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The feed water pressure at this point is about 20 to 30 pounds per square inch. The R/O process requires pressure to force the water through a semi-permeable membrane, i.e. overcome the osmotic pressure. To provide this added pressure requirement, high pressure pumps are located between the micron filters and the R/O units. The pumps are a "manifolded" design. This means that all five of the pumps are available to pressurize the manifold pipe which supplies the feedwater to the R/O units.
The flow to the R/O units is regulated with a flow control valve based on the amount of product water to be produced by the unit. Feed pressures to the units are expected to rise as the membranes age. Initial feed pressures were 100 psi with a five year projected pressure of 120 psi. This pressure is also affected by the degree of salinity of the feedwater. As the raw water quality decreases, or a brackish water supply is further developed, more pressure will be needed to produce the same quantity of water.
The R/O membrane is a basic separation process. When the feedwater enters the membrane it is separated in "Product" and "Concentrate" water. The concentrate is 17% of the feed flow into the unit and is highly concentrated with the ions from the feedwater. The concentrate is then disposed of through a separate pipeline to the City's wastewater treatment plant where it is treated and becomes part of the effluent utilized by the Reclaimed Water System. The R/O membranes pass 83% of the feedwater through the membrane as it removes calcium and magnesium ions in the water. This water is called the "Product" water. It is then piped to the next step of treatment.
R/O flow sheet
Flow Sheet
R/O reaction mechanism
Flow Sheet