Pre-treatment of wastewater retentate to mitigate fouling on the pressure retarded osmosis (PRO) process

Abstract

Abstract Pressure retarded osmosis (PRO) has been widely investigated to harness the osmotic energy from salinity gradient. Seawater desalination brine (SWBr) and wastewater retentate (WWRe) are employed as the draw and feed solutions respectively due to a large salinity difference between them and reuse of wastewater, but the fouling caused by WWRe on PRO membranes is too severe. In order to mitigate fouling on PRO membranes, several pre-treatment methods, such as ultrafiltration (UF), nanofiltration (NF) and low-pressure reverse osmosis (LP-RO), were adopted and compared in the current study. The filtrate from each pre-treatment method was used as the feed solution in the subsequent PRO tests. Results showed that without pre-treatment, the PRO process at 15\u202fbar produced a power density of 2.92\u202fW/m2 which was only 40% of its initial power density after a 6-h test. The UF pre-treatment was not helpful in fouling mitigation, but NF and LP-RO were effective to remove foulants and scalants. Using NF-filtrates, the PRO process could produce a stable power density of 7.3\u202fW/m2. It was a 10.7% drop from the initial power density. Using LP-RO-filtrates, the PRO process could produce a stable power density of 8.4\u202fW/m2, which was only a 7% drop from the initial power density. NF was recommended to be the optimal pre-treatment method in the future study because it had a higher permeability than LP-RO. The major foulants and scalants were also identified by analysing filtrate samples and fouled membranes. Calcium and phosphate ions formed hydroxyapatite which caused severe scaling inside the PRO membrane. Silica, however, might not cause fouling or scaling in this PRO process.