245. The Process.—Septic action is a biological process caused by the activity of obligatory or facultative anaërobes as the result of which certain organic compounds are reduced from higher to lower conditions of oxidation, some of the solid organic substances are rendered soluble, and a quantity of gas is given off. Among these gases are: methane, hydrogen sulphide, and ammonia. The biologic process in the septic tank represents the downward portion of the cycle of life and death, in which complex organic compounds are reduced to a more simple condition available as food for low forms of plant life. The disposal of sewage by septic action, when introduced, promised the solution of all problems in sewage treatment. Septic action is now better understood, and it is known that some of the early claims were unfounded.
The principal advantage of septic action in sewage treatment is the relatively small amount of sludge which must be cared for compared to that produced by a plain sedimentation tank. The sludge from a septic tank may be 25 to 30 per cent and in some cases 40 per cent less in weight, and 75 to 80 per cent less in volume than the sludge from a plain sedimentation tank. The most important results of septic action and the greatest septic activity occur in the deposited organic matter or sludge. The biologic changes due to septic action which occur in the liquid portion of the tank contents are of little or no importance. The installation of a septic tank, although it may fail to prevent the nuisance calling for abatement, has a remarkable psychological effect in stilling complaints. Among other advantages are the comparative inexpensiveness of the tanks and the small amount of attention and skilled attendance required. The tanks need cleaning once in 6 months to a year. If properly designed no other attention is necessary.
The septic tank has fallen into some disrepute because of the better results obtainable by other methods, the occasional discharge of effluents worse than the influent, the occasional discharge of sludge in the effluent caused by too violent septic boiling, and on account of patent litigation. This last difficulty has been overcome as the Cameron patents expired in 1916. Occasionally the odors given off by the septic process are highly objectionable and are carried for a long distance. These odors can be controlled to a large extent by housing the tanks. Over-septicization must be guarded against as an over-septicized effluent is more difficult of further treatment or of disposal than a comparatively fresh, untreated sewage. An over-septicized or stale sewage is indicated by the presence of large quantities of ammonias, either free or albuminoid, frequently accompanied by hydrogen sulphide and other foul-smelling gases. The oxygen demand in an over-septicized sewage is greater than that in a fresh or more carefully treated sewage.
246. The Septic Tank.—A septic tank is a horizontal, continuous-flow, one-story sedimentation tank through which sewage is allowed to flow slowly to permit suspended matter to settle to the bottom where it is retained until anaërobic decomposition is established, resulting in the changing of some of the suspended organic matter into liquid and gaseous substances, and a consequent reduction in the quantity of sludge to be disposed of.[[151]] It is to be noted that a continuous flow is essential to a septic tank. Small tanks containing stagnant household sewage are called cesspools, although sometimes erroneously spoken of as septic tanks.
Septic and sedimentation tanks differ in their method of operation only in the period of storage and the frequency of cleaning. The period of flow in a septic tank is longer and it is cleaned less frequently. The results obtained by the two processes differ widely. A septic tank can be converted into a sedimentation tank, or vice versa, by changing the method of operation, no constructional features requiring alteration. The purpose of the tank is to store the sludge for such a period of time that partial liquefaction of the sludge may take place, and thus minimize the difficulty of sludge disposal. For this reason the sludge storage capacity of a septic tank is sometimes greater than would be necessary for a plain sedimentation tank.
| TABLE 82 | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Efficiencies and Performance of Septic Tank at Columbus, Ohio | ||||||||||||||
| (Report of Sewage Purification, by G. A. Johnson, Nov. 10, 1905) | ||||||||||||||
| Month, 1904–1905 | Aug. | Sept. | Oct. | Nov. | Dec. | Jan. | Feb. | March | April | May | June | Avg. | ||
| Temperature, degrees F. | ||||||||||||||
| Influent | 69 | 70 | 65 | 60 | 54 | 51 | 48 | 50 | 57 | 61 | 67 | |||
| Effluent | 69 | 68 | 64 | 59 | 52 | 48 | 45 | 49 | 57 | 62 | 68 | |||
| Oxygen consumed, parts per million: | ||||||||||||||
| Influent | 49 | 50 | 52 | 47 | 43 | 51 | 44 | 47 | 53 | 33 | 40 | 47 | ||
| Effluent | 40 | 36 | 40 | 39 | 37 | 35 | 37 | 39 | 50 | 34 | 33 | 38 | ||
| Per cent removal | 18 | 28 | 23 | 15 | 16 | 31 | 16 | 17 | 6 | –3 | 18 | 19 | ||
| Organic nitrogen, parts per million: | ||||||||||||||
| Influent | 6.5 | 8.2 | 9.3 | 8.4 | 8.8 | 8.5 | 6.7 | 6.4 | 7.9 | 6.1 | 6.7 | 7.8 | ||
| Effluent | 7.3 | 5.5 | 6.0 | 7.4 | 8.2 | 7.0 | 5.4 | 5.5 | 5.2 | |||||
| Per cent removal | –12 | 32 | 35 | 12 | 7 | 18 | 19 | 14 | 25 | 30 | 19 | 19 | ||
| Free ammonia, parts per million: | ||||||||||||||
| Influent | 9.7 | 12.2 | 12.4 | 16.3 | 14.7 | 10.8 | 8.3 | 9.9 | 12.3 | 6.9 | 8.3 | 11.7 | ||
| Effluent | 10.5 | 11.5 | 12.4 | 17.2 | 14.3 | 11.1 | 8.9 | 10.7 | 14.9 | 9.0 | 8.7 | 12.1 | ||
| Per cent removal | –8 | 6 | 0 | –6 | 3 | –3 | –7 | –8 | –21 | –23 | –5 | –3 | ||
| Residue on Evaporation, parts per million: | ||||||||||||||
| Total: | ||||||||||||||
| Influent | 990 | 952 | 993 | 961 | 989 | 949 | 890 | 850 | 1067 | 912 | 945 | 946 | ||
| Effluent | 935 | 891 | 893 | 916 | 925 | 886 | 843 | 782 | 895 | 800 | 835 | 873 | ||
| Per cent removal | 6 | 6 | 10 | 5 | 6 | 6 | 5 | 8 | 16 | 12 | 12 | 8 | ||
| Volatile: | ||||||||||||||
| Influent | 231 | 184 | 162 | 175 | 156 | 167 | 156 | 168 | 212 | 122 | 162 | 166 | ||
| Effluent | 206 | 160 | 129 | 148 | 137 | 137 | 134 | 137 | 147 | 103 | 144 | 139 | ||
| Per cent removal | 11 | 13 | 20 | 15 | 12 | 18 | 14 | 18 | 31 | 16 | 11 | 16 | ||
| Mineral: | ||||||||||||||
| Influent | 759 | 768 | 831 | 786 | 833 | 782 | 734 | 682 | 855 | 700 | 783 | 780 | ||
| Effluent | 729 | 731 | 764 | 768 | 788 | 749 | 709 | 645 | 748 | 697 | 691 | 734 | ||
| Per cent removal | 4 | 5 | 8 | 2 | 5 | 4 | 3 | 5 | 11 | 1 | 12 | 6 | ||
| Cubic yards wet sludge per million gallons: | 0.10 | 1.24 | 1.09 | 1.17 | 0.65 | 0.63 | 0.57 | 1.34 | ||||||
| Per cent removal of suspended matter: | ||||||||||||||
| Total | 59 | 54 | 56 | 51 | 42 | 48 | 32 | 47 | 56 | 67 | 53 | 50 | ||
| Volatile | 60 | 41 | 48 | 52 | 44 | 55 | 47 | 47 | 62 | 80 | 15 | 48 | ||
| Fixed | 75 | 65 | 60 | 51 | 40 | 38 | 19 | 48 | 53 | 64 | 67 | 51 | ||
| Gas evolved, cubic feet per day: | 29 | 14 | 41 | 50 | ||||||||||
247. Results of Septic Action.—The results obtained from the septic tanks at the Columbus Sewage Experiment Station are given in Table 82. The effluent is higher than the influent in free ammonia, but the reduction of other constituents, particularly suspended matter, is marked.
Septic action is sensitive to temperature changes, and to certain constituents of the incoming sewage. Cold weather or an acid influent will inhibit septicization. In winter the liquefaction of sludge may practically cease, whereas in summer liquefaction may exceed deposition. The amount of gas generated is a measure of the relative amount of septic action. The rapid generation of gas in warm weather disturbs the settled sludge and may cause a deterioration of the quality of the effluent because of the presence of decomposed sludge. The results in Table 82 show the effect of cold weather on the process. In warm weather the violent ebullition of gas sometimes causes the discharge of sludge in the effluent, resulting in a liquid more difficult of disposal than the incoming sewage. Since septic action is dependent on the presence of certain forms of bacteria, where these are absent there will be no septic action. Sewage generally contains the forms of bacteria necessary for this action but it has occasionally been found necessary to seed new tanks in order to start septic action.
The sludge from septic tanks is usually black, with a slight odor, though in some cases this odor may be highly offensive. The sludge will flow sluggishly. It can be pumped by centrifugal pumps and it will flow through pipes and channels. It has a moisture content of about 90 per cent and a specific gravity of about 1.03. It is dried with difficulty on open-air drying beds, and it is worthless as a fertilizer. The composition of some septic sludges are shown in Table 83.
248. Design of Septic Tanks.—The sedimentation chambers of a septic tank are designed on the same principles as the sedimentation basins described in Art. 240. The velocity of flow should not exceed one foot per minute. The channels should be straight and free from obstructions causing back eddies. The ratio of length to width of channel should be between 2 : 1 to 4 : 1 with a width not exceeding 50 feet, and desirably narrower. The depths used vary between 5 and 10 feet, exclusive of the sludge storage capacity. Hanging baffles should be placed, one before the inlet and the other in front of the outlet, so as to distribute the incoming sewage over the tank, and to prevent scum from passing into the outlet. The baffles should hang about 12 inches below the surface of the sewage. Intermediate baffles are sometimes desirable to prevent the movement of sludge or scum towards the outlet. The placing of baffles must be considered carefully as injudicious baffling may lessen the effectiveness of a tank by so concentrating the currents as to prevent sedimentation or the accumulation of sludge. Baffles should be built of concrete or brick, as wood or metal in contact with septic sewage deteriorates rapidly. In designing the sludge storage chambers it may be assumed that one-half of the organic matter and none of the mineral matter will be liquefied or gasified. The net storage volume allowed is about 2 to 3 cubic yards per million gallons of sewage treated. Variations between 0.1 and 10.0 cubic yards have been recorded, however. If grit is carried in the sewage to be treated, it should be removed by the installation of a grit chamber before the sewage enters the septic tank.