Effect of sample filtration on the quality of monitoring data reported for organic compounds during wastewater treatment

Accurate quantification of organic wastewater compounds (OWCs) is essential for assessing their removal efficiency in wastewater treatment plants (WWTPs) and for calculating discharge rates into effluent-receiving surface waters. In this study, we undertook a theoretical evaluation of the effect of sorption and sample filtration on data quality. Filtration of samples, while commonly practiced, may preclude a potentially significant fraction of chemical mass from both chemical measurements and mass flow analyses for WWTPs. Sorption theory dictates that analyte losses from sample filtration are notable for hydrophobic organic compounds (HOCs) featuring a pH-dependent logarithmically transformed organic carbon-water distribution coefficient (log D_OC) of ≥3.0. Among a total of 33 organic wastewater compounds considered, the extent of sorption to filterable materials ranged from 22% for bisphenol A diglycidyl ether (BADGE) to 99% for di-(2-ethylhexyl)phthalate (DEHP). Sample filtration also was demonstrated to have a profound impact on the outcome of chemical fate and behavior studies. When the chemical mass residing on filterable particulates was considered, the concentration spread (range) doubled between maximum and minimum concentrations reported for raw wastewater. Furthermore, removal efficiencies of WWTPs calculated for HOCs increased by as much as 62% just by changing the method of accounting. We conclude that some of the data spread reported in the literature concerning chemical mass loadings, contaminant concentrations in raw sewage, and removal efficiencies of similarly designed WWTPs is driven not by actual differences in sewage composition, geographic locations and treatment units but by sample processing protocols and the method of mass accounting.