Sewage could be dumped in ocean - Colin Hovey

The Wanganui wastewater treatment plant. Photo / File / Stuart Munro

A retired engineer who helped design the Wanganui wastewater treatment plant says the solution is to put the fresh excess waste straight out to sea while the problem is being sorted.

Colin Hovey said the problem was not going to go away while the council continued to allow the excess loadings to come into the plant.

"It's not toxic material, and we did it for 20 years.

"The council can apply for an emergency consent from Horizons, and there won't be any lasting environmental effects for the short time."

Since December 8 the wastewater plant has been overloaded with material as protein waste and hydrogen sulphide have been dumped into the system. It has left a stench hanging over parts of the city.

The tragedy of the plant breakdown was the mechanical failure of the aerators, Mr Hovey said.

The original 17 aspirator-type aerators worked to oxygenate the loadings in the top four metres of the ponds where the bacteria broke down the matter.

The matter then becomes inert and drops to the bottom of the ponds where more anaerobic bacteria continues its work.

Mr Hovey said the treatment ponds were designed for extra capacity and there was more than enough for the different loads coming in at the time.

The issue at the ponds - which is causing the stink - is that it takes three days for the plant bacteria to crank up to deal with any surge in the loads.

But Mr Hovey said he did not know if any material from the bottom had come to the top. He called the material that settled on the bottom of the ponds "bug bones''.

"The bacteria can't react rapidly.'' A reading of the oxygen was monitored at the treatment plant and extra aeration can be provided if the reading suggests it is necessary.

Mr Hovey was involved in the design of the plant with principal designer Dave Stewart. He said Mr Stewart was one of most experienced New Zealand scientists who designed treatment plants.

"There are oxidation ponds outside cities that have not been cleared for up to 50 years."

Furthermore, trade waste was food for bacteria, and Mr Hovey did not see any issue with the sulphides, because the bugs adapted to the material they worked on.

Before he retired, Mr Hovey said, the council monitored the different industries over 40-50 days a year in 4/5 time periods.