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The pathogenic organisms may be transmitted through water. It is important, therefore, that the drinking water is fit for consumption. Purification ( reclamation) of drinking water is brought in from a body of water into a mixing chamber where water is mixed with coagulating chemicals. Then it is passed through sand filter and chlorinated before it is released to the consumers. The bacteriological examination is carried out to test water for pollution with pathogenic organisms. Likewise, the sewage which may contain high levels of organic matter and pathogenic organisms is treated by a combination of systems which include oxidation and chlorination prior its disposal in a body of water. These procedures are described in this chapter.
Water plays an important role in the transmission of disease agents especially those caused by the enteric group, in which the infection takes place via the gastrointestinal tract and the causative agents are discharged with the feces. Most of the important diseases, thus transmitted are typhoid fever, bacillary dysentery, amoebic dysentery, paratyphoid fever and cholera. Other diseases like infectious hepatitis, leptospirosis, poliomyelitis and other infections may also be transmitted by water. It is important, therefore, that sewage must be disposed of properly to avoid contamination of water supplies and that the water for consumption is free from all the pathogenic organisms.
From a hygienic point of view, water may be classified as follows:
A. Potable water.
satisfactory for drinking purposes.
B. Polluted water.
does not have a pleasant appearance, taste, or odor. The water may or may not contain the pathogenic organisms; it may contain wastes from agriculture and industry.
C. Contaminated water.
contains dangerous chemical or microbial agents. Its tastes, odor and appearance may still be normal.
The U.S. Public Health Department has set up certain standards for drinking water which include specifications relating to physical and chemical properties, source and protection of the water, bacteriological quality, and limits on concentration of radioactivity.
Coliform bacilli, Streptococcus facalis and Clostridium perfringens are present in the intestines of men and animals and the presence of any of these organisms in water is considered contamination from fecal material. All three organisms, therefore, are used as an indicator of pollution. The coliform bacilli are almost exclusively used in this country while both Str. fecalis and Cl. perfringens are used in Europe as indicators for pollution.
The standard procedure for Total Coliform MPN tests for examination of water has been adopted by the Public Health Association.
(1) The presumptive test
lactose broth is inoculated with 10 ml, 1 ml., and 0.1 ml. of water sample respectively. The volume of the smallest inoculum of water sample producing fermentation gives a crude estimate of the number of coliform bacilli present in the water sample.
(2) Confirmed test
consists of inoculation of the sample showing fermentation with production of gas on selective media such brilliant green lactose bile broth and EMB.
(3) Completed test
the colonies developing on eosin-methylene blue agar are called typical which are nucleated, with or without metallic sheen; atypical colonies are opaque, unnucleated, mucoid, and pink after 24 hours of incubation, or negative (all others).
Formation of gas in the secondary tube of lauryl tryptose broth within 48 + 3 hr. and demonstration of gram-negative, non spore-forming, rod-shaped bacteria in the agar culture constitute a satisfactory completed test demonstrating the presence of a member of the coliform group.
This consists of the filtration of the water sample through a filter. The filter disc which retains the bacteria is transferred to a pad containing a liquid media such as Endo medium. This is followed by incubation. The bacterial colonies which grow on the surface are identified and counted.
In purification of drinking water for municipalities, three major operations are generally carried out (Fig. 1).
A. Sedimentation and flocculation
The water from the source is pumped to large reservoirs where water is allowed to stand. Substances like iron salts and alum are added to form a sticky, flocculent precipitate to help settle down solid materials including microorganisms. Thus the water is clarified.
B. Filtration
The clarified water is then passed through sand filters which are of various types. The filters can remove 99 percent of the bacteria present in the raw water.
C. Chlorination
After filtration the water is treated with chlorine. The chlorineated water is then stored in underground cisterns. Highly polluted water may be made potable, crystal clear and almost sterile by these procedures.
Sewage is an important vehicle in the transmission of enteric infection and therefore, its disposal is of great importance. The sewage is processed in treatment plants so as to render it harmless before it is discharged into any body of water. Many methods of sewage purifications are in use. In general, the following procedure is used to bring about the decomposition of complex organic compounds into amino acids, monosaccharides, and the like, which are oxidized completely to carbon dioxide and water in the case of carbon and hydrogen and to nitrite and nitrate in the case of nitrogen.
The following steps are used in the disposal of sewage:
A. Screening
The raw sewage is pumped into the grit chambers where large objects are either removed or broken into small pieces.
B. Sedimentation
The material is held in large vats and the solid materials settles down at the bottom. At this point water and solids are separated and handled separately.
l. Sludge
settled sludge is removed from the settling tanks by means of mechanical scrapers and pumped to externally heated digesters. Digested sludge, elutriated and conditioned with ferric chloride is vacuum filtered for use as a soil conditioner. Sludge gas, produced by the digestion of the sludge, is used for running the gas engines in the pump room, and as a fuel for other heating purposes in the plant.
2. Aeration of fluid
the clarified fluid is subjected to various aeration treatments. The common ones are activated sludge, aerating filters and oxidizing lagoons.
a. Activated sludge
- In order to maintain aerobic conditions,the compressed air is forced through sewage in a tank. Particles of suspended matter is flocculated which is followed by formation of gelatinous masses known as activated sludge. This process results in clarification of the material, its enzymatic decomposition and decrease in bacterial contents. The fluid part of the activated sludge-sewage mixture is passed into a final settling tank where it is removed.
b. Aerating filters
In this process, the fluid is allowed to trickle and splash continuously or intermittently through artificial beds of broken stone. The filtrate is collected and the organisms feed upon and help oxidize the organic matter of the sewage.
c. Oxidation lagoons
raw sewage or preliminarily treated wastes are carried to oxidation ponds or lagoons in order to achieve aerobic oxidation.
d. Chloration and disposal
After aerobic oxidation, chlorine is added to the supernatant which is dumped into the body of water such as river. (Fig. 2)
Study questions 1. Name several diseases which are transmitted through the media of water.2. Does the bacteriological examination of water detect pathogens? Explain.
3. Why sewage is considered of such great sanitary significance?
4. What are the reasons for using Eschericha coli as an indicator organism in the bacteriological analysis of water?
5. Describe the procedure for purification of drinking water.
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