A key characteristic of decentralized greywater treatment and reuse is the high variability both in nutrient concentrations and flow. The variability in flow leads to stagnant water in the system and causes short-term fluctuations in the effluent water quality. Automated monitoring tools provide data to understand the underlying mechanisms behind the dynamics and to implement adapted control strategies. In this study, we investigated the fluctuations in a building-scale greywater treatment system comprising a membrane bioreactor followed by a biological activated carbon filter. Short-term dynamics in the effluent of the biological activated carbon filter were monitored with automated flow cytometry and turbidity and the impact of these fluctuations on different hygiene-relevant parameters in the reuse water was evaluated. Continuous biofilm detachment into the stagnant water in the biological activated carbon filter led to temporarily increased turbidity and cell concentrations in the effluent after periods of stagnation. The fluctuations in cell concentrations were consistent with a model assuming higher detachment rates during flow than during times with stagnant water. For this system, total cell concentration and turbidity were strongly correlated. Further, we could show that the observed increase in cell concentration was not related to a deterioration of organic carbon concentration, and the concentration of the opportunistic pathogens P. aeruginosa and L. pneumophila. We showed that turbidity measurements are sensitive to detect changes in the effluent water quality and can be used as a valuable tool to monitor the fluctuations caused by intermittent flow. The intermittent flow did not lead to an increase in opportunistic pathogens and from this study there are no indications that stagnant water in the BAC need to be prevented.
