UFC 3-240-13FN
25 May 2005
Figure 2-8. RO Unit
2-2.7.1
RO Pretreatment. Material that can potentially foul (or plug) the
membrane will interfere with the RO process and must be removed before the water
contacts the membrane. Foulants include suspended and colloidal solids, iron, metal
oxides, scale, and biological materials. It is important to determine a Silt Density Index
(SDI), a measurement of suspended materials in the water, prior to using the water in
the RO unit. Only water determined to be within the manufacturer's acceptable SDI
range should be used. Pretreatment may be required to achieve an acceptable SDI.
2-2.7.1.1
SS Removal. SS can be removed by filtration, usually using a sand filter
followed by a cartridge filter. The addition of filter aids may be necessary to achieve
acceptable filtration. In certain instances, coagulation and clarification may be required
before the filtration step.
2-2.7.1.2
Iron Oxide Removal. Iron oxides are the result of the oxidation of
dissolved ferrous salts or corrosion of the equipment. The first process can be
controlled by aerating the water prior to its being filtered, the second by threshold
treatment with sodium hexametaphosphate before any aeration. For proper operation,
these iron oxides must be removed prior to the water contacting the membrane.
2-2.7.1.3
Scale Prevention. Scale-forming salts are concentrated as a result of the
RO process just as they are during an evaporative process. To prevent scale, the
water's pH is adjusted by adding either acid to produce a pH of between 5.0 and 6.5, or
a scale inhibitor such as sodium hexametaphosphate or any of the phosphonates
(PBTC [2-phosphonobutane-1,2,4-tricarboxylic acid], HEDP [1-hydroxyethylidene 1,1-
diphosphonic acid], or AMP [amino-tri (methylene) phosphonic acid]). The solubility of
scale-forming salts controls the rejection rate of the water that cannot be processed
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