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Water is recycled (mouth ----> intestines ----> feces ----> water ----> mouth), especially in big cities. Yet, not until this century were measures taken to purify the water supply adequately. Dramatic reductions in the incidence of typhoid fever and other water-borne diseases correlate with advances in water treatment (Figure 15.53). The coliform test was introduced in 1905, filtration methods in 1906, chlorination in 1913.
A. Materials that Should Not Be Discharged Into Lakes and Rivers Without Treatment:
1. Pathogens (in feces)
2. Organics:
a. Non-biodegradable - Living organisms can eat but they cannot degrade these materials. Therefore non-biodegradables persist in the environment, and as they move up in food chains, many of them concentrate in the fat of aminals.
b. Biodegradable - organisms whose growth is limited by their carbon source will proliferate if large quantities of the source are suddenly made available. For example, sulfate (SO42-) reducers are limited by carbon. Therefore, if a carbon source is discharged into the environment, these organisms multiply. Moreover, sulfate reduction generates a toxic gas, hydrogen sulfide (H2S). In addition, as these organisms grow, their predators and other organisms that live on their decayed materials also grow and deplete the oxygen supply. Consequently, other aerobic organisms such as fish starve for oxygen and die. This phenomenon is known as eutrophication.
3. Phosphate - the growth of organisms such as algae and cyanobacteria, that fix their own carbon and nitrogen, is limited by phosphate. Thus if a phosphate source is introduced into the environment, these primary producers will proliferate and eutrophication may occur.
4. Nitrogen sources (NH4+, NO3-, NO2-) - similarly, the introduction of nitrogen sources into the environment will allow nitrogen-limited organisms to grow, initiating eutrophication. Fertilizers are rich in such nitrogenous materials.
The main point is that releasing untreated, nutritious materials into the environment may favor the growth of certain microorganisms, upset the normal balance of ecosystems, and kill many larger creatures that require oxygen to live, such as fish.
B. Water-Borne Pathogens
1. In developed countries:
a. Water-borne pathogens have essentially been eradicated.
b. Such organisms may reappear during natural disasters (floods, earthquakes) if water supplies become contaminated.
c. Contamination may occur by accidental mixing of water flowing through pipes having treated and untreated water.
2. In non-developed countries:
a. Many countries have poor or non-existent water treatment facilities.
b. It is wise to avoid the various sources of polluted water. The drinking water may contain potentially hazardous pathogens. In addition, vegetables washed in water, and shellfish that concentrate the microorganisms from the polluted waters in which they live, may be trouble.
3. Prevention
a. Boiling water at 100oC for ten minutes will exterminate virtually all contaminating pathogens.
b. Inoculation against specific pathogens provides immunity against infection.
4. Notable water-borne pathogens (Table 15.6):
| a. Bacterial | b. Protozoan | c. Viral |
| (i). Salmonella typhi - causative agent of typhoid fever.
(ii). Vibrio cholerae - causative agent of cholera. The acidic gastric (stomach) juices provide some protection against this organism. Infection requires 109 organisms in an acidic environment but only 104 organisms in neutral environments. This disease strikes children more often than adults because the stomachs of children are generally less acidic. |
(i). Giardia lamblia - causative agent of giardiasis,
a severe form of gastroenteritis. This organism has a cyst phase that is
not killed by chlorine (Cl2). Cases of contamination do occur,
even in the U.S., especially in relatively unpopulated areas. This is because
this organism has a cyst phase that is not killed by chlorine. If the filtration
step of water treatment is omitted, as occurs in some relatively small
towns, such cysts may contaminate the drinking water.
(ii). Entamoeba histolytica - may cause ulcers and abscesses in the liver and lungs. If untreated, brain damage may occur. |
(i). Hepatitis A - a single-stranded RNA virus that infects
the liver. The resulting liver inflammations are usually not fatal.
(ii). Poliovirus - another single-stranded RNA virus. A worldwide vaccination program has made infections by this virus extremely rare. |
5. Symptoms and treatment
a. Different organisms may cause similar symptoms. Most water-borne pathogens cause severe dehydration and diarrhea.
b. An expedient method is needed to identify the offending organism reliably so that the proper medication may be administered. Culturing of organisms for identification is expensive and generally takes longer than the ill person would like! Alternatives such as detection of the causative agent by immunofluorescence, ELISAs, or PCR, can be used for more rapid identification (within minutes to hours).
C. Assay of Waste Treatment
The Biochemical Oxygen Demand (BOD) assay provides a measure of the levels of biologically degradable organic material present in water samples. The idea is to measure decreases in oxygen concentration following the oxygen-dependent oxidation of organics by bacteria, i.e. organics + O2 -----> CO2. The procedure is as follows:
1. The water to be assayed is vigorously shaken so that it is completely saturated with oxygen.
In oxygen-saturated water, the concentration of oxygen is 10 mg/L.
2. The vessel is sealed and left at 20oC for five days. During this time, any bacteria in the water oxidize organic material in that water, reducing the amount of oxygen present.
3. An oxygen electrode is then used to measure the amount of oxygen remaining in the sample.
The amount of oxygen consumed (BOD) is difference of the initial and final concentrations of oxygen in the sample:
[O2]consumed = [O2]initial - [O2]final
e.g. if after 5 days at 20oC a sample contains 4 mg/L of oxygen, then the amount consumed is:
[O2]consumed = [O2]initial - [O2]final = 10 mg/L - 4 mg/L = 6 mg/L
4. To assay the efficiency of a wastewater treatment procedure, the results of BOD assays are compared for water samples taken before and after the treatment. By definition, the efficiency of treatment is expressed as the percent decrease in BOD (i.e. the percent decrease in oxygen consumed).
Percent decrease in BOD = 100 x (BODbefore - BODafter)/(BODbefore)
e.g. Suppose the BOD is 6mg/L before a treatment and is 2 mg/L after treatment, what is the percent decrease in BOD?
The percent decrease in BOD is = 100 x (6 mg/L - 2 mg/L)/(6 mg/L) = 66%
Thus a high percentage decrease in BOD indicates that an effective purification has been achieved.
D. Sewage Treatment
There are three levels of sewage treatment, although only the first two are commonly employed (Figure 17.51).
1. Primary treatment
Primary treatment is a strictly physical process wherein sewage is filtered through screens or grates to remove large solid materials. These solids are incinerated, buried or composted. Primary treatment decreases the BOD by 30-40%. The liquid is further cleaned up by secondary treatment.
2. Secondary treatment
Involves the microbial digestion of organic material and removal of microorganisms. First, the liquid from primary treatment is allowed to settle for several hours to separate insolubles ("sludge") from the liquid fraction. The liquids are treated aerobically by the "activated sludge" process while the sludge is treated anaerobically in "sludge digesters" (see details below).
| a. Aerobic secondary treatment - The "activated
sludge" process (Figure 17.53). Waste water is vigorously oxygenated to promote growth of aerobes, especially slime forming bacteria such as Zooglea ramigera. These organisms oxidize much of the organic material, producing carbon dioxide. In addition, they form a layer of slime called "floc" ("activated sludge") that traps other organisms and adsorbs soluble organics. The floc contains fecal protozoa, enterics, pathogens, viruses etc. After the floc is given five to ten hours to settle, it is sent off to a sludge digestor for further digestion (see below). Some of the floc is retained for use as an inoculum to re-populate the tank for processing the next batch of material. The activated sludge process decreases the BOD by 75-90%. |
b. Anaerobic secondary treatment - Sludge
digestion (Figure 17.52). Sludge digestors encourage the growth of anaerobes, including methanogens. Sludge contains a large amount of insoluble organics (20-100 g/L) that are microbially digested. The degradative chemistry is similar to the anaerobic decomposition of carbon and the biochemistry of the rumen. Digestion occurs at 35-37oC, at pH 6.8 for 14-30 days. Products include the gases methane and carbon dioxide which are released into the atmosphere through gas outlets. This methane is often recovered and used to power the pumps and maintain the temperature of the sludge digestor. The undigested sludge solids settle and are incinerated or buried. Sludge digestion decreases the BOD in the remaining liquid fraction by > 90%. |
Usually, after secondary treatment, the wastewater is released into the environment. This water is far from "pure". Materials remaining in the water include: most of the waste phosphates, half of the nitrogenous wastes, many organics, including some pesticides and insecticides that are suspected carcinogens.
3. Tertiary treatment
A distant dream. Due to high cost, this is not widely done. Tertiary treatment methods purify the wastewater further until it is drinkable. The following chemical and physical methods are used for removing:
a. Non-biodegradable organics:
(i) Activated charcoal binds non-specifically to many of the dissolved organics and remaining particulates, removing them from the water.
(ii). Chlorine (Cl2) oxidizes and precipitates organics that can be removed after they settle.
b. Phosphate:
Selective precipitation removes phosphate from solution as insoluble salts. These precipitates, which also carry along most remaining microorganisms, are allowed to settle and are then removed.
(i) Addition of calcium chloride (CaCl2) - leads to the formation of calcium phosphate [Ca3(PO4)2], that comes out of solution.
(ii) Addition of ferric chloride (FeCl3) leads to the formation of ferric phosphate (FePO4), that comes out of solution.
c. Nitrogen:
Treatment with the strong base lye (i.e. NaOH), converts the ammonium ions (NH4+) to ammonia gas (NH3) that bubbles out of solution.
NH4+ + OH- -----> NH3 gas + H2O
d. Microorganisms:
Chlorination kills any remaining microorganisms.
E. Potable (Drinkable) Water
1. Assay of water "purity" - the coliform test (1905)
Coliform bacteria include rod-shaped , gram-negative, lactose-fermenting species that do not form spores. Many enterics fit these criteria, including E. coli and Salmonella sp. Coliforms are used to indicate if the water contains fecal contamination. Although not necessarily pathogens, coliforms are useful as indicators of true pathogens present in feces that we might not have an assay for.
Indicator organisms are widely used in microbiology; criteria for a good indicator organism are that it must:
a. Coliform test procedure:
A standard volume (100 mL) of water is passed through a filter that retains bacteria. The filter is transferred to a plate containing media selective for coliforms and incubated overnight. Colonies grow on the media, each arising from a single coliform contaminant present in the original water sample (Figure 15.51).
b. EPA guidelines:
(i) Water samples must be assayed > 20 times/month.
(ii) The water is bad and the public must be alerted if > 4 colonies/100 mL are found in >5% of the samples.
2. Preparation of potable water involves several steps:
a. Settling - Water from rivers or underground wells is allowed to settle, to separate and subsequently remove large insolubles (sand, silt, etc.).
b. Coagulation - Aluminum and ferric ions are added and the pH is raised. This results in the precipitation of aluminum hydroxide [Al(OH)3] and ferric hydroxide [Fe(OH)3]. As these substances precipitate from the solution, they carry along microorganisms and adsorbed organics. After settling, the supernatant is collected.
c. Filtration - Next, the water is filtered through sand. This further removes organisms and organics by adsorption. This is an especially important step for removing protozoa that have a cyst phase, e.g. giardia lamblia, because these cysts are often resistant to chlorine.
d. Chlorination - Treatment with chlorine (Cl2) kills off any remaining microorganisms.
"Pure" drinking water prepared in this way is free of microorganisms. Some organics are probably still present. The water most likely came from a body of water (e.g. a river) where wastewater that only underwent secondary treatment was dumped.