Use of UV Light for Disinfection of Water

Introduction

UV has become widely accepted as an alternative to chemicals for water disinfection. UV disinfection is a completely clean technology that is harmless to humans, animals and aquatic life and produces no unwanted disinfection by-products. It is highly effective at permanently destroying virtually all microorganisms, including those resistant to chlorine, such as Cryptosporidium parvum.

How UV disinfection works

UV is the part of the electromagnetic spectrum between visible light and X-rays. The specific portion of the UV spectrum between 200 - 400 nm has a strong germicidal effect, with peak effectiveness at 265nm. At these wavelengths UV kills microorganisms by penetrating cell membranes and damaging their DNA and other intracellular molecules, making them unable to reproduce and effectively killing them.

The different classes of UV are:

• UV vacuum: 40 - 200 nm
• UVc area: 200 - 280 nm
• UVb area: 280 - 315 nm
• UVc area: 315 - 400 nm

Types of UV Lamp Technology

A typical Aquionics 'in-pipe' UV disinfection system consists of a UV lamp housed in a protective quartz sleeve, which is mounted within a cylindrical stainless steel chamber. The water or wastewater to be treated enters at one end and passes through the chamber before exiting at the other end.

There are two main types of UV technology, based on the type of UV lamps used: low pressure and medium pressure. Low pressure lamps have a monochromatic UV output (limited to a single wavelength at 254nm, whereas medium pressure lamps have a polychromatic UV output (between 185-400nm).

Effect of UV on Microorganisms

The DNA of microorganisms absorbs UV light and is destroyed by it. Maximum absorption occurs at both 200nm and 265nm and not at 254nm, the wavelength produced by low pressure lamps and often wrongly assumed to be optimum wavelength for killing microorganisms. At 200nm most absorption occurs in the 'backbone' DNA molecules of ribose and phosphate. At 265nm, UV absorption mainly occurs in the nucleotide bases: adenine, guanine, cytosine and thymine (and uracil in the case of RNA). The most common products resulting from damage by UV radiation are thymine dimers, which are formed when two adjacent thymine molecules become fused. The formation of these dimers and other photoproducts prevents the DNA from being able to replicate, effectively killing the cell.

In addition to DNA and RNA, UV also causes photochemical reactions in proteins, enzymes and other molecules within the cell. Absorption in proteins peaks around 280nm, and there is some absorption in the peptide bond (-CONH-) within proteins at wavelengths below 240nm. Other biological molecules with unsaturated bonds may also be susceptible to destruction by UV - examples include coenzymes, hormones and electron carriers. The ability of UV to affect molecules other than DNA and RNA is particularly interesting in the case of larger microorganisms such as fungi, protozoa and algae. In these microorganisms, although UV may be unable to penetrate as far as the DNA, it could still have a lethal effect by damaging other molecules.

Parameters influencing the effect of UV

UV dose is a combination of UV lamp power and exposure time. Lamp power is measured in UV intensity (mW/cm²) and depends on the initial UV intensity of the lamp and the UV intensity at a certain distance from the lamp. In practice, UV intensity is very much dependent on the water quality, which is influenced by the level of UV-absorbing compounds in the water. In addition, the quantity of water to be treated and its speed defines the required exposure time in a UV reactor. The sum of UV intensity multiplied by the exposure time results in the calculated UV dose.

1 - UV dose

Just as UV has different effects on the various components within a microorganism, it also has differing effects on different microorganisms, which each have their own specific sensitivity to UV light. A bacterium, for example, is much more sensitive to UV light than a mould or algae. This sensitivity is expressed by a D10-value, which shows the UV dose needed for a 90% reduction rate of the specific micro-organism.

UV dose is a combination of UV lamp power and exposure time. Lamp power is measured in UV intensity (mW/cm2) and depends on the initial UV intensity of the lamp and the UV intensity at a certain distance from the lamp. In practice, UV intensity is very much dependent on the water quality, which is influenced by the level of UV-absorbing compounds in the water. In addition, the quantity of water to be treated and its speed defines the required exposure time in a UV reactor. The sum of UV intensity multiplied by the exposure time results in the calculated UV dose.

2 - UV transmittance

A simple indication of the absorbance of UV light by water is the transmission value of the water (T10), which is the total value of all absorbing components in the water, such as suspended organic materials, and minerals such as iron and magnesium – both dissolved and undissolved. If the transmission value is known, the calculation of UV intensity in the UV reactor can be calculated and the size of the reactor determined. Depending on the size or volume of the reactor, the UV dose can be calculated.

3 - Quantity of water to be treated

The quantity of water to be treated defines the size of a reactor. The choice of the size of a reactor is limited by factors such as headloss, pipeline sizes, and the UV dose required.

4 - Water temperature

Low pressure UV lamps

As the surface temperature of a low pressure UV lamp is relatively low, the influence of water temperature is significant. The optimal water temperature is around 20 degrees Celsius, with all temperatures below or above 20 degrees Celsius resulting in lower UV output by low pressure lamps. At temperature below five degrees Celsius, UV output becomes unpredictable and low pressure lamps fail to start.

Medium pressure UV lamps

Medium pressure UV lamps, with a higher surface temperature, are not influenced by the surrounded water temperature and can operate effectively at temperatures ranging from -20 degrees Celcius to +80 degrees Celsius.

5 - Repair mechanism of UV damage

The need to recover from or repair UV damage is common to virtually all microorganisms that are regularly exposed to UV light in nature. Known as reactivation, the process can take place in both light and dark conditions and is called, respectively, photoreactivation and dark repair. The ability to reactivate varies significantly depending on the type of UV damage inflicted and by the level of biological organisation of the microorganism. The repair mechanism is not universal and there are no clearly defined characteristics determining which species can repair themselves and those which cannot.

The part of cells most vulnerable to UV damage is the DNA and RNA. This is due partly to its unique function as the depository of the cell's genetic code, and also because of its highly complex structure and large size. It is hardly surprising therefore that all known molecular repair mechanisms have evolved to act upon the macromolecular nucleic acids, particularly DNA. In photoreactivation, repair is carried out by an enzyme called photolyase which reverses the UV-induced damage, while in the case of dark repair it is carried out by a complex combination of more than a dozen enzymes. To begin reactivation (both light and dark), these enzymes must first be activated by an energy source - in photoreactivation this energy is supplied by visible light (300-500nm), and in dark repair it is provided by nutrients within the cell. In both cases, reactivation is achieved by the enzymes repairing the damaged DNA, allowing replication to take place again.

Common strains of E. coli contain about 20 photolyase enzymes, each of which can repair up to five thymine dimers per minute - this means that, in a single cell, up to 100 such dimers can be repaired per minute. 1mJ/cm² of UV produces approximately 3000-4000 dimers (Oguma, 2002) so, theoretically, damage induced by 1mJ/cm² of UV can be repaired in just 30 minutes.

Low pressure UV lamps have traditionally been used in water treatment plants because their UV output at 254nm closely matches the absorption peak of DNA bases at 265nm. Recent research, however, has shown that E. coli DNA is capable of photoreactivation after exposure to low pressure UV, but not after exposure to medium pressure UV. Further studies concluded that polychromatic, medium pressure UV radiation is much more effective than monochromatic low pressure UV at causing permanent, irreparable damage to the DNA of E. coli.

The implications of these findings are far-reaching. For any industry where UV is used to disinfect water or effluent, the operator needs to be sure that the treatment is permanent. This is especially the case when the treated liquid will subsequently be exposed to light. The applications affected by these findings include any where the treated water or effluent is subsequently exposed to light. Examples include wastewater, bottled water, fisheries and swimming pools. Also important, due to the possibility of dark repair, are drinking water and process water applications.