Low pressure membrane filtration is playing an increasingly important role in the reclamation of municipal wastewater. However, membrane fouling remains a critical factor affecting the efficiency of the membranes in the filtration of the biologically treated secondary effluent. The potential of ultrasound (US) was investigated for membrane cleaning and feed pre-treatment in the microfiltration of municipal activated sludge-lagoon effluent. Application of US (45 kHz, 107 W) to the fouled membranes for greater than 5 min gave greater cleaning efficiency with flux recovery of over 80% compared with backwashing (74%). The better performance of US was attributed to the cavitational shear forces induced by US which dislodged the cake layer and loosened the materials clogging the membrane pores. When US cleaning was followed by backwashing, flux recovery was enhanced further, this was due to the dislodged foulants being effectively flushed away from the membrane surface and pores by backwashing. However, flux recoveries decreased with successive fouling and cleaning cycles, the decrease being greater for backwashing (64% for US after 5 cycles, three times that for backwashing). These results were consistent with membrane analysis by attenuated total reflectance - Fourier transform infrared (ATR-FTIR) and scanning electron microscopy (SEM). The increased irreversible fouling was due to compaction of the foulants and their increased affinity to the membrane surface and pores during the operation under pressure. When used as feed pre-treatment, sonication led to some dissolution and fragmentation of the particulate matter in the effluent, these smaller particles had a negative impact on permeate flux by causing rapid pore blockage. However, sonication led to decreased irreversible fouling. ATR-FTIR spectra of membranes fouled with sonicated effluent showed that US modified the characteristics of the effluent organic matter (EfOM) by altering the structure/conformation of proteins, which apparently reduced their potential to adsorb to the membranes. US treatment followed by Al3+-based coagulation using aluminium sulphate or polyaluminium chlorohydrate (ACH) achieved a greater increase in permeate flux compared with coagulation alone. This effect was greater at relatively higher turbidity (8.0 NTU) compared with lower turbidity (i.e., 1.0 and 3.6 NTU). This increase was due, at least in part, to the breakdown of the particles to form more nuclei for coagulation. Furthermore, the physico-chemically modified EfOM may have enhanced interaction with the hydrolysed Al species, which led to the formation of a cake layer with lower filtration resistance. These changes influenced the size of the coagulation flocs, increasing the diameter by 20–30%. Flux recovery was greater when US was used before ACH, and decreased only slightly with successive fouling and cleaning cycles. The enhanced cleaning performance may be due to the lower affinity of ACH flocs for the membrane materials. As alum treatment alone achieved good flux recovery, there was not a significant benefit from employing US for mitigating irreversible fouling. This work demonstrated the potential of US for mitigating membrane fouling. Further work on the determination of the detailed mechanisms associated with enhancing coagulation would enable optimisation of the system with regard to process, cost and energy efficiency
