The disposal of sewage, in a convenient and sanitary manner is a problem of serious importance in the equipment of isolated dwellings with modern household conveniences. The manner of heating, lighting and of water supply are questions of selection among a number of established systems, but the problem of sewage disposal must in a great measure be determined by local conditions. Unless the natural surroundings are such as will permit sewage to be emptied directly into a stream of considerable volume, the problem of its safe disposal becomes one of serious importance.

Sewage is understood to mean the fluid waste from the kitchen, toilet and laundry and has nothing whatever to do with garbage. Sewage disposal has to do with conducting away the house waste and disposing of it in a sanitary manner. Sewage disposal does not necessarily have anything to do with sewage purification; although a sewage disposal plant may be so constructed as to discharge a purified effluent, it usually is understood to have to do alone with its disposal in a manner that does not offend the aesthetic sense. A simple sewage plant is anything that will take the sewage away from the house in such a way as to produce no unsightly accumulations that will decay and produce offensive odors.

A sewage purification plant is one in which the raw sewage from the house drain is first liquefied, after which the liquid is passed into a filter where it undergoes a process of bacterial disintegration and the organic matter reduced to the inorganic state, where no further change is possible. The water which flows from such a filter is clear and sparkling, and is often taken for spring water. The degree of purification given to the sewage will depend on the style of filter and the length of time necessary for the water to pass through it.

Sewage is composed of organic matter in a fluid or part fluid condition, contained in a large volume of water. It is not usually the dark, heavy, foul-smelling fluid that is imagined by many, but a turbid liquid possessing only a few of the qualities usually ascribed to sewage. Under favorable conditions practically all of the organic matter will be readily dissolved and the sewage will become entirely liquid.

As a liquid, the raw sewage is in the most favorable condition for rapid decay and if left standing in the air it soon develops properties that render it highly objectionable.

The decay of all organic matter is a process of disintegration that ultimately ends in the elements from which it came. In the disposal of sewage, the aim is to permit this disintegration to take place under conditions that will be least offensive to the aesthetic sensibilities, and in some cases to render it free from harmful properties should there be present the bacteria of communicable disease.

The successful disposal of sewage from cities is accomplished under a great variety of conditions. It is much easier to arrange for sewage purification on a large scale than in a small way. The reason for this is that in the care of a city the sewage-disposal plant is under the supervision of a competent person, whose business it is to see that the conditions are kept at the highest efficiency. Private plants are left almost entirely without care, until they fail from causes that are usually preventable. Sewage may be successfully purified under a great many conditions, but no type of plant has as yet proven itself successful that does not receive intelligent attention.

The most successful of small sewage disposal plant is the septic tank system alone or in connection with an adequate form of bacterial filter. Cesspools are not to be countenanced by people of intelligence. The cesspool has been so universally condemned by authorities on sanitation, that all intelligent people look upon it as a thing filthy beyond description. Although the septic tank is little more than an improved cesspool, the condition under which it acts is entirely different from that which takes place in the latter and with care and watchfulness, it may be made to work to a degree of perfection that is surprising. The one great cause of the failure of small sewage-disposal plants is the lack of proper care.

The process of sewage purification as now practised in the most successful plants is largely mechanical, but bacterial action plays a part of great importance in the completion of the process. It consists of two stages: the tank treatment, in which the sewage is liquefied; and the process of filtration where the liquefied sewage—commonly called the effluent—from the septic tank undergoes a process of filtration and bacterial purification.

The Septic Tank.

—The septic tank alone, as used for sewage disposal, is often termed a sewage purifying plant, when in reality it is only intended to change the sewage into a form in which it can be readily carried away. The word septic means putrifying, and when applied to sewage disposal it furnishes a convenient term but has nothing to do with purification. The septic tank furnishes only the first stage of the purifying process, and although its effluent may be clear and possess little odor, it is nevertheless unpurified. The septic tank discharges an effluent of more or less completely digested sewage, instead, as in the cesspool, of permitting it to remain a constantly festering mass, to be slowly absorbed by the earth.

The sewage is first collected in a tank of sufficient size to contain the discharge from the house for 24 hours. In the process of digestion which the sewage undergoes while in the tank, it is rendered almost entirely liquid; at the same time it is acted upon by the bacteria that are developed, and that tend to reduce the sewage to its elemental form. The effluent liquid which passes out of the tank is almost colorless and possesses relatively little odor.

The tendency of the change which takes place in the tank is to nitrify the organic matter but under ordinary conditions the action is not fully complete. The effluent sometimes undergoes but little change except to be reduced to a liquid. If the effluent is now allowed to flow into a ditch where it will stand in pools, further putrification will take place with its resulting annoyance. In case the septic tank is to be used alone, the effluent should be conducted to a stream for final disposal. A septic tank must be built to accommodate a certain number of people and of sufficient size to take care of the entering sewage. The action which goes on in the tank will render the contents almost entirely fluid, and under good conditions the sewage will be completely digested. When working properly, a thick scum will form on the surface, through which filters the gases that are liberated in the process of disintegration. The formation of the scum is an indication that the filter is doing its best work. Should the tank be required to take care of more sewage than it can conveniently handle, the scum will not form and the effluent will be turbid because of the undigested matter.

The change that takes place in the sewage while it remains in the tank is first that of being liquefied and then disintegrated by bacterial action. That such a change does take place is evidenced by the residue that is found in the tank in the process of cleaning. This is a black granular substance, composed mostly of humus and commonly known as sludge. The amount of accumulated sludge is relatively small, and the operation of cleaning is not necessary more than twice a year and is not the disagreeable task one might suppose.

The Septic Tank With a Sand-bed Filter.

—In places where the use of the septic tank alone is not possible, it sometimes happens that the natural conditions are such as will permit the effluent to be drained directly into the soil. With such a condition, the effluent goes into a filter bed composed of gravel or other loose material, where it undergoes still further bacterial action and if the process is complete, the water which comes from the filter bed is clear and odorless. Under good conditions it is clear sparkling water and contains but a small amount of impurities.

Septic tanks are made in many forms but that illustrated in Figs. 151 and 152 is commonly used. In Fig. 151 the tank is shown in position to receive the sewage from the house drain, where it is to undergo the first treatment and then to be conducted to a filter bed made of porous tile, set in loose soil. The tank is shown in detail in Fig. 152. It is a cemented brick cistern with an opening to the surface that contains a double cover as a protection during cold weather. A brick partition divides the tank into spaces G and H, that contain volumes that are to each other as 1 to 2. The tank is of such size as will hold a volume of sewage equal to 24 hours’ use; that is, it is expected that any portion of sewage will remain in the tank for that length of time. The sewage enters at A, in such a way as will give the least disturbance of the liquid of the tank. An opening C allows the liquid to pass from H into G, where any additional sewage entering H will displace an equal amount in G, which will pass out at B to the filter bed. The partition D is high enough so that the scum that forms on the surface will not pass directly into the space G. The entrance and exit pipes are made of vitrified sewer tile with the openings below the surface.

As the sewage enters the tank A, a considerable portion will sink to the bottom, while some will float to the top where a thick scum will gather. By far the greatest portion of solids will be readily dissolved in the water and the remainder will be still further reduced to liquid form by bacterial solution. The process of disintegration that goes on evolves a considerable amount of carbon dioxide and ammonia which filters through the scum. The process that now goes on in the tank is that of liquefying the organic matter and changing it from organic to the inorganic state.

The bacteriologist recognizes in the process of sewage disintegration the work of two classes of bacteria, the aerobic or those bacteria that work by reason of air and do their work only in its presence and the anaerobic or those that work in the absence of air. In the action of the sewage-disposal plant both kinds of bacteria are at work. If, in the final stage where the sewage passes into the filter, air can be carried into the earth the action will be hastened.

It is evident that, since the sewage entering the tank is almost entirely dissolved, under ideal action this system would give very little trouble, but actually as the sewage enters the tank the disturbance caused by the incoming water forces some of the undigested matter into the outlet and being carried into the filter bed it will be deposited at the first opportunity. This will cause the filter bed to fill up with undigested sewage at the point nearest the entrance, and in course of time it will stop the pipe because of this accumulation.

To avoid such an occurrence, tanks have been built in which an automatic siphon discharges the effluent whenever a certain quantity has collected. Such a tank is shown in Fig. 153. With this arrangement, the sewage enters the first tank at A, and passes into the second tank at B. At S is shown an automatic siphon, so made that when the effluent has collected to the height of the water line, the siphon automatically discharges the contents of the tank. This is known as a dosage tank because periodically a dose of the effluent is discharged into the filter bed. The volume discharged is sufficient to fill the greater portion of the bed, and force out the air in the loose soil. As the water filters from the bed the air is drawn in to take its place and gives the bacteria which work in the presence of air an opportunity to do their work. The work done by this filter bed is first to filter out any suspended matter carried in the effluent which will lodge on the surface of the filter material and then to undergo the slow process of integration, and to permit the oxidation of the dissolved sewage. If this matter is deposited faster than it disintegrates then the filter will fill up and finally refuse to work.

The Septic Tank and Anaerobic Filter.

—In places where the use of the simple septic tank is not possible and where the character of the soil will not permit of a natural sand-bed filter, an anaerobic filter may be constructed through which to pass the effluent from the septic tank.

The anaerobic filter is one in which anaerobic bacterial action is given opportunity to reduce the organic matter in the sewage to its elemental condition. The filter may be constructed in any form that will permit the process of filtration to be carried out in a way that will afford good anaerobic action. The extent to which the purification is to be carried will determine the form and size of the filter.

In Fig. 154 is shown such a plant, where a combined septic tank and anaerobic filter discharges its effluent into a filter ditch in which the purifying process is continued through a bed of gravel of any desired length. The figure illustrates a plant that was designed for a country residence. The septic tank and anaerobic filter are located relatively as shown in the drawing, the filter ditch following the course of a roadway. The water is finally discharged into a little stream, where it mingles with the water from a spring, and flows through a meadow.

The septic tank in Fig. 154 is quite similar in construction to the others described except that a section of sewer tile takes the place of the brick wall between the two parts of the tank. The opening O, through which the effluent is discharged, is located a little above the center of the tank.

The anaerobic filter is a tank, rectangular in cross-section, made with brick walls and cemented on the inside. The effluent from the septic tank enters the anaerobic filter in a chamber, that is separated from the main tank by a wooden grating against which rests the filter material. As indicated in the drawing, the bottom is filled with coarse material; stones or broken tiles about 4 inches in diameter. Above this is a layer of material about 2 inches in diameter and above that another layer of 1-inch material; the top is made of gravel. This forms the anaerobic filter, in which takes place the bacterial action away from the presence of air. The interspaces in the filter material allows the effluent from the septic tank to seep through and deposit the particles of matter held in suspension. The arrangement is such as is best suited to the anaerobic action. Here, shut away from the light and air, the organic matter in the effluent undergoes disintegration just as would happen in the earth.

It is evident that some of the matter that should remain in the septic tank and be removed as sludge will be carried into the anaerobic filter. This will, of course, form an insoluble deposit that will accumulate and in the course of time the filter will become clogged. It should be expected that such a filter will ultimately need renewing, for this reason the top is made of a slab of reinforced concrete that may be raised to allow the removal and refilling of the filter material.

The automatic siphon discharges the water from the chamber S, whenever it fills. The discharged water from the siphon is conducted into a drain tile, placed in a ditch filled with gravel or other loose material, which serves as an additional filter and in which the water undergoes a still further purification. This filter ditch is constructed as indicated in longitudinal section. The water from the siphon enters the tile C and seeping through the filling is drained away in the tile shown at D.

The tiles are not set close together, but the joints are left open and covered by pieces of broken tile as shown in H. This is to prevent the filter material from entering the tile and thus stopping the ready flow of the water.

The filter ditch of the plant will be constructed according to the contour of the ground and will follow the natural drainage. The course of the ditch—if it is desired to use one—will accommodate itself to the character of the ground. The final discharge of the water will be determined by the natural drainage.

That a plant of this kind will work perfectly when new is is beyond a doubt but that it will continue indefinitely to give perfect satisfaction is not reasonable to expect. The septic tank will require cleaning, probably once a year. The anaerobic filter will require renewing at intervals, depending on the amount of sewage the filter is required to take care of and the rate at which the plant is worked, probably once in 4 or 5 years. If the septic tank is of insufficient size to readily digest the sewage, the accumulation of sludge in the anaerobic filter will be greater than should occur.

It would be only reasonable to suppose that the siphon will sometimes refuse to discharge. Even though it is an automatic siphon, circumstances may cause it at times to fail to act. For this reason the manhole is placed so that the siphon may be inspected and repaired, should it be necessary. It must not be supposed that once such a plant is in place that all of the work is over. The success of a good sewage-disposal plant of this kind demands eternal vigilance.

In the level areas where the possibilities of natural drainage is not good, it sometimes occurs that plants such as those described are not permissible. To overcome such conditions the plant in Fig. 155 represents an installation where the effluent is carried several hundred feet through a drain tile before it is finally discharged into an outlet. This plant is made up of two separate tanks, the first acting as a septic tank, while the second tank is a settling chamber. The water from the second chamber is pumped by windmill power and discharged into a drain tile at the required height through which it is carried to the place of final deposit.

Limit of Efficiency.

—Much that has been written on the subject conveys the impression that the septic tank alone, used under various conditions, will eliminate disease germs and all offending features of sewage and render it a pure water with a small amount of residue remaining in the tank. That such is not the case is all too evident to many who have constructed plants expecting perfect results and have attained only partial success.

It is not reasonable that a plant giving satisfaction under the usual conditions could accomplish its purpose under stress of work. It is quite evident that the amount of sewage from any source cannot be constant. It is equally evident that the effluent from the plant cannot always be the same; but with reasonable limits of variation, a suitably designed tank ought to take care of the sewage from a house at all times and discharge an effluent that is reasonably clear and without offending odor.

It should be kept in mind that, as commonly used, the chief office of the septic tank is to do away with the things that offend the senses, and not to make an effluent that might serve as drinking water. It must also be kept in mind that if the disease germs enter the plant because of sickness in the house that there is every possibility that the germs will be in the discharged water.

The plant must be located as is directed by the natural surroundings but the drainage must be away from buildings and particularly from wells.

Small sewage plants are reasonably efficient and add immensely to the comfort and healthful conditions of the home. They are not perfect in their action but there is excellent reason to believe that the plant of ideal construction will yet be attained.

In a flat country where drainage is difficult, the form of plant must be modified to suit the prevailing conditions but some form of working plant can always be devised. Small plants do not give so efficient results as those of large size but they do very acceptable work. To do good service they must receive attention but the actual amount of labor they demand is small. Small sewage-disposal plants are not expensive nor difficult to construct, and for the amount of labor and money expended they give returns that cannot be estimated.

In determining the character of plant to be constructed, in any particular place, local conditions will in a great measure decide the type to be used. The degree of purity to which it will be necessary to reduce the effluent will depend on the location of the plant and the means of final disposal. If the effluent can be run into a stream of sufficient volume, the septic tank alone will probably answer the purpose.

The septic tank reduces sewage to a liquid form which has some odor. It may be carried away in an open ditch which has good flow, but if allowed to collect in pools it will undergo further putrescence and be objectionable.

It may be possible to use a small creek for final disposal but one in which the effluent from a septic tank alone would be objectionable. In such a case the use of the septic tank combined with an anaerobic filter would probably give a permissible degree of purity.

With a plant composed of a septic tank and anaerobic filter, sewage is rendered almost free from odor and the effluent will not undergo further putrescence when collected in pools.

In many cases it is desired to purify the effluent still further, either because of lack of means for final disposal or because the effluent would contaminate the water into which it is discharged. In such cases the plant will consist of the septic tank, an anaerobic filter and a filter ditch or sand-bed filter. The effluent from such a plant will be clear sparkling water that might be mistaken for spring water.

The design and construction of sewage-disposal plants has been made a subject of investigation in a number of State engineering experiment stations. In addition, manufacturers of cement have prepared descriptive literature that is sent gratis on application. These bulletins contain detailed information as to the working properties and construction of private plants to suit the various conditions of disposal. The following is taken by permission of the Universal Portland Cement Co. from their bulletin on “Concrete Septic Tanks.”