Membrane Fouling During the Microfiltration of Primary and Secondary Wastewater Treatment Plant Effluents

Microfiltration (MF) has emerged as a useful process for concentrating fine particles and clarifying wastewater. The loss of membrane flux due to fouling, however, is one of the main impediments in the economical development of membrane processes for use in water and wastewater treatment. The nature and extent of fouling when used for wastewater is strongly influenced by three factors: biomass characteristics, operation conditions, and membrane characteristics. This study is an extension of a previous research on application of microfiltration in treating combined sewage overflow (CSO) and focused on the fouling mechanism of microfiltration of primary effluent and secondary effluent from municipal wastewater treatment plants, the former of which was used herein to simulate CSO. A 3-step test was designed to fractionate membrane, cake and irreversible fouling resistances in membrane filtration using a stirred dead-end cell system. The results shows that the dominating fouling mechanism for microfiltration of primary / secondary effluents is cake resistance, whereas irreversible fouling dominates the fouling of tap water filtration, indicating a totally different fouling pattern although waste tap water is regarded as one of the major component of municipal wastewater. The suspended solids in both primary and secondary effluents were investigated to be moderately compressible particles by showing a compressibility index n around 0.4-0.6. The irreversible fouling of membrane filtration, as the term suggests, is hard to be removed and results in unrecoverable permeate flux decline in practice although it occupies a comparatively small fraction of the total hydraulic resistances in sewage effluent filtration than deposited cake. A multi-cycle test was conducted to investigate how the irreversible resistance will vary over repeated cycles of membrane filtration and surface cleaning like a long-term practical application. The result of a 17-cycle test showed that the irreversible resistance gradually increased from initial 5% to about 21% of total resistance after six cycles and then basically remained constant at this plateau. This study is valuable to industrial application and design of membrane processes, especially in the application of microfiltration of CSO or primary/secondary effluents as a tertiary treatment method.