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The integration of one anaerobic reactor in the mainstream (AMSR) of a pre-denitritication-MBR was evaluated with the aim to achieve simultaneous sludge minimization and phosphorous removal. The excess sludge production was reduced by 64% when the AMSR was operated under 8h of hydraulic retention time (HRT). The highest nutrients removal performances referred to organic carbon (98%), nitrogen (90%) and phosphorous (97%) were obtained under 8 h of HRT. In contrast, prolonged anaerobic-endogenous conditions were found to be detrimental for all nutrients removal performances. Similarly, the lowest membrane fouling tendency (FR=0.65•10 11 m-1 d-1) was achieved under 8 h of HRT, whereas it significantly increased under higher HRT. The highest polyphosphate accumulating organisms kinetics were achieved under HRT of 8 h, showing very high exogenous Prelease (46.67 mgPO4-P gVSS-1 h-1) and P-uptake rates (48.6 mgPO4-P gVSS-1 h-1), as well as a not negligible P-release rate under endogenous conditions at low COD/P ratio (≈ 1).

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The main goal of this study was the evaluation of the excess sludge reduction in a MBR for biological nitrogen removal (BNR) through the implementation of the Oxic-Settling-Anaerobic (OSA) process. For this purpose, a MBR pilot plant (42 L volume) was realized according to a pre-denitrification scheme. The whole experimentation was divided into two periods, named Period 1 and Period 2, respectively. In Period 1 the pilot plant was started-up and the excess sludge production was evaluated. In Period 2 the plant configuration was partially modified by inserting an anaerobic reactor into the return activated sludge (RAS) line to realize an OSA configuration. In Period 1, the Y obs resulted equal to 0.39 gVSS g −1 COD removed , in accordance with the reference values for MBR plants reported in the literature (Wang et al. 2013). Similarly, all the kinetic and stoichiometric parameters, for both autotrophic and heterotrophic biomass, resulted in line with those reported in a MBR with a pre-denitrification scheme (Lubello et al. 2009). In contrast, in Period 2 the Yobs showed a significant decrease, reaching a pseudo steady-state value of 0.17 gVSS g −1 COD removed at the end of the experiments, highlighting a reduction of 55% compared to Period 1.