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.