Validity of chlorine-wall reaction models for drinking water distribution systems

Chlorine concentrations in water distribution systems are generally predicted by combined models of reactions in bulk water and at pipe walls. The structure of the widely used EPANET wall-reaction models is questioned, as they do not reproduce the variation in wall-reaction rate with decreasing chlorine observed in real pipelines. The microbially mediated wall-reaction model (EXPBIO) is structurally valid. EXPBIO was extended to calculate the mass-transfer coefficient in individual pipes, rather than using a single fitted value. Smooth- and rough-pipe versions were formally validated against observed chlorine data from the Mirrabooka pipeline, where rough-pipe predictions better matched lower observed chlorine concentrations. In a medium-sized rough pipe, the mass-transfer coefficient doubled between 10 and 30 & DEG;C. In the real pipeline, chlorine concentration decreased much faster with distance downstream at higher temperature, due to increasing microbial activity and mass-transfer of chlorine. System simulations to search for improved seasonal chlorine dosing strategies need to include these effects.