Development of the CDWQ-E2 post-treatment water quality model

Culligan, Liam Patrick
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Drinking water distribution systems are considered to be the main source of drinking water contamination yet to be fully addressed. The deterioration of water quality within distribution systems is a result of a complex set of interrelated factors, currently not fully understood by water utilities, as their effect on water quality must be considered simultaneously, which creates considerable difficulty. Routine sampling and monitoring of water can play an important role in ensuring that water quality does not deteriorate within a distribution system. However, solely taking such an approach can only provide limited information, as sampling can only provide information regarding water quality at sampling locations and monitoring cannot be used to predict future water quality. The use of deterministic mathematical models is one of the best tools available to both researchers and engineers in order to gain greater insight into the numerous interrelated processes affecting water quality within distribution systems and to determine effective ways of maintaining high quality water within distribution systems. Consequently, the Expanded Comprehensive Disinfection and Water Quality Model, Version 2 (CDWQ-E2) has been developed to utilise the latest advances in the fields of microbial growth and residual disinfectant decay modelling in order to provide greater insight into the relationship between the biological, chemical, and hydraulic factors affecting water quality. Various tools have been incorporated into the CDWQ-E2 model to enhance its interpretative capabilities. A theoretical distribution system is used to show the fundamental soundness of the model and to exhibit its features. The results of the simulations performed demonstrate the importance of both the maintenance of an adequate disinfectant residual and low substrate concentrations in order to keep biological growth below acceptable limits. Furthermore, the simulated results testify to the potential for nitrification to occur in distribution systems utilising chloramine as a secondary disinfectant, which can establish a positive feedback loop with regard to monochloramine decay and have severe implications for drinking water quality.
A research report submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in partial fulfillment of the requirements for the degree of Master of Science in Engineering Johannesburg, 2015