Redox potential as a method to evaluate the performance of retention soil filters treating combined sewer overflows.

Retention soil filters (RSFs) protect water bodies from pollutant loads originating from combined sewer overflows (CSOs) by filtering the wastewater through a filter layer having a depth of 0.75 to 1 m. The microbiological processes in the filter material are influenced by the redox potential (Eh). This potential is a strong indicator of the prevailing environmental conditions and the possible type of microbial activity. Previous investigations of filter bodies have been confined to constructed wetlands (CWs) with regular intermittent wastewater inflow. Compared to CWs, RSFs are characterized by higher oxygen availability due to alternating operating and dry periods. This study aimed to determine the Eh in RSFs and investigate its influence on the removal efficiency for different substances. We established a conceptual model for the standard Eh curve following a loading event, and the variations to this standard in two depths and between treatments. Correlations were determined with a canonical correlation analysis between the pollutant removal of COD, ammonium, phosphorous, E. coli, somatic coliphages and diclofenac and the Eh. Although the removal efficiency is influenced by several additional operating factors such as the preceding dry period, filter age and the respective inflow concentrations, our results show that the Eh is an adequate approach to assess the removal efficiency of RSFs for these substances.