Emma Mason, Nicholas Phillips, Amanda Sabatino
The City of Cockburn has struck on a bright idea: harness the sun to filter Lake Yangebup of chemicals and rid the local community of the midge menace.
“It is a nutrient stripping basin,” says the city’s environmental services manager and the system’s mastermind Chris Beaton.
Mr Beaton describes a nutrient stripping basin as an artificial winding wetland stream, populated by sedges, shrubs and trees, designed to draw the nutrients out of the water.
“There is a lot of these basin type filters around the place, but this one is novel because it uses solar power to power a pump that takes water out of the lake, pushing it back out through the nutrient stripping basin,” he says.
“Most just rely on storm water flowing in to make them work, so they only work when it rains and they only treat the water coming from road runoff or from drains.”
Situated in the City of Cockburn, Lake Yangebup is part of the Beeliar Wetland chain and receives runoff from the drains in the South Jandakot area.
Historically the lake has been a poorly managed ecosystem, having suffered arsenic contamination from old wool scouring works, which used to be located on the water banks.
Currently the lake collects up to two tonnes annually of nitrogen and high amounts of phosphorous in the form of fertiliser runoff, used on nearby agriculture and suburban gardens.
The water makes its way to the lake through groundwater flows and several storm drains, causing the lake to become hyper-eutrophic (nutrient rich).
“It’s even more than hyper-eutrophic if there is such a thing,” Mr Beaton says.
“These nutrients lead to algae blooms, which supply a food source for the midge, the midge then hatch and fly towards the first light they see.
“At times there can be tens of millions of them hatching out of the lake.
“We have had local businesses basically pull the pin and leave because they could not effectively operate with the midge around.”
Up until now the City of Cockburn has relied on expensive bi-annual applications of S-methoprene; a pesticide that controls the midge.
“Every time we applied the S-methoprene it cost tax payers roughly $25,000,” Mr Beaton says.
“Comparatively this project cost $150,000.”
Some of that cost came from the state Natural Resource Management Community Grants Program.
Members of the Yangebup community have complained that chemical odours affect them for days after treatment.
“When the council treats the lake the smell of chemicals gets to you for days,” says nearby A1 Boxes and Parts business owner Val Fernandes.
But he notes the midge are worse.
“Sometimes when I work late and go home around 8 or 9pm and drive down Tamara drive, the whole street is swarming with midge,” he says.
“I think the problem is improving though.”
The process is unique in a number of ways that might have wider use across other water bodies in Perth.
Mr Beaton says the active solar pump could be applied to other filter systems dotted around the city.
There are other benefits too.
“It works during the day and then it stops pumping during the night,” Mr Beaton says.
“This is good because the water can just sit in the basin overnight and release some of its sediment through the water column, allowing for some of the nutrients to be taken up by the plants.”
Mr Beaton notes that the system in its current state would take about 10 to 15 years to filter the whole lake.
“You can probably put a few more pumps on the lake and the filters can be surrounded by picnics and barbeques so kids can go float their paper boats down the running water,” he says.
“It is not just something that you can do to treat nutrients, but it is also adding something the local environment and community can use as a fun place.
“It provides habitat which is another benefit of revegetation and the plants in the system grow so fast, it is just gangbusters.”
UWA Professor and expert in water resources and environmental engineering Anas Ghdouani is a little more critical of the project.
“Local governments are trying to solve a problem that is originating elsewhere and they need a co-ordinated solution,” Professor Ghdouani says.
“If nutrient rich water is flowing from Council A to Council B, and Council B is left with the cleaning bill, how fair is that?
“Instead of this treatment or drains feeding directly into the lake, we should be having urban streams that cleanse the water before it gets to the lake.
“If it gets to the lake and it’s dirty, full of nutrients, you almost need the water to go through a desalinisation plant because it’s wastewater.”
Mr Beaten says Cockburn has engaged in intensive community education programs to help let people know that adding fertilisers to their lawn, “that’s ultimately going to end up in the wetlands”.
“We are inviting the community to be involved in the vegetation process and have started recontouring the drains that are already coming into Yangebup Lake and turning them into living streams,” he says.
“So, basically, turning those drains into nutrient treatment drains like this one.”
Mr Beaton notes that the system is not perfect.
“Sometimes the outflow from the basin is producing more nitrogen and phosphorous than it is consuming, sometimes it produces less,” he says.
“It varies considerably.”
Mr Beaton says the basin might be intercepting nutrient rich groundwater.
“There is still the occasional algae bloom, but the total nitrogen and the total phosphorous levels are starting to fall,” he says.
The lake is owned by the WA Planning Commission.
Photography: Nicholas Phillips