Drinking Water Treatment and Testing

Large cities generally obtain their drinking water from surface waters such as lakes or rivers. Because surface water may serve as the receiving water for another city's sewage effluent, drinking water treatment is intimately connected to sewage treatment. The quality of the surface water is also affected by the characteristics of the watershed, the land over which water flows into the river or lake. Even pristine rivers are likely contaminated with feces of animals that inhabit the watershed.

Smaller communities often use groundwater, pumped from a well, as a source of drinking water (figure 31.5). This water occurs in aquifers, water-containing layers of rock, sand, and gravel, that is replenished as water from various sources seeps through the soil. Because aquifers are not directly exposed to rain, animals, and the atmosphere, they are somewhat protected from contamination. However, poorly located or maintained septic tanks and sewer lines, as well as sludge or other fertilizer can lead to groundwater contamination.

Public water systems in the United States are regulated under the Safe Drinking Water Act of1974, amended in 1986 and 1996. This gives the Environmental Protection Agency (EPA) the authority to set drinking water standards in order to control the level of contaminants in drinking water. Standards are modified in response to new concerns; for example, new regulations now govern the maximum levels of Cryptosporidium oocysts, Giardia cysts, and enteric viruses in drinking water. ■ Cryptosporidium, p. 626

Water Treatment Processes

The treatment of metropolitan water supplies is designed to provide a safe water supply by eliminating pathogenic microbes as well as harmful chemicals (figure 31.6). First, water flows into a reservoir and is allowed to stand long enough for the particu-late matter to settle. The water is then transferred to a tank where it is mixed with a flocculent chemical, such as aluminum potassium phosphate, or alum. Alum causes materials still suspended in the

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792 Chapter 31 Environmental Microbiology: Treatment of Water, Wastes, and Polluted Habitats

Causative Agents
Figure 31.5 Groundwater The water in the aquifer is replenished as water from various sources seeps through the soil. Poorly located or maintained septic tanks and sewer lines, as well as sludge or other fertilizers, can lead to groundwater contamination.

liquid to coagulate, forming aggregates that slowly sink to the bottom. As the clumps settle, they remove unwanted materials from the water, including some bacteria and viruses.

Following the flocculation, the water is filtered, often through a thick bed of sand and gravel, to remove various microorganisms including bacteria and protozoan cysts and oocysts. Organic chemicals that may be harmful or impart undesirable tastes and odors can be removed by additional filtration through an activated charcoal filter, which adsorbs dissolved chemicals. Not only does filtration physically remove various particles, but microorganisms growing in biofflms on the filter materials use carbon from the water as it passes. This lowers the organic carbon content of the water, resulting in less microbial growth in pipes delivering the water. ■ filtration, p. 122 ■ diatomaceous earth, p. 302

Finally, the water is treated with chlorine or other disinfectants to kill harmful bacteria, protozoa and viruses that might remain. A concern with using chlorine, however, is that some of the by-products formed when chlorine reacts with organic material may be carcinogenic. In response to this concern, ultra violet irradiation and ozone are increasingly being used as alternatives, but a small amount of chlorine must still be added to prevent problems associated with post-treatment contamination. Note that disinfection of waters with a high organic content requires more chlorine because organic compounds consume free chlorine. ■ ultraviolet irradiation, p. 123 ■ ozone, p. 120 ■ chlorine, p. 118

Water Testing

A primary concern regarding the safety of drinking water is the possibility that it might be contaminated with any of a wide variety of intestinal pathogens, such as those discussed in chapter 24. It is not feasible to test for all of the pathogens, however, so indicator organisms function as a surrogates. These are microbes that are routinely found in feces, survive longer than intestinal pathogens, and are relatively easy to detect and enumerate. The most common group of bacteria used as indicator organisms in the United States are total coliforms, lactose-fermenting members of the family Enterobacteriaceae, including E. coli. The group is functionally defined as facultatively anaerobic Gram-negative,

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Alum mixed with water

Alum mixed with water

Sedimentation With Alum

Step 1 Water is held in a series of reservoirs, where large materials sediment. Aluminum potassium sulfate (alum) may be added to cause flocculation of organic matter, which then settles out.

Step 2 The water is then filtered through beds of sand, which removes almost all of the bacteria. Filtration through activated charcoal may be used to remove toxic or objectionable organic materials.

Step 3 Finally, chlorination is used to disinfect the water, killing any pathogens that might remain. It is possible to treat drinking water with ultraviolet irradiation or with ozone so that it does not have to be chlorinated. Some cities also add fluoride to protect against dental cavities.

Figure 31.6 Steps in the Treatment of Metropolitan Water Supplies

Step 1 Water is held in a series of reservoirs, where large materials sediment. Aluminum potassium sulfate (alum) may be added to cause flocculation of organic matter, which then settles out.

Step 2 The water is then filtered through beds of sand, which removes almost all of the bacteria. Filtration through activated charcoal may be used to remove toxic or objectionable organic materials.

Step 3 Finally, chlorination is used to disinfect the water, killing any pathogens that might remain. It is possible to treat drinking water with ultraviolet irradiation or with ozone so that it does not have to be chlorinated. Some cities also add fluoride to protect against dental cavities.

Figure 31.6 Steps in the Treatment of Metropolitan Water Supplies

31.2 Drinking Water Treatment and Testing allowing identification of coliform colonies. (figure 31.7b). ■ selective media, p. 93 ■ differential media, p. 94

i ONPG/MUG test. A water sample is added to a minimal medium containing two relevant substrates, ONPG (o-nitrophenyl-/i-d-galactopyranoside) and MUG (4-methylumbelliferyl-^-d-glucuronide). Organisms that can degrade lactose hydrolyze ONPG, yielding a yellow-colored compound; thus, all coliforms turn the medium yellow. E. coli produces an enzyme that hydrolyzes MUG, generating a fluorescent compound. Thus, a sample can be assayed simultaneously for the presence of both total coliforms and E. coli.

i Presence/absence test. This uses a medium similar to that of the MPN method, but no attempt is made to estimate the number of total coliforms in the sample. Instead, a 100 ml water sample is added to a concentrated, lactose-containing broth that contains a vial to trap gas. If gas is produced, the broth is then tested to confirm the presence of coliforms.

rod-shaped, non-spore-forming bacteria that ferment lactose, forming acid and gas within 48 hours at 35°C. Although these organisms are routinely present in the intestinal contents of warm-blooded animals, certain species can also thrive in soils and on plant material. Thus, the presence of coliforms does not necessarily imply fecal pollution. To compensate for this shortcoming, fecal coliforms, a subset of total coliforms more likely to be of intestinal origin, are also used as indicator organisms. The most common fecal coliform is E. coli. Note that although some strains of E. coli can cause intestinal disease, the organism is used in water testing merely to indicate fecal pollution.

Methods used to detect total coliforms in a water sample include:

■ Most probable number (MPN) method. This is a statistical assay of cell numbers. It employs successive dilutions of a water sample in tubes of lactose-containing broth that have a vial to trap gas (see figure 4.15). Tubes in which gas is produced are tested to confirm the presence of coliforms (figure 31.7a).

■ Membrane filtration. A water sample is passed through a filter that retains bacteria (see figure 4.14), concentrating the bacteria in a known volume of water before they are plated. The bacteria are then plated on a lactose-containing selective and differential agar medium

The goal for the number of total coliforms in drinking water is zero. The total coliform rule establishes a maximum number of positive samples (100 ml) permitted. That maximum relates to the number of samples routinely collected by the water system. This ranges from 1 to 480 samples per month, depending on the size of the population served by the system. Systems that collect at least 40 samples per month are in violation if more than 5% are total coliform-positive in a month. Systems that collect fewer samples are in violation if more than one sample tests positive per month. If a sample tests positive, repeat samples within 24 hours are mandated. Total coliform-positive samples are also tested for either fecal coliforms or E. coli. If a water system exceeds the monthly total coliform limit, the system must notify the state and the public. Notification is also required if either of two sequential samples that test positive for total coliforms also test positive for fecal coliforms or E. coli.

Because of limitations of total coliform and fecal coliform assays in predicting contamination with protozoan cysts and oocysts, alternatives are being explored. Other microbes that may be used as indicators of fecal pollution include enterococci, some Clostridium species, and certain types of bacteriophages. Work is under way to develop nucleic acid probes and polymerase chain reaction (PCR) methods to detect pathogens, including viruses, in water. ■ nucleic acid probes, pp. 225, 236 ■ PCR, pp. 229,239

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794 Chapter 31 Environmental Microbiology: Treatment of Water,Wastes, and Polluted Habitats

Water sample

Presumptive test

Lactose broth inoculated and incubated 24 hours.

No gas produced. Negative presumptive test.

Gas produced. Positive presumptive test.

EMB plate inoculated and incubated 24 hours.

Coliform colonies form. Positive confirmed test.

No coliform type colonies formed. Negative confirmed test.

Completed test: Lactose broth and agar slant inoculated and incubated.

Lactose broth

Agar slant

No gas produced. Negative completed test.

Membrane filter on a filter support.
Water sample filtered through membrane filter (0.45 ^m).
Membrane filter removed and placed in plate containing the appropriate medium.

Incubation for 24 hours.

Incubation for 24 hours.

Gas produced

Gram-negative rods present: no endospores

Coliform group present. Positive completed test.

Typical coliform colonies.

Figure 31.7 Methods Used for Testing Water Contamination of water is indicated by the presence of total coliforms. (a) Traditional method for detecting total coliforms. In the first step, the water sample is added to lactose broth and incubated for 24 hours.The production of gas is a positive presumptive test.The next step is to confirm that the gas-forming organisms in the sample are coliforms.To do this, the 24-hour culture is streaked onto an EMB (eosin-methylene blue) plate and incubated for 24 hours. E. coli grown on EMB gives characteristic colonies with a metallic sheen.To complete the investigation, lactose broth and an agar slant are inoculated.The presence of Gram-negative rods and the absence of endospores on the agar slant and the presence of gas production in the lactose broth constitute a positive completed test. (b) The membrane filter procedure is used for direct recovery of indicator bacteria from water. Which method would allow easier detection of more dilute populations of microorganisms?

Typical coliform colonies.

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Responses

  • raija
    Why do bacterial colonies recover from drinking water and grown on emb media .?
    2 years ago
  • berhane
    What the causative agents for the pollution of water?
    2 years ago
  • fethawi
    What are causative organisms in polluted water?
    1 year ago

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