Comparative analysis of energy intensity and carbon emissions in wastewater treatment in USA, Germany, China and South Africa

Currently almost all wastewater treatment plants (WWTPs) require a large amount of energy input to process the influent, mostly as electricity, and the associated carbon emissions are in aggregate significant. In order to achieve carbon neutrality, it is important to understand direct and indirect carbon emissions generated by WWTPs. Here, we focused on electricity use in WWTPs as it is a major source of carbon emissions. Specifically, we compared the electricity intensity and associated carbon emissions of WWTPs in four countries: the USA, Germany, China, and South Africa. We found that 100% energy self-sufficient WWTPs are feasible by a combination of increased energy efficiency and energy harvesting from the wastewater. Carbon emissions of WWTPs depend strongly on the electricity fuel mix, wastewater treatment technologies, treatment capacity, and influent and effluent water quality. A few WWTPs operating in developed countries (USA and Germany) have already achieved almost 100% (or higher) electricity self-sufficiency through energy efficiency and harvesting biogas and electricity. In comparison with Germany, WWTPs in the USA are more heterogeneous and the range of unit carbon emission intensity is much wider. In some areas where the organic content in wastewater is lower and less biogas is produced, it is still possible to achieve energy self-sufficiency by using thermal energy from wastewater. Industrial wastewater in China in general consumes more electricity and the carbon intensity of electricity is also higher, resulting in much higher unit carbon emissions as compared with other countries. In megacities such as Shanghai, larger capacity of centralized WWTPs can decrease the unit carbon emissions significantly. These findings provide a global perspective on the state of WWTPs and are helpful to improve the understanding, designing and operating of WWTPs from the perspective of achieving carbon neutrality.