As Chinese cities face mounting dumps of sewage sludge, an expert has found a new way to make the toxic material into bricks and cement at low cost.

A professor with the Environmental and Biological Geochemistry Institute of the Zhejiang University in east China's Zhejiang Province, Weng Huanxin, has developed a method of transferring sludge into bricks and cement.

Sludge is threatening to besiege more than 700 cities in China. Beijing [Images] alone produces 1,000 tonnes of sludge a day, Shanghai 700 tonnes and Shenzhen 300 tonnes, Xinhua news agency reported.

It is estimated that China's sludge amount will rise at an annual rate of 10 to 15 per cent in the following years.

Beijing is expected to discharge over two million tonnes of sewage every day in 2008, which is likely to yield more than 2,000 tons of sludge if all the sewage is treated.

Most of the sludge is just piled up in open air or landfilled, which occupies too much land and may cause pollution. Direct burning will produce toxic fumes.

According to the new technology, sludge can be dried at a low temperature, under which toxic components are permanently fixed and won't volatilise any more. It is then made into hard bean-sized granules.

The granules, mixed with clays, can be burnt into lightweight bricks. Since each granule contains 1,500 kilocalories of heat, they themselves can contribute to burning and help save fuels.

Meanwhile, small holes emerging inside the granules after burning will reduce the weight of bricks and enhance their resistance against pressure.