How to calculate and test modern UV equipment for water, air and surface disinfection?

UV dose is the main process parameter to evaluate the effectiveness of UV equipment.

UV dose refers to the amount of UV radiation delivered to a surface (surface dose) or to a volume (volume dose).

Today, the term surface dose or fluence is commonly used worldwide. Tables indicating surface doses for various microorganisms are open to the public. The most complete and current information is published by the International Ultraviolet Association (IUVA).

Earlier, the concept of volume dose was sometimes applied to calculate the equipment for air disinfection in volume. This approach is not correct, since it contains too many assumptions that make it unfeasible in practice. For example, it presumes that UV radiation is evenly distributed throughout the volume. It is also considered that the air in the room is mixed intensively and evenly (which is not a common practice) and the room has a regular shape (However, many rooms have an oblong or other complex shapes).

It is easy to measure the delivered UV dose when a surface is treated by an open UV irradiator. For this purpose you only measure the UV intensity on this surface using a special UV sensor and then set the exposure time.

However, you cannot directly measure the UV dose provided by a UV unit, through which a certain volume of water or air passes. You can either calculate or measure it indirectly. The UV intensity in a UV unit can be measured with a special UV sensor (such an instrument is rarely used in air systems, but it is common in water disinfection systems); but there are some difficulties with measuring the exposure time.

The average exposure time can be estimated as the internal volume of the UV unit divided by the flow rate of water or air, which passes through this volume. This is quite rough estimation since the difference in velocity in various cross areas of this volume is not taken into account – it is assumed that the velocity is equal throughout. Average UV dose is based on average exposure time and average UV intensity. Average UV dose has been in practice for a long time. Although now it is replaced by estimated UV dose. The exposure time for estimated UV dose is calculated based on different CFD models. They require serious computer simulation but their fidelity is higher.

UV equipment in many countries shall be certified to prove its effectiveness before it can be used or go on sale. For this purpose a bio assay method is applied. This is a test of real equipment in real operating conditions with the use of biological markers (microorganisms). These are microorganisms with known UV doses for different levels of disinfection. The field and full-scale test carried out under strict control determines the effective UV dose of equipment. This dose is known as Reduction Equivalent Dose (RED) in America and as Reduction Equivalent Fluence in Europe. It is measured in mJ/cm2. In an ideal scenario the obtained RED and estimated design UV dose should be the same.

Therefore, modern methods of UV dose calculation should be applied UV equipment development. A liable manufacturer of UV equipment always specifies the UV dose for the given flow rate of air or water passing through a UV system.

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Ultraviolet technology for water, air and surface disinfection is based on germicidal effect of UV-C radiation. 

UV radiation is electromagnetic radiation between x-rays and visible light. UV wavelengths range from 100 to 400 nanometer.

The UV wavelengths are divided in 4 groups, each with a different germicidal effect – UV-A (315–400 nm), UV-B (280–315 nm), UV-C (200–280 nm) and Vacuum UV (100–200 nm).

Ultraviolet in

Within the UV spectrum, UV-C range is considered the strongest UV radiation, which is easily absorbed by DNA, RNA and proteins. This range is often called germicidal due to its high disinfection efficiency against bacteria and viruses. The highest germicidal effect occurs at 205-280 nm and the maximum germicidal sensitivity of microorganisms at 265 nm. 

The germicidal effect is based on photon absorption by DNA and RNA molecules. Photochemical reaction provokes dimerization of DNA and RNA bonds, which inhibits the ability of microorganisms to replicate. This process is known as inactivation of microorganisms.

Mechanism of
UV disinfection

UV disinfection technology can be applied for potable water supply, wastewater treatment as well as for air and surface disinfection applications.

The major advantages of this technology:

  • high efficiency against a wide range of microorganisms including chlorine resistant ones (viruses and protozoa oocysts);
  • no impact on physical, chemical and organoleptic properties of water and air; no by-products; no dangerous overdosing;
  • low capital costs, power consumption and operational costs;
  • UV systems are compact and easy to operate; no need for special operational safety precautions.

Main industrial available sources of UV radiation are low pressure amalgam lamps and mercury medium pressure lamps. Medium pressure lamp technology have higher power per lamp (several kW) but significant lower efficiency (9-12%) in comparison to low pressure lamp technology with efficiencies between of 35-40% and power per lamp up to 1000 watt.

UV systems equipped with amalgam lamp technology generally have a little larger physical footprint but they are significantly more energy efficient.

The design of UV application depends on the required UV dose, flow rate and physical and chemical parameters of media to be disinfected. The facility design criteria, flexible, economical and sustainable operation are also the decisive design parameters.