Most U.S. public water supplies today are disinfected with either chlorine or chloramines (chlorine mixed with ammonia). 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. Unfortunately, chlorine and its related compounds are oxidizing agents and can have harmful effects on medical and industrial procedures or processes. Even low concentrations of chlorine can rupture a patient's red blood cells when going through hemodialysis.
These compounds can also cause stress cracks in stainless steel, alter and damage active pharmaceutical agents, or lead to undesirable by-products. Water treatment systems are no exception; reverse osmosis membranes and ion exchange resins will deteriorate and degrade when chlorine is present.
Removal of Chlorine
Free chlorine and chloramines can be removed in several ways which are described below:
Adsorption Dechlorination can be performed with many types of activated carbon, but granular activated carbon (often 12 x 40 mesh size), or GAC, is the form most commonly used in large water treatment filters. Free chlorine removal is the result of residence time in contact with the carbon, rather than filter surface loading, so standard designs have flow rates of 2 to 3 gpm/ft3 of bed. Chloramines are significantly harder to remove than chlorine. Chloramines requires an extended period of contact with the activated carbon, referred to as empty bed contact time (EBCT).
Activated carbon mainly utilizes surface adsorption to remove chlorine and similar compounds, but also reacts with it and becomes consumed directly. It has been reported that 1 pound of carbon will react with 6 pounds of chlorine. If this is an accurate prediction, then water containing 1 ppm of chlorine will contain 8.3 pounds of chlorine per 1,000,000 gallons and will consume 1.4 pounds of carbon.
Keep in mind that the adsorption rate can be reduced if sediment and organic molecules foul the GAC's minuscule pores. How quickly the filter fouls generally determines the replacement frequency of GAC beds. As a practical matter, most people who utilize this type of dechlorination method switch out the GAC beds annually or semi-annually.
In cases where the consequences of chlorine breakthrough could be serious (i.e. hemodialysis), at least two 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.
Unfortunately, GAC can become home to bacterial growth once the chlorine is removed by the upper inch or so of the bed. The organic material that the GAC filters, acts as sustenance for bacteria, which can colonize within the pores of the carbon granules. In addition, the colonization can become even more rapid if the filtered water temperature is warm. In cases where the prevention of bacterial contamination is critical, such as in pharmaceutical or semiconductor water treatment systems, steam or hot water sanitizable filter vessels are used. Downstream treatments can also be used, such as distillation or ultraviolet disinfection.
Chemical Dechlorination – Reduction reactions occurring from sulfites, bisulfites, or metabisulfites can also remove chlorine. Chemical reduction prevents a bacterial breeding ground from being introduced upstream of the rest of the water treatment system. However, downstream treatments such as deionizers may become burdened by certain ions (e.g. chloride, sodium, sulfate, etc.) that are introduced or produced through chemical reduction. An oxidation-reduction potential (ORP) or continuous chlorine monitor is usually a necessity. Equipment to modulate the chemical feed into the feedwater might be required as well. This method of dechlorination also requires the handling of dangerous 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.
Ultraviolet Dechlorination – Another way to remove chlorine is ultraviolet irradiation. This is a high intensity method of chlorine removal that uses broad spectrum ultraviolet irradiation to dissociate free chlorine and chloramines, turning them into hydrochloric acid. The ultraviolet total organic (TOC) reduction process utilizes different nanometer wavelengths for specific compounds. For instance, free chlorine will normally be reduced with 185 nanometer wavelengths, while chloramines require wavelengths of 245-365 nanometers. The ultraviolet dose required for dechlorination is 15-30 times higher than for ultraviolet disinfection. Ultraviolet dechlorination does not propagate bacteria, and is highly effective at simultaneously disinfecting water and reducing the total organics.