How to size a UV system

Posted on August 1, 2023 How to size a UV system

 

How to Size a UV System: A Step-by-Step Guide with Real Calculations :

Sizing a UV disinfection system isn’t just about matching numbers – it’s about ensuring water safety, compliance, and system efficiency.

In this guide, you’ll learn exactly how to calculate:

  • The correct UV dose
  • The flow rate
  • The UV transmittance (UVT) needed

with a real-world calculation example at the end.

Factors to consider while sizing a UV system

 

Why Sizing Matters

UV disinfection is a proven technology in water and wastewater treatment. But its effectiveness depends on proper sizing. If undersized, pathogens survive. If oversized, you’re wasting power and money.

Expert Insight:

“Over 70% of UV system performance issues come from incorrect sizing or poor maintenance.” – Alfaa UV Engineering Team

 

components of a UV system

Step 1: Calculate the Required UV Dose (mJ/cm²)

What is UV Dose?

UV dose is the amount of UV light energy delivered to microorganisms. It’s measured in mJ/cm² (millijoules per square centimeter).

Typical UV Dose Requirements:

Application UV Dose (mJ/cm²)
Drinking Water 40 – 45
Wastewater 30 – 100
Process Water (Food/Beverage) 50 – 150

Formula:
UV Dose = UV Intensity (mW/cm²) × Exposure Time (sec)

 

Step 2: Determine Your Flow Rate

Flow rate tells us how much water passes through the system per second.

Common Units:

  • m³/hr (cubic meters per hour)
  • LPM (liters per minute)

Example Conversion:

If your plant processes 500 m³/day:
→ 500 / 24 = ~20.8 m³/hr
→ 20,800 L/hr

 

Step 3: Measure UVT (UV Transmittance)

What is UVT?

UVT is the percentage of UV light that can pass through water. The cleaner the water, the higher the UVT.

Water Type Typical UVT (%)
Treated drinking water 90 – 95
Process water 85 – 92
Secondary wastewater 55 – 70

Use a UVT meter to measure UVT at 254 nm wavelength.

 

Example: Sizing a UV System for 500 m³/day Plant

Let’s say you have a plant processing 500 m³/day of treated drinking water.

  • Flow Rate: 20.8 m³/hr
  • Target UV Dose: 40 mJ/cm²
  • Measured UVT: 92%

Based on these numbers, you can now work with UV manufacturers to select a system with:

  • Reactor size that accommodates ~21 m³/hr
  • UV lamps with sufficient intensity
  • A reactor chamber designed for the measured UVT

✅ You’ve now successfully sized your system to meet disinfection standards.

 

FAQs

1. How is UV dose calculated in water disinfection?
UV dose = Intensity × Exposure time. Covered in Step 1 above.
2. What UVT is good for drinking water?
Above 90% is ideal for clear, treated water.
3. Can I use the same UV system for wastewater and process water?
No. Different applications require different doses and flow dynamics.

 

Final Thoughts

Sizing a UV system isn’t a guessing game. By understanding your UV dose, flow rate, and UVT, you make a system that’s safe, sustainable, and long-lasting.

Need help?

Reach out to the experts at Alfaa UV for custom solutions backed by over 20 years of experience.

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