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 works that serve whole towns. To serve the city of Waco, which has a population of about 100 000, would require almost 100 hectares of tanks of fish, Drenner says. To improve on this, he will try out a new system this summer that uses taller tanks, about 3 metres deep. The idea is to reduce the land area needed by increasing the number of fish in each tank. "It's analogous to saving land by building high-rise flats," he says.
Drenner hopes the revamped system will provide a cheap means by which village and farming communities can clean up their sewage, especially in tropical climates where the temperatures better suit the fish. He says it will take two years to find out whether it is practical.
The fish might also be a safe source of food, provided the original sewage is free of contaminants such as pesticides and heavy metals. "They taste excellent," he says. If the water does contain heavy metals, however, the fish would have to be treated as toxic waste and disposed of by burning or burial in landfills.
Specialists in water treatment say space and the low rate of removal are the main obstacles to the system being widely used. "It's fun and provocative, but I wouldn't regard it as practical in most cases, the demand for space being the key point," says Rod Palfrey of the process technology group at Britain's Water Research Centre in Swindon, Wiltshire.
Palfrey says that the most practical and efficient way to remove phosphorus is to precipitate it out with iron or aluminium salts. Another method, being developed in Australia (New Scientist, 15 January, p 16), uses a clay spray to lock up phosphorus in clay particles where algae cannot feed on it.
Author: Andy Coghlan
Source: Environmental Science and Technology (vol 34, p 522)
New Scientist issue 26th February 2000
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