CHAPTER IV

SUB-SURFACE IRRIGATION

The disposal of sewage by the method of sub-surface irrigation, sometimes known as the Waring system, consists in its distribution by means of open jointed tiling over a comparatively large area of soil and at a depth of a few inches beneath the ground surface. The sewage should first be passed through settling tanks to remove as much as practicable of the suspended matters contained in it, as explained in the chapter on settling tanks. The partially clarified effluent from the settling tank should then be collected in a dosing chamber, or separate compartment of the settling tank, and discharged intermittently, preferably by means of a siphon, into the sub-surface irrigation system. This intermittency of discharge of accumulated quantities of effluent is necessary for an even distribution of the effluent throughout the entire system of sub-surface tiling, and for a continuous and successful operation of the system as a whole. It has been found necessary, also, to alternate the discharge of effluent from the dosing or siphon chamber over different portions of the irrigation area. One siphon is all that is necessary to install in the siphon chamber for sub-surface irrigation systems, and if the settling tank has two compartments, as shown in Fig. 3, Chapter II, the single siphon would be placed in the centre of the chamber.

The principle involved in this method of sewage purification is that of any general method of sewage reduction in whatever form carried on, namely, its oxidation or nitrification. This oxidation, or breaking down of the organic matter in the sewage, is accomplished in this case, as in the case of intermittent sand filters, contact beds, and sprinkling filters, through the agency of bacterial action.

Householders have long been familiar with the fact that although solids contained in sewage may have been discharged for long periods of time into a cesspool, the latter, if located in dry, porous soil, did not seem to become filled with the solid residue. This is due to the liquefaction of the solid matter in the sewage after its discharge into the cesspool, and to the seepage and bacterial reduction of the liquid matter in the surrounding soil. To replace the cesspool and eliminate the insanitary conditions which, in most instances, result from its use, other methods have been devised which utilize the agencies of nature to the best advantage. Thus the sedimentation and, in some cases, the liquefaction of the solid matters in sewage are carried on in specially designed settling tanks which are easily cleaned and which provide for greater efficiency in settling out suspended matters than the cesspool. Similarly, the filtration of liquids from cesspools through the soil is replaced by the scientific method of sub-surface irrigation, which is much more efficient in three distinct ways: (1) the limited seepage area represented by the walls of the cesspool is increased many times by distributing the effluent from the settling tank over a large area of soil in a system of sub-surface tiling; (2) the bacterial reduction is more effective, since it has been found that the bacterial action necessary to purify sewage takes place in the upper layers of the soil and is almost absent at depths of five feet or more; (3) the soil is given an opportunity to rest and to dry out by alternately using different portions of the sub-surface irrigation system. In the cesspool the seepage of the effluent and whatever bacterial action takes place in the surrounding soil must go on continuously, which often results in clogging of the soil and overflowing of the cesspool.

The purification and final disposition of sewage by means of sub-surface irrigation is the method best adapted to the single residence, and oftentimes to the hotel or institution, if soil conditions are favorable and proper area is available. This system requires less oversight in its operation than the various forms of artificial filters. Furthermore, the sewage is entirely hidden from sight after it leaves the settling tank, and this is usually desirable near private residences and on the grounds of country homes, country clubs, and summer hotels. Also, where the sewage must be treated in close proximity to a residence or hotel or at a point on the windward side of a residence, this method, more effectually than any other, precludes the possibility of a nuisance resulting from the operation carried on, since the settling-tank effluent is at no point exposed directly to the air. Furthermore, its cost is less than that of other works for final treatment of sewage, and, finally, the system is more easily installed.

The method is in reality modified broad irrigation, but the sub-surface irrigation field can be utilized much more effectively and with considerably less attention than a broad irrigation area, and, as noted above, is less liable to be the cause of a nuisance or to be the means of spreading infectious disease through the agencies of flies and other insects.

If an area of sandy soil is available on which to locate the sub-surface irrigation field, if the settling tank and siphon chamber have been correctly built, and if the sub-surface tiling system has been properly laid, the success of the system is well assured. On the other hand, failure is certain if either broad irrigation or sub-surface irrigation methods of sewage disposal are attempted on stiff, impervious clay soils. Between the ranges of porosity of soil represented by these limits there are many soils in which sewage may be successfully disposed of by sub-surface irrigation. A sandy or gravelly loam will, without question, successfully care for sewage effluent when such effluent is properly distributed by sub-surface tiling, and even in a rather heavy soil the effluent from a settling tank may often be disposed of satisfactorily by providing for a greater length of sub-surface tiling per person served by the settling tank than that which would suffice in the more porous soils. However, if the soil is very heavy so that surface water does not readily seep away, or if the ground-water level is within two or three feet of the surface, this method is not suitable and some form of filter, described in the succeeding chapter, should be used for final treatment of sewage.

When soil conditions and the area available are favorable to this method and such a system is to be installed, the irrigation area selected should be at the point where the ground-water level is lowest, and this will generally be on a plateau or bench at the head of a slope of ground. The relative elevation of the ground surface should, of course, be low enough to insure operating head or fall to operate the siphon in the chamber adjoining the settling tank and to distribute the effluent by gravity to the sub-surface tiling. If the soil is composed of loose gravel, or lies over a limestone or shale formation, the location of the irrigation area should be selected with a view to preventing the contamination of any wells or springs which may exist on the premises,—that is, the area should be on lower ground, and as far removed from wells as is convenient.

As will be explained later, the length of sub-surface tiling necessary to receive a given quantity of sewage effluent should vary, within certain limits, with the character and porosity of the soil, thus requiring larger quantities of effluent to be delivered from the siphon or dosing chamber in the case of the more compact soils. Also the size of this chamber should be determined with reference to the number of sections into which the sub-surface tiling system is divided.

TABLE II

FOR USE IN THE CONSTRUCTION OF SUB-SURFACE IRRIGATION SYSTEMS IN SANDY OR VERY POROUS SOILS

Dimensions given are for inside measurement. ══════╤═══════╤═══════╤═══════╤═══════╤═══════╤════════╤════════╤════════ Number│ Total │No. of │ Mean │ Mean │ Depth │Diameter│ Dis- │Distance of │Length │ Sect- │ Width │Length │ of │ of │charging│ from Per- │ of │ions in│ of │ of │Siphon │ Siphon │Depth or│Roof to sons │3–inch │ Sub- │Siphon │Siphon │Chamber│ (Inch- │Maximum │ Top of Served│ Sub- │surface│Chamber│Chamber│ from │ es). │Depth of│ Wall by │surface│System.│(Feet).│(Feet).│Roof of│ │Effluent│between Sewer.│Tiling.│ │ │ │ Tank │ │ above │Settling │ │ │ │ │(Feet).│ │ Lower │Tank and │ │ │ │ │ │ │Edge of │ Siphon │ │ │ │ │ │ │ Siphon │Chamber │ │ │ │ │ │ │ Bell │ (Inch- │ │ │ │ │ │ │ (Inch- │ es). │ │ │ │ │ │ │ es). │ ──────┼───────┼───────┼───────┼───────┼───────┼────────┼────────┼──────── 4│ 140│ 2│2´ │2´ │ 2´ 4˝│ 3˝│ 13˝│ 12˝ 8│ 280│ 2│2´ 6˝ │2´ 6˝ │ 2´ 4˝│ 3˝│ 13˝│ 12˝ 12│ 420│ 2│2´ 6˝ │4´ │ 2´ 4˝│ 3˝│ 13˝│ 12˝ 15│ 525│ 2│3´ 6˝ │4´ │ 2´ 4˝│ 3˝│ 13˝│ 12˝ 25│ 875│ 2│3´ │4´ │ 3´ 2˝│ 5˝│ 23˝│ 12˝ 35│ 1225│ 2│3´ 8˝ │4´ 6˝ │ 3´ 2˝│ 5˝│ 23˝│ 12˝ 50│ 1750│ 2│4´ │6´ │ 3´ 2˝│ 5˝│ 23˝│ 12˝ 75│ 2625│ 2│3´ │6´ │ 3´ 2˝│ 5˝│ 23˝│ 16˝ 100│ 3500│ 2│4´ │7´ │ 3´ 2˝│ 5˝│ 23˝│ 16˝ 125│ 4375│ 3│5´ 6˝ │8´ │ 3´ 2˝│ 5˝│ 23˝│ 16˝ 150│ 5250│ 3│7´ │8´ │ 3´ 2˝│ 5˝│ 23˝│ 16˝ 175│ 6125│ 3│7´ 6˝ │8´ │ 3´ 9˝│ 6˝│ 30˝│ 16˝ 200│ 7000│ 3│8´ │8´ │ 3´ 9˝│ 6˝│ 30˝│ 18˝ ──────┴───────┴───────┴───────┴───────┴───────┴────────┴────────┴────────

TABLE III

FOR USE IN THE CONSTRUCTION OF SUB-SURFACE IRRIGATION SYSTEMS IN THE HEAVIER LOAMS (NOT CLAY OR HARDPAN)

Dimensions given are for inside measurements. ══════╤═══════╤═══════╤═══════╤═══════╤═══════╤════════╤════════╤════════ Number│ Total │No. of │ Mean │ Mean │ Depth │Diameter│ Dis- │Distance of │Length │ Sect- │ Width │Length │ of │ of │charging│ from Per- │ of │ions in│ of │ of │Siphon │ Siphon │Depth or│Roof to sons │3–inch │ Sub- │Siphon │Siphon │Chamber│ (Inch- │Maximum │ Top of Served│ Sub- │surface│Chamber│Chamber│ from │ es). │Depth of│ Wall by │surface│System.│(Feet).│(Feet).│Roof of│ │Effluent│between Sewer.│Tiling.│ │ │ │ Tank │ │ above │Settling │ │ │ │ │(Feet).│ │ Lower │Tank and │ │ │ │ │ │ │Edge of │ Siphon │ │ │ │ │ │ │ Siphon │Chamber │ │ │ │ │ │ │ Bell │ (Inch- │ │ │ │ │ │ │ (Inch- │ es). │ │ │ │ │ │ │ es). │ ──────┼───────┼───────┼───────┼───────┼───────┼────────┼────────┼──────── 4│ 300│ 2│ 2´ 5˝ │ 3´ │ 2´ 4˝│ 3˝│ 13˝│ 12˝ 8│ 600│ 2│ 3´ │ 5´ │ 2´ 4˝│ 3˝│ 13˝│ 12˝ 12│ 900│ 2│ 4´ │ 5´ │ 2´ 4˝│ 3˝│ 13˝│ 12˝ 15│ 1125│ 2│ 4´ │ 6´ 6˝ │ 2´ 4˝│ 3˝│ 13˝│ 12˝ 25│ 1875│ 2│ 4´ │ 6´ │ 3´ 2˝│ 5˝│ 23˝│ 12˝ 35│ 2625│ 2│ 4´ │ 4´ 6˝ │ 3´ 2˝│ 5˝│ 23˝│ 16˝ 50│ 3750│ 2│ 4´ 6˝ │ 6´ │ 3´ 2˝│ 5˝│ 23˝│ 16˝ 75│ 5625│ 2│ 6´ │ 7´ │ 3´ 2˝│ 5˝│ 23˝│ 16˝ 100│ 7500│ 2│ 7´ │ 8´ │ 3´ 2˝│ 5˝│ 23˝│ 16˝ 125│ 9375│ 3│ 8´ │10´ │ 3´ 9˝│ 6˝│ 30˝│ 20˝ 150│ 11250│ 3│ 9´ │12´ │ 3´ 9˝│ 6˝│ 30˝│ 20˝ 175│ 13125│ 3│10´ │12´ │ 4´ 2˝│ 8˝│ 35˝│ 20˝ 200│ 15000│ 3│11´ │12´ │ 4´ 2˝│ 8˝│ 35˝│ 20˝ ──────┴───────┴───────┴───────┴───────┴───────┴────────┴────────┴────────

The dimensions of siphon chambers to effectively deliver the effluent in proper volumes to the sub-surface irrigation system are given in the following tables, which indicate widths of siphon chambers to agree in general with the widths of the settling tanks to serve a given number of persons, as shown in Chapter II. These tables of dimensions for siphon chambers provide for two different capacities where the same number of persons are served by the sewer, depending on the total lengths of sub-surface tiling required, which in turn depend on the character of the soil in which the sub-surface system is laid. The tables provide for a division of the sub-surface tiling system into two parts up to a system for 100 persons, and into three parts for a greater number of persons. These tables also show the total length of lateral distributing tiling in the sub-surface irrigation system necessary to distribute over a sufficient area at the irrigation field, in both sandy soils and in the heavier loams, the various quantities of sewage to be treated in the different-sized tanks and discharged from the siphon chambers. The tables also indicate the diameter of the siphon and the discharging depth of each siphon.

As discussed in Chapter III, the siphon, in discharging, may draw upon the upper 4 to 8 inches of sewage in the settling tank without interfering with the efficiency of the tank. The dimensions of siphon chambers for 75 or more persons in Table II, and for 35 or more persons in Table III (see page 59), provide for such a draught upon the settling-tank contents of from 4 to 8 inches when the siphon discharges. This will decrease the cost of the plant somewhat and provide for a more efficient form of siphon chamber. The last column in each table provides for the proper height of dividing-wall between the settling tank and siphon chamber to allow the drawing down of the settling-tank contents as noted above.

The sub-surface irrigation or distributing system consists of a main carrier or effluent sewer leading away from the siphon chamber to the irrigation field, of two or more branches of this main carrier, and of parallel lines of lateral distributing tiling extending at intervals of 4 to 6 feet from the branch carriers, or, in some locations, from each side of the branch carriers.

The frontispiece shows the relation between the several portions of a sub-surface irrigation system. The house sewer is shown leading to the settling tank, and from the siphon chamber adjoining the settling tank the main carrier or effluent sewer is shown leading to a diverting manhole from which the effluent is carried at each discharge of the siphon to the lateral lines of sub-surface tiling by the two branch carriers.

Fig. 26 shows in plan and section a sub-surface irrigation system. The section, which is drawn to a larger scale than the plan, shows the settling tank and the adjoining siphon chamber. From this siphon chamber the effluent sewer carries the discharge from the siphon to the diverting manhole, at which point the effluent is diverted to the different portions of the sub-surface tiling.

In Fig. 27 is shown in plan the diverting manhole and a small portion of the sub-surface tiling system together with a section through the diverting manhole and one of the lines of distributing tiling.

The main carrier should be of vitrified tile sewer pipe with cemented joints, and should always have two or more branches at the irrigation field in order to allow the use of different portions of the field in turn for three days or a week at a time, thus allowing one of the portions of the field to be resting for corresponding periods. The branch carriers should be of vitrified tile also, and should have cemented joints. If the diameter of the siphon is 5 inches, the main carrier should be of 8–inch vitrified tile with a fall of at least 6 inches per 100 feet in order to quickly carry the dose from the siphon chamber to the several lines of sub-surface tiling forming the distributing system. With 3–inch siphons, a 6–inch main carrier may be used, but the gradient or fall of the main carrier should then be at least 12 inches per 100 feet, owing to the smaller capacity of the 6–inch pipe. In placing the siphon in position, when the siphon chamber is being built, care should be taken to see that the trap or U-shaped pipe is set plumb or in a vertical position. Concrete should then be placed around the siphon to hold it in proper position and at the proper height, and the trap should be filled with water before the bell is placed in position. The bell should then be placed in position over the long leg of the trap to prevent the materials used in construction from being dropped into the siphon. The siphon should be set so that the lower edge of the bell, or of that portion of the bell under which the effluent is to flow, is three inches above the floor of the siphon chamber.

In laying the distributing system, every second or third length of the branch carriers, according to the porosity of the soil and the spacing of the lines of distributing tiling, should consist of a Y-branch (see Fig. 28), to which a one-eighth bend (see Fig. 29) should be fitted if the lines of lateral tiling are to be laid at right angles to the main carriers, as shown in Fig. 27; or the lateral tiling may be fitted directly to the Y-branch if the lateral lines are to be led away from the carrier at an angle of 45°, as shown in Fig. 30. The Y should branch from the lower portion of the pipe, as shown in Fig. 28.

The lateral tiling should be of three-inch agricultural tile (see Fig. 31), laid with a space of one-quarter inch between each length and with a piece of tar paper or a half-collar of larger diameter pipe, as shown in Fig. 32, placed over the joints to prevent clogging of the pipe with earth. In the heavier soils the lateral lines of sub-surface tiling are sometimes set in trenches eight to fourteen inches deep and about twelve inches wide, filled with broken stone or gravel placed around the tiling to within two or three inches of the ground surface, as shown in Fig. 32. This allows the effluent to seep away more readily, but while of advantage in those soils the provision is not necessary in the more porous soils.

It is generally found that a sufficient length of sub-surface tiling should be laid to provide for not more than one to three gallons of effluent per day for each linear foot of tiling. In sandy soils there should be at least thirty to forty feet of tiling for each person served by the sewer, with six feet of space between the lines of tiling. This length per person should be increased up to seventy or eighty feet for the more compact sandy or gravelly loams, or the lighter clay loams, with the lateral tiling spaced four feet apart. It is not considered feasible to attempt to dispose of sewage by sub-surface irrigation in soils which will not care for effluent when the greater lengths of tiling per person, as stated above, will not prevent the appearance of effluent on the surface. If, however, after the installation of a sub-surface system in a rather heavy soil, it is found that proper seepage of the effluent does not occur, the lateral branches may sometimes be lengthened and the system then found to operate satisfactorily.

The lines of lateral tiling should be laid with the invert, or bottom of the pipe, inside, from six inches to one foot below the surface, as shown in Fig. 27. They should be parallel with the contours or at right angles with the slope of the field, and should have a gradient or fall of one-sixteenth of an inch to the foot when laid in sandy soil or sandy loam, and of not more than one thirty-second of an inch to the foot when laid in the heavier loams. To obtain such gradients for the sub-surface tiling it is sometimes necessary to lay out the trenches along irregular or curved lines, as shown in Fig. 33. The tiling should be laid near the surface, as stated, and never deeper than twelve inches. The temperature of the sewage will prevent its freezing even in very severe winter weather, especially when the ground is covered with snow.

To provide for diverting the flow from the siphon chamber first into one of the two portions into which the sub-surface system is divided, and then, after an interval of three days or a week, into the other portion of the system, at the point where the main carrier is to branch, a ten-inch iron pipe casting (see Fig. 34), with its lower portion forming the body of a double Y-branch of six-inch or eight-inch pipe, may be placed, having a swinging blade or gate attached inside in a vertical position. When, for example, the effluent has been passed for a week into section B of the sub-surface system, the gate _C_, shown in Fig. 34, may be swung to the dotted position and the effluent, at each discharge of the siphon chamber, will then pass through the branch carrier _A_ to section A of the sub-surface system; or a double Y-branch of iron pipe (see Fig. 35) or a cross may be placed at this point on the main carrier when there are to be three sections of the sub-surface system, and valves may be placed on the three branches of the main carrier thus formed to permit of alternately shutting off the flow to the various sections of the sub-surface tiling system (see also Fig. 30). Perhaps the simplest and most serviceable device, however, for alternately resting different portions of the irrigation field is a diverting manhole with stop planks or wooden sluices sliding in grooves in the concrete walls or in a wooden frame, as shown in Fig. 27. (See also Fig. 43, Chapter V.)

Where the ground-water level is not very deep below the surface, or a clay or hardpan stratum occurs at a depth of a few feet, it is advisable to underdrain the irrigation field by lines of open-jointed tiling laid at right angles to the lateral distributing tiling and spaced about fifteen feet apart. (See Fig. 36.)

These underdrains should be placed at least four feet below the surface, and inspection pipes should be placed over the outlets of the underdrains or at the points where they discharge into a main underdrain, in order to afford opportunity to determine if all portions of the irrigation field are properly caring for the effluent. To provide for the placing of the inspection pipes, a length of vitrified tile with a Tee may be placed on each line of underdrain tiling near its junction with the main underdrain. On this Tee, two or three lengths of vitrified tile may be set, reaching to the ground surface and provided with a removable wooden cover or a vitrified tile cap. This provision for inspection is necessary where underdrains must be laid and where the pollution of a stream is to be prevented, since it is often found that through the activities of burrowing animals direct outlets from the distributing tiling to the underdrains are formed and the final effluent is therefore not sufficiently purified by seepage through the soil. It is desirable for this reason to omit the underdrains when possible, and in some instances a blind ditch may be constructed around two or three sides of the field in order to intercept the ground-water flow and to lower the ground-water level at the field, thus better insuring proper seepage of the effluent distributed by the sub-surface tiling.

The essential features of the sub-surface irrigation system of sewage disposal have been outlined above, and it may be said that this method is especially adapted to the residence or single house. The method may be employed with success to dispose of sewage from country clubs and summer hotels, provided the soil conditions are favorable and proper areas may be utilized. In these cases the comparatively short period during each year in which the system is in use and the resulting long periods of rest give opportunity for a recuperation of the soil and permit the use of this system in comparatively large installations where, under continuous operation, a different method of disposal would be indicated. It should be borne in mind, however, that when any doubt arises as to the suitability of the soil to care for sewage by this method, and especially where considerable expense would be involved in the installation of the system, competent engineering advice should be sought by property owners before the installation is undertaken. In fact, it is advisable in the case of all large plants of this type to employ the services of a sanitary engineer to lay out the system, since the matter of accurate gradients and proper operating arrangements then becomes very essential to the success of the undertaking.

While it is not generally advisable to arrange for the disposal of sewage by sub-surface irrigation when the number of persons served by the sewer exceeds two hundred, this method will be found a most satisfactory one if the general conditions at any point are favorable to its use as heretofore described, and in such cases the adoption of this system is strongly recommended to the owners of residences, summer camps, summer hotels and boarding-houses, and to the managers of moderate-sized institutions and of country clubs who must meet the problem of properly disposing of sewage on their own premises.