The role nitrogen plays in making algae grow too fast in the Manawatu River is likely to be in the spotlight as a summer of research into the harmful effects of Palmerston North's treated sewage discharge nears an end.

Horizons Regional Council is four to six weeks away from drafting changes to the city council's discharge consent conditions designed to improve the health of the river.

Horizons' general manager of strategy and regulation Nic Peet said most testing to help diagnose the problem was complete, and results were being analysed before the changes could be proposed.

The key nutrient suspected of continuing to stimulate algal growth is soluble inorganic nitrogen (SIN).

The main form of SIN found downstream of the wastewater treatment plant is ammonia.

The amount of ammonia allowed in the discharge is already limited, because it is highly toxic in concentrations and acts as a nutrient. It is also found in household bleach and used in fertiliser in lower doses.

The raised SIN levels come despite extra alum dosing this season to remove dissolved reactive phosphorus (DRP). Some of the research is also looking at whether river sediments absorb DRP at times of high river flows, and releases it when the river level drops.

Even the algae has been collected and is being tested to find out whether it absorbs and stores extra DRP when it is abundant, to be used when supplies in the water are lower. Another theory is that it is the balance of DRP and SIN that work together in certain proportions to encourage algal growth.

Mr Peet said while Horizons hoped to have enough information within the next four to six weeks to be able to inform the city council what had to change, other research would also help the city council work out what it needed to do differently to meet new standards.

The city council is already working through a $1.127 million action plan to support research and to improve several parts of the Totara Rd waste-water treatment plant operation to make it work properly.

Removing an accumulation of sludge that was restricting the effectiveness of the aeration ponds was expected to gobble up about $700,000 of that budget.

However, city council water and waste services manager Rob Green said trials had shown an alternative process for removing water held within the sludge had been successful and provided a cheaper way of removing and disposing of the sludge. Although the project would cost less, it would not be carried out until the new financial year.

A malfunction of the wetland pond that happened because of a buildup of sludge early in the summer was also being fixed.

About two-thirds of the sludge had been removed, and the pond, which had been bypassed while it was out of action, would be back in use next month, Mr Green said.

- © Fairfax NZ News

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