Council changing tack on sewage treatment upgrade

May 13th, 2014
[by David Armstrong]

The use of laser sensing from an aeroplane has provided Council with information meaning it must change its strategy on upgrading the Motueka wastewater plant.

Councillor Barry Dowler told Grey Power Motueka's AGM yesterday that the plan proposed last year for ground infiltration of waste water would not work adequately, and instead land discharge systems will be developed.

The topographic information was obtained by Lidar scanning, in which equipment in an aeroplane shone laser light onto the land and analysed the reflected light. This showed with great detail and accuracy the humps and low points which are hard to spot by eye on the ground.

While the new information showed that the effects of a river stopbank breach would not be as severe as believed, it also demonstrated that the Rapid Infiltration Basins (RIBs) that TDC had proposed last year would not be effective, and in fact would make things worse in the case of heavy rainfall.

Council's working party last year put forward its preferred option to discharge to land through RIBs, and a trial of a small RIB was under way (see our earlier story here).

The RIB takes the wastewater that has initially been treated at the inlet stage and allows the remaining solids to settle for a few days, then the water is flushed out to either another RIB (if there is a series of them) or straight to groundwater.

Barry said a workable solution for Motueka's over-stressed sewage treatment plant was a high priority for Council because its resource consent expires in 2017 and it must be shut down if no answer is found by then.

He estimated a cost of $35 million to shift the facility to higher land to the west of Motueka, which would not be affordable at present. Instead a solution that will do for another 30 years would need to be found and resource consent sought for that.

Council's data predicts that sea level rise will not be a problem for the sewage plant until about 50 years from now. A resource consent for the present site for another 30 years will buy time to come up with a long-term solution.

"Tender documents are out now for de-sludging the ponds, which is very much overdue already," Barry told the Grey Power audience of about 20 people. The sludge will be put into large bags and left to dry for a few years before final disposal.

This will take about three months, during which time Council engineers will come up with measures that will buy those extra 30 years.

Barry said some improvements likely to happen include getting more oxygen into the waste by stirring it mechanically, constructing as floating wetland so roots can extract nutrients from the liquid beneath, using baffles to regulate flows, and maybe a UV plant in the final pipe to the estuary outflow.

The main problem with the existing wastewater treatment system is its inability to handle the extra input during rain events. "The present setup is okay for Motueka now and for current developments, such as new growth in West Motueka," he said. "But it can't handle extra stormwater getting into the pipes from rain and raised water table."

The sewage pipes are old and are developing cracks which allow rainwater in during storms, overwhelming the treatment works.

Barry said people may be complaining that Council doesn't spend money in Motueka, but in fact about $600,000 each year is spent just maintaining the stormwater system.

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