Biological activated carbon filter for greywater post-treatment

Abstract

Greywater reuse offers significant water saving potentials. The better the reclaimed water quality, the more flexible the possible water reuse and the higher is the fresh water saving potential. Biological activated carbon (BAC) filters can be used as a post-treatment step to improve the quality of the reclaimed greywater. Building-scale greywater treatment and reuse must be able to cope with highly variable nutrient concentrations and flows, which together with irregular reuse patterns lead to periods of water stagnation in the treatment system. In this thesis, we investigated the influence of intermittent flow on: i) biodegradation and adsorption capacity in the BAC filter, ii) the overall total organic carbon (TOC) removal in the BAC filter, and iii) fluctuations in the quality of the produced water. Further, this thesis aims at improving the understanding of the relative importance of sorption and biodegradation for long-term TOC removal in BAC filters and how these processes complement each other, for example to buffer high influent TOC concentrations. In the chapters two to four, the main results from this thesis are presented. In the second chapter, we studied the influence of intermittent flow on TOC removal in lab-scale BAC filters. There was no significant difference in the performance of BAC filters receiving constant or intermittent flow in terms of removal of overall TOC, TOC weight fractions, and bacterial growth potential. Furthermore, no significant differences between the biological activity and the sorption capacity of BAC filters with constant and intermittent flow were observed. Peaks in influent TOC concentrations were attenuated by a combination of biodegradation and sorption as a temporary sink. In chapter three, we monitored short-term fluctuations in the effluent of a building-scale BAC filter with automated online flow cytometry and with a turbidity sensor. Temporarily increased cell concentrations and turbidity in the effluent water after periods of stagnation can be well explained with continuous biofilm detachment in the BAC filter. The observed increase in cell concentrations was neither related to a deterioration of TOC concentration nor to hygiene-relevant parameters such as the concentration of the opportunistic pathogens L. pneumophila and P. aeruginosa. A strong correlation between turbidity and cell concentrations showed that a turbidity sensor is a valuable tool to monitor fluctuations in the water quality after the BAC filter. In chapter four, we studied the long-term TOC removal over more than 900 days of operation of the building-scale BAC filter used as greywater post-treatment after a membrane bioreactor. The TOC removal performance depended on the operational time of the BAC filter, the influent TOC concentration, and for the upper part of the ii BAC filter also on the empty bed contact time. A carbon mass balance over the entire period showed similar importance of biodegradation and sorption on the overall TOC removal. Biodegradation was the main removal mechanism in the upper part of the BAC filter, while remaining sorption buffered high influent TOC concentrations in the lower part. The BAC filter required minimum maintenance in the form of backwashing after more than 800 days of operation for long-term stable TOC removal. Overall, we did not observe negative effects of intermittent flow on the operation and the effluent hygiene-relevant water quality parameters of the BAC filter. Therefore, we do not see a need to buffer intermittent flow. A generous filter design with low average filtration rates and long average empty bed contact times is important to provide low effluent TOC values despite highly variable flow and influent TOC concentrations. Long-term stable TOC removal performance with minimum maintenance is possible due to both biodegradation and sorption in the BAC filter. In conclusion, the BAC filter as post-treatment for greywater after a membrane bioreactor is a low-cost treatment step, which is simple to operate and improves the quality of the reclaimed water.