Assessment of causes of irreversible fouling in powdered activated carbon/ultrafiltration membrane (PAC/UF) systems

Powdered Activated Carbon (PAC) has been successfully used in conjunction with membrane ultrafiltration (UF) to reduce taste, odour, colour and other concerns caused by organic material present in raw drinking water sources. PAC addition also typically significantly reduces the extent of fouling in hybrid PAC/UF systems. However, in some cases, PAC addition can have a significant negative effect by increasing irreversible membrane fouling. The present study was developed to assess the cause of irreversible fouling in a system for which PAC addition had a negative effect. The first part of the study evaluated if irreversible fouling could potentially be due to the breakdown of PAC particles in a PAC/UF membrane system. Particle size analysis of the virgin PAC suggested that pore plugging was unlikely because the PAC was too large. However, when exposed to relatively high shear conditions typical of UF systems, the size of the PAC was observed to decrease to a range comparable to that of the size of the pores in the UF membranes used. However results from the analysis of a bench scale PAC/UF system suggested that irreversible fouling was not solely due to the direct plugging of membrane. The decrease in the permeability for the irreversibly fouled system with PAC was observed to be linear over time, suggesting that irreversible fouling was possible due to the formation of a cake layer by PAC. This was confirmed with FESEM imaging. The second part of the study aimed to characterize the foulants in the cake layer on PAC/UF hollow fibres. FESEM and SEM-EDX were performed to obtain insight on the characteristics of the membrane surface of the hollow fibres. Sonication and a novel solubilisation technique were used to analyze the foulants in/on the membrane surface of the hollow fibres. Organic and inorganic material extracted from the fouled membrane suggested that these have great influence on the PAC cake layer formation on membrane fibres. The results from the study indicate that PAC does not cause irreversible membrane fouling in PAC/UF systems by itself, but it may facilitate the absorption of organic and inorganic material causing irreversible fouling.Applied Science, Faculty ofCivil Engineering, Department ofGraduat