Minimum Disinfectant Residual Calculator
Estimate the free chlorine residual and dose needed to meet your selected endpoint target. Example values are illustrative, not recommended operating limits. All concentrations use an as-Cl₂ basis.
Calculator inputs
Results
Equations
C(t) = C₀ exp(−kt). Required initial residual = Ctarget exp(kt). Required applied dose = required initial residual + immediate demand. From measurements: k = ln(C₁/C₂) / Δt. Here k is a positive loss coefficient; EPANET uses a negative coefficient for decay.
Worked example
To retain 0.20 mg/L after 24 hours with k = 0.05 h⁻¹, the required initial residual is 0.6640 mg/L. With 0.50 mg/L immediate demand, the applied dose is 1.1640 mg/L. A current initial residual of 1.00 mg/L falls to 0.3012 mg/L after 24 hours and reaches 0.20 mg/L after 32.19 hours.
Scope and choosing inputs
This free chlorine first-order screening model does not select a legal minimum or establish CT or microbial inactivation. Enter your system's applicable target. Chloramine formation, breakpoint chlorination, nitrification, pH-dependent effectiveness and disinfection byproducts are outside its scope.
C₀ is measured after immediate demand is satisfied. Include only demand consumed before that point, to avoid counting losses twice. For decay fitting, use matching water samples at known times after that initial stage, without redosing or mixing. Equal positive readings imply zero decay; increasing or zero readings cannot establish a positive first-order decay coefficient.
Bottle tests measure bulk-water decay. Pipe walls, biofilms, temperature and mixing can change decay in the system. A single water age represents a parcel or plug-flow estimate, not a mixed tank or a network simulation. Flow converts dose to a continuous daily active-chlorine mass. Use the chemical feed calculator for solution strength and density.
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Equation values and input guide
Enter an applicable endpoint target and representative water age. Supply a fitted decay coefficient or estimate it from paired samples. Separate immediate demand from the subsequent decay period when estimating applied dose.
- C₀, C(t), Ctarget
- Initial residual after immediate demand, predicted residual after elapsed time, and desired endpoint residual. All use mg/L as Cl₂.
- k, t
- Positive first-order loss coefficient and elapsed water age. Their units must match so kt is dimensionless; the selectors convert time and coefficient units.
- exp, ln
- Natural exponential and natural logarithm. exp(−kt) is the fraction of initial residual remaining in the model.
- C₁, C₂, Δt
- Earlier and later residual measurements and elapsed sample time used to estimate k. Use comparable samples after immediate demand, without added disinfectant or mixing.
- Immediate demand
- Concentration consumed before the initial residual measurement, in mg/L as Cl₂. Add it once to the calculated required initial residual.
- Water flow
- Converts applied dose to active chlorine mass per day for continuous operation. It does not determine water age in this tool.
Use the unit selectors beside the inputs. Units shown in the equations must be consistent when checking the math by hand.