J Water SRT Ð Aqua Vol. 48, No. 6, pp. 219±226, 1999 Evaluation of chlorine decay kinetics expressions for drinking water distribution systems modelling G. J. Kastl*, I. H. Fisher and V. Jegatheesan, Australian Water Technologies, PO Box 73, West Ryde, NSW 2114, Australia ABSTRACT: The decay of chlorine in drinking water involves a complex set of reactions that is usually simpli®ed to ®rst order kinetics in models of water quality in distribution systems. However, to be useful in optimising chlorine dosing regimes, the kinetics expression should accurately describe the shape of the chlorine decay curve for dierent chlorine doses and be able to simulate re-chlorination. After considering the nature of the reactions involved in chlorine decay, ®ve simpli®ed reaction schemes were evaluated for their suitability to describe chlorine concentration in bulk water. Each scheme was ®tted to a sample of experimental data of chlorine decay in raw water obtained from Warragamba Dam (the major source of water supplied to Sydney, Australia). A scheme involving two parallel reactions of organic carbon compounds with chlorine is both necessary and sucient to satisfy the requirements of modelling chlorine decay accurately. INTRODUCTION Chlorine concentration in drinking water diminishes with time. The rate of the decay in bulk water is usually described as a ®rst order reaction [1±5]. This is done with full knowledge that it is not a real mechanism, since the reaction rate of chlorine in pure water in the dark is negligible in comparison to its reaction rate in natural waters. This indicates that chlorine is reacting with some impurities present in natural waters rather than decom- posing on its own. Experiments indicate that reaction rate coecient k (expressed as reaction rate over chlorine concen- tration) is a function of the initial chlorine dose and amount of chlorine already reacted with this particular water. The ®rst order approximation is based on the assumption that chlorine reacts with organic contaminants in water, which are present in much larger quantities than chlorine. Therefore the concentra- tion of organic contaminants is not signi®cantly changed during the reaction with chlorine and the reaction rate is only proportional to the concentration of chlorineÐthe so-called pseudo-®rst-order reaction. This scheme for describing chlorine decay may be sucient for some applications; namely where the chlorine dose is not varied and when a description is required only from some time after dosing. The main advantage of the ®rst order reaction scheme is its simplicity and available analytical solution, with no need to keep track of any other concentrations but chlorine. It seems that this advantage is the decisive factor even in cases when the ®rst order reaction does not ®t the experimental data very well at all. Furthermore the same ®rst order expression is used to describe chlorine decay in a distribution system of pipes and service reservoirs where chlorine decay is due to reaction in bulk water and reaction on the surfaces enclosing it [2,4]. It was suggested that reaction rate on these surfaces can be correlated with the bulk reaction rate [6]. Another study by Biswas et al. [3], considered radial and axial mass transfer with the ®rst order kinetics to describe chlorine decay in pipes. Here the assumption is that the ®rst order kinetics is correct but deviation in pipes is caused by mass transfer and surface reaction. Similarly the value of the ®rst order rate coecient k is aected by variations in hydrody- namic conditions (laminar/turbulent ¯ow) [7]. This deviation is possibly caused by deviation from the ®rst order reaction kinetics. Changing of the ¯ow pattern then changes reaction time over which reaction rate is measured and since the reaction is not of ®rst order, the reaction rate coecient k will seemingly change with the ¯ow. The purpose of this paper is to develop and evaluate a suitable kinetic expression, which faithfully describes chlorine decay in bulk water. Sound representation of the kinetics of chlorine decay in bulk water is considered to be a fundamental element for modelling chlorine decay in a distribution system. Thus, this paper deals with the modelling of free chlorine. Further, the nitrogenous compounds will also react with chlorine, either reducing it to chloride or forming combined chlorine. The authors have presented elsewhere [8,9] the reac- tion of chlorine with these nitrogenous compounds and the subsequent decomposition of combined chlorine in addition to the reaction of chlorine with organic matter. Also, if iron (in the form of ferrous ion) or manganese (as manganous ion) is present in the bulk water they will contribute to the decay of chlorine. This can be modelled in a similar way to that of organic or nitrogenous compounds. However, if surface water is used to supply the distribution system, the concentrations of nitrogenous compounds, iron and manganese will be generally small compared with that of organic compounds. Chlorine decay is also known to be in¯uenced by tempera- ture. One value of activation energy (E) for all reactions can be # 1999 IWSA 219 *Correspondence: E-mail: george.kastl@awtpl.com.au