Long term effects of complete sludge retention in a membrane bioreactor for municipal wastewater treatment

The experimental work showed the feasibility of operating membrane bioreactors with complete sludge retention for long timeframes without any loss of treatment performance. Flux sustainability was investigated and a critical TMP value independent of the sludge concentration was observed, above which the fouling rate rapidly increased. This same pattern of loss of membrane permeability was evident both during the transient growth phase and during the steady state, suggesting that different causes may have the same effect. From a biological point of view, the experimental data showed that the biomass evolves towards a minimized maintenance organic loading rate, resulting in proportionality between the volumetric loading rate (VLR) and the equilibrium biomass concentration. When designing new plants, this feature allows for defining the biomass concentration based on the VLR.