Pollutant risk to seawater plant
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Faecal contamination and as many as 15 indicator pharmaceuticals, as well as an array of industrial and household chemicals, have been detected close to the site of the City of Cape Town's proposed desalination plant.
The City is planning to desalinate seawater to provide drinking water as a response to its devastating drought.
But a new study led by Professor Leslie Petric of the Environmental and Nano Science (ENS) group at the University of the Western Cape has revealed the high microbial load being discharged into the ocean daily and the “growing chemical pollution risk to the near-shore coastal environment and thus to the desalination plant's intake water”.
The study involved the sampling of seawater and marine organisms collected offshore and in rock pools at 22 different points near Granger Bay, which is near the proposed site and recreational beaches, in June, July and August this year.
“Faecal contamination of the seawater was evident from the presence of indicator gut microbes, including Escherichia coli and Enterococcus bacteria.
“As many as 15 indicator pharmaceuticals, as well as industrial and household chemicals, were found in the seawater and were highly bio-accumulated in marine organisms such as mussels, limpets, sea urchins and starfish.
“These microbes and chemicals indicate the probable presence of pathogens, and many chemicals of emerging concern in the seawater serving as intake water to the proposed desalination plant.”
The research paper, "Desalination and seawater quality at Green Point, Cape Town: A study on the effects of marine sewage outfalls", was published this week in the SA Journal of Science.
The authors included Jo Barnes, Lesley Green, Adeola Abegunde, Melissa Zackon and Cecilia Sanusi from the Universities of Cape Town, Stellenbosch and the Western Cape.
Their research noted “intermittent high levels of microbial pollution” and 15 pharmaceutical and common household chemicals identified and quantified in the background seawater and bio-accumulated in marine organisms.
“These indicator microbes and chemicals point to the probable presence of pathogens, and literally thousands of chemicals of emerging concern in the seawater. Their bio-accumulation potential is demonstrated.
“In respect of proposed desalination, the findings indicate that desalinated seawater must be subjected to treatment protocols, capable of removing both bacterial loads and organic chemical compounds.”
The study, write the researchers, was motivated by the ongoing drought in the Western Cape, which led to the City's proposal to produce drinking water via seawater desalination plants.
“The terms of reference provided in the tender documents make the assumption that the tens of millions of litres a day of untreated sewage effluent discharged into the ocean, via the marine outfalls located around the Peninsula, are dispersed out to sea and that intake seawater to the desalination plants will contain only inorganic salts, and not organic chemical pollutants or micro-organisms.
“However, kayakers, long-distance swimmers and citizen groups have claimed that untreated effluent from the marine outfalls washes back to shore in specific conditions.
"Where positive independent E coli counts have been demonstrated the city has argued that the E coli results are a result of stormwater run-off.”
In the paper, they write how there is growing evidence that certain emerging contaminants could affect human and environmental health.
Persistent organic pollutants include pharmaceutical and personal health-care products such as over-the-counter and prescription drugs and household products such as soaps, detergents, disinfectants, perfumes, dental care products, skin and hair products and surfactants, as well as these compounds’ degradation products.
“Given the diversity of contaminants shown to be ubiquitously present in the intake water in such close proximity to the marine outfall in Green Point, it's probable the water recovered from desalination may still be contaminated with traces of complex pollutants after the reverse osmosis process. This probability represents a public health issue,” the authors state.
Adequate disinfection and monitoring of the efficacy of tertiary treatment to ensure complete decomposition of harmful pharmaceuticals and other chemicals is essential “to ensure the water supplied to the city is not toxic.
“Even if most of the compounds were removed by the reverse osmosis step, they are not destroyed and remain in the brine retentate. Returning these compounds to the sea, as is planned, only to be filtered indefinitely while toxic compounds build up in marine life, is a futile exercise.”
In the long term, they write, it would be technically more efficient and cost-effective to prevent the sewage from entering the ocean in the first place.
“With a rise in the use of chemical compounds on a daily basis, and many thousands of regulated and unregulated emerging contaminants being discharged and detected in the aquatic environment, many of which exceed the recommended reference dose of various regulators, great caution is needed.”
They write how the idea of sending a sewer pipeline out to sea was approved when the volumes of effluent being discharged to the ocean were relatively small, based on the “incorrect assumption that the ‘solution to pollution is dilution’ and at a time when the variety and volume of manufactured and pharmacological compounds impacting the sewage was far lower than the current situation”.
Compact new treatment systems that can treat the sewage to high standards and recover the water before discharge to the ocean can eliminate the need for desalination.
The costs of the chemical and pharmaceutical compound clean-up ought to be borne by the companies producing the substances “as they have not been contributing to the clean-up pollutants in water systems”.
Retailers and consumers of pharmaceuticals and common household chemicals, too, need to review their contribution to the growing pollution of ocean ecologies.
“While the city has vigorously opposed the politics of ‘poo flingers’ such as Andile Lili, who have dumped human waste to force the argument about improved sanitation in Khayelitsha and elsewhere, the city itself is daily depositing a volume of many Olympic-size swimming pools into the ocean.
“One might indeed quip that in terms of the current sewage management infrastructure, ‘Je suis Andile Lili’ (I am Andile Lili).
“The convergence of sanitation activism in seaside suburbs and shack settlements in a time of drought suggests that the City’s water should be understood as one hydrological system, and therefore managed as a single ecology, not via the separation of environment, sanitation and water supply,” they conclude.