The biggest asset in the Selwyn District has an issue with the wrong kind of water.
Stormwater is flowing into the Pines Wastewater Treatment Plant in Rolleston, Selwyn District Council’s single largest asset, during heavy rain - a breach of the council’s own bylaws.
In a joint statement, Selwyn Water chief executive, Alex Cabrera, and council infrastructure and property director, Tim Mason, said there is always water that gets into the wastewater network through natural water flows during any flood event.
“It is impossible to assess how much of the additional water was natural ingress of water because it was a flood.
“We do not connect wastewater and stormwater systems, however, in the past connections were not always built to the same regulatory requirements as today.
“The council and Selwyn Water are working together to identify these in older areas of the network and fix them.”
The treatment plant on the outskirts of Rolleston receives wastewater from Darfield, Leeston, Lincoln, West Melton, Prebbleton, and Rolleston and is operated by Corde, a Selwyn’s District Council owned company.
At a recent council briefing, Corde’s water and parks general manager Vance Perrin said the flows through the plant are around 11,500m3 per day normally, but during downpours earlier this year the pipe network to the plant was running at 39,000m3.
He said 23,000m3 was received directly into the plant, which is consented for a maximum of 25,000m3 per day, and 16,000m3 had to be diverted into network storage.
Perrin said the network wasn’t maxed out but “we wouldn’t have wanted it to continue on for much longer”.
“It shows the impacts that weather events can have on this infrastructure.”
Perrin confirmed it was because “stormwater is entering the network”.
Wilshire's question was alluding to council policy, updated earlier this year, which bans the connection of stormwater to the wastewater system.
It was something he had raised in his personal submission on the Stormwater, Drainage and Watercourse Bylaw in April.
He said he was told that stormwater and wastewater networks are separate and stormwater should not be discharged to the wastewater network.
It was also noted in the staff response to his submission that during large rain events, surface water ingress into the wastewater network can occur through low gully traps – which is illegal.
Ashburton has suffered similar issues.
Neil Mcann, Ashburton District Council infrastructure manager, said the treatment facility at Wilkins Road experienced high flows during heavy rain earlier this year.
Staff suspected illegal discharges into gully traps and low-lying gully traps were part of the problem, along with infiltration in the pipe network.
Council investigated areas of Tinwald and discovered downpipes discharging into gully traps and low-lying traps, in both older and newer houses he said, as well as infiltration in the pipe network and manholes.
These issues will be further investigated when the council develops a wastewater strategy, which will prioritise future work based on cost and benefit.
