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612 P. Kajitvichyanukul et al.
The advantages of RO over other POU/POE systems are as follows:
l RO is relatively insensitive to flow and total dissolved solids (TDS) level. In addition, it can remove
a variety of ions and metals and very fine suspended particles such as asbestos that may be found in
water. It is also effective in removing some detergents; some taste, color, and odor-producing
chemicals, certain organic contaminants; and some pesticides.
l RO is suitable for small systems with a high degree of seasonal fluctuation in water demand.
l Water treated by RO does not adversely affect people on sodium-restricted diets.
Some of the limitations of RO are as follows:
l Reverse osmosis units are slow and produce more wastewater. A little more than 1 gallon of
potable water is produced every 6 h, and 3–5 gallons of wastewater are generated in that time.
l High pressure (and the associated electrical energy costs) is required to operate the unit.
l High level of pretreatment is required in some cases.
l RO requires routine maintenance.
The main purpose of RO treatment in POU/POE system is generally to reduce the levels of
naturally occurring substances that cause water supplies to be unhealthy or unappealing (foul
tastes, smells, or colors) and the substances that have contaminated the water supply, resulting
in possible adverse health effects or decreased desirability. Before installing the RO and other
treatment units, the water has to be tested to identify the bacteria and level of minerals that are
present. The water test analysis and interpretation will provide information about both naturally
occurring substances and those resulting from human activity. The intended use of the water
(drinking only, drinking and cooking, laundry, or all household uses) will also help in
determining the extent of treatment needed and the type of system to select. However,
treatment of contaminated water supplies should be considered only as a temporary solution.
The best solution is to remove the source(s) of contamination and/or obtain a new water supply.
For the POU/POE system, RO is listed as an SSCT by USEPA for copper, lead, fluoride,
antimony, barium, beryllium, cadmium, chromium, selenium, and thallium. In addition, RO
is also suggested to receive further investigation for its potential application for nitrate
removal. In fact, RO is an effective method for reducing the concentration of total dissolved
solids, including a variety of ions and metals and very fine suspended particles such as
asbestos that may be found in water. It is also effective in removing some detergents; some
taste, color, and odor-producing chemicals; certain organic contaminants; and some pesticides.
List of the impurities that can be reduced by RO is shown in Table 14.5 (30). However,
the rate of reduction of each specific compound will depend on the RO membrane type and
the system’s operating conditions.
Various major contaminants can be removed by RO in POU/POE units. Details and the
requirements in pollutant removal for each contaminant are provided below.
Hardness: Hardness of water is a measure of the amount of minerals, primarily calcium
and magnesium, it contains. Water that contains more than 200 mg/L or 200 parts per million
(ppm) as calcium carbonate (CaCO3), or 12 grains per gallon, is considered to be hard and
may cause plumbing and laundry staining.
Total dissolved solids: The TDS test measures the total amount of dissolved minerals in
water. The solids can be iron, chlorides, sulfates, calcium, or other minerals found on the