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 OF process.