Two-stage reverse osmosis: optimal element configuration and energy savings

Abstract

RO desalination can help to ensure secure water resources now and in the future, but the process remains energy intensive. Improving RO’s energy efficiency is thus an important step towards achieving a sustainable water supply. While innovations in membrane and pump technology are not likely to substantially decrease the energy consumption of the RO process, improved system designs have real potential to bring RO closer to its thermodynamic performance limit. Two-stage systems can substantially lower RO energy consumption.

In a fixed size two-stage reverse osmosis (RO) system with eight membrane elements, the elements can be shared between the two stages in seven distinct element configurations. In this work, we investigate the optimal element configuration (system design) of a two-stage RO system. We isolate the energetic benefits of staging by comparing the energy consumption of a two-stage RO system to that of a single-stage RO system with the same system size and freshwater productivity. The optimal element configuration will place at least half of the elements in the first stage; the exact configuration depends on feed salinity, recovery ratio, and membrane permeability.

Previous studies on the energetic benefits of two-stage RO have not accounted for both the system size and the effects of concentration polarization. We evaluate systems with an average system flux comparable to today's systems and account for frictional losses and the effects of concentration polarization. This results in a more realistic evaluation of the energetic benefits of two-stage RO.

More energy can be saved by adding a stage when the thermodynamic least work of separation is larger. Therefore, energy savings from adding a second stage grow as recovery ratio increases. Significant energy can be saved with high salinity feeds at relatively low recovery ratios. We find that significant energy can be saved with the simplest two-stage RO design, at a system flux similar to today's RO plants and accounting for the effects of concentration polarization.

We perform a brief economic analysis to compare the relative capital expenses to the reduction in specific energy consumption (SEC) associated with a two-stage RO plant. We find that two-stage RO is probably not viable for seawater desalination at today’s typical recovery ratios. If recovery ratios can be pushed up to 60%, two-stage RO could become viable with favorable financing terms and high cost of electricity.