The main limitation of the ultrafiltration (UF) process identified in drinking water treatment is membrane fouling which leads to high capital and operations costs and, in some cases, threatens the viability of the technology. The nature of fouling may be classified as either reversible or irreversible. Reversible fouling, caused by solids accumulation on the membrane surface, is typically remediated by intermittent hydraulic backwashes. In comparison, irreversible fouling requires a cumbersome chemical cleaning of the membrane that reduces the plant efficiency. Adsorption of natural organic matter (NOM) in the membrane pores has been found to be a cause of irreversible fouling. However, UF operating conditions also impact the degree of irreversible fouling. This study examined the impact of several operating parameters on fouling including; flux, concentrate velocity in hollow fibers, backwash frequency, and transmembrane pressure. The results of a pilot study were used to analyze fouling mechanisms other than organics adsorption and raw water quality, during filtration of a surface water. A hydrophilic cellulose derivative membrane and a hydrophobic acrylic polymer membrane were used to conduct these tests. Experiments were conducted to isolate the effect of transmembrane pressure from other variables such as filtrate volume, filtration time, and concentrate velocity. Results demonstrate that hydraulic backwash effectiveness decreased as the transmembrane pressure applied in the previous filtration increased. For instance, backwash efficiency in terms of membrane flux recovery was reduced by 50 percent when the transmembrane pressure was increased from 0.4 bar to 1.4 bar. This paper discusses hypotheses for the explanation of this phenomenon and examines future research needs.