(378c) Water Purification with Closed Circuit Reverse Osmosis

The demand for clean water for
industrial, agricultural and domestic use is increasing around the world. Reverse
osmosis (RO) is an important method for water purification and desalination. Advances
in membrane performance have increased permeate recovery, improved reliability
and reduced energy consumption, however, broader application of RO has been
limited, in part, by its energy requirements, costs, risks of fouling and
scaling and operational challenges posed by source water changes. An improved
technology that further increases permeate recovery, lowers costs, lowers energy
consumption and increases the reliability and flexibility of RO would help advance
the beneficial and efficient uses of locally-produced, sustainable water
sources.

Closed circuit (CC) RO is being
applied for water purification and desalination in commercial installations in several
international locations. CC RO uses standard equipment and membranes in a batch-like
configuration that operates at a lower average feed pressure than conventional
RO, thereby reducing energy consumption. The process exposes membranes to
varying feed compositions as the system concentrates and dilutes and provides
high cross flow across short membrane arrays. These conditions improve membrane
performance by improving flux distribution and enhancing cross flow. The
process also has the potential to prevent fouling and scaling and disrupting
bio-film formation and scale precipitation which, in turn, may reduce chemical
use and membrane clean-in-place frequencies. The permeate recovery percentage can
be adjusted at the control panel without modification of system hardware,
giving operators the means to adjust system performance as necessary to cope
with source water changes. A recovery range of 15 to over 97% has been
demonstrated in single-stage units, and energy consumption reduced by 35%.

The objective of this study is to
evaluate CC RO as an alternative water treatment process to conventional RO for
achieving high recovery and brine minimization. The author presents the
technical principles of the CC RO processes and operating data from running plants.
Process design and performance models are developed and validated with the
purpose of adapting CC RO for new applications. This paper will help
stakeholders better understand this emerging technology and the potential general
applicability of CC RO processes in desalination and water reuse.