A NEW APPROACH FOR WATER QUALITY NETWORK MODELLING
A case study of a regional chloraminated distribution system
S Moradi, C WK Chow, D Cook, M Drikas, P Hayde, R Amal
Publication Date (Web): 7 May 2018
Chloramine is commonly used as a disinfectant instead of chlorine to meet regulations regarding formation of disinfection by-products in drinking water, particularly in Australia. The chloramine dose is crucial to ensure the water is safe to drink, but also needs to be considered based on taste and odour problems arising from use of high chloramine doses. In chloraminated drinking water systems, monochloramine decay occurs due to chemical and microbiological reactions. Maintaining a chloramine residual throughout the water distribution system is important in ensuring microbiologically safe water is supplied at the customer’s tap. Modelling of disinfectant residual in treated water distribution systems is aimed at creating a better understanding of the effect of water quality on the disinfection consumption and can serve as a decision making tool for effective water quality control. This study developed a new approach for water quality network modelling (WQNM) to enable estimation of monochloramine residual in real drinking water distribution systems using Bentley commercial hydraulic package (Water GEMS). The approach is based on using chemical and microbiological factors that affect chloramine decay rate. The model is based on an organic character (SUVA) as chemical factor, a laboratory measure of the microbiological decay of monochloramine (Fm) as microbiological factor, initial monochloramine concentration to the network, and hydraulic retention time (HRT) of the water samples through the distribution systems. The applicability of the model for estimation of monochloramine residual was tested on a large regional chloraminated water distribution system (WDS), Tailem Bend to Keith (TBK) WDS in Australia through statistical test analysis between the experimental and estimated data. Water samples were collected at twelve different locations from TBK WDS, and distribution system sampling sites focused on major tanks located at varying distances in the distribution system. Electronic Geographic Information Systems data was used to develop the hydraulic water network model of the TBK WDS in Water GEMS.
To develop a WQNM into EPANET Multi-Species Extension, a software tool recently added in Water GEMS, chloramine decay was assumed to be described by the sum of two first-order equations, in which the first part describes a rapid decay and the second simulates a slower decay. It was also assumed that the monochloramine demand after 2-days for water samples from TBK WDS can be expressed by chemical (SUVA) and microbiological (Fm) parameters. After calibrating the model parameters based on 70% of water samples from TBK WDS, all the equations were solved via a fifth order Runge-Kutta method with automatic time step control to estimate monochloramine residual through different sampling locations in the distribution system. The developed model was able to identify locations where nitrification occurred in selected WDS in Australia, and therefore this modelling approach has the potential to be used by water treatment operators as a decision support tool in order to manage chloramine disinfection. This numerical approach for monochloramine residual estimation based on the water quality and dosing conditions can be considered as a pre-warning method to control disinfection dosing process in drinking water system.
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