Impact of sludge flocs on membrane fouling in membrane bioreactors

Membrane bioreactors (MBR) are widely used for wastewater treatment, but membrane fouling reduces membrane performance and thereby increases the cost for membranes and fouling control.
Large variation in filtration properties measured as flux decline was observed for the different types of sludges. Further, the flux could partly be reestablished after the relaxation period depending on the sludge composition. The results underline that sludge properties are important for membrane fouling and that control of floc properties, as determined by the composition of the microbial communities and the physico-chemical properties, is an efficient method to reduce membrane fouling in the MBR.
High concentration of suspended extracellular substances (EPS) and small particles (up to 10 µm) resulted in pronounced fouling propensity. The membrane fouling resistance was reduced at high concentration of divalent ions such as calcium and iron. Furthermore, it was shown that the ratio between cations and EPS was important for the fouling potential of the sludge. A high ratio between divalent ions and EPS reduced membrane fouling as soluble EPS were adsorbed and bound within the sludge flocs. Strong compact flocs reduced membrane fouling, and more compact and strong flocs were formed if the concentration of divalent ions were high.

Sludge was fractionated by centrifugation providing supernatant with soluble EPS and colloidal particles but without flocs. Filtration test on untreated sludge and supernatant showed that the flux decline was most pronounced for the supernatant indicating that sludge flocs had a positive effect on permeate flux. Mixing sludge flocs and supernatant in different ratios shows that the permeate flux increase with sludge floc concentration. This could be due to the scouring effect of the flocs or the formation of more loose fouling layers on the membranes.
The study showed that sludge with strong compact sludge flocs and low concentration of suspended EPS and colloidal particles gave lower fouling propensity. It is known that the composition of the microbial community to some extent determines floc structure, which was not further determined here. However, presence of strong flocs are usually also supported when the concentration of divalent ions is high, the ionic strength is low and the reactor is well-aerated.