Investigating Chlorate Occurrence in Hungarian Drinking Water: A Survey Analysis of Treatment Plant Practices

The revised EU Drinking Water Directive, which designates chlorate as a regulated parameter, presents a significant challenge for Hungary, where breakpoint chlorination with sodium hypochlorite is widely used for ammonium removal. Because hypochlorite decomposes during storage, half of the surveyed plants reported chlorate concentrations exceeding the national limit of 0.25 mg/L. A survey conducted on 208 drinking water treatment plants evaluated current operational practices and mitigation strategies. Chlorate occurrence was primarily associated with systems using sodium hypochlorite as the main oxidant, whereas technologies relying on chlorine gas or biological ammonium removal showed no relevant chlorate occurrence, even when hypochlorite was applied for postdisinfection. Large variability in hypochlorite handling, particularly delivery frequency, residual stock, storage volume, and cleaning practices, strongly influenced chlorate levels, with small systems being the most vulnerable. No relationship was observed between chlorine demand and chlorate concentrations, underscoring that chlorate occurrence in the finished water is governed by operational practices, hypochlorite quality, storage conditions, and dosing protocols.