The normal constituents of domestic wastewater are shown in Table 2.3. The parameters shown in the table are the ones normally used to characterize organic wastes found in municipal wastewaters. As indicated, untreated domestic wastewater is categorized as weak, medium, and strong.


In addition to the physical and chemical characterization of water and wastewater, it is important that the microbiological constituents be also addressed. The constituent microbiological characterizations to be discussed in this section include the following: bacteria, protozoa, and viruses. In addition, qualitative and quantitative tests for the coliform bacteria will also be addressed. The treatment then proceeds to viruses and protozoa. The treatment on protozoa will include discussion on Giardia lamblia, Cryptosporidium parvum, and Entamoeba histolytica.

The basic units of classifying living things are as follows: kingdom, phylum, class, order, family, genus, and species. Organisms that reproduce only their own kinds constitute a species. The genera are closely related species. Several genera constitute a family. Several related families form an order and several related orders make a class. A number of classes having common characteristics constitutes a phylum. Lastly, related phyla form the kingdom. Ordinarily, only two kingdoms exist: plant and animal; however, some organisms cannot be unequivocally classified as either a plant or an animal. Haeckel in 1866 proposed a third kingdom that he called protist to include protozoa, fungi, algae, and bacteria. Protists do not have cell specialization to perform specific cell functions as in the higher forms of life.

At present, it is known that the protists bacteria and cyanobacteria are different from other protists in terms of the presence or absence of a true nucleus in the cell. This leads to the further division of the protists.

The protists fungi, algae, and protozoa contain a membrane-enclosed organelle inside the cell called a nucleus. This nucleus contains the genetic material of the cell, the DNA (deoxyribonucleic acid) which is arranged into a readily recognizable structure called chromosomes. On the other hand, the DNA of bacteria is not arranged in an easily recognizable structure as the chromosomes are. The former organisms are called the eucaryotes; the latter, the procaryotes. Therefore, two types of protists exist: the eucaryotic protists and the procaryotic protists. The eucaryotes are said to have a true nucleus, while the procaryotes do not.

A third type of structure that does not belong to the previous classifications is the virus. Although viruses are not strictly organisms, microorganisms, in general, may be classified as eucaryotic protists, procaryotic protists, and viruses.

2.3.1 Bacteria

Bacteria are unicell procaryotic protists that are the only living things incapable of directly using particulate food. They obtain nourishment by transporting soluble food directly from the surrounding environment into the cell. They are below protozoa in the trophic level and can serve as food for the protozoa. Unlike protozoa and other higher forms of life that actually engulf or swallow food particles, bacteria can obtain food only by transporting soluble food from the outside through the cell membrane. The nutrients must be in dissolved form; if not, the organism excretes exoenzymes that solubilize the otherwise particulate food. Because of the solubility requirement, bacteria only dwell where there is moisture. Figure 2.5 shows a sketch of the bacterial cell.

Bacteria are widely distributed in nature. They are found in the water we drink, in the food we eat, in the air we breathe; in fact, they are found inside our bodies themselves (Escherichia coli). Bacteria are plentiful in the upper layers of the soil, in our rivers and lakes, in the sea, in your fingernails—they are everywhere.

Bacteria are both harmful and beneficial. They degrade the waste-products produced by society. They are used in wastewater treatment plants—thus, they are beneficial. On the other hand, they can also be pathogenic. The bacteria, Salmonella typhosa, causes typhoid fever; Shigella flexneri causes bacillary dysentery. Clostridium tetani excretes toxins producing tetanus. Clostridium botulinum excretes the toxin causing botulism. Corynebacterium diphtheriae is the agent for diphtheria.

Cell wall preven*"

Cytoplasmic membrane regulates transport of food into and waste production out of cell.

Cytoplasm containing RNA controls anabolism, manufactures and recycles enzymes, stores food

Slime layer of organic polymers varies in thickness with age of the cell and other environmental conditions; stores food and binds food and other bacteria into food.

Cytoplasmic membrane regulates transport of food into and waste production out of cell.

Cytoplasm containing RNA controls anabolism, manufactures and recycles enzymes, stores food destruction by outside^ shear forces; may be ~ 10-50% of cell weight.

Slime layer of organic polymers varies in thickness with age of the cell and other environmental conditions; stores food and binds food and other bacteria into food.

FIGURE 2.5 Structure of a bacterial cell.

Bacteria come in three shapes: spherical (coccus), rod-shaped (bacillus), and spiral-shaped (vibrio, spirillum, and spirochete). A vibrio is a spiral organism shaped like a coma. A spirillum is also a spiral organism whose long axis remains rigid when in motion; the spirochete is also yet another spiral organism whose long axis bends when in motion.

The cocci range in size from 0.5 fim to 2 fim in diameter. The smallest bacillus is about 0.5 fim in length and 0.2 fim in diameter. In the opposite extreme, bacilli may reach to a diameter of 4 f m and a length of 20 f m. The average diameter and length of pathogenic bacilli are 0.5 f m and 2 f m, respectively. The spirilla are narrow organisms varying in length from 1 fim to 14 fim.

The shape of the bacteria is maintained by a rigid cell wall. The cytoplasm of the cell has a high osmotic pressure that, without a rigid cell wall, can easily rupture by the diffusion of outside water into the cell. The rigidity of the cell wall is due to its chemical makeup of which the chief component is mucocomplex, a polymer of certain amino sugars and short peptide linkages of amino acids. Depending upon the type of bacteria, the bacteria may also contain techoic acid and mucopolysac-charide, or lipoprotein and liposaccharide.

Directly beneath the cell wall is a membrane called the cytoplasmic membrane that surrounds the cytoplasm. In the eucaryotes, an organelle called mitochondrion, and, in the photosynthetic eucaryotes, an organelle called chloroplast are the sites for the electron-tranport and the respiratory enzyme systems. The bacteria do not have the mitochondrion nor the chloroplast, but the functions of these organelles are embedded within the sites in the cytoplasmic membrane. The cytoplasm is the living material which the cell is composed of, minus the nucleus.

Many bacilli and all spirilla are motile when suspended at the proper temperature in a suitable medium. The organ of locomotion is the flagella. The bacterium may have one flagella, few, or many arranged in a tuft. The flagella may protrude at one end or both ends of the organism. True motility is seldom observed in the cocci.

The bacteria may be motile in one medium and nonmotile in another; also, it may be motile at one temperature but nonmotile at another. Salmonella typhosa, a bacillus that causes typhoid fever, moves at a rate of about 2,000 times its length in one hour.

Under unfavorable conditions, some species of bacteria, mostly bacillus such as the Bacillus and Clostridium, assume a form within the cytoplasm that is resistant to environmental influences adverse to bacterial existence. This form is called spores. The cocci and spirilla rarely exhibit this behavior—this form being confined only to about 150 species of the bacilli. The most important pathogenic, spore-forming bacteria are those causing tetanus, gas gangrene, botulism, and anthrax.

Only two members of the procaryotic protists exist: the regular bacteria and the blue-green algae. Although the blue-greens possess chlorophyl, their characteristics are those of bacteria possessing no true nucleus. Blue-greens are called cyanobacteria and are true bacteria, the procaryotic protists.

According to body surface reaction, bacteria are classified into Gram-positive and Gram-negative bacteria. This method of classification was introduced by Hans Christian Gram, a Danish physician working in Berlin. The reaction of the bacterial surface to staining by crystal violet or gentian violet treated with iodine characterizes this method. For the Gram-positive bacteria, the stain cannot be removed by flooding with alcohol, acetone, or aniline; for the Gram-negative bacteria, on the other hand, the stain is removed by these solvents.

Bacteria reproduce by binary fission. The cell elongates and a constriction is formed; genetic materials are pushed through the constriction and the genetic blueprint is transcribed creating a new daughter cell. The bacterium divides at the constriction. It requires from 15 to 30 minutes for the newborn cells to attain adulthood and restart the cycle of the binary division.

Starting with one cell, let N be the number of cells resulting from the binary fission of one cell at any time. Then

where n is the number of generations.

Application of the above equation will show that one bacterium will reproduce to

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