Rotorua has now reached a point of diminishing returns regarding wastewater treatment.

Increased capacity was needed in light of projections of an increased population count over the next 30 years.

Rotorua Lakes councillors were told this fact at the first committee meeting of the year in outline by infrastructure manager, Stavros Michael.

Discharging wastewater into Whakarewarewa Forest is about to end and the council requires a resource consent to discharge wastewater into Lake Rotorua.

But it and the increased capacity of the discharge has polarised elements in the Rotorua community, concerned at what is seen as a deterioration in the health of the lake.

During a presentation to the operations and monitoring committee of the council, Stavros said there was a need as the district grew to increase the council's wastewater treatment.

The question is why discharging into Whakarewarewa Forest was no longer viable?

'The continued discharge into the forest while performing very well since 1991 is that now we have come to the point where we have diminishing returns,” Stavros says.

If relocation from the forest was to be attempted, what kind of new system was going to be established?

This would mean that there was a need for the district as it grew an increased capacity of the council's treatment to make sure, because the council could no longer rely on the land, it was fully treated and safe before it was returned to the environment.

For now, Rotorua produced 7 billion liters of wastewater which contained 360 tonnes of nitrogen and 40 tonnes of phosphorous, Stavros says. Wastewater should not contain more than 50 tonnes of nitrogen.

The forest's efficiency is decreasing, he says.

With projected growth over the next 30 years, the district would generate almost 25 million litres of wastewater a day, in increase of 25% by today's standards.

'If we were to stay in the forest, the forest itself would not have the capacity to achieve a nutrient reduction required to meet the depth agreement. On top of that, was owner objection for staying in the forest.

'We do need a new wastewater system that will remove all the nutrients discharged,” says Stavros.

Resource consents to discharge wastewater into Lake Rotorua will be submitted to the Environment Court mid to late March, Michael said. Meanwhile the public will be furnished with facts and information through various media outlets.

Meeting chairman Charles Sturt says considerable ill-informed comment had been made in the community and Stavros Michael had set the record straight. People did not understand the work that had gone behind the scenes.

Cr Rob Kent says while he quality of water entering the lake was of a better quality 'than anything else going into the lake”, what happened if anything went wrong in rough weather and effluent entered the lake untreated in the event of storm surges?

'Focusing on what happens over the next 30 years, we have taken into account the climate change effects and obviously the experience we've had in recent years as to storm surges,” Stavros says.

'The capacity of the new treatment plant will allow us to almost double our current capacity and that requires a treatment storage capacity … that will allow us to treat up to 72,000 litres of effluent each day.”

A snapshot of the presentation included:

■ Rotorua waste water treatment plant among the best in New Zealand for Nitrogen (N) removal, treating wastewater to high standard.

■ Wastewater treatment plant to date the biggest contributor to reducing N going in to Lake Rotorua.

■ Land treatment system established in Forest 1991 but N stripping by the soil has decreased and excess water is affecting the land and trees.

■ 2013 – Environment Court directs RLC to investigate alternative options for disposal of recovered water following concerns from tangata whenua.

■ The council has an easement over 433 hectares in the forest but no ongoing resource consent.

■ The council has been irrigating there since 1991 and during that time the land has been returned to CNI by the Crown through its Waitangi Tribunal settlement.

■ CNI and the council agreed to end spraying in the forest because it is not culturally acceptable and is no longer environmentally sustainable.

■ Upgrade treatment plant processes s that all wastewater is full treated and safe before the water is returned to the environment, including: Increased capacity and provision for storm flow storage; additional phosphorous removal; ultra-filtration; UV treatment; land contact bed.

■ Discharging into Whakarewarewa is no longer viable.