Tech breakthrough for toxic dam
New hope for Pretoria and other strained and parched municipalities
ITTED against a device with the powerful battery of an F16 fighter jet, a CAT scan motor and the ability to distribute thousands of litres of water every day, toxic blue-green algae that once plagued a Pretoria dam has had little chance to thrive.
For Leanne Coetzee, who runs the water purification works at Rietvlei Dam, proof of the steady success of the SolarBee project is in the return of clean, algae-free water to the dam, which supplies 6 percent of Pretoria’s drinking water.
“It used to be horrible here and it used to stink, but not any more,” says Coetzee, as the 15 Solarbees – one is down for repairs – orbit like water satellites in the dam.
The solar-powered machines, a product of US technology, circulate water in Rietvlei, disturbing the habitat of dangerous cyanobacteria, until recently so prolific the dam was ranked among the most eutrophic dams in SA.
For decades, a worsening load of pollutants and excess nutrients, particularly phosphates and nitrates, entered the system from the dam’s urbanised upper catchment.
Each Solarbee pumps about 40 000 litres of water a minute, sucking it in and pushing it out in a stretch of 12ha. And that’s the trick.
“One thing we’ve never been able to do before is disturb the habitat of the cyanobacteria in a dam,” says Coetzee, of the City of Tshwane’s scientific services division. “If you come home and there’s only an inch of water in your house, you won’t feel very comfortable.
“That’s the whole idea of the Solarbees. If we switch off the Solarbees, I think we’ll get a significant (algal) bloom.”
At Rietvlei, algal blooms were first sighted in the 1970s, mainly microcystis, a species of cyanobacteria, or blue-green algae, that fast became the dominant population in the dam. It produces dangerous toxins that can cause liver and brain damage.
Coetzee calls the microcys-
Ptis, which resemble peas and can move themselves up and down, the dam’s “problem child”. “You can’t chlorinate and get rid of them like with other species of algae,” she says, noting how they release toxins when they perish.
“We’ve seen a change in the algae population since we put in the Solarbees. Now they (the microcystis) have dropped, and we’re getting other algae, a golden-brown algae, but it’s fine because you can whack it with chlorine.”
She shows a section of before and after photographs. The first batch are of the murky pea-soup that the purification works had to battle in 2007. “This is when you’ve got trouble,” she remarks, showing the next batch, the clear, blue water of 2011.
With the roll-out of the Solarbees, there has been a gradual, continuous improvement in the water quality.
“I believe it is working and we now have much more stable water.
“We used to have fish kills and the canoeists complained the water smelt like sewage, which it did. But now they want to buy us another Solarbee.”
Riaan Marais, the manager of the Rietvlei nature reserve, says the Solarbees have worked wonders to reduce the algae load in the dam.
“As soon as the wind blew off the dam, you could smell it… but the problem is we still have the same amounts of pollution entering the system.
“Whatever happens upstream ends up in the dam. If there’s a big sewage spill or industrial spills, it gets concentrated by the dam.”
The Solarbees, he believes, cannot take all the credit for Rietvlei’s recovery, pointing out how for several years now, Rietvlei’s wetland has been the subject of an intensive restoration project run by Working for Wetlands. This has helped improve the quality of the water that flows into the dam.
“This all means that cleaner water ends up in the dam because of the wetland and Leanne can make cleaner water to drink.”
The introduction of online turbidity units on the air filters at the works, as well as the introduction of granular activated carbon, have also played their part, says Coetzee.
Good rainfall helps to dilute pollutants, and the improved wastewater treatment works at Kempton Park have also helped reduce the load of pollutants in the dam.
But some problems remain. A report in 2007 by the Water Research Commission noted how the dam is highly contaminated with endocrine-disrupting compounds arising from pesticides, fertilisers, personal care products, heavy metals, industrial chemicals and pharmaceuticals.
These have caused reproductive damage and sex deformities in the reserve’s animals.
Coetzee points out that this is not unique to Rietvlei.
“Bear in mind that the control group (for the study) couldn’t find a clean site, not even in Antarctica. You can’t just say we have the problem – Antarctica does, Europe does, it’s everywhere.
“I’m not denying we found the compounds and we are working on it. But to run suddenly screaming for the hills won’t help. You must be realistic in the risk.”
She adds that there is talk of Solarbees being rolled out in other eutrophic and hypertrophic dams – 35 percent of SA’S dams are in this blighted condition, including in Loskop and Roodeplat dams – but the inhibitive cost of the technology is likely to stand in the way.
Rietvlei, she suggests, is the largest full-scale applicant of Solarbees in the world, “simple” technology already used in 300 lakes and dams in the US.
It has been somewhat of a revolutionary step, though crediting the success of the Solarbees with the recovery of the Rietvlei Dam does have its critics.
“What we needed is something that could handle the problem now.
“It’s not destined to solve the problem in as much as manage the problem,” shrugs Coetzee.
She is now completing a study for North West University comparing the past 20 years of ecological data with historical data to better understand the influence of the Solarbees on the improved condition of the dam.
“It looks very promising. The Solarbees have helped our water quality, but I need to prove this scientifically.”