Techno-economic evaluation of various RO plus PRO and RO plus FO integrated processes

Pressure retarded osmosis (PRO) and forward osmosis (FO) have been proposed to integrate with seawater reverse osmosis (RO) in order to reduce the overall energy consumption of desalination. However, their economic advantages in comparison to conventional RO have not been validated. This study aims to use well developed mass transfer and economic models to investigate the technical and economic feasibilities of four potential RO and PRO/FO hybrid processes. They are (1) RO and open-loop PRO (referred to as RO + oPRO) to harvest the osmotic energy using energy recovery devices (ERDs), (2) RO and closed-loop PRO (RO + cPRO) to harvest the osmotic energy and recycle the pressurized and diluted brine as seawater feed to RO, (3) RO and FO post-dilution (RO + FO) to dilute and recycle the RO brine, and (4) seawater feed pre-dilution by FO before entering RO (FO + RO) to reduce the RO operating pressure. RO + oPRO takes advantage of the osmotic energy and reduces the RO operating expenditure (OpEx), but requires huge additional capital expenditure (CapEx) that renders it uneconomical. RO + cPRO reduces both OpEx and CapEx because it eliminates the need of additional ERDs and downsizes the seawater intake, pretreatment and brine discharge units. RO + FO achieves the same CapEx saving as RO + cPRO but at an increased OpEx. FO + RO dilutes the seawater, reduces the RO operating pressure and significantly lowers OpEx, especially at a high RO recovery. Based on the analysis, we believe that RO + cPRO and FO + RO are the two most economical and promising processes to reduce both OpEx and CapEx of seawater desalination.