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Proposed Plan To Reduce Impact Of The Wastewater Discharge

27 Tuesday November

Proposed Plan To Reduce Impact Of The Wastewater Discharge On the Manawatū River

Palmerston North City Councillors will debate a plan to reduce the impact of the city’s wastewater discharge on the Manawatū River on Wednesday night.

Chief executive Paddy Clifford says the aim of the project is to start addressing matters related to compliance with the current resource consent as identified in the Joint River Monitoring Programme – in particular the growth of algae, bacteria and microbes (periphyton) and the impact on mayflies and other members of the macroinvertebrate community that are leading to a significant adverse effect in the River at times of low flow.

The action plan is estimated to cost $1.127million dollars this financial year and an estimated $435,000 to $635,000 in the following year.

“I’m pleased to report the plan should be able to paid from within existing budgets,” says Mr Clifford. “Staff identified offsets where staff savings have arisen and in terms of Council’s overall debt position being less than budget projection, which has led to a lower level of interest expense.”

If adopted by Councillors the plan will be implemented immediately to take advantage of the summer low flow conditions in the River.

Water and waste service manager Rob Green says while the plan addresses the concerns raised in the Joint Monitoring Programme it will also help address the more general water quality matters that might be applicable in the future.

The plan comprises a mixture of further investigations, trials and specific improvements to treatment process and a consent review process. “We’ve structured it in a way to ensure all work is undertaken in a progressive and sequential way,” says Mr Green.

“Once the causes of the periphyton growth are better understood the Council will be in an improved position to make decisions about operational changes or any new facilities required.”

Rob Green says the plan is tentative in nature, especially from year two onwards, due to the uncertainty surrounding the causes of the observed effects in the River.

An allowance of $100,000 has been included in the proposed Action Plan in year 2013/14 to initiate an investigation into best practicable option although the bulk of any expenditure including any new capital work could be expected in the term of the 2015-25 LTP.



Proposed Action Plan
The investigations include:

What are the likely causes of higher periphyton growth. There are three aspects to this investigation:
• Why is there higher than expected Dissolved Reactive Phosphorus in the River?
• What is ‘kick starting’ periphyton growth?
• Does removing DRP at higher flows help to reduce periphyton growth?

Why is there higher than expected DRP in the River? Again several likely possibilities have been suggested for the higher than expected DRP in the River:
• Possible sorption / desorption
• Total phosphorus converting to DRP once the discharge enters the River
• Ground water contributing from leaking ponds or the landfill

What is ‘kick starting’ periphyton growth? The investigation will use bioassay techniques (inserting trays with jars containing different enhancements into the River for a period of about two weeks and measuring the growth of periphyton for each preparation).

Does removing Dissolved Reactive Phosphorus (DRP) at higher river flows help to mitigate periphyton growth? It is proposed to trial this during a significant recession period this coming summer. It is estimated that this will require dosing for approximately 22 extra days. There will also be additional sludge disposal and monitoring costs. The monitoring of the periphyton growth during this period will be integrated with Horizons regular monitoring programme.

Conduct a review of:

The effectiveness of Wastewater Treatment processes. Since the last thorough review of the wastewater treatment processes was undertaken around 1998/99 there has been growth in the city and other discharges connected to the treatment plant.

This review is to look at the loadings of the different contaminants coming into the plant and compare this with the capacity of the treatment processes to cope.

This action provides for reviewing the efficiency of the different process and will involve analysis of on line data as well looking at the aerated ponds biological processes and any other aspects that impact on treatment efficiency such as aerated pond flow circulation.

Immediate actions:

Desludging of aerated ponds. The aerated ponds are estimated to have lost about 25% of their capacity due to sludge accumulation. Biological processes in the ponds are not working as well as they should be and consequently there is a lower wastewater quality particularly with respect to BOD, suspended solids and E.coli levels as a result.

Over the past year a technique of bio-remediation has been applied to the sludge and decreased its density but not its volume. Council has since approved a plan to increase the trial period so that the contractor can increase the remediation process, at no cost to Council.

If the bio-remediation technique does prove to be successful then it will mean a large cost saving over resorting to mechanical means – as indicated this could cost up to about $700k.
In the event of it still not being successful in recovering the full lost volume in the ponds there would be savings in the cost of removing by mechanical means due to the lower density of the sludge so the total cost is not likely to be any greater.

Pilot use of clarifier at higher River flows without dosing. It is considered worthwhile to undertake a four week trial of operating the clarifier without dosing at higher River flows when the wastewater would not normally pass through the clarifier. It is not expected that this will make a very significant difference, possibly resulting in a small decrease in BOD and suspended solids. The value of trialling this would be to understand whether this facility could be used in the future in a strategic way and only a small cost is involved.

Promotion of phosphorus free detergents. Twenty per cent of DRP in the wastewater stream is estimated to come from trade waste sources and 80% from domestic sources. Of the phosphorus from domestic sources recent studies suggest that about 70-80% is from humans and about 20-30% from detergents. Promotion and advice to the community about the advantage of Phosphorus free detergents would help to make a small contribution to this issue as well as making the community aware of the source of the issue.


The results of the various investigations and review of treatment plant efficiency together with an understanding of any new consent standards that PNCC is likely to be required to meet will be brought together around June / July 2013.

Mitigation may require both operational changes to the current facility or if further investment is required in any new facility then a consideration of what is the best practicable option to meet the desired outcome.

Other desirable operational changes may also result from this programme of work and will be reported to the Council.

Should the assessment suggest that further treatment processes are required to reduce adverse effects to acceptable levels or meet new consent standards then further work will need to be initiated on what the best practicable option is to meet these requirements?

Consent condition 23B requires PNCC to review the best practicable option for treating and disposing of wastewater (including land disposal systems) in the 13th year of the consent, scheduled for July 2016.

To view the Action Plan please visit: http://www.pncc.govt.nz/media/1909549/agenda_council_28-11-12.pdf


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