Your Council

Wastewater improvement

Thursday September 26 2013

Palmerston North is working to improve how we deal with the wastewater from around the city.

Ahhurst Sewer

One quality treatment

Palmerston North City Council is working closely with Horizons Regional Council, and other Manawatū River Leaders' Accord signatories, to reduce the number of point discharge sources. Wastewater assets have a long lifespan and it is important that the pipeline and pumps are designed to cope not just with today's flows, but future proofing our wastewater systems for the next 50 years.

• Linton residential, Linton Army Camp and Manawatū Prison 
• New Zealand Pharmaceuticals 
• Aokautere residential 
• Longburn industrial 
• Ashhurst
• Longburn residential

By diverting these six point source discharges through the wastewater treatment plant it means the localised effects are removed from the river, and it has allowed for a significant improvement in treatment thereby improving overall river water quality. Upon completion of the project there will only be one point source discharge of treated wastewater within the city reach of the Manawatū River.

Bunnythorpe - Mangaone Stream

Planning is underway on a $1.5 million project that will connect the Bunnythorpe wastewater reticulation with the Totara Road Wastewater Treatment Plant. Currently Bunnythorpe is connected to the Feilding wastewater treatment plant. This is not satisfactory as in heavy rain it can overflow into the Mangaone Stream.


New Zealand's first pilot trial of a WETOX sludge removal plant is underway at the treatment plant. Using a technique known as wet air oxidation it captures beneficial sludge products and reduces the overall volume of sludge needed to be disposed of. The WETOX plant has potential to recover phosphorus and alum that the treatment plant then uses to settle certain hazardous chemicals and suspended solids and bacterial colonies. Funded by the Ministry for the Environment and a private company the trial is expected to be completed in late 2013.


Early 2013 the City Council installed a deodouriser system at the treatment plant to contain odours from the plant from affecting nearby properties. The $35,000 system comprises a series of standpipes that spray a fine mist of deodouriser into the air to prevent odours from leaving the site. A weather station helps to control the system.

De-sludging aerated lagoon

Prior to the installation of the clarifier, alum was directly added to the aerated lagoons to control phosphorus discharge to the river. A by-product of this was the formation of alum sludge, which settled on the bottom of one of the aerated lagoons. The presence of the sludge is affecting the performance of the treatment plant and so in 2014 it will be removed.

Wastewater Action Plan

Palmerston North City Council and Horizons Regional Council's Joint Monitoring Report found that the wastewater discharge was the likely cause of the higher rate of periphyton growth downstream and that the impact on the macroinvertebrate community was leading to a significant adverse effect. Following the report's recommendations the City Council adopted a $1.7M Action Plan for reducing the impact of the city's wastewater on the Manawatū River. The more significant elements of this plan included:

Engineering consultants MWH have been engaged to review the treatment processes and advise on potential options that could mitigate the impact of the wastewater discharge on periphyton growth in the river. A number of low cost process improvements have been identified that will improve the efficiency of the processes but will not bring about any significant improvement in the discharge quality. Upgrade options have also been identified to reduce the level of particulate Phosphorus in the discharge and the level of soluble inorganic nitrogen (SIN) in the discharge. Both of these upgrades would help to reduce periphyton growth in the river.  The cost for treating for particulate Phosphorus is estimated at $2.9M. The options for reducing SIN range from $11M to $42M depending on the level of SIN removal required and the process chosen.

Love your river

Palmerston North City Council's 2013 educational campaign Love Your River highlights to the general public how their everyday actions can have a direct effect on wastewater and the health of the Manawatū River. Research shows that up to 30% of phosphorous in domestic wastewater derives from household habits. Changing consumer behavior towards using phosphorous-free detergents, composting food scraps, and washing their car on the lawn, are just some of the ways residents can help reduce phosphorous loads sent to the wastewater treatment plant.