UV Dose & System Sizing

UV dose

UV light output at 254 nanometres is known as UVC light (germicidal region), and has the ability to inactivate all known microorganisms, bacteria, pathogens, virus and moulds, including chemically resistant organisms such as Cryptosporidium and Giardia.  UV dose is measured in mj/cm2. Systems are typically sized in two ways: -

1.  To provide a minimum level of UV intensity.  This sizing ensures that the UV intensity at the furthest location from the UV lamp (UVC light source) is the minimum dose the system will provide.  This UV dose is known as a wall dose. 

2. An average dose is takes into account the wall dose (furthest distance from the UV lamp) and the area immediately in front of the UV lamp where the UV output is greatest.  The sizing method calculates and average dose for the system. 

Sizing UV Systems.

Sizing a UV system is based around water quality.  For sizing UV disinfection systems, water quality is measured in UVT (Ultraviolet Transmittance).  This measures the ability for the UV light to penetrate a fluid.  If the water is of high quality e.g. borehole water the UVT will be high - around 95% UVT.  If the water is low quality e.g. river water, the UVT will be low around 80% UVT.  It is important to note that a fall in just a few % UVT can double the amount of UV intensity required to provide disinfection.