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Independent Inquiry into WWTP
Published: 12 Sep 2016
Author: Cr Rob Vinsen


Independent Inquiry into WWTP

Mr Domm’s report is predictable given the Terms of Reference he was given.

It tells us what we already know – that the MWH designed plant was untried and innovative, that it had design faults, and that it was incapable of meeting the Resource Consent. It gives Council’s advisors  opinions  , but does not give  MWH’s opinion. That’s what an inquiry should do – hear both sides and adjudicate on it.  Mr Domm said that he had no contact with MWH at all, so how can any conclusion be taken out of this report on the questions that really matter such as whether  Council managed and operated the plant properly which is very disappointing.

I supported the need for this enquiry in the misguided belief that the question “ is the current plant salvageable” would be independently addressed. That Mr Domm did not address this question – the only question that really matters now – is not his fault. The Terms of Reference ruled out seeking technical advice and on answering this key question.

We know  that components of the MWH plant were not up to processing the effluent  sufficiently – so, of course, we need to modify the plant. However, the report offers no professional  opinion that supports Cardno BTO’s  advice that the $20M investment in the insitu plant should be written off and to start again.  They just say that the plant is faulty and wasn’t working as Council was led to believe it would.

The report confirmed  two basic  design flaws in the insitu plant.

One is  the depth of the lagoons . 

This is the only innovation that was unprecedented.  Aerated Lagoons are the most common WWTP process worldwide, and MWH had just finished the Palmerston North aerated lagoon  plant prior to designing  Whanganui’s. In Palmerston North they built a separate sludge storage lagoon, but for Whanganui  the designers thought that they could save costs by simply making the  lagoons  four meters deeper to store the sludge for up to twenty years. This innovation has failed for various reasons – but other professional opinion say’s you  address this by modifying the lagoon  to manage the sludge on a continual and daily basis.  

The other identified fault is the poor mixing of the biomass in the main lagoon.

The decision in 2005  to go away from the standard four rectangular shaped ponds to two kidney shaped ponds appears to have been an inadvisable one, as it led to low retention times  and short circuiting. But both  MWH  and Cardno (in its original report )suggested modifying this fault by partitioning the pond into three zones using rubber curtains, and changing the aerators to a surface mounted type  that wouldn’t drag the sludge up to the surface causing odours.

It is tragic – for ratepayers especially- that no resolution of the key questions  are offered by the inquiry. It is simply a chronological document of the process that happened , one that was already known to those that took an interest.

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