In large wastewater treatment plants, the large amounts of excess sludge can be reduced to less than 1 % in a series of successive treatment steps: sludge thickening, dewatering, digestion, drying and incineration.
The key advantage from sludge digestion is the production of biogas which is used in cogeneration units which produce heat and electricity. The heat is used to bring the digesters at operation temperature while the electricity is used as a power source. In Marrakech and Fez, anaerobic digestion of the municipal wastewater treatment sludge generates up to 70 % of the wastewater treatment plants energy needs while reducing the carbon footprint and greenhouse gas emissions.
In both wastewater treatment plants the cogeneration solution meets the Clean Development Mechanism (CDM) requirements of the Kyoto Protocol office.