The current annual output of sludge is up to about 70 million tons, including about 35 million tons of municipal sludge and 35 million tons of industrial sludge. Sludge is harmful, and the actual effective sludge treatment and disposal rate is less than 25%. Therefore, we must promote the sludge treatment and disposal industry and reduce secondary pollution. What are the characteristics of industrial sludge, which accounts for a large proportion?