Calcium hypochlorite

(endorsed 2005)

Calcium hypochlorite is a drinking water disinfectant used only for small systems.

General description

Calcium hypochlorite, $\text{Ca(OCl)}_{2}$, is a white crystalline solid. It has a specific gravity of 2.35, decomposes in water and alcohol, is not hygroscopic and is practically clear in a water solution. The chemical is a highly active oxidiser and is relatively stable. The oxidising capability of 1 g calcium hypochlorite (65% strength) is equivalent to the oxidising capability of 0.65 g chlorine gas.

Calcium hypochlorite is available commercially as a dry solid, with a strength of up to 74% available chlorine. In this form, it loses about 0.013% of its strength per day under normal storage conditions, although the rate can be higher if the chemical is in contact with water or is exposed to the atmosphere. It is also available in a tablet form for use in automatic feed equipment at low-flow treatment plants or for dosing of in-system reservoirs.

Appropriate handling materials for calcium hypochlorite include glass, ceramics, fibreglass-reinforced plastic, polyethylene, polyvinyl chloride. Rubber-lined containers can also be used.

Chemistry

Calcium hypochlorite is formed by the addition of chlorine to a slurry of ‘milk of lime’ (calcium hydroxide).

Calcium hypochlorite granules dissolve in water to form hypochlorous acid (HOCl), which partially dissociates to the hypochlorite ion ($\text{OCl}^{-}$).

$$\text{Ca(OCl)}_{2}+\text{2H}_{2}\text{O}\rightarrow\text{2HOCl}+\text{Ca(OH)}_{2} \newline \text{Ca(OCl)}_{2}\rightarrow\text{Ca}^{2+}+\text{2OCl}^{-} \newline \text{OCl}^{-}+\text{H}_{2}\text{O}\leftrightarrow\text{HOCl}+\text{OH}^{-}$$

As with the addition of chlorine gas, the relative distribution of hypochlorous acid and hypochlorite ion resulting from the addition of calcium hypochlorite to water will depend on pH and temperature.

Calcium hypochlorite is a base and therefore raises the pH of water, whereas chlorine gas produces an acidic reaction that lowers the pH of the solution. The extent of the pH change depends on the alkalinity of the water.

Typical use in Australian drinking water treatment

Calcium hypochlorite is generally used as a disinfectant in smaller water treatment plants or in new water mains or in-system reservoirs.

As a disinfectant in water systems, calcium hypochlorite must be dissolved in water before it is added to the main supply. Doses usually range from 1 to 5 mg/L (as available chlorine), with 2–3 mg/L typical. Selection of the appropriate chlorine dose should take into account the C.t (disinfectant concentration × contact time) and chlorine residual required, and the levels of disinfection by-products likely to be formed. A free chlorine residual of ≥0.2 mg/L throughout the distribution system is preferred. Superchlorination (doses of 10 to 50 mg/L) may be used to disinfect or clean tanks and pipelines.

Contaminants

The purity of chemicals used in Australia for the treatment of drinking water varies, depending on the manufacturing process. Chemical contaminants that may be present in calcium hypochlorite include:

• aluminium

• arsenic

• barium

• cadmium

• chromium

• fluoride

• iron

• lead

• magnesium

• manganese

• mercury

• nickel

• selenium

• silica

• silver

Residual and by-product formation in drinking water

When employed in drinking water treatment, calcium hypochlorite should be used in such a way that any contaminant or by-product formed by the use of the chemical does not exceed guideline values in the Australian Drinking Water Guidelines.

The use of calcium hypochlorite as a disinfectant results in the formation of free chlorine, combined chlorine residuals and disinfection by-products. The by-products formed include trihalomethanes (THMs), haloacetic acids (HAAs), haloacetonitriles (HANs), haloketones, chloral hydrate and chloropicrin. Although many specific chlorine disinfection by-products have been identified, many of the total organic halogens are as yet unidentified.

Among the many factors affecting the species formed as disinfection by-products are pH, temperature and levels of total organic carbon (TOC), bromide and chlorine. THMs (e.g. chloroform, bromodichloromethane, dibromochloromethane and bromoform) are the most widely known chlorination by-products. Chlorinated THM, HAA and HAN species are generally found at higher levels than brominated species; however, brominated species predominate in waters containing high levels of bromides.

The disinfection by-products most likely to occur and to be of concern to health are total THMs and THM species, total HAAs and HAA species.

Status

Calcium hypochlorite was endorsed by the NHMRC for use as a drinking water treatment chemical in 1983. The revision undertaken in 2003 did not change the status of this chemical for the treatment of drinking water.

References

ANSI (American National Standards Institute)/AWWA (American Water and Wastewater Association) Standard no B300-99. AWWA CD-ROM (April 2003). Details at <www.awwa.org>

Clesceri LS, Greenberg AE and Eaton AD (eds) (1998). Standard Methods for the Examination of Water and Wastewater, 20th Edition. American Public Health Association, Washington, DC.

Connell GF (1996). The Chlorination/ Chloramination Handbook. Water Disinfection Series, American Water Works Association, Denver.

White GC (1992). Handbook of chlorination and alternative disinfectants, 3rd edition. Van Nostrand Reinhold, New York.