Attacking Wanganui's waste treatment ponds with microbes appears to be working, but it will be a few more weeks before a truer picture emerges.
Last month the Wanganui District Council revealed the treatment plant was not functioning as it should and, as a result, the level of bacteria discharged into the sea off South Beach was higher than the Horizons Regional Council discharge consent allowed.
At the same time the district council announced it was going to start a process of "bio-augmentation" that would introduce bacteria to accelerate the breakdown of waste in the holding ponds. That process was begun two weeks ago and since then microbes have been regularly pumped into the ponds behind the city's airport.
WDC infrastructure manager Mark Hughes said dosing of the ponds started on October 30.
"At the moment there are five dosing 'stations' in the top bigger pond and three in the bottom pond," Mr Hughes said. "It seems to be working okay but it will be a few weeks before we're able to get meaningful results," he said.
The "good" bacteria are being produced in a sealed shipping container next to the main pond. A sign on the container names it as a "transportable microbial reactor facility".
An average of between 28,000 and 32,000cu. m of solid waste is pumped into the treatment ponds every day. The treatment plant, was commissioned in 2007 and marked the final stage of the city's $120million wastewater separation and treatment project.
But from the start it has been dogged with mainly mechanical problems. Soon after it started, some of the original aerators used to stir up sludge in the ponds failed. Replacement aerators were installed but some of them failed as well. Then a few weeks ago, 12 more aerators failed, leaving only seven of the total of 23 in working order.
The WDC had budgeted for ongoing maintenance of the treatment plant in its district plan and that included the bio-augmentation process, although that was not planned to happen until 2015. But increasing amounts of suspended solids in the ponds meant the final step of sterilising the treated faecal coliforms using ultraviolet lights was not working properly. That was when the WDC started breaching its discharge consents.
As well as discharge failings there had been a steady stream of complaints about odour from the ponds. Mr Hughes said the council was still getting the occasional complaint about smells and those were being addressed. "We're looking at treating the odour at the inlet end of the main pond and expect to have that process under way by November 19," he said.
The same company providing the bio-augmentation to the WDC will cultivate the specific microbes to tackle the odour problem. Mr Hughes said constant and regular tests were being carried out at the treatment plant and on Castlecliff Beach and South Beach to monitor the faecal coliform levels and constant checks were being made in the sea beyond the outfall pipe off South Beach.
He said that from an operational point of view, problems with the aerators may have been resolved with the use of the microbes. He said staying with the microbe dosing programme could mean only four or five aerators would be needed in future.
What will help make that possible is the third stage of the improvement programme at the plant and that involves separating the large pond into three smaller ponds. "It means putting three heavy rubber curtains in the top pond to create those three zones. It means we'll be better able to manage the treatment process," Mr Hughes said.
He said all the signs were that things were on the improve.
"The point we want to stress is that there is certainly no heightened health risks," he said.