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Monitoring the performance data is one of the most important tasks for keeping a reverse osmosis, (RO) system operating consistently. The main parameters which are tracked are the differential pressure for each stage, the permeate flow rate, the % salt rejection, and the RO unit recovery.

Differential Pressure

For most RO systems in operation, the change in pressure drop, (DP) across the membranes is the leading indicator of fouling.  Typically, the increasing DP occurs before the permeate flow rate is affected. By the time a decline in permeate flow is detected; the membranes should have been cleaned. A 15% or more increase in the DP indicates that cleaning is needed.

The most typical fouling associated with increasing pressure drop is from particulate matter and organics which will show changes in the 1st stage differential before the 2nd stage becomes fouled. Alkaline detergent cleaning is effective on particulate and organic fouling.

Biological fouling from bacteria will usually result in rapid DP increase in a matter of a week or two.  A severely fouled system will have the bacteria throughout the membrane groups resulting in an even pressure drop increase in the stages or arrays.  Alkaline cleaning followed by biocide treatment is recommended for bacteria fouling.

Backend fouling in the last stage or array is an upset condition related to the concentration affect of the water.  This can happen with the loss of antiscalant feed or some contaminant in high concentration beyond the reasonable saturation point. Silica fouling, hardness scaling, and iron fouling may start at the concentrate end of the RO membranes.  Make corrections in pretreatment and use acid cleaner as the first cleaning cycle. 

Permeate Flow Rate

If the RO system has changing temperatures or operates at various pressures, normalizing the permeate rate is done.  A decline of 5-10% in permeate flow rate means that fouling has occurred at or near the surface of the membrane and the fluid dynamics have been disrupted.  It is important to realize that as the net operating pressure declines, the permeate flow rate will decline accordingly.  Certain contaminants such as aluminum are attracted to the membrane surface and can result in loss of permeate flow rate.  Clean the RO membranes with both acid and alkaline cleaners and check feed/exit flow rates for adequate element cross-flow.

% Salt Rejection

In many cases, the RO membranes become fouled to the point of cleaning without any significant change in salt rejection performance.  This is due to the fact that much of the foulant material is inert (such as silt) and does not carry ionic charge.  In cases where the rejection is affected by the contaminants in the water, ionic interference is occurring at the active charged amine sites on the surface of the membrane.  Concentrations of metal ions such as iron, manganese, and copper can film the surface of the membrane and result in reduced rejection.  A 2% change is significant.  Acid cleaning is usually most effective in removing metal and mineral contaminants.

% Recovery

The recovery should be tracked and the concentrate rate adjusted if necessary to maintain the design recovery within limits.  If the system has a declining permeate rate over time, the operating recovery is allowed to be reduced in order to maintain adequate cross-flow rates through all the membrane elements.  Try to maintain 20 GPM for 8” elements and 5 GPM for 4” elements as the minimum concentrate rate.