Reducing biosolids from a membrane bioreactor system: Assessing the effects on carbon and nutrient removal, membrane fouling and greenhouse gas emissions

This study presents the effects on carbon and nutrient removal, membrane fouling and greenhouse gas (GHG) emissions of an Oxic-Settling-Anaerobic (OSA) - Membrane Bioreactor (MBR) pilot plant fed with real wastewater. The influence of three sludge return internal ratios (IR) was investigated by testing 45, 75 and 100%. The results showed that with the increase of IR, the biological sludge production substantially decreased by 85.8% due to the combination of cell lysis and endogenous metabolism. However, a worsening of ammonia removal efficiencies occurred (from 94.5 % to 84.7 with an IR value of 45 and 100%, respectively) mostly due to the ammonia release caused by cell lysis under anaerobic conditions. The N2O emission factor increased with the rise of IR (namely, from 2.17% to 2.54% of the total influent nitrogen). In addition, a variation of carbon footprint (CF) (0.78, 0.62 and 0.75 kgCO2eq m-3 with 45, 75 and 100% IR, respectively) occurred with IR mainly due to the different energy consumption and carbon oxidation during the three periods. The study's relevance is to address the optimal operating conditions in view of reducing sludge production. In this light, the need to identify a trade-off between the advantages of reducing sludge production and the disadvantages of increasing membrane fouling and GHG emissions must be identified in the future.