Adsorption Processes 8
9.7 Ultraviolet light
There is a growing interest in the use of ultraviolet (UV) light to disinfect drinking water as it has been shown to inactivate a wide range of micro-organisms including Cryptosporidium (see Table 9.2) without producing any disinfection by-products. UV light is widely used in the UK to disin-fect wastewater discharges at coastal sites but its use in drinking water applications was primarily limited to small groundwater systems.
Ultraviolet radiation is an electromagnetic radiation of slightly higher frequency than visible light but rather lower than that of X-rays. It is responsible for the tanning effect of sunlight and, it is argued, is a major cause of malignant melanoma or skin cancer. The UV spectrum is arbitrar-ily divided into three bands according to the wavelength of the radiation It is the lowest wavelength (and therefore highest frequency) radiation, in the UVC band, that has the strongest biocidal properties.
UVA 400–315 nm UVB 315–280 nm UVC 280–200 nm Visible 400–700 nm
Table 9.5 UV lamp characteristics.
Parameter Low pressure Medium pressure High pressure
UV wavelength (nm) 185 and 254 240–300 240–300
Max output (W m−2) 60 500–2000 >2500
Typical lamp life (hr) 8000 4000 4000
Optimum temp (◦C) 50∗ 0–100 0–100
∗Output from a low-pressure lamp falls off as the surface temperature of the lamp varies from 50◦C in either direction.
UV radiation is produced commercially by the use of mercury vapour, antimony and xenon lamps. Normally the lamp is enclosed in a protec-tive quartz sleeve and its output is expressed as the UV radiation power measured at the outer surface of the sleeve in W m−2of surface area. Com-mercial lamps fall into two principal categories: low pressure and medium pressure. Their characteristics are summarised in Table 9.5 and output shown in Figure 9.3.
Ultraviolet irradiation owes its bactericidal effect to its ability to pen-etrate the cell and act directly on the nuclear DNA. The radiation does not destroy the bacterial cell material, but disrupts the DNA by causing adjacent chemical groups on the double helix of the DNA molecule to fuse and prevent the molecule from replicating. This means that the bacterium is unable to reproduce and is thus inactivated or not viable. It is, however, wrong to think in terms of bacteria being killed by UV because it has been demonstrated that exposure of a UV-treated bacterial cell to visible light (300–500 nm) causes photo-reactivation which reverses the effects of the UV dose. UV dose is often measured in millijoules per square centimetre or milliwatts per square centimetre (10 J m−2= 1 mJ cm−2= 1 mW cm−2)
150 200 250 300 350 400 450 500 550 600 650 700 750 800 Wavelength / nm
Spectral emittance (rel)
(a) (b)
Fig. 9.3 Spectral distribution from (a) low-pressure and (b) medium-pressure lamps.
Table 9.6 Reduction in E. coli at increasing UV dose.
UV dose, mJ cm−2 Reduction in viable count (%)
5.4 90
10.8 99
16.2 99.9
21.6 99.99
and the relationship between UV dose and inactivation rate is logarithmic as Table 9.6 shows for E. coli.
The understanding of UV technology has developed enormously in the past 5 years due to the application of computational fluid dynamics to optimise UV reactor design and by challenging UV reactors with resis-tant microbes to demonstrate performance. This has built a confidence in UV reactors’ ability to inactivate microbes under a range of realistic water treatment plant operating conditions; i.e. water quality, flow rate and turbidity. A wide range of UV reactors have been designed to efficiently deliver the necessary dose to inactivate micro-organisms (see Figure 9.4).
The USEPA has accepted that UV reactors, which have been indepen-dently validated, can be used for Cryptosporidium protection. The USEPA has published comprehensive guidelines on validation and the issues to be addressed in UV reactor design. This means that UV reactors offered
Fig. 9.4 UV installation at Victoria, British Columbia (with kind permission from Trojan Technologies).
Table 9.7 Examples of potable water treatment works where UV is being used for Cryptosporidium inactivation (data courtesy of Trojan UK).
Bioassay dose
Flow (m3hr−1) Water source (mJ cm−2) Performance (inactivation) Seattle, Washington 28 000 Surface water 40 3 log Cryptosporidium Victoria, British Columbia,
Canada
24 000 Surface water 40 2 log Cryptosporidium Rotterdam, The Netherlands 28 400 Surface water 70 1.5 log clostridia+
Cryptosporidium
Albany, New York 6300 Surface water 40 2 log Cryptosporidium
commercially should be able to demonstrate the ability to deliver a bioas-say dose which is internationally accepted. There are other internationally recognised standards for bioassay validation like the DVGW in Germany and O-NORM in Austria.
UV will effectively disinfect waterborne pathogens with a UV dose of 40 mJ cm−2and achieve a 4 log reduction, an exception is the adenovirus which with a UV dose of 40 mJ cm−2 gives a 1.5 log inactivation, but adenovirus is highly susceptible to a low chlorine dose. A chlorine resid-ual should still be used for protection in the distribution system and there is growing interest (see Table 9.7) in a multi-barrier approach to disin-fection of municipal drinking water, with UV as the primary disinfectant and a chlorine residual providing protection in the distribution system.
The major factors affecting the performance of the UV process include (i) UV transmission, (ii) turbidity, (iii) hydraulics and (iv) foulants such as iron, organic matter and calcium carbonate. The transmissivity of the water is of crucial importance in determining how much UV power must be applied to ensure that all the water in the cell is exposed to the desired dose; and the flow rate, which a given lamp unit will treat, is directly pro-portional to the transmissivity of the water. Light transmission is affected by a number of characteristics of the water. Colour in the water absorbs UV and visible light, iron salts are oxidised and ‘use up’ UV radiation, whilst turbidity scatters the light. Suspended solids in the water may shield micro-organisms from the radiation and may also coat the quartz sleeve, reducing the intensity of UV radiation reaching the water. Scale deposition has the same effect. Solids are particularly significant when virus removal is required, since viruses are associated mostly with the suspended solids.