How RO Membranes Are Configured and Arranged in the RO System?

Performance

Many FRP membrane housings with membrane elements are arranged in parallel.

According to production needs, many RO membrane elements can be arranged into single-pass, double-pass or multi-pass. In every pass, membrane elements can be arranged into 1-stage, 2-stage and even multi-stage. In the single-pass process, the feed is filtered only once through the RO membrane elements within a single pressure vessel. In the double-pass process, the feed is filtered twice through RO membrane elements within different pressure vessels and so on. In the water treatment industry, single-pass 2-stage and single-pass 3-stage arrangements are commonly used. In addition to adhering to the membrane supplier's design principles, membrane element selection and arrangement shall also take raw water quality, users' requirements on product water quality, recovery and other factors into consideration.

RO system single-pass three-stage arrangement flow chart

Multi-Stage Arrangement

Multi-stage arrangements are mainly used where a larger water treatment capacity and a higher recovery are desired, including industrial water desalination.

Generally installed behind the high pressure pumps, multi-stage arrangement makes RO membranes in the same pass installed in parallel. In this way, the concentrate from the 1st stage process becomes the feed water to 2nd stage of RO membrane elements, and the concentrate from the 2nd stage process becomes the feed water to 3rd stage of RO membrane elements and so on. In this process, the permeate produced from all stages are drained out directly. As a result, the more stages of RO membranes, the higher recovery of the system. The amount of the concentrate decreases as the stage number increases. To ensure the same flow of the concentrate on the surface of membranes at all stages, the number of FRP membrane housings in parallel can be reduced to equalize the feed flow entering every membrane element, typically, configured in 2:1 array arrangement.

RO system double-pass arrangement flow chart

Multi-Pass Arrangement

Multi-pass arrangements are mainly where higher permeate qualities are desired and are generally installed behind high pressure pumps.

In this case, the permeate from the 1st pass process becomes the feed water to the 2nd pass, and the permeate from the 2nd pass process becomes the feed water to the 3rd pass. In this process, the 2nd pass concentrate returns to the 1st pass as the feed water where the concentrate quality is better than the raw feed water. In this way, high purity fresh water is obtained after multi-pass RO systems.

This process is suitable for:

Occasions where a higher permeate quality is desired;
Occasions where ion exchange cannot be used as post-treatment;
Occasions where a higher bacteria, pyrogen and organic matter removal is desired, for example, water for pharmaceuticals;
Occasions where a higher reliability is required.