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UV dose requirements for drinking water, pharmaceutical purified water, food & beverage, STP effluent reuse, and UVGI air disinfection — with the regulatory standard that applies to each.
The Fundamentals
UV dose (also called UV fluence) measures the amount of UV-C energy at 253.7 nm delivered to a unit area of water. It is expressed in millijoules per square centimetre (mJ/cm²). A higher dose means more energy delivered — and a higher degree of pathogen inactivation.
Dose is determined by the UV-C output of the lamp, the exposure time of the water to the lamp, and the UV transmittance (UVT) of the water. Systems must be sized to deliver the required dose at end-of-lamp-life (EOLL) — not just at commissioning — under the minimum expected UVT of the source water.
Key formula
UV Dose (mJ/cm²) = UV-C Intensity (mW/cm²) × Exposure Time (s)
Reactor UV dose is validated by CFD modelling and/or bioassay testing using MS2 bacteriophage. Alpha UV System provides dose calculation reports from IIT Patna engineers with every system.
UV-C wavelength
253.7 nm
Peak germicidal effectiveness
WHO drinking water dose
40 mJ/cm²
4-log inactivation minimum
Lamp output at EOLL
~80% of initial
Philips UV-C lamps rated
UVT: clear RO water
95–98%
Best-case source water
UVT: secondary STP effluent
55–70%
Worst-case — size for this
Quartz sleeve fouling factor
0.85–0.90
Applied to dose calculation
WHO 4th Ed.
Min. Dose
40 mJ/cm²
Design Target
40–55 mJ/cm²
Log Reduction
4-log (99.99%)
CDSCO Schedule M 2025
Min. Dose
100 mJ/cm²
Design Target
100–400 mJ/cm²
Log Reduction
≥4-log (Schedule M)
FSSAI / HACCP
Min. Dose
40 mJ/cm²
Design Target
40–80 mJ/cm²
Log Reduction
4-log (HACCP CCP)
CPCB Discharge Norms
Min. Dose
30 mJ/cm²
Design Target
30–50 mJ/cm²
Log Reduction
≤100 MPN/100ml
WHO Pool Guidelines
Min. Dose
25 mJ/cm²
Design Target
25–40 mJ/cm²
Log Reduction
3–4-log
ASHRAE 62.1 / CDC HICPAC
Min. Dose
1–10 mJ/cm² (air)
Design Target
Pathogen-specific
Log Reduction
2–4-log air pathogens
Design targets include margin over minimum for lamp aging, quartz fouling, and UVT variation. Contact us for dose calculation specific to your water quality and flow rate.
Different pathogens require different UV doses for inactivation. System design is driven by the most UV-resistant target pathogen in your water source.
| Pathogen | 3-log dose | 4-log dose | Notes |
|---|---|---|---|
| E. coli | 3 mJ/cm² | 6 mJ/cm² | Most chlorine-sensitive bacteria |
| Salmonella typhi | 4 mJ/cm² | 7 mJ/cm² | Typhoid — key drinking water pathogen |
| Legionella pneumophila | 2 mJ/cm² | 3 mJ/cm² | Cooling towers, HVAC condensate |
| Rotavirus | 30 mJ/cm² | 40 mJ/cm² | Most UV-resistant common virus |
| Adenovirus | 60 mJ/cm² | 90 mJ/cm² | Most UV-resistant human virus |
| Giardia lamblia | 5 mJ/cm² | 8 mJ/cm² | Chlorine-resistant protozoan |
| Cryptosporidium parvum | 10 mJ/cm² | 22 mJ/cm² | Chlorine-resistant — UV is required |
Source: USEPA UV Disinfection Guidance Manual (2006); Hijnen et al. (2006); Bolton & Cotton, The Ultraviolet Disinfection Handbook (2008).
Critical Design Variable
RO / DI water
95–99%
Standard lamp loading
Deep well / groundwater
85–95%
+10–20% lamp loading vs. RO
Surface / river water (filtered)
75–88%
+25–40% lamp loading
STP/ETP secondary effluent
55–70%
+60–100% lamp loading
Always size for minimum UVT, not average. If your STP effluent UVT drops to 55% after heavy rainfall events, your UV system must deliver 40 mJ/cm² at 55% UVT — not at the dry-season average of 68%. Alpha UV System measures UVT on-site before finalising system design.
WHO Drinking Water Guidelines (4th edition) recommend a minimum UV dose of 40 mJ/cm² for 4-log (99.99%) inactivation of pathogenic bacteria, viruses, and protozoa. BIS IS 10500:2012 does not specify a UV dose directly, but achieving microbiological compliance with IS 10500 requires a UV dose that delivers 4-log inactivation under worst-case operating conditions (end-of-lamp-life, fouled quartz sleeve, minimum UVT in source water). All Alpha UV System drinking water products are validated to 40 mJ/cm² at end-of-lamp-life.
UV transmittance (UVT) measures how much UV-C light at 253.7 nm passes through 1 cm of water. Clear drinking water (RO or deep well) typically has UVT of 90–98%. River water or post-filtered surface water typically has UVT of 75–90%. STP secondary effluent typically has UVT of 55–70%. As UVT decreases, less UV-C reaches pathogens in the water. UV systems must be sized for the minimum expected UVT in the water source — not average or best-case values. Undersized UV systems will fail to deliver the required dose during low-UVT conditions.
UV dose (mJ/cm²) = Lamp UV-C output (mW) × Exposure time (seconds) ÷ Flow path area (cm²). In practice, UV dose in a reactor is calculated using computational fluid dynamics (CFD) models or validated by bioassay testing (using MS2 bacteriophage as a UV-resistant surrogate). The installed dose must account for lamp aging (UV output degrades to ~80% of initial by end of rated life), quartz sleeve fouling (typically 10–20% reduction), and the water UVT range. Alpha UV System provides a UV dose calculation report with every system, prepared by IIT Patna engineers.
CDSCO Revised Schedule M 2025 does not specify an exact UV dose, but the requirement is that UV disinfection must achieve the microbiological limits for purified water: total aerobic microbial count (TAMC) ≤100 CFU/ml, total yeast and mould count (TYMC) ≤10 CFU/ml, and absence of specified organisms. In practice, pharmaceutical UV systems are designed to deliver 100–400 mJ/cm² to ensure reliable inactivation of drug-resistant biofilm organisms and the margin required for IQ/OQ/PQ validation. Alpha UV System pharmaceutical systems come with IQ/OQ/PQ documentation templates and dose calculation reports.
For CPCB-compliant STP effluent reuse, the target is ≤100 MPN/100ml total coliform, which typically requires 30–50 mJ/cm² at the water's actual UVT (usually 55–70% for secondary effluent). For agricultural irrigation reuse under CPCB guidelines, a slightly lower dose may be acceptable. For industrial process water reuse, the dose depends on the intended use. Alpha UV System sizes STP/ETP UV systems based on measured UVT in the effluent — not assumed values.
A new Philips UV-C lamp at commissioning delivers 100% of its rated UV output. By end of rated life (9,000 hours for standard, 12,000 hours for long-life Philips lamps), UV output degrades to approximately 80% of initial — this is the End-of-Lamp-Life (EOLL) output. UV systems must be sized to deliver the required dose at EOLL, not just at commissioning. If a system is sized only for commissioning conditions, it will fail to deliver target dose for the last 30–40% of the lamp's operating life. All Alpha UV System sizing calculations use EOLL lamp output.
The UV dose calculation report prepared by IIT Patna engineers includes: system design flow rate (LPH), water UVT range (minimum, typical, maximum), lamp model and rated UV-C output (mW at 253.7 nm), number of lamps, lamp spacing, reactor geometry, UV dose at commissioning (mJ/cm²), UV dose at EOLL (mJ/cm²), margin over target dose (%), and the pathogen log-inactivation achieved at EOLL. The report references WHO, USEPA, and CDSCO guidelines as applicable. CDSCO auditors and HACCP auditors accept this format.
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IIT Patna engineers provide dose calculation reports accepted by CDSCO, FSSAI, and CPCB auditors.
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