Working with energy and mass balances: a conceptual framework to understand the limits of municipal wastewater treatment.

At present all municipal waste water treatment plants (WWTPs) are energy consumers. Electrical energy requirements for oxygen transfer are large in secondary biological systems. Nevertheless, from a thermodynamic point of view chemical oxygen demand (COD) is an energy source. Combustion of every kilogram of COD releases 3.86 kWh of energy. In this manuscript some measures are presented, from a conceptual point of view, in order to convert the actual concept of wastewater treatment as an 'energy sink' to an 'energy source' concept. In this sense, electrical self-sufficiency in carbon removal WWTPs could be obtained by increasing the sludge load to the anaerobic sludge digester. Nitrogen removal increases the energy requirements of WWTPs. The use of a combined two-stage biological treatment, using a high loaded first stage for carbon removal and a second stage combined nitrification-anammox process for nitrogen removal in the water line, offers a way to recover self-sufficiency. This is not a proven technology at ambient temperature, but its development offers an opportunity to reduce the energy demand of WWTPs.