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Case Study 2: Report on LNEC / EPAL joint activity: test and optimization of the integrated water quality model
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Passage of the Safe Drinking Water Act in 1974 and its Amendments in 1986 (SDWAA) is changing the way water is treated and delivered in the United States. Under the SDWAA the U.S. Environmental Protection Agency (EPA) is required to regulate chemical contaminants and pathogenic microorganisms in drinking water. Emphasis has shifted from a primary concern with treated drinking water to attainment of standards at the point of consumption. Two regulations promulgated under the SDWAA, the Surface Water Treatment Rule (SWTR) and the Total Coliform Rule (TCR) specify treatment and monitoring requirements that must be met by all public water suppliers. This paper will examine the effect of various system variables on chlorine residual propagation. A recently proposed model (EPANET) will be utilized to examine the extent of fluid velocity and pipe radius on chlorine demand. The effect of these variables on the maintenance of chlorine residuals will be demonstrated. It will be shown that the same variables that affect the propagation of chlorine residual levels can potentially affect disinfection efficacy and the formation of disinfection by-products.
The Techneau WP5.5 Water Quality Model (Coelho, 2008) is a water quality modeling platform based on an event-driven, hydrodynamic water supply network model. It provides a basis for the integration of the results of Techneau research being carried out into specific physical-chemical and microbiological processes taking place in the water and its environment, the network (Wricke et al, 2007).
Free chlorine consumption in distribution systems is due both to chemical reactions occurring in the bulk phase and at the pipe walls. Knowledge of the relative importance of these various reactions is needed in order to improve chlorine decay modeling. Experimental results carried out in this study make it possible to propose a hierarchical classification of the main parameters involved in the free chlorine decay observed in distribution systems. Corrosion of metallic pipe appears to be a major parameter, while synthetic materials are of little influence. The rate of chlorine decay in bulk phase can be estimated according to the TOC and the temperature. Influence of biofllms depends on the BDOC content of water, and on the pipe diameter. Chlorine decay due to corrosion phenomena must be modeled according to a zero order kinetics, while chlorine decay due to other parameters can be modeled according to a first order kinetics with respect to chlorine
Changes in chlorine residual concentrations in water distribution systems could be used as an indicator of microbial contamination. Consideration is given on how to model the behavior of chlorine within the distribution system following a microbial contamination event. Existing multispecies models require knowledge of specific reaction kinetics that are unlikely to be known. A method to parameterize a rate expression describing microbially induced chlorine decay over a wide range of conditions based on a limited number of batch experiments is described. This method is integrated into EPANET-MSX using the programmer's toolkit. The model was used to simulate a series of microbial contamination events in a small community distribution system. Results of these simulations showed that changes in chlorine induced by microbial contaminants can be observed throughout a network at nodes down-stream from and distant to the contaminated node. Some factors that promote or inhibit the transport of these chlorine demand signals are species-specific reaction kinetics, the chlorine concentration at the time and location of contamination, and the system's unique demand patterns and architecture.
Several computer programs for modelling water distribution networks have been developed which incorporate a facility for modelling chlorine decay. Problems have been experienced with the calibration and durability of these models due to both temporal and spatial variability in the decay constants. Chlorine will decay either due to reactions at the pipe wall or due to reactions in the bulk water. The aim of the work presented in this paper is to investigate the factors which influence bulk decay. Over 200 determinations of bulk chlorine decay against time were performed on waters taken from 32 sampling locations within the Severn Trent region, U.K. The bulk decay constant was observed to show significant variation with temperature, the initial chlorine concentration and the organic content of the water. An equation was derived relating these parameters which could be used to update the decay constants in network models and improve their durability.
The disinfection of water supplies for domestic consumption is often achieved with the use of chlorine. Aqueous chlorine reacts
with many harmful micro-organisms and other aqueous constituents when added to the water supply, which causes the chlorine
concentration to decay over time. Up to a certain extent, this decay can be modelled using various decay models that have
been developed over the last 50+ years. Assuming an accurate prediction of the chlorine concentration over time, a measured
deviation from the values provided by such a decay model could be used as an indicator of harmful (intentional) contamination.
However, most current chlorine decay models have been based on assumptions that do not allow the modelling of another species,
i.e. the species with which chlorine is reacting, thereby limiting their use for modelling the effect of a contaminant on
chlorine. This paper investigates the use of genetic programming as a method for developing a mixed second-order chlorine
decay model.
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