Membrane and Desalination Technologies - TCE Moodle Website

624 P. Kajitvichyanukul et al.
water produced decreases 1–2% for every decrease in degree below the standard temperature
of 77 F. An RO system supplied with well water at a temperature of 60 F produces only three
quarters of the volume it would produce at 77 F(30). Increased feed water temperature also
results in lower salt rejection or higher salt passage due to a higher diffusion rate for salt
through the membrane.
As described earlier about the mechanism of RO process, the major factor controlling the
efficiency in contaminant removal is water pressure. RO can work properly if there is enough
water pressure to overcome the osmotic pressure. Moreover, the RO system must work
against back pressure created in the storage tank as it fills with water and compresses the
air in the tank. The net water pressure at the RO membrane can be calculated by subtracting
back pressure and osmotic pressure from the feed line pressure. If the net water pressure at the
membrane is lower than 15 psi, treated water production is less efficient and contaminant
rejection rates are lower (30).
Water pressure is also a function of the water flux across the membrane. Water pressure
increases in direct relationship to increases in water flux across the membrane. In addition,
increased water pressure results in increased salt rejection. If the water pressure is not enough,
auxiliary pumps or booster pumps can be added to the RO system to raise and maintain the
pressure. Consequently, the quality and quantity of water produced can be achieved. Highquality
RO systems have valves that shut off the flow whenever storage tank pressure reaches
two-thirds of the feed pressure; at this point, low net water pressure can result in low rejection
rates.
3.5.3. Rejection Rate
Most reverse osmosis systems operate in the 75–80% recovery efficiency range. For
example, this means that a unit operating at 75% recovery efficiency will produce 75 gallons
of pure treated water and 25 gallons of wastewater from 100 gallons of raw feed water.
To determine the efficiency of RO unit in contaminant removal, the term rejection percentage
(% rejection) is always used. The rejection percentage is the percentage of a particular
contaminant that does not move through, or is rejected by, the membrane. RO membranes are
rated for their ability to reject compounds from contaminated water. A rejection rate is
calculated for each specific ion or contaminant as well as for reduction of TDS. The rejection
percentage for nitrate can be as high as 90%. However, the initial concentration of contaminants
entering the process has to be considered to determine the efficiency of the RO process
(see above, Types and initial concentrations of the contaminants in feed water). It is important
to know the specific requirements for water quality when selecting the RO system for drinking
and cooking purposes. For example, high rejection rates are essential when high nitrates or
lead concentrations in the water must be brought below the EPA maximum contaminant or
action levels (30). Moreover, when water contains more than one contaminant, the rejection
rate for each contaminant may be reduced or one of the contaminants may be reduced in
preference to the other contaminant. For example, cases have been reported where water
supplies containing either high TDS levels or high sulfates in combination with nitrates show
no decrease in nitrates after treatment.