Sewer pipe ‘overwhelmed’

Half a million litres of sewage may not have entered a Napier estuary if an outfall pipe from a new $21 million wastewater plant was working to capacity.

Napier City Council decided to discharge the wastewater into the Ahuriri inlet after heavy rains from former Cyclone Debbie overwhelmed the system.

The council said an Olympic swimming pool’s worth of wastewater (about 2.5 million litres) was discharged on the afternoon of April 5. Twenty per cent (about 500,000 litres, or 20 milk tankers) was sewage.

It had been necessary to discharge ‘‘a small amount of wastewater’’ into the estuary in order to keep it from flowing into city’s streets, the council added.

Hawke’s Bay Regional Council investigated but chose not to prosecute the city council.

Information obtained under the Local Government Official Information and Meetings Act reveals that the spill would not have occurred if the capacity of the ocean outfall at the city’s wastewater plant had been operating as designed.

The outfall pipe, built about 50 years ago, flows into the ocean from a $21.5m biological trickling filter wastewater plant that began operating n 2014.

The released documents show that the outfall was designed with a capacity of 1500 litres per second but only operates at 1200 litres per second. The volume passing through Napier’s wastewater pump stations is limited by this constraint.

The regional council was concerned because the wastewater system should have been been

"It is important that we ... don't simply shift a problem from one area to another." Council manager Chris Dolley

able to cope with the weather event, which it said ‘‘was not of such significance to occur on only the very rarest of occasions’’.

Asked for an explanation by the regional council, the city council’s asset strategy manager, Chris Dolley, replied by email that ‘‘the size of the installed equipment was insufficient to transfer the quantity of wastewater generated by the event out to sea through the Ocean Outfall’’.

The wet-weather flow was five times the dry-weather flow, and had consisted of household wastewater, groundwater that had infiltrated the wastewater system, and stormwater illegally connected to the wastewater flow, Dolley said.

The city council was developing various ways of fixing its wastewater system.

This included a review of the decision to ‘‘derate’’ the outflow capacity and to look at building on-site storage ponds at the wastewater plant which could hold the overflow and treat it and dispose of it through the outflow after a flood event.

Yesterday, Dolley said it would take several years to get the wastewater system to a standard that could cope with an event such as April’s.

The first step would be to complete a detailed model of the whole network, because ‘‘it is important that we understand the whole network and don’t simply shift a problem from one area to another’’.

A tender for this would would be awarded soon, he said.

Dolley said it wasn’t clear when or why the outfall began operating at 1200 l/s and not 1500 l/s and this would be investigated.

Any on-site storage would be determined by the level of protection required and physical constraints of the site, he said.

The council’s Long Term Plan includes $3.8million of work on sewer capacity over the next three years, though none of it involves the outfall.

Between the years 2026 and 2029, the council has budgeted $11.4m to replace or upgrade the outfall.

A further $3.8m will be spent on reducing inflow and infiltration into the wastewater pipes over 30 years.