In a previous post, we discussed the use of chlorine gas as a disinfectant. Now we will look at the alternative forms of chlorine gas: Calcium Hypochlorite, Sodium hypochlorite, and Chloramines. Every disinfectant has its positive and negatives so choosing the right disinfectant for your system takes planning and time.
Calcium Hypochlorite (65 – 70 percent)
This product is manufactured in a powder, granular or tablet form. Many of us know this product as HTH which is a trade name. Calcium hypochlorite Ca(ClO)2 tends to raise the pH of the water and is a weaker disinfectant than gaseous chlorine. Flammable hazards exist when in contact with oxidizers. This product should be stored in a cool, dark area because it degrades when exposed to light or heat and carries a shelf life of 2 years. It is frequently used in distribution system disinfection applications but more treatment facilities are using it as a primary disinfectant because of safety concerns concerning chlorine.
Sodium Hypochlorite (5.25 – 15 percent)
A liquid (NaOCl) that is frequently referred to as bleach. Formerly used extensively in well systems and for the disinfection of distribution system pipes and fittings but more and more surface water treatment facilities are using sodium hypochlorite as their primary disinfectant. As with HTH, sodium hypochlorite should be stored in a cool, dark place because of the degradation effects of heat and light. This product has a shelf life of 60 to 90 days.
Formed by the addition of ammonia with chlorine. Chloramines are approximately 80 times weaker than chlorine gas as a primary disinfectant and it requires a very long contact time to achieve pathogen inactivation. Chloramine can last up to 27 days in the distribution system when a solution, much longer than chlorine. Since the chlorine is in the combined form, the formation of THMs and HAAs is limited. Also, because chloramines never reach breakpoint, the minimum disinfectant residual for chloraminated systems is 0.5 mg/L. The chlorine to ammonia ratio, pH and the rate of decay must be carefully monitored and controlled to prevent nitrification, taste and odor, increased lead and copper levels and biological regrowth in the distribution system.
All three types of disinfectants can treat your drinking water. It is up to you as the operator of the plant to determine which method will produce the best water quality and the right price.