Also in: Disaster Response and Recovery
Disinfection Contact Time from a Baffling Factor
The CT a disinfection step earns depends on T, the time by which only 10% of the water has passed through.
Example
You enter
- Vessel volume from round
- Volume at minimum operating level (gal) 0
- Round tank inside diameter (ft) 40
- Rectangular tank length (ft) 0
- Rectangular tank width (ft) 0
- Minimum operating depth (ft) 30
- Peak hourly flow (gpm) 347
- Baffling condition (EPA Table 4-2) unbaffled
- Entered baffling factor (0.1-1.0) 0
- Outlet free-chlorine residual (mg/L, optional) 0
You get
- Vessel volume (gal) 282000
- Tdt (min) 813
- Baffling factor 0.1
- T10 (min) 81.3
Checked against published worked examples from: U.S. EPA.
Details, formula, and sources
ScopeThe CT a disinfection step earns depends on T, the time by which only 10% of the water has passed through, and a vessel reaches it far sooner than its volume suggests. This calculator derives T the way EPA 815-R-20-003 does: the theoretical detention time is the volume at the MINIMUM operating level divided by the PEAK hourly flow, and T is that time multiplied by the Table 4-2 baffling factor -- 0.1 unbaffled, 0.3 poor, 0.5 average, 0.7 superior, 1.0 plug flow. The manual's own Example 4-1, a 40 ft diameter clearwell at a 30 ft minimum depth carrying 347 gpm with no baffling, holds about 282,000 gal, a detention time of 813 minutes, and a contact time of 81.3 minutes -- a tenth of the naive figure. Emergency systems are where this matters most. A 20,000 gal pillow bladder with one inlet and one outlet fed at 50 gpm has a 400 minute detention time but only 120 minutes of contact time if it is merely poor, and 40 if it mixes completely; at 1.0 mg/L free chlorine at the outlet that is a CT of 120 or 40 mg-min/L. A length of pipeline after the injection point is close to plug flow and is often worth more than a large tank. The calculator shows T at every factor so the arrangement can be compared, and the CT when an outlet residual is entered. The factor is a judgment of the vessel's hydraulics, and a tracer study supersedes it. It does not compute the required CT for a log inactivation, pH, and temperature, the chlorine demand, or residual decay, and it does not design baffles. EPA 815-R-20-003, the primacy agency, and the operator of record govern.
Formula or tableTDT = volume at the minimum operating level / peak hourly flow (min); T (t10) = TDT x BF (EPA Eq. 4-3); BF = 0.1 unbaffled, 0.3 poor, 0.5 average, 0.7 superior, 1.0 plug flow (EPA Table 4-2), or an entered tracer-study factor; CT = outlet residual x T.
Source and editionEPA 815-R-20-003, Disinfection Profiling and Benchmarking Technical Guidance Manual (June 2020), Section 4.4, Equation 4-3, Table 4-2, and Example 4-1, read at epa.gov on 2026-09-30. The primacy agency's tracer-study or baffling-factor determination governs.
Free accessA free EPA guidance manual; the five published baffling factors are restated as selectable values and the arithmetic is volume over flow.
What governsEstimate. Operator of record and primacy agency govern.
API field namesvessel_shape, volume_gal, diameter_ft, length_ft, width_ft, min_depth_ft, peak_flow_gpm, baffling_condition, custom_baffling_factor, residual_mg_l, vessel_volume_gal, tdt_min, baffling_factor, t10_min
- Minimum operating level the volume when the tank is drawn down, not the nominal capacityEPA 815-R-20-003 Section 4.4
- Peak hourly flow the flow when the least water is in the tank and the most is leavingEPA 815-R-20-003 Section 4.4
- A tracer study supersedes the factor the Table 4-2 factors are judgments of the hydraulics, and the primacy agency may require a study or assign a factorEPA 815-R-20-003 Table 4-2