Wavelength, Angstrom

FIGURE 17.14 Electromagnetic spectrum.

Lamp bulbs are typically 0.75 m to 1.5 m in length and 15-20 mm in diameter. Mercury vapor is sealed inside this bulb. To produce the radiation, electric current, which generates an electric arc, is passed through the bulb. In actual use in the treatment of water and wastewater, the lamps are either suspended over the liquid to be disinfected or immersed in it. When immersed, the lamps are encased in quartz tube to prevent the cooling effect of the liquid on the lamps. The right-hand side portion of Figure 17.13 shows an ultraviolet lamp assembly; the left-hand side shows an actual installation located below floor level.

Figure 17.14 shows the electromagnetic spectrum. From the figure, the ultraviolet light occupies a band along the spectrum extending from 30 A to 3650 A (3 nm- 365 nm). The bactericidal wave range extends from 2000 A to 2950 A (200 nm-295 nm);

the optimum wave range extends from 2500 A to 2700 A (250 nm-270 nm). As mentioned before, the UV-B range is < 320 nm. The other "half" of the UV spectrum is called the UV-A range and extends from 320-365 nm. UV-A is not carcinogenic.

Note: A very small "window" of spectrum is available for visible light. 17.7.1 Unit Operations in UV Disinfection

The unit operations in UV disinfection, among other things, involve the generation of the ultraviolet radiation and its insertion into or above the flowing water or wastewater to be disinfected. The contact time is very short, being in the range of seconds to a few minutes. Thus, the contact tank for UV disinfection is short relative to the contact tanks for chlorination and ozonation which require the serpentine configuration to satisfy the contact time required. The contact time and UV intensity is best determined by a pilot plant study. Intensity is normally expressed as milliwatts or microwatts per square centimeter of projected area. To be effective, the "sheet" of flowing liquid should be thin so that the radiation can penetrate. The equation for UV disinfection follows the universal law of disinfection given by Equation (17.2).

Example 17.21 A total flow of 1000 m /d is to be disinfected using UV. What should be the cross-sectional area of the channel in order to maintain self-cleaning velocity? What would be the total combined length of the channel if the contact time has been determined by a pilot study to be 1.0 min?

Solution: Cross-sectional area = 1000/24(60)(2.0) = 0.34 m , assuming a self-cleaning velocity of 2.0 m/min. Ans

Length of channel = 1.0(2) = 2.0 m. The flow in the tank will not be exactly plug flow, so increase channel length by 10%. Thus, length of channel = 2.0(1.10) = 2.2 m. Ans

Example 17.22 A study was conducted to determine the optimum UV intensity and contact time to be used to design a UV contact tank. For a 99% kill, the results are as follows:

Contact Time (sec) Intensity (mW/cm2)

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