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    Activated Carbon Filters



Chlorine or chloramines (chlorine combined with ammonia) are the methods of choice for disinfection in virtually all US public water supplies. For disinfection, chlorine is generally added as a gas or liquid (typically as sodium hypochlorite) to produce a free chlorine residual of 0.5 parts per million (ppm) to 2.0 ppm. Chlorine and its related compounds are relatively strong oxidizing agents and have a deleterious effect on many industrial and medical processes or procedures. For example, these compounds, even in very low concentration, can cause rupturing of the red blood cells in patients undergoing hemodialysis. They can produce unwanted by-products or alter or destroy active agents if present in pharmaceutical manufacture and can cause stress cracks in stainless steel. They also adversely affect many water treatment systems; for example, they slowly degrade ion exchange resin and will quickly destroy reverse osmosis membranes.

Removal of Chlorine

Free chlorine and chloramines can be removed in several ways which are described below:

Absorption – Many forms of activated carbon can be used for dechlorination. But, granular (often 12 x 40 mesh size) activated carbon (GAC) is most commonly used in large water treatment filters. Typical design has flows of 2 to 3 gpm/ft3 of bed because free chlorine removal is a function of residence time in contact with the carbon rather than filter surface loading. Chloramines are two to three times more difficult to remove than chlorine alone and require longer time in contact with the activated carbon, referred to as empty bed contact time (EBCT). Activated carbon removes chlorine and related compounds primarily by surface adsorption but also reacts with it and becomes consumed directly. It has been reported that one pound of carbon reacts with 6 pounds of chlorine. If this is an accurate prediction, then water containing 1 ppm of chlorine will contain 8.3 lbs of chlorine per 1,000,000 gallons and will consume 1.4 lbs of carbon.

It should be noted, however, that the minute pores that give GAC its ability to rapidly react with chlorine compounds can become fouled with sediment or large organic molecules, thus reducing the speed of adsorption. This generally governs the replacement frequency for GAC beds. As a practical matter, many users of GAC for dechlorination replace the beds semi annually or annually based on longstanding experience. Where chlorine breakthrough could have serious consequences (e.g., hemodialysis), two or more filters are installed in series and chlorine is monitored between the filters.

When breakthrough occurs, the polishing filter is placed into the primary position (physically or by opening and closing valves) and a fresh filter is placed in the polishing position. GAC has the major disadvantage of serving as a gracious host for bacterial growth once the chlorine is removed in the upper inch or so of the bed. Its pores provide plenty of surface area for bacterial colonization, the organics it filters from the water provide food and, if the filtered water is warm, the log growth rate is markedly increased. Where bacterial contamination is critical (e.g., pharmaceutical or semiconductor), steam or hot water sanitizable filter vessels are used. Downstream treatment such as Ultraviolet disinfection or distillation may be utilized.

Chemical – Chlorine can be eliminated by reduction reactions using sulfites, bisulfites or metabisulfites. Chemical reduction avoids introducing a bacterial breeding ground upstream of the rest of the water treatment system. However, this method introduces or produces ions (e.g., sodium, sulfate, chloride) which place a slightly increased load on downstream treatment such as deionizers. It typically also requires installation of a continuous chlorine or oxidation-reduction potential (ORP) monitor and possibly a controller to modulate the chemical feed into the feedwater. It also entails handling hazardous and odorous powders and / or liquids. These reducing agents react with oxygen in the air and water and thus have to be reconstituted frequently due to loss of solution strength.

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