As stated under the preceding plate, the use of the cesspool is a practice to be followed only as a last resort, when no better method can be employed. At best, however, the cesspool is a crude, filthy affair, although in times past it has served an important purpose. The use of the septic tank is to-day leading to the disuse of cesspools, and it seems to be only a matter of time when the latter will be largely a thing of the past.
One form of septic tank is shown in Fig. A, Plate 49. The house sewage is discharged into the first of the three compartments of the septic tank, this compartment being commonly known as the grit chamber, and in which the most important action of the tank takes place. From the grit chamber the liquid portion of the sewage overflows into the second, or settling chamber, and from this into the third or discharge chamber, from which the effluent may be disposed of in a number of different ways, which will be considered later.
Septic Tank & Automatic Sewage Siphon
All three compartments of the septic tank are necessarily watertight, the leeching process not being employed in connection with the septic tank. The action of the septic tank does not result in separating the solids from the liquids by mechanical means, the action being entirely of a chemical nature. The reduction of sewage by means of the septic tank is by the action of certain bacteria which live and multiply in all fresh sewage. By means of this bacterial action, all forms of organic and vegetable matter are transformed from solids into liquids known as nitrates. Ordinarily this action effects the change from solid to liquid within a few hours. Even substances of such hard nature as bones, leather, etc., may be thus changed in form, although the time required is very much greater than in the case of substances of softer nature.
The septic tank is made generally of sufficient size to hold about a day's accumulation of sewage. The action of the class of bacteria which act upon sewage requires neither light nor air; in fact, both light and air should not be allowed to enter the septic tank. A certain amount of warmth must be maintained in order to provide for the proper action of the bacteria, although no • special arrangement to provide heat is necessary. There is considerable heat present in all house sewage, and the sinking of the tank underground provides an additional amount, as also the action of the bacteria itself. To secure the best results, the sewage which enters the septic tank should be well diluted.
The presence of a supply of air in the septic tank not only stops the action of the bacteria, but allows the contents to putrefy, as in the use of the cesspool. Without the presence of air, obnoxious gases do not form, and therefore, even when opened for a short time, the septic tank does not throw off foul odors and gases in any amount.
In starting a septic tank there is nothing to be done of a special nature, after the plant has been made ready, beyond the admission of sewage to it. For the tank to reach a high state of efficiency, however, requires a sufficient length of time to elapse for the bacteria to breed and form in sufficient numbers. This period varies with conditions that are present, from one to three weeks. On the surface of the sewage standing in the tank, a thick coating or scum of vegetable and animal matter soon forms, in which the bacteria breed and perform their work of disintegration.
Upon the under side of this scum their action is particularly strong, the solids being transformed within the space of a few hours into liquids, which are in the form of ammonia compounds.
The scum on the surface of the sewage varies greatly in thickness, but is sometimes of such an amount and so compact that the weight of a person can be sustained upon it. The bacteria also form upon the sides of the tank, thus attacking the sewage from every direction.
The numbers of these bacteria are so great as to be inconceivable, millions of them being present in a very small volume of the sewage.
In order that the best results may be obtained, the bacteria should be disturbed as little as possible. They adhere to almost any rough substance, but upon glass and similar surfaces they do not seem' to be able to gain a hold. Great care should be taken against breaking or disturbing the scum in any way. Therefore, the inlet, as it enters the grit chamber, should discharge through a bend to a point well below the surface of the sewage, as shown in Fig. A.
Metallic and other substances upon which the bacteria are unable to act, settle to the bottom of the grit chamber, which should be cleaned out occasionally, and for this purpose each chamber of the septic tank should be provided with a 24-in. iron cover, fitting tightly into an iron frame securely embedded in the masonry.
From the grit chamber the liquids collecting in that compartment overflow into the settling chamber. This overflow should be so constructed as to transfer the liquids with the slightest possible disturbance of the contents of the settling chamber. The method of overflow shown in Fig. A is a good one to follow, as it allows the liquid to trickle over as it collects. The process of disintegration is continued in the settling chamber, although to a much less extent than in the grit chamber, for the reason that the sewage has been so far purified in the latter that there is not the substance present in the settling chamber to give life to the countless numbers of bacteria that exist in the settling chamber. In many plants, a third chamber is added, into which the effluent overflows before reaching the discharging chamber, the septic action being less in each successive chamber, owing to the increasing purity of the liquids.
From the last settling chamber the effluent overflows into a discharge chamber usually, although in some cases it is discharged directly from the settling chamber to the final place of disposal.
A great factor in the successful operation of the septic tank is the formation of the scum on the surface of the sewage. This scum not only provides working ground for the bacteria, but aids in preventing the penetration of light and air when the cover is removed, and holds the heat contained in the sewage and prevents the striking through of colder air. This scum sometimes reaches a thickness of over a foot and a half. After the effluent reaches the discharge chamber, or in some cases the last settling chamber, the method of final disposal must be determined, the decision being made with due regard to the existing local conditions. If a running stream or ravine is convenient, the solution is often easily made by discharging the sewage into such a natural disposing medium or ground.
When the effluent reaches the discharge chamber, it has been purified to a great extent, but not entirely, and unless some natural means of disposal, such as a stream, is at hand, it is necessary to make provision for the carrying on of this final purifying process, which is commonly known as filtration.
A method quite commonly employed, consists in discharging the effluent into a specially prepared trench close to the surface of the ground, or with its upper face open.
For ordinary residences, a trench 18 to 20 ft. in length and 3 or 4 ft. in depth should be sufficient, the trench being made of correspondingly larger dimensions when greater amounts of liquid must be cared for. At the bottom of the trench a thick layer of broken stone should be filled in, and above this a layer of gravel. Above the gravel a layer of coarse sand is sometimes used. Into this trench the liquid from the septic tank is discharged, and provision should be made for distributing it as evenly over the filtering bed as possible, in order that no one part of the trench may be called upon to perform a greater amount of work than is its share. If too large an amount of liquid is delivered at one point, it cannot be properly cared for by the filtering material, and is therefore not properly purified.
This form of disposal is sometimes carried further, by collecting the water filtered through the trench into an under drain, and from this pipe discharging it into a second filter. From the second filter the water may be pumped out onto the surface instead of allowing it to leech away into the soil. When pumped from the second filter, the sewage which entered the septic tank, has been transformed into an absolutely pure form. That this is true may be seen from the fact that such pump water has in some instances been used for drinking purposes.
Sometimes the liquid discharged from the discharge tank is deposited over the surface of the ground, where filtration and the purifying action of the sun's rays complete the final purifying operation.
This practice is not generally practicable, however, for various obvious reasons, among which are the lack of sufficient exposed surface of light soil, the proximity of other dwellings, the difficulty of securing an even distribution over the surface, etc.