Effect of pressure on fouling of microfiltration membranes by activated sludge

AbstractThe effect of pressure on fouling layers formed by diluted activated sludge was studied in real time with ultrasonic reflectometry by fouling microfiltration membranes for 300 min at constant pressure (0.15 bar (15 kPa) and 0.25 bar (25 kPa)), as well as by performing a series of pressure-step experiments in which the pressure was instantaneously increased from 0.15 to 0.25 bar. For the constant pressure experiments, the change in ultrasonic signal amplitude was inversely proportional to the degree of fouling as represented by changes in permeability and the amount of deposited material. This finding was verified by a series of replicated filtration experiments of 15, 30, and 60-min duration at 0.15 bar, which indicated a statistically significant correlation between the degree of fouling (quantified by permeability loss and post-mortem characterization metrics) and ultrasonic amplitude change. Diluted activated sludge filtrations at varying pressures revealed that there is less ultrasonic amplitude reduction at higher pressure where the resistance of the fouling layer is higher. This finding reflects the formation of a hydrated fouling deposit that serves as an impedance matching layer with the water-filled membrane that produces lower reflection at lower pressure. Thus, the fouling layer is thought to be less hydrated and denser at higher pressures, which confirms that the fouling layer is a compressible structure. Given that fouling layer mechanical behavior may well influence membrane filtration performance, it may be possible to improve membrane bioreactor filtration by engineering fouling layer compressibility.