Hydraulic retention time for basins at a given flow, and disinfection contact time: T10 from EPA baffling factors, achieved CT, and the ratio against a required value. Runs entirely offline; inputs persist in this browser via localStorage.
Use limits: The required CT value comes from the regulatory tables (temperature, pH, disinfectant, and log-inactivation credit sought) or from the permit — this tool doesn't reproduce those tables; it computes what the basin achieves so you can compare. The baffling factor is a claim about hydraulics that the primacy agency may require a tracer study to substantiate. Compliance determinations belong to the operator of record and the permit, not a field calculator.
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Effective volume
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Theoretical HRT (V/Q)
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T10 (HRT × BF)
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CT achieved (residual × T10)
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CT ratio (achieved / required)
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Max flow that still meets required CT
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How the Numbers Are Derived
The math
HRT (min) = V (gal) / Q (GPM)
T10 = HRT × BF — the time the fastest 10% of the water actually spends in the basin, which is what disinfection credit is based on. CT = C_residual × T10, compared against the required CT for the target log inactivation. The max-flow line inverts the chain: Q_max = V × BF × C / CT_req.
EPA baffling factor descriptions
BF
Condition
What it looks like
0.1
Unbaffled
Agitated or mixed basin; inlet and outlet close together; clearwell with no baffles
0.3
Poor
Single or no intra-basin baffles; unbaffled inlet/outlet
0.5
Average
Baffled inlet or outlet with some intra-basin baffling
0.7
Superior
Perforated inlet baffle, serpentine or perforated intra-basin baffles, weir or perforated launder outlet
These are the EPA SWTR guidance defaults. A tracer study can substantiate a higher (or reveal a lower) T10/T ratio than the table gives — the custom entry is for that.
Field notes
Volume should be the effective volume at the actual operating level — a clearwell run at half depth has half the CT the drawings imply.
During startup, low plant flows make CT easy; the check that matters is at design peak flow, which is why the max-flow output exists — it's the operating envelope in one number.
Residual decays through the segment. Using the outlet residual is the conservative convention; using the inlet residual flatters the number.
Multiple segments in series (basin + pipeline) each get their own CT and the credits sum — run this tool per segment.
References
US EPA — Surface Water Treatment Rule guidance manuals (CT tables, baffling factors). https://www.epa.gov/dwreginfo/surface-water-treatment-rules
EPA Disinfection Profiling and Benchmarking Guidance Manual — T10 determination and tracer studies.
State primacy agency guidance and the facility's operations permit — governing requirements.