Ultra Violet Disinfection
Ultra Violet Disinfection
Disinfection by the Ultraviolet (UV) Process
Ultraviolet radiation’s germicidal properties were recognized for many years before the first attempt was made to use it for disinfection of water in 1919. The killing action of ultraviolet light is similar to sunlight that kills bacteria in surface waters.12
Among available procedures, the ultraviolet process finds frequent application for unsafe well and surface water supplies in homes and on farms. The UV process is a practical means of disinfection for a one-or-two-faucet water supply, and will effectively destroy potentially harmful bacteria and viruses. The UV radiation causes a cross-linking within organism’s DNA that is disruptive enough to inactivate the cell.13
The UV water-disinfection system consists of one or more ultraviolet lamps enclosed in a quartz sleeve. Ultraviolet lamps produce electromagnetic radiation with a wavelength of 2537 angstroms (A). The lamps are similar to fluorescent lamps, but they contain no phosphorescent coating on the inside of the tube to convert the ultraviolet radiation to visible light. The quartz sleeve surrounding each lamp protects it from the cooling action of the water, since the lamps must maintain a certain level of heat to produce the necessary killing intensity.14
The germicidal effect of UV radiation results from the intensity of the lamp combined with the length of exposure. The intensity of the light produced by a conventional germicidal lamp is sufficient to kill microorganisms in a fraction of a second. The light’s intensity is, of course, reduced with distance and by the medium through which it passes. In addition to the effect of turbidity, minute traces of iron compounds that commonly exist in water reduce the light transmission. Therefore, the water should be filtered before it passes into the UV lamp chamber to eliminate the possibility of an organism escaping in the shadow of a particle. UV units are more effective when the flow is turbulent, because the organisms receive more uniform exposure.15For these reasons, UV water purification units are designed so that water passes in a relatively thin layer around the lamps.
Each unit is designed so that the water flows at a particular rate. The flow must be regulated to meet this rate to ensure that all organisms present receive adequate exposure.16
A typical domestic-size ultraviolet device (Figure 10-7) consists of a UV lamp, a quartz jacket or panel, a flow chamber (where the emitted UV rays are radiated into the stream), a ballast, and a light-intensity meter for monitoring.
The UV process does not distort the taste of water or the pH value. It generally avoids creating new chemical complexes, such as might occur with a chemical disinfectant.
To obtain good operating performance of the UV device, the water should be prefiltered so only the cleanest water passes through the UV flow chamber. The bulb should be removed at least four times per year and cleaned, as well as the quartz or plastic jacket, so the water will be exposed to full-strength UV rays. Hardness and iron or manganese, if present in the feedwater, can prematurely coat the quartz sleeve and prevent the UV radiation from penetrating the water.
The bulbs should be replaced at least annually (or as recommended), as most are rated for not more than 9,000 hours of operation (365 days equal 8,760 hours). These smaller UV devices for single-faucet applications are rated at less than one gpm (3.8L/min) flow and consume a steady 10 to 35 watts of electricity.
Disinfection by Distillation
The distillation process described earlier in this chapter is yet another means of obtaining small volumes of microbiologically-safe drinking water. This heat-sterilization
Typical Domestic-Style Ultraviolet System
process is among the easiest to operate by homemakers and requires only conventional 110-to120-volt AC electric current for countertop models. No chemicals are used- just heat, which creates steam to disinfect the product water and deionizes the water at the same time. The distillation process, perhaps the most readily understood disinfection method among lay people and consumers, appeals to many as a viable mode of treatment. One limiting factor of this process is the extra time needed to produce one gallon (3.8L) of processed water when compared to ultraviolet, ozonation, chlorination and iodination.
Disinfection by Chemical Feed
Ozone Feed Treatment
The ozone (O3 ) oxidation process, over 100 years old and widely used among European countries, has been used mostly in municipal water and wastewater treatment in the United States. However, there appears to be a place for this unique process in residential or business market. Scaled- down, safe, and dependable versions of this process are becoming available for small (5-15 U.S. gallons per minute) flow rate ranges. Ozone, as an oxidating step, will also provide extremely powerful disinfection (see Table 8-3). The ozone process leaves no residual taste/odor, such as occurs with chlorine and
Typical Dry Chemical Chlorinator Installation at Wall Head
chloramine feeds. Where the atmosphere can be used, a very inexpensive source of oxygen can be utilized to generate ozone on site. 20 Some trace by-products (e.g., formaldehyde and bromoform) can be generated with the ozone process, however.
Ozone must be generated on site, at point of injection into the water or waste stream. It cannot be packaged, stored, and shipped as ozone. Commercial and industrial ozonation systems generally use liquid oxygen as the feed to generate the O3 gas, usually passing the oxygen through a dryer unit first. Domestic-size ozonation units use air with a desiccant-type dryer and depend upon the available free oxygen in the atmosphere, which on average runs around 20-21 percent. Corona discharge ozone generators produce O3 - air concentrations greater than one percent, whereas ultraviolet light ozone generators yield less than one-tenth of one percent ozone in the airstream. The higher the concentration of O3in the airflow bubbled into the water, the more ozone will transfer into the water. Bubble (O3) contact time and total bubble surface area are also critical to the resultant ozone transfer into water. 21 The most vital step in ozonation is the proper and full distribution of the generated O3 into the water stream.
Ozone (0 3 ) is not very stable and, accordingly, an O 3 residual in the treated water cannot be maintained, as can be done with chlorine, iodine, and bromine. For this reason to establish full disinfection on a continuing basis, municipal water supplies must have downstream supplemental chlorine feed in conjunction with ozone. The ppm-mg/L level of chlorine feed is very low, of course, because the primary oxidation step is accomplished by the ozone treatment. The modern, 600 million gallons per day municipal water treatment plant for the city of Los Angeles, which began operation in 1986, employee two-step disinfection process. Ozone provides the primary disinfection step, with limited downstream chlorine feeds to establish a low Cl 2 residual. This facility is one of the largest ozone systems in the world, and this two-step process is capable of meeting product water quality below EPA limits for trihalomethanes.
Oxidizing Catalytic Media
Because chemical and atmospheric conditions vary greatly from region to region in North America, many products and techniques have been tested and used to try to reduce the "troublesome trio"
over the last three quarters of the 20th century. Among filtering materials used are those called chemically reactive media (see Table 1-4, Chapter One These catalytic media speed up chemical reactions by serving as a catalyst.