This study explores the effects of water quality simulation results by embedding axial dispersion into the classical advective-reactive model in a water distribution system. The Eulerian-Lagrangian operator-splitting method is employed to solve the model with axial dispersion. Satisfactory results were obtained after the numerical solution was tested against the analytical and other numerical solutions. The water quality simulation results without the reaction item show that when water velocity is low (or Peclet numbers are small), dispersion is dominant and significantly affects the numerical simulation results. The contaminant concentration at downstream node gradually increased with time along the upstream pipelines from the source, which is particularly reflected in the terminal. The simulation results show that the biomass concentration may demonstrate synthetic effects of axial dispersion and reaction, i.e. mutual promotion, given the multicomponent (substrate, residual chlorine, and microbial biomass) reaction-transport processes. It is particularly reflected in the low flow velocity.