Monitoring Aromatic Organics for Optimizing Coagulation
Organics Monitoring (TOC)
Monitoring Nitrates in Drinking Water & Wastewater
Importance of Measuring UVT for UV Disinfection
Disinfection By-Products (DBP) Precursor Monitoring
Water Distribution System Security Monitoring

Importance of Measuring UV Transmittance (UVT) for UV Disinfection

UV disinfection systems disinfect water using UV light at the 254 nm wavelength. UV light at this wavelength actually destroys the DNA of microbiological material in the water which prevents dangerous viruses such as cryptosporidium and e-coli from reproducing and causing harm.

UV Disinfection Dose and UV Transmittance (UVT)

The effectiveness of a UV disinfection system is determined by the UV dose that the UV disinfection system is able to deliver to the water. The UV dose is dependent primarily on the combined effects of the UV light intensity, the exposure time of the system and the UV transmittance (UVT) of the water.

The intensity of the UV light source depends on the size and type of UV lamp used and the power supplied to the lamp. The exposure time of the system depends on the flow rate of the system. The flow rate is often regulated between multiple UV disinfection systems operating in parallel, and there is often a manufacturer specified maximum flow rate for the system. The UVT of the water being disinfected can significantly affect the effective UV dose delivered to the water by the UV disinfection system and it varies over time and from site to site.

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.

Turbidity and Colour

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 the UVT will be okay. 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, an example of this is pesticides.

Changing UVT

Often one water source can contain a varying 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. The Spring and Fall months typically have the lowest UVT values due to melted snow runoff and decaying plant matter.

Due to the effects of UVT on the performance of UV disinfection systems, it is vital the UVT of the water be known for every UV disinfection system application to ensure proper treatment.

Real Tech Solutions

Real Tech has several products that are commonly used to ensure the correct operation and performance of UV disinfection systems. The Real UV254 'M' series monitors allow continuous, real-time testing of the UVT of the water. The UVT is conveniently output via a 4-20mA communication interface either directly to the UV disinfection system or to a PLC. Real Tech offers several models within the 'M' series to meet any specific site requirements. The M2000 series is generally appropriate for monitoring UVT in wastewater applications when used in conjunction with Real Tech's Pump System. For most general water qualities the Real UV254 M3000 can be used and in high purity drinking water applications the M4000 model is ideal. The M4000 makes use of a longer path length flow cell resulting in higher sensitivity for higher purity water applications.

In addition to monitoring UVT in real-time, it is also often necessary to perform UVT testing on grab samples. The Real UV254 'P' series of portable meters are ideal for providing on-site UVT tests of grab samples within a couple of minutes. The Real UV254 P100 is useful for most wastewater applications while the P200 model is designed for drinking water UV disinfection applications. Real Tech also offers the Real UV254 P300 for higher purity drinking water applications. The Real UV254 'P' series can be used for servicing existing UV disinfection systems or for determining UV disinfection system design requirements at future potential installation sites. The meters can also be used to validate the performance of Real Tech's continuous monitors, such as the Real UV254 M3000 and M4000.