Process Variant for the Treatment of Anaerobic Digester Effluent with a Membrane Bioreactor

The present study investigated treatment of anaerobic digester effluent with a membrane bioreactor (MBR). Nitrogen removal rates of 98% and remaining ammonium concentrations below 1 mg/L were obtained. The content of recalcitrant chemical oxygen demand (COD) represents an obstacle for direct discharge quality. Hence, particle removal via microfiltration or ultrafiltration for subsequent nanofiltration or reverse osmosis cannot be avoided. To maintain the activated sludge concentration at a constant level, ranging between 8 and 9 g/L, the amount of surplus sludge to be withdrawn from the MBR had to equate to the amount of influent into the reactor at a hydraulic retention time of 12 days. Therefore, the application of a classical MBR system with submerged membranes was not appropriate. A new approach consisted in using the surplus sludge and the supernatant after settling as a source material for filtration experiments. Although the settling properties of the surplus sludge were not excellent, the sludge was reduced to half of its volume after 24 h of settling. At the same time, however, increased ammonium concentrations of 5.1 mg/L were observed. Due to the low flux of 13L/(m(2)h) obtained with submerged membranes, the application of external ceramic microfiltration and ultrafiltration modules was investigated. This resulted in much higher flux rates, up to 170 L/(m(2)h). In this context, the study showed that the cross flow velocity is the essential parameter to guarantee high flux rates, whereas the transmembrane pressure did not contribute to an increased permeate flux once the value exceeded 0.5 bar.