Effects of sludge retention time on the performance of a membrane bioreactor treating municipal sewage

The performance of a bench scale membrane bioreactor (MBR) fed on real municipal wastewater was monitored over the long-term in order to investigate optimal operating conditions in terms of process parameters. The experimental MBR was operated for a total period of 4 years under the same conditions except for the sludge retention time (SRT). Five tests were run after fresh start-ups, and different SRT ranging between 20 days and complete retention were adopted. This paper focuses on the SRT influence on parameters related to sludge filterability, membrane cleaning and biological activity. The average biomass concentration under steady state conditions increased for increasing SRT with a logarithmic function. In order to evaluate the sludge filterability, the capillary suction time (CST) and sludge resistance to filtration (SRF) were monitored over time showing decreasing trends during the transient phase and average steady state values comparable with those reported for conventional activated sludge. The CST and SRF steady state values were minimized for SRT in the range 40-80 days. The viscosity increased with increasing SRT and appeared to be related to the suspended solids content. The membrane cleaning frequency decreased towards 1 cleaning every 2 months as the system approached the steady state. The experimental data suggested the adoption of the CST for early warning of the cleaning needs. The respirometric tests showed that the membrane bioreactor could be operated at the higher SRT without drawbacks on the biodegradation activities. (C) 2007 Elsevier B.V. All rights reserved.