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 LÀ1 dÀ1 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.

Correlations between SRT, biomass growth, biomass activity and membrane cleaning requirements where evaluated by comparing the results of five different experimental runs. Common and constant values of volumetric loading rate and fluxes were always maintained and the SRTs were varied between 20 days and complete retention. Results showed a good correlation between the organic loading rate and the SRT. A minimum OLR value of 0.06 gCOD gVSSÀ1 dÀ1 was approached for SRT tending to complete retention, and this should correspond to maintenance metabolism. When very low SRT were explored, the system did not allow for OLR higher than about 0.5 gCOD gVSSÀ1 dÀ1. This correlation between SRT and OLR can be very useful in designing and operating MBRs, especially if a general applicability could be demonstrated. Respirometric measurements indicated that the decrease of activity expected for increasing sludge ages was observed to be not relevant, with halved values found when passing from SRT ¼ 20 days to complete retention. The study of membrane cleaning requirements has shown that the steady state cleaning frequency was scarcely dependent on the sludge age and about one membrane cleaning every 2 months was required on average. Investigation on the physical properties of the sludge indicated that the capillary suction time can be a suitable early warning parameter for the cleaning schedule, especially during growth and transient phases when the fouling rate seems to be higher. Further investigations are required to confirm the validity of the proposed trends under different operational conditions (hydraulic and volumetric loading rates, nitrification–denitrification, etc.). This extended study, together with a suitable modeling approach, could be very helpful in design and operation of MBRs for municipal wastewater treatment.