Effects of process operating conditions on performance and membrane fouling in submerged anaerobic membrane bioreactors (AnMBRs)

Abstract

Submerged anaerobic membrane bioreactor (SAnMBR) technology is becoming an attractive alternative for industrial wastewater treatment as it improves the biological process while allowing for the recovery of energy through biogas production. However, membrane fouling presents one of the main drawbacks of the technology. It is generally believed that operating below the critical flux can reduce the fouling rate. Another drawback hampering the wide-spread application of the SAnMBR technology is its competitors, mainly conventional high-rate anaerobic systems, like up-flow anaerobic sludge blanket (UASB) reactors which are known for their high OLRs (up to 40 kgCOD/m3.d). This Thesis focused on two parts. In the first part, investigated the effects of sparging rates (8.72, 6.0 and 4.38 m3/m2.h) and mixed liquor total solids (MLTS) concentrations (15, 10.6, 7.7 and 5.7 g/L) on the critical flux in a short term study. The study concluded that both MLTS concentration and sparging rate are key factors to be considered for optimization of the SAnMBR. Critical flux increased as the sparging rate increased for all MLTS concentrations, but no further increase was noted above (6.0 m3/m2.h). The relationship between the critical flux and MLTS concentrations of 5.7, 7.7 and 10.6 g/L can be approximated by a linear relationship for all the sparging intensities tested (the critical flux decreased as MLTS concentrations increased), but no further decrease in the critical flux was noted at MLTS concentration of 15.0 g/L. The short term study was followed by a long term study to validate the concept of the critical flux. The SAnMBR was operated at sub-critical flux but membrane fouling occurred (decreased flux and increased TMP) within five weeks operation mainly caused by a gel layer formation. The results suggest that the concept of critical flux is not valid for SAnMBRs. In the second part, a high-rate SAnMBR was developed. The results were stunning as the SAnMBR’s performance surpassed the UASB’s performance. The SAnMBR maintained organic loading rate (OLR) of 39.85 ± 1.14 Kg COD/m3.d for more than 100 days at chemical oxygen demand (COD) removal efficiency of more than 99.7% (with very low effluent COD concentration of 42 ± 17 mg/L) and excellent biogas production (0.39 ± 0.07 L CH4 /g COD removed) and CH4 composition of 66.89% ± 1.52. The results suggest that SAnMBR can compete with UASB and achieved superior effluent quality for system closure or water reuse. In part I, no membrane fouling was developed under stable operation for over 300 days. In part II, gel layer formation was the main mechanism of fouling. Inorganic fouling was more important than organic fouling in both part I and II studies.