Influence of organic loading rate on the performance of ultrafiltration and microfiltration membrane bioreactors at high sludge retention time

This paper presents the results of 220 days of two bench scale submerged membrane bioreactor (MBR) experiments using micro (MF-MBR) and ultrafiltration (UF-MBR) flat sheet membranes. Both systems operated with a volumetric loading rate of between 0.4 and 1.3 kg COD m−3 d−1. Results showed that the growth rate of MLSS increased when the loading rate increased and that results related to removal efficiency rendered values higher than 93% chemical oxygen demand (COD) removal in both MBRs, indicating their suitability for operating at different loading rates. As regards membrane fouling, it was observed that the total number of chemical cleanings performed in the two MBRs was similar. The cleaning frequency decreased when the concentration of mixed liquor suspended solids (MLSS) and organic loading increased in both MBRs. MF membranes worked better at extended periods of operation and high concentrations of MLSS, whereas the UF-MBR maintained lower permeability at the beginning of the experiments at MLSS concentrations lower than 2–3 g/L. Dead-end filtration experiments showed that the highest filtration resistance was found at the highest soluble extracellular polymeric substance (EPS) concentrations, that is, at low sludge ages or low MLSS concentrations, indicating that membrane fouling is attributed to a large extent to the existence of this kind of EPS.

► MBR technology is suitable for operating at different volumetric loading rates.
► The results obtained in the MF-MBR were better than those obtained in the UF-MBR.
► Both MBRs achieved high values of organic matter removal, greater than 90%.
► Membrane cleaning frequency decreased while concentration of SS increased.
► Growth rate of SS at high loading rate was higher than that at medium and low load.