Chloral hydrate (the hydrated form of trichloroacetaldehyde) is formed in water treatment as a by-product of the reaction between chlorine and naturally occurring organic material. Although a clear link between adverse health effects and levels of chloral hydrate in drinking water has not been established, the World Health Organisation (WHO) has set a Provisional Guideline Value of 10 µg l−1. This paper reviews the potential for the formation and removal of chloral hydrate in drinking water treatment processes.

Two possible formation mechanisms for chloral hydrate are the reaction between amino acids and chlorine and the reaction between the aldehydes (formed when natural organic material is oxidised) and chlorine. Chloral hydrate can decay in alkaline conditions to give chloroform and in oxidising conditions to give trichloroacetic acid. Although the optimum conditions for the minimisation of trihalomethanes (THMs) may be different from the optimum conditions for the minimisation of chloral hydrate, the removal of organic precursors is common to both. Enhanced coagulation has the potential to reduce chloral hydrate levels in the treated water and to be the most efficient, convenient and cost effective method of reducing chloral hydrate levels in the treated water. Free chlorine is necessary for significant formation of chloral hydrate.

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