Measuring UV Percent Transmittance (UVT)

UV disinfection systems disinfect water using UV light at the 254nm wavelength. The effectiveness of a UV disinfection system is dependent on the UV does that the UV disinfection system is able to deliver to the water. The UV does can be determined primarily from the combined effects of the UV light intensity, the exposure time of the system and the UVT of the water.

The intensity of the UV light source depends on the type of UV lamp and the power supplied to the lamp and is usually determined by the manufacturer. The period of time the water is exposed to the UV light depends on the flow rated of the system. The flow rate may be adjustable in large systems, or it may be set by the manufacturer to a maximum flow rate using a flow restrictor. The UVT is dependent on the specific site and can significantly affect the effective UV dose delivered to the water by the UV disinfection system.

 UVT is related to the quantity of organics, colloidal solids and other material in the water which absorb and scatter the UV light as it passes through the water. In a UV disinfection system, if the UVT of the water is too low, then the UV light is not able to penetrate the water as efficiently, thereby reducing the effective UV dose delivered by the system. This is why manufacturers usually state a minimum UVT for a UV disinfection system, below which the system will not function properly.

It is often thought that if the water appears clear to the naked eye, or if the turbidity is low then this means the UVT will be high. However, this is not correct. Turbidity is a measure of the quantity of suspended solids in the water and is not related to the organics or fine particles that tend to affect UVT. It is true that colour usually does indicate the presence of organics in the water such as tannins and humic material. However, it does not follow that just because the water doesn’t appear coloured that it will have high UVT. This is because some organics and other matter can be present in the water that cause low UVT but do not add any visible colour to the water.

The UVT is primarily affected by quantities of different types of organic matter in the water. Often one water source can contain a varied amount of organic material, and therefore will have different UVT values at different times. This is especially true for surface water applications such as a lake or river. Different kinds of weather can potentially affect the UVT of a water source, which tends to cause significant changes in UVT from season to season.

Due to the effects of UVT on the performance of UV disinfection systems, it is vital to the UVT of a water sample be known for every UV disinfection system application to ensure proper treatment, The Real UVT’s portability and ease of use make it an invaluable tool for those companies utilising UV water and waste water disinfection systems.