Influence of intermittent flow on removal of organics in a biological activated carbon filter (BAC) used as post-treatment for greywater

Abstract

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 in 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.

Biological activated carbon filter for greywater post-treatment: Long-term TOC removal with adsorption and biodegradation

“…One specific characteristic of the system in this study is the intermittent flow leading to times with stagnant water. Hess et al., (2020) showed that intermittent flow does not significantly influence removal performance. However, the water source can make an important difference because of the varying composition of the organic carbon ( Liang et al., 2007 ).…”

“…This fits with the observation reported here that the varying EBCT had a significant influence on removal performance in the upper parts of the filter but not across the overall filter, for which the EBCT was always sufficiently long. One characteristic of decentralized greywater treatment is its highly variable flow ( Hess et al., 2020 ): treatment steps therefore cannot be dimensioned for one specific flow. If good effluent water quality needs to be ensured at all times, a generous BAC filter design with low average filtration rate and long average EBCT is therefore needed to be able to deal with variable flow and thus variable EBCTs.…”

“…Our assumption for calculating the equilibrium load on the GAC neglects slower sorption into smaller pores and assumes that it is mainly the surface and the larger pores of the GAC that are relevant to TOC removal ( Erdem et al., 2020 ). Previous research showed that the assumption of constant equilibrium loading is feasible despite variable influent concentration ( Hess et al., 2020 ).…”

“…The biomass was quantified with the method described in ( Velten et al., 2007 ) and as reported previously ( Hess et al., 2020 ). In short, 1 g wet weight of GAC particles was transferred in a muffled glass vial with 10 mL of 0.2 µm filtered (Millex-GP, Merck Milipore Ltd) tap water.…”

Stagnation leads to short-term fluctuations in the effluent water quality of biofilters: A problem for greywater reuse?

“…One promising technology for buffering influent variability in biodegradable and adsorbable compounds is the biological activated carbon (BAC) filter. Highly variable flow and stagnation have been shown to have no negative impact on total organic carbon (TOC) removal ( Hess et al. 2020 , McKie et al.…”