Reverse osmosis (RO) is a ...

Reverse osmosis (RO) is a process that forces a feedwater stream containing dissolved impurities (salts and organics) through a semipermeable membrane into two separate streams: one of removed solids (concentrate or reject) and one of purified water, producing soft permeate water.

As the feedwater passes through the membrane, the ions and organics are left behind in the reject and the permeate remains with 96 to 98 percent fewer impurities. Permeate recoveries are typically limited to 75 percent because of the limited solubility of the dissolved salts left behind in the reject.

Normalized permeate flow is one of the most sensitive forecasters of trouble in an RO system. Fouling can reduce the permeate flow rate. However, just measuring the permeate flow rate is insufficient because it varies with the feedwater temperature, feed pressure, permeate pressure and feedwater conductivity (total dissolved solids, or TDS). Though all RO membrane manufacturers have their equations for determining the normalized permeate flow , several parameters are needed to develop this complex calculation: temperature, net driving pressure, pressure drop, permeate, feedwater TDS and permeate flow. This calculation corrects for temperature and pressure variations. The calculation, known as normalized permeate flow, adjusts the daily data readings to what they would be if the system were operating at startup pressure at 25°C. This allows for daily comparisons of RO performance. Seasonal feedwater temperature variations can make fouling trends difficult to detect if the normalized permeate flow calculation is not used. For example, RO feedwater derived from surface sources becomes warmer during the springtime. This increase in feedwater temperature affects membrane performance by increasing the permeate flow rate. If the membrane elements are fouling at the same time (fouling causes a decrease in permeate flow rate), it is unlikely that this will be noticed until a change in seasons. At this time, the membrane elements may be severely fouled, with the permeate flow rate severely restricted.

Percentage rejection= (feed Conductivity-Permeate Conductivity)/

                                          Feed Conductivity