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Thread: modelling contaminant decay

  1. #1
    Join Date
    Dec 2016

    modelling contaminant decay

    InfoWater can be used to simulate the decay of contaminants such as chlorine. There are several reference guides about reasonable decay constant k for chlorine.
    I would like to analyze the decay of other potential contaminants such as bacteria, virus or biotoxins.

    Should I use the Chemical/temp option or the Source tray option?
    Is there any literature or references about bulk decay and wall decay for such contaminants?

  2. #2
    Forum Moderator

    Innovyze Employee

    Join Date
    May 2015

    We would suggest you do a literature review of EPANET to see if you can find references on trying to model such decay items as bacteria, viruses and biotoxins.

    I don't think anyone has ever tried to model these mostly because 1) they are very difficult to measure in the field (this is why coliform bacteria presence/absence test are used as a surrogate) and 2) Water utilities add a disinfectant to minimize and deactivate contaminants and simply maintain a residual as protection from the viruses/bacteria/biotoxins growing and multiplying. 3) Since there is no easy way to measure the viruses/bacteria/biotoxins it would be very difficult to actually "validate" or "calibrate" such a model in most real world systems except for under carefully controlled lab conditions making it impractical for many systems to use it.

    However, from a theoretical perspective one likely "could" model these although it likely would be very difficult. Key things to know would include several things
    1) Rates of growth for each viruses/bacteria/biotoxins in both the bulk water and pipe walls
    2) Inactivation rates for each in both the bulk water and pipe walls
    3) Initial concentrations of each viruses/bacteria/biotoxins at all points in the system (or just run long enough to achieve Steady state potentially)

    Since there are multiple species this would likely have to be done using a multi species type analysis as well, and would likely require that the model already had a calibrated disinfectant model already available.

    By the way any reference values for chlorine decay would need to be field validated and the pipe wall decay factors generally have to be field fit to actual measured data used in the field. No one value in a book would adequately represent most water systems as the water chemistry of the unique water and its temperature will play a large roll in the decay. Most bulk decay values are measured by Jar Tests for each water source and pipe decay coefficients are field fit using actual field measurements which is often a very extensive field gathering endeavor.

    This in fact is why so many use Water age as a surrogate as the factors utilities can control can generally be replicated in a water age analysis by simply reducing the amount of time the water can either lose chlorine or grow disinfection by products. Impacts of modifying operation can be evaluated and this is usually "close enough" for many systems although it is not a perfect surrogate especially when chlorine redosing is occurring as this "resets the clock" on how much time would then be a concern.

    Anyway, some thoughts for you to consider as you pursue this option. It is "possible" but may not be "practical" to effectively model this.

    Patrick Moore

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