Deposition of extracellular polymeric substances (EPS) and microtopographical changes on membrane surfaces during intermittent filtration conditions

Extracellular polymeric substances (EPS) play a significant role in modifying surface characteristics, eventually creating conditions suitable for bacterial attachment. The purpose of this study was to investigate the fouling potential of the EPS present in the effluent of an aerobic membrane bioreactor during intermittent filtration conditions. The aerobic rotational membrane system (ARMS) is a novel compact reactor which is designed to convert ammonia nitrogen in concentrated wastewater to nitrates with high conversion rates. The effluent from the reactor contains significant amounts of dissolved substrates which include EPS produced by the biofilm and salts. A series of cross-flow filtration tests were conducted with intermittent once a day short filtration periods using SEPA CFII membrane test cell and an RO membrane. The intermittent tests runs were conducted for overall filter use times of 1, 2, 4, and 6 days. Progression of the microtopographical changes, amount of EPS accumulation, and flux characteristics were evaluated. The images taken by atomic force microscopy (AFM) showed a layer of soft deposits forming over a strong sublayer firmly covering the membrane surface within a short time. The sublayer consisted of distinct modular units which were firmly attached to each other and to the membrane surface. The amount of EPS deposited on the membranes increased with use time and the membranes became significantly hydrophilic. The membrane flux declined gradually after each daily intermission until the 5th ± 1 day, then a small increase in flux was observed. The flux increase may be due to dislodging of some of the deposited material from the membrane surface due to shearing during cross-flow filtration conditions.