (LAWA), which suggests people stay out of coastal waters for two to three days after rain, regardless of the time of year.