UV Disinfection Calculator
Calculate UV dose and reactor sizing for pathogen inactivation based on flow rate, UVT, and target log reduction.
About this calculator
Design UV Dose is built from the USEPA UV Disinfection Guidance Manual's (EPA 815-R-06-007, November 2006) Table 1-2 dose requirements for Cryptosporidium, Giardia, and viruses (the manual's virus figures are based on adenovirus, the most UV-resistant enteric virus commonly used to set the requirement). That table's dose-response is not a straight line — each additional half-log of required inactivation costs progressively less marginal dose for the two protozoa but progressively more for viruses, which is why the calculator interpolates between the manual's tabulated points rather than multiplying a flat per-log figure. Bacteria has no entry in that EPA table at all — the manual doesn't set a bacterial dose requirement because bacteria are markedly more UV-sensitive than the tabulated organisms, so selecting Target Pathogen "Bacteria" falls back to a general published E. coli sensitivity estimate instead of an EPA-sourced figure.
Design UV Dose also always increases with Target Log Reduction and Safety Factor, and always decreases as UV Transmittance (UVT) rises — cleaner, more UV- transmissive water needs less design margin to deliver the same dose. Flow Rate has no effect on Design UV Dose at all (dose is a per-area figure independent of throughput); it instead drives Required Power, UV Lamps Required, and the downstream cost estimates, which are simplified engineering approximations rather than figures pinned to a specific published reactor-sizing standard — real UV reactor selection requires vendor-specific validated dose-delivery data, not just this calculator's dose-and-flow arithmetic.
Inputs
Results
Design UV Dose
21 mJ/cm²
Required Power
11.4 kW
≈ 11 microwaves
Figures current as of 2006. Source: U.S. EPA, Ultraviolet Disinfection Guidance Manual for the Final Long Term 2 Enhanced Surface Water Treatment Rule (UVDGM), EPA 815-R-06-007, November 2006
How to Use This Calculator
- Enter Design Flow Rate (MGD) — use the maximum design flow for conservative sizing.
- Select Target Pathogen (Cryptosporidium, Giardia, or virus) and set the Target Log Reduction required by your permit.
- Enter UV Transmittance (UVT %) from your source water or post-filter measurements — lower UVT requires more power.
- Set Safety Factor (typically 1.2-1.5) and review Design UV Dose (mJ/cm²) calculated for the log credit claimed.
- Review Required Power (kW), UV Lamps Required, and Reactor Vessels for equipment procurement and electrical planning.
- Check Estimated Capital Cost and Annual Operating Cost for budget development and life-cycle cost comparisons with chlorination.
How the result changes with Target Log Reduction
| Target Log Reduction | Design UV Dose | Required Power |
|---|---|---|
| 1.5 | 7 mJ/cm² | 3.7 kW |
| 2.25 | 13 mJ/cm² | 6.8 kW |
| 4.5 | 51 mJ/cm² | 27.5 kW |
| 6 | 88 mJ/cm² | 47.3 kW |
What each input means
- Design Flow Rate
- Peak flow rate through the UV reactor.
- Target Pathogen
- Cryptosporidium, Giardia, and virus doses use the EPA UVDGM Table 1-2 requirements. Bacteria has no EPA UVDGM entry (it's markedly more UV-sensitive than the tabulated organisms) and uses a general published E. coli sensitivity estimate instead.
- Target Log Reduction
- Desired log reduction (3-log = 99.9% inactivation, 4-log = 99.99%).
- UV Transmittance (UVT)
- Percentage of UV light transmitted through a 1-cm water sample at 254nm. Lower UVT requires more power.
- Safety Factor
- Design safety factor applied to the required dose. Typical: 1.2-2.0.
How this is calculated
Worked example, using the default values
- Identify Input Parameters5 parametersDesign Flow Rate = 3, Target Pathogen = 4, Target Log Reduction = 3, UV Transmittance (UVT) = 85, Safety Factor = 1.5 = 5 input(s) provided
- Calculate Design UV Dose21 = 21
- Calculate Required PowerRequired Power11.4 = 11.4
- Calculate UV Lamps RequiredUV Lamps Required76 = 76
- Calculate Reactor VesselsReactor Vessels4 = 4
Figures and sources
- UV dose (mJ/cm²) required per 0.5-log inactivation increment for Cryptosporidium, Giardia, and viruses (Table 1-2) (2006) — U.S. EPA, Ultraviolet Disinfection Guidance Manual for the Final Long Term 2 Enhanced Surface Water Treatment Rule (UVDGM), EPA 815-R-06-007, November 2006
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Where does the Design UV Dose figure for Cryptosporidium and Giardia come from?
It's interpolated from Table 1-2 of the USEPA UV Disinfection Guidance Manual for the Final Long Term 2 Enhanced Surface Water Treatment Rule (UVDGM), which tabulates required UV dose in mJ/cm² at 0.5-log increments up to 4.0-log inactivation for Cryptosporidium, Giardia, and viruses. The calculator linearly interpolates between the manual's tabulated points for log-reduction targets that fall between them.
Why does Design UV Dose behave differently for viruses than for Cryptosporidium at the same log reduction?
The EPA's tabulated dose-response curves aren't the same shape for each organism. Virus doses climb steeply with each additional half-log of inactivation credit (based on adenovirus, a relatively UV-resistant virus), while Cryptosporidium and Giardia doses climb more gently — UV light is highly effective against those protozoa even at high log-reduction targets, so achieving the same log credit costs far less dose for them than for viruses.
Why is there no EPA-cited dose figure for the Bacteria option?
The EPA UVDGM's Table 1-2 covers only Cryptosporidium, Giardia, and viruses — it doesn't set a bacterial dose requirement because bacterial reduction is normally sufficient when a system is already sized for virus inactivation, since bacteria tend to be markedly more UV-sensitive. Selecting Bacteria uses a separate, general published E. coli sensitivity estimate instead of an EPA-table figure.
Does increasing Flow Rate change my Design UV Dose requirement?
No — Design UV Dose is a per-area figure (mJ/cm²) that depends only on the target pathogen, log-reduction target, UV Transmittance, and Safety Factor, not on how much water is flowing through the reactor. Flow Rate instead drives Required Power, UV Lamps Required, and the reactor-sizing and cost outputs, since more flow needs more total UV energy delivered per unit time to hit the same dose.
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