EPANet’s water quality simulator can track the growth or decay of a substance by reaction, as it travels through a distribution system. In order to do this, it needs to know the rate at which the substance reacts, and how this rate might depend on substance concentration. Reactions can occur, both within the bulk flow and with material along the pipe wall; see below:
In the above example, free chlorine (HOCl) is shown reacting with natural organic matter (NOM) in the bulk phase, and is also transported through a boundary layer at the pipe wall, to oxidize iron (Fe) released from pipe wall corrosion. Bulk fluid reactions can also occur within tanks. EPANet allows a modeller to treat these two reaction zones separately.
Bulk reactions and Wall reactions are now discussed separately.