The application of footprints for assessing the sustainability of wastewater treatment plants: A review

Wastewater treatment plants (WWTPs) play an important role within the urban water cycle in preventing the natural water environment and human health from being negatively affected by human activities. However, wastewater treatment processes such as the discharge of effluent and indirect emissions resulting from energy or chemical production, also negatively affect the environment. Footprints have been used to track human influence on the environment through different areas of concern and have been applied in assessing the sustainability of WWTPs. A comprehensive review of footprint assessment for evaluating the wastewater treatment process in WWTPs was investigated in this article. The review showed that the carbon footprint and the water footprint were the two of the most popular footprints for evaluating the sustainability of WWTPs, and other footprint (such as nitrogen and phosphorus footprints) assessment applications were also introduced to assess the eutrophication of water bodies. To promote the application of footprint assessment on WWTP, this article regulated the study goals, frameworks, system boundary, data treatment methods, resultant interpretation process. The pros and cons of footprint assessments were discussed and investigated in detail, several suggestions were come up with for the improvement of footprint assessments. The analyzation of footprint assessments under different WWTPs revealed that wastewater treatment technologies and scales significantly impacted the footprints. Moreover, the research hotspots identified by way of a keyword network diagram showed that researchers tended to propose new indicators and models for the grey water footprint and the carbon footprint respectively, and water-carbon-energy nexus was a promising direction for future studies. Finally, two possible application scenarios, element tracing through modified footprints and management improvement through mathematical optimization, were proposed to optimize the operation strategies of WWTPs. Overall, footprint assessment research for WWTPs is still at the initial stage and needs further exploration and improvement, e.g. introducing new tools, exploring system variation under various scenarios, combining other footprints, and implementing the nexus assessment.