208. Physical Characteristics.—Sewage is the spent water supply of a community containing the wastes from domestic, industrial, or commercial use, and such surface and ground water as may enter the sewer.^[118] Sewages are classed as: domestic sewage, industrial waste, storm water, surface water, street wash, and ground water. Domestic sewage is the liquid discharged from residences or institutions and contains water closet, laundry, and kitchen wastes. It is sometimes called sanitary sewage. Industrial sewage is the liquid waste resulting from processes employed in industrial establishments. Storm water is that part of the rainfall which runs over the surface of the ground during a storm and for such a short period following a storm as the flow exceeds the normal and ordinary run-off. Surface water is that part of the rainfall which runs over the surface of the ground some time after a storm. Street wash is the liquid flowing on or from the street surface. Ground water is water standing in or flowing through the ground below its surface.

Ordinary fresh sewage is gray in color, somewhat of the appearance of soapy dish water. It contains particles of suspended matter which are visible to the naked eye. If the sewage is fresh the character of some of the suspended matter can be distinguished as: matches, bits of paper, fecal matter, rags, etc. The amount of suspended matter in sewage is small, so small as to have no practical effect on the specific gravity of the liquid nor to necessitate the modification of hydraulic formulas developed for application to the flow of water. The total suspended matter in a normal strong domestic sewage is about 500 parts per 1,000,000. It is represented graphically in Fig. 149. The quantity of organic or volatile suspended matter is about 200 parts per 1,000,000. It is shown graphically in the smaller cube in Fig. 149.

The odor of fresh sewage is faint and not necessarily unpleasant. It has a slightly pungent odor, somewhat like a damp unventilated cellar. Occasionally the odor of gasoline, or some other predominating waste matter may hide all other odors. Stale sewage is black and gives off nauseating odors of hydrogen sulphide and other gases. If the sewage is so stale as to become septic, bubbles of gas will be seen breaking the surface and a black or gray scum may be present. Before the South Branch of the Chicago River was cleaned up and flushed this scum became so thick in places, particularly in that portion of the Stock Yards where the river became known as Bubbly Creek, that it is said that weeds and small bushes sprouted in it, and chickens and small animals ran across its surface.

A physical analysis of sewage should include an observation of its appearance, and a determination of its temperature, turbidity, color, and odor, both hot and cold. The temperature is useful in indicating certain of the antecedents of the sewage, its effect on certain forms of bacterial life, and its effect on the possible content of dissolved gases. Temperatures higher than normal are indicative of the presence of trades wastes discharged while hot into the sewers. A low temperature may indicate the presence of ground water. If the temperature is much over 40° C. bacterial action will be inhibited and the content of dissolved gases will be reduced. Turbidity, color, and odor determinations may be of value in the control of treatment devices, or to indicate the presence of certain trades wastes, which give typical reactions. Since all normal sewages are high in color and turbidity, the relative amounts of these two constituents in two different sewages has little significance regarding the relative strengths of the two sewages or the proper method of treating them. A fresh domestic sewage should have no highly offensive odor. The presence of certain trades wastes can be detected sometimes in fresh sewages, and a stale sewage may sometimes be recognized by its odor.

Sewage is a liability to the community producing it. Although some substances of value can be obtained from sewage^[119] the cost of the processes usually exceed the value of the substances obtained. Where it becomes necessary to treat sewage the value of these substances may be helpful in defraying the cost of treatment.

209. Chemical Composition.—Sewage is composed of mineral and organic compounds which are either in solution or are suspended in water. In making a standard chemical analysis of sewage only those chemical radicals and elements are determined which are indicative of certain important constituents. Neither a complete qualitative nor quantitative analysis is made. A sewage analysis will not show, therefore, the number of grams of sodium chloride present or any other constituent. A complete standard sanitary chemical analysis will report the constituents as named in the first column of Table 71. The quantities of these materials found in average strong, medium and weak sewages are also shown in this table. These values are not intended as fixed boundaries between sewages of different strengths. They are presented merely as a guide to the interpretation of sewage analyses.

The principal objects of a chemical analysis of sewage are to determine its strength and its state of decomposition. The influents and effluents of a sewage treatment device are analyzed to aid in the control of the device and to gain information concerning the effect of the treatment. Chemical and other analyses, in connection with the desired conditions after disposal, will indicate the extent of treatment which may be required. The standard methods of water and sewage analysis adopted by the American Public Health Association have been generally accepted by sanitarians. These uniform methods make possible comparisons of the results obtained by laboratories working according to these standards.

210. Significance of Chemical Constituents.—Organic nitrogen and free ammonia taken together are an index of the organic matter in the sewage. Organic nitrogen includes all of the nitrogen present with the exception of that in the form of ammonia, nitrites, and nitrates. Free ammonia or ammonia nitrogen is the result of bacterial decomposition of organic matter. A fresh cold sewage should be relatively high in organic nitrogen and low in free ammonia. A stale warm sewage should be relatively high in free ammonia and low in organic nitrogen. The sum of the two should be unchanged in the same sewage.

Nitrites (RNO₂) and nitrates (RNO₃)^[124] are found in fresh sewages only in concentrations of less than one part per million. In well-oxidized effluents from treatment plants the concentration will probably be much higher. Nitrates contain one more atom of oxygen than nitrites. They represent the most stable form of nitrogenous matter in sewage. Nitrites are not stable and are reduced to ammonias or are oxidized to nitrates. Their presence indicates a process of change. They are not found in large quantities in raw sewage because their formation requires oxygen which must be absorbed from some other source than the sewage. In an ordinary sewer or sluggishly flowing open stream this absorption cannot take place from the atmosphere with sufficient rapidity to supply the necessary oxygen.

Oxygen consumed is an index of the amount of carbonaceous matter readily oxidizable by potassium permanganate. It does not indicate the total quantity of any particular constituent, but it is the most useful index of carbonaceous matter. Carbonaceous matter is usually difficult of treatment and a high oxygen consumed is indicative of a sewage difficult to care for. The amount of oxygen consumed, as expressed in the analysis, is dependent on the amount of oxidizable carbonaceous matter present, the oxidizing agent used, and the time and temperature of contact of the sewage and the oxidizing agent. It is essential therefore that the test be conducted according to some standard method, since the results are of value only as compared with results obtained under similar conditions.

Total solids (residue on evaporation) are an index of the strength of the sewage. They are made up of organic and inorganic substances. The inorganic substances include sand, clay, and oxides of iron and aluminum, which are usually insoluble, and chlorides, carbonates, sulphates and phosphates, which are usually soluble. The insoluble inorganic substances are undesirable in sewage because of their sediment forming properties which result in the clogging of sewers, treatment plants, pumps, and stream beds. The soluble inorganic substances are generally harmless and cause no nuisance, except that the presence of sulphur may permit the formation of hydrogen sulphide, which has a highly offensive odor. The organic substances are: carbohydrates, fats, and soaps, which are carbonaceous and are difficult of removal by biological processes; and the nitrogenous substances such as urea, proteins, amines, and amino acids. The inorganic and organic substances may be either in solution or suspension or in a colloidal condition.

Volatile solids are used as an index of the organic matter present, as it is assumed that the organic matter is more easily volatilized than the inorganic matter. The amount of volatile inorganic matter present is usually so small as to be negligible.

Fixed solids are reported as the difference between the total and volatile solids. They are therefore representative of the amount of inorganic matter present.

Suspended matter is the undissolved portion of the total solids. High volatile suspended matter is an indication of offensive qualities in the nature of putrefying organic matter, whereas fixed suspended matter is indicative of inoffensive inorganic matter. It is difficult to obtain a sample of sewage which will represent the amount of suspended matter in the sewage, since a sample taken from near the surface will contain less inorganic matter and grit than a sample taken near the bottom.

Settling solids are indicative of the sludge forming properties of the sewage and of the probable degree of success of treatment by plain sedimentation. Volatile settling solids indicate the property of the formation of offensive putrefying sludge banks. There is no chemical test which will indicate the scum-forming properties of sewage. Fixed settling solids indicate the presence of inorganic matter, probably gritty material such as sand, clay, iron oxide, etc.

Colloidal matter is material which is too finely divided to be removed by filtration or sedimentation, yet is not held in solution. It can sometimes be removed by violent agitation in the presence of a flocculent precipitate, as in the treatment with activated sludge, or by the flocculent precipitate alone, as in chemical precipitation, or by the acidulation of the sewage so as to precipitate the colloids. Colloidal matter is probably the result of the constant abrasion of finely divided suspended matter while flowing through the sewer or other channel. High colloidal matter may therefore indicate a stale sewage, or the presence of a particular trades waste. Colloids are difficult of removal. For this reason, where sewage is to be treated, turbulence in the tributary channels should be avoided.

Alkalinity may indicate the possibility of success of the biologic treatment of sewage, since bacterial life flourishes better in a slightly alkaline than in a slightly acid sewage. Within the normal limits of the amount of alkalinity in sewage the exact amount has little significance in sewage analyses. Sewages are normally slightly alkaline. An abnormal alkalinity or acidity may indicate the presence of certain trades wastes necessitating special methods of treatment. A method of sewage treatment may be successful without changing the amount of alkalinity in the sewage since the amount of alkalinity is not inherently an objection.

Chlorine, in the form of sodium chloride, is an inorganic substance found in the urine of man and animals. The amount of chlorine above the normal chlorine content of pure waters in the district is used as an index of the strength of the sewage. The chlorine content may be affected by certain trades wastes such as ice-cream factories, meat-salting plants, etc., which will increase the amount of chlorine materially. Since chlorine is an inorganic substance which is in solution it is not affected by biological processes nor sedimentation. Its diminution in a treatment plant or in a flowing stream is indicative of dilution and the reduction of chlorine will be approximately proportional to the amount of dilution.

Fats have a recoverable market value when present in sufficient quantity to be skimmed off the surface of the sewage. Ordinarily fats are an undesirable constituent of sewage as they precipitate on and clog the interstices in filtering material, they form objectionable scum in tanks and streams, and they are acted on very slowly by biological processes of sewage treatment. Although fats are carbonaceous matter they are not indicated by the oxygen consumed test because they are not easily oxidized. They are therefore determined in another manner; by evaporation of the liquid and extracting the fats from the residue by dissolving them in ether.

Relative stability and bio-chemical oxygen demand are the most important tests indicating the putrefying characteristics of sewage. Since stability and putrescibility have opposite meanings the relative stability test is sometimes called the putrescibility test. The relative stability of a sewage is an expression for the amount of oxygen present in terms of the amount required for complete stability.

A relative stability of 75 signifies that the sample examined contains a supply of available oxygen equal to 75 per cent of the amount of oxygen which it requires in order to become perfectly stable. The available oxygen is approximately equivalent to the dissolved oxygen plus the available oxygen of nitrate and nitrite.^[125]

The relative stability numbers, given in Table 72, are computed from the expression, S = 100(1 - 0.794t) in which S is the stability number and t is the time in days that the sample has been incubated at 20° C. The bio-chemical oxygen demand is more directly an index of the consumption of available oxygen by the biological and chemical changes which take place in the decomposition of sewage or polluted water. As such it is a more valuable, though less easily performed test than the test of relative stability.

The methods for the determination of the relative stability and the bio-chemical oxygen demand are given to show more clearly what these tests represent. The procedure in the relative stability test is to add 0.4 c.c. of a standard solution of methylene blue to 150 c.c. of the sample. The mixture is then allowed to stand in a completely filled and tightly stoppered bottle at 20° C. for 20 days or until the blue fades out due to the exhaustion of the available oxygen. There are three methods in use for the determination of the bio-chemical oxygen demand;^[127] the relative stability method, the excess nitrate method, and the excess oxygen method. In the relative stability method the sample to be treated should have a relative stability of at least 50. If it is lower than this the sample should be diluted with water containing oxygen until the relative stability has been raised to or above this point. The oxygen demand in parts per million is then expressed as

in which O' is the oxygen demand, O is the initial oxygen in parts per million (p.p.m.) in the diluting water or sewage, P is the proportion of sewage in the mixture expressed as a ratio, and R is the relative stability of the mixture expressed as a decimal. For the effluents from sewage treatment plants, polluted waters, and similar liquids, the total available oxygen expressed as the sum of the dissolved oxygen, nitrites, and nitrates, divided by the relative stability expressed as a decimal will give the bio-chemical oxygen demand. The excess nitrate method requires the determination of the total oxygen available as dissolved oxygen, nitrites, and nitrates and the addition of a sufficient amount of oxygen in the form of sodium nitrate to prevent the exhaustion of oxygen during a 10–day period of incubation. At the end of the period the total available oxygen is again determined. The difference between the original and the final oxygen content represents the bio-chemical oxygen demand. The excess oxygen test requires the determination of the total available oxygen as before, and the addition of a sufficient amount of oxygen, in the form of dissolved oxygen in the diluting water, to prevent exhaustion of the oxygen in a 10–day period of incubation. The difference between the original and final oxygen content represents the bio-chemical oxygen demand. Theriault concludes as a result of his tests, that the relative stability and excess nitrate methods are open to objections but that the excess oxygen method yields very accurate and consistent results with as little or less labor than is required by other methods.

Dissolved oxygen represents what its name implies, the amount of oxygen (O₂) which is dissolved in the liquid. Normal sewage contains no dissolved oxygen unless it is unusually fresh. It is well, if possible, to treat a sewage before the original dissolved oxygen has been exhausted. Normal pure surface water contains all of the oxygen which it is capable of dissolving, as shown in Table 73. The presence of a smaller amount of oxygen than is shown in this table indicates the presence of organic matter in the process of oxidation, which may be in such quantities as ultimately to reduce the oxygen content to zero. Normal pure ground waters may be deficient in dissolved oxygen because of the absence of available oxygen for solution. The presence of certain oxygen-producing organisms in polluted or otherwise potable surface waters may cause a supersaturation with oxygen however.

The dissolved-oxygen test for polluted water is probably the most significant of all tests. If dissolved oxygen is found in a polluted water it means that putrefactive odors will not occur, since putrefaction cannot begin in the presence of oxygen. It is possible for different strata in a body of water to have different quantities of dissolved oxygen, and putrefaction may be proceeding in the lower strata before the oxygen is exhausted from the upper strata. The oxygen content of a river water will indicate the ability of the river to receive sewage without resulting in a nuisance.

211. Sewage Bacteria.—A slight knowledge of the nature of bacteria is necessary in order that the biological changes which occur in the treatment of sewage may be understood. Bacteria are living organisms which are so small that it is difficult or impossible to study them either with the eye alone or with the aid of powerful microscopes. They are studied by means of cultures, stains, and certain characteristic phenomena such as the production of a gas, the production of a red colony on litmus lactose agar, etc. Bacteria occur in three forms: spherical, called coccus; cylindrical, called bacillus; and spiral, called spirillum. In size they vary from the largest at about 1
10,000 of an inch to sizes so small as to be invisible under the most powerful microscope. An ordinary size is 1
25,000 of an inch. The cylindrical or rod bacteria are about four times as long as they are wide. Some bacteria possess the power of motion due to the presence of flagella or hairs which can be moved and cause the cell to progress at a rate as high as 18 cm. per hour, but usually the rate is very much less than this. The composition of the bacterial cell has never been definitely determined.

Bacteria are unicellular plants. They possess no digestive organs and apparently obtain their food by absorption from the surrounding media. Reproduction is by the division of the cell into two approximately equal portions. This reproduction may occur as frequently as once every half hour and if unchecked would quickly mount to unimaginable numbers. The natural cause limiting the growth of bacteria is the generation by the bacterium of certain substances such as the amino acids, which are injurious to cell life. The exhaustion of the food supply is not considered as an important cause of inhibition of multiplication. The products of growth of one species of bacteria may be helpful or harmful to other forms. Where the products are helpful the effect is known as symbiosis, and where harmful the effect is known as antibiosis. In sewage the presence of both aËrobic and anaËrobic bacteria is usually mutually helpful and the condition is an example of symbiosis. The aËrobes, sometimes called obligatory aËrobes, are bacteria which demand available oxygen for their growth. The anaËrobes, or obligatory anaËrobes, can grow only in the absence of oxygen. There are other forms that are known as facultative anaËrobes (or aËrobes) whose growth is independent of the presence or absence of oxygen.

Spores are formed by some bacteria when they are subjected to an unfavorable environment such as high temperatures, the absence of food, the absence of moisture, etc. Spores are cells in which growth and animation are suspended but the life of the cell is carried on through the unsuitable period, somewhat similar to the condition in a plant seed.

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.

Metabolism is the life process of living cells by which they absorb their food and convert it into energy and other products. It is the metabolism of bacterial growth that in itself or by the production of enzymes hastens the putrefactive or oxidizing stages of the organic cycles in sewage treatment. Bacteria can assimilate only liquid food since they have no digestive tract through which solid food can enter. The surrounding solids are dissolved by the action of the enzymes, the resulting solution diffusing through the chromatin or outer skin, and being digested throughout the interior cytoplasm.

Bacteria are sometimes classified as parasites and saprophytes. The parasites live only on the growing cells of other plant or animal life. The saprophytes obtain their food only from the life products of living organisms and do not exist at the expense of the organisms themselves. Facultative saprophytes (or parasites) may exist on either living or dead tissue.

The decomposition of sewage may be divided into anaËrobic and aËrobic stages. These conditions are usually, but not always, distinctly separate. The growth of certain forms of bacteria is concurrent, while the growth of other forms is dependent on the results of the life processes of other bacteria in the early stages of decomposition.

When sewage is very fresh it contains some oxygen. This oxygen is quickly exhausted so that the first important step in the decomposition of sewage is carried on under anaËrobic conditions. This is accompanied by the creation of foul odors of organic substances, ammonia, hydrogen sulphide, etc.; other odorless gases such as carbon dioxide, hydrogen, and marsh gas, the latter being inflammable and explosive; and other complicated compounds. An exception to the rule that putrefaction takes place only in the absence of oxygen is the production of other foul-smelling substances by the putrefactive activity of obligatory and facultative aËrobes. Hydrogen sulphide may be produced apparently in the presence of oxygen the action which takes place not being thoroughly understood.

The biolysis of sewage is the term applied to the changes through which its organic constituents pass due to the metabolism of bacterial life. Organic matter is composed almost exclusively of the four elements: carbon, oxygen, hydrogen, and nitrogen (COHN) and sometimes in addition sulphur and phosphorus. The organic constituents of sewage can be divided into the proteins, carbohydrates, and fats. The proteins are principally constituents of animal tissue, but they are also found in the seeds of plants. The principal distinguishing characteristic of the proteins is the possession of between 15 and 16 per cent of nitrogen. To this group belong the albumens and casein. The carbohydrates are organic compounds in which the ratio of hydrogen to oxygen is the same as in water, and the number of carbon atoms is 6 or a multiple of 6. To this group belong the sugars, starches and celluloses. The fats are salts formed, together with water, by the combination of the fatty acids with the tri-acid base glycerol. The more common fats are stearin, palmatin, olein, and butyrine. The soaps are mineral salts of the fatty acids formed by replacing the weak base glycerol with some of the stronger alkalies.

The first state in the biolysis of sewage is marked by the rapid disappearance of the available oxygen present in the water mixed with organic matter to form sewage. In this state the urea, ammonia, and other products of digestive or putrefactive decomposition are partially oxidized and in this oxidation the available oxygen present is rapidly consumed, the conditions in the sewage becoming anaËrobic. The second state is putrefaction in which the action is under anaËrobic conditions. The proteins are broken down to form urea, ammonia, the foul-smelling mercaptans, hydrogen sulphide, etc., and fatty and aromatic acids. The carbohydrates are broken down into their original fatty acid, water, carbon dioxide, hydrogen, methane, and other substances. Cellulose is also broken down but much more slowly. The fats and soaps are affected somewhat similarly to the hydrocarbons and are broken down to form the original acids of their make up together with carbon dioxide, hydrogen, methane, etc. The bacterial action on fats and soaps is much slower than on the proteins, and the active biological agents in the biolysis of the hydrocarbons, fats, and soaps are not so closely confined to anaËrobes as in the biolysis of the proteins. The third state in the biolysis of sewage is the oxidation or nitrification of the products of decomposition resulting from the putrefactive state. The products of decomposition are converted to nitrites and nitrates, which are in a stable condition and are available for plant food. It must be understood that the various states may be coexistent but that the conditions of the different states predominate approximately in the order stated. In the biolysis of sewage there is no destruction of matter. The same elements exist in the same amount as at the start of the biolytic action.

214. The Nitrogen Cycle.—Nitrogen is an element that is found in all organic compounds. Its presence is necessary to all plant and animal life. The nitrogenous compounds are most readily attacked by bacterial action in sewage treatment. The non-nitrogenous substances such as soaps and fats, and the inorganic compounds are more slowly affected by bacterial action alone. The element nitrogen passes through a course of events from life to death and back to life again that is known as the Nitrogen Cycle. It is typical of the cycles through which all of the organic elements pass.

Upon the death of a plant or animal, decomposition sets in accompanied by the formation of urea which is broken down into ammonia. This is known as the putrefactive stage of the Nitrogen Cycle. The next state is nitrification in which the compounds of ammonia are oxidized to nitrites and nitrates, and are thus prepared for plant food. In the state of plant life the nitrites and nitrates are denitrified so as to be available as a plant or animal food. The highest state of the Nitrogen Cycle is animal life, in which nitrogen is a part of the living animal substance or is charged from protein to urea, ammonia, etc., by the functions of life in the animal. Upon the death of this animal organism the cycle is repeated. The Nitrogen Cycle, like the cycle of Life and Death, is purely an ideal condition as in nature there are many short circuits and back currents which prevent the continuous progression of the cycle. The conception of this cycle is an aid, however, in understanding the processes of sewage treatment.

215. Plankton and Macroscopic Organisms.—In general the part played by these organisms in the biolysis of sewage is not sufficiently well understood to aid in the selection of methods of sewage treatment involving their activities. The presence in bodies of water receiving sewage, of certain plankton which are known to exist only when putrefaction is not imminent, indicates that the body of water into which the discharge of sewage is occurring is not being overtaxed. The control of sewage treatment plant effluents so as to avoid the poisoning of fish life or the contamination of shell fish is likewise important. The study of plankton and macroscopic life in the treatment of sewage is an open field for research.

216. Variations in the Quality of Sewage.—The quality of sewage varies with the hour of the day and the season of the year. Some of the causes of these variations are: changes in the amount of diluting water due to the inflow of storm water or flushing of the streets or sewers; variations in domestic activities such as the suspension of contributions of organic wastes during the night, Monday’s wash, etc.; characteristics of different industries which discharge different kinds of wastes according to the stage of the manufacturing process, etc. In general night sewage is markedly weaker than day sewage in both domestic and industrial wastes, but in specific cases the varying strength depends entirely upon the characteristics of the district. Some analyses are given in Table 74, which emphasize these points.

References:

1.

1908 Report of the Ohio State Board of Health.

2.

Report on Sewage Purification at Columbus, Ohio, by G. A. Johnson, 1905.

3.

Report on Industrial Wastes from the Stock Yards and Packingtown in Chicago, by the Sanitary District of Chicago. 1921.

4.

Report of the Baltimore Sewerage Commission, 1911.

217. Sewage Disposal.—Previous to the development of the water-carriage method for removing human excreta and other liquid wastes the solid matter was disposed of by burial and the liquid wastes were allowed to seep into the ground or to run away over its surface. Following the development of the water-carriage system, which necessitated the development of sewers, the problem of ultimate disposal was rendered more serious by the concentration of human excreta together with a large volume of water. The unthinking citizen believes the problem of sewage disposal is solved when the toilet is flushed or the bath tub is drained. The problem may more truly be said to commence at this point.

It would appear that the simplest method of disposal of sewage would be to discharge it into the nearest water course. Unfortunately the nature of sewage is such that it may be either highly offensive to the senses or dangerous to health or both, when discharged in this manner. Only the most fortunate communities are favored with a body of water of sufficient size to receive sewage without creating a nuisance.

The problems of sewage disposal are to prevent nuisances causing offense to sight and smell; to prevent the clogging of channels; to protect pumping machinery; to protect public water supplies; to protect fish life; to prevent the contamination of shell fish; to recover valuable constituents of the sewage; to enrich and to irrigate the soil; to safeguard bathing and boating; for other minor purposes; and in some cases to comply with the law. Sewage may be treated to attain one or more of these objects by methods of treatment varying as widely as the objects to be attained.

218. Methods of Sewage Treatment.—In studying the subject of sewage treatment it must be borne in mind that it is impossible to destroy any of the elements present. They may be removed from the mixture only by gasification, straining or sedimentation. Their chemical combinations may be so changed, however, as to result in different substances than those introduced to the treatment plant. It is with these chemical changes that the student of sewage treatment is interested.

The methods of sewage treatment can be classified as mechanical, chemical and biological. These classifications are not separated by rigid lines but may overlap in certain treatment devices or methods. Mechanical methods of treatment are exemplified by sedimentation, and screening. Chemical precipitation and sterilization are examples of chemical methods. The biological methods, the most important of all, include dilution, septicization, filtration, sewage farming, activated sludge, etc. If for any reason it is desired to treat sewage by more than one of these methods the procedure should follow as nearly as possible the order of the occurrence of the phenomena in the natural biolysis of sewage. For example, in one treatment plant the sewage would first pass through a grit chamber where the coarse sediment would be removed, then through a screen where the floating matter and coarse suspended matter would be removed, then to a sedimentation basin where some finer suspended matter might settle out, then to a digestive tank where the solid matter deposited would be worked upon by bacterial action and partially liquefied. Simultaneous to the liquefaction of the deposited solid matter the liquid effluent from the digestive tank might proceed to an aËrating device to expedite oxidation, then to an aËrobic filter, and finally to disposal by dilution.