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Membrane Technologies for Point-of-Use and Point-of-Entry Applications 623
requirements, (c) the water pressure to meet the supply and rejection rates (this value has to be
considered to determine whether the RO unit will be operated on the normal operating
pressure of a home water system or using a booster pump), (d) the rejection rate or the
percentage of contaminants to be removed by the membrane, and (e) the monitoring or testing
method of the system’s performance. Details of the major factors in operating an RO system
in POU/POE are provided below.
3.5.1. Types and Initial Concentrations of the Contaminants in Feed Water
Types and initial concentrations of the contaminant entering the RO process are the crucial
factors that determine the quality of the purified water. Overall water quality affects the
efficiency of an RO system and its ability to remove specific contaminants. In operating an
RO unit, it is required that the water source entering the RO units must not contain more than
10 grains of hardness (0.2 mg/L of iron and less than 0.25 mg/L of manganese). The TDS
should be about 500 ppm. The higher the TDS, the greater the osmotic pressure and the lower
the recovery rate of treated water (Table 14.6).
Moreover, the initial concentration of contaminants is also affected depending on the
rejection rate. Rejection rates need to be high enough to reduce the contaminant level in the
untreated water to a safe level. For example, if the initial concentration of nitrate is relatively
high as 120 mg/L, 90% of nitrate removal would still leave 10% or 12 mg/L of nitrate in
the treated water. This value is greater than the maximum contaminant level (MCL), which is
10 mg/L for nitrate allowed in drinking water. Thus, the rejection rate of nitrate at 10% from
this RO unit is not an appropriate value for purified water. Water with very high levels of
nitrates (such as 100 mg/L) would remain near or above the health standard levels even after
treatment. However, high concentrations of nitrate levels are not expected in feed water and
indicate unusual problems that require special investigation and handling. The NSF recommends
that special designs be used for RO units where the NO3 N level exceeds 40 mg/L
(nitrates as used in this part refers to nitrate-nitrogen or NO3 N).
3.5.2. Water Pressure and Temperature
The standard at which manufacturers rate RO system performance is 77 F, 60 pounds per
square inch (psi), and TDS at 500 ppm (30). Membrane productivity is very sensitive to
changes in water temperature. As water temperature increases, water flux increases due to the
higher diffusion rate of water through membrane. It is reported that the amount of treated
Table 14.6
Best available technologies (include RO) for radionuclides (40 CFR 141.66(g), 142.65(a))
Contaminant Best available technology
Combined radium-226 and radium-228 Ion exchange, reverse osmosis, lime softening
Uranium Ion exchange, reverse osmosis, lime softening,
coagulation/filtration
Beta particle and photon radioactivity Ion exchange, reverse osmosis