The most common use of chlorine in water treatment is to disinfect water. As a disinfectant, it has drawbacks, but it also has advantages. Other methods of disinfection such as ultraviolet and ozonation are effective disinfectants but they do not provide a residual to prevent pathogen regrowth as chlorination does. When treatment plants are distant from the point of use, chlorination is the best way to provide safe water to the end user. Municipal water providers usually rely on measurements of “chlorine residual”—the amount of chlorine remaining in the water after it reaches its destination—as proof of safety. Residual requirements vary, but typical residual goal would be for 0.2 to 1 mg/L.
In addition to disinfection, chlorine can be effectively used to oxidize iron, manganese and hydrogen sulfide to facilitate their removal, to reduce colour in water, and to aid in such treatment processes as sedimentation and filtration.
Chlorine and pH
In general terms, the lower the pH of the water, the more effective chlorine is as a disinfectant. Again, speaking generally, a reason for dosing effectively is that chlorination raises the pH of water, so overdosing often raises the pH to levels where chlorine does not work effectively as a disinfectant. More is not always more powerful.
Types of Chlorine Used in Water Treatment
“Pure chlorine” is seldom used for water treatment.
The pure chemical element has the physical form of a diatomic green gas. The name chlorine is derived from chloros, meaning green, referring to the color of the gas. Chlorine gas is two- and one-half times as heavy as air, has an intensely disagreeable suffocating odor, and is exceedingly poisonous. In its liquid and solid form, it is a powerful oxidizing, bleaching, and disinfecting agent.
This element is a part of the halogen series forming salts. It is extracted from chlorides through oxidation and electrolysis.
Calcium hypochlorite is an inorganic compound with formula Ca(ClO). It is the main active ingredient of commercial products called bleaching powder, chlorine powder, or chlorinated lime, used for water treatment and as a bleaching agent. This compound is relatively stable and has greater available chlorine than sodium hypochlorite (liquid bleach).
Calcium hypochlorite is a general oxidizing agent and therefore finds some use in organic chemistry. For instance, the compound is used to cleave glycols, α-hydroxy carboxylic acids and keto acids to yield fragmented aldehydes or carboxylic acids. Calcium hypochlorite can also be used in the haloform reaction to manufacture chloroform.
Sodium hypochlorite is most often encountered as a pale greenish-yellow dilute solution commonly known as liquid bleach or simply bleach, a household chemical widely used (since the 18th century) as a disinfectant or a bleaching agent. The compound in solution is unstable and easily decomposes, liberating chlorine, which is the active principle of such products. Indeed, sodium hypochlorite is the oldest and still most important chlorine-based bleach.
Sodium hypochlorite solutions have been used to treat dilute cyanide waste water, such as electroplating wastes. In batch treatment operations, sodium hypochlorite has been used to treat more concentrated cyanide wastes, such as silver cyanide plating solutions. Toxic cyanide is oxidized to cyanate (OCN−) that is not toxic, idealized as follows:
CN− + OCl− → OCN− + Cl−
Sodium hypochlorite is commonly used as a biocide in industrial applications to control slime and bacteria formation in water systems used at power plants, pulp and paper mills, etc., in solutions typically of 10–15% by weight.