Relationships between mixed liquor properties, operating conditions and fouling on two full-scale MBR plants

Two full-scale membrane bioreactors (MBRs) were monitored to investigate the impact of mixed liquor properties and operating conditions on the filtration performance. Large differences were observed between the two MBRs. At the Guéthary MBR, the F:M ratio increased gradually up to a factor of 2.5, but no impact was observed on the filtration process, even though the mixed liquor resistance to filtration increased. However the diluted sludge volume index (DSVI) and the protein concentration in the mixed liquor supernatant were correlated to the filtration performance. In comparison, none of the mixed liquor properties seemed to impact the filtration process at the Grasse MBR, which was subjected to sudden changes in biological and filtration operating conditions: one of the two aeration tanks was taken off line during two months, and the setpoint for coarse bubble air flow rate was changed 3 times. Nevertheless, the mixed liquor properties were affected by the changes in the biological process and the filtration performance responded to changes in the coarse bubble air flow rate. This work clearly demonstrated the strong impact of the operating strategy and the difficulty to generalize the results of fouling studies between full-scale plants, and from lab/pilot scale to full-scale.

Research Highlights
► DSVI may be a relevant of fouling potential (when no metal dosing occurs).
► Strong impact of the operation strategy on MBR fouling (SRT, F:M, coarse bubble air flow rate).
► Difficulty to generalize the results of fouling studies.
► Full-scale MBRs use much lower coarse bubble specific air flow rates than lab/pilot scale MBRs.