Ultraviolet Explained

Training UV Engineers

UV Disinfection Basics

Strong sunlight is known to kill bacteria, viruses, moulds and spores. Almost a century ago scientists identified the part of the spectrum responsible for this well-known effect as primarily in the UV-C spectrum.

What is Ultra Violet (UV) light?

UV light is a naturally occurring component of solar radiation. It falls in the region between visible light and X-Rays in the electromagnetic spectrum between the wavelengths 100nm – 400nm. UV can be further categorized into UV-A, UV-B and UV-C and Vacuum UV.

Ultra Violet (UV) light

UVC / Radiation

What is UV-C light?

UVC between the wavelengths of 220nm and 290nm is recognised as having significant ‘germicidal’ properties. UV-C light is almost entirely filtered out by the Earth’s atmosphere, so to utilize its germicidal properties we have to artificially generate it here on earth using commercially produced UV lamps.

How does a UV-C light disinfect?

When UV light meets a microorganisms it penetrates its DNA, destroying the adenine and thymine bonds effectively inactivating bacteria, viruses, spores and moulds, by stopping them from multiplying and causing infection.


How do we generate UVC Light?

There are two mainstream UVC lamp technologies used in Industrial and Municipal applications:

  1. Amalgam lamps offer a monochromatic (Single wavelength) output at 254nm with 30%+ conversion of electricity to UVC. These lamps are efficient but their energy density is low which means they are relatively low power (100 to 800 W) and long. They are used when efficiency is key, but larger multi-lamp systems can be bulky and difficult to maintain.
  2. Medium Pressure lamps offer a polychromatic output across a wide spectrum. This can be useful to match the sensitivity of a target organism, but they have a lower energy efficiency (~15%). Conversely they are high powered (1kw to 24KW) and short which means you need less lamps in a smaller reactor.

These two lamp technologies offer a choice of reactor design which allows us to weigh the pros and cons of size and efficiency for a given application. As we manufacture both types of lamp we are uniquely placed to assess these competing characteristics to arrive at our “Application Optimised UV” solutions.

Low Pressure

UV Low Pressure Lamp

Medium Pressure

UV Medium Pressure Lamp

Medium Pressure v Low Pressure: You decide between high efficiency or compactness depending on your situation

Ultraviolet Lamps

1 Medium Pressure Lamp Output = 5 Low Pressure Lamps Output


Benefits of using Low Pressure or Medium Pressure lamps:

Low PressureMedium PressureSupporting Comments
Low power useLow pressure lamps are more efficient, but are lower powered
Efficient at higher flow ratesMedium pressure lamps run at a much higher power density to low pressure lamps so one medium pressure lamp can treat a much higher flow than a single low pressure lamp.
Space restrictionsMedium pressure lamps for the same UV output are around a third of the length of a low pressure lamp and so systems are much smaller
Lamp lifeLow pressure lamps typically last 9000 to 15000 hours while the latest generation medium pressure lamps last about 9000 hours
Save on maintenance & sparesFor the same conditions medium pressure UV systems generally have a smaller footprint and use fewer lamps than Low pressure system
Disinfection efficiencyMedium pressure has a higher UV light energy output and not only breaks a microbe’s DNA bond but also ruptures the cell wall. Some microorganisms are much more sensitive to these multiple wavelengths produced by medium pressure lamps.
Low running temperatureLow pressure lamps run at about 120°C, whereas medium pressure runs at 600°C to 800°C
High water temperatureMedium pressure UV systems are hardly affected by the water temperature, whereas low pressure can only operate between 5-40°C
Status ‘ON’ after no water flowIn many cases LP systems can operate for longer without any water flow than MP systems.

Key Features of a UV treatment System

Key Features of a UV System

UV Chamber: This is the reactor through which fluid flows and comes into contact with the UV Light. It has an inlet and an outlet with connections to suit your pipework and is designed and tested as a pressure vessel. Chambers can be Axial flow, Cross flow or U shaped.

UV lamp inside quartz sleeve: The UV lamp is sealed within a quartz sleeve protecting it from the water or fluid whilst allowing the UVC wavelengths to pass through. Some quartz can be doped to manage the wavelengths that pass. The quartz is held in position by lamp flanges or a supporting framework.

UV Intensity sensor: This instrument measures the intensity (W/cm2) of UV light passing through the water and arriving at the sensor. This intensity is used to calculate the UV dose (mJ/cm2) that is being delivered.

Temperature sensor: A hardwired trip switches the system off at high temperatures, whilst a built in thermocouple provides a temperature measurement on the control panel.

Auto-wiper: Like a wiper on a car cleans your windscreen an auto-wiper can be used to clean the quartz without having to interrupt the treatment process. Wipers are used in water pre-treatment and waste water systems where solids or scale deposits can form. Wipers are not required post RO or demineralisation. A chemically assisted wiper is available on some models.

Power supply & Control cabinet: Provides the user with information on the performance of the UV system and controls power to the lamps.

UVT Meter: Some systems will require a sensor to measure changes in the income water UV transmission (UVT). This feeds into the system controller and regulates the power to ensure that the required dose is maintained. Our PQ EO, PQ AF and PQ AL ranges have UVT corrections built in to validated algorithms and do not require a separate UVT instrument.

Flow meter: If your flow varies and you want to optimise your energy consumption you will also need a flow signal into the UV so that the power can be adjusted to match the flow rate.


How is a UV system sized & selected

There are three key parameters to consider when selecting and sizing a UV disinfection system:

  • Water quality
  • Water flow rate
  • Pathogen(s) to be inactivated

To understand the principles of UV system selection fully, each of these parameters need be examined in more detail.

UV Transmission (UVT)

The nature and quality of the water to be disinfected is critical and of all the water quality parameters, UV transmittance (UVT) is the most important. UVT determines how easily UV-C light penetrates the water to get to the organism.

Ultraviolet Crossflow

UVT is measured by taking a water sample in a quartz cuvette and passing UV light at 254nm through the sample. The percentage of UV light that penetrates this sample is referred to as the sample’s ‘UVT’. Most typically, the cuvette used has a path length of 10mm, in which case the UVT reading is referred to as being the ‘T10’ value. Other parameters such as biological oxygen demand (BOD), chemical oxygen demand (COD), turbidity and total dissolved solids (TSS) may be an indication of the water quality and the UVT range to be expected, but only a direct measurement can be relied up as there is no correlation between these parameters and UVT.

If the water quality varies over time then a single spot sample can be unreliable, so samples over a representative period should be taken. Then make sure that the UV system you select has the capability to deliver the dose required under those conditions.

Total Suspended solids (TSS) and total dissolved solids (TDS) or salinity are also important. TSS is particularly important because excessive TSS can result in a phenomenon known as ‘shielding’, whereby the pathogens are shielded from the UV-C light by particles suspended in the water. TDS/salinity is important because, at very high levels, attention must be paid to the construction materials of the UV system to avoid corrosion.

Flow rate and residence time

UV Inactivation is a very rapid process requiring only seconds which means that UV treatment can offer a very compact and rapid treatment compared to alternative disinfection technologies.

But if the dose is defined as Intensity times residence time then for any given reactor volume, the slower the flow rate of the water through it, the longer the UV exposure time and vice versa. So the maximum and minimum flow rate of the water are key performance parameters. This is why many UV systems now have the ability to adjust the power output of the lamps in relation to changes in water flow rate. By doing so, energy may be conserved when water flow rates are lower than peak flows.

When determining maximum and minimum flow rates, it is important to establish the instantaneous flow rates, as this determines the instantaneous minimum and maximum UV exposure times. Daily and hourly flow rates are usually misleading in this respect, as they can mask important ‘peaks and troughs’ in the instantaneous flow rate, resulting in erratic calculations of the true UV exposure. In turbulent flow conditions the residence time of organisms will vary and hence the dose received by each organism will be different. Any given reactor will have a characteristic Gaussian dose distribution so beware of using average dose calculations in critical disinfection processes as these are gross simplifications and can give a false sense of security.


Pathogens to be inactivated

Different pathogens have differing resistance to UV. Some are more susceptible than others and so require different amounts of UV-C exposure for inactivation. There may even be a differing spectral response i.e. the dose required can depend on the wavelength of UVC applied. In order to correctly size and select a UV system, it must be established which pathogen(s) are to be inactivated and what is there UV response characteristic.

What does inactivation truly mean? Does it mean that every single pathogen that ever passes through the UV system will be inactivated? In reality, this is impossible, sterility is not economically achievable. Indeed, this is true regardless of what disinfection method is used, whether it is UV, chlorine or anything else. What is possible is to reduce the pathogen count by a predictable amount. This predictable amount is referred to as a ‘log’ reduction (as in logarithmic reduction). A 1 log reduction will see the pathogen of interest reduced by 90% from the influent level. A 2 log reduction will see a 99% reduction, 3 log by 99.9%, and so on. Scientists have calculated the amount of UV exposure required to inactivate a whole range of different pathogens by various log reductions. Examples appear in the table below.

Micro-bugs that ultraviolet eradicates

UV dose

A UV application requires a specified UV dose to achieve its required level of treatment. UV dose is the amount of UV energy per unit area that falls on a surface. It is calculated as:

Dose = Intensity x Time (mJ/cm²)*

* Intensity is determined by the lamp power and Time of exposure to UV. We will recommend the appropriate UV dose for each application, taking into account the lamp ageing and fluid transmittance, the target organism and the reduction required. In the turbulent flow conditions of UV reactor not every particle of water or organism has the same residence time which means that performance assessment is either a statistical calculation using Computer Fluid Dynamics (CFD) modelling or direct empirical measurement using a bioassay.

UV Dose Requirements – millijoules per centimeter squared (mJ/cm2)1


1 40 CFR 141.720 (d)(1)

The amount of UV-C delivered to inactivate a pathogen has been referred to as ‘UV exposure’. In fact the correct term for this exposure is ‘UV dose’ or ‘UV fluence’. The relationship between UV fluence and log reduction, as illustrated in the table above, is described as a pathogen’s ‘Dose Response Curve’. As UV dose is the most common term for UV exposure, this is what will be used from here onwards.

It is important to note from the table above that the UV dose required to inactivate a given pathogen to a given log reduction is rarely linear. A common mistake is to take the UV dose required to achieve a 1 log inactivation and simply multiply it in order to calculate a higher log reduction. Although one very common pathogen, E. coli, has a dose response curve that is almost linear, most are not.


Pathogen barrier or Hygiene maintenance – what’s your need?

For regulated applications such as drinking water the answer is easy, you have a defined target pathogen and you have to achieve a given log reduction, and often a validation protocol is laid down. (See Validations)

In industrial applications your choices are wider. Do you want:

  • Pathogen barrier at the inlet
  • Pathogen as the final treatment to act as a final barrier
  • To reduce bio burden on your treatment equipment
  • To maintain hygiene in high quality water you have already generated

These considerations will help you select a dose level and whether your UV needs to be validated or not or whether a hygienic design is required.

Share your treatment needs with us and we will recommend a design philosophy. As a first approach we would suggest you consider a pathogen barrier at the beginning of your process to ensure that they do not populate your downstream equipment and then use a hygiene maintenance philosophy to control secondary contamination after treatment and in distribution and storage. If your process is critical we recommend a bioassayed or validated pathogen barrier as proof of performance. See our PQ systems.

Our hygienic designs will help maintain low bacterial counts when part of a hygienically designed system. i.e. a system which is continuously circulated at velocities > 1m / sec, are kept below 20C, have no dead legs, and are crevice free and highly polished. Routine CIP will also be required to ensure that biofilms do not build up, but a good UV will extend the period between CIPs reducing down time and running costs.


Validations and Bioassays

UV Disinfection System Validation

Bioassays are becoming increasingly popular in the UV industry. They provide an independent third party evaluation of a UV systems performance against a defined protocol. This provides peace of mind that a product will do what it is supposed to and provide a level playing field for purchasers and vendors to assess the merits of competing products. The control system of our Bioassayed PQ products will tell you directly if you are achieving the design dose by referring to a third party algorithm built into the controller.

Validations used in regulated environments such as drinking water are designed with safety in mind and often prescribe methods for dealing with real world uncertainty, for example the accuracy of instrument readings or statistical variations in sampling or microorganism sensitivity. Validation factors are therefore added over the top of bioassay results to cater for uncertainty.

Validated units are therefore often more expensive than non-validated units. Not just because of the significant cost of performing the validation but also because of the added power required to be sure that the uncertainties are covered.

Validation Protocols:

This validation is based on the US EPA’s UV Design Guidance Manual. It is intended for drinking water applications but is often applied elsewhere as it is one of the most comprehensive validations available. It provides for performance qualification against a range of Microorganisms including Chlorine resistant organisms such as Cryptosporidium and Giardia. It also caters for Virus reduction. This validation protocol provides a methodology to control to a given UV dose to cater for given target organisms.

DVGW’s validation is a more rigid “certification” prescribing the conditions under which the UV will provide a dose of 40 MJ Bactillus Subtiluss. The systems will alarm when the intensity set point goes below that 40 MJ. It is a robust and simple approach but provides less flexibility to optimize design for different target organisms.

NWRI, is a waste water and a reuse validation where UV is tested on effluent following different levels of pre-treatment such as sand filters and Membranes. It is designed to protect drinking water catchment areas, aquifers and sensitive irrigation projects such as golf courses.

The Norwegian Veterinary Institute regulates UV systems used in Fish farming in Norway and other aligned countries. It assesses the performance of UV against fish disease organisms based on Bioassays and issues certification following extended real-world site trails.

NSF 61 certifies that UV system materials are suitable for contact with water.

NSF 50 certifies that a UV is suitable for us in recreational water facilities such as pool and splash pads. I twill also certify that the product meets the MAHC (Model Aquatic Health care) requirements.