A case-study on the accuracy of mass balances for xenobiotics in full-scale wastewater treatment plants

Removal efficiencies of micropollutants in wastewater treatment plants (WWTPs) are usually evaluated from mass balance calculations using a small number of observations drawn from short sampling campaigns. Since micropollutant loads can vary greatly in both influent and effluent and reactor tanks exhibit specific hydraulic residence times, these short-term approaches are particularly prone to yield erroneous removal values. A detailed investigation of micropollutant transit times at full-scale and on how this affects mass balancing results was still lacking. The present study used hydraulic residence time distributions to scrutinize the match of influent loads to effluent loads of 10 polar micropollutants with different influent dynamics in a full-scale WWTP. Prior hydraulic modeling indicated that a load sampled over one day in the effluent is composed of influent load fractions of five preceding days. Results showed that the error of the mass balance can be reduced with increasing influent sampling duration. The approach presented leads to a more reliable estimation of the removal efficiencies of those micropollutants which can be constantly detected in influents, such as pharmaceuticals, but provides no advantage for pesticides due to their sporadic occurrence. The mismatch between sampled influent and effluent loads was identified as a major error source and an explanation was provided for the occurrence of negative mass balances regularly reported. This study indicates that the accurate determination of global removal values is only feasible in full-scale investigations with sampling durations much longer than 1 day. In any case, the uncertainty of these values needs to be reported when used in removal assessment, model selection or validation.