Highly variable flow has to be expected in decentralized greywater treatment and can lead to intermittent operation of the treatment system. However, few studies have addressed the influence of variable flow on the treatment performance of a biological activated carbon filter (BAC). In this study, we investigated the influence of intermittent flow using small-scale BAC columns, which treat greywater as a second treatment step following a membrane bioreactor (MBR). Three operating strategies to respond to variable flow were evaluated. The activated carbon was characterized before and after the experiments in terms of biological activity and sorption capacity. The performance of the BAC filters was assessed based on total organic carbon (TOC) removal, TOC fractions and growth potential. No significant differences were observed between constant flow compared to on-off operation with intermittent flow over the range of tested influent concentrations. Peaks with high TOC during 24 h periods were attenuated by sorption and biological degradation. Adsorbed TOC was released after switching back to normal concentrations for influent concentrations more than 5 times higher than usually observed, the BAC functioned as a temporary sink. In line with these results, the high influent TOC values led to increased biological activity on the filter but did not influence the sorption capacity. The experiments showed that intermittent flow does not negatively impact the performance of a BAC and that there is no need for additional equalization tanks to buffer the variable flow, for example in household-scale greywater treatment.
