Disinfection Residual Effects of Water Distribution of Systems Materials

The Havelock North Inquiry has focused additional attention on residual disinfection levels in potable water systems, especially in smaller systems, and those which to date may not have been treated.

Several disinfection methods are commonly used; chlorine, chloramine, and chlorine dioxide; to obtain a residual value considered suitable to meet the requirements of the NZ Water Guidelines for water quality.

However, these disinfectants behave in different ways when considering the effect on common materials used within the distribution system. Acting as oxidizing agents, the residual disinfectants alter the water properties and in particular circumstances, can lead to premature component failures in practice. Major water systems are regarded as having a nominal 100 year design lifetime, and various AS/NZS Product Standards are developed on this basis.

At the same residual level in the distribution system, a change between disinfectant types can result in accelerated failure, and some disinfection methods should be avoided when considering existing distribution system materials.

The paper examines the effect of the residual disinfectants on the oxidation reduction potential of the water, and the resultant effect on a number of metallic, and non-metallic materials.

The mechanism of oxidative degradation is explored, and a typical life cycle is demonstrated. This involves both the behavior of the disinfectant residuals, and the individual material type response.