Feed Water Salinity

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. 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…show more content…
Over operating time, the feed pressure is adjusted to compensate for fluctuation of feed water temperature, salinity and permeate flux decline due to fouling or compaction of the membrane. For the purpose of specifying the high pressure pump, it is usually assumed that specific flux of the membrane will decline by about 20% in three years. The pump has to be designed to provide feed pressure corresponding to the initial membrane performance and to compensate for expected flux decline. If the RO system is equipped with centrifugal pump, then the conventional approach is to use an oversized pump and regulate feed pressure by throttling (partially close the feed valve). Today an increasing number of RO system use electric motors with variable speed drives, which enable adjustment of flow and feed pressure of the pump over a wide range with very little loss in efficiency. The variable speed drive reduces unproductive pressure losses which were common in the past. Some RO systems use positive displacement pumps (piston or plunger pumps) as a high pressure process pump. The positive displacements pumps enable regulation of feed pressure at constant pump output, with little change of pump efficiency. Positive displacement pumps are less common in RO systems due to capacity limitation of this pump type, high frequency of required…show more content…
FEED WATER TEMPERATURE: Change in feed water temperature results in the change in the rate of diffusion through the membrane. The change in the permeate flux rate with temperature is described by the following equation: TCF = exp(K*(1/(273+t) - 1/298)) (1) Where TCF is temperature correction factor, K is a constant characteristic for a given membrane material, and t is feed water temperature in degrees Celsius. In this equation, a temperature of 25°C is used as a reference point, with TCF = 1. The change in flux rate with temperature for polyamide membranes is shown in the following graph.
The rate of change is about 3% per degree. Since RO systems are design to operate at constant output, feed pressure is adjusted to compensate for water flux changes when temperature changes. The salt diffusion through the membrane changes with temperature approximately at the same rate as water flux. Because permeate flux is maintained constant, permeate salinity changes accordingly to the temperature

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