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Development of a pulsed light approach as a novel solution in drinking water treatment

Due to the fact that Cryptosporidium is resistant to current water treatment processes the need to develop new methods for the water industry has arisen. One such method is the use of ultra violet (UV) light.

Ultraviolet radiation/light lies between visible light and X-rays on the electromagnetic spectrum. Ultraviolet light (UV) is divided into UVA (400-320 nm), UVB (320-280 nm), UVC (280-200 nm) and vacuum UV (VUV) (200-100) radiation penetrates the cell membranes to impact directly on DNA molecules.   Nucleic acid absorbs light energy at 240 to 280 nm with an absorption maximum at 265 nm (UVC), when DNA or RNA absorbs this energy dimers are formed. The most regular dimers formed are those of cyclobutane between nearby pyrimidines (CPD) on the same DNA strand.  Higher doses of UV light also cause protein damage leading to a loss of structure and function and also can result in cell lysis. UV energy is absorbed by proteins at 280 nm and there is some absorption by the peptide bond within protein structures at 240 nm. Additional important biological molecules with unsaturated bonds e.g. hormones, coenzymes and electron carriers may also be vulnerable to destruction by UV. This is an important factor in larger organisms such as fungi and protozoa.

The earth’s atmosphere prevents UVC, also known as germicidal UV, from reaching the earth’s surface. For this reason for disinfection purposes artificial sources of generating UVC are needed. Producing UV radiation requires electricity to power UV lamps which consists of a quartz tube containing an inert gas (e.g., argon) and a small amount of liquid mercury. When a voltage is applied to the lamp, some of the liquid mercury vapourises. Free electrons and ions then collide with the gaseous mercury atoms, “exciting” the mercury atoms into a higher energy state. The excited mercury atoms return to their ground (normal) energy state by discharging energy as UV light. Mercury is favourable for UV disinfection because it emits light in the germicidal wavelength range (200 – 300 nm). The UV light produced depends on the concentration of mercury atoms in the UV lamp, which is directly related to the mercury vapour pressure. UV disinfection uses either low pressure (LP) lamps at a wavelength 253.7 nm or medium pressure (MP) lamps at wavelengths from 180 to 1370 nm or lamps which emit high intensity pulses of light. There are numerous sources of UV radiation, however the most common is the electric arc and mercury lamp which provide continuous UV light.

Continuous wave UV

Inactivation of organisms with continuous wave UV light is performed by using low-pressure (LP) mercury lamps designed to emit light at 254 nm i.e. monochromatic light. Due to the distinct disinfection method of UV (the absorption of UV energy at 254 nm by DNA) traditional UV disinfection systems consisted of low pressure lamps that produce this monochromatic radiation. However, in the late 1990s medium pressure (MP) UV lamps were introduced because they emit polychromatic light including the germicidal wavelengths (200 to 300 nm). There is usually no difference in the disinfection ability between these lamps. But there are advantages and disadvantages to each. MP lamps have a higher germicidal output than LP lamps, and so require fewer lamps for disinfection. A novel approach to water disinfection using a pulsed UV system is the focus of this study. This PUV method has the potential to provide a broader range of UV wavelengths coupled with a better penetration rate than conventional UV systems.  An increased penetration rate allows for the treatment of less clear substances such as water with an increased turbidity.

Pulsed UV light 

Pulsed light (PL) is produced by storing electrical energy in a capacitor and releasing it as a short high intensity. A modest energy input of a few joules (J) can result in high peak-power dissipation of about 107-108 W. The electrical energy is applied to a xenon flash-lamp in which the energy ionises the gas to create plasma that expands to fill the lamp. Outer shell electrons are stripped away and intense pulses of UV light are emitted. The efficacy of the pulse system is attributed to the unique effects of high peak power and broad spectrum UV content coupled with the ability to control pulse duration and frequency. The light produced by the lamp includes broad spectrum wavelengths form UV to near-infrared; during each pulse the system delivers a spectrum that is 20,000 times more intense than sunlight at the earth’s surface. The UV dose can be adjusted by increasing or decreasing the frequency of the pulsing. Preliminary research findings suggests that pulsed light is effective for killing bacteria, fungi, and viruses and the killing effect is much higher in a much shorter time than with continuous UV treatment (Rowan et al. 1999). Therefore, this study focuses on the use of a pulsed light approach for actively and repeatedly inactivating C. parvum.

 

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