memberResources REVERSE OSMOSIS (RO) After storage, water may next undergo reverse osmosis treatment. Reverse osmosis is used to remove excess dissolved solids and a variety of organic contaminants. Reverse osmosis systems generally reject 90-98% of dissolved salts (ionic contaminants) and 98-100% of dissolved organic contaminants (except small molecules such as THMs) in source water. This is particularly useful with source water high in dissolved solids (800 ppm or greater). To produce purified water, reverse osmosis may be followed by deionisation through a mixed-bed deioniser. Reverse osmosis, or R.O., refers to a technology relying on a high pressure pump and special membrane, called semi-permeable membranes, to reverse the natural phenomenon of osmosis. The following illustrates osmosis and osmotic flow. The skin of a raisin is a natural semi-permeable membrane, e.g., placing a raisin in water causes it to swell. Water passes through the raisin skin from a place of relatively pure water to the inside with its high concentrations of protein and other large, soluble chemicals. The large molecules cannot pass through the skin (membrane), while smaller water molecules can. The force or pressure generated by the water as it passes through a semi-permeable membrane is called osmotic pressure. By applying pressure against the osmotic flow of water, pure water can be forced back through the membrane. This flow is the opposite, or “reverse,” of osmosis. Reverse Osmosis employs synthetic membranes, which are more selective than natural membranes. The synthetic materials used in semi-permeable membranes allow water molecules to pass through but block most ions or organic molecules. The amount of pressure needed to reverse the osmotic flow depends on the concentration (ppm) of contaminants. For example, the osmotic pressure of sea water is about 400 psi; therefore, a pressure of 400 psi must be applied to sea water to force even one drop through a semi-permeable membrane. As a rough guide, osmotic pressure equals about 1 psi per 100 ppm TDS. In an R.O. unit three flows can be identified-feed, concentrate, and permeate. Feed refers to water entering the R.O. unit for treatment. Concentrate, or reject Solution, refers to the concentrated solution of contaminants which is unable to pass through the membrane. Permeate, or recovery, refers to the higher purity water which passes through the membrane. As was noted before, the removal of contaminants is not 100%; a small fraction pass through the membrane and is referred to as “leakage” or “passage.” Types of RO Membranes There are four classes of polymers used to produce synthetic semi-permeable membranes for use in RO: polyamide; thin film composite (TFC); cellulose acetate; and cellulose triacetate.

The thin film composite membrane is constructed of polyamide-type materials. Each type of membrane material has its advantages and applications. However, for bottled water production, the most common membranes are either cellulose-acetate, polyamide or TFC. Table 1 provides a comparison of the operational characteristics of these membranes. Table 1 - Reverse Osmosis Membrane Characteristics Membrane

pH

Chlorine

Biological

Temperature

Material

Stability

Resistance

Resistance

Limits

Cellulose

2-8

≤1.5 ppm

Poor

2-35°C

Polyamide

4-11

≤0.1 ppm

Good

2-35°C

Thin Film

2-11

≤0.1 ppm

Good

2-45°C

4-8

≤ 1.5 ppm

Fair-good

2-30°C

Acetate

Composite Cellulose Triacetate

Cellulose acetate costs less than polyamide-based membranes. But, as noted in Table 1, cellulose-based membranes are susceptible to degradation by bacteria and other micro-organisms. Cellulose acetate has a low chemical resistance and cannot be sued above pH 8. This also limits the ability to clean the membrane, because strong cleaning agents cannot be used. On the other hand, cellulose acetate membranes are more resistant to oxidizers such as chlorine and ozone than are the polyamide-based membranes. The new TFC membranes exhibit good chemical resistance and the highest temperature stability; however, TFC membranes are more prone to fouling and are degraded by oxidizers. Special TFC membranes have been developed to remove certain contaminants, such as nitrates, not readily removed by traditional membranes. RO membranes are usually supplied in a spiral-would element, because other configurations are more susceptible to fouling.

Contaminant Removal Efficiencies by RO Table 2 provides typical percentage removal of inorganic constituents by Reverse Osmosis. Table 2 - Typical Percentage Removal of Selected Inorganics by Reverse Osmosis Contaminant

Percent Removal

Arsenic

99

Barium

95

Bicarbonate

98

Boron

83

Bromide

95

Cadmium

98

Calcium

99

Chloride

95

Chromium

95

Copper

99

Cyanide

82

Fluoride

95

Iron

99+

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Lead

99+

Magnesium

99

Manganese

99+

Mercury

97

Molybdenum

97

Nickel

99+

Nitrate

95

Nitrite

90+

Permanganate (