These are EPANET’s two first-order chlorine reaction coefficients.
Bulk decay is a property of the water (organics, iron, manganese, ammonia) and acts everywhere
including inside the OHR. Wall decay is a property of the pipe (corrosion, tuberculation, biofilm),
acts only in pipes, and bites harder in small-diameter pipes.
Bulk decay sets the overall residual level; wall decay sets how fast it falls off towards the tail end.
| Bulk decay (per day) | Use when |
|---|
| 0.1–0.2 | Clean treated groundwater, effective iron/manganese removal, cool season. |
| 0.3 (default) | Typical treated tubewell supply — safe default when no bottle test exists. |
| 0.4–0.8 | Untreated or partially treated surface/canal water, or groundwater with residual iron, manganese or ammonia reaching the network. Fully treated surface water is normally lower. |
| 0.8–1.0+ | High organics, water above ~30 °C, or long storage in a lightly-drawn OHR. |
| Wall decay (m/day) | Use when |
|---|
| 0.00–0.05 (default 0.05) | Newly commissioned network — all HDPE/UPVC/cement-lined DI, no biofilm history. |
| 0.1 | Mixed or few-year-old network in normal condition. |
| 0.2–0.5 | Older unlined GI/CI, visible tuberculation, chronic low residual or known biofilm. |
Bulk decay roughly doubles for each 10 °C rise, so a value from a winter bottle test should be
re-checked for summer. Measure bulk decay directly: fill amber bottles with no headspace at the chlorination
point, hold at distribution temperature, read free residual at 0/1/2/4/8/24/48 h, and take the slope of
ln(C/C₀) against time. Wall decay cannot be bottle-tested — fix bulk decay first, then adjust wall decay
until modelled residual matches field FRC readings at the tail ends.
Sources. EPANET 2.2 User Manual (US EPA) — typical first-order wall coefficient
0.25–1.0 ft/day (= 0.076–0.30 m/day), negligible for new plastic or cement-lined pipe ·
Rossman, Clark & Grayman, J. Environmental Engineering 120(4), 1994 — the mass-transfer
wall-decay model EPANET implements ·
Vasconcelos et al., Journal AWWA 89(7), 1997 — field-measured bulk decay constants ·
AWWA Manual M32 — calibration against field data ·
IS 10500 / CPHEEO — 0.2 mg/L minimum free residual at the consumer end.