What is UV Transmittance (UVT)? – A Water Doctor Explains

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What is UV transmittance (UVT)? And why is it important to know about UV transmittance when sizing a UV system?

You’ll find everything you need to know in this guide.

📌 Key Takeaways:

  • UV transmittance is a measure of how much light passes through the water, in %.
  • It’s important to understand UV transmittance when sizing a UV water purifier to make sure you buy a system with a high enough UVT for efficient disinfection of your water source.
  • UV transmittance is the amount of UV light that travels through a water sample, while UV absorbance, is the amount of UV light that a water sample absorbs.

🤔 What Is UV Transmittance? Quick Definition

UV transmittance (UVT) is a measure of what percentage of ultraviolet light can pass through the water.

Specifically, it’s a measure of how much ultraviolet light at 254 nanometers (nm) is capable of passing through a standard water path length of 10 mm.

UVT is a percentage (%) measurement. The percentage of light that travels through the sample of water helps you to get an understanding of the quality of the water sample.

Even if water looks clean and pure to the naked eye, it might contain impurities that block or absorb UV light. The number of dissolved impurities in the water – including colloidal solids, iron, and other substances – will determine the UVT.

So, if a water sample had a UVT of 10%, that would mean that only 10% of light was able to pass through the sample. It’d be much murkier than a water sample with a 90% UVT, which would allow 90% of water to pass through the sample.

Diagram detailing UV transmittance process

🔎 Why UV Transmittance Matters

UV transmittance matters because it gives you the information you need to choose the right UV equipment to effectively disinfect a water sample based on its indicated quality.

UVT affects a UV system’s performance. Let’s say you have a UVT that’s lower than originally sized for. In this case, the UV light intensity would be lower because less light would pass through the sample, meaning that the efficiency of the disinfection process would be hindered.

The UV transmission of a water column might change from one day to the next, especially in municipal drinking water or wastewater treatment plants.

Routinely measuring UVT means that specialists can monitor and adjust the UV dose to ensure the UV disinfection system can continue to inactivate pathogenic microorganisms present in the water at all times.

📖 How To Measure UV Transmittance

UVT is measured by determining the amount of UV light at 254nm that can pass through a water with a predefined path length. In most cases, the path length is standard, at 10mm.

This can then be compared to the amount of UV light that passes through distilled water with the same path length.

In the below table, we’ve shared the typical UVT values for different water sources:

Water SourceStandard UVT (254nm @10mm)
Tap Water69% to 96%
Purified Water≥ 99%
Salt Water69% to 90%
Pool Wateraround 95%
Primary Treated Wastewater19% to 44%
Secondary Treated Wastewater44% to 60%
Tertiary Treated Wasetwater60% to 69%

Various equipment can be used to perform UVT measurements, and the most common is the UV transmittance (UVT) monitor. This electronic device uses LED technology and a single-beam technique to deliver a quick, accurate UVT reading without the need for a reagent.

How uv transmittance is measured

📐 How To Size A UV System Using UV Transmittance

Once you know the UVT of a water sample, it’s then easy to size a UV disinfection system that has a suitable power consumption, lamp number and UV strength, and other features to efficiently disinfect the water.

That means you’ll be able to invest in the right system size, without unnecessarily overspending, in the knowledge that it’ll provide the disinfection you need for your water source.

Keep in mind that different pathogenic microorganisms are inactivated with different UV doses, so take this into account when you’re sizing a UV disinfection system. Also make sure to assess the other factors that may affect your decision, like your water flow rate, the UV system location, and more.

🆚 UV Transmittance vs UV Absorbance

UV transmittance and UV absorbance are often both mentioned in a manufacturer’s information about a UV purifier. Don’t mix these terms up – they both have different definitions and uses.

We know that UV transmission is a measurement of the amount of UV light that travels through a water sample. UV absorbance, on the other hand, is the amount of UV light that a water sample absorbs. Both of these are affected by the concentration of particulates and dissolved solids in the sample water.

UV absorbance is an effective way to characterize the specific impurities within a water sample.

You don’t need to worry too much about UV absorbance when you’re shopping for a UV disinfection system. This measurement is typically used by scientists or water experts to analyze water quality changes or predict the formation of disinfection byproducts.

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📑 Final Word

UVT measurement provides insight into water quality that’s invaluable in choosing a UV system that offers the right UV intensity to effectively disinfect a water supply.

When you know UVT, alongside factors including UV intensity and flow rate, you can calculate the right UV dose delivery for your water treatment requirements.

  • Jennifer Byrd
    Water Treatment Specialist

    For 20+ years, Jennifer has championed clean water. From navigating operations to leading sales, she's tackled diverse industry challenges. Now, at Redbird Water, she crafts personalized solutions for homes, businesses, and factories. A past Chamber President and industry advocate, Jennifer leverages her expertise in cutting-edge filtration and custom design to transform water concerns into crystal-clear solutions.

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