MASS AND ENERGY FLOWS IN A MUNICIPAL WASTEWATER TREATMENT PLANT EXPRESSED AS CALORIFIC VALUES

The Municipal Wastewater Treatment Plant of Yverdon-les-Bains, Switzerland is a conventional plant serving 32' 000 population equivalents and composed of primary sedimentation, the activated sludge process and clarification. Waste sludge is treated by anaerobic digestion and the resulting biogas is used for co-generation to produce heat and electricity. Using operational data and calorific value determinations, mass and energy balances were made for the unit processes related to anaerobic digestion. The results show that, in terms of calorific value, half of the potential energy entering the WWTP as dissolved or suspended solids is lost in the centrifuge cake. Moreover, the amount of energy input required to operate the plant is almost the same as the amount of energy recovered from the produced biogas.