Wessex Water finished a six- week trial of a fertiliser made from dried sewage sludge last week, and pronounced that the problem of toxic heavy metals such as zinc, cadmium and lead is not as great as feared. Gareth Jones, director of science and quality at Wessex Water, said the fertiliser, which looks like grey coffee granules, is a safe alternative to artificial fertilisers.
'There is no pollution, it is odour-free and is rich in phosphates and nitrogen. The levels of heavy metals contained in the granules are lower than those found in many other fertilisers such as fish meal and dried blood,' he said.
Previous research had indicated that sewage sludge, which is derived from domestic and industrial waste, is rich in heavy metals which can persist in the soil for centuries. But Wessex Water said heavy metal in its fertiliser was well within legal limits.
The Wessex Water granules are produced by drying out fermented sludge in a tumbling drum that is infused with hot air. The sludge is heated to about 90C, which kills most harmful bacteria, viruses and human parasites in the waste. Methane from the fermenting sludge is used as heating fuel. Keith Fitzgerald, environmental regulation manager at Wessex Water, said the resulting fertiliser was worth about pounds 22 a ton to farmers, although initially it was being offered free.
Field trials conducted by the government Agricultural Development and Advisory Service at a farm near Chippenham, Wiltshire have shown the granules can perform as well as artificial fertilisers, Mr Fitzgerald said. However, nitrogen will have to be added if farmers intend to use the granules alone for longer periods, he said.
All the water companies are looking at methods of dealing with sewage sludge because of a ban on dumping at sea that comes into effect in 1998. Wessex said it could end sea dumping by the end of this year. Many other companies, however, intend to incinerate the sludge on land.Reuse content