Sludge is an inevitable byproduct of wastewater treatment, which, if treated improperly, will extensively pollute the environment. Due to the complex composition, sludge, which contains pathogenic microorganisms, parasitic eggs, toxic heavy metals and refractory substances, causes secondary pollution to water, soil and atmosphere. It also poses a serious threat to the ecological environment, if not applied with harmless treatment. In addition, sludge landfill squeezes the scarce land, resulting in waste of resources. However, sludge is a misplaced resource, not waste. If properly utilized, the sludge can be used as a valuable fuel or feedstock.