Chlorine Residual Decay (First-Order)
First-order residual C(t) = C0 x exp(-k t), time to a target residual, and an optional booster distance.
Example
You enter
- Initial free chlorine (mg/L) 2
- Decay-rate constant k (1/hr) 0.1
- Elapsed time (hr) 10
- Target residual (mg/L) 0.2
You get
- Residual at elapsed time 0.73576
- Time to reach target 23.0259
Details, formula, and sources
Per EPA 815-R-02-020 and AWWA M14; 40 CFR 141.74 governs the extremity residual.
First-order decay C(t) = C0 x exp(-k x t). Time to target = ln(C0 / target) / k. Booster distance = distribution velocity x time-to-target (when a velocity is entered).
EPA 815-R-02-020 (Effects of Water Age on Distribution System Water Quality); AWWA M14.
epa.gov and awwa.org.
Estimate. Operator of record and primacy agency govern.
Field names used by the API: initial_mg_l, decay_k_per_hr, time_hr, target_mg_l, residual_mg_l, time_to_target_hr
- Decay model bulk first-order C(t) = C0 e^(-kt)EPA 815-R-02-020 water-age model
- Typical k 0.05-0.20 1/hr depending on TOC and temperatureAWWA M14 / EPA water-age studies
- Extremity target 0.2 mg/L default detectable residualEPA 40 CFR 141.74