Treated effluent used for irrigation
April 6, 2004
Native trees are thriving at North Shore City Council's Rosedale wastewater treatment plant after being watered with
treated effluent.
An irrigation trial on two planted areas totalling about 4000 square metres has been going since late February.
As well as meeting resource consent requirements, the trial explores other ways to re-use treated effluent, and the
effect it has on plants and soil.
The council's works and environment committee chairperson, Joel Cayford, says re-use of effluent is common in developed
countries. He says trials in North Shore City have long been something the community has wanted.
"The public has made it clear that it wanted council to explore other ways of disposing of treated effluent - rather
than pumping it all out to sea," he says.
"North Shore City Council has spent millions upgrading the treatment plant, and adding ultraviolet disinfection, so the
effluent now meets very high standards, high enough for specific nonpotable (not for drinking) applications - such as
irrigation," Councillor Cayford says.
"Although only a small amount of treated effluent is being re-used in this trial, much more could be in future once we
find out what its effects are on North Shore soil and land types.
"It makes little sense to build another Waikato Pipeline to irrigate North Shore golf courses and parks with drinking
water, if we can safely use treated effluent for the purpose, complete with its dissolved fertiliser nitrate and
phosphate minerals."
The average daily discharge of treated effluent into the sea is 50,000 cubic metres.
The two trial sites are being closely monitored. The levels of phosphorus,
faecal coliform and nitrogen are tested by council staff monthly, and the visual growth, rain and wind levels and water
volume monitored. Independent scientists from AgResearch will test the phosphorus and nitrogen balance, traces of heavy
metals in the plant leaves, and soil on an annual basis.
Results on the effect of the treated effluent on the plants and soil will be available at the end of the three-year
trial.
Joel Cayford says the cost of installing the pipes, the effluent irrigation pump and filter, and the control systems was
about $150,000.
Another trial site is currently under construction on land within North Shore City's Rosedale Treatment Plant site. As
with the two other sites, only native plants, including puriri, titoki, karaka, pittosporum, pohutukawa, totara and
flax, will be planted there in May.
ENDS