Sludge harmless treatment is the key technical problem that requires prompt solutions in China and even the whole world. This paper discusses problems and disadvantages existing in the sludge treatment of China. On the basis of the inventions that utilize the residual heat of the smoke fume to dry the sludge with low temperature, the paper comes up with the harmless solution of dealing with waste by waste, which means recycle the waste. This solution combines the sludge harmless treatment with energy saving and emission reduction, which can further facilitate the development of the city sludge disposal.
The sludge (referred to as municipal sludge) is a kind of sediment that is generated during the purification process of the sewage treatment plant for the municipal sewage and industrial waste water. This harmful solid waste should be disposed in a proper way, otherwise it will cause serious pollutions to the environment.
The municipal sludge contains a great deal of heavy metal elements, which cannot be decomposed by the microorganism and will be accumulated in the body of the organism. Therefore, heavy metal is the major harmful material in the sludge. A 5-year monitoring shows that the heavy metal content in the municipal sludge will change with the time and the region, which will also be affected by the source of the industrial waste water and its proportions.
The heating value and moisture content of the sludge. The municipal sludge has a high heating value because of the organic substances it contains. The heating energy is the only valuable resource that can be recycling in the sludge and is positively correlated to the content of the organic substances. The sludge has about 30%——45% content of the organic substances, with the heating value between 1200-2500 kilocalorie / kilogram. The use of the sludge heating energy depends on its moisture content. Only if the sludge moisture content drops below 30%, its heating energy can be used.
The water contained in the sludge exists in the form of interstitial water, capillary water, absorbed water and bound water. Most proportions of the interstitial water can be removed by concentration process in the treatment plant. Then the mechanical dehydration can get rid of interstitial water and part of the capillary water, which will reduce the moisture content of the sludge to up to 80%. The sludge with 80% moisture content looks like a paste which becomes a total harmful solid waste and requires a complete treatment.
