How to model DBP formation in WaterGEMS or WaterCAD?
There are two general ways to model this with WaterGEMS.
The simpler way is to model DBP formation as a constituent that grows to reach an equilibrium concentration. You would start with a calibrated EPS model and then define a bulk and wall reaction rate and equilibrium concentration. You can try to get a handle on the bulk and equilibrium values with some bottle tests in the lab. If you want to have confidence in the model, you will eventually need to do some field calibration work until you can reasonably match the field measurements. You can adjust the reaction rate for temperature using K2 = K1 Theta^(T2-T1) where Theta is usually on the order of 1.04 and K1 is the rate constant at temperature T1.
A more thorough but difficult approach is to use our multi-species extension which enables you to more accurately model the reactions between chlorine, DBP's and precursors. You need to set up rate reaction equations for the interactions between the three (or more) constituents and determine the reaction constants for your water. This feature is available for the SELECTseries 5 release and later.
Note - Using the Concentration Limit Field
The concentration limit was implemented in EPANET to model disinfectant by-product (DBP) formation. It only applies to the bulk reaction rate. The DBP formation reactions occur in the bulk fluid, not at the wall.
When you mix chlorine with natural organic matter (NOM), which are also referred to as DBP precursors, you create DBP. This causes DBPs to increase. However, given sufficient chlorine, the NOM will get used up and the concentration will level off at the concentration limit (CL).
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