Council to Tackle Long-standing Stormwater Issue in Waitara

Excess water in Waitara’s sewerage system is about to be tackled in a two-pronged approach by New Plymouth District Council.

By reducing the amount of stormwater in the sewerage system, the cost of sewage treatment will decrease – and could lead to a significant saving in the Waitara to New Plymouth sewer pipeline project.

Flows through the town’s sewer reticulation rise substantially after heavy rainfall. This points to stormwater getting into the sewer pipes through inflow (water coming from improper stormwater connections) and infiltration (from groundwater seeping into the pipes through cracks).

The most common inflow problem is from gully traps that aren’t high enough, says Manager Compliance Scotia Boelee.

“Gully traps are outside homes and are a safety valve for the sewerage system, to ensure any water backflow doesn’t come out through the shower drain or toilet,” she says.

“However when the overflow level of the gully dish isn’t at least 25mm above paved surfaces or 100mm above unpaved surfaces, surface water during rainfall can flow into the sewerage system.

“To fix this, our inspectors are contacting Waitara residents to advise that we’ll be checking their properties’ private drainage to ensure they meet the Building Act requirements.

“The less water that gets into the sewerage system, the lower the volume of sewage we need to treat – which is a cost saving.

“It also means that by the time we come to build the Waitara to New Plymouth sewerage line we can build it to handle a lower flow, which is another cost saving.”

The inspections will be conducted neighbourhood by neighbourhood, with residents advised by letter before an inspector visits.

To rectify the issue of infiltration by groundwater, the Council is checking the reticulation pipework to identify any pipes that need to be repaired or replaced.

“Waitara has quite a high water table which exacerbates the groundwater problem,” says Ms Boelee.

“It’s a similar situation to the one we faced in Inglewood. We focused on inflow and infiltration there as well, and now overflows from the sewer system to the Kurapete Stream have been substantially reduced in the past four years.