Pre-depositing versus mixing powdered activated carbons for gravity-driven membrane systems during treated domestic wastewater filtration: Permeability stabilization and removal performance

Abstract

Pre-deposition of powdered activated carbons (PACs) is an effective method to improve the permeate quality of gravity-driven membrane (GDM) system but always with a limited or even negative effect on its fouling behavior. PAC dosing strategies play dominant roles on the fouling of microfiltration and ultrafiltration membranes under constant flux mode, owing to the alteration of PAC-membrane interactions. Hence, how to further increase the membrane flux in the PAC-GDM system needs more investigation. In this study, the effects of PACs on the fouling by treated domestic wastewater were evaluated during GDM filtration using three different PAC dosing strategies: pre-depositing PACs onto membrane surfaces, and mixing PACs with bulk feeds in reactors totally before filtration and mixing PACs with bulk feeds in reactors periodically during filtration. PACs in three dosing modes all improved the removal of organics, but showed conflict effects on the fouling. Unlike mixed PACs increasing the final flux by more than two times than that obtained in the control system, pre-deposited PACs showed a dramatically negative effect on the flux development with a final value decreased by 75.0%. As demonstrated by the scanning electron microscopy images of biofilm structures and the contents of dissolved organic carbon and adenosine triphosphate in reactors, pre-deposited PAC layer aggravated the accumulation of organics and microorganisms with the formation of a stratified and compact structure, which hindered their back-transport to the bulk water. However, mixed PACs induced the formation of a uniform and porous biofilm, which favored the retained organics and microorganisms migrating back into concentrated waters, especially for pre-mixed PACs. This study links PAC dosing strategies, physical structures of biofilms and spatial distributions of organics and microorganisms, with practical relevance for the hydraulic performance of GDM systems, hopefully leading to a wider application of PAC-GDM systems in wastewater treatment.