Exciting new technology used to help clean up Rotorua lakes

Tuesday 29 March 2005 Environment Bay of Plenty and Rotorua District Council are investigating technology right at the forefront in New Zealand to try and clean up the Rotorua lakes.

Rotorua lakes project coordinator Paul Dell says many of the solutions being considered or tested are leading edge and innovative. “We have a huge challenge facing us and we need to think outside the square,” he explains.

Projects include the construction of major in-lake structures like the 700-metre diversion wall proposed for Lake Rotoiti and another that, if it goes ahead, would channel the Hamurana Stream outflow away from Lake Rotorua.

Environment Bay of Plenty is currently trialling ways to chemically treat phosphorus in streams that flow into different lakes. At Lake Rerewhakaaitu, it is also testing a new method for removing dissolved phosphorus from streams by running the water through “socks” of cleaned steel slag.

On its part, Rotorua District Council will spend $50m to $60m over the next few years on sewerage upgrades and treatment to reduce the amount of nutrients entering the lakes. In another innovation, the district council has started injecting methanol into its sewage treatment reactor to increase microbial activity and remove more nitrogen from the effluent before it is discharged to the Whakarewarewa forest. Environment Bay of Plenty’s Proposed Water and Land Plan, now almost finalised, will also help prevent further increases in nitrogen and phosphorus, this time from land use activities.

Mr Dell says innovative thinking is needed to improve quality in the Rotorua lakes, particularly in Lake Rotorua and Lake Rotoiti. Lake Rotoiti’s problems are likely to be improved with a single action, by diverting the water flowing into it from Lake Rotorua. “But Lake Rotorua will need a lot more than a single solution – it is going to take a whole range of actions to reduce the risk of algal blooms in that lake.”

Streams, fed by groundwater reserves, are constantly supplying nitrogen and phosphorus to Lake Rotorua. Because of this, Environment Bay of Plenty is trialling ways to remove nutrients from four streams in the lake catchment. As part of the process, all projects will have to be thoroughly tested for adverse environmental effects.

Puarenga and Utuhina Streams Environment Bay of Plenty is seeking consents to trial the use of Alum dosing as a way to remove phosphorus from the Puarenga and Utuhina streams on the southern side of Lake Rotorua. The chemical binds to dissolved phosphorus, causing it to cluster and sink to the lakebed.

Hamurana Stream Hamurana Stream puts a large volume of water into Lake Rotorua so has quite significant nitrogen and phosphorus inputs. One idea being investigated is to build a wall along the northern side of the lake, just out from the lake edge, that would channel the Hamurana Stream’s outflow towards the Ohau Diversion, rather than letting it mix with Lake Rotorua’s water. Environment Bay of Plenty has commissioned a detailed investigation into the impact this would have on the lake’s trout fishery, and the viability of the project will depend on the results of this and other factors, including costs and community expectations.

Tikitere Diversion The Tikitere geothermal field, east of Lake Rotorua, puts around 25 tonnes of ammonium (a form of nitrogen), into Lake Rotorua every year via the Waiohewa Stream. Environment Bay of Plenty and Rotorua District Council are investigating ammonium treatments as well as the piping of this geothermal flow away from Lake Rotorua to a location north of the Ohau Channel outlet. This will prevent the nitrogen fuelling blue-green algal growth in Rotorua and Rotoiti, and affecting the Kaituna River.

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