UV Dose & System Selection

Sizing UV Systems and Calculating the Correct Wavelength for Disinfection

UV dose

UV light output at 254 nanometres is known as UVC light (germicidal region). UVC light 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 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 an average dose for the system.

Validated RED Dose. The term RED – Reduction Equivalent Dose is used for validated UV systems that have undergone independent, 3rd party bioassay testing. The testing uses live surrogate microorganisms e.g. MS2 and T1 to physically test and ascertain ‘real world’ reactor performance. Validated RED doses take into account UV chamber design, hydraulic flow efficiencies, lamp positioning and intensity zones to guarantee the performance of a UV system at achieving bacterial log reductions of microorganisms e.g. a 40 RED will achieve 99.99% reduction. Safety factors such as lamp aging and quartz fouling factors are also included in the UV dose equation to provide a conservative and guaranteed performance.


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 clarity of the water to ultraviolet light, relative to that of distilled water (taken to be 100%).  Specifically, this property is measured in a quartz cuvette at a fixed UV wavelength of 254 nm.  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.


Typical Application Typical UVT % Normal Required Dose
Drinking Water 90% - 95% 25 mJ/cm2 RED - 40 mJ/cm2 RED 
Wastewater 50% - 80% 30 mJ/cm2 Wall Dose 
Swimming Pools 94% 60 mJ/cm2 Average 
Aquaculture 85% - 90% 320 mJ/cm2 Average 
Rain Water Harvesting - Irrigation 80% - 85% 150 mJ/cm2 Wall Dose 
Rain Water Harvesting - Process 80% - 85% 30 mJ/cm2 Wall Dose 
Process Water - Towns Mains 90% 30 mJ/cm2 
Make-up Water - Towns Mains 90% 30 mJ/cm2 
Cooling Loops 70% - 90% 30 mJ/cm2 



How to Specify UV Systems

The use of UV light has now become standard practice in a vast range of industries, including municipal drinking water and wastewater treatment, Industrial process and manufacturing, offshore and marine and aquatics. Ultraviolet disinfection systems are used to ensure water is free from harmful organisms and is a proven, regulated and environmentally friendly technology.

In order to provide the best Ultraviolet system to suit your needs, the following variables are required to accurately size a UV system for a specific application. The following information is helpful but not essential to providing a quotation:

  • Flow e.g. 150 m3/hr
  • Application e.g. Drinking Water / Waste Water / Swimming Pools
  • Water Source e.g. Borehole / Surface Water / Final Effluent
  • UVT (Ultraviolet Transmittance) e.g. 90% UVT
  • Details of pre treatment e.g. Filtration
  • Intended use e.g. Irrigation / Drinking Water / Process Water
  • Known biological challenge e.g. Cryptosporidium reduction
  • Known dose standard e.g. 30 mJ/cm2 minimum / 60 mJ/cm2 average
  • Treatment Cycle e.g. Single pass treatment or re-circulation

Connect with an Expert

Contact Us