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.
| TABLE 71 | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Chemical Analysis of Sewages | |||||||||||
| (Parts per million) | |||||||||||
| From Report on Industrial Wastes from the Stock Yards and Packingtown, Chicago by the Sanitary District of Chicago in 1921, page 231. | |||||||||||
| Typical Analyses | Boston 1905–7 | Columbus 1904–5 | Waterbury, Conn. 1905–6 | Gloversville, N. Y. 1908–9 | Worcester, Mass. 1908 | Chicago, 39th St. Residential 1909–12 | Chicago, Center Avenue. Industrial. Day Sewage 1913 | ||||
| Strong | Medium | Weak | |||||||||
| Nitrogen as Organic Nitrogen | 35 | 20 | 10 | 9.1 | 9.0 | 14.8 | 23.0 | 7.8 | 79 | ||
| Free Ammonia | 50 | 30 | 15 | 13.9 | 11.0 | 7.8 | 12.0 | 22.2 | 9.1 | 22 | |
| Nitrites | 0.10 | 0.05 | 0.0 | 0.0 | 0.09 | 0.14 | 0.38 | 0.10 | 0.49 | ||
| Nitrates | 0.40 | 0.20 | 0.1 | 0.20 | 0.20 | 1.52 | 0.88 | 0.33 | 3.04 | ||
| Oxygen consumed | 75 | 50 | 30 | 56[[120]] | 51[[121]] | 46[[120]] | 95[[120]] | 117 | 43 | 268 | |
| Oxygen demand | 300 | 200 | 100 | ||||||||
| Chlorine | 175 | 100 | 15 | 2300 | 65 | 48 | 158 | 57 | 40 | 1100 | |
| Suspended matter | 500 | 300 | 150 | 135 | 209 | 165 | 406 | 258 | 144 | 605 | |
| Volatile | 91 | 79 | 115 | 229 | 166 | 90 | 46 | ||||
| Fixed | 44 | 130 | 50 | 177 | 92 | 54 | 144 | ||||
| Alkalinity | 200 | 100 | 50 | 125 | 350 | 41 | 233 | 212 | 291 | ||
| Fats | 40 | 20 | 25 | 26 | 48 | 23[[122]] | 198[[123]] | ||||
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.