Optimization of two-stage seawater reverse osmosis membrane processes with practical design aspects for improving energy efficiency

While single-stage is the general configuration for seawater reverse osmosis (SWRO), the two-stage design can increase the overall recovery of an SWRO system. Due to its high-recovery operation, the specific energy consumption (SEC) of two-stage SWRO is higher than that of single-stage. Thus, the two-stage configuration has not been extensively applied in the current desalination market. In contrast, recent studies have reported that the two-stage design can lower the SEC of SWRO compared to that of single-stage. However, the analyses were biased towards SEC, and the practical design aspects (e.g., permeate quality, water flux, and design ratios) were not systemically considered. Thus, this study examines the applicability of a two-stage SWRO system with a capacity of 100,000 m³/d that employs 1200 pressure vessels (PVs). Two-stage SWRO actually consumed a greater amount of energy than that of single-stage for typical SWRO recovery with the same number of PVs. In contrast, single- and two-stage SWRO produced permeate similar in quality, while the two-stage exhibited superior water-flux distribution along the PVs. Additionally, optimal ratios of permeate flow rate and number of PVs were determined by energy recovery devices type, where the ratio of 1:2 was selected for the reverse osmosis system with a pressure exchanger and 2:1 for that with a Pelton turbine. Considering SEC and other operational aspects, the use of two-stage SWRO was feasible at a 50–70% recovery rate.