UV TRANSMITTANCE MONITORS:

UV transmittance (UVT) of the water plays an important role in UV disinfection. There are a couple of methods to determine UVT (USEPA, 2003). One method is to take grab samples from the UV reactor and analyze them with a bench-top spectrophotometer. The second method is on-line UVT monitoring. A UVT monitor typically calculates the UVT with intensity readings taken by the UV intensity sensors at different locations away from the lamp. The design and operation of the on-line UVT monitor will be specific to the equipment manufacturer. The following photo shows an example of a UVT monitor.

Severn Trent Online Transmittance Monitor

Installation of the UVT monitor requires electrical connections as well as sampling ports and drains for water samples.

UV transmittance readings are useful in troubleshooting reactor problems, and in some cases determining whether the reactor is operating within validated conditions. UVT monitors are also used in systems that use the UV intensity and UVT setpoint and calculated dose control strategies for monitoring the delivered UV dose. (USEPA, 2003)

Performance of the UVT monitor should be checked weekly by comparing readings with grab samples analyzed by a spectrophotometer. The weekly grab sample should be taken from a location near the UVT sampling point. The frequency of checks may be changed after a year’s worth of data has been collected. If UVT is part of the control strategy the recommended recording frequency for UVT is every four hours. (USEPA, 2003)