Design Parameters Affecting Performance

The performance of membrane elements operating in a reverse osmosis system is affected by the feed water composition, feed temperature, feed pressure, and permeate recovery ratio. Membrane compaction and fouling also affect membrane performance. Performance at a given set of system operating parameters can be calculated from nominal membrane performance at reference test conditions. I. FEED WATER SALINITY Some RO systems experience fluctuation of feed water composition during operation. This may be due to seasonal fluctuation of feed water salinity supplies, or due to intermittent operation of a number of water sources of different salinity. As long as different feed water compositions will not require a change in the system recovery ratio, changing feed water composition will affect only the required feed pressure and permeate water salinity. The chart below shows the change in required feed pressure and projected permeate salinity as a function of feed salinity for a system operating at an average flux rate of 15 gfd and recovery rate of 85%. Calculations were made for two membrane types: ESPA and CPA2. The ESPA membrane has specific flux of 0.24 gfd/psi net, which is twice as high as specific flux of CPA2 membrane. It can be seen that for both membranes feed pressure and permeate salinity increase with feed salinity in a similar way. The rate of increase in permeate salinity is higher than the rate of increase in feed pressure. If the different feed water also contains concentrations of sparingly soluble salts higher than in the design feed water, then the recovery ratio may have to be reduced to avoid the possibility of precipitation of scale from the concentrate stream.