Reverse Osmosis systems trouble shooting

Over the past thirty years Reverse Osmosis (R.O.) technology has become a standard water desalination technique. RO is now applied in a wide variety of industries for the production of pure and low saline process water. RO is also widely used for the desalination of seawater and brackish water for the production of potable water. Reverse osmosis systems can typically remove up to 99% of total dissolved solids.

The principles of RO technology is removing dissolved salts by pressurizing the raw feedwater through semi permeable membrane to divide the feed stream into product water (low saline) stream which pass through the membrane and concentrate (reject) water stream usually let to drain.



A properly designed and operated RO system, including appropriate pretreatment processes and equipment such as media filters, clarifiers, and chemical dosing systems, will provide a reliable source of purified water for many years. However, some factors can negatively affect the RO system performance, productivity and salt rejection capability.

The performance declines can result from problems or changes arising in one or more of three major areas:

1- Mechanical failures / system mistakes such as pump or valve failures, improper system recovery, improper pretreatment equipment, etc.

2- Chemical changes in the RO feedwater.

3- Microbiological changes in the RO feed water.



As a result of one or more of these occurrences, the RO system may experience a loss of salt rejection and/or productivity.

Some performance losses are irreversible while others can easily be remedied if the causes are diagnosed accurately and appropriate action is taken.



MONITORING REQUIRED FOR DIAGNOSIS



In order to properly diagnose the symptoms or causes for RO performance changes, it is very necessary to routinely collect relevant system operation data for pretreatment, RO unit and posttreatment equipment.



Upon start up of a new RO system, a data log sheet should be established to determine baseline performance. Data sheets serve to alert to the need for maintenance and/or replacement for some items. Collecting data shall require test kits and calibrated hand-held instruments, such as pH meter and TDS meter.



The most important RO performance variables that should be considered are:



1- Product water flow rate.

2- Differential pressure across membrane pressure vessels. (i.e. pressure difference between feed pressure and reject pressure)

3- Percent of salt rejection.



Percent of salt rejection can be determined by the following equation:

% Salt rejection = [ ((CF+CR)/2) -CP] X 100

[ (CF+CR)/2]

% Salt passage = 100 - (%salt rejection)

Where,

CF = Feed TDS

CR = Reject TDS

CP = Permeate TDS



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