Estimation of chloramine decay in drinking water distribution systems

When compared to free chlorine, chloramines represent an established secondary disinfectant that may be utilized to reduce disinfection by-product formation and improve residual stability. Maintaining an acceptable residual in distribution systems however may be challenging due to potential high maintenance costs associated with uni-directional flushing and swabbing. This study was designed to identify the primary sources of chloramine demand for a distribution system receiving surface and groundwater. Decay in the bulk water was quantified using a bottle point method. Bench-scale pipe section reactors (PSRs) were utilized to evaluate decay associated with cast‑iron and polyvinyl chloride (PVC) pipe sections that had been harvested from a distribution system, as well as to allow comparisons with new pipe material. Bulk water decay was compared to that observed in PSRs at flow rates of 0.03 m/s and 0.09 m/s, representative of distribution system conditions. Pipe material type and associated surface conditions were identified as the most critical factors influencing decay. Observed first-order decay rates for harvested cast‑iron pipe with an established microbial community and metal/silicate scale were significantly higher than those for PVC. In contrast, first-order decay rates for harvested PVC were comparable to new pipe, with water velocity exerting a negligible impact. Overall, these results reinforce the importance of determining the primary sources of chloramine decay in distribution systems.