Gisborne wetland trials underway

It’s science in action down at the Gisborne Wastewater Treatment Plant in Banks Street.

It’s science in action down at the Gisborne Wastewater Treatment Plant in Banks Street.

IN THE BARREL: Grasses and rushes for the site get a head start.
CHECKING: CIBR environmental microbiologist Dr Jacqui Horswell (left) and NIWA scientist Dr Chris Tanner check to see the raupo plants have viable roots before planting in barrels at the Banks Street Wastewater Treatment Plant.

Rows of blue barrels filled with several varieties of wet-loving native species have gained pride of place as their every centimetre of growth is watched with anticipation, measured, analysed . . . and admired.

Sixty barrels in all – half of them using gravel as their growing medium, half using bark. Two main species were planted late last year, raupo (Typha orientalis) and a tall rush (Schoenoplectus tabernaemontani); with another three just in case – cutty grass (Carex geminata), marsh club-rush (Bolboschoenus fluviatilus) and native rush (Machaerina articulata).

The plants were initially watered with ordinary water to get them established and then low-dose treated wastewater from the Biological Trickling Filter (BTF) treatment plant. Settled biosolids will be applied early next year. The idea is to see which plants grow best and in what type of medium.

Trial run

NIWA’s Dr Chris Tanner and CIBR’s Dr Jacqui Horswell are driving the trial with major input from Wastewater Technical Advisory Group (WTAG) members and council staff. Based on their science knowledge and overseas experience, they recommend further treating the effluent from the BTF tank in a process using wetlands.

Currently, solids are screened through the milliscreening unit. The resulting liquids go to the BTF tank for processing, trickling down through a series of plastic haybale-like blocks. This mimics water running down a stony stream whereby the slimy build-up helps transform the effluent into plant-like matter.

Treated effluent is currently discharged 1.8km out to sea via an outfall pipe. The scientists recommend putting this treated effluent into a tank, to let it settle by gravity into liquid effluent and settled biosolids or sludge (like liquids and solids). Experiments show 85 percent of solids settle within an hour. These solids could then head for a planted sludge (biosolids) drying bed wetland for treatment.

The remaining watery effluent, largely free of solids, would feed into an effluent treatment wetland where the natural process of slowly trickling through extensive plantings would further treat it.

Dr Tanner says trials for the sludge-drying wetlands – using plants in barrels and lysimeters – will look at how native species tolerate Gisborne’s settled biosolids and how they might decontaminate it.

Connecting traditional and modern practices

“The idea is to reduce the sludge on site and treat the final effluent with wetlands to a level seen as sufficient to protect the environment and health to meet cultural requirements. It’s connecting traditional and modern practices on a city scale, and putting the life force back into the environment.”

In the planted sludge drying beds, bacteria and microbes around plant roots help decontaminate the sludge, get rid of disease-causing organisms (pathogens) and reduce contaminants like emerging organic contaminants (eg, triclosan, the antibacterial, antifungal agent found in soaps, detergents and cosmetics). The sludge-drying wetlands have the capacity to treat sludge for up to 10 years, and produce a clean, usable sludge.

Sludge-drying wetland systems have been used in Europe over the past 25 years. Research is promising but they have not been previously applied in New Zealand and experience with sludge from a BTF plant is limited. Dr Tanner said overseas systems cannot be simply imported to the Gisborne or New Zealand environment.

“Things don’t work the same here. Plants used successfully overseas are noxious weeds here. Every sludge and effluent is different. New Zealand, and Gisborne itself, has a specific climate with different rainfall, humidity and temperatures.

“This is a low-energy, natural process that could produce high-quality water and reduce costs to the community.”

Rows of blue barrels filled with several varieties of wet-loving native species have gained pride of place as their every centimetre of growth is watched with anticipation, measured, analysed . . . and admired.

Sixty barrels in all – half of them using gravel as their growing medium, half using bark. Two main species were planted late last year, raupo (Typha orientalis) and a tall rush (Schoenoplectus tabernaemontani); with another three just in case – cutty grass (Carex geminata), marsh club-rush (Bolboschoenus fluviatilus) and native rush (Machaerina articulata).

The plants were initially watered with ordinary water to get them established and then low-dose treated wastewater from the Biological Trickling Filter (BTF) treatment plant. Settled biosolids will be applied early next year. The idea is to see which plants grow best and in what type of medium.

Trial run

NIWA’s Dr Chris Tanner and CIBR’s Dr Jacqui Horswell are driving the trial with major input from Wastewater Technical Advisory Group (WTAG) members and council staff. Based on their science knowledge and overseas experience, they recommend further treating the effluent from the BTF tank in a process using wetlands.

Treated effluent is discharged 1.8km out to sea via an outfall pipe. The scientists recommend putting this treated effluent into a tank, to let it settle by gravity into liquid effluent and settled biosolids or sludge (like liquids and solids). Experiments show 85 percent of solids settle within an hour. These solids could then head for a planted sludge (biosolids) drying bed wetland for treatment.

The remaining watery effluent, largely free of solids, would feed into an effluent treatment wetland where the natural process of slowly trickling through extensive plantings would further treat it.

Dr Tanner says trials for the sludge-drying wetlands – using plants in barrels and lysimeters – will look at how native species tolerate Gisborne’s settled biosolids and how they might decontaminate it.

Connecting traditional and modern practices

“The idea is to reduce the sludge on site and treat the final effluent with wetlands to a level seen as sufficient to protect the environment and health to meet cultural requirements. It’s connecting traditional and modern practices on a city scale, and putting the life force back into the environment.”

In the planted sludge drying beds, bacteria and microbes around plant roots help decontaminate the sludge, get rid of disease-causing organisms (pathogens) and reduce contaminants like emerging organic contaminants (eg, triclosan, the antibacterial, antifungal agent found in soaps, detergents and cosmetics). The sludge-drying wetlands have the capacity to treat sludge for up to 10 years, and produce a clean, usable sludge.

Sludge-drying wetland systems have been used in Europe over the past 25 years. Research is promising but they have not been previously applied in New Zealand and experience with sludge from a BTF plant is limited.

“Things don’t work the same here. Plants used successfully overseas are noxious weeds here. Every sludge and effluent is different. New Zealand, and Gisborne itself, has a specific climate with different rainfall, humidity and temperatures.

“This is a low-energy, natural process that could produce high-quality water and reduce costs to the community.”

Experts lend a hand

Gisborne has a wealth of professional expertise working on its behalf to find the best way to further treat the city’s wastewater to a higher level. Working alongside Gisborne District Council staff and Wastewater Technical Advisory Group (WTAG) members are scientists:

Dr Jacqui Horswell, who leads the sludge drying wetlands part of the project. She is an environmental microbiologist and science leader at ESR, leading the Centre for Integrated Biowaste Research. She deals in solids; her colleague Chris Tanner, in water.

She and her CIBR and NIWA colleagues will build smaller trial sludge drying wetlands and look at how to make them work better to achieve the results the community wants.

Dr Horswell is one of New Zealand’s leading experts on microbial contaminants (eg, pathogens) in human waste, and has a wealth of waste and land management sector experience.

Dr Chris Tanner is an aquatic ecologist and principal scientist (aquatic pollution) at NIWA, Hamilton. His research over the past 25 years focused on understanding the ecology of wetland systems and the processes of removing pollutants.

He has worked in a range of natural wetlands and lakes but his particular interest lies in using constructed wetlands to address sanitation and water quality issues for communities, farmers and industry. He has consulted widely on the design and management of wetlands and led demonstration projects on farms and in small communities and marae across New Zealand, and in village sanitation projects in Fiji.

Dr Jason Park is a wastewater scientist (Aquatic Pollution Group) at NIWA, Hamilton. His NIWA research over the past 12 years focused on energy production and nutrient recovery from domestic and agricultural wastewaters.

His work in algal wastewater treatment using High-Rate Algal Ponds (HRAPs) combined with biofuels production has brought this technology closer to being realised.

He is currently assessing the characteristics of wastewater and Biological Trickling Filter sludge, such as its settling and de-watering properties, to investigate the feasibility of sludge and effluent treatment wetlands for Gisborne.

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