Council releases previously confidential wastewater details

Published on 17 May 2016

17 May 2016

Whanganui District Council Chief Executive Kym Fell today released previously confidential information relating to the Council’s wastewater treatment plant.

“Expert advice obtained by the Council which expressed the opinion that the previous plant was never going to work properly has until now been withheld from public disclosure because it was legally privileged and formed part of our case against the plant designer MWH,” Mr Fell said.

“However, now that settlement with MWH has been completed there is no reason for the information to remain confidential and it is in the public interest to release it.

“It will assist the community to understand why the Council pursued a claim against MWH and the reasons why the Council has been so meticulous in considering all viable options for building a new treatment plant.”

The information released is a presentation made by Humphrey Archer of CH2M Beca to the Council on 28 October 2015. Mr Archer was engaged as an independent expert as part of the review of the design for the new wastewater treatment plant. Mr Archer also reviewed the MWH design for the failed treatment plant. The information in his report was previously confidential and privileged.

The key opinions expressed by Mr Archer in the report are:

• The ‘optimised lagoon process’ had not been used in other treatment plants. It attempted to combine all parts of the wastewater treatment functions in one lagoon. This included stormwater storage capacity, an aerobic upper layer and an anaerobic sludge storage layer. The normal process for wastewater plants comprises a fully-mixed aerated lagoon followed by one to three partially-mixed aerated lagoons. The partially-mixed lagoons have two environments – an aerobic upper layer and an anaerobic base layer. These are known as ‘facultative’ or ‘stratified’ lagoons.

• Significant errors were made in the estimated sludge storage volume. This meant the sludge storage capacity was exceeded in about 2009/10, two years after the plant was commissioned. There had been some differing estimates of when sludge storage would reach capacity but these reports indicated it would occur in 13 to 20 years.

• The required aeration energy was significantly underestimated.

• The aeration disturbed the sludge layer and prevented full anaerobic digestion of the sludge. Increased aeration would have caused more disturbance of the sludge layer.

Mr Archer’s presentation is available here (PDF, 2.4MB)