📖 How To Use
How to Use This UV Disinfection Calculator
Sizing a UV disinfection system correctly takes three inputs and under a minute:
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Choose your mode: Batch Tank or Continuous Flow
Use Batch Tank if you're disinfecting a stored volume (e.g., a rooftop tank or cistern). Use Continuous Flow if water passes through a UV reactor inline — like a point-of-entry system or a municipal line.
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Select your target pathogen
Each pathogen requires a different minimum UV dose in mJ/cm² to achieve 4-log (99.99%) inactivation. Cryptosporidium needs only 10 mJ/cm²; adenovirus needs 100. If your target isn't listed, choose "Custom dose" and enter the value from your water quality report or regulatory standard.
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Enter tank volume (batch) or flow rate (continuous)
For batch mode: the total volume you need to disinfect. For flow mode: the maximum flow rate the system must handle at peak demand.
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Enter UV lamp output and system efficiency
Lamp output is in electrical watts — check your lamp's spec sheet. System efficiency (typically 70–85%) accounts for quartz sleeve losses, water transmittance, and lamp aging. Default to 80% if unsure.
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Read your results
The calculator outputs required dose, minimum exposure time, UV intensity at your system geometry, log reduction value (LRV), and a pass/fail status based on your inputs.
Important: This calculator assumes ideal plug-flow conditions. Real reactors have hydraulic inefficiencies. Apply a minimum safety factor of 1.5× to the calculated lamp power for certified installations. Always validate with biodosimetry testing for regulated water systems.
📐 The Formula
UV Disinfection Formula Explained
UV disinfection works by delivering enough radiant energy to destroy a pathogen's DNA, preventing reproduction. The critical metric is UV dose (also called UV fluence):
UV Dose (mJ/cm²) = UV Intensity (mW/cm²) × Exposure Time (s)
Effective UV Power (mW) = Lamp Output (W) × Efficiency (%) × 1000
UV Intensity (mW/cm²) = Effective Power (mW) ÷ Treatment Volume (cm³) × path length
For batch tanks:
Exposure Time (min) = [Dose × Volume (L) × 1000] ÷ [Effective Power (mW) × 60]
For continuous flow:
Max Flow Rate (L/min) = [Effective Power (mW) × 60] ÷ [Dose × 1000]
Log Reduction Value (LRV) represents the order-of-magnitude reduction in pathogens. 1 LRV = 90% killed, 2 LRV = 99%, 3 LRV = 99.9%, 4 LRV = 99.99%.
Minimum UV Dose Reference Table
| Pathogen | 1-log (mJ/cm²) | 2-log (mJ/cm²) | 4-log (mJ/cm²) | Source |
|---|
| E. coli | ~3 | ~6 | 25–40 | EPA/NWRI |
| Giardia lamblia | ~5 | ~10 | 22–30 | EPA UVDGM |
| Cryptosporidium | ~2.5 | ~5 | 10–12 | EPA LT2 |
| Adenovirus | ~8 | ~18 | 90–100 | EPA UVDGM |
| Legionella | ~4 | ~8 | 30–36 | NWRI |
| Cholera (V. cholerae) | ~3 | ~6 | 22–26 | WHO |
Note on Adenovirus: UV dose for adenovirus is 2.5–10× higher than for bacteria. If your water may contain adenovirus (common in surface water and some groundwater), size your system accordingly — a lamp sized for E. coli will be badly undersized.
🎯 Use Cases
When to Use UV Disinfection
UV disinfection is most appropriate when:
Rooftop and Household Storage Tanks
Static stored water is vulnerable to algae, bacterial regrowth, and insect contamination. A batch UV treatment before distribution provides a chemical-free disinfection step — no chlorine taste, no byproducts. Size the lamp so the entire tank volume receives the required dose within a manageable treatment window (typically 4–8 hours).
Borehole and Well Water
Groundwater contaminated with coliforms or Giardia from agricultural runoff is a common issue in rural areas. UV is ideal here since the water typically has low turbidity (high transmittance), maximising lamp efficiency. Run continuous-flow UV inline at the pump outlet before any storage.
Rainwater Harvesting Systems
First-flush diverters remove the worst contamination, but harvested rainwater still carries bacteria from roof surfaces and bird droppings. UV treatment before use is especially important in potable applications — use our First Flush Diverter Calculator alongside this tool to size both steps.
Swimming Pools and Spas
UV dramatically reduces the chlorine demand in pools by destroying chloramines and providing primary disinfection. Size the system for the full recirculation flow rate — typically 4–6 complete turnovers per day — and accept lower doses (25–40 mJ/cm²) since residual chlorine supplements the treatment.
What UV Does NOT Do
UV does not remove dissolved chemicals, heavy metals, nitrates, or improve taste and odour. It has no residual effect — water can be recontaminated after treatment. Always combine UV with physical filtration (≥5 micron) upstream, and consider a carbon filter downstream for taste. For chemical contamination, see our Water Filter Flow Rate Calculator.
❓ FAQ
Frequently Asked Questions
What UV dose do I need to disinfect drinking water?
For general drinking water disinfection targeting bacteria and protozoa (Giardia, Cryptosporidium), a minimum dose of 40 mJ/cm² is the standard used by the EPA and most international guidelines. If adenovirus is a concern — common in surface water — you need 100 mJ/cm² or more. For Cryptosporidium alone, 10 mJ/cm² achieves the required 4-log reduction per EPA LT2 rules.
How do I calculate UV dose for a tank?
UV Dose (mJ/cm²) = UV Intensity (mW/cm²) × Exposure Time (seconds). For a batch tank, rearrange to find the exposure time needed: Time (s) = [Required Dose × Tank Volume (cm³)] ÷ [Effective Lamp Power (mW)]. The effective lamp power equals the electrical wattage × system efficiency (typically 70–85%). This calculator does all of that automatically — just enter your values.
What is a typical UV system efficiency?
Most UV systems operate at 70–85% efficiency. Losses come from the quartz sleeve (5–10%), UV transmittance of the water (varies enormously — crystal clear groundwater may hit 95%+ UVT, turbid surface water can drop below 50%), and lamp aging (output drops ~20% over the lamp's rated life). For sizing purposes, use 80% as a conservative default, and replace lamps annually regardless of remaining rated hours.
Does UV disinfection kill all bacteria and viruses?
At adequate dose, UV inactivates virtually all waterborne pathogens. However, "inactivate" is not the same as "kill" — UV damages DNA so organisms can't reproduce, but they remain in the water. Some organisms have DNA-repair mechanisms that can partially recover from low-dose UV exposure in the presence of visible light (a phenomenon called photoreactivation). This is why UV must deliver the full required dose in a single pass, not spread across multiple sub-threshold exposures.
How long should I run a UV lamp in a storage tank?
This depends entirely on lamp power and tank volume. A 40W lamp (at 80% efficiency, 32W effective) treating a 5,000-litre tank to 40 mJ/cm² requires approximately 6.25 hours of continuous operation. Use the Batch Tank mode on this calculator to get the exact time for your setup. Smaller tanks and higher-wattage lamps reduce treatment time proportionally.
What watt UV lamp do I need for my flow rate?
Required lamp power (W) = [Flow Rate (L/min) × Required Dose (mJ/cm²) × 1000] ÷ [60 × System Efficiency]. As a rough guide: a 25W lamp handles roughly 2–3 L/min for general disinfection; a 75W lamp can handle 6–9 L/min; a 150W lamp suits 12–18 L/min. Always size up — running a UV system at 100% capacity leaves zero safety margin for peak flows, turbid water, or lamp aging.
Does turbidity affect UV disinfection performance?
Yes — turbidity is one of the most critical factors. Suspended particles scatter and absorb UV light, dramatically reducing the effective dose delivered to pathogens. The standard recommendation is to pre-filter water to below 1 NTU (ideally <0.5 NTU) before UV treatment. At 5 NTU, effective UV transmittance can drop by 30–50%, meaning your lamp must work proportionally harder to deliver the same dose. Always measure water clarity before sizing a UV system.