SEPTIC TANKS

A tight, underground septic tank with shallow distribution of the effluent in porous soil generally is the safest and least troublesome method of treating sewage upon the farm, while at the same time more or less of the irrigating and manurial value of the sewage may be realized.

The late Professor Kinnicutt used to say that a septic tank is "simply a cesspool, regulated and controlled." The reactions described under the captions "How sewage decomposes" and "Cesspools" take place in septic tanks.

In all sewage tanks, whatever their size and shape, a portion of the solid matter, especially if the sewage contains much grease, floats as scum on the liquid, the heavier solids settle to form sludge, while finely divided solids and matter in a state of emulsion are held in suspension. If the sludge is retained in the bottom of the tank and converted or partly converted into liquids and gases, the tank is called a septic tank and the process is known as septicization. The process is sometimes spoken of as one of digestion or rotting.

History.—Prototypes of the septic tank were known in Europe nearly 50 years ago. Between 1876 and 1393 a number of closed tanks with submerged inlets and outlets embodying the principle of storage of sewage and liquefaction of the solids were built in the United States and Canada. It was later seen that many of the early claims for the septic process were extravagant. In recent years septic tanks have been used mainly in small installations, or, where employed in large installations, the form has been modified to secure digestion of the sludge in a separate compartment, thus in a measure obviating disadvantages that exist where septicization takes place in the presence of the entering fresh sewage.

Purposes.—The purposes of a septic tank are to receive all the farm sewage, as defined onpage 1, hold it in a quiet state for a time, thus causing partial settlement of the solids, and by nature's processes of decomposition insure, as fully as may be, the destruction of the organic matter.

Limitations.—That a septic tank is a complete method of sewage treatment is a widespread but wrong impression. A septic tank does not eliminate odor and does not destroy all organic solids. On the contrary, foul odors developed, and of all the suspended matter inthe sewage about one-third escapes with the effluent, about one-third remains in the tank, and about one-third only is destroyed or reduced to liquids and gases. The effluent is foul and dangerous. It may contain even more bacteria than the raw sewage, since the process involves intensive growths. As to the effects upon the growth and virulence of disease germs little is known definitely. It is not believed that such germs multiply under the conditions prevailing in a septic tank. If disease germs are present, many of their number along with other bacteria may pass through with the flow or may be enmeshed in the settling solids and there survive a long time. Hence the farmer should safeguard wells and springs from the seepage or discharges from a septic tank as carefully as from those of cesspools.

Further treatment of effluents.—The effluent of a septic tank or any other form of sewage tank is foul and dangerous. Whether or not the solids are removed by screening, by short periods of rest, as in plain or modified forms of settling tanks, or by longer quiescence, as in septic tanks, the effluent generally requires further treatment to reduce the number of harmful organisms and the liability of nuisance. This further treatment usually consists of some mode of filtration. In the earliest example of such treatment the sewage was used to irrigate land by either broad flooding or furrow irrigation. By another method the sewage is distributed underground by means of drain tile laid with open joints, as illustrated in Figures27and30.

Artificial sewage filters are composed of coarse sand, screened gravel, broken stone, coke, or other material, and the sewage is applied in numerous ways. Since, filtration is essentially an oxidizing process requiring air, the sewage is applied intermittently in doses.[9]

[9]Artificial filters of various types are well described and illustrated in Public Health Bulletin No. 101, "Studies of Methods for the Treatment and Disposal of Sewage—The Treatment of Sewage from Single Houses and Small Communities." U. S. Public Health Service, December, 1919.

If properly designed and operated, filters of sand, coke, or stone are capable of excellent results. Under the most favorable conditions it is unwise to discharge the effluent of a sewage filter in the near vicinity of a source of water supply. Under farm conditions filters are usually neglected or the sewage is improperly applied, resulting in the clogging and befouling of sand filters and the discharge from stone filters of an effluent which is practically as dangerous and even more offensive than raw sewage. Moreover unless the filters are covered there are likely to be annoying odors, and there is always the possibility of disease germs being carried by flies where sewage is exposed in the vicinity of dwellings. Hence it seems more practical for the farmer, avoiding the expense of earth embankments or masonry sides and bottom for a filter bed, to waste the tank effluent beneath the surface of such area of land as is most suitable and available. This method of applying sewage to the soil or subsoil is often spoken of as subirrigation, but subsoil distribution of sewage is different in principle and practice from subirrigation for the increase of crop yields. Subirrigation is rarely successful unless the land is nearly level, the topsoil porous and underlaid with an impervious stratum to hold the water within reach of plant roots,and unless a relatively large quantity of water is used and the work is skillfully done. On the other hand, the quantity of sewage on farms being small, it may be wasted in hilly ground, which should be as porous, deeply drained, and dry as possible.

Parts of a system.—The four parts of a septic-tank installation with subsurface distribution of the effluent are outlined inFigure 17: (1) The house sewer from house to tank; (2) the sewage tank consisting of one or more chambers; (3) the sewer from tank to distribution field; (4) the distribution field, where the sewage is distributed and wasted, sometimes called the absorption field. These parts will be discussed in the order named, although the last should have the first consideration.

Fig. 17.—Parts of a septic-tank installation

House sewer.—The length will vary with the slope of the ground and position of buildings, well, and distribution field. Fifty to 100 feet is a fair length; a greater is still more sanitary. Wherever possible the house sewer should be laid straight in line and grade.Figure 18shows how this work may be done. Suppose the distance from A to E be 100 feet; that grade boards be set 25 feet apart crosswise of the trench at A, B, C, D, and E; that the ground at A be 4 feet lower than at E; that the top of the sewer be 2½ feet below the surface of the ground at A and 4½ feet below the surface of the ground at E; the fall of the sewer between A and E is 2 feet (4 + 2½ - 4½ = 2). If the fall in 100 feet be 2 feet, in 25 feet it is one-fourth as much, or 6 inches. Hence, grade board B is 6 inches higher than grade board A, C is 6 inches higher than B, and so on to E. The top edges when all the boards are set with a carpenter's level and fastened in position should be in line. The grade thus established may be any convenient height above the top of the proposed sewer, and the measuring stick used to grade the pipe is cut accordingly. This height is usually a certain number of whole feet. Fixing the line of the sewer is a mere matter of setting nails in the top edges of boards A and E directly over the center of the proposed sewer and tightly stretching a fish line or grade cord; nails should be set where the cord crosses boards B, C, and D.

Fig. 18.—Setting line and grade for house sewer. To the observer at A the top edges of the grade boards appear as one; the half-driven nails are set to line

If the cellar or basement contains plumbing fixtures, the house sewer should enter 1 to 2 feet below the cellar floor. If all plumbing fixtures are on the floors above, the sewer may enter at no greater depth than necessary to insure protection from frost outside the cellar wall. Digging the trench and laying the pipe should begin at the tank or lower end. The large end of the pipes, called the hub, should face uphill, and the barrel of each pipe should have even bearing throughout its length. Sufficient earth should be removed from beneath the hubs to permit the joints to be made in a workmanlike manner.

The house sewer may be vitrified salt-glazed sewer pipe, concrete pipe, or cast-iron soil pipe. The latter, with poured and calked lead joints makes a permanently water-tight and root-proof sewer, which always should be used where the vicinity of a well must be passed; 4, 5, or 6-inch pipe may be used, depending mainly on the fall and in less degree on the quantity of sewage discharge. As a measure of economy the 4-inch size is favored for iron pipe. If vitrified pipe is used, either the 5 or 6-inch size is preferable, as these sizes are made straighter than the 4-inch size and are less liable to obstruction. Of the two the 5-inch size is preferable. The fall in 100 feet should never be less than 2 feet for 4-inch size, 1½ feet for 5-inch size, 1 foot for 6-inch size.

Figure 19shows methods of making good joints.A,B,C,D,E,F, andGare ordinary sewer pipe joints;H, cast-iron soil pipe.

Fig. 19.—How to make good joints. See text for directions and specifications

Ashows the use of a yarning iron to pack a small strand of jute into the joint space, thus centering the pipes and preventing the joint filler running inside. The joint surfaces should be free of dirt and oil. The jute is cut in lengths to go around the pipe; a small strand is soaked in neat Portland cement grout, then twisted and wrapped around the small end of the pipe to be pushed into the hub of the last pipe laid. After the pipe is pushed home the jute is packed evenly to a depth of not over ½ inch, leaving about 1½ inches for the joint filler. Old hemp rope or oakum dipped in liquid cement or paper may be used, in place of jute, and the packing may be done with a thin file or piece of wood.

Bshows the use of a rubber mitten or glove to force Portland cement mortar into the joint space. The mortar should be thoroughly and freshly mixed in the proportion of one volume of cement to one volume of clean sand and should be pressed and tamped to fill the joint completely.

Cshows a section of finished joint. The fresh mortar should not be loosened or disturbed when laying the next pipe.

Dshows method of pouring a joint with grout, which is quicker, cheaper, and better than using a rubber mitten. A flexible sheet-metal form or mold, oiled to prevent the grout sticking, is clamped tightly around the joint and is completelyfilled with grout consisting of equal parts of Portland cement and clean sand mixed dry, to which water is added to produce a creamy consistency. The pipes should not be disturbed and the form should not be removed for 24 hours.

Eshows a section of grouted joint, well rounded out, strong, and tight.

Fshows the use of a pipe jointer for pouring a hot filler. The pipe jointer may be an asbestos or rubber runner or collar or a piece of garden hose clamped around the pipe leaving a small triangular opening at the top. The jointer is pressed firmly against the hub, and any small openings between the jointer and pipe are smeared with plastic clay to prevent leakage of the filler. A clay dike or funnel about 3 inches high built around the triangular opening greatly aids rapid and complete filling of the joint space. The filler may be a commercially prepared bituminous compound or molten sulphur and fine sand. The former makes a slightly elastic joint; the latter a hard unyielding joint. With good workmanship both kinds of joint are practically water-tight and root-proof, and cost about the same as cement mortar joints. The filler is heated in an iron kettle over a wood, coke, or coal fire. It should be well stirred, and when at a free running consistency should be poured with a ladle large enough to fill the joint completely at one operation. As soon as the compound cools the jointer is removed. Sulphur-sand filler is made by mixing together dry and melting equal volumes of ordinary powdered sulphur and very fine clean sand, preferably the finest quicksand. A 5-inch sewer pipe joint requires from three-tenths to nine-tenths of a pound (according to the kind of pipe) of sulphur, worth 3 to 5 cents per pound, and a like quantity of sand. From ½ to 1½ pounds of bituminous filler are required for a 5-inch pipe joint.

Gshows section of finished joint.

Hshows the use of a pouring ladle in making lead joints in cast-iron soil pipe. This pipe is in lengths to lay 5 feet, and the metal of the barrel is ¼ inch thick. The joint is yarned with dry jute or oakum, as described above, and is poured full with molten, soft, pig lead to be afterwards driven tightly with hammer and calking tools. About ¾ pound of lead for each inch in diameter of pipe is required. Prepared cements of varying composition have proved effective and, as they require no calking, are economical. Among the best is a finely ground, thoroughly mixed compound of iron, sulphur, slag, and salt.

Iis a homemade pipe jointer or clay roll for use in pouring molten lead. A strand of jute long enough to encircle the pipe and the ends to fold back, leaving an opening at the top, is covered with clay moistened, rolled and worked to form a plastic rope about 1 inch in diameter. The jointer gives the very best results but must be frequently moistened and worked to keep the clay soft and pliable. The jointer shown inFis frequently used for pouring lead joints.

Obstructions in house sewers are frequent. Among the causes are broken pipes, grade insufficient to give cleansing velocities, newspaper, rags, garbage, or other solids in the sewage, congealing of grease in pipes and main running traps (house sewer traps), and poor joint construction whereby rootlets grow into the sewer and choke it. Good grade and good construction with particular care given to the joints, will avert or lessen these troubles. The sewer should be perfectly straight, with the interior of the joints scraped or swabbed smooth. When the joint-filling material has set, the hollows beneath the hubs should be filled with good earth free of stones, well tamped or puddled in place. It is important that like material be used at the sides of the pipe and above it for at least 1 foot. The back filling may be completed with scraper or plow. No running trap should be placed on the house sewer, because it is liable to become obstructed and it prevents free movement of air through the sewer and soil stack. Conductors or drains for rain or other clean water should never connect with the house sewer, but should discharge into a watercourse or other outlet.

Where obstruction of a house sewer occurs, use of some of the simple tools shown inFigure 20may remedy the trouble. It is not likely that farmers will have these appliances, except possibly someof the augers; but some of them can be made at home or by a blacksmith, and most of them should be obtainable for temporary use from a well-organized town or city sewer department. The purpose of the several tools shown is indicated in the notation.

The tank.—The septic tank should be in an isolated location at least 50 to 100 feet from any dwelling. This is not always possible, because of flat ground, but in many such instances reasonable distance and fall may be secured by raising both the house sewer and tank and embanking them with earth. Cases are known where tanksadjoin cellar or basement walls and the top of the tank is used as a doorstep; in other cases tanks have been constructed within buildings. Such practices are bad. It is difficult to construct an absolutely water-tight masonry tank, and still more difficult to make it proof against the passage of sewer odors.

Fig. 20.—Sewer-cleaning tools—how to use them.A, Ordinary 1½ or 2 inch auger welded to a piece 1¾-inch extra strong wrought pipe about 5 feet long: the stem is lengthened by adding other pieces of pipe with screw couplings, and is fitted with a pipe handle; all cleaning work should proceed upstream;B, twist or open earth auger;C, ribbon or closed earth auger;D, spiral or coal auger;E, ship auger;F, root cutter;G, sewer rods, with hook coupling, usually of hickory or ash 1 or 1¼ inches in diameter and 3 or 4 feet long;H, gouge for cutting obstructions;I, scoop for removing sand or similar material;J, claw, andK, screw, for removing paper and rags;L, scraper;M, wire brush for removing grease, drawn back and forth with a wire or rope;N, homemade wire brush (for a 5-inch sewer use a 1½-inch wooden pole to which is securely tacked a piece of heavy rubber, canvas, or leather belting or harness leather 5½ by 8 inches, spirally studded, as shown, with ordinary wire nails 1½ inches in length)

In Northern States, particularly in exposed situations, it is desirable to have the top of the tank 1 to 2 feet underground, thus promoting warmth and uniformity of temperature in the sewage. In Southern States this feature is less important, and the top of the tank may be flush with the ground. Every tank should be tightly covered, for the reasons above stated and to guard against the spreadof odors, the transmission of disease germs by flies, and accidents to children.

Considerable latitude is allowable in the design and construction of septic tanks. No particular shape or exact dimensions can be presented for a given number of people. One family of 5 persons may use as much water as another family of 10 persons; hence the quantity of sewage rather than the number of persons is the better basis of design. Exact dimensions are not requisite, for settlement and septicization proceed whether the sewage is held a few hours more or a few hours less. As to materials of construction, some form of masonry, either brick, building tile, rubble, concrete, or cement block, is employed generally. Vitrified pipe, steel, and wood have been used occasionally.

Fig. 21.—One-chamber septic tank—does nothing more than a tight cesspool. Brick construction, heavily plastered inside; size suitable for 180 to 280 gallons of sewage daily (nominally 4 to 7 persons)

A plant for use all year round should have two chambers, one to secure settlement and septicization of the solids and the other to secure periodic discharge of the effluent by the use of an automatic sewage siphon. The first chamber is known as the settling chamber, the second as the siphon or dosing chamber. The siphon chamber is often omitted and the effluent is allowed to dribble away through subsurface tile, as illustrated inFigure 16. The latter procedure is not generally advised, but may be permissible where the land slopes sharply or has long periods of rest, as at summer houses and camps.

The septic tanks shown in this bulletin are designed to satisfy the following conditions:

1. Water consumption of 40 gallons per person per day of 24 hours.

2. A detention period of about 24 hours; that is, the capacity of the settling chamber below the flow line is approximately equal to the quantity of sewage discharged from the house in 24 hours.

3. Where a siphon chamber is provided, its size is such that the dose of sewage shall be approximately equal to 20 gallons per person; that is, the capacity of the siphon chamber between the discharge and low-water lines is roughly equal to the quantity of sewage discharged in 12 hours.

A simple one-chamber brick tank suitable for a household discharging 180 to 280 gallons of sewage daily is shown inFigure 21. A small two-chamber tank constructed of 24-inch vitrified pipe, suitable for a household discharging about 125 gallons of sewage daily, is shown inFigure 22. A typical two-chamber concrete tank is shown inFigure 23. Excepting the submerged outlet, all pipes within the tank and built into the masonry are cast-iron soil pipe with cast-ironfittings. Vitrified or concrete sewer pipe and specials are generally used, as they are frequently more readily obtainable and a slight saving in first cost may be effected. Cast iron is less liable to be broken in handling or after being set rigidly in masonry, and the joints are more easily made water-tight. The submerged outlet is midway of the depth of liquid in the settling chamber. The inside depth of the siphon chamber is the drawing depth of the siphon plus 1 foot 5 inches.

The following table gives the principal dimensions with quantities of materials for four sizes of tank as illustrated inFigure 23:

Dimensions and quantities for septic tanks.

Siphons.—Reference has already been made to the vital importance of air in sewage filtration. If the spaces within a filter or soil are constantly filled with water, air is excluded, and the action of the filtering material is merely that of a mechanical strainer with its clogging tendency. The purpose of a siphon is twofold: (1) To secure intermittent discharge, thus allowing a considerable period of time for one dose to work off in the soil and for air to enter the soil spaces before another flush is received; (2) to secure distribution over a larger area and in a more even manner than where the sewage is allowed to dribble and produce the conditions of the old-fashioned sink drain—namely, a small area of water-logged ground.

Fig. 22.—Two-chamber septic tank, simple and inexpensive, constructed of 24-inch vitrified sewer pipe, size suitable for 125 gallons of sewage daily (nominally 3 persons).A, House sewer;B, settling chamber, made of doubleTbranch and one length of straight pipe, each 3 feet long and 2 feet in diameter, supported by 4 inches of concrete, all joints made water-tight;C, submerged outlet, consisting of a metalTslipped into the sewer-pipe branch;D, wire screen ¼-inch mesh;E, siphon chamber made of oneTbranch 3 feet long and 2 feet in diameter;F, siphon;G, 3-inch overflow;H, sewer to distribution field;I, tight cover with lifting ring;J, concrete protection around sewer-pipe hubs

Fig. 23.—Typical two-chamber concrete septic tank. (See table for dimensions and quantities for different sizes)

Three types of sewage siphon are shown inFigure 24. In all, the essential principle is the same: A column of air is entrapped between two columns of water; when the water in the chamber rises to a predetermined height, called the discharge line, the pressure forces out the confined air, destroying the balance and causing a rush of water through the siphon to the sewer. The entire operation is automatic and very simple. The siphons shown are commercial products made of cast-iron; they have few parts and none that move, and the whole construction is simple and durable. The table (fig. 24) lists stock sizes adapted to farm use. Manufacturers furnish full information for setting their siphons and putting them in operation. For example, take type 2,Figure 24: (1) Set siphon trap (U-shaped pipe) plumb, making E (height from floor to top of long leg) as specified; (2) fill siphon trap with water till it begins to run out at B; (3) place bell in position on top of long leg, and the siphon is ready for service. Do not fill vent pipe on side of bell.

Type 2INCHESDiameter of siphonA3344Diameter of outletB4444Drawing depthC13151417Depth to floorD16¼18¼17¼20¼Height above floorE7¼9¼8¾11¾Clearance under bellF2222Inside bottom of outlet, to discharge lineG20½22½22¾25¾Discharge line, to top of wallHDepth of outlet sumpILength and width of outlet sumpJDiameter of carrier (R),and minimum fall (S)in feet per 100 feetR4444S2 ft.2 ft.7 ft.8 ft.R5555S1½ ft.1½ ft.2 ft.2½ ft.R6666S1 ft.1 ft.1 ft.1 ft.Type 3INCHESType 1INCHESDiameter of siphonA33444Diameter of outletBDrawing depthC131514175Depth to floorD1717192215Height above floorE1313151819Clearance under bellFInside bottom of outlet, to discharge lineG19212025Discharge line, to top of wallH6688Depth of outlet sumpI13131112Length and width of outlet sumpJ18191918[Transcriber Note: The data for Type 1 Siphon has been added to the table.]Fig. 24.—Three types of sewage siphon. The table gives dimensions for setting standard 3 and 4 inch siphons; also the appropriate size and grade of the sewer to carry the siphon discharge

[Transcriber Note: The data for Type 1 Siphon has been added to the table.]

Fig. 24.—Three types of sewage siphon. The table gives dimensions for setting standard 3 and 4 inch siphons; also the appropriate size and grade of the sewer to carry the siphon discharge

The overhead siphon, type 3,Figure 24, may be installed readily in a tank already built by addition of an outlet sump. If properly setare handled, sewage siphons require very little attention and flush with certainty. Like all plumbing fixtures they are liable to stoppage if rags, newspaper, and similar solids get into the sewage. If fouling of the sniffing hole or vent prevents the entrance of sufficient air into the bell to lock the siphon properly, allowing sewage to dribble through, the remedy is to clean the siphon. Siphons are for handling liquid; sludge if allowed to accumulate will choke them.

Submerged outlet.—The purpose of a submerged outlet is to take the outflow from a point between the sludge at the bottom and the floating solids or scum. The outlet inFigure 23may be readily made of sheet metal by a tinsmith. Wrought iron or steel pipe with elbows or light lead pipe may be used, the pipe being set in the concrete and left in place. Sometimes a galvanized wire screen (¼-inch mesh) is fitted over the inner end to prevent large solids leaving the settling chamber and possibly clogging the siphon or distribution tile. If a screen is used it should be easily removable for cleaning.

Manhole frame and cover.—The frame and cover shown inFigure 23are stock patterns made of cast-iron and weighing about 250 pounds per set. The cover is 21 inches in diameter; it is tight and, on account of its weight, is unlikely to be disturbed by small children. The frame or rim is about 7 inches high and 31 inches in longest diameter. If desired, light cast-iron cistern or cesspool covers obtainable from plumbing supply houses, homemade slabs of reinforced concrete (seeFigure 25), or wooden covers (seeFigure 21) may be used.

Fig. 25.—Homemade reinforced concrete covers. (1) Slabs placed crosswise permit uncovering the whole tank for cleaning, but as inspection is somewhat difficult, cleaning is the more likely to be neglected; (2) manhole, 18 inches square; cover, 22 by 22 by 3 inches thick, easy to make and to slide or lift from the opening

Overflow.—The purpose of an overflow is to pass sewage to the distribution field should the siphon stop working. The overflow (fig. 23) is a 3-inch riser pipe with top 3 inches above the discharge line and the bottom calked or cemented into the side outlet of aTbranch. The run of theTbranch should correspond with the size of the sewer from the tank to the distribution field. If this sewer is 4-inch pipe, a 4 by 3 inchTbranch is used, the 4-inch spigot end of the siphon being calked or cemented into the branch, as shown inFigure 23; if the sewer is 5-inch, a 5 by 3 inchTbranch is used and connected to the siphon with a 5-inch to 4-inch reducer (in vitrified specials the equivalent is a 4-inch to 5-inch increaser); if the sewer is 6-inch, a 6 by 3 inchTbranch is used and connected to the siphon with a 6-inch to 4-inch reducer.

Concrete work.—Before excavation for the tank is begun, two wooden forms should be built for shaping the inside of the settling and siphon chambers. In most instances the ground is fairly firm, so that the lines of excavation may conform to the outside dimensions of the tank, the back of the walls being built against the earth.The forms may be made of square-edged boards, braced and lightly nailed, as shown inFigure 26. The forms should have no bottom. If it is desired to lay the sides and covering slab in one operation, the top of the forms must be boarded over. All pipe and manhole openings should be accurately placed and cut. The faces of the forms may be covered with paper or smeared with soap or grease to facilitate removal later.

Fig. 26.—Forms for concrete work—how to use themClick on image to view larger size.

1 Make the forms as shown and to the dimensions required byFigure 23and the table on p. 29; nails to be driven from the inside and left projecting for drawing with a claw hammer.2. Excavate to lines 6 or 8 inches, as may be required, outside of the forms and to the depths required for both chambers.3. Pour settling chamber floor and place form thereon.4 Pour settling chamber walls to level of siphon chamber excavation, inserting submerged outlet pipe at the proper height. 5. Block siphon and short pipes to correct line and grade, and fill with concrete around the trap.6. Pour siphon chamber floor, and place the form thereon.7 Continue pouring all walls to their full height, inserting the inlet pipe when the concrete reaches that elevation.8. Do not remove forms till the concrete is hard; with favorable weather, forms for walls only may be removed in 1 to 2 days; forms supporting a cover slab should remain 1 to 2 weeks.

1 Make the forms as shown and to the dimensions required byFigure 23and the table on p. 29; nails to be driven from the inside and left projecting for drawing with a claw hammer.

2. Excavate to lines 6 or 8 inches, as may be required, outside of the forms and to the depths required for both chambers.

3. Pour settling chamber floor and place form thereon.

4 Pour settling chamber walls to level of siphon chamber excavation, inserting submerged outlet pipe at the proper height. 5. Block siphon and short pipes to correct line and grade, and fill with concrete around the trap.

6. Pour siphon chamber floor, and place the form thereon.

7 Continue pouring all walls to their full height, inserting the inlet pipe when the concrete reaches that elevation.

8. Do not remove forms till the concrete is hard; with favorable weather, forms for walls only may be removed in 1 to 2 days; forms supporting a cover slab should remain 1 to 2 weeks.

The ground should next be excavated to the proper depth for placing the floors in both chambers. The settling chamber floor, being the lower, should be placed first. Effort should be made to secure water-tight work, a feature of especial importance where leakage might endanger a well or spring. A concrete mixture of 1:2:4 is generally preferred (1 volume cement, 2 volumes sand, 4 volumes stone). The ingredients should be of best quality and thoroughly mixed. The concrete should be poured promptly and worked with a spade or flat shovel to make the face smooth and eliminate pockets or voids within the mass.[10]Before the settling chamber floor has hardened the form should be set upon the floor and the concrete work continued up the sides. The pipe form for the submerged outlet should be set. When the side walls of the settling chamber have reached the bottom of the excavation for the siphon chamber, the siphon trap with its connecting branch and short piece of pipe should be set to proper line and grade and blocked in position. The floor of the siphon chamber should now be poured and the form for that chamber placed thereon, leaving a 6-inch or 8-inch space (according to the thickness of the division wall) between the ends of the two forms. Pouring of all side walls and the top slab should continue without stop, making the entire structure a monolith.

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