Complexity of Chloramine Decay Kinetics in Premise Plumbing

Nitrification-driven chloramine decay kinetics have largely been unquantified in premise plumbing, which is particularly vulnerable to opportunistic pathogen growth. Here, we carried out complementary experiments in an at-scale premise plumbing rig with mature biofilms (>4 years age) with influent residuals of <0.2, 0.25, 0.5, 1.0, and 2.5 mg/L as Cl₂ and sterile glass jars, with and without an inoculum containing nitrifying bacteria. Chloramine decay was complete after 8 h of stagnation in all PEX rig pipes (n = 16), tested over a range of diameters (1/4-3/4") and flow rates (0.25-2.2 gpm), with decay rates increasing in situations with higher nitrification rates. The jar experiments revealed that chloramine actually persisted better at higher (37-39 °C) than lower (19-30 °C) temperatures, contrary to standard temperature-adjusted kinetic assumptions, presumably because nitrifiers are inhibited at higher temperatures. Contrary to assumptions made in conventional models, chloramine decay was only effectively modeled as first order in 8/24 cases in the rig experiment (R² > 0.9). The best fit chloramine decay reaction order varied among the rig pipes from 0.88 to 2.74, depending on chloramine dose and exposure time, hydraulics, and modeling method.