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.