Multi-Solute Water Quality (MSQ) Modelling

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July 10, 2020 | Luca Serena

4 minute read

Innovyze announces the Beta Release of a new water quality module in InfoWorks WS Pro normal package, named MSQ (Multi-Solute Water Quality).

The MSQ module matches the EPANET MSX extension and allows the end-user to model complex reactions between multiple chemical and biological species, such as auto-decomposition of chloramines to ammonia and the formation of disinfection by-products.

This article is intended to show the capabilities of the new MSQ module focussing on the specific issue of modelling chlorine decay, starting from a brief description of the phenomena and then comparing the former “Basic” approach of a single-constituent decay versus more advanced methodologies provided by the new MSQ module. 

Modelling Chlorine Decay

Chlorine is commonly added to water as a disinfectant, at the last stage of treatment and often during distribution, to prevent bacterial growth; an overdose could produce harmful levels of by-products, whereas not enough would promote bacterial growth.

The chemistry of the reactions between chlorine and the organic materials present in water is complex and system-specific. Although extensively studied, these reactions are poorly understood throughout the extent of distribution systems.

The chlorination of organic matter present in raw water supplies results in formation of THMs (trihalomethanes) in drinking water as a disinfection by-product; the rate and degree of THM formation normally increases mainly as a function of the chlorine reacted since dosing, temperature, and pH. The THM concentration also increases in the distribution system as the water moves out from the water treatment plant due to the continued presence of chlorine residuals.

Chloroform is classified as carcinogenic to humans; this is the THM present in greatest concentration in drinking-water. The other THMs are even more hazardous to health. Because of this, the total THM level should be monitored and kept below the tolerable daily intake, hence the importance of modelling.

Single-constituent decay models

One of the earliest, simplest and initially most popular approaches for modelling chlorine decay was labelled the first-order modelling method.

According to first-order kinetics, there is only one component involved. Therefore, in modelling with first-order kinetics, chlorine concentration is assumed to decrease over time of its own accord and it does not consider any other species with which chlorine is reacting.

This is what is available in InfoWorks WS Pro under the “Basic” water quality tools, where the general first-order kinetic expressions for reservoir chlorine decay in bulk water is expressed as follow:

where ccl is chlorine concentration at time t, Cl0 is initial chlorine concentration [mg/L] and k is the decay constant.

The decay constant (k) can be defined within the Network object under the WQ Reaction Coefficient.

Despite the simplicity and easiness of this model, it does not accurately represent the fast chlorine decay rate that occurs after initial dosing of real raw and treated waters. The decay constant also needs to be re-evaluated to represent the slower decay rate that occurs after each booster dose.

Reactive-constituent decay models

These problems are overcome by adopting a reactive-constituent model of chlorine decay (also known as the parallel second-order model, first published in 1999 and since shown applicable to a wide range of waters and operating conditions (e.g. by Fisher et al. 2017). This model is formed by considering the reaction of chlorine with two types of natural organic matter – fast and slow reacting – and consists of two simultaneous parallel reactions with the overall second-order kinetics as follows:

where FRA is the concentration of Fast reacting Reducing Agents and SRA is the concentration of Slow reacting Reducing Agents in the water.

The second-order reaction rates could be given as follows:

where CS, CF and kS, kF are concentrations of the reacting agents and their rate constants respectively for Slow and Fast reacting agents. CTHM is the total THM concentration generated from the two parallel reactions and yTHM is the THM yield per unit of chlorine reacted.

Rate constant coefficients are invariant, for given water quality. Once determined from simple lab experiments, no further adjustment is needed for different initial doses, temperatures or successive booster doses.

The first-order term [in square brackets above] is not part of the parallel second-order model. It is occasionally needed when the lab. experiments are not run for as long as the maximum water age in the system.

(Reference: Fisher, I., Kastl, G., Sathasivan, A. (2017). A comprehensive bulk chlorine decay model for simulating residuals in water distribution systems Urban Water Journal 14 (4), 361-368)

InfoWorks WS MSQ allows you to implement your own equations to efficiently model reactive constituents in the comfort of your InfoWorks WS Pro user interface.

The solute data object is normally used when carrying out a water quality simulation and it contains data parameters which will be used to calculate substance reaction rates.

In WS Pro v5.0 (or below) the Solute Data Object looks as follow:

The Solute Object has now been expanded to include the formerly “Basic” water quality module and a new “MSQ” section which provides the ability to input user-defined solute objects, constants, variables as well as reservoir and pipe equations:

Initial source concentration can be defined within the Control Object on a new Water Quality (MSQ) section in the property sheet:

Network Validation now includes the Solute Object to check all parameters prior to running the simulation.

Result comparison between single-constituent (Basic) and reactive constituent (FCL) water quality results is shown below, looking at the source (Reservoir KRES32) and at two nodes in the distribution network.

When following the reactive constituents model, we can now start to see THM concentration appearing as FCL decays:

A theme can now be created to display THM concentration throughout the network.


Built on top of EPANET MSX extension, InfoWorks WS Pro MSQ allows the end user to model complex reactions in a water distribution system within the advanced and user-friendly InfoWorks interface.

Please contact us if you would like to become a beta tester or have any questions.

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Tags: Infoworks Ws Pro

About the Authors

Luca Serena

Luca Serena

Product Manager


Luca Serena is product manager for Innovyze's Water Distribution product family.