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(558bm) Effective Methods to Pre-Treat Wastewater Retentate and Reuse the Treated Wastewater Retentate on the Pressure Retarded Osmosis (PRO) Process

Authors: 
Yang, T., National University of Singapore
Wan, C. F., National University of Singapore
Xiong, J., Ngee Ann Polytechnic
Gudipati, C. S., Separation Technologies Applied Research and Translation (START) – NTUitive
Chung, T. S., National University of Singapore
Abstract

Pressure retarded osmosis (PRO) is a promising technology to extract the osmotic energy from salinity gradient. Seawater desalination brine (SWBr) and wastewater retentate (WWRe) are proposed as the draw and feed solutions respectively due to the purpose of reuse of wastewater and their large salinity difference. However, due to the complicated constituents of the WWRe, severe fouling is caused on PRO membranes, which dramatically impedes the development of PRO technology. 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. All of the membranes used in this work were hollow fiber membrane and they were spun in-house. Both NF and LPRO membranes 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. Results indicated that after 6-hour PRO test at 15 bar, the power density of the fouled membrane dropped to 40% of its initial power density while NF-filtrate and LPRO-filtrate reduced the power density drop from 60% to only 10.7% and 7%, respectively. The 1-day FO test in PRO mode showed the same trend in the pattern of power density, which further proved that NF and LPRO pre-treatments were effective in fouling mitigation. 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.