1. INTRODUCTTIONΒΆ

EPANET [Rossman, 2000, Rossman et al., 2020] is a widely used program for modeling the hydraulic and water quality behavior of drinking water distribution systems. Its water quality component is limited to tracking the transport and fate of just a single chemical species, such as fluoride used in a tracer study or free chlorine used in a disinfectant decay study. In addition, the longitudinal dispersion process, which can play an important role in affecting the water qualities at the dead ends of a water distribution system, is not modeled in EPANET. This manual describes an extension to the original EPANET that allows it to model the advection, dispersion and reaction of any system of multiple, interacting chemical and biological species. This capability has been incorporated into both a stand-alone executable program as well as a toolkit library of functions that programmers can use to build custom applications. This set of software tools is referred to as EPANET-MSX, where MSX stands for Multi-Species Extension.

Many water quality problems in distribution systems can only be analyzed by using a multi-species approach. Consider the following descriptive examples:

  • Free chlorine disinfectant is lost in bulk solution due mainly to oxidation-reduction reactions involving HOCl and OCl-, and natural organic matter (NOM). The NOM itself is a heterogeneous mixture of organic compounds (e.g., humic and fulvic acids) of varying chemical characteristics. Current single-species models, however, must model free chlorine loss under the assumption that all other reactants are in excess and thus their concentrations can be considered constant. This limitation is responsible for the widespread observation that the water-specific decay rate constant of the common first-order model is not a constant at all, but rather varies significantly with chlorine dose (a clear indication of model structure error). The formation of regulated chlorination by-products, which result from free chlorine and NOM interactions, presents yet another set of reaction mechanisms involving multiple interacting species.

  • Mono-, di-, and tri-chloramine result from interactions between free chlorine species and ammonia, and are increasingly used as residual disinfectants. These chloramines also interact with NOM, though the reactions are slower than those for free chlorine. Thus chloramine decay in distribution systems involves multiple interacting chemical species, which a single-species model is forced to simplify as a quasi-first order reaction. Further, ammonia may be produced by auto-decomposition of chloramines, which is of significant practical importance for understanding nitrification episodes in distribution systems and storage tanks. Nitrification models may need to consider attached-growth nitrifying biofilms, suspended nitrifying biomass, and the electron donor (ammonia), electron acceptor (oxygen), and carbon source that supports microbial growth.

  • For the relatively common situation where more than one water source supplies a distribution system, current models are not able to represent meaningful differences in source water quality, as they relate to water quality evolution in the distribution system. Modelers must try to compensate for this limitation by assigning bulk decay rate coefficients to specific pipes, according to which source supplies them. Such an approach has obvious deficiencies when attempting to model distribution system zones where sources blend together, and these zones are sometimes the focus of water quality issues.

None of these examples can be accurately modeled by using the single-species capabilities of the current EPANET program. This shortcoming provides the motivation to extend EPANET so that it can model reaction systems of any level of complexity.

Another feature in EPANET-MSX 2.0 is the option to include the dispersion process in the water quality modeling analysis of a water distribution system. Both EPANET and the original EPANET MSX model the transport of a single species by solving the one-dimensional advection-reaction equation; while EPAENT-MSX 2.0 solves a set of one-dimensional advection-dispersion-reaction equations to analyze water quality transport of mutiple interacting species.

The changes and updates that have been made in version 2.0 of EPANET-MSX include:

  • Improved water quality mass balance and mass balance reporting as in EPANET 2.2.

  • Dispersion modeling as an option to be included in water quality simulation.

  • OPENMP parallelization for both reaction and dispersion simulation codes.

  • A batch file (runvc.bat) is provided to automatically launch the Visual Studio C/C++ compiler and compile the reaction equations.

The following sections of this manual describe the conceptual framework used by EPANET-MSX to model multiple reacting species within a distribution system, provide instructions on how to use both the command line and toolkit versions of EPANET-MSX, give a complete description of the format of an MSX input file, and describe several example applications in detail. The appendices describe each function in the EPANET-MSX toolkit, the format of its binary output file, and the meaning of its error codes.