Wastewater Options

About the trials

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About the trials

Trials are taking place at the Banks Street wastewater treatment plant to find out how well wetlands in this region can treat the liquid and solids (sludge) that comes out of the biological trickling filter (BTF) plant.

The trials – joint funded by Council and the Centre for Integrated Biowaste Research (CIBR) – include looking at which native plants grow best when watered with treated wastewater and whether these plants grow better in bark or gravel.

Two main species – raupo (Typha orientalis) and a tall rush (Schoenoplectus tabernaemontani – have been planted in 60 half-barrels. Half are growing in gravel; half are growing in bark. Another 3 species are also being trialled – cutty grass (Carex geminata), marsh club-rush (Bolboschoenus fluviatilus) and native rush (Machaerina articulata). 

The plants are watered with low-dose treated wastewater from the BTF. They'll be shifted in late 2015 into large vessels containing gravel, then moved in early 2016 to ESR’s Kenepuru Science Centre in Porirua. Gisborne’s sludge (the solid stuff) will then be applied. Every day, the scientists will water, monitor and test for what’s absorbed, what runs off, what evaporates and what’s removed.

What will the trials show?

The scientists are looking at how well the native plants put up with, and clean up, Gisborne’s settled solids. They’re also looking at how solids settle, how water drains through, and how nitrogen, phosphorus and biochemical oxygen demand (BOD5) are reduced or removed. Biochemical oxygen demand is the amount of oxygen that aerobic microorganisms need to break down organic matter in a sample of wastewater.

What types of contaminants are in sludge and effluent?

Bacteria, viruses, heavy metals and emerging organic contaminants including the antibacterial agent triclosan found in soaps, detergents and cosmetics. Other emerging contaminants include ingredients in products such as insect repellents, shampoo, sunscreen, toothpaste, pain killers and pharmaceuticals.

What else happens at the Banks Street treatment plant site?

The Banks Street site is also home to a miniature floating treatment wetlands, comprising a thick fibrous mat that floats on a bed of treated wastewater. Aquatic plant roots grow through the floating mat and into the water beneath, providing a large surface area for nutrients to be integrated, biofilms to grow and fine particles to be trapped. This ecosystem enables remaining particles of solids to be eaten by worms, leeches and snails.

What other work is going on to help decide the feasibility of wetlands?

Wetland design work – led by Dr Chris Tanner, NIWA, and Dr Jacqui Horswell, CIBR, part of ESR.
Biosolids contaminants – led by WTAG’s Gordon Jackman and Northcott Research Consultants director Dr Grant Northcott.
Viruses – led by Gisborne Medical Officer of Health Dr Bruce Duncan
Sensitive materials – led by wetland project manager Robson Timbs
Alternative use of wetlands-treated water – led by WTAG’s Murray Palmer and Peter Williamson.

What will happen to the underwater pipe going out to sea?

The pipe will stay if the wetlands option goes ahead. Treated industrial wastewater will still flow through the pipe to the sea. It’s also important to keep the pipe in working order and to have it as an option in emergencies.

Wetland Trial

L-R John Mackay and Dr Bruce Duncan check the growth of plants growing in barrels as part of the trial


Councillor's tour of the trial - February 2015

Media releases

February 2016 - Wetlands trial could set benchmark

August 2015 - Wetlands could remove viruses from wastewater