Cleaning of ultrafiltration (UF) membranes is usually performed with addition of powerful oxidizing agents such as aqueous chlorine. The results of cleaning are assessed by flux levels, and complete restoration of initial flux indicates that the membrane is usable for further operation. The cleaning is performed in short time periods using high concentrations of cleaning agent, and that operation often results in partial loss of membrane integrity. Pristine and chlorine-cleaned 20 kDa cellulose acetate membranes were compared to find how the cleaning influences the retention of organic matter. The experiments were performed on secondary effluents of Ashkelon municipal wastewater treatment plant (Southern Israel) with average total organic carbon level of 14 mg/L. The effluents were pretreated by 130 mg/L FeCl3 center dot 6H(2)O and 0.6 g/L powdered activated carbon. The results of current study indicate that despite partial disintegration, flux drop and loss of mechanical strength, proper pretreatment is effective for retention of organic matter by UF membranes. Minor difference in organic content of permeates of pristine and chlorine-treated membranes were observed. The possible pathways of gradual degradation of cellulose surface, with formation of carboxyl, aldehyde and ketone end groups were described. Enlargement of membrane's pore size was followed with streaming potential measurements.