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The deposition of powdered activated carbon (PAC) on an ultrafiltration (UF) membrane surface, a common phenomenon in the PAC/UF hybrid process, may have a pronounced influence on UF operation. In this study, the influencing factors and mechanisms of the deposition of PAC were systematically investigated. The influencing factors included the organic species, membrane material, PAC size, filtration volume, and cleaning water. The experimental results suggested that PAC itself was rarely deposited on the membrane surface, forming a monolayer. However, in the cases when organic matter was heavily deposited on the membrane surface (e.g., humic acid (HA) and sodium alginate (SA) in the presence of 0.5 mmol/L calcium ions), multilayers of PAC and the organic matter formed; thus, the mass of the deposited PAC significantly increased. The deposition of PAC was barely affected by the membrane material, and decreased as the PAC size increased. Physical cleaning with pure water could effectively detach the deposited PAC. The HA adsorbed by the PAC and the membrane hindered the deposition of PAC because this part of the HA decreased the attractive force between the PAC and the membrane. However, the HA rejected by the membrane promoted the deposition of PAC, as evidenced by the fact that the mass of the deposited PAC increased as the mass of the deposited HA increased. The PAC could be "trapped" in the layer of HA when it formed a cake layer with the HA rejected by the membrane.

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