Effect of solid retention time on sludge filterability and biomass activity: Long-term experiment on a pilot-scale membrane bioreactor treating municipal wastewater

A membrane bioreactor (MBR) pilot-plant fed by municipal wastewater, was continuously operated for 450 d at different solid retention times (SRT: 25, 50 and 75 d) with the aim of identifying possible relations between transmembrane pressure (TMP) evolution and a set of parameters describing physico-chemical sludge characteristics such as capillary suction time (CST), soluble microbial products (SMP), extracellular polymeric substances, critical flux (Jc), oxygen diffusion, biomass kinetic and stoichiometric coefficients (measured by respirometric techniques). Data analysis confirms that MBRs can operate with high SRT ensuring good treatment performances and, from the biological point of view, no effective loss of biodegradation activities. During the study changes in sludge filterability properties for the different operating conditions were observed. Among the different microbial products that characterise the colloidal materials of the mixed liquor, only the carbohydrate fraction of the SMP was correlated with CST and Jc and showed a clear dependence on SRTs. However, the same information can be obtained by a much simpler analysis such as the chemical oxygen demand (COD) fine fraction (defined as the difference between the mixed liquor soluble COD and the effluent COD). TMP trend showed the typical evolution proposed in the local flux theory. Jc and CST trends can be used to predict sludge filterability. Higher sludge fouling propensity was observed at operating conditions when the highest sludge activity was also observed, suggesting a direct relation between fouling and microbial metabolism. Moreover, the addition of the anoxic process seemed to deteriorate the sludge filterability.

► A MBR was continuously operated at different solid retention times for 450 d.
► SRT effects on sludge filterability and biomass activity were evaluated.
► The carbohydrate fraction of the SMP was correlated with CST and Jc.
► Jc and CST trends can be used to predict sludge filterability.
► The addition of the anoxic process seemed to deteriorate the sludge filterability.