The residual slushy matter resulting from municipal and industrial wastewater treatment processes, for instance from the chemical and paper industry. This type of waste must be dealt with in a safe and effective manner especially if it contains hazardous material.
Wastewater sludge initially contains around 70% of water. Before being able to use them in cement kilns they must be thickened (partly dewatered) using natural gas and electricity to dry the sludge. After drying, the sludge still contains around 30% of moisture. Waste incinerators do not require the sludge to be dried beforehand.
Wastewater sludge is one of the five waste streams considered in this assessment next to solvents & waste oils, filter cake from wastewater treatment, paint and ink residues, and fluff.