Toward Energy Neutrality in Municipal Wastewater Treatment: A Systematic Analysis of Energy Flow Balance for Different Scenarios

To reduce operational costs and carbon footprints, it is highly desirable to achieve energy neutrality in municipal wastewater treatment plants (WWTPs). We reviewed more than 100 nitrification/denitrification (N/DN)- and anaerobic ammonia oxidation (ANAMMOX)-based wastewater treatment systems. The energy consumption performance of N/DN systems ranged from 0.3 to 4 kWh/kg of COD and from 5 to 15 kWh/kg of N, while those of ANAMMOX-based systems ranged from 1 to 5 kWh/kg of COD and from 0.5 to 1.5 kWh/kg of N. According to an energy balance analysis of typical domestic wastewater (COD = 500 mg/L; TN = 50 mg/L) treatment, the conventional N/DN process consumes an average of 0.5 kWh/m³ (1.78 MJ/m³) more energy than the amount recovered from the digestion and incineration of its sludge. However, if wastewater is pretreated using a chemically enhanced primary treatment or anaerobic treatment (AT), subsequent ANAMMOX-based wastewater treatment systems may realize WWTP energy autarky or even electricity outputs of ≤0.17 kWh/m³. In such a nexus of energy recovery, the biogas generation from the AT or the digestion of sludge would be a more effective way to recover energy than the incineration of dewatered digestates. The combination of early stage COD capture and ANAMMOX is a promising approach to achieving sustainable energy performance in future WWTPs.