A pilot plant study of a polysulfone ultrafiltration (MWCO of 30 kD) tubular membrane process was conducted for the treatment of reservoir water. The membrane separation system was operated in the cross-flow filtration mode at 4.7 m/s and the feed-and-bleed mode for a long term of 4000 h without chemical cleaning and backwashing. The results showed that the behavior of permeate flux of the membrane system operated in the feed-and-bleed mode was similar to that of membrane systems with a periodic backwashing. At the beginning of filtration, bleeding of highly concentrated retentate caused a significant increase in permeate flux by 20%. However, as filtration progressed over time, the permeate flux of the fouled membrane was almost independent of the change in concentration of retained materials by bleeding the retentate. Three distinct stages in permeate flux decline were observed as follows: (1) sharp decrease from 120 to 30 l/m(2)/h in 1250 h due to a rapid build-up of a fouling layer, (2) gradual decrease to 15 l/m(2)/h in 2800 h due to the role of tangential shear induced by cross-flow velocity, and (3) stable permeate flux until 4000 h due to the establishment of a pseudo-steady-state condition. Permeate quality was stable, regardless of concentrating and diluting retentate in each cycle and fouling for a long duration of filtration. Rejection efficiencies for ultraviolet absorbance at 260 nm (UV260) and dissolved organic carbon (DOC) were around 58% and 49%, respectively. The measured turbidity and concentration of suspended solids in bleed water agreed with calculated values from a simple mass valance, while the measured DOC and UV260 of bleed water were much lower than calculated values.