LIST OF FIGURES

A sub-surface irrigation sewage-disposal plant[Frontispiece]
FIGURE PAGE
1.Plan of settling tank[15]
2.Longitudinal section of settling tank[17]
3.Sketch of settling tank with longitudinal partition wall[19]
4.Forms used for building side walls for concrete tank[23]
5.View of settling tank, showing baffles, sludge pipe, drain pipe, and inlet and outlet pipes[26]
6.Section showing tank with concrete roof and form for constructing roof[28]
7.Form for manhole opening[30]
8.Plan and longitudinal section of modified Imhoff tank[33]
9.Vertical cross-section of modified Imhoff tank[34]
10.Sludge valve for floor of tank[38]
11.Sludge valve for side wall of tank[39]
12.Sluice gate valve made by Coffin Valve Co[40]
13.Ordinary gate valve[40]
14.English slide valve with wedge-lock handle[41]
15.Flap valve attached to length of sewer pipe[42]
16.Flap valve with metallic seat attached[43]
17.Flap valve with loose-link hinges[44]
18.Intermittent dosing apparatus made by Ansonia Manufacturing Co.[45]
19.Simplest form of automatic siphon[46]
20.Van Vranken automatic siphon[47]
21.Miller automatic siphon[48]
22.Double alternating siphons of the “Merritt” type[49]
23.Triple alternating siphons of the Miller type[50]
24.Single “Merritt” automatic siphon[51]
25.Air-lock siphon for admitting and releasing sewage from each one of four beds in regular order[52]
26.Plan and section of sub-surface irrigation system[61]
27.Plan and section of a portion of a sub-surface irrigation system[62]
28.Y-branch of vitrified tile pipe[64]
29.Eighth bend of vitrified tile pipe[64]
30.Sub-surface tiling[65]
31.Photograph of tile laid as if for sewage disposal[66]
32.Sub-surface tiling with broken stone or gravel surrounding pipe[67]
33.Sub-surface systems on irregular ground[68]
34.Special casting of double Y-branch with swinging gate[69]
35.Double Y-branch with valves on branches of main carrier[70]
36.Sub-surface tiling system with underdrains[71]
37.View of sand-filter beds for village in Massachusetts[75]
38.Layout for intermittent sand filtration[78]
39.Intermittent sand-filtration beds[79]
40.Portion of distributing troughs for sand filters[80]
41.General view of disposal plant at Bedford Reformatory[81]
42.View of sand filter with distribution trough. Settling tank is at the end of the bed[82]
43.View of diverting manhole[83]
44.Plan of diverting manhole[84]
45.Five-way diverting manhole[85]
46.General plans of contact-bed system near Albany, N. Y., opposite page[89]
47.View of sprinkling filter at Dansville, Pa., in winter[95]
48.Distribution of sewage and arrangement of check levees on a hillside[106]
49.Distribution of sewage on a hillside of moderate slope[107]
50.Square beds for orchards according to some Western practice[108]
51.Grain-field in spring in process of irrigation[109]

PRACTICAL METHODS OF

SEWAGE DISPOSAL

FOR

RESIDENCES, HOTELS AND INSTITUTIONS

CHAPTER I
INTRODUCTORY

The problem of sewage disposal for a single house differs from the corresponding problem for a city chiefly in two ways: first, because in the city it is becoming, if it has not, indeed, already become, a necessity, and city authorities, though somewhat reluctantly, are willing to grant the necessary appropriation to secure engineering advice which will solve the problem in a scientific as well as economic fashion. In the case of a single house, whether a farm-house or a villa, the necessity of employing competent engineering advice has not been generally recognized, and no attempt has been made to solve the problem of sewage disposal in a scientific manner.

Cesspools have been considered the only way of caring for sewage in places where a running stream was not available, or where attempts were made to protect such a stream from pollution, and while, in these last few years, crude attempts have been made to utilize the so-called septic tank, such attempts have generally been so unintelligent that the results have been anything but satisfactory. Since it has been understood that insects, such as flies and mosquitoes, play an important part in the transmission of disease, the danger of overflowing cesspools and of open ditches in which stagnant sewage is present, has been appreciated; also the higher standards of living which have made themselves felt throughout the rural community have demanded in farm-houses and country homes sanitary conveniences which have hitherto been wanting.

Gradually every house is using more and more water for various purposes, and living conditions, which in the past tolerated a scanty supply drawn from a pump, are no longer endured. The increased water supply and the demands of extended plumbing mean a greater amount of sewage—so great an amount that, in many cases, soils which could receive and digest the waste waters from houses supplied by wells are clogged and made impervious by this greater amount.

Further, the danger to wells from the infiltration of cesspools is more feared, and it is understood as never before that in order to maintain the highest degree of health in a family the drinking-water used must be above suspicion and not subject to contaminating influences in the vicinity.

Again, communities are being aroused to the intrinsic value of maintaining streams in a pure condition—partly because of the value of fish and ice coming from the streams themselves, and partly on the broad ground that watercourses belong to the country as a whole, and must be kept pure for the sake of succeeding generations, not spoiled for them on account of the selfishness of a few at the present time.

Thus it is that to-day the problem of sewage disposal, while arousing general interest, is recognized as one which requires more than the common sense of an average person, that the force and principles involved are understood to be not those in common use, and that, for successful disposal of sewage, special knowledge and judgment are required.

Whatever the character of the sewage and whatever the kind of soil available for treatment, the method of dealing with sewage most obvious to most people has been to discharge the sewage directly into the nearest watercourse. This has been the practice of cities as well as of individual houses in the past, and the practice is very difficult to check because of the economy of this method of disposal. In many cases there is no objection to this method, and where a large stream is available, where no use is made further downstream of the waters for drinking purposes, and where the volume of water in the stream is sufficient to dilute the sewage to a point where no odors or objectionable appearances result, it would seem most uneconomical to adopt any more complicated method of disposal than by simply carrying the outfall pipe into the main bed of the stream.

In New York State, and in a number of other States, the number of which is continually increasing, such direct discharge, however, is not permitted by law except under certain conditions. In New York State it is required that any house, butter or cheese factory, manufacturing establishment, or village shall obtain the permission of the State Commissioner of Health before such a method of discharge be adopted, and in order to obtain this permission it must be definitely shown that the conditions of the stream are such that no reasonable objection to this method could be urged. The policy of the various Departments of Health in the United States is gradually becoming more and more rigorous in the matter of prohibiting the discharge of crude sewage into watercourses, and it is wise to make very sure that the discharge of sewage into streams is above the suspicion of a nuisance before adopting this as a suitable method. Rather would it seem better to provide for some method of treatment and allow only purified sewage to go into the stream than to run the risk of being forced in a few years to reconstruct the entire line of outfall pipe, with perhaps an entire reconstruction of the plumbing within the house.

The problem of treatment is the question of so modifying the character of a large volume of dirty water that it shall neither injure the quality of any drinking-water into which it may be discharged, nor cause objectionable odors, nor present disagreeable appearances in any body of water into which it may be emptied.

In order to properly understand a reasonable method of treatment some consideration must be given to the composition of sewage. This is chiefly water with which is mixed a small amount of animal, vegetable, and mineral matter. Roughly speaking, the amount of mineral dirt is about one tablespoonful to a barrelful of water, and the combined amount of animal and vegetable matter amounts to another tablespoonful. It seems almost impossible that so small a quantity of organic matter as one tablespoonful in a barrel of water could cause offense in any way, and yet engineers, city officials, and householders know by bitter experience that, when spread out on the surface of the ground or when allowed to stand in pools, water so polluted will undergo putrefaction resulting in most disagreeable odors and in complete stagnation. The problem of sewage treatment, then, consists in removing from the barrelful of water, the tablespoonful of organic dirt, whether animal or vegetable, in such a way that no odors shall be occasioned by the process and at the same time so that the cost of the process may be a reasonable one.

Unfortunately, the greater part of this organic matter is in solution, dissolved, like salt in water, so that, though undeniably present, it must be removed by some process more complicated and less obvious than that of simple straining. It would be comparatively simple if the polluting substances remained floating or suspended in the water. Then they could be strained out through a fine sieve or settled out in a tank, either with or without the aid of chemicals. But for particles in solution, straining, by itself, is useless and, while in large plants frequent use is made of sieves as a complement to the main process of purification, in small plants it is of so little value as hardly to deserve consideration.

Another factor enters to lessen the value of the use of screens or sieves in an installation for a single house. A great deal of the organic matter found in sewers requires both agitation and time for its subdivision into particles small enough to be acted upon in any process of purification adopted. If a screen is used, large particles of putrescible matter are held on the screen since not enough time has existed to break down their mass, and thus the screen itself becomes a most emphatic disturbance and a most objectionable feature of the purification plant.

For efficient purification, therefore, some method of reducing and modifying the character of organic solids, particularly those in solution, must be selected. In seeking a method by which this may be accomplished, scientific men found years ago that this very process was being carried on continually by natural forces, although at a very slow rate of purification. All organic matter, however formed and wherever present, is subject to the natural forces of decay. Fruits, vegetables, and meats of all kinds, exposed to the air, rapidly lose their original character and form and in the course of time disappear entirely. Except for this provision of nature, the accumulation of organic wastes since the beginning of the earth’s occupation by human beings would be so great that the earth would be uninhabitable on account of the deposits of waste matter which would have formed by this time. Nature, then, recognizes the need of disposing of organic wastes, and her method is the one which apparently must be followed by human beings if successful treatment is to be secured.

Only a few decades ago, it was found that this process of decay was due to the activity of very small organisms known as bacteria, and their agency was proved by experiments which showed that if vegetables or meat were kept free from bacteria, no decay, fermentation, or putrefaction took place. It was proved that the air itself was not responsible because in certain experiments air was allowed to enter through a filtering medium fine enough to strain out the bacteria and no decay took place, although oxygen and air were both freely admitted. It is well understood by the housewife that fruits can be kept indefinitely if they are cooked sufficiently to kill any bacteria present and then sealed in bacteria-free, air-tight jars. When such preserves spoil, it is because some bacteria were left in the jar or have since been admitted through an imperfect top. When decay is allowed to proceed, the obvious result is, first of all, a softening of the material, as in the case of a rotten apple, a liquefaction, as it is more technically known. Following that part of the process is a gradual breaking down of the material, the residue being of an earthy character which is assimilated by the soil into which it falls.

The bacteria required for the putrefaction of organic matter are among the most widely distributed of all the micro-organisms. They are always found in the air, except on mountain tops, in deserts, and over the ocean. They are very numerous in surface waters, such as streams and ponds, and their relative number everywhere increases as the amount of organic matter increases, so that the greater the need for them the greater is their number. It has been found that the great majority of these bacteria require air for their energetic development, and this fact is most important when it comes to the practical construction of a piece of apparatus for making use of these bacteria. It has also been found that, for several reasons, these bacteria work most effectively in the soil and can take care of a larger quantity of organic matter there than elsewhere. This is partly because in the surface layers of the soil, particularly where that soil has been cultivated, a great number of the particular bacteria involved in decay are always to be found. Pure, clean sand from the desert contains almost none of these beneficent bacteria. Rich garden soil is fairly teeming with them, so that, curiously, the more organic matter and the more bacteria present in any soil, the more active that soil will be in taking care of other organic matter.

Then, again, the soil particles, particularly in sandy soil, are so separated as to allow between them a certain and appreciable amount of air, and by means of this air the activity of the bacteria is made continuous and the products of their activity utilized. Without such an admission of air, the bacteria are choked and diminish rapidly in numbers. There is, however, a definite degree of purification and a certain quantity of organic matter which can be taken care of by the bacteria incident to any particular soil. Up to that quantity purification proceeds more or less satisfactorily according to the intelligence shown in feeding the bacteria in such a way as suits their convenience. If, however, that quantity be exceeded, all purification stops, the bacteria are apparently discouraged, and no further improvements can be expected. A fine-grained soil will not be so useful as a coarse-grained soil because the former does not allow sufficient air in the interstices of its soil particles. Another practical reason for not making use of soils of fine grains is that such soils can absorb only a small amount of liquid because of the mechanical construction of the material. On the other hand, soils whose grains are too coarse are undesirable because their mechanical construction is such that the liquids containing organic matter in solution pass through so rapidly that time enough is not given for bacterial action.

As a result of the principles just enumerated, it may be said that there are three distinct and essential conditions for the successful disposal of sewage through the soil. These three conditions are, first, a rate of application suitable to the soil which it is proposed to use; second, an interrupted or intermittent delivery of the sewage so that the bacteria can obtain, between consecutive doses of sewage, the necessary amount of oxygen for their own preservation and well-being; and, third, a resting period in which is carried forward that intimate association between the partly decomposed organic matter and the oxygen or air present in the pores of the soil by which the final oxidation is obtained.

The rate of application varies, as already indicated, with the size of particles found in the soil, and it should also vary with the purification desired. The larger the particles, the higher may be the rate of application, but less efficient will be the process. With grains of sand as fine as 1⁄200 of an inch, and with a rate of application not greater than five gallons per square yard of surface per day, filtration through such an area has been proved to be capable of removing from the foulest sewage all the objectionable material and converting the liquid into what is an equivalent of the purest spring water. If the rate appropriate to this particular soil is exceeded, the efficiency decreases, and the unmistakable and inevitable result is to stop all purification and convert the filter into a stagnant cesspool. If, to take the other extreme, the soil particles are increased until they are as large as hen’s eggs, then, if the rate of application is not greater than 200 gallons per square yard of surface per day, and if the method and rate of application are suitable to this large amount, the resulting effluent is sufficiently freed from its objectionable matter so that the liquid can be turned into any body of water without danger of odors or other nuisance. If this rate is exceeded, or if the method of application is not carefully considered, the resulting effluent is foul in the extreme and the process itself becomes a nuisance.

It can be seen by this brief explanation that it is not possible to assign any particular rate of application to any particular kind of treatment, since in all the methods of purification which have been worked out considerable variation in the details of that particular method have been practised. It will be possible, therefore, in succeeding chapters to indicate by the size of filters recommended only limiting or average values for rates of purification, since those rates are always dependent upon other factors than the particular method being discussed. It must also be remembered that soils may exist which have no porosity whatever, and through which it is impossible for sewage to make its way. Such soils are not available for sewage purification, and, no matter how small the rate or how careful the method of application, such areas will fail to produce any practical purification. Soils like clay, peat, and fine water-deposited silt are of this sort. Clay soils may sometimes become pulverized by cultivation so that they will ultimately be able to take care of a moderate amount of sewage. In such a case it is possible to dispose of sewage successfully in the top six inches of soil which, by continual cultivation, has been made out of the stiff clay. In such cases, the difficulty is not that of oxidizing the sewage, but that of taking care of the effluent, which must be held between the cultivated soil and the raw clay underneath.

The second requirement mentioned is secured by discharging the sewage onto the soil area at intervals, the number of doses per day depending upon the size of particles in the bed. There has been a general principle established that the size of these doses ought to be smaller as the size of the particles increases, so that, whereas in the case of sand beds the total daily dose is usually divided into from one to three parts and each part delivered onto the bed with an appropriate interval, in the case of coarser materials used for sprinkling filters, the time interval between doses is much reduced and in some installations recently constructed in England that interval has been measured in seconds. The variations in the rate of flow of sewage onto any filter, however, are so great that any such requirement as designing discharging apparatus to work at intervals of a few seconds is useless, and if as small an interval as one minute is provided for the coarsest material for the maximum rate of flow at any time of the day, the installation will probably be successful for the lesser rates occurring at other times of the day. As an indication of the way in which this modification is made, it is customary, when the size of soil particles is that of peas, to make the interval between successive discharges about one hour, so that the dose applied at any one time would be equal to 1⁄24 of the daily volume. With gravel filling, the particles being the size of English walnuts, the interval between doses is shortened to five minutes, and the amount of any one dose is thus made 1 about 1⁄280 of the total daily volume. With the coarser filling, as when a size as large as hen’s eggs is used, the interval would be cut down to about one minute. It should be added that the intervals last mentioned are characteristic only of some devices used for dosing sprinkling filters and that there is a wide divergence of practice among engineers when dealing with any particular size of sand or stone particles in all kinds of filter beds.

The third requirement, namely, the occasional resting of the bed, is met by providing some additional area over that theoretically required, so that the flow may be diverted from part to part of the total area (which is usually divided into beds for this purpose), and in this way each part is allowed, in turn, a period for resting. For example, if the required area be divided into two beds and a third bed added equal in area to one of the two and a regular rotation of dosing be practised, each bed would rest not only the time between the regular twelve-hour period dosing, but might also be given a complete rest, occasionally, for an extended period. This third requirement is probably less imperative with the coarser particles and there are many examples of coarse-grained beds which have been continuously operated for a period of years. It is found, however, that with such treatment clogging is inevitable, and that such clogging is partially relieved by a period of rest somewhat proportional to the length of time the beds have been operated. It is, then, only shortsighted policy to economize at the beginning and attempt to save money by not building an additional area, since the clogging of the whole plant is bound to occur in the course of time, and then another plant must be built or the material forming the bed taken out, washed, and replaced. Otherwise the sewage must go unpurified to the outfall while the bed is recovering from the long period of overwork.

It is convenient to divide sewage purification into two processes, the preliminary process and the final, or finishing process, and, while the preliminary process, in itself, never accomplishes purification, yet it is of considerable value in facilitating and increasing the rate and efficiency of that purification. The most common preliminary treatment is sedimentation, by which the larger solids in suspension are allowed to settle in a tank or tanks so that the filter beds later used are relieved from the accumulation of those deposits. Under the name of septic tank such a receptacle for suspended solids has been exploited as a complete method of purification, and many underground tanks have been constructed in various parts of the country which have, at the time of their installation, been considered competent to furnish all the necessary purification. When it is remembered that less than one-half of the organic matter in sewage is in suspension and that the best results in any sort of a tank succeed in depositing only one-half of those suspended solids, it can readily be seen that a tank, whether called septic or settling, cannot be a complete method of treatment. In reality, such a tank does little more than take out from the sewage the greasy material and a certain proportion of the suspended matter. Whatever part of this is organic matter may, by a particular arrangement of the tank, be considerably reduced in quantity, so that the intervals of cleaning can be extended, but in every tank the removal of the deposits is necessary, and subsequent treatment is required if adequate purification is accomplished.

The final, or finishing, process may be carried out according to any one of several methods. It may be done by discharging the tank effluent into a system of agricultural drains laid just below the surface of the ground, called sub-surface irrigation. It may be done by removing the top soil from a bed of sand placed by nature, and needing little except suitable surface distribution to insure the most efficient purification.

For a small plant, instead of a sand filter, for which the sand is found naturally in a suitable location, an artificial filter may be built by preparing an enclosure and carting in sand for filling.

Where no sand is available, or where its use would be uneconomical, broken stone may be used to ensure final treatment. With stone, on account of its large voids, the enclosure must either be water-tight, and the outlet pipe must be provided with a valve or other device so that the sewage under treatment may be held in the enclosure or tank long enough to deposit the solids in suspension and to be acted on by the bacteria concerned. This method is known as the contact bed treatment. Or, finally, the desired results may be obtained by spraying the sewage onto a deep layer of broken stone, the method being called the sprinkling filter treatment.

The choice of the final treatment, in any particular case, depends on the character and slope of the ground, on the availability and cost of sand or of broken stone, and on the amount of sewage to be treated. It is hoped that the following pages will give to the reader both an intelligent appreciation of the advantages and disadvantages of each of the several methods of sewage purification discussed, and also sufficient insight into the necessary details of construction so that the method chosen can be put into successful operation.

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.

Fig. 1.—Plan of Settling Tank.

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.

Fig. 2.—Longitudinal Section of Settling Tank.

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 Inside Width (Feet).Mean Inside Length (Feet).Depth (Feet).[[1]]
4 3 45
8 3 75
12 4 7.55
15 4 85
25 4105
35 4.5125
50 6125
75 6156
100 7176
125 817.56
150 8186
175 8206
200 8226
250–-2 compartments in tank, each5.5186
300–-2 compartments in tank, each5.518.57
350–-2 compartments in tank, each6197
400–-2 compartments in tank, each6198
450–-2 compartments in tank, each6228
500–-2 compartments in tank, each6248

[1]. 12 inches greater than depth of sewage.

Fig. 3.—Sketch of Settling Tank with Longitudinal Partition Wall.

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.

Fig. 4.—Forms Used for Building Side Walls for Concrete Tank.

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.

Fig. 5.—View of Settling Tank, Showing Baffles, Sludge Pipe, Drain Pipe, and Inlet and Outlet Pipes.

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.

Fig. 6.—Section Showing Tank with Concrete Roof and with Form for Constructing 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.

Fig. 7.—Form for Manhole Opening.

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.

Fig. 8.—Plan and Longitudinal Section of Modified Imhoff Tank.

Fig. 9.—Vertical Cross-Section of Modified Imhoff Tank.

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.

CHAPTER III
VALVES, SIPHONS, AND SIPHON CHAMBERS

It was explained in Chapter I that one of the essentials of successful sewage purification is an intermittent application of the sewage to the beds in which bacteria are to act. This intermittent action is secured by providing a small additional tank or by setting aside a part of the settling tank and by installing therein some kind of mechanism for the purpose of changing the more or less regular flow into an intermittent or periodical flow. The proper capacity of this tank will be considered later in the chapters dealing with the several methods of final purification. Now it may be said only that the size depends on the amount of sewage to be cared for per day and on the size of the dose demanded by the purification method. The size of dose depends directly upon the method of treatment and on the size of the particles in the beds intended to receive the sewage. On sand beds, for example, it is customary to discharge the sewage from the dosing tank three times a day, although many plants operate with a daily discharge. The size of the dosing tank, however, in the latter case has to be three times as large as in the former, and it is usually worth while to take the additional trouble of having more frequent operation in order to save the cost of the larger tank.

The simplest method of construction of the dosing tank is to make it a part of the sedimentation tank by means of a cross wall, which latter must be strong enough to withstand the pressure of the water on one side when the dosing tank is empty. (See Figs. [2] and [3].) There is no objection to this tank being separate and some distance away from the sedimentation tank, and sometimes, for convenience in distributing the sewage from the dosing tank onto several beds in turn, the dosing tank is placed at the centre of a group of beds with the settling tank outside. If the dosing tank is a part of the main tank, the sewage flows into it over a dividing wall between the two tanks or through a pipe laid through this wall, while if the tank is separate from the other, then a longer pipe connection is required.

Fig. 10.—Sludge Valve for Floor of Tank.

It is economical to arrange that the level of the sewage in the dosing tank, at the time when that tank discharges, shall be at the level of the sewage in the settling tank, since then no head is lost. It is better still to arrange the mechanism in the dosing tank so that the level of the sewage there at the time of its discharge will be from four to eight inches higher than the normal level in the settling tank. The effect of this is to back up the sewage and raise the general level in the settling tank, and when the dosing tank discharges there is drawn off not only the sewage in that tank, but also an amount in the large settling tank equivalent to that which is above the normal level of the sewage there. The advantage of this is plain in that it reduces the necessary volume of the dosing tank by that of the back water in the settling tank, and, while it was thought at one time that such a frequent variation in the level of the main tank might affect injuriously the scum which forms there, and perhaps also the bacterial action going on in the tank, there seems to be no real reason why this method may not be used with considerable advantage in economy of construction.

The bottom of the dosing tank, which is preferably made of concrete, should have a slope toward the point from which the outlet pipe leads, thus enabling the outward rush of sewage to carry off any material which would otherwise settle in the bottom and perhaps decompose there.

Fig. 11.—Sludge Valve for Side Wall of Tank.

The simplest method of operating the dosing tank is by means of a hand valve fastened either to the floor of the chamber or to the bottom of the outside wall. Fig. [10] shows a simple form of a valve suitable for the floor and intended to be operated by a rod extending up through the sewage to the outside air. Such a valve can be made at any local foundry, the bearing surfaces turned up in any machine shop, and a piece of leather for packing purchased at any hardware store. Such a design, however, is not suitable for a large valve or for a great depth of water, since the pressure on the valve is dependent on the weight of the column of water acting on its area. If the outlet pipe is six inches in diameter, the diameter of the upper surface of this valve would be about ten inches, and the area of the top of the valve would be about half a square foot, so that, with six feet of water above, weighing 62½ pounds per square foot, the weight on the valve to be lifted would be 186 pounds, rather more than could be lifted by one man. Under such conditions it would be necessary, using such a valve, to rig a lever, the fulcrum being fastened to the edge of the tank, the short end of the lever to the rod, and the long end so arranged as to reduce the load in the ratio of about one to four. Fig. [11] shows another type of valve intended to be set into the side of the tank with the floor sloping rapidly toward the low point at which this valve is set. These valves require better workmanship and are preferably purchased from one of the dealers in valves who make this type as one of their regular stock forms. Fig. [12] shows the design made by the Coffin Valve Company, of Troy, N. Y., and a similar form of valve is made by the Caldwell-Wilcox Company, Newburg, N. Y. For a six-inch pipe, these valves are so made that the danger of the moving parts rusting together is avoided by having one surface bronze or some similar noncorrosive metal. Fig. [13] shows an ordinary gate valve generally used for water works, but applicable to sewerage works. Such a valve is shown in Fig. [5].

Fig. 12.—Sluice-Gate Valve made by Coffin Valve Co.

Fig. 13.—Ordinary Gate Valve.

Fig. [14] shows a form made in England and largely used as a cheap valve for the purpose of emptying a tank rapidly. The peculiarity of the valve is that a sidewise motion of the long handle locks the valve into position so that the moving part of the valve may be readily set at any height. The one shown in the figure is taken from the catalogue of the Adams Hydraulic Company, Westminster, London, and is listed in their catalogue at $6.50 for a six-inch pipe.

Fig. 14.—English Slide Valve with Wedge-lock Handle.

Fig. [15] shows another type of valve which is supplied by some firms making sewer pipe and consists, as may be seen, of a light moving valve which is attached to a projection cast on the top of the vitrified tile pipe in such a way that the valve comes to an even seat on the bevelled end of the pipe. It is found that with pressure acting against the valve the thin metal of which it is composed is pressed against the pipe so that little, if any, water or sewage will escape. The valve can easily be opened by attaching a cord or chain to the ring at the lower edge of the valve, and when released the valve shuts automatically. This is a very cheap and convenient design, and answers every purpose for emptying tanks by hand.

More elaborate structures of the same general type have been made, using cast iron as the metal, the stationary collar with the bevelled end being built into the masonry wall of the tank. This type of flap valve is faced with bronze, and the bearings or joints have bronze bushings. A satisfactory valve of this sort can be made at a local foundry and machine shop, but there is danger that the valve will not be water-tight. Fig. [16] shows such a valve with the metal seat which is intended to be bolted into the masonry of the tank wall.

Fig. 15.—Flap Valve Attached to Length of Sewer Pipe.

Fig. [17] shows another form of this same sort of valve, taken from the catalogue of the Adams Hydraulic Company, and noteworthy because of the loose-link connection at the upper part of the valve, the object of this being to prevent the valve closing at the upper part without, at the same time, closing at the bottom.

If the dosing tank is to work automatically and independently of human agency, an arrangement which is always preferable, there must be installed some mechanism which takes the place of the valve operated by hand. This mechanism is in almost every case a siphon which is put into action when the water level reaches a certain height, and which discharges rapidly until the water falls to a point when air is admitted to the inside of the siphon pipe, thereby interrupting the flow.

Fig. 16.—Flap Valve with Metallic Seat Attached.

There is on the market a dosing apparatus which does not involve a siphon, and which is shown in Fig. [18]. This is made by the Ansonia Manufacturing Company, 30 Church Street, New York City, and its operation may be described as follows: It consists of two floats connected by means of a chain which passes over a wheel supported in the upper part of the chamber. As the water in the chamber rises, the left-hand float shown in the drawing rises and the right-hand float falls, thereby communicating a rotary motion to the wheel. A projection on this wheel at a certain point when the left-hand valve has reached the desired height communicates with an inside portion of the wheel, to which a chain connected with the valve is attached. Thus the valve is opened at the right height, and remains open until the water has fallen to the bottom of the chamber. Then the left-hand float falls, and the apparatus is ready to repeat the operation. This apparatus, for a small installation, will probably cost, set up in place, about $15.

Fig. 17.—Flap Valve with Loose-link Hinges.

Fig. [19] shows the simplest form of siphon arranged to discharge water from a tank. It will be noticed that it consists of an inverted bent pipe, one leg being longer than the other, and extending into a pool of water formed in the end of the discharge pipe. When the water level in the tank reaches the bent portion of the siphon pipe, the water begins to flow out, and will continue to flow until air is drawn in at the lower end of the short leg. This stops the flow and the tank begins to fill again.

Fig. [20] shows another method of working the siphon and insuring its rapid initial action. This is known as the Van Vranken flush tank, and the feature of this arrangement is the movable bucket, which in one position seals the lower end of the longer leg. Then, however, the siphon begins to act, and the bucket, which is hung on trunnions, is disturbed and its contained water is dumped out. This allows the escape of the water in the longer leg and insures a vigorous starting up of the siphon into action.

Fig. 18.—Intermittent Dosing Apparatus made by Ansonia M’f’g Co.

A more simple type, however, is the inverted siphon arrangement, developed perhaps most completely by the Pacific Flush Tank Company under the Miller patents. Fig. [21] shows their ordinary design, the upper part of the siphon being replaced by a bell and the discharge starting when the level of the water in the long leg of the siphon has been depressed sufficiently to reach the curved part of the pipe. The principle on which this siphon works is as follows:

Fig. 19.—Simplest form of Automatic Siphon.

When the water rises in the tank above the lower edge of the bell, the air which remained between the water in the siphon pipe and in the bottom of the tank is confined, and, as the water rises, is gradually compressed. The effect of this compression is to force down the water in the long leg of the siphon and to hold down the level of the water inside the bell lower than the level outside. When sufficient head of water in the tank is secured, the water inside the pipe will be forced down to the curved part of the pipe, and, the siphon being so designed, the water level inside the bell will be just at the top of the same pipe, but on the outside. Any slight additional height then allows the contained and compressed air to escape around the bend in the pipe, suddenly relieving the pressure and allowing the water to enter the pipe from under the bell readily. Thus the siphon starts and continues to flow until the water level falls so that the air is drawn in under the bell. That stops the action of the siphon and the tank fills again. These siphons are generally sold in two pieces, the cast iron bell and the curved pipe being the factory product. At the plant they have to be set in place, generally bedded in concrete and properly connected with the outlet pipe. For a small installation a three-inch or four-inch siphon is ample, and will cost, delivered, from $10 to $15, depending on the freight.

Fig. 20.—Van Vranken Automatic Siphon.

Fig. [22] shows two siphons with auxiliary air-pressure chambers installed in the same chamber for the purpose of automatically diverting the flow from one bed to another. This may be done more simply by installing two ordinary siphons of the Miller or similar type. If one of these siphons is filled half full when the tank is empty, that siphon will discharge first because of the amount of water already present in the U-shaped tube. During the filling of the tank previous to its discharge, the other siphon will partially fill, so that when the tank begins to fill for the second time the second siphon is half full and the first nearly empty. In this way alternate action is secured and the discharge takes place as often as the tanks fill.

Fig. 21.—Miller Automatic Siphon.

Fig. 22.—Double Alternating Siphons of the Miller Type.

Fig. [23] shows three similar siphons installed with some auxiliary piping attached for the purpose of making the periodic discharge more positive. These small auxiliary pipes are so put together that there is an auxiliary siphon passing under the edge of the bells. When one siphon discharges, the auxiliary siphon of the corresponding large siphon is filled with water, and at the same time part of the water in the auxiliary siphon of the other is discharged, so that it will be the first to operate at the next filling. When the water is forced to the bottom of the small siphon, it is blown out through the vent pipe, and, the air following, the large siphon is started.

Fig. 23.—Triple Alternating Siphons of the Miller Type.

Fig. 24.—Single “Merritt” Automatic Siphon.

Fig. [24] shows an automatic discharging siphon made by the Merritt Company, of Camden, N. J., and embodying a different principle. The main discharge pipe is built in the form of a “U” tube, the longer leg containing an auxiliary small air pipe, with a return bend at its lower end. When the chamber starts to fill, this small pipe bend or seal is filled with water, so that the rising water confines and compresses air in both the large and small “U” pipes. In time, and at any desired height, determined by changing the relative lengths of the parts of the small pipe siphon, the seal is broken and the air escaping draws air enough from the large pipe to start it in action. The method has an advantage in that it requires no deep excavation, and the mechanism can be set after the siphon chamber is built.

Fig. 25.—Air-lock Siphon for Admitting and Releasing Sewage from each one of Four Beds in Regular Order.

Fig. [25] shows a method of securing the alternate discharge of sewage by siphons whose action depends upon an air trap, each siphon being of the type shown in Fig. [24]. The installation of the figure is further complicated by the fact that it is arranged to discharge sewage from the four contact beds as well as to discharge sewage onto the beds. The compartments, and the piping connected therewith, at the four corners operate to admit the sewage from the central channel onto the four beds in rotation. The four square wells between the corner wells operate to empty those beds in turn into the pipe shown at the centre of the drawing, the pipe leading to the nearest stream. The operation may be described as follows: Sewage enters at the top of the drawing, and from the inlet channel flows into the siphon channels marked A. A1 is ready to discharge if bed No. 4 was the last one to fill, since, when that bed filled, the small bell D4 forced the siphon A1 open. Sewage therefore flows through siphon A1 into bed No. 1. As the sewage level rises in bed No. 1, the outlet siphon from bed No. 2, G2, is locked by the air pipe from B1 so that bed No. 2 will be ready for the next dose. Also the air pipe from the bell H1 opens the siphon G4, and allows bed No. 4 to drain into the outlet drain. Also bell D1, when bed No. 1 is full, opens the siphon A2 through the connecting air pipe so that bed No. 2 begins to fill as soon as bed No. 1 is full. And finally bell C1 locks the siphon A1, and stops further flow into bed No. 1. The other beds operate in the same way in turn.

The manufacturers of siphons are always glad to advise prospective buyers of the proper arrangement of siphons and the details of placing, with dimension sketches.

As a summary, it may be pointed out that in any installation, one of the three methods above described may be adopted.

1. A simple valve worked by hand may be adopted and the alternate distribution of the sewage regulated by choice of the several valves placed at the head of the several discharge pipes.

2. An automatic discharge mechanism may be installed which will operate regularly and intermittently, but lacking any automatic selection of the bed onto which the discharge is to be made. These siphons will discharge as often as the tank fills, but the particular valve must be opened in order that the discharge may take place onto any one of the several beds.

3. An apparatus may be installed which will both discharge intermittently and will also automatically select different beds in turn onto which the discharge shall take place. It may even discharge onto contact beds and also empty those beds, entirely automatically.

Which of these mechanisms shall be selected depends upon the amount of money available and on the value to be placed on the freedom from constant care which an automatic installation gives. Not that a sewage-disposal plant may be ignored because an automatic mechanism has been installed. No machine is infallible, and sewerage machinery may give out or stop working just as that for any other purpose. But instead of a daily routine of duties which may not be interrupted, by means of automatic apparatus one may avoid everything except casual inspections and periodic cleaning.

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 of Persons Served by Sewer.Total Length of 3–inch Sub-surface Tiling.No. of Sections in Sub-surface System.Mean Width of Siphon Chamber (Feet).Mean Length of Siphon Chamber (Feet).Depth of Siphon Chamber from Roof of Tank (Feet).Diameter of Siphon (Inches).Discharging Depth or Maximum Depth of Effluent above Lower Edge of Siphon Bell (Inches).Distance from Roof to Top of Wall between Settling Tank and Siphon Chamber (Inches).
414022´2´2´ 4˝3˝13˝12˝
828022´ 6˝2´ 6˝2´ 4˝3˝13˝12˝
1242022´ 6˝4´2´ 4˝3˝13˝12˝
1552523´ 6˝4´2´ 4˝3˝13˝12˝
2587523´4´3´ 2˝5˝23˝12˝
35122523´ 8˝4´ 6˝3´ 2˝5˝23˝12˝
50175024´6´3´ 2˝5˝23˝12˝
75262523´6´3´ 2˝5˝23˝16˝
100350024´7´3´ 2˝5˝23˝16˝
125437535´ 6˝8´3´ 2˝5˝23˝16˝
150525037´8´3´ 2˝5˝23˝16˝
175612537´ 6˝8´3´ 9˝6˝30˝16˝
200700038´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 of Persons Served by Sewer.Total Length of 3–inch Sub-surface Tiling.No. of Sections in Sub-surface System.Mean Width of Siphon Chamber (Feet).Mean Length of Siphon Chamber (Feet).Depth of Siphon Chamber from Roof of Tank (Feet).Diameter of Siphon (Inches).Discharging Depth or Maximum Depth of Effluent above Lower Edge of Siphon Bell (Inches).Distance from Roof to Top of Wall between Settling Tank and Siphon Chamber (Inches).
43002 2´ 5˝ 3´2´ 4˝3˝13˝12˝
86002 3´ 5´2´ 4˝3˝13˝12˝
129002 4´ 5´2´ 4˝3˝13˝12˝
1511252 4´ 6´ 6˝2´ 4˝3˝13˝12˝
2518752 4´ 6´3´ 2˝5˝23˝12˝
3526252 4´ 4´ 6˝3´ 2˝5˝23˝16˝
5037502 4´ 6˝ 6´3´ 2˝5˝23˝16˝
7556252 6´ 7´3´ 2˝5˝23˝16˝
10075002 7´ 8´3´ 2˝5˝23˝16˝
12593753 8´10´3´ 9˝6˝30˝20˝
150112503 9´12´3´ 9˝6˝30˝20˝
17513125310´12´4´ 2˝8˝35˝20˝
20015000311´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.

Fig. 26.—Plan and Section of Sub-surface Irrigation System.

Fig. 27.—Plan and Section of a Portion of a Sub-surface Irrigation System.

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.

Fig. 28.—Y-Branch of Vitrified Tile Pipe.

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].

Fig. 29—Eighth Bend of Vitrified Tile Pipe.

Fig. 30.—Sub-surface Tiling.

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.

Fig. 31.—Photograph of Tile laid as if for Sewage Disposal.

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.

Fig. 32.—Sub-surface Tiling with Broken Stone or Gravel Surrounding Pipe.

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.

Fig. 33.—Sub-surface System on Irregular Ground.

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].)

Fig. 34.—Special Casting of Double Y-Branch with Swinging Gate.

Fig. 35.—Double Y-Branch with Valves on Branches of Main Carrier.

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.

Fig. 36.—Sub-surface Tiling System with Underdrains.

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.

CHAPTER V
SEWAGE FILTERS

It has been shown that the selection of the type of plant best suited to solve the sewage-disposal problem at any given place depends on several factors and can be safely made only after a consideration and study of such local conditions as the character of the soil, the area available, the presence and nearness to the surface of ground water, and the local topographical conditions. If sub-surface irrigation is not feasible, when, for instance, the soil is nearly impervious to water or when, in the case of a wet soil, adequate underdrainage is not possible, some form of artificial filter must be constructed to complete the reduction of the sewage where the effluent from the settling tank may not properly be discharged directly into a stream.

If such a filter is to be constructed, the kind most suitable depends, in turn, upon several factors, such as the degree of purification to be attained, the suitability of the available areas or locations for the different types of filters, the operating head or fall available, and the relative cost of the sand, gravel, broken stone, or furnace slag which may be used as material for the filter bed.

With respect to the degree of purification of sewage that is desired it may be said that, of the three general methods of sewage purification, namely, intermittent sand filtration, treatment in contact beds, and filtration through sprinkling or trickling filters, the first method produces the most highly purified effluent. Such an effluent, if from a properly constructed and operated sand filter, may generally be considered sufficiently purified to allow its discharge into a stream, even if the stream is subsequently used as a source of potable water supply. In some instances, however, subsequent sterilization or disinfection of the effluent may be required, particularly if the waterworks intake is relatively near the point of discharge from the sewage filter, or if the flow of the stream is small in comparison with the sewage flow. However, if a stream used as a source of potable water supply receives the effluent from a properly operated sand filter, the further safeguarding of the quality of the water should generally be accomplished by filtration or sterilization of the water supply, or both.

In many cases the local conditions are such that contact beds or sprinkling filters may be constructed more easily or more economically than sand filters and, at the same time, the lesser efficiency of the contact bed or the sprinkling filter, owing to the fact that the stream is not used for water supply, may not preclude the adoption of these latter types of plants.

However, where natural deposits of sand of not too finely divided particles occur or where such sand may be readily procured, intermittent sand filters are most satisfactory for the final treatment of sewage.