212. Organic Life in Sewage.—Living organisms, both plants and animals, exist in sewage. Bacteria are the smallest of these organisms. Others, which can be studied easily under the microscope or can be seen with difficulty by the naked eye but which do not require special cultures for their study, are classed as microscopic organisms or plankton. Organisms which are large enough to be studied without the aid of a microscope or special cultures are classed as macroscopic. The part taken in the biolysis of sewage by macroscopic organisms belonging to the animal kingdom, such as birds, fish, insects, rodents, etc., which feed upon substances in the sewage is so inconsequential as to be of no importance. Both plants and animals are found among the macroscopic organisms.
Organisms in sewage may be either harmful, harmless, or beneficial. From the viewpoint of mankind the harmful organisms are the pathogenic bacteria. Their condition of life in sewage is not normal and in general their existence therein is of short duration. It may be of sufficient length, however, to permit the transmission of disease. The diseases which can be transmitted by sewage are only those that are contracted through the alimentary canal, such as typhoid fever, dysentery, cholera, etc. Diseases are not commonly contracted by contact of sewage with the skin nor by breathing the air of sewers. It is safe to work in and around sewage so long as the sewage is kept out of the mouth, and asphyxiating or toxic gases are avoided.
The beneficial organisms in sewage are those on which dependence is placed for the success of certain methods of treatment. These organisms have not all been isolated or identified.
The total number of bacteria in a sample of sewage has little or no significance. In a normal sewage the number may be between 2,000,000 and 20,000,000 per c.c. and because of the extreme rapidity of multiplication of bacteria a sample showing a count of 1,000,000 per c.c. on the first analysis may show 4 to 5 times as many 3 or 4 hours later. A bacterial analysis of sewage is ordinarily of little or no value, since pathogenic organisms are practically certain to be present, there is no interest in the harmless organisms, and the helpful nitrifying and aërobic bacteria will not grow on ordinary laboratory media. Occasionally the presence of certain bacteria may indicate the presence of certain trades wastes. In general, the total bacterial count, as sometimes reported, represents only the number of bacteria which have grown under the conditions provided. It bears no relation to the total number of bacteria in the sample.
The presence of bacteria in sewage is of great importance however, as practically all methods of treatment depend on bacterial action, and all sewages which do not contain deleterious trades wastes, contain or will support the necessary bacteria for their successful treatment, if properly developed.
213. Decomposition of Sewage.—If a glass container be filled with sewage and allowed to stand, open to the air, a black sediment will appear after a short time, a greasy scum may rise to the surface, and offensive odors will be given off. This condition will persist for several weeks, after which the liquid will become clear and odorless. The sewage has been decomposed and is now in a stable condition. The decomposition of sewage is brought about by bacterial action the exact nature of which is uncertain.
It[[129]] is well established that many of the chemical effects wrought by bacteria, as by other living cells, are due, not to the direct action of the protoplasm, but to the intervention of soluble ferments or enzymes.
Enzymes are soluble ferments produced by the growth of the bacterial cell.
In[[130]] many cases the enzymes diffuse out from the cell and exert their effort on the ambient substances ... in others the enzyme action occurs within the cell and the products pass out, (for example) ... the alcohol-producing enzymes of the yeast cell act upon sugar within the cell, the resulting alcohol and carbon dioxide being ejected.
Other chemical effects may be brought about by the direct action of the living cells, but this has never been well established.