Biological processes have been widely used for the treatment of both domestic and indus-trial wastewaters.In such biological processes,pollutants are converted into pollution-free substances by microorganisms through oxidation-reduction reactions.Thus,how to quan-tify the internal oxidation-reduction properties wastewaters and seek out targeted counter-measures is essential to understand,operate,and optimize biological wastewater treatment systems.So far,no such approach is available yet.In this work,a novel concept of electron neutralization-based evaluation is proposed to describe the internal oxidation-reduction properties of wastewater.Pollutants in wastewater are defined as electron donor substances(EDSs)or electron acceptor substances(EASs),which could give or accept electrons,respec-tively.With such an electron neutralization concept,several parameters,i.e.,electron resid-ual concentration(R),economy-related index(E and Er),and economical evaluation index(Y and Yr),are defined.Then,these parameters are used to evaluate the performance and eco-nomic aspects of currently applied wastewater treatment processes and even optimize sys-tems.Three case studies demonstrate that the proposed concept could be effectively used to reduce wastewater treatment costs,assess energy recovery,and evaluate process perfor-mance.Therefore,a new,simple,and reliable methodology is established to describe the oxidation-reduction properties of wastewater and assess the biological wastewater treat-ment processes.