Scientists sound warning over pollutants in treated sea water

04 December 2017 - 06:24 By Dave Chambers
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WET AND WILDThe Gugulethu swimming pool is one of a few open this summer since water restrictions were implemented in Cape Town.
WET AND WILDThe Gugulethu swimming pool is one of a few open this summer since water restrictions were implemented in Cape Town.
Image: Esa Alexander

Desalinated sea water poses a probable health risk to Capetonians, say scientists from the Western Cape's top universities.

The city's three marine sewage outfalls, which send more than 36million litres of effluent a day into the ocean, contain chemicals that mean "great caution is needed" in the desalination process, which is touted as the answer to Cape Town's water crisis, the environmental scientists warn.

A team led by Professor Leslie Petrik, of the environmental and nano-science group at the University of the Western Cape, writes in the December edition of the SA Journal of Science: "It is probable that the water recovered from desalination may still be contaminated with traces of complex pollutants after the reverse osmosis [desalination] process. This probability represents a public health issue."

Team member Lesley Green said: "None of us would like the results of our lab tests to be what they are in the context of this crisis."

But the City of Cape Town said it was confident that desalination plants in the Cape Town harbour, the V&A Waterfront, Strandfontein and Monwabisi, which are due to start operating in February, would produce safe water.

Xanthea Limberg, Cape Town's mayoral committee member for water and waste services, said desalinated water would have to meet the same standards as dam water.

"The water produced from desalination will be tested daily for conformance with the standard," Limberg said.

"Complex chemical pollutants will not be present in sufficient quantities to place the public at more risk than they would be just living in an urban environment where these chemicals are freely used."

However, Petrik said: "Our drinking water standards are not adequate because they do not [include] maximum regulated levels for the persistent chemical compounds. The city can say the water complies with the regulation yet that would not prove [its] safety."

A CSIR report commissioned by the city concluded a fortnight ago that "no immediate ecological disaster is imminent as a result of the effluent discharge".

Stormwater runoff may be a significant source of faecal-indicator bacteria

But the two-year CSIR study, for which samples were collected between March 2015 and September 2016, was not concerned with the use of sea water for desalination. It agreed with Petrik's finding that faecal contamination from the effluent outfalls might reach the shoreline.

"The uncertainty stems from the fact that stormwater runoff might be a significant source of faecal-indicator bacteria to the shoreline, as is the case in most cities," it said.

"Regardless of the source of the bacteria, their counts in shoreline water samples at many sites were high enough to suggest a significant periodic risk to human recreation in near-shore and shoreline waters."

Petrik's study, conducted with colleagues from the University of Cape Town and Stellenbosch University, said city council tender documents for desalination plants "make the assumption that the tens of millions of litres a day of untreated effluent discharged into the ocean are dispersed out to sea and that in-take sea water to the desalination plants will contain only inorganic salts, and not organic chemical pollutants or microorganisms".

Sea water samples collected at 22 points near Granger Bay in June, July and August proved this assumption untrue, they said.

"The results are consistent with kayakers' claims that on occasion the water is a health risk," he said.

Petrik's team also analysed limpets, mussels, sea urchins, starfish, sea snails and seaweed for evidence of pollutants, and said they found high levels of chemical compounds that provided evidence of long-term exposure.

"The significant increase in their levels in 2017 against our findings in 2015 is noteworthy," they said.

"None of these compounds would normally be found in sea water and should definitely not be present in these marine organisms. With the exception of caffeine, all are manufactured substances.

"[This] is a clear indication of faecal pollution of the shoreline, and that additional chemical substances are likely in the sea water.

"The full impact of chronic exposure to pharmaceuticals, antibiotics and cleaning products on the marine food chain and human health is not fully known, but their ubiquitous presence in trace levels in desalination intake water poses a potential risk to human health."

Limberg said Petrik was correct to highlight the accumulation of new synthetic chemicals as an area that needed further research.

"There is currently no acute health risk posed by the accumulation trace elements of common chemical compounds," she said.

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