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Ultra Violet (UV) light is produced naturally by the sun, but if it is generated artificially in a secure and controlled manner, it can be harnessed to provide complete bio-security assurances to industry and utilities in the treatment of drinking or process water.

In the context of water treatment, UV light has two major effects:

  • A biological action in which microorganisms are inactivated, providing effective disinfection of water;
  • A chemical effect in which unwanted chemicals are broken down for easy removal in later processes.

This article describes the nature of these effects and how they are harnessed in water treatment systems, how to ensure the optimum germicidal properties and to transmit the light effectively to pathogens held in water.

What is Ultra Violet light?

Ultra Violet (UV) light is electromagnetic radiation with wavelengths between 100 nm and 400 nm, shorter than those of visible light but longer than X-rays. The UV spectrum can be sub divided further into the following regions:

  • UV-A 320–400 nm
  • UV-B 280–320 nm
  • UV-C 200–280 nm
  • Vacuum UV 100–200 nm

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Figure 1: The Electromagnetic Spectrum showing the relative positions of UV and visible light

UV-A and -B are familiar to most people as the cause of suntans and sunburn, and from which sun cream offers protection. UV-C is also produced in sunlight but is filtered almost entirely by the Earth's ozone layer and atmosphere.

UV light also possesses the ability to disinfect air and water, an ability which increases as the wavelength decreases. Both UV-A and UV-B can disinfect water but long exposure times are needed. The wavelength range of 200–300 nm (primarily in the UV-C region) is the most useful for practical disinfection purposes, and is often known as "germicidal UV". Organisms absorb UV light strongly across this range, peaking at a wavelength of 265 nm (Figure 5).

How does UV light disinfect water?

The mechanisms whereby UV light disinfects water are different from those of chemical disinfectants such as chlorine. Chemical disinfectants attack cell structures, damaging their metabolism and preventing growth. UV attacks the molecular structure of the cells themselves, the nucleic acids DNA and RNA, causing mutations that prevent the replication process that is essential for cells to survive.

DNA and RNA are the molecules that control the creation of proteins, a vital component of living cells. DNA consists of two polymer strands entwined in a helical arrangement (the famous "double helix"). These two strands are linked by small molecules called bases that are joined together by hydrogen bonds, a type of chemical bond that often occurs between molecules containing hydrogen atoms. There are four bases in DNA: adenine, thymine, cytosine and guanine. They can only bond to each other in certain combinations; cytosine can only bond to guanine, and adenine to thymine. This restriction is the basis for the storage of the genetic code that is used to create an organism's proteins.

UV light breaks the hydrogen bond linking the adenine and thymine base pair and forms a covalent bond between the thymine groups (Figure 2). The new covalent bond is much stronger than the hydrogen bond it replaces, and so it prevents the DNA replication process that is needed to create proteins; this in turn stops the pathogen's cells from reproducing. If cell reproduction stops, then a pathogen has a typical lifetime of only a few minutes, and so it is rendered harmless.

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Figure 2: UV light causes mutations in DNA which prevent the replication effect required for cells to survive

On exposure to UV light, the level of a pathogen's inactivation depends on two factors:

  • The UV dose delivered to the pathogen, which is defined as UV intensity × exposure time; and
  • The pathogen's susceptibility to this UV dose, which varies for every organism.

Some organisms contain enzymes that can repair the mutations caused by UV light in the DNA molecule. These organisms require higher UV doses to ensure that the required bio-security of the water is achieved. However, common water borne pathogens such as Cryptosporidium and Giardia have not been observed to restore their activity after UV treatment.

What are the pros and cons of disinfection by UV light?

Disinfection by UV light offers many advantages over conventional disinfection methods:

  • It requires no chemicals.
  • It does not leave a taint or taste in the liquid.
  • It can be more efficient than chlorine.
  • It is cheaper than ozone disinfection.
  • It is effective against bacteria, moulds, spores and viruses, including chlorine resistant species such as Cryptosporidium and Giardia.

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Figure 3: Electron microscope images of common water-borne pathogens Cryptosporidium (left) and Giardia (right)

Its primary disadvantage is that it offers no residual disinfection in the same way as a chemical treatment such as chlorination. In other words, the disinfection effect only endures while water is exposed to the UV light source. Any contamination that occurs after this time will not be inactivated because the UV light is no longer present. Therefore, water disinfected with ultraviolet light must be transported and delivered so as to avoid further contamination after leaving the chamber.

Testing and Validation of UV Systems

The lack of residual disinfection means that it is critical to ensure that the required UV dose is delivered while the water is in the chamber. Public safety bodies such as municipal water authorities have established methods of validating the germicidal performance of UV systems to protect public safety. Systems are tested, measured and validated by independent, external measurement organisations.

The testing process uses, for example, challenge organisms such as MS2-phage, which is harmless to humans but is deactivated by UV light in a similar way as more dangerous pathogens. By measuring the reduction in the active MS2 organisms achieved by the UV system, the actual UV dose delivered to the water can be determined and then the ability to deactivate other pathogens can be predicted. More and more UV systems are now being subjected to this type of validation, both in municipal drinking water applications and for various industrial processes such as swimming pools and food and beverage production.

How does UV light remove chemicals from water?

UV light has the ability to break certain chemical bonds, and thereby help in the reduction of unwanted chemicals in water. The energetic UV light photons cause the two electrons in the chemical bond to separate from each other. This process is called photolysis. In many cases, the remaining fragments of the original molecule are less harmful and may be removable by other processes, such as absorption onto filters containing activated carbon.

The energy needed to break a chemical bond depends on the atoms joined together by the bond. Only photons with energies at or above this level will be able to break that particular bond. The energy of a photon increases as the wavelength decreases, and so the germicidal UV light range of 200–300 nm contains a range of photon energies.

The breaking of the bond usually leaves a single, unpaired electron on each fragment of the original molecule; this is a very reactive and unstable situation and so the fragments quickly react with other available molecules such as water itself, or will bind to the surface of particles such as activated carbon.

Photolysis has many industrial applications:

  • Chloramine destruction in swimming pools – Chloramines are formed when chlorinated water reacts with chemicals introduced by bathers (such as sweat or saliva), and can lead to eye and skin irritation if present in sufficient quantities. UV can selectively destroy chloramines without compromising water quality.
  • Dechlorination in process waters – Processes in the food and beverage industry often require chlorine-free water, and UV can be used alongside other methods such as activated carbon to break down and remove chlorine-containing disinfectant molecules.
  • Deozonation of process waters – Ozone is an effective disinfectant but reacts readily with many other chemical compounds, so must usually be removed from food and beverage streams to prevent unwanted reactions with other ingredients. UV can be used both to remove ozone and also for disinfection as an alternative to ozonation.

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Figure 4: Energetic UV photons can break certain chemical bonds in molecules

How is the UV light produced?

Nearly all current commercial industrial and municipal UV disinfection systems use mercury vapour lamps to produce the UV light for disinfection. These are very similar in design to standard fluorescent light tubes. The output of UV is sensitive to the applied voltage, therefore many UV lamps use special power supplies (known as ballasts) to ensure a constant supply so that the correct output is maintained.

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Figure 5: Conventional UV lamp with wiper mechanism

There are several different types of UV lamps commonly used to supply germicidal UV.

Low-pressure lamps

Low-pressure (LP) lamps are monochromatic, meaning that they generate UV light at a single wavelength. For mercury vapour lamps, this wavelength is 254 nm. (There is also an emission at 185 nm but it is normally filtered out by the quartz glass housing of the lamp.) They have high efficiencies (typically 35% UV-C) but low power density (typically 1 W/cm2). They contain relatively small quantities of mercury to generate the low vapour pressure needed during operation (typically 0.01 mbar).

Low-pressure lamps are well-suited to water disinfection applications because they deliver a single wavelength that is very close to the optimum for disrupting DNA with high electrical efficiency. Their reduced power density only allows them to cope with fairly low water flow rates, and they are less suitable for breaking down chemical impurities because they do not deliver photons with a sufficiently wide range of energies to break a useful range of chemical bonds.

Amalgam lamps

Amalgam UV lamps are also known as low-pressure high-output (LPHO) lamps. They are low-pressure lamps that contain an alloy of mercury with another metal such as indium, instead of the pure liquid mercury found in LP lamps. This alloy releases mercury vapour at higher temperatures, which means that the lamp can run at higher electrical power and consequently operate a higher power density (typically 2.5 W/cm2).

Amalgam lamps have a similar range of applications as LP lamps but can cope with higher flow rates of treatment water.

Medium-pressure lamps

Medium-pressure (MP) lamps contain much greater amounts of liquid mercury than low pressure lamps, and run at higher temperatures and electrical power input. This means that their power density is high (typically 30 W/cm2), but the UV-C efficiency is low (typically 13%). However, the higher vapour pressure of mercury (typically 1–6 bar) causes MP lamps to emit UV light across a wide range of wavelengths (185–400 nm). This means that, in addition to water disinfection applications, MP lamps are useful for breaking down chemical impurities because they deliver photons capable of breaking a wide range of chemical bonds by photolysis.

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Figure 6: The output spectrum from conventional UV lamps overlaps with the wavelengths absorbed by pathogens and DNA

How is the UV light delivered?

UV water treatment systems typically use a continuous flow chamber design, where the water to be treated passes the UV lamp at a known flow rate. The design of the chamber's flow characteristics is very important to ensure that as much of the water as possible is exposed to the full output of the lamp. Computational Fluid Dynamics (CFD) modelling is a useful tool in tracking the pathways of water through a chamber and preventing under-exposure to the UV light. This has become even more important as the number of systems being subjected to independent validation have increased.

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Figure 7: CFD analysis of an L-shaped chamber shows highly uniform liquid velocity around the UV lamp (in grey), indicating that the entire liquid stream should receive the calculated UV dose

Another important factor in delivering the UV light to the process water is the UV transmittance of the water. Particles and dissolved chemicals can absorb the UV light from the lamp and prevent it penetrating the entire volume of the chamber. This effect can be very significant, especially in larger chambers, and may mean that multiple lamps are required. UV intensity sensors can be used to adjust the lamp power if needed to boost the output if process conditions change.

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Figure 8: Typical multi-lamp chamber with a dedicated UV intensity sensor for each lamp

Finally, the cleanliness and age of the UV light source are also important in optimising the output. In most systems, the lamp is contained within a quartz sleeve to provide extra protection to the water supply in the event of the lamp breaking. As the system is used, the outside of this sleeve can become fouled by deposits from the water, which may become baked on over time due to the operating temperature of the system. In many cases, wiper mechanisms are provided which act like a car's windscreen wiper and keep the sleeve surface clean (Figure 5). However, the intrinsic output from conventional lamps themselves reduces over time, because the UV light eventually causes darkening of the lamp envelope (solarisation). Maintenance of the system is therefore necessary to keep it operating at optimum efficiency.

Summary

UV water treatment is a clean and sustainable technology for disinfecting water containing a range of pathogens. In some applications, it can also rapidly break down a range of chemical impurities. Its unique attributes allow it to act as either an alternative or a complement to other chemical disinfection systems, and it is used in a wide variety of applications in the leisure, food and beverage and pharmaceutical industries. In some countries, it is also used in municipal drinking water systems, which has led to independent validation methods being developed whose use is now being extended to other applications. This maturing of the industry, together with the attractive environmental and economic benefits of the technology, mean that UV water treatment is well-positioned to contribute significantly to meeting the world's growing need for clean and safe water.