Combined Forward Osmosis and Membrane Distillation System for Wastewater Treatment and Reuse Open Access
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Sustainable solutions for clean, affordable water are becoming increasingly important due to water scarcity problem in different parts of the world. Membrane processes can present an effective approach to address this issue. Recently developed membrane technologies such as forward osmosis (FO) and membrane distillation (MD) have demonstrated many advantages over traditional pressure driven processes. This research evaluated the performance of a combined forward osmosis (FO) and membrane distillation (MD) process for sustainable wastewater treatment and reuse.Three bench-scale membrane modules were setup in the lab, one forward osmosis (FO) system, one membrane distillation (MD) system and one combined FO-MD system. Water flux was characterized by various operational parameters. FO membrane flux increased with increasing draw solution concentration and temperature. The MD membrane flux was not significantly affected by the draw solution concentration; however, draw solution temperature was the predominant factor. The FO-MD process demonstrated more than 99.9% rejection of combined chemical components when treating ammonium, COD, arsenic and synthetic wastewater. FO alone was not able to reject arsenic efficiently and was supported by MD to achieve more than 99.9% rejection. Secondary effluent from Blue Plains Advanced Wastewater Treatment Plant was used in the FO-MD system, and produced high quality permeate.The fouling and cleaning behavior of FO and MD membranes were investigated for treating domestic wastewater. Results showed that flux decline caused by organic foulants in wastewater was up to 88% recoverable for FO membrane by cleaning with tap water. For MD membrane, almost no flux recovery was achieved with tap water. Long-term (10 d) tests showed consistent performance of FO membrane by rejecting the contaminants. However, organic foulants reduced the hydrophobicity of the MD membrane, caused wetting problem and allowed contaminants to pass through. Deteriorating water quality of the Chesapeake Bay has imposed strict regulations of nutrient discharge for the wastewater treatment plants. The liquid extract from solids handling processes, also known as sidestream, can contribute 15 to 30 percent of the total nutrient load. To evaluate the performance of FO-MD system for sidestream treatment, a batch mesophilic anaerobic digester was constructed in the lab. The FO-MD system was able to achieve almost 100% rejection of solids and acetic acid, and more than 98% rejection of NH3-N from the feed solution. The high rejection of NH3-N was mainly achieved due to the FO process. The solids in the feed solution contribute to fouling problem in FO. However, 76% flux recovery was achieved by cleaning with tap water. FO membrane also demonstrated excellent performance with 33% more water recovery, when cleaned for 15 min in every 24 h. Water flux model for the combined FO-MD system was developed and validated with the lab data. The model helped understand the inherent flux balancing mechanism of the combined FO-MD process, and to quantify the fouling effect in the FO process.Overall, the combined FO-MD process demonstrated high rejection efficiency and low fouling potential; thus can be an effective solution for sidestream treatment to achieve low nutrient discharge limits.
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