1、外文资料DRINKING WATER TREATMENT ANDWATER SECURITYC. P. Gerba, K. A. Reynolds, and I. L. Pepper Rivers, streams, lakes, and aquifers are all potential sources of potable water. In the United States, all water obtained from surface sources must be filtered and disinfected to protect against the threat of
2、 microbiological contaminants. Such treatment of surface waters also improves values such as taste, color, and odor. In addition, groundwater under the direct influence of surface waters such as nearby rivers must be treated as if it were a surface water supply. In many cases however, groundwater ne
3、eds either no treatment or only disinfection before use as drinking water. This is because soil itself acts as a filter to remove pathogenic micro organisms, decreasing their chances of contaminating drinking water supplies. At first, slow sand filtration was the only means employed for purifying pu
4、blic water supplies. Then, when Louis Pasteur and Robert Koch developed the Germ Theory of Disease in the 1870s, things began to change quickly. In1881, Koch demonstrated in the laboratory that chlorine could kill bacteria. Following an outbreak of typhoid fever in London, continuous chlorination of
5、 a public water supply was used for the first time in 1905 (Montgomery,1985). There gular use of disinfection in the United States began in Chicago in 1908. The application of modern water treatment processes had a major impact on water-transmitted diseases such as typhoid in the United States (see
6、also Chapter 11).The following sections describe conventional water treat-ment that is practiced in the public sector (e.g., municipal water supplies).28.1 WATER TREATMENT PROCESSESModern water treatment processes provide barriers, or lines of defense, between the consumer and waterborne disease. Th
7、ese barriers, when implemented as a succession of treatment processes, are known collectively as a treatment process train (Figure 28.1). The simplest treatment process train, known as chlorination, consists of a single treatment process, disinfection by chlorination (Figure 28.1a). The treatment pr
8、ocess train known as filtration, entails chlorination followed by filtration through sand or coal, which removes particulate matter from the water and reduces turbidity (Figure 28.1b). At the next level of treatment, in-line filtration, a coagulant is added prior to filtration (Figure28.1c). Coagula
9、tion alters the physical and chemical state of dissolved and suspended solids and facilitates their removal by filtration. More conservative water treatment plants add a flocculation (stirring) step before filtration, which enhances the agglomeration of particles and further improves there moval eff
10、iciency in a treatment process train called direct filtration (Figure. 28.1d). In direct filtration, disinfection is enhanced by adding chlorine (or an alternative disinfectant, such as chlorine dioxide or ozone) at both the beginning and end of the process train. The most common treatment process t
11、rain for surface water supplies, known as conventional treatment, consists of disinfection, coagulation, flocculation, sedimentation , filtration, and disinfection (Figure 28.1e).As already mentioned, coagulation involves the addition of chemicals to facilitate the removal of dissolved and suspended
12、 solids by sedimentation and filtration. The most common primary coagulants are hydrolyzing metal salts, most notably alum Al2(SO4)3 14H2O, ferric sulfateFe2(SO4)3, and ferric chloride (FeCl3). Additional chemicals that may be added to enhance coagulation are charged organic molecules called polyele
13、ctrolytes; these include high-molecular-weight polyacrylamides, dimethyldially-ammonium chloride, polyamines, and starch. These chemicals ensure the aggregation of the suspended solids during the next treatment step, flocculation. Some times polyelectrolytes (usually polyacrylamides) are added after
14、 flocculation and sedimentation as an aid in the filtration step.Coagulation can also remove dissolved organic an dinorganic compounds. Hydrolyzing metal salts added to the water may react with the organic matter to form a precipitate,or they may form aluminum hydroxide or ferric hydroxidefloc parti
15、cles on which the organic molecules adsorb. Theorganic substances are then removed by sedimentation and filtration, or filtration alone if direct filtration or in-line filtration is used. Adsorption and precipitation also remove inorganic substances.Flocculation is a purely physical process in which
16、 the treated water is gently stirred to increase interparticle collisions, thus promoting the formation of large particles. After adequate flocculation, most of the aggregates settle out during the 1 to 2 hours of sedimentation. Microorganisms are entrapped or adsorbed to the suspended particles and removed during sedimentation (Figure 28.2).Sedimentation is another purely physical process, involving the gravitational settling of suspended particles that are d
