Membrane bioreactor biomass characteristics and microbial yield at very low mean cell residence time

Membrane bioreactors (MBRs) are an exciting and evolving technology that replaces gravity sedimentation with micro- or ultra-filtration. MBRs are typically operated at low mean cell residence times (MCRTs), but there are cases when operating at very low MCRT may be more beneficial. In this study, a laboratory-scale MBR and SBR were operated in parallel and at very low MCRTs (3 d, 2 d, 1 d and 0.5 d) to assess the relative bioreactor performance, biomass characteristics, and microbial yield. This study confirmed that the MBR maintains higher solids levels and better overall effluent quality than conventional bioreactors at all MCRTs tested. The MBR biomass particles were approximately 10 µm, which was significantly smaller than those of the SBR under all operating conditions tested. The MBR sludge typically did not dewater as well as that of the SBR. As the MCRT was decreased, the SBR particle size became smaller and the dewaterability improved, which supports the notion that smaller particles dewater better because there is less bound water present. The MBR sludge was more hydrophobic, which should result in more sorption of organic micro-pollutants like pharmaceutical compounds. These experiments also showed that the MBR biomass true yield was higher than that of the SBR. This study expands the MBR dataset available for very low MCRT operation.