Counter Flow Reverse Osmosis (CFRO) Systems

Around the Osmosis basis, variations of different types of systems have been developed in the market.  The diagram below shows a few common types.

Couterflow Reverse Osmosis (CFRO) is a process which adds a counter flow on the permeate side of the RO membrane, to reduce of the osmotic pressure on the permeate side to reduce the net driving pressure required for the permeate to from the feed/concentrate to the permeate side. Thus it doesn't need to add pressure on the feed pump used in nromal RO System.

At its core, CFRO is a membrane-based, pressure driven brine concentration technol- ogy, which shares many properties with other membrane-based chemical separation technologies, such as RO and FO. Like RO, permeate flows from the side of high concentration and high hydraulic pressure, across the membrane, to the side of low concentration and low hydraulic pressure. However, unlike RO, CFRO employs two feed streams instead of one. The first feed stream, which is referred to as the con- centrate stream, is analogous to the feed stream in an RO system. The concentrate stream is dewatered as permeate flows through the semi-permeable, salt-rejecting membrane, and leaves the module with a reduced mass flow rate and increased con- centration. On the opposite side of the membrane is another saline feed stream, which we call the diluate stream. This stream is diluted as permeate flows through the membrane, and the stream leaves the module with increased mass flow rate and reduced concentration.

CFRO process is suitable to be applied when zero liquid discharge is requred, because the brine side can achieve TDS up to 260,000 mg/L. This process also increase recovery ratio compared to typical Reverse Osmosis process.

The down side of this process, is the complicated control to maintain the flow and pressure balance within the stages.

Although the CFRO process helped to achieve higher recovery ratio and higher concentration on the brine side using high pressure pumps and pressure vessels same as RO systems, the overall energy requirement to produce each cubic meter of permeate water is high. The capital investment is also very high.