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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 focusing 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, to prevent bacteria growth; an overdose could be harmful, whereas not enough would promote bacteria growth.
The chemistry of the reactions between chlorine and the organic materials present in water is complex and, although extensively studied, poorly understood throughout the extent of distribution systems.
The chlorination of organic matter present in raw water supplies results in THMs (Trihalomethanes) formed in drinking water as a disinfection by-product; the rate and degree of THM formation normally increases mainly as a function of the chlorine concentration, 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 possibly carcinogenic to humans; this is the THM present in greatest concentration in drinking-water. Because of this, total THM level should be monitored and stay 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 first order modelling method.
According to first order kinetics, there is only one component involved. Therefore, modelling with first order kinetics, chlorine concentration is assumed to be decreased over time by itself and it does not consider 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 has not represented a good data fit in different applications
Reactive-constituent decay models
An additional step could be taken by adopting a reactive-constituent model (also known as parallel second order model) of chlorine decay. This is formed by considering two types of natural organic matter reaction with chlorine – 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.
Rate constant coefficients are invariant, for given water quality once determined in simple lab. experiments, no further adjustment needed for different initial dose or temperature.
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.