Comparison of Configurations for High-Recovery Inland Desalination Systems

Abstract

Desalination of brackish groundwater (BW) is an effective approach to augment water supply, especially for inland regions that are far from seawater resources. Brackish water reverse osmosis (BWRO) desalination is still subject to intensive energy consumption compared to the theoretical minimum energy demand. Here, we review some of the BWRO plants with various system arrangements. We look at how to minimize energy demands, as these contribute considerably to the cost of desalinated water. Different configurations of BWRO system have been compared from the view point of normalized specific energy consumption (SEC). Analysis is made at theoretical limits. The SEC reduction of BWRO can be achieved by (i) increasing number of stages, (ii) using an energy recovery device (ERD), or (iii) operating the BWRO in batch mode or closed circuit mode. Application of more stages not only reduces SEC but also improves water recovery. However, this improvement is less pronounced when the number of stages exceeds four. Alternatively and more favourably, the BWRO system can be operated in Closed Circuit Desalination (CCD) mode and gives a comparative SEC to that of the 3-stage system with a recovery ratio of 80%. A further reduction of about 30% in SEC can be achieved through batch-RO operation. Moreover, the costly ERDs and booster pumps are avoided with both CCD and batch-RO, thus furthering the effectiveness of lowering the costs of these innovative approaches.

A perspective on reverse osmosis water desalination: Quest for sustainability

“…In each cycle, salt buildup in the RO system during the period of complete concentrate recirculation is purged during subsequent period of concentrate discharge. In addition to enhanced operational flexibility, recent theoretical analysis under idealized plug‐flow conditions suggests that for a single stage RO, the approach can potentially be utilized as an alternative to using an energy recovery device . Nevertheless, existing analysis have yet to consider the impact of non‐ideal flow conditions in real RO systems, especially during the concentrate withdrawal period.…”

Self-Powered Desalination of Geothermal Saline Groundwater: Technical Feasibility

“…The SEC of real brackish water RO systems is much higher than that given by Equation (11), and the analogous expressions in Table 3 of reference [23], for multistage systems. This is because of several frictional and other non-essential losses including: electrical losses in pump motor, hydraulic losses in pump, frictional losses in pipes, concentration polarization, pore friction loss etc.…”

“…To minimise the essential losses, a batch RO system would be preferable to a multi-stage continuous flow design. The batch system does not require a separate ERD and is equivalent in performance to an infinite number of stages, even though it may contain only one RO module [23,48]. The batch system does, however, require valves in addition to pumps and these should be selected carefully for low cost and reliable operation.…”

“…Differentiation of Equation (11) shows that the minimum SECsideal = 2Posm occurs at r = 0.5. For those cases where multiple stages or ERD have been used, formulae for SECsideal given by Qiu and Davies (Table 3 of reference [23]) have been used.…”

“…The calculation of osmotic pressure Posm also takes into account the dissociation of the salts present and experimentally determined values of osmotic coefficient, as detailed in the Appendix. The expression for the ideal minimum specific energy consumption of desalination at recovery r, based on the van't Hoff expression, has been given elsewhere as [23]:…”