The correlations among wastewater internal energy, energy consumption and energy recovery/production potentials in wastewater treatment plant: An assessment of the energy balance

Abstract

Given the economic and environmental importance of energy use in wastewater treatment plants (WWTPs), the need to assess the energy balance of WWTPs has become a growing concern. Previous studies have suggested that energy balance or even net energy production may be achieved in WWTPs under specific conditions. However, information regarding the energy consumption and the energy recovery/production potential in WWTPs as a function of the influent characteristics is still very limited. In this paper, by exploring the correlations among wastewater internal energy, energy consumption and energy recovery in WWTPs, a novel net energy consumption (NEC) model was developed for predicting the energy self-sufficiency level of WWTPs. From our results, exponential regression showed a high accuracy in predicting the annual energy consumption, the annual excess sludge production and the bioreactor footprints in WWTPs. Wastewater with more internal energy which is determined by influent chemical oxygen demand (COD) concentration and flow rate, not only leads to higher energy consumption in WWTPs, but also results in an increase in the excess sludge production, bioreactor footprints and wastewater volume. This means that the WWTPs could achieve energy saving or even net energy production by incorporating sludge incineration, photovoltaic (PV) generation and thermal energy recovery. By combing regression analysis with theoretical formula, the annual net energy demand of WWTPs reached −0.187–0.466 kWh·m−3 in the range of wastewater condition studied (the influent COD concentration range of 60–800 mg·L−1 and the flow rate range of 1296–100,000 m3·d−1). The NEC model reveals that the net zero energy consumption may be achieved by integrating the better understandings of wastewater internal energy, energy conversion methods and environmental media energy, which is of value to policy makers for the planning of new WWTPs and provides theoretical support for the selection of available energy recovery methods.