Could fish clean up sewage works?

A FEW tankfuls of fish could put a stop to the devastating blooms of algae caused when sewage works discharge effluent into rivers and lakes.

Most sewage works remove solids from raw sewage and get rid of organic matter with the help of sludge-dwelling bacteria. But few remove dissolved phosphorus and nitrogen from detergents and fertilisers. When too much of these nutrients get into fresh water, they can trigger explosive growths of algae. All the oxygen in the water is used up when the algae decompose, suffocating aquatic life. Some blooms also generate dangerous toxins.

So Ray Drenner, a biologist at the Texas Christian University in Fort Worth, has developed a waste-water purification system in which algae consume these polluting nutrients before the water is discharged. Fish then nibble away at the algae, incorporating the nutrients into their bodies or excreting them. The faeces sink to the bottom of the tank for regular collection and disposal.

In Drenner's system, which he developed with Laura Rectenwald of Baylor University in Waco, the outflow from a sewage treatment works passes through a series of tanks containing an algae-eating African fish called Tilapia mossambica (pictured above and right). The tanks are fitted with vertical plastic screens on which periphyton algae, which thrive on nitrogen and phosphorus, grow (see Diagram, right).

"The fish spend the whole day grazing the screens, cropping them almost to a fine velvety layer," says Drenner. The layer of algae constantly regrows.

Drenner stresses that the system will work only after sewage has undergone conventional treatment. The fish would serve as a "polishing" step to clean up the water before its final discharge. When fed with water from a sewage works in Waco, the system removed 82 per cent of the phosphorus and 23 per cent of the nitrogen.

In its current form, the system occupies a large area, making it unsuitable for treating water from big treatment

Contact: Claire Bowles
New Scientist

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