Low-Concentration Ozonation as a Feed Pretreatment Strategy to Reduce Organic Fouling in Pressure-Retarded Osmosis

Feed water pretreatment presents a significant issue in salinity-gradient energy production by pressure-retarded osmosis (PRO). Conventional pretreatment methods require an energy input comparable to the energy output of PRO, thus narrowing the feasibility window for power production. In this work, we investigate continuous low-dosage ozonation of the feedwater as a strategy to reduce organic membrane fouling in PRO. For this, we run laboratory experiments using humic acid, alginic acid, and natural surface water as foulants. For alginic acid and surface water, ozone at a concentration of 0.1 mg L–1 substantially reduced membrane fouling and recovered the water flux of the fresh membrane. For humic acid, some residual fouling remained. Tests showed that the ozonation of humic acid leads to low-molecular-weight residues that might explain the persistent fouling. To assess the feasibility of the method, we tested the ozone compatibility of the membrane by means of Fourier transform infrared spectroscopy (FTIR) and estimated the power requirement for ozone production. FTIR showed some oxidation of the membrane material, but the membrane performance remained unaltered. The power requirement for ozonating the feed at 0.1 mg L–1 is estimated to be 0.01 kWh/m3.