Application and toxicity of sodium dichloroisocyanurate

Introduction

Sodium dichloroisocyanurate(NaDCC) is the sodium salt of a chlorinated hydroxytriazine and is used as a source of free available chlorine, in the form of hypochlorous acid, for the disinfection of water. It is widely used as a stable source of chlorine for the disinfection of swimming pools and in the food industry. It is also used as a means of disinfecting drinking-water, primarily in emergencies, when it provides an easy-to-use source of free chlorine, and, more recently, as the form of chlorine for household point-of-use water treatment[1].

Picture 1 Sodium dichloroisocyanurate powders

Sodium dichloroisocyanurate has recently been approved by the United States Environmental Protection Agency and the WHO for the routine treatment of drinking water. Like other forms of chlorine, NaDCC produces hypochlor[1]ous acid, a well-known oxidizing agent. Bound with cyanuric acid, however, the compound presents certain advantages over NaOCl as a water disinfectant. It may also offer other advantages in terms of stability, safety, up-front cost and convenience.

Basic chemistry

While both NaOCl and NaDCC rely on HOCl as the active agent, there are important differences in the performance of the two compounds. Unlike NaOCl which releases all of its chlorine as FAC, NaDCC releases only approximately 50% of the chlorine as FAC, the balance remaining as ‘‘reservoir chlorine’’ (bound) in the form of chlorinated isocyanurates . When the FAC is used up, the equilibrium is disturbed, immediately releasing further FAC from the ‘‘reservoir’’ until the total available is used up. Thus, the stabilized chlorine in NaDCC acts as a reservoir of HOCl which is rapidly released when the free available chlorine is depleted.

Application

Sodium dichloroisocyanurate used for disinfecting drinking-water should be of adequate purity so that there is no increase in any inorganic or organic contaminants in the drinking-water. The amounts of sodium dichloroisocyanurate used should be the lowest consistent with adequate disinfection, and the concentrations of cyanuric acid should be managed to be kept as low as is reasonably possible.

Toxicity

Studies of the toxicity of sodium cyanurate are appropriate for assessing the safety of sodium dichloroisocyanurate, because any residues of intact sodium dichloroisocyanurate in drinking-water would be rapidly converted to cyanuric acid on contact with saliva[2]. Both sodium dichloroisocyanurate and sodium cyanurate have low acute oral toxicity. Sodium cyanurate does not induce any genotoxic, carcinogenic or teratogenic effects. The NOEL from which the guideline value was derived was based on multiple lesions of the urinary tract (calculi and hyperplasia, bleeding and inflammation of the bladder epithelium, dilated and inflamed ureters and renal tubular nephrosis) and cardiac lesions (acute myocarditis, necrosis and vascular mineralization) in male rats exposed at the next higher dose.

All chlorine products have some level of toxicity; this is what renders them such effective microbicides. When chlorinated water is ingested, however, the available chlorine is rapidly reduced by saliva and stomach fluid to harmless chloride ions salts. This is true for all sources of chlorine, including both NaOCl and NaDCC. The unique characteristic of the isocyanurates is cyanuric acid, the carrier that allows the chlorine to be contained in a solid, stable and dry form. It is the potential toxicity of such cyanuric acid, therefore, that required review by regulatory agencies prior to the approval of NaDCC for the routine treatment of drinking water

Microbial effectiveness

As noted above, NaDCC is an alternative source of FAC (HOCl). Accordingly, the significant body of evidence on the antimicrobial action of chlorine is as relevant to NaDCC as it is to NaOCl and other sources of chlorine. While certain bacterial spores have shown greater resistance to NaDCC, thus at least suggesting the potential for differences in activity based on the chlorine donor, no differences have been reported in respect to waterborne pathogens. Susceptibility to hypochlorous acid has been established with respect to a wide variety of bacteria, including Escherichia coli, Salmonella dysenteriae, Shigella sonnei, Campylobacter jejuni, Yersinia enterocolitica; viruses, including hepatitis A, poliovirus, rotavirus, adenovirus and calicivirus; helminthes; and protozoa, including cysts of Entamoeba histolytica and Giardia lamblia. Microbicidal activity is a function of chlorine concentration and contact time. At doses of a few mg/l and contact time of about 30 min, free chlorine inactivates more than 4 logs of most waterborne pathogens. Cryptosporidium has demonstrated considerable resistance to chlorination  and Mycobacterium has also been reported as resistant . It should also be noted that in some cases, certain viruses have also exhibited greater resistance to chlorine and chlorine compounds than common bacterial indicators of faecal contamination. This may have implications for determining the required concentration and contact time required to kill or deactivate potential pathogens in the untreated water collected for use in emergency and development settings.

Reference

1 Guidelines for drinking-water quality: fourth edition incorporating the first and second addenda

2 Thomas Clasen 1, Paul Edmondson. Sodium dichloroisocyanurate (NaDCC) tablets as an alternative to sodium hypochlorite for the routine treatment of drinking water at the household level. Int J Hyg Environ Health. 2006 Mar;209(2):173-81.

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