This research focused on the combined fouling by humic acid (HA) and powdered activated carbon (PAC) which possibly occurred in the PAC-ultrafiltration (PAC-UF) hybrid process. The membrane fouling and the HA transmission were systematically investigated with HA (Aldrich, 10 mg/L) and PAC (with different sizes and dosages) in the presence and absence of 0.5 mM calcium. Experimental results suggested that PAC and HA exhibited a significant synergistic fouling effect when they formed a fouling cake together. In the absence of calcium, the adsorption of HA on PAC surface increased the repulsive interaction between PAC particles, and thus a fragile cake layer was formed by the PAC-HA mixture. In the presence of calcium, however, a dense cake layer was formed due to particle bridging by HA-Ca-HA associations. The combined fouling effect increased as the PAC dosage increased and the PAC size decreased, both in the presence and absence of calcium, suggesting that the steric effect play an important role in the combined fouling. In the absence of calcium, an unexpected phenomenon that the HA transmission increased in the presence of PAC suggested that in addition to the steric effect, the hindered foulant back diffusion effect was also involved in the combined fouling. (C) 2015 Elsevier B.V. All rights reserved.