Next time you go for a swim in the seas around Hong Kong, or eat seafood caught there, you risk picking up antibiotic-resistant superbugs discharged in sewage, tests by University of Hong Kong engineers show.

A study by Professor Tong Zhang, of the university’s Department of Civil Engineering, found the city’s sewage treatment plants are hot spots of antibiotic resistant bacteria.

“Antibiotics enter the public sewers via urine and faeces, and are transported to wastewater treatment plants,” he explains.

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The plants house huge reactors that act as incubators for growing bacteria, which clean the sewage by feeding on pollutants. Unfortunately, this brew provides the perfect breeding ground for antibiotic resistance. Exposed to high concentrations of antibiotics, resistant bacteria proliferate.

The problem is magnified in countries such as India and China with a lot of drug factories. Wastewater from the factories, which contains high concentrations of antibiotics, mixes with municipal sewage in the public sewers.

In Hong Kong, treated sewage is disinfected with chlorine, meant to kill the bacteria, before it is released into watercourses and the sea. But, according to Tong, this does not completely sterilise the treated sewage and some bacteria survive.

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“In the sea, swimmers are at risk of picking up disease-causing bacteria that have acquired the genes for resistance,” he says. “Others might catch them from infected seafood. If people get sick, the bacteria will multiply in their bodies and be added to the sewage system, starting the process all over again.”

A system that uses membranes to filter antibiotics and bacteria from the water has been implemented in some countries. Tong would like to see this tech­nology deployed to clean the antibiotic-rich wastewater coming out of local hospitals.

“It’s expensive, but the costs of inaction could be much higher,” he says.

The findings come amid growing global concern about the spread of superbugs resistant to most forms of antibiotic - drugs used in humans to treat a wide range of illnesses and prevent infection during childbirth, surgery and organ transplants and also used widely in agriculture. China is the world’s biggest user and producer of antibiotics.