Energy costs of running UV disinfection systems are one of the biggest expenditures of a wastewater treatment works, accounting for up to 25% of the total energy usage.
Low pressure UV systems have been around for over twenty years but tend to be restricted to works where the volume of water treated is less than 20Ml/day as they are usually made up of a large number of low power (<100W) lamps emitting UV wavelengths at 254nm. The bigger the flow, the larger number of lamps needed. The challenge in such circumstances is the balance between energy costs and effective disinfection, proper control of such systems is critical.
Controlling energy usage in a UV system is dependent upon flow rate, lamp power and UV transmittance (UVT) of the water being subjected to the UV light.
UVT will vary over time and is dependent upon the concentration of organic material as well as the turbidity of the water. The more organics and the higher the turbidity, the more time the lamps will need to be on in order to provide a sufficient dose of UV light to the water. The more UV light absorbed, the lower the UVT. The UVT measurement is carried out by measuring the ratio of emitted light (usually at 254nm) to that received by a detector after the light has passed through the water sample and is usually characterised as a percentage (%UVT).