A mathematical model that accounts for simultaneous transport of subst
rates, disinfectants and microorganisms has been developed to predict
substantial changes in quality of distributed water. The model consist
s of a set of mass balance equations for organic substances, ammonium
nitrogen, oxidized nitrogen, dissolved oxygen, alkalinity, biomass, an
d disinfectants in the bulk liquid phase and within the biofilm under
laminar and turbulent flow conditions. This model is validated by comp
aring its solutions with the numerical solutions in the literature and
then is applied for predicting the behavior of a typical water treatm
ent plant effluent through a distribution pipe. The flow properties an
d disinfectant consumption rate at the pipe wall play a significant ro
le in the determination of potable water quality in the distribution s
ystem.