A pilot study on pressure retarded osmosis operation and effective cleaning strategies

A SWRO-PRO pilot was built to systematically investigate the osmotic power generation from state-of-the-art thin-film composite (TFC) hollow fiber membrane modules. For performance benchmarking, three feed pairs were examined. They were (1) synthetic brine and tap water, (2) real seawater reverse osmosis brine (referred to as SWBr) and tap water, and (3) real SWBr and wastewater retentate (referred to as WWBr) feed pairs. All feed pairs had the capabilities to produce power densities ranging from 5 W/m² to 5.7 W/m², which were obtained based on draw solution pressure and NaCl concentration of 15 bar and 0.8 M, respectively. These results were equal or more than the required power density of 5 W/m² proposed by Stafkraft. Various cleaning strategies at pilot level were adopted and proven to be effective and feasible to sustain module performance. Online cleaning approaches; namely, backwash, clean-in-place (CIP) and maintenance cleaning (MC) were performed to evaluate the cleaning efficiencies of the fouled membrane modules. The effectiveness of various cleaning agents was also evaluated using (1) tap water, (2) 200 ppm HCl solutions at pH 2.3 and (3) 200 ppm NaOCl solutions at pH 10.5. The results showed that for the case of SWBr and WWBr feed pair, acid maintenance cleaning was able to fully recover the water flux or power density, whereas caustic cleaning was only able to recover the water flux up to 80%. Chemical-free cleanings by backwash and CIP could only recover water flux from 42% to 51%. The introduction of intermittent CIP cleanings at a fixed interval was found to be effective in slowing down the water flux decline, thereby prolonging the operational duration.