Surface Water Treatment Rule CT check: compare CT-achieved (C·T10) against the CT-required you read from the governing table, summed over sequential disinfection segments. Runs entirely offline; inputs persist in this browser via localStorage.
This calculator does not certify compliance. The state primacy agency and the system's approved monitoring plan certify compliance — this tool only does the arithmetic on numbers you supply. It deliberately does not embed the SWTR CT tables: you must read each CT-required value from the current 40 CFR 141 Subpart H tables (or your state's adopted version) and the USEPA Guidance Manual, for your exact disinfectant, temperature, pH, and residual. Required CT shifts steeply with temperature and pH — a value from the wrong table cell is a wrong answer that looks right.
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Sum of inactivation ratios
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Effective log inactivation achieved
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Target log inactivation
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Conditions on record
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How the Numbers Are Derived
CT-achieved and T10
CT-achieved = C · T10, with residual C in mg/L and T10 in minutes. T10 is the detention time the fastest 10% of water experiences: T10 = (contact volume ÷ peak hourly flow) × baffling factor. In consistent field units, T10 (min) = volume (gal) ÷ flow (gpm) × BF. The baffling factor is the ratio T10/T that accounts for short-circuiting — 1.0 only for true plug flow like a pipe.
Ratios, sequential segments, and log inactivation
For each segment the inactivation ratio is CT-achieved ÷ CT-required, where CT-required is the value you read from the SWTR table for the log target and conditions. A segment that delivers the full required CT has a ratio of 1.0. Sequential segments (e.g. a pipeline, then a clearwell, then the distribution entry point) add their ratios, and compliance requires Σ ratios ≥ 1.0. The effective log inactivation achieved is log-target × Σ ratios — so a summed ratio of 1.0 exactly meets the target log.
Why this tool makes you enter CT-required
The SWTR CT tables are large lookup tables that vary steeply and non-linearly with temperature, pH, and residual. Transcribing them from memory into a calculator is how subtle errors get institutionalized. So this tool asks you to read each CT-required straight from the governing table for the exact cell your conditions fall in, and it does the ratio arithmetic on that. The conditions fields above exist only to make sure you pull the right cell — and to put those conditions on the printout.
Field notes
Peak hourly flow governs T10, not average day — the worst case is the busiest hour, when contact time is shortest.
Baffling-factor presets are for planning. A tracer study (step-dose or slug) measures the real T10/T for a specific basin at a specific level; use it for reporting where it exists.
Measure the residual at the outlet of the segment it credits — the concentration that survived the contact, not what went in.
Temperature is the big lever. The same basin that passes in August can fail in February because required CT roughly doubles as water goes cold — profile the cold-water condition.
Filtration provides most of the Giardia/virus credit; disinfection typically covers the remainder. Set the log target to the disinfection portion your plan assigns, not the whole plant requirement.
References
USEPA — Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water Sources (the CT tables and T10/baffling guidance).
40 CFR 141 Subpart H — Surface Water Treatment Rule (and the state primacy agency's adopted version).
The system's approved disinfection profiling/benchmarking and monitoring plan — governing for what must be demonstrated.