26 September 2013

Filtration system to improve Palmerston North’s wastewater discharge

A $2.9 million disk filtration system will reduce the effect of the Palmerston North wastewater discharge on the growth of periphyton (algae) in the Manawatū River at low flow.

Last night, Palmerston North City Councillors approved the project after considering a number of options. Over the past 12 months Councillors have received regular briefings and workshops on the project which has been driven by scientific research.

The peer reviewed report, carried out by independent consultant Keith Hamill, found:
• Existing levels of dissolved reactive phosphorus (DRP) removal needed to be maintainted to mitigate periphyton growth but that treatment for DRP at higher flows provided little actual benefit.
• Particulate phosphorus in the discharge also contributes to periphyton growth by converting to DRP in the river and by settling as sediment in the periphyton mat aiding growth.
• At river flows below 20-30m3/s nitrogen in the form of soluable inorganic nitrogen (SIN) was limiting periphyton growth.

Water and waste services manager Rob Green says the Totara Road Wastewater Treatment Plant has “a good record of compliance and we aim to keep it that way.”

“The disk filtration system will reduce the amount of particulate phosphorus from the discharge which will reduce the amount of nutrients available for periphyton preventing it from growing as fast.”

Palmerston North City Council acting chief executive Ray Swadel says the science has shown that the discharge is not having a significant adverse effect on aquatic life in the reach of the river as a whole, and that it is not causing a reduction in life supporting capacity of the river.

“However,” he says, “the discharge is having more of an effect than was anticpated when Horizons initially gave Council consent.”

Palmerston North Mayor Jono Naylor says over the years Council has demonstrated its commitment to meeting its requirements as part of the Manawatū River Leaders Accord through numerous wastewater projects.

“Since 2003 Council has introduced ultra violet light treatment and a clarifier. We’ve also closed some of the smaller plants around the city with the aim of ensuring one high quality standard of treatment and one discharge point.”

“We’re committed to improving the quality of the discharge. Guided by science we’ve agreed to an affordable financially prudent response.”

The project will be mostly self funding as it will lead to savings of between $140,000 and $200,000 per annum due to a reduction in alum dosing. It comes on top of the $1.7M allocated to the Wastewater Action Plan.

Rob Green says today, Council lodged its response to HRC’s formal review of the Consent conditions, including the plans to build the disk filtration system.

It’s anticipated the public will have the opportunity to make submissions on the review later this year before the hearing is held next year.

Mr Green says irrespective of the outcome of the formal review Council will:
• desludge the aerated lagoon as part of the Wastewater Action Plan
• complete the Ashhurst to Palmerston North wastewater pipeline
• start work on the Longburn (residential) to Palmerston North wastewater pipeline
• continue to promote the Love Your River campaign
• work toward the commissioning of the disk filtration system in the 2015/16 financial year.

In 2003 Council was granted a 25 year consent for the wastewater discharge and embarked on a $16million upgrade of the treatment plant. A clarifier was built and alum dosing commenced which significantly improved the quality of the discharge into the river by reducing the amount of dissolved reactive phosphorus or DRP in the discharge. An ultra violet light treatment process was introduced which allowed Council to meet bacteriogical standards.

Alongside these projects Palmerston North has been closing small treatment plants around the city, insisting on one high quality treatment process – Ashhurst and Longburn (residential) should join the system next year.

In 2011 HRC assessed the discharge as not complying and causing a significant adverse effect on aquatic life due to its impact on periphyton growth and the macro-inverterbrate community during low flow conditions. This led to a joint PNCC-HRC Monitoring Programme by consulting ecologist Keith Hamill.

The Consent conditions recognise there would be more periphyton growth downstream of the plant however Mr Hamill’s report found it was growing at a rate faster than expected causing mayflys to move downstream and for caddisflys to move in. The report also found more DRP in the river than could be explained by the discharge alone.

As a result of the report Horizons Regional Council is carrying out a formal review of the Consent conditions and Palmerston North City Council adopted a $1.7M Wastewater Action Plan. The plan included a review of treatment processes and mitigation options, an increase in alum dosing at higher flows, investigation into the causes of the higher periphyton growth downstream of the plant, de-sludging of the aerated ponds (due to occur later this year) and a marketing campaign aimed at encouraging residents to use non phosphorus based detergents.

Keith Hamill’s studies continued throughout last summer during the worst drought in 70 years, thereby providing a unique opportunity to study the effects at extended periods of low flow. His full report is available at http://www.pncc.govt.nz/media/2163129/pncc_wwtp_investigations_2013_revised_v2-signed.pdf.

ENDS

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