The influence of temperature on chlorine bulk-decay rates in drinking water

Abstract

Chlorine is used worldwide to assure the microbiological safety of drinking water in transport and distribution systems. There, its concentration is usually kept within the range of 0.2 mg L-1 to 1 mg L-1, in order to protect public health (WHO, 2011). However, chlorine concentration decays as the water travels through the transport and distribution systems. Such decay is mainly due to chemical reactions of chlorine with the water natural organic matter (NOM) and, generally to a minor extent, with the pipes´ wall material. The former, the so-called bulk decay (kb), is a key parameter in the modelling of chlorine behaviour in drinking water systems. In addition to water NOM contents and reactivity (Hua et al., 1999), kb is mainly affected by the water temperature (Powell et al., 2000). Powell et al. (2000) found that 10ºC raises in water temperature may lead to 2.5 fold increases in kb values. However, increments as small as 5ºC may double the rates of bulk chlorine decay (Kastl et al., 1999). Such phenomenon is particularly important in Continental countries where the water temperature spans from close to freezing to very high. Ongoing climate changes, including global warming and higher frequency and severity of extremes, such as heat and cold waves, are likely to aggravate such conditions and to extend them to other areas including the Mediterranean regions like Algarve (Portugal). There the distribution systems water temperature may range from 13ºC in winter to ca. 21ºC in summer (Pina et al., 2010). In this study the effect of temperature on kb was investigated for raw and clarified waters of surface origin. The latter had undergone ozonation and coagulation/flocculation/sedimentation. The waters were from Alcantarilha and Tavira drinking water treatment plants (DWTP), located in Algarve.