Critical flux aspect of air sparging and backflushing on membrane bioreactors

Membrane bioreactors (MBR) combine biological processes with membrane filtration. The main obstacle to efficient operation remains the deterioration of membrane permeability with time. One possible approach to the fouling issue is sub-critical flux operation. Other options are air sparging and backflushing. This study investigated the impact of air sparging and backflushing on flux enhancement, as well as exploring the relationship between use of these strategies and critical flux. The research was accomplished in pilot plant scale using a 70 L reactor fed with glucose-based synthetic wastewater at temperatures around 20°C and MLSS of approximately 10 g/L. Results showed that air sparging and backflushing each increased the flux in the MBR. Using both strategies at low transmembrane pressures (TMP) yielded the most substantial flux enhancement (factor 2) at sub-critical conditions. Without enhancement, no critical flux could be identified for permeate flow rates of less than 2/3 of the limiting flux, and the flux dropped to 20% of the limiting flux after 8 days in pseudo-steady state. With a combination of air sparging and backflushing at low TMP (38 kPa), it was possible to maintain flux for the same time frame at about 40% of the limiting flux. The fact that no fouling occurred indicates (by definition) sub-critical flux.

1. The limiting flux for the experimental setup was determined to be about 20 L/(m2 h) when no enhancement techniques were applied. 2. It was impossible to identify critical flux for conventional filtration (NON) of the chosen synthetic wastewater at about 2/3 of the limiting flux at various operation conditions. 3. It is assumed that, if a critical flux for conventional filtration (NON) exist, it must be close to the pseudo-steady state flux, which is 4 L/(m² h) after about 8 d. 4. Stepwise pressure increase for NON-enhanced filtration of synthetic wastewater can not overcome the flux decline at MLSS of ca. 10 g/L and CFV of ca. 1.3 m/s. The permeability decreases by about 94% within 5 d. For NON-enhanced constant pressure filtration the permeability decreases by at least 60% within ca. 9 d. With BF enhancement and TMP rise, it is possible to stabilize the flux for roughly over 7 d. However, the perme- Fig. 8. Comparison of fouling number for NON-enhanced flux and enhanced application with a combination of AS + BF. NON-enhanced flux had a substantially higher CFV. AS+BF enhancement featured high or low TMP. MLSS ranged between 8.3–10.8 g/L.