Modelling chlorine residual and trihalomethane profiles in water distribution systems after treatment including pre-chlorination

Accurate, efficient distribution system models predict profiles of chlorine and trihalomethane concentrations using chemical reactions. Constant coefficients defining reaction-rates are typically derived from laboratory decay-tests conducted days after sampling from a particular water treatment plant (WTP). The problem is any post-filter chlorine residual due to pre-chlorination will have reacted completely beforehand. Consequently, the derived reaction-model will not accurately represent water exiting the WTP filters. Two methods to include these missing reactions were successfully developed. They have similar accuracy but the second method is less efficient as it adds a third reactant and requires estimation of two more coefficients. Both require an additional decay-test at the WTP, commencing immediately after sampling. Due to pre-chlorination, the linear increase in total trihalomethane (TTHM) formation with increasing chlorine demand (after primary disinfection) commences at some positive TTHM concentration. The TTHM concentration measured at the laboratory must be reduced by that resulting from the post-filter residual reactions, to accurately represent TTHMs leaving the WTP. Where treatment does not involve pre-chlorination, some downstream chlorine demand does not produce TTHMs. This was accurately modelled by identifying a fraction of fast reactant as non-productive. The work extends applicability and improves accuracy of chlorine/trihalomethane modelling of distribution systems.