Permeate recovery and flux maximization in semibatch reverse osmosis

Abstract

Most reverse osmosis (RO) and nanofiltration (NF) processes operate as once through or plug flow (PF) systems in which permeate recovery, flux and cross-flow are coupled. Such processes are well suited for applications where permeate recovery percentages are limited and where well pretreated source waters of uniform composition are available. Some membrane filtration applications, such as ultrafiltration, operate as batch systems. Batch operations allow independent manipulation of permeate recovery, flux and cross-flow, and are therefore well suited for high recovery and/or problematic source waters. However, the application of batch operation to RO and NF has been limited because of the increase in energy requirements and reduction in permeate quality that can result. In addition, batch systems can require higher capacity components and more membrane elements and housings than comparable continuous or plug flow systems to compensate for downtime between batches. New semibatch or continuous batch processes for RO or NF applications are emerging. These processes provide high adjustable recovery rates, independently adjustable cross-flow and resistance to and even reversal of fouling and scaling. They can consume less energy and require fewer membrane elements than PF systems. High recovery operation reduces both concentrate production and source water pumping and pretreatment requirements. High cross-flow, reduced lead element flux, more even flux distribution and salinity cycling can reduce the effects of fouling and scaling and the associated chemical and cleaning requirements. These features are particularly beneficial for inland desalination, wastewater concentration and water reuse applications. This paper considers process designs and projected membrane performance of semibatch RO systems for brackish and low salinity source waters. The potential advantages of new process design features that can be employed in batch RO but are not practical for conventional RO, such as short membrane arrays and high recovery single stage designs, are evaluated. Particular focus is given to the operating conditions and performance of individual membrane elements in multi-element membrane arrays. Comparisons are made to the performance of conventional PF RO designs. The analysis will help engineers of RO and NF systems to understand the new capabilities, optimization considerations and potential new applications for this emerging process technology.