Composition of Sewage.—When a house is supplied with running water, the ordinary method of disposing of most household wastes is to transport them by flowing water to a disposal plant. The mixture of household wastes and water is called sewage. The amount of sewage that is produced in a house is nearly equal to the quantity of water that is used in the house.
Sewage consists of the waste water from kitchens, laundries, and bathrooms, and of human excretions mixed with a large quantity of water. Each 1000 parts of sewage contains only about 1 or 2 parts of solid matter, of which about one-half is suspended in the water and one-half is dissolved. Somewhat more than one-half of the solids of sewage is decomposable organic matter, and the remainder consists of substances (such as minerals) which will not decompose. There is not much difference in the average composition of samples of sewage from various sources, whether they are taken from a large public sewer or from the discharge pipe of a small house.
Sewage contains 1,000,000 or 2,000,000 bacteria in each cubic centimeter. About 10 per cent. of the bacteria are colon bacilli from the human intestine. Some may be disease germs that were discharged from the bodies of human beings. Most of the bacteria are the ordinary ones of decay and putrefaction.
Dangers from Sewage.—Sewage is dangerous to health principally on account of the disease germs which it contains. The principal danger is that the sewage may convey the disease germs into a well or stream or ether source of water-supply. The danger is not proportional to the number of germs that enter the water. A few in a glass of drinking-water are almost as dangerous as large numbers. The great volume of water with which human excretions are mixed in sewage makes it probable that disease germs will be carried into a water-supply from an elaborate sewer system more readily than they will from the undiluted excretions of a primitive disposal place. The conveniences of plumbing and of a sewage system are accompanied by an increased responsibility for care in the final disposal of the sewage.
Sewage flowing into a body of water is a menace to the health of those who bathe in the water. There is a possibility that oysters taken from sewage-laden water may contain disease germs. The pollution of salt waters at seaside resorts may affect the health of visitors from inland towns, and of people of communities in which oysters are received from polluted waters. The sewage disposal system of a community may have an influence on the health of people living far away from the community.
Another danger from sewage is that house-flies may transmit disease germs from it to food or to the mouths and eyes of persons.
An argument for the proper disposal of sewage is that it is offensive to sight and smell, and is a nuisance that is not to be tolerated in respectable communities. Cesspools overflowing on the ground are among the principal nuisances with which a rural health officer has to deal.
Disposal Systems.—The problem in sewage disposal is to remove the decomposable organic substances and the bacteria from the water, and to destroy them so that they will not be offensive to the senses or dangerous to health. An unsuccessful disposal method that has been thoroughly tested is that of holding the sewage in a tank and treating it with chemicals, such as copperas or alum, which will coagulate the solids so that they will either float on the surface or settle to the bottom. The clarified liquid may then be drawn off. This system is expensive, uncertain, and impractical, and a health officer will seldom need to give it consideration.
Another system is the distribution of the sewage over the surface of farm lands. This, too, has been thoroughly tried, and has nearly always proved unsuccessful. The sewage has very little fertilizing value; a large area is required for its disposal; the cost of labor makes the system expensive; and the presence of disease germs renders the crops dangerous for human food. A health officer will do right if he condemns a system of surface disposal because of danger to health and of expense.
A third system of sewage disposal is to discharge the sewage into a lake, or river, or bay, or other body of water. This is a dangerous method of disposal, especially if the body of water is used as a source of water-supply. The natural purification of the water depends largely on the oxidizing action of the oxygen that is naturally dissolved in the water. If the proportion of sewage to the water into which it is discharged is as 1 to 200, the quantity of oxygen in the water will be decreased to such a degree that some kinds of fish cannot live in the water. If the proportion of sewage to water is as 1 to 50, the quantity of oxygen will be decreased to such a degree that putrefaction may take place. But disease germs may remain alive in the water even though the water does not become offensive to the senses. One of the great problems with which departments of health have to deal is the prevention of pollution of bodies of water with sewage.
Nearly every system of sewage disposal with which a health officer has to deal depends for its action on bacterial decomposition, with a further purification by oxidation or by filtration through the soil. If the purification is complete, the organic substances in the sewage will either be removed or completely oxidized to carbon dioxid, water, and nitrates, sulphates, phosphates, and other minerals which are naturally found in ground water; and the water which flows away will be free from bacteria. It is possible to purify sewage to such a degree that it is fit for use as drinking-water.
The devices which are commonly used for purifying sewage are: 1, a collecting tank; 2, subsurface irrigation pipes; 3, a contact bed; 4, a sprinkling filter; 5, a sand filter; and 6, a chlorinating apparatus. These devices are often used in various combinations, as, for example, a collecting tank, a sprinkling filter, and a chlorinating apparatus.
Settling Tank.—Every efficient system of sewage disposal makes use of a collecting tank in which the raw sewage is received. A collecting tank is not filled and then emptied before it receives more sewage, but the sewage flows through it continuously. It is made of sufficient size to hold at least the quantity of sewage that is produced during twelve or twenty-four hours. The sewage therefore remains in the tank for from twelve to twenty-four hours, and during that time it undergoes two processes: 1, sedimentation, and 2, putrefaction. While sewage is flowing slowly through a collecting tank, the heavy particles of solid matter sink to the bottom, and the lighter ones float on the surface. For this reason a collecting tank is often called a settling tank or sedimentation basin. An efficient tank will usually remove somewhat less than one-half of the solid matters that are suspended in the sewage.
An active bacterial action also takes place in a collecting tank. The action is one of putrefaction, and for this reason the receptacle is often called a septic tank. The result of the putrefaction is to liquefy some of the solid bits of matter that float in the sewage, and to decompose some of the substances that are dissolved in it. The action is rapid during the first few hours that a particular mass of sewage remains in the tank, but after that period of time the action is slow. There is no advantage in retaining the liquid part of sewage in a septic tank for a longer time than a day. A septic tank will remove about half of the decomposable substances which are contained in sewage, and the effluent will undergo further offensive decomposition unless it is subjected to a greater degree of purification.
Cesspool.—The cesspool is the type of sewage disposal plant in which a health officer is especially interested, for it usually constitutes the entire disposal system of houses which are not connected with a public sewer system. A cesspool is an underground septic tank from which the liquid slowly escapes through the soil. The actions which take place in it are sedimentation and putrefaction. The liquid which escapes is slowly filtered through the soil, and its organic matter is slowly oxidized by the oxygen which penetrates the soil. The effluent finally reaches the underground water. The degree to which it is purified will depend largely on its quantity, the character of the soil, and the depth at which the ground water is reached. Fine sand makes an efficient filter. Coarse gravel, or a fissure in rock, allows the liquid to pass through almost unchanged. If the quantity is considerable, the purifying capacity of the soil may be exceeded. If the ground water lies near the surface of the soil, the purifying action will be slow, and the effluent will not soak away from the cesspool.
Safely of a Cesspool.—The safety and efficiency of a cesspool will depend principally on: 1, the capacity of the soil to absorb the effluent; 2, its nearness to a well or spring or other source of water-supply; and, 3, the care with which it, is maintained. If houses are near together, the soil may he saturated with sewage to such a degree that all the ground water is polluted. Under these conditions cesspools may still be safe if all the wells are closed and only a public water-supply is used. If the soil has not sufficient capacity to absorb the effluent, the installation of subsurface irrigation pipes will often solve the problem of the final disposal of the liquids.
Construction of a Cesspool.—A cesspool is usually constructed with circular sides and arched top, and is built of brick or stone laid without mortar. No bottom is laid in it, and perforations are sometimes left in its sides to allow the liquid to soak away readily. A standard size for a cesspool of an ordinary house is 7 feet in diameter and the same in depth.
It is necessary to provide a cesspool with a cover which fits tightly enough to exclude flies and mosquitoes. Uncovered cesspools breed a large proportion of the mosquitoes which annoy the people of villages. It is not necessary that the cover should be perforated for the escape of gases.
It is economic to construct two connected cesspools. The outlet pipe of the first cesspool is provided with an elbow which extends downward about 2 feet in order to draw oil only the portion of the liquid which is comparatively clear. The bottom of the first cesspool soon becomes clogged with sediment, but the bottom of the second cesspool should remain porous for years. The expense of frequently cleaning a single cesspool will soon exceed the cost of a second one.
Maintenance of a Cesspool.—A cesspool that is acting properly will act continuously for months and years. About a foot of sediment will collect on the bottom, and a foot of scum will float on the surface. This quantity of sediment and scum will not increase, for the processes of putrefaction will go on continuously, and will slowly reduce the solid matters to gases and liquids. The perfect action of a cesspool requires that the bacterial action shall be as active as possible. Some persons put chlorid of lime or other antiseptics into their cesspools, expecting to lessen the offensiveness of the sewage. About the only effect of the antiseptic is the undesirable one of restraining the liquefying action of the bacteria, thus increasing the quantity of sediment and clogging the cesspool until it overflows. Antiseptics in a cesspool do more harm than good.
When the bottom of a cesspool becomes clogged, the proper remedy is to pump out the liquid contents and remove a few inches of the earth until clean soil is reached, and replace it with clean sand. If the bottom soil is simply turned over, the mud which clogs the soil remains, and the condition of the cesspool is soon as bad as ever.
Large quantities of gases, such as marsh gas, are produced during the process of putrefaction. The mixture of these gases with air is explosive, and serious accidents have resulted from lowering a lantern into the cesspools too soon after they were opened.
Subsurface Irrigation.—When the capacity of the soil to dispose of liquids is limited from any cause, such as the nearness of the ground water to the surface or the close texture of the subsoil, a system of subsurface irrigation is often used as an accessory to a cesspool. It is also well adapted as the main feature of the disposal plant of a large country house or a small institution. The system consists of small pipes of agricultural tiling laid with open joints in rows 3 to 6 feet apart a foot or two beneath the surface of the soil. The pipes receive the effluent from a cesspool or septic tank, and distribute it into the upper layers of the soil where the oxidizing and nitrifying bacteria are especially abundant and active. The pipes may be laid in a front yard with benefit to the lawn. An acre of subsurface irrigation tiling will take care of from 15,000 to 25,000 gallons of sewage effluent daily. The heat of the sewage will prevent the soil around the pipes from freezing even when the ground elsewhere is frozen.
Construction of a Subsurface Irrigation Bed.—The tiling of a subsurface irrigation bed is laid with a slope of about 4 inches in 100 feet in order that the in-flowing sewage will not rush to the outer end of the system, but will distribute itself uniformly through all the tiling. The joints are wrapped with a thin layer of excelsior, and their upper surfaces are covered with tarred paper in order to exclude sand and yet allow space for the escape of the liquid.
The raw sewage from a house is received into a dosing tank whose capacity is equal to that of the subsurface pipes. An automatic device discharges the contents of the filled tank suddenly in order that the whole pipe system may be flooded. While the tank is refilling, the liquid in the pipes soaks away, oxygen penetrates the ground, and the soil becomes prepared for a new dose of sewage. The service of a sanitary engineer is usually required in constructing a subsurface irrigation system.
When irrigation tiling is laid in order to increase the capacity of a cesspool located in a low area, it is usually impossible to construct a dosing tank. The effluent then drains into the tiling constantly, and there is a probability that a sediment will collect in the joints and finally clog them. One remedy for this condition is to build the cesspool above ground, sufficiently elevated to allow the construction of a dosing tank.
Elements in a Disposal Plant.—It is not economic to dispose of large quantities of sewage effluent by direct absorption into the soil. The plan that is usually adopted by a large institution, or a village, or a city is to subject the effluent to purification to a degree that it may be safely discharged into a stream or other body of water. Four processes of purification that are commonly employed are: 1, sedimentation in a septic, or settling, tank; 2, oxidation by aerobic bacteria; 3, filtration; and 4, chlorination. A septic or settling tank is nearly always used in every system for the preliminary treatment of the sewage.
Imhoff Tank.—When a large quantity of sewage is treated in a septic tank, there is a considerable accumulation of sediment called sludge. An Imhoff tank is a special form of receiving basin that is designed to liquefy the maximum amount of sludge, and to require the removal of the sludge with the least frequency. It consists of a deep concrete tank which is divided into an upper and a lower compartment. The bottom of the upper compartment opens into the lower by a narrow slit. The raw sewage is received into the upper compartment, and as it slowly flows along, its sediment falls into the lower compartment and there undergoes putrefactive decomposition. The action which takes place in the upper compartment is principally a sedimentation of the coarser solids with but little action by the anaerobic bacteria on the finer solids or the dissolved substances. The greater part of the purification of the liquid is accomplished later by aerobic bacteria to whose action the effluent is subjected either in a contact bed, or a sprinkling filter, or a sand filter.
Contact Bed.—A contact bed consists of a large tank about 3 feet deep filled with pieces of stone about the size of hen’s eggs. The effluent from the sedimentation tanks is oxidized by aerobic bacteria which cling to the stones. After three or four hours the liquid is drawn off and the bed is allowed to lie empty for a few hours in order that a new supply of oxygen may penetrate the beds.
A contact bed will remove about two-thirds of the organic matter and bacteria that is in the liquid. One acre of beds will treat about 500,000 gallons of sewage daily or about the quantity that is produced by 5000 people.
Sprinkling Filter.—A broken-stone bed in which the sewage is sprinkled upon the surface is called a sprinkling filler. Oxidation in it takes place rapidly owing to the intimate contact of the liquid with the air, and to the thin sheets in which it trickles clown between the stones. An efficient sprinkling filter will act about three times as rapidly as a contact bed, and will remove over 80 per cent. of the solids and bacteria from the sewage.
Sand Filter.—A sand filter consists of a bed of sand, usually about one-quarter of an acre in area and 4 feet deep, surrounded by an embankment, and underdrained. The effluent from the collecting tank undergoes oxidation and filtration in it, and if the bed is working properly over 90 per cent. of the solids and bacteria are removed. The effluent is discharged upon a sand filter in a sudden gush only once or twice a day. The liquid soaks away quickly, and the nitrifying bacteria then act on the filtrate for some hours. If the bed is continuously soaked with sewage it will have little or no purifying action, owing to the impossibility of oxygen penetrating it. A sand filter is usually constructed in several beds, and the effluent is discharged upon each in rotation. An acre of sand filter will dispose of about 100,000 gallons of sewage daily, or about the quantity that is produced by 1000 persons. A sand filter is economic in sandy soils where land is cheap.
After a sand filter has been in use for some weeks, a scum of grease and fine solids forms on the surface and clogs the sand. It is necessary that each bed should be thrown out of use frequently and dried in order that the scum and an inch or two of the surface sand may be removed.
Late in the fall the surface sand of a filter must be deeply furrowed and ridged in order to prevent freezing. A layer of ice forms on the surface and is supported by the ridges. The ice protects the sewage and soil beneath it from freezing. A sand filter that is properly ridged will remain in good condition throughout a severe winter.
Chlorination.—The final process in the complete purification of sewage is the sterilization of the effluent with chlorin in the form of chlorid of lime or of liquid chlorin. The chlorin acts by combining with the hydrogen of the impurities, thus forming hydrochloric acid and liberating oxygen in an active form. The oxygen immediately combines with the organic matter and bacteria and oxidizes them. The quantity of chlorin that is needed will depend on the amount of impurities that are in the liquid. From 1 to 10 parts of chlorin in each 1,000,000 parts of sewage are usually required.
Tests of Efficiency.—A health officer can perform two tests to determine the efficiency of a sewage disposal plant. The first test is the determination of the stability of the effluent, or its liability to undergo further decomposition and putrefaction. If the coarse particles are removed from sewage and the total amount of organic matter is reduced to a quarter of its original amount, the effluent will usually be stable. The test may be performed by taking a jar of the purified sewage and setting it aside for several days in a room of ordinary temperature. If no turbidity or offensive odor develops, the sewage is stable, and will not undergo decomposition when it is discharged into a body of water or upon a filter-bed.
The effluent from a collecting tank or from a contact bed is seldom stable. That from a sprinkling filter or a sand filter is usually stable if the system is properly operated.
A second test of the efficiency of a sewage disposal system is the determination of the number of colon bacilli in the treated sewage. Intestinal bacteria are rather long lived in sewage, but they do not multiply in number. A disposal plant will reduce the number of colon bacilli in about the same degree that it reduces the quantity of organic matter. The reduction in the number of colon bacilli per cubic centimeter as the sewage passes through the disposal plant is therefore a reliable indication of the efficiency of the purification process, and of the destruction of disease germs which may be in the sewage. A health officer can have the test done by sending samples of the sewage to a laboratory in sterile 2-ounce bottles. A health officer must take a series of samples, one from the stream of sewage before it enters the collecting tank, one from the stream of effluent that flows from the tank, one from the effluent from the contact bed, or sprinkling filter, or sand filter, and one from the effluent after final sterilization. These samples will show a progressive diminution in the number of bacilli, and none will be found in the last sample if the system is 100 per cent. efficient.
Choice of a System of Disposal.—The system of disposal that is needed for a particular locality will depend largely on what disposition is finally made of the effluent. If the effluent is discharged into a large body of water, an Imhoff tank with chlorin treatment of the effluent will constitute an efficient system. If the effluent from a disposal plant is discharged into a small stream which is used as a source of water-supply, it may be necessary that the disposal plant shall consist of a collecting tank, a sprinkling filter, a sand filter, and a chlorinating apparatus. The disposal plant must be adapted to the particular locality which it serves.
Sewer System.—A sewer system consists of collecting pipes or sewers, and a disposal plant. A health officer has little to do with the sewers directly, except when they are broken or obstructed. The construction and maintenance of sewers are engineering problems with which an engineering department has to deal.
A health officer has very little power over a private sewer system, except to require that the disposal shall be done in a sanitary manner. He cannot require a householder or the managers of an institution to install any particular type of sewer system. But a health officer who understands sewer systems will often be asked to give advice regarding their installation and management. If a health officer condemns a sewer system at a private residence, or an institution, he should be able to advise the owners or managers what system to install in its place, where to locate the disposal plant, and how to manage the system.
Manufacturers and contractors often exploit patented systems which are merely complicated adaptations of well-known principles. A householder can usually get a practical plumber to design a simple, inexpensive system which is adapted to the soil of the locality. It is a good plan for a health officer to consult the plumbers regarding the costs of standard sewer pipes, cesspools, and disposal plants in his locality. A health officer can also promote public health by advising the plumbers regarding the standard methods of sewage disposal which are efficient and economic in his locality.
Public Sewer System.—When a city, village, or congested district needs a public sewer system, the health officer is the person from whom the people and officials naturally seek advice. The manner of establishing a sewer district is prescribed by statute law in some of the states. For example, the New York State law relating to sewers in towns is found in the town law, Art. 11, Secs. 230-248; and the law relating to village sewers is found in the village law, Art. 11, Secs. 260-278, and in the public health law, Art. 3, Sec. 21a; and the relation of sewage to stream pollution and potable waters is found in the public health law, Art. 5, Secs. 73-87. It is the duty of a health officer to know the laws relating to sewers and sewage in his district, and to advise the people and officials concerning them.
The question sometimes arises of the advisability of the construction and operation of a public sewer system by a private person or corporation who would charge for its use. It is not wise to make the use of a sewer dependent on money payments. If the water-supply of a house is cut off, the occupants can obtain a temporary supply from a neighbor without undue hardship or danger; but if a house is cut off from sewer connections, the resulting pollution of the house and soil may be dangerous to health.
Designing and constructing a public sewer system are problems of engineering and business which are under the jurisdiction of the business department of a municipal government. It is the duty of a health officer to give advice regarding the sanitary problems to be solved. But the decision regarding the solution of the problems does not lie with him or with the board of health, hut with the engineering and business departments.
Operation of a Disposal Plant.—The operation of a disposal plant is under the control of the business departments of a municipal government, but it is the duty of a health officer to make inspections and take bacteriologic samples of the sewage in order to have records of the efficiency of the plant. The health officer of a rural community is the official who is best qualified to take samples and to secure their examination.