Floating wetlands to treat sewage

A new sewage treatment option has been decided on by the Central Hawke's Bay District Council.

The new programme will use floating wetlands on existing oxidation ponds, the plant roots growing down into the wastewater. The plants will take up nutrients, and organisms growing on the roots will treat the sewage.

The effluent (treated sewage) will then be dosed with chemicals and passed through a sand filter to remove phosphorus, and finally irradiated with ultra-violet light to kill pathogens. The final effluent, expected to be close to drinking-water standard, will discharge into the Tukituki and Waipawa Rivers.

The final effluent quality is expected to comply with the stringent standards of resource consents for such treatment plants that have to be met by September 30, 2014. "This has been the biggest decision this council has had to face in a number of years" said Central Hawke's Bay Mayor Peter Butler. "In coming to this option ... we thought hard about what is right for the environment and affordable to the ratepayers of Central Hawke's Bay."

The construction cost for the sewerage treatment upgrade project to be budgeted in the council's Long Term Plan (LTP) was $6.2million, and included other work required to bring parts of the existing sewerage treatment plants up to the latest standards.

While deciding how to meet the higher resource consent standards the council considered five options that could be built under the council's existing wastewater consents including worm farm treatment, sequential batch reactors, a modified Bardenpho plant, anaerobic digestion; plus a sixth option of forest treatment as proposed by the Hawke's Bay Regional Council, which would have required a new resource consent. The cost to the council of those options ranged from estimates of $7.7million to $11million with increased operating costs between $293,000 and $379,000.

"The investigations and consultation carried out to reach this result have been extremely robust, and the end result, with considerable savings in cost to the ratepayers and the significantly improved quality of the treated effluent going into the rivers, was a great result for the efforts of councillors, staff, and the community as a whole."

The Hawke's Bay Regional Council chief executive Andrew Newman said the district council would need to satisfy it that the design of the floating wetlands proposal would deliver the required standards by the 2014 deadline. "We will be writing to the Central Hawke's Bay District Council to reiterate the requirement for it to meet the required standards of its discharge consent within two years," he said.