Influence of sludge retention time on biomass characteristics and cleaning requirements in a membrane bioreactor for municipal wastewater treatment

The study is aimed at defining possible correlations between sludge retention time (SRT), biomass growth, biomass activity and membrane cleaning requirements in a submerged membrane bioreactor (MBR). For the experimental campaign, a lab-scale MBR was continuously fed on pre-settled municipal wastewater and operated to obtain complete nitrification. Volumetric loading rate of 1.2 gCOD L-1 d-1 was maintained by fixing the permeate flux and diluting the influent wastewater as required. Experimental runs were performed adopting SRT of 20, 40, 60 and 80 days. Results were integrated with data from a previous experiment with complete sludge retention. A significant correlation between SRT and organic loading rate has been observed. Results of the respirometric measurements suggested that the biomass activity is weakly affected by the sludge age. Membrane cleaning requirements also appear to be scarcely dependent on SRT and on the average membrane cleaning was required about every 2 months. Further investigation of the sludge physical properties indicate that the capillary suction time can be considered as a suitable early warning parameter for the cleaning schedules. The findings allow the conclusion that SRT longer than 40 days could be considered for reliable operation of MBRs for municipal wastewater treatment.