Serially connected forward osmosis membrane elements of pressure-assisted forward osmosis-reverse osmosis hybrid system: Process performance and economic analysis

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Journal Article
Desalination, 2018, 448 pp. 1 - 12
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© 2018 Elsevier B.V. Due to the improved dilution of draw streams, employing pressure-assisted forward osmosis (PAFO) to the hybrid system of forward osmosis (FO) followed by reverse osmosis (RO) for seawater desalination has been expected to reduce the overall economics. However, replacing FO with PAFO causes an additional energy cost in the seawater dilution step which inevitably leads to a question that PAFO-RO hybrid is truly an economically beneficial option. More importantly, though serial connection of FO elements improves the dilution of initial draw water, this economic benefit is also compensated with the additional membrane. To rationalize its overall performance and economic benefit, thorough performance and economic evaluations were conducted based on actual pilot-scale PAFO operations for serial connection of up to three 8040 FO elements. The results showed the FO-RO hybrid is not an economically feasible option unless a significant unit FO element cost cut-down is guaranteed. Meanwhile, PAFO-RO showed benefits with regards to target RO recovery and unit FO element cost, particularly when two FO elements are serially connected (SE2). It was found that PAFO-RO, indeed, has higher economic potential than FO-RO. A graphical overlapping method suggested in this work can help determine optimal serial configuration and operating conditions of PAFO-RO.
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