Development of a batch reverse osmosis (RO) system for high-recovery Prof. Phillip Davies University of Birmingham, Great Britain

Scarcity of freshwater in drylands necessitates use of alternative resources such as brackish groundwater. However, processes to desalinate and upgrade groundwater are typically energy intensive and expensive when it comes to providing the large quantities of water needed for irrigation.  And excessive extraction of groundwater may cause depletion and salinisation of aquifers. Here we report on a batch reverse osmosis (RO) system that aims to achieve high-efficiency, high-recovery desalination of groundwater. High recovery is important to conserve groundwater and to minimize discharge of waste brine. Whereas high recovery has conventionally been achieved using multistage RO systems, batch RO uses just one stage. After reviewing briefly recent developments in batch RO technology, we introduce the system under development at University of Birmingham. It uses a free piston arrangement and just three valves to operate cyclically in two phases:  pressurisation then purge-refill. Theoretically batch RO could achieve ideal minimum energy consumption in desalination. Non-ideal correction factors describe the departure from ideal performance, giving a simple mathematical model of the system. The model has been used to design batch RO systems using only algebraic (not differential) equations implemented in a spreadsheet tool. Five prototype batch RO systems have been constructed in Birmingham, UK. The most recent uses a single 8-inch spiral wound membrane module to give an output of about 10 m 3 per day. Preliminary experiments results will be reported at the conference, and there will be a discussion comparing the system to continuous- flow and other types of batch or semi-batch RO system.