Pii: S0043-1354(01)00063-x

One of the most common problems encountered in water treatment applications of membranes is fouling. Natural organic matter (NOM) represents a particularly problematic foulant. Membranes may be fouled by relatively hydrophilic and/or hydrophobic NOM components, depending on NOM characteristics, membrane properties, and operating conditions. To maximize flux recovery for an NOMfouled ultrafiltration membrane (NTR 7410), chemical cleaning and hydraulic rinsing with a relatively high cross-flow velocity were investigated as cleaning strategies. The modification of the membrane surface with either an anionic or a cationic surfactant was also evaluated to minimize membrane fouling and to enhance NOM rejection. Foulants from a hydrophobic NOM source (Orange County ground water (OCGW)) were cleaned more effectively in terms of permeate flux by acid and caustic cleanings than foulants from a relatively hydrophilic NOM source (Horsetooth surface water (HT-SW)). An anionic surfactant (sodium dodecyl sulfate (SDS)) was not effective as a cleaning agent for foulants from either hydrophobic or hydrophilic NOM sources. High ionic strength cleaning with 0.1M NaCl was comparatively effective in providing flux recovery for NOM-fouled membranes compared to other chemical cleaning agents. Increased cross-flow velocity and longer cleaning time influenced the efficiency of caustic cleaning, but not high ionic strength cleaning. The membrane was successfully modified only with the cationic surfactant; however, enhanced NOM rejection was accompanied by a significant flux reduction. # 2001 Published by Elsevier Science Ltd. Key words}cleaning strategies, NOM-fouled UF membrane, cleaning agents, the membrane surface modification INTRODUCTION In recent years, pressure-driven membrane processes have emerged as treatment alternatives for the removal of natural organic matter (NOM) (Hong and Elimelech, 1997; Cho et al., 1999). Despite the strong potential of membranes in water treatment, one of the common problems encountered in applications is membrane fouling. Membrane fouling is caused by pore blocking due to foulants such as organics, minerals, colloids, microbial contaminants, and particles on the membrane surface, or by cake formation (Song, 1998). These membrane foulants cause a deterioration in membrane performance indicated by flux decline. NOM components associated with membrane foulants have been found to be composed of hydrophobic acids, hydrophobic neutrals, and hydrophilic neutrals/bases through characterization of NOM-fouled membranes by contact angle, zeta potential, and attenuated total reflectionFourier transform infrared (ATR-FTIR) spectra measurements, as well as molecular weight (MW) distribution and XAD-8/4 resin fractionation of extracted NOM foulants (Cho et al., 1998). Besides flux decline, fouled membranes may exhibit different NOM rejection behavior due to foulants on the membrane surface and, even after cleaning, some foulants can still remain to reduce permeability as well as change the surface charge and hydrophobicity of the membrane surface. The surface of a fouled membrane can potentially be recovered by cleaning agents, in terms of hydrophobicity based on contact angle as well as (negative) surface charge based on zeta potential, resulting in changes of NOM rejection and flux-decline trends after cleaning (Zhu and Nyström, 1998). Many approaches have been studied to minimize membrane fouling; these include pretreatment of feed 1