This paper investigates the impacts of different components in backwash water on ultrafiltration (UF) fouling control. Natural surface water was used as feed water, different backwash waters with different Ca2+ and Na+ concentrations were prepared by dosing CaCl2 and NaCl into demineralized water. Furthermore, UF permeate containing mainly natural organic matter (NOM) with minimal cations (Ca2+ and Na+) was produced by dialysis and used for backwashing as well. Thus, the efficiency of different backwash waters on UF fouling control was evaluated. Results show that the presence of both divalent (Ca2+) and monovalent cations (Na+) in backwash water reduces the fouling control efficiency. Since the negative charges of UF membranes and NOM compounds are screened by the cations during filtration, NOM can easily deposit on the surface of the UF membrane, causing fouling. When the mono- and divalent cations are absent in the backwash water, the charge-screening effect around the negatively charged UF membrane and NOM is reduced, increasing the repulsion force between them. in addition to the charge-screening effect, the absence of calcium in backwash water can also reduce the calcium-bridging effect between the membrane and NOM, increasing the fouling control efficiency of the backwash. Measurements of the streaming potential indicate that backwashing with demineralized water can maintain the negative charge of the membrane. Organic compounds in the backwash water do not influence the fouling control efficiency of the backwash. (C) 2009 Elsevier B.V. All rights reserved.