Chlorate is a disinfection by–product (DBP) that can arise where sodium hypochlorite, calcium hypochlorite, chlorine dioxide or onsite electrolytic chlorination (OSEC) are used for disinfection. Although there is a legal requirement for water companies to minimise DBP formation, there is no current prescribed concentration or value for chlorate in drinking water in the EU, although a future drinking water standard of 0.25 mg / L has been proposed. The chlorate concentration data held by the DWI is limited.
Controls on chlorate in drinking water are in place through national conditions of use for OSEC and chlorine dioxide and purity requirements under the British Standard for sodium hypochlorite. These controls should ensure that the World Health Organisation (WHO) provisional guideline value for chlorate (0.7 mg / L) is not exceeded. A recent joint report of the Food and Agriculture Organisation of the UN and the WHO Expert Committee on Food Additives has proposed an acceptable daily intake of 10 µg / kg body weight (BW) for chlorate due to its acute toxicity. Using standard assumptions, this would correspond to a health-based value of ~ 0.3 mg / L for chlorate, which is below the current WHO provisional guideline of 0.7 mg / L (WHO, 2011). In addition the European Food Safety Authority (EFSA) has recently reviewed whether the default MRL of 0.01 mg / kg that applies to all food and feed commodities is achievable for chlorate at the current time (EFSA Draft Act, 2019). It has concluded that the level corresponding to the 95th percentile of the occurrence data reflecting levels that are realistically achievable when good manufacturing practices are used should be used as a temporary MRL. This may impact the value for drinking water determined by the EU.