Effect of sludge age on the performance of a membrane bioreactor: influence on nutrient and metals removal

In this paper the effect of different SRT on the performances of an ultrafiltration pilot MBR operating with real wastewaters was tested to analyse both the ability of the system to remove nutrients and micropollutants and the possible decrease in waste sludge production. Increasing MLSS from 4 to 9 g/l reduced biomass production by 84% and increasing MLSS from 9 g/l to 17 g/l reduced biomass production by 75%. The progressive sludge mineralization was clear since the VSS decreases from 75% TSS to 52% TSS. The industrial fraction of the influent affected the denitrification in all the three periods. The effluent quality increased only when passing from short SRT to long SRT. Ag, Cd and Sn removal was >99% in all the runs. Cu removal was 72–89%. Hg removal was >90% while Pb had varying behavior due to its inconstant presence in the influent (50–65%). B and Se seemed not to be efficiently retained by the biomass in both experimental conditions (0–28% and 0–31%, respectively). Arsenic was a major concern (33–37%). The next step of the research will focus on the possibility of enhancing As removal by looking at alternative technologies to integrate with the UF process.

The effect of different sludge ages on the performances of an UF-hollow fiber pilot-scale MBR was tested. The following evidence was found: C increasing the reactor biomass concentration reduced biomass production by up to 84% together with the decrease of MLVSS fraction to 52%TSS; C the partial stabilization of the activated sludge when applying very high SRT leads to a low content of the volatile matter and a decrease in the biogas production. Therefore, it seems sensible to operate to a biomass concentration <10 g/l so to produce a not stabilized sludge and exploits the benefits originated from the application of the anaerobic digestion process in terms of energy production. However, major efforts to comprehend this problem need to be done in future work; C the reduction of biomass viability did not affect nitrification kinetics when passing from Run 1 to Run 2: AUR increased from a minimum of 0.13 to 2.51 mgNH3N/gVSSh. A decrease was observed in Run 3 with an average AUR value of 1.15 mgNH3N/gVSSh but without affecting the final effluent quality; C on the basis of nitrogen mass balances 190 d SRT seemed the most efficient operational condition since NUR reached a value of 0.74 mg/gVSSh while in the other two periods only 0.2 mg/gVSSh was achieved; C the effluent quality increased only when passing from short SRT to long SRT; on the contrary, a slight quality decrease was observed when moving to very long SRT; C metals removal increased with SRT enhancement for Al, Ag, Ba, Cd, Co, Fe, Hg, Mn, Ni, V and Zn; As, B, Cu, Pb and Se removal generally decreased with longer sludge ages; As was a major concern since its removal ratio was 33–37% and its concentration exceeded the standard limits.