CHAPTER II

THE SETTLING TANK AND ITS CONSTRUCTION

As has been stated, a most effective preliminary step in the treatment of sewage is to pass it through a properly designed settling tank in order that the grosser solids and suspended matters as far as possible may be deposited there and finally disposed of separately from the liquid sewage. This partial removal of the suspended matters, amounting to about fifty per cent. in well-designed and carefully operated tanks, very materially aids in the final treatment of sewage on filters or on sub-surface irrigation areas by preventing clogging of the filters or of the piping in the irrigation system.

In connection with the larger settling tanks for hotels or institutions, it is sometimes advisable to pass the sewage first through a screen chamber before it is discharged into the settling tank, in order that the grosser suspended solids may be collected more easily than from the tank; but, as has been pointed out, screening of sewage is not necessary at small disposal plants, and in fact is not generally advisable owing to the continual labor involved in removing and disposing of the screenings, and no description of screening plants will therefore be given.

The old method of discharging sewage and house wastes into loose-walled cesspools on all occasions and under all sorts of conditions is rapidly changing, as is desirable. True, in certain locations, where ample area is available, where the soil is dry and porous, and where neither springs nor wells nor the soil near dwellings will be contaminated thereby, cesspools may be safely used. In other locations a small expenditure of time and money will provide the means by which nature’s processes of reduction of the organic matter in sewage may be carried on much more efficiently and satisfactorily than ever can be the case in a cesspool.

The scheme for properly disposing of sewage at any point should therefore include its sedimentation in a settling tank of proper construction and ample capacity, whether its final treatment is to be effected by sub-surface irrigation, intermittent sand filtration, contact beds, or sprinkling filters. Where the sewage effluent is to be discharged into a stream or body of water of comparatively large flow or volume, and where that stream is not subsequently used as a potable water supply, it is sometimes permissible to subject the sewage to settling tank treatment only. Such partial treatment, however, should be arranged for only as a temporary measure, and the tank should be so constructed with respect to the elevation of adjacent areas that works for final treatment of sewage, when required, may be constructed as advantageously as possible. Moreover, in the more progressive States, as noted in Chapter I, the purity of streams is being carefully safeguarded, and the general tendency of public health officials is to require more complete treatment of sewage before its discharge into a watercourse than is accomplished by settling tanks.

The settling tank for residences and institutions, as shown in Fig. 1, should have a capacity of from five to fifteen cubic feet for each person served by the sewer in order that proper time of detention in the tank may be allowed for the sedimentation of the suspended matters in the sewage. The depth of the tank should be from five to eight feet, and its width should generally be from one-third to one-half the length. Fig. 2 shows a longitudinal section of the settling tank and siphon chamber.

The following table gives the dimensions of tanks which should be adopted to provide a proper time of detention of sewage, based on the number of persons to be served:

TABLE I

DIMENSIONS FOR SETTLING TANKS ══════════════════════════════════════╤══════════╤══════════╤══════════ Persons Served by Sewer. │ Mean │ Mean │ Depth │ Inside │ Inside │(Feet).[1] │ Width │ Length │ │ (Feet). │ (Feet). │ ──────────────────────────────────────┼──────────┼──────────┼────────── 4 │3 │ 4 │ 5 8 │3 │ 7 │ 5 12 │4 │ 7.5 │ 5 15 │4 │ 8 │ 5 25 │4 │10 │ 5 35 │4.5 │12 │ 5 50 │6 │12 │ 5 75 │6 │15 │ 6 100 │7 │17 │ 6 125 │8 │17.5 │ 6 150 │8 │18 │ 6 175 │8 │20 │ 6 200 │8 │22 │ 6 250 –-2 compartments in tank, each │5.5 │18 │ 6 300 –-2 compartments in tank, each │5.5 │18.5 │ 7 350 –-2 compartments in tank, each │6 │19 │ 7 400 –-2 compartments in tank, each │6 │19 │ 8 450 –-2 compartments in tank, each │6 │22 │ 8 500 –-2 compartments in tank, each │6 │24 │ 8 ──────────────────────────────────────┴──────────┴──────────┴──────────

Footnote 1:

12 inches greater than depth of sewage.

The dimensions of settling tanks given above provide for longer periods of detention in the case of the smaller tanks than in that of the larger, an excess which is necessary on account of the greater fluctuation in the flow of sewage reaching the smaller tanks. The larger tanks may be better and more conveniently operated if they are divided by a longitudinal partition wall as shown by Fig. 3, and arranged for in the table for tanks serving 250 or more persons. This provision is not so necessary in the case of the smaller tanks, especially if they are to be installed at summer resorts or country homes occupied for only a few months in the summer. If, however, the tanks are to be operated continuously they may have two chambers for greater convenience in removing sludge. The flow through one compartment may then be stopped by closing a valve placed on the inlet pipe to that compartment, or by inserting one of the stop-planks or sluices in a diverting chamber, as shown in Fig. 3, at the left of the tank and inserting a ten-inch board in the groove over the outlet weir wall of the compartment to be cleaned. The entire flow of sewage is then passed through the other compartment while the first is being cleaned. This division of the tank into two compartments is sometimes desirable in the case of the smaller tanks and may easily be accomplished. For instance, instead of a tank 6 feet by 12 feet, two compartments may be arranged for, each 3 feet 6 inches by 10 feet; and instead of a tank 8 feet by 20 feet, two compartments may be constructed, each 5 feet wide and 16 feet long.

The settling tank should be located as far as conveniently possible from the dwelling, and especially from any wells or springs, in order that leakage of sewage, which may always occur, will not lead to the contamination of a water supply or of the soil near the residence. It may not be possible in every case to locate such tanks more than fifty feet away from the house or from the well, but the distance should never be less than this, and when located at this minimum distance from the dwelling or from a well, especial care should be used to make the tank water-tight.

The walls of the tank should preferably be constructed of concrete, although they may be built of brick or wood. The last material is often the cheapest, and tanks constructed of lumber will last for several years without renewal. The concrete tank, however, is more easily made water-tight, and is a permanent structure. The walls of the tank, when the height is less than 8 or 10 feet, should be 8 inches thick at the top, and should have a batter on the inside of 1½ inches per foot of height. If the tank is to be built with two compartments, the partition wall should be 10 or 12 inches thick at the top and should have a batter on both sides.

The tank should generally be placed with its top at the level of the ground surface, and the sewer from the house should enter the end of the tank with its flow line or invert 12 inches below the top of the walls. The house sewer or drain should have a grade or fall of not less than 9 inches in 100 feet. Preferably, the sewer should be laid at the above minimum grade for at least 50 feet or so before it enters the tank in order to prevent excessive velocity in the sewage flow at this point. At the entrance to the tank the sewer should be provided with an elbow so that the sewage will be discharged downward below the surface. Similarly, if an outlet pipe from the tank is used, as shown in Fig. 5, this pipe should pass through the wall at the outlet end of the tank, one foot below the top of the tank, and should also be provided with an elbow which will start from below the surface.

Where a siphon is to be used to discharge the effluent from the tank onto a filter or into a system of sub-surface tiling, the separate chamber in which the siphon must be placed may be built as an extension of the settling tank so that the end wall of the settling tank will serve as one of the walls of the siphon chamber.

The siphon chamber floor may be placed considerably above the level of the floor of the tank as shown in Figs. 2 and 3, since a sufficiently large quantity of effluent for dosing a filter or a sub-surface irrigation system may be collected in the chamber of reduced depth thus provided. This shallower construction saves excavation and also reduces the operating head or fall, which latter is sometimes hardly equal to the demands of the subsequent treatment. The capacity needed in this chamber for different installations will be given later in the discussion of sewage filters and sub-surface irrigation systems.

Having determined upon the dimensions of the tank and selected the site, the construction is commenced by making the excavation about four feet wider and longer than the outside dimensions of the tank and siphon chamber combined, in order to provide room for setting the forms for placing the concrete, provided concrete is to be used in its construction. With brick walls an additional width and length of two feet is needed.

Fig. 4 gives an illustration of the forms to be used in constructing the walls for concrete tanks, the cut at the left showing a view of the form to be used when the tank is constructed either partly or wholly above the natural ground surface, or below the surface in loose soils, and the cut at the right showing a view of the form to be used when excavation for the tank is made in rock, hardpan, or clay. The top width of the walls should be 8 inches, and the bottom width should be 8 inches plus 1½ inches for each foot of height. Thus, for a wall 6 feet high the bottom width should be 17 inches,—the inside face of the wall having a batter of 1½ inches per foot of height. This batter is necessary, when the tank is constructed below the ground surface, to withstand the lateral earth pressure when the tank is empty. If the tank is to be constructed above the ground surface, the outside wall should be battered and the inside wall made vertical, since the pressure which the wall must withstand is then only from the liquid within the tank. The partition wall between the settling tank and siphon chamber should be 10 or 12 inches thick at the top, depending on its height, and should have a batter on both sides.

To set up the forms for the concrete walls, stakes 2 inches by 4 inches and about 2½ feet long are first driven on each side of the bottom of the wall, and 6 inches away from the wall as laid out, at intervals of 2 feet. Pieces of scantling, 2 inches by 4 inches and with a length equal to the height of the wall, are then placed in upright position and securely nailed to these stakes. The inner scantling are then inclined and temporarily fastened at the top by a short nailing piece to the outer row so as to leave an opening of 10 inches between each pair of scantling. Additional stakes are then driven from 2 to 4 feet from the wall on each side, as shown in the illustration, and braces 2 inches thick and 3 inches or 4 inches wide are nailed to these stakes and to the upright and inclined scantling. One-inch boards are then lightly nailed to the scantling, as shown, the boards making up the inside face of the form being placed in sections of two feet in order to afford opportunity for thorough tamping of the concrete as the form is being filled. The concrete is then placed between the boarded sides of the form in 6–inch layers and well rammed.

The concrete should be composed of one part by measure of Portland cement to two and a half parts of clean, sharp building sand and five parts of broken stone or clean gravel. The cement and sand should first be thoroughly mixed, while dry, to an even color and then wet and tempered to a soft mortar. The broken stone or gravel, after having first been thoroughly wet, should be spread evenly over the batch of mortar and the mass shoveled over at least three times to insure a thorough coating of the stones with mortar. The concrete thus made may then be placed in the forms in six-inch depths and thoroughly rammed until water covers the surface.

When it is essential that the tank be water-tight, and, in fact, in constructing all tanks, each layer of concrete should be placed between the forms, when possible, before the concrete in the layer previously placed has set. If the work of placing the concrete is of necessity interrupted, before placing another layer the surface of the older concrete should first be sprinkled and swept with a stiff broom and a thin coating of neat cement mortar (containing no sand) should then be washed over the surface of the concrete.

It may be noted that a barrel of Portland cement (equal to four bags) contains 3.8 cubic feet, so for concrete with the proportions of cement, sand, and stone as specified above, for each barrel of cement used there should be used 9.5 cubic feet of loose sand and 19 cubic feet of loose stone; and for each cubic yard of concrete required there will be needed 1.30 barrels (or 5.2 bags) of cement, 0.46 cubic yards of sand, and 0.92 cubic yards of stone if the stone is fairly uniform in size and contains forty-five per cent. of voids. With stone or gravel less uniform in size, less cement and sand is required. The cement and sand, made into mortar, will fill the voids or open spaces in the mass of broken stone. (For further details see Chapter VII.)

As shown in the illustration (Fig. 4), the foot of each upright and inclined scantling should be placed at the proposed elevation of the floor of the tank, and the boarding should not be carried below this level. Then, if the excavation for the wall has been carried to a level 6 or 8 inches lower than the floor of the tank, the concrete when being placed between the forms will spread under the bottom of the forms, making a footing for the wall on the outside and better insuring a water-tight joint when the floor is laid against the inside foot of the walls.

In making the excavation for the tank, after reaching the proposed level for the floor a trench should be cut around the floor space to a depth of 6 to 8 inches below the floor level. The width of this trench should be such as to extend from 6 to 8 inches inside and an equal distance outside the wall at the floor level. After the walls have been constructed as described, the forms should be left in place for at least 24 hours, to allow the concrete to set, and then removed. The excavation inside the walls should then be carried 6 inches below the floor level, the soil well tamped, and a 6–inch layer of concrete placed to form the floor of the tank. It is well to sprinkle all concrete daily until it has thoroughly set.

If the type of siphon selected has a U-shaped pipe extending below the floor of the siphon chamber, it will be necessary to set the siphon in position while the floor is being laid and the discharge pipe in position while the wall is being laid. The siphon should be so placed that the bottom of the bell over the longer leg is 3 inches above the floor of the siphon chamber or of the sump in the siphon chamber if such a depression is made in the construction of the floor.

The floor of the tank should slope toward the inlet end at a rate of one-half inch per foot of length in order to facilitate the removal of sludge when the tank is being cleaned. This will result in providing a somewhat greater depth at the inlet end of the tank than is shown by the tables, and a lesser depth at the outlet end, leaving the depth at the centre of the tank as shown. The inlet and outlet pipes to the tank, which should be of cast iron, should be placed in position through the forms while the walls are being laid.

When it is desired to have an outlet pipe from the tank near the bottom (see pipe A, Fig. 5), for the purpose of drawing off the supernatant liquid, and so saving the labor of removing the liquid by pail when the tank is being cleaned, this pipe should be of cast iron, 4 inches in diameter and fitted with a valve and valve rod placed outside the tank, and should also be placed in position during the construction of the tank. The valve rod, or stem, should reach to the surface of the ground through a 3–inch pipe casing. The lower outlet pipe should be extended around the siphon chamber to discharge into the effluent pipe leading away from this chamber, when possible. This lower outlet pipe should leave the tank at least one foot above the floor and sometimes at a higher elevation, in order to discharge into the sewer leading to the irrigation field or to the filter.

Pipe B in Fig. 5 shows a sludge pipe which may be laid to a suitable site for disposing of sludge from the tank when the slope of the land will permit the draining of the sludge by gravity into trenches or onto a sludge bed. This sludge pipe should be fitted with a valve and valve stem, and the valve may be inside the tank, as shown in the illustration, or outside the tank, as shown on pipe A. If such an arrangement for disposing of sludge is possible, it is manifestly unnecessary to provide pipe A as shown in Fig. 5, since the supernatant liquid as well as the sludge may then be piped to a sludge bed or pit. This bed should be shallow, but of ample capacity to hold the entire contents of the settling tank. The sludge may then be drawn off about every six weeks, thereby operating the tank as a settling tank rather than as a septic tank. It will be found after scum of a certain thickness has formed on the surface of the sewage in the tank that the thickness will not materially increase.

The roof of the tank should preferably be of concrete reënforced with iron rods, although it may be of brick arches or of two-inch planking. The use of brick for the roof is not advisable, however, since the forms for the construction of the arches are rather difficult to make, and brick roofs are apt to be broken down sooner or later through the action of frost. A wooden roof, also, must be renewed at intervals and is not as satisfactory as a concrete roof.

A section of a tank with a concrete roof is shown by Fig. 6, together with the temporary form built up inside the tank on which to lay the roof. The form is built by setting 2–inch by 4–inch scantling on wedges along the walls of the tank in pairs 18 inches apart and bracing these at the foot. Boards 1½ inches thick and 10 inches wide are then nailed across the tank to the tops of the scantling, the top edges of the boards being 1 inch below the top of the walls. A false roof is then made of boards nailed lengthwise of the tank to the 10–inch boards, and a layer of concrete 2 inches thick is then placed on the floor thus made, reaching over the top of the walls to the outside edges. Iron rods, ¾ of an inch thick and spaced 1 foot apart, are then placed on the concrete across the tank and reaching to within 1 inch of the outside edges of the walls. More concrete is then placed over the first layer to a total depth of 6 inches or 8 inches, depending on the width of the tank, the concrete being well rammed as it is placed. After the concrete has set, the wedges may be knocked from under the upright scantling and the form taken down and removed through the manhole. The manhole covers and frames, as shown in the illustrations in Chapter III, may be cast at local foundries or purchased through sewer-pipe dealers.

To provide manholes or openings through the roof into the tank and into the siphon chamber, round openings 2 feet in diameter should be cut in the false roof while it is being laid, the distance between the pairs of scantling at this point being made 2 feet. The manhole frames should then be so placed that the flange or base of the frame will be imbedded to a depth of 2 inches in the roof when completed. The manhole at the entrance end of the tank should be located at one side of the entrance pipe and over the valve on the sludge pipe. To provide the necessary opening through the concrete roof below the manhole frame, an eight-sided wooden form, as shown in Fig. 7, with an inner diameter of 2 feet and a height equal to 2½ inches less than the thickness of the roof, is placed over the opening in the false roof. On this wooden form the manhole frame is placed and the concrete laid around the form and over the flanges of the manhole frame. Two of the ¾-inch iron rods should be placed across the tank close to each side of the wooden form after the first 2–inch layer of the concrete roof has been placed.

When it is desired to carry the manhole some distance above the level of the top of the roof to provide for a rather deep earth covering for the tank, the eight-sided wooden form may be made deeper as desired, and another larger, similar form built for the outside form of the necessary concrete manhole well. The space between the two forms may then be filled with concrete and the manhole frame set on the octagonal-shaped wall thus formed.

In order to insure a more uniform flow of sewage through the tank and thus reduce the velocity of flow in all portions to a minimum, baffle boards of 2–inch planks should be placed across the tank near the inlet pipe and near the outlet pipe, as shown in Fig. 5. These boards are set in grooves formed in the concrete by nailing 1–inch by 3–inch strips to the inside form when the tank wall is constructed. These baffles also serve a useful purpose by reducing the disturbance of the scum as the sewage enters the tank and by preventing the escape of scum from the tank.

The boards should extend to a depth of one foot below the inlet and outlet pipes, and should usually be placed 12 to 18 inches from the ends of the tank. Where the effluent from the tank is to be collected in a siphon chamber adjoining the tank, it is preferable to provide a weir or wall between the tank and the siphon chamber. The top of this wall should be one foot below the roof to allow the effluent to flow over this wall from the tank into the siphon chamber. In this case no outlet pipe from the tank is used, and the baffle boards should extend downward 12 inches below the level of the sewage in the tank. These baffle boards should be carried up to a level with the top of the tank walls.

It is advisable to provide an overflow pipe from the siphon chamber which should leave this chamber at an elevation of 3 or 4 inches above that of the inlet pipe to the tank, and which should, by means of an elbow, be extended down outside the chamber to connect with the sewer into which the siphon discharges. This is desirable in order to provide an overflow in case the siphon becomes clogged or fails to operate.

Where a tank must of necessity be located near a residence, any nuisance due to odors may be prevented by inserting a 4–inch galvanized-iron conductor-pipe through the roof of the tank, and carrying this pipe up into the air 20 or 30 feet along a tree trunk or the side of a building.

If a sub-surface irrigation field is to be laid out, the tank should preferably be near the proposed location of the sub-surface irrigation area (see Fig. 26, Chapter IV), although the effluent may be carried to the sub-surface irrigation field from a settling tank located at some distance from such field. Since the sewage enters the tank near the top of the tank and the effluent discharges from the siphon chamber at a considerable distance below the top of the tank, it is of advantage to place the settling tank on sloping ground, if possible, so that one end will be wholly in excavation and the other will be partly above the natural ground surface. This reduces the depth of trenching and provides for more readily distributing the effluent by gravity from the tank through the sub-surface tiling which is laid just below the surface of the ground. The tank must always be higher than the distributing field to allow for the flow of sewage, and it is desirable to have the tank buried in the ground if possible in order to keep the temperature of the sewage as high as possible in winter. These ideal conditions are not always to be attained.

The one important point to be kept in mind if the settling tank is to be properly operated and not allowed to develop into a nuisance is that the sludge or sediment must be removed from the bottom of the tank at intervals before the effective capacity of the tank is so reduced that the proper sedimentation of the sewage is impossible. The frequency of cleaning necessary varies in different cases, but usually the tank should be emptied and cleaned at intervals of from three months to one year, and where the contour of the ground allows the sludge to be readily drawn off into trenches or to a sludge bed, cleaning should be practised every five or six weeks.

There is, perhaps, little need for cleaning the tank as often as once in six weeks, but it is generally found and has been affirmed in court testimony that the removing of the sludge from a settling tank once every six weeks will prevent septic action from taking place, and the tank will then be operated as a settling tank and not as a septic tank. This is desirable in view of the fact that royalties have been claimed under certain patents on septic tanks. As explained on p. 11, the important function of the tank is to settle out suspended solids, while the processes that take place in the septic tank but not in the settling tank are of minor importance, and it is advisable therefore to operate these tanks as settling tanks when possible.

In no case should the sludge be allowed to accumulate until it fills more than one-quarter of the tank. The sludge may be disposed of by burying in trenches or ploughing under, or it may be spread on the surface at points remote from highways and dwellings or sources of water supply. The depth of accumulated matter in the tank should frequently be tested at the inlet end by using a pole or stick.

In reference to the preliminary treatment of sewage in tanks, it should be noted that the most recent development in the design of sewage-disposal plants has been the improved method of sedimentation of sewage represented by the Imhoff or Emscher tank. A modified design of this tank is shown in plan and longitudinal section in Fig. 8, and a cross-section of the tank is shown in Fig. 9. The principle employed is to provide a separate chamber for storing the sludge which results from the sedimentation of the suspended matters in the sewage, this chamber being almost entirely separated from the portion of the tank in which the sedimentation takes place. This separation of the sludge from the flowing sewage is accomplished in the tank shown by inserting in the tank, parallel with the side walls, two inner partitions _AA_, which are vertical for a few feet below the surface of the sewage and then slope toward the centre line of the tank, but, as shown by Fig. 9, do not meet at the centre line, the one passing a few inches under the other. The opening or slot thus formed between the two inner partitions allows the suspended matters which settle out of the sewage flowing through the upper compartment to pass into the lower or sludge compartment and there remain in a quiescent state until removed from the tank. The object of this separation of the sludge from the flowing sewage is to prevent the gas bubbles which emanate from the sludge during its decomposition from rising through the flowing sewage and interfering with the process of sedimentation going on in the upper compartment, and to provide for a more complete decomposition or “digestion” of the sludge. The gas bubbles on rising from the deposited sludge strike the sloping lower sections of the inner partitions and are deflected to the portions of the tank next to the outside walls. A sludge pipe leads away from the bottom of the hopper-shaped sludge compartment, and at intervals of from one to four weeks the valve on this sludge pipe is opened for a short time and a small portion of the accumulated sludge is allowed to be forced out onto a sludge-drying bed by the weight of the sewage in the tank. The portion of the sludge thus removed has, of course, remained in the tank the longest time, generally five or six months, and has had the fullest opportunity to be reduced and rendered inodorous and easy to dispose of.

This method of sedimentation was first experimented with about twelve years ago by Mr. H. W. Clark at the Lawrence Experiment Station of the Massachusetts State Board of Health, then partially developed by Dr. W. Owen Travis, of Hampton, England, and finally worked out by Dr. Ing. Carl Imhoff in connection with the disposal of sewage in the Emscher River district in Germany. The method has been extensively and successfully used in Germany, and similar tanks are now being installed in this country. While these tanks are probably more effective than septic tanks and the usual type of settling tanks in the removal of suspended matters in sewage, their chief value will undoubtedly be found in the rendering of the sludge less odorous and more easily handled. This form of settling tank is covered by patents, and a moderate royalty is charged on tanks of this type.

A description of the Imhoff tank has been here included since it represents an important development in sewage disposal and helps to solve what has heretofore been one of the main difficulties of sewage disposal, especially for cities and villages, namely, the satisfactory and convenient disposition of sludge; but it is not considered that their construction is advisable or warranted where only a small quantity of sewage is to be treated, and settling tanks to treat sewage contributed by less than, say, two hundred persons would generally be constructed as previously described.