Mass Imbalances in EPANET water-quality simulations

EPANET does not always conserve constituent mass during water-quality simulations. The failure to conserve mass can result in significant errors in estimated constituent concentrations. My co-authors and I have prepared a paper that documents the occurrence of mass imbalances, explains why they occur, provides recommendations for minimizing mass imbalances, and presents a preliminary water quality routing algorithm for use in EPANET that always conserves mass. The paper may be of interest to anyone who performs water-quality simulations using EPANET. It has been submitted to Drinking Water Engineering and Science and is currently under review. The draft manuscript, entitled “Mass imbalances in EPANET water-quality simulations”, can be obtained from the journal’s website at https://www.drink-water-eng-sci-discuss.net/dwes-2017-28/. Comments on the manuscript can also be posted on the website. The abstract for the paper is given below.
ABSTRACT. EPANET is widely employed to simulate water quality in water distribution systems. However, in general, the time-driven simulation approach used to determine concentrations of water-quality constituents provides accurate results only for short water-quality time steps. The use of an adequately short time step may not always be feasible. Overly long time steps can yield errors in concentration estimates and can result in situations in which constituent mass is not conserved. The absence of EPANET errors or warnings does not ensure conservation of mass. This paper provides examples illustrating mass imbalances and explains how such imbalances can occur. It also presents a preliminary event-driven approach that conserves mass with a water-quality time step that is as long as the hydraulic time step. Results obtained using the current approach converge, or tend to converge, to those obtained using the preliminary event-driven approach as the water-quality time step decreases. Improving the water-quality routing algorithm used in EPANET could eliminate mass imbalances and related errors in estimated concentrations. The results presented in this paper should be of value to those who perform water-quality simulations using EPANET or use the results of such simulations, including utility managers and engineers.