Comparing the effects of pre-deposited and pre-mixed powdered activated carbons on algal fouling during ultrafiltration

This investigation compared the effects of pre-deposited and pre-mixed powdered activated carbons (PACs) in submerged ultrafiltration membrane reactors on the distribution of extracellular organic matter (EOM) and its contribution to algal fouling during the filtration of algae-rich water. Experimental results indicate that pre-mixed PAC mitigated algal fouling, whereas pre-deposited PAC exhibited the opposite effect. The porous fouling layer in the pre-mixed case favored the retention of rejected EOM in the concentrated solution owing to a high backward migration rate. Conversely, the dense fouling layer in the pre-deposited case impeded the backward diffusion of EOM, which caused the majority of the rejected EOM to accumulate on the surface and at the bottom of the fouling layer. This difference in EOM distribution led to different cleaning efficiencies. Physical cleaning easily removed both algal cells and pre-mixed PAC from the membrane surfaces, whereas physical and chemical cleanings were both ineffective for pre-deposited PAC, owing to the stronger PAC-EOM attractive force and the NaClO-induced change in the chemical bonds of the PAC and even the membranes. Moreover, decreased algae growth rates were observed in the reactors with PAC as a result of changes in the illumination and the cake layer structure, which could further intensify the contrast between the effects of pre-deposited and pre-mixed PACs on fouling.