(558bn) Pre-Treatment of Wastewater Retentate to Mitigate Fouling on the Pressure Retarded Osmosis (PRO) Process
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 bar produced a power density of 2.92 W/m2 which was only 40% of its initial power density after a 6-hour 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 W/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 W/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. Both NF and LPRO membranes used in this work were thin-film composite (TFC) hollow fiber membranes and they were fabricated into modules with 1 inch in module diameter. Using the same module design and fabrication method, Separation Technologies Applied Research and Translation (START) has been able to fabricate TFC hollow fiber modules with up to 4 inch in diameter. 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.