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CHAPTER 28 Wastewater Treatment, Water Purification, and Waterborne Microbial Diseases. Wastewater Microbiology and Water Purification Public Health and Water Quality.
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Wastewater Treatment, Water Purification, and Waterborne Microbial Diseases
Microbial culture methods for evaluating the health significance of polluted drinking water were not practiced until coliform-counting procedures were developed and adapted in about 1905.
Until then, water purification was limited to filtration to reduce turbidity. Although filtration significantly decreased the microbial load of water, many microorganisms still passed through the filters. In about 1910, chlorine was discovered to be an extremely efficient disinfectant for large water supplies.
Drinking water quality is determined by counting coliform bacteria. Strict adherence to uniform microbiologic standards makes this method a reliable and reproducible indicator of fecal contamination in all public water supplies in the United States.
Filtration and chlorination of water supplies significantly decreases microbial load. Application of water purification methods to drinking water is the most important public health measure ever devised (Figure 28.2).
Wastewater enters a treatment plant and, following treatment, the effluent water—treated wastewater discharged from the wastewater treatment facility—is suitable for release into surface waters such as lakes and streams or to drinking water purification facilities.
Wastewater treatment is primarily concerned with treating sewage and industrial wastes to reduce the biochemical oxygen demand (BOD) to acceptable levels.
Figure 28.5 shows anoxic secondary wastewater treatment, and Figure 28.6 shows aerobic secondary wastewater treatment processes.
Primary, secondary, and tertiarywastewater treatment involves physical, biological, and physicochemical processes.
Drinking water plants employ industrial-scale physical and chemical systems that remove or neutralize biological, inorganic, and organic contaminants from a variety of natural, community, and industrial sources.
To further ensure that residual chlorine levels are maintained throughout the distribution system, most municipal water treatment plants also introduce ammonia gas with the chlorine to form the stable, nonvolatile chlorine-containing compound chloramine.
Figure 28.8 traces the flow of raw water (untreated water) through a typical treatment scheme.
Raw water is first pumped from the source to a sedimentation basin where anionic polymers, alum (aluminum sulfate), and chlorine are added. After mixing, the particles continue to interact, forming large, aggregated masses, a process known as flocculation.
Sediment, soil, sand, mineral particles, and other large particles settle out. The sediment-free water is then pumped to a clarifier (coagulation basin), a large holding tank where coagulation takes place.
The alum and anionic polymers form large particles from the much smaller suspended solids. The resulting water is potable, finished water, free of chemical and biological contamination.
Drinking water and recreational water may both be sources of waterborne pathogens. In the United States, the number of disease outbreaks due to either of these sources is relatively small in relation to the large number of exposures to water (Tables 28.1, 28.2).
Worldwide, lack of adequate water treatment facilities and access to clean water contribute significantly to the spread of infectious diseases.
Vibrio cholerae is a pathogen that causes cholera, an acute diarrheal disease resulting in severe dehydration.
Giardiasis and cryptosporidiosis are spread by the chlorine-resistant cysts of Giardia intestinalis and Cryptosporidium parvum, respectively, in drinking water and recreational water contaminated by the feces of infected humans or animals.
Legionella pneumophila is a respiratory pathogen that causes legionellosis and Pontiac fever. L. pneumophila grows to high numbers in warm water and is spread via aerosols. The prevalence of legionellosis is not decreasing, and infections are underreported.
Typhoid fever, viral infections, and amebiasis are important waterborne diseases. Waterborne typhoid fever and viral illnesses, although still common diseases in developing countries, have been controlled by effective water treatment in developed countries.
Amebic dysentery caused by Entamoeba histolytica is a worldwide problem that affects millions of people. Meningoencephalitis is a rare but serious condition caused by Naegleria amebiasis.