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, say researchers at the Texas Christian University in Fort Worth. 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 fertilizers. 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.
Ray Drenner , a biologist at Texas Christian, 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. Their wastes 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. The tanks are fitted with vertical plastic screens on which periphyton algae, which thrive on nitrogen and phosphorus, grow.