Understanding UV Dose

UV dose is a very important, yet often poorly understood concept with residential UV system consumers. The UV dose that is applied by any UV system is critical in understanding the level of protection that will be offered by the UV system. UV dose is the measure of the amount of germicidal light that is absorbed by a particle as it passes through a UV system. The higher the UV dose, the greater the level of protection.

UV dose is represented in a number of different, but equivalent ways with "millijoules" (mJ) being the most common unit. The list below shows a number of different waterborne pathogens and the UV dose required to achieve 99.9% destruction. Not all UV systems are created equal and you'll see from the list below that a UV system that does not deliver an adequate UV dose may not give you the level of protection you need.

Organism Destruction to 99.9% by UV

e. coli bacteria 6.6 mJ

coliform bacteria 6.6 mJ

Legionnaires bacteria 12.3 mJ

Salmonella bacteria 15.2 mJ

Rotavirus 24 mJ

Adenovirus 40 mJ

UV dose is impacted by several factors: lamp intensity, fouling, water flow rate, and water quality. In the paragraphs below, each one of these topics will be explored:

Lamp Intensity Impacts UV Dose

Lamp intensity is a measure of how much light is produced by the lamp per unit of lamp surface area. Lamp intensity is usually dictated by the type of lamp technology that is used. The most common lamp types used in UV water systems are listed here by increasing intensity: Low Pressure (LP), Low Pressure High Output (LPHO), Amalgam, Medium Pressure (MP). Of these four types only the first three are used in residential or small industrial and commercial systems. Medium pressure lamps produce far too much heat to be practical in all but the largest municipal applications. Understanding the lamp intensity differences helps to explain how a smaller UV system can out-perform a larger system. A system using a 15" long LPHO lamp can outperform a system using a 24" LP lamp.

Fouling Impacts UV Dose

UV lamps are never exposed directly to the water. UV lamps are typically enclosed by a quartz sleeve that insulates the lamp from the water. Over time this quartz sleeve can become coated with minerals naturally found in the water. If this mineral coating becomes too thick it can prevent the UV light being generated by the lamp from ever reaching the water and the dangerous organisms it may contain. The heavier the degree of fouling, the lower the UV dose. Periodic cleaning of the quartz surfaces will reduce this effect. In many cases pre-treatment (such as a water softener) will be used with the UV system to ensure the mineral level in the water is low and that fouling is kept to a minimum. Some UV systems such as the Hallett 15xs and Hallett Upstream 10-50 incorporate an automatic quartz cleaning device into the design of the system.

Flow Rate Impacts UV Dose

The length of time that an organism spends in front of a UV lamp is directly related to the water flow rate through the system. The faster the flow rate, the less time the organism spends in front of the lamp and the lower the UV dose. NSF certified UV systems, such as the TrojanUVMax PRO 20 are equipped with a flow restrictor that ensures the flow rate never exceeds the design capacity of the system. Water Quality Impacts UV Dose How Do I Decide Which UV System to Buy The majority of UV manufacturers now produce charts that show the UV dose delivered by their systems. These charts usually show the dose delivered for various flow rates and make assumptions about the water quality