Wastewater Project


We are building a new wastewater treatment plant for our city's domestic and industrial wastew​ater. 

Wastewater plant construction contract signed

Whanganui District Council Chief Executive Kym Fell says a significant milestone in building a new wastewater treatment plant has been reached with the signing of the construction contract with Hawkins Infrastructure on 20 September 2016.

Read more in our media release

Wastewater inquiry report released

The independent review into the circumstances and Council processes which led to the failure of the old wastewater treatment plant has been completed and was released on 10 September 2016.

Read our media release about the report

Read the independent review report (PDF, 9.5MB)

MWH Rectification Proposal released

The Whanganui District Council has released MWH’s 2014 proposal for an alternative design to reconfigure and upgrade the failed wastewater treatment plant. There have been a number of requests for this document and although we are making this available, it is important to put it into context around the events of the time.

More information is available in our media release about the proposal

MWH Rectification Proposal (PDF, 332KB)

New wastewater treatment plant underway

The Council, at an extraordinary meeting held on 9 August 2016, confirmed its earlier decision in March 2016 to proceed with the construction of the new wastewater treatment plant, bringing to an end several years of deliberations over the best way to replace the existing failed plant.

Read more in our media release

10-Year Plan 2015-2025 amended

​On 30 June 2016 the Council adopted some amendments to the 10-Year Plan 2015-2025, including new resolutions around the wastewater treatment plant. The Council noted that new information provided during recent discussions with major trade waste users may lead to changes to our current wastewater scheme. Should a more affordable solution arise, the Council has the flexibility to accommodate any changes within the 2016/17 Annual Plan. Those changes would be reported back to the Council before a decision to proceed with any wastewater scheme occurs.

Read our media release about the Council's decisions

 Inquiry into wastewater treatment plant

In July 2016 the Council agreed to proceed with an independent inquiry into the circumstances and Council processes which led to the failure of the old plant, from evaluation of the original concept to the acceptance of the need to commission a new plant. This inquiry is about the processes that were followed rather than technical details of the old plant which have already been thoroughly investigated and made public. It is also not concerned with the decision to build the new plant.

 Mayor Main responds to queries - 8 June 2016

Mayor Annette Main has provided information and answers to questions to clear up misunderstanding in the community about the wastewater treatment plant. It is a response to matters raised in the Wanganui Chronicle in an editorial and a letter to the editor. Read more here.​

Expert opinion on previous treatment plant

On 17 May 2016 Whanganui District Council Chief Executive Kym Fell 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)

 New treatment plant process

Check out how our new wastewater treatment plant process will work - how it will deal with the wastewater coming in and the foul air and sludge that is produced in the process. If you click on the image below it will take you to a PDF diagram so you can see how the process will work.

Diagram showing how the new wastewater treatment plant will deal with wastewater, foul air and sludge 


​March​​ 2016

Green light for new treatment plant
On 9 March 2016, the Whanganui District Council confirmed that a new wastewater treatment plant will be built. The plant will include a sludge drying facility, which, while initially adding a capital cost, will reduce annual operating costs.  Construction of the plant will begin in July this year and it will be up-and-running within three years

Fe​bruary 2015​

An update on this work was reported to the Council on 25 February 2015 and it was decided that further work is necessary to ensure the new plant is affordable for current and future ratepayers and our commercial and industrial users.

These decisions will have a long-term impact on our community. There are no shortcuts in this process so it’s important to take the time to get it right once and for all.

Read more about the 25 February 2015 decisions here

July 2​​​01​​5

The Council discussed options for the treatment plant on 29 July 2015 and decided to amend the design approved in June 2014 by incorporating a dryer. This will reduce costs and provide more options for disposal of sludge produced by the plant. The Council has also commissioned an affordability study which is being carried out by Massey University.

Read the 29 July 2015 decisions here

​Octobe​​​r 2015

At the Council meeting held on 28 October 2015, the Council resolved:

That prior to proceeding further with the Cardno developed design a report is commissioned from CH2M Beca for further consideration by Council that asks the question:
Is the Cardno developed design the optimum process when considering both environmental and cost efficiency issues for the Whanganui wastewater treatment plant.

That the Chief Executive be delegated to develop a report on the timeline, scope and cost of such a report and report back to the next Council meeting.


The existing plant had been experiencing problems for some time, with measures put in place in October 2012 to try to remedy the problems which caused odours throughout parts of the district at times.

The Council commissioned wastewater specialists Cardno BTO to advise on short-term and long-term measures to address the issues with our wastewater treatment plant. Cardno's first report on the long-term options was presented to the Council on 29 April 2013.

Cardno BTO report on evaluation of long-term improvements for consent compliance (PDF, 4.7MB)

We had to remove the sludge from the existing ponds before we could build the new plant so the old treatment plant is not operating and wastewater is being discharged to the sea, via the ocean outfall, ​under emergency provisions. In the new treatment plant, the sludge will be contained in a covered pond and regularly removed so there will not be ongoing odour problems.

Cardno BTO's report on the design was peer reviewed by AECOM and it was also reviewed by an independent expert from CH2M Beca. These reports were presented to the Council on 19 June 2014 and the design approved.

Design reports by Cardno BTO, AECOM and CH2M Beca (PDF,  5.5MB) 

Media releases an​​​d ​​​FAQs

Page reviewed: 03 Apr 2018 1:18pm