H. Burdett Cleveland

“ ESTIMATES OF COST In order to estimate the cost of installing a sewage-disposal plant and of treating continuously the sewage from any residence, certain fundamental assumptions are always necessary. In the first place, the unit cost of the manual labor which forms so large a part of the total cost of construction must be known for the particular time and place, and perhaps no item in the cost of construction is so important as this. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ In order to plant vegetables on such a hill, small furrows may be made along the hill and laid out with great care so that the flow of sewage in the furrows shall be only at a slow velocity, so slow that the soil can absorb the moisture as the water passes along. By zigzagging this furrow back and forth down the hillside, vegetation on the hill will receive the benefit of the water, and if any of the sewage succeeds in reaching the bottom of the hill, it will be so purified that it may be safely discharged into any depression or watercourse there found. ”

“ Irrigation, of course, cannot prevent rainfall, and it may be that after a copious soaking of the ground with the irrigating water a heavy rain may follow, resulting in an excess of moisture as bad for the ground as none at all. The possibility of irrigation cannot prevent excessive rains at the time of harvest, but the advantages of being able to control the soil moisture during the period of growth are more than enough to counterbalance any possible disadvantages. ”

“ 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. ”

“ Inasmuch as the variation in sizes of the individual particles of rounded gravel is such that a dense concrete results naturally, it is quite reasonable both to increase the amount of stone and decrease the amount of cement if that variation in size seems to be one which will produce a dense mixture. Thus one cubic yard of stone, one-third cubic yard of sand, and four bags of cement may be used and will, under favorable conditions, result in a good concrete. ”

“ 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. ”

“ In the case of any sewage-disposal project of considerable magnitude, where any doubt exists as to the suitability of the sand available for use in sand filters, analyses of representative samples of the sand should be arranged for, and competent engineering advice should be sought before any large outlay is incurred. In general, however, it may be said that any clean, sharp sand suitable for building use is suitable for sand-filter beds in any situation. ”

“ 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. ”

“ The sewage from an ordinary household, on the basis of 10230 gallons per head per day, amounts to 180 gallons per day, or about 5,400 gallons per month, or 720 cubic feet. This amount of water would cover an acre of ground to a depth of a little less than one-fourth of an inch, and it is plain that in order to have the sewage of a single house furnish the necessary amount of water for successful crop growing, the area required is only about one-twelfth of an acre, or an area about 60 feet square. ”

“ In most cases, ordinary blasting powder is suitable for rock excavation in small quantity. It lifts the rock rather than shatters it, and is more convenient and safe to handle. Forty-per-cent dynamite is to be recommended where the rock is very seamy so that quick-acting explosive is essential, and also where the rock is very hard, so that black powder tends to blow out the hole rather than to shatter the rock. ”

“ For sand filters, either natural or artificial, the tank must be emptied as with sub-surface irrigation, and, in addition, the surface must be scraped occasionally, and at the approach of winter furrows must be dug. Perhaps two days’ time would be all that would be needed for a plant dealing with the sewage of a single family. A broken-stone bed requires no attention for seven or eight years, and then the stone has to be shovelled out, washed, and replaced. ”

“ In the smaller plants accommodating up to one hundred and fifty persons, it is hardly necessary to provide for more than three beds, thus allowing opportunity for each one to rest for one week in every three to six weeks, which will result in a temporary increase of fifty per cent in the rate of application of effluent to the remaining beds. In the case of the larger plants, especially if they are to be operated continuously, it is better to construct five beds so that two pairs of two beds each may be used alternately, leaving one bed, or twenty per cent of the total area, out of use. ”

“ The cost of shovelling sand through a screen depends upon the amount of material which has to be rejected, since only a certain proportion of the sand is available for all that is shovelled. The cost of this shovelling is again about fifteen cents per cubic yard of material shovelled, and if one-third of it is coarse gravel which has to be rejected, one and one-half yards would have to be screened for every yard of sand available, and the cost would, therefore, be twenty cents for screening, a total cost of sand in the wagon of thirty-five cents per yard. ”

“ He places the limits of population, the sewage from whom would be cared for on an acre, between 1,000 persons per acre and 100 persons per acre. More recent experience, together with constant observation of farms established in the early period of the practice, indicates that the higher value is too great, and that where agricultural processes alone are considered, 100 persons per acre is a suitable maximum value for irrigation on sandy loam, and that 40 persons per acre is a suitable number where the soil is inclined toward density and fine texture. ”

“ Between the ranges of porosity of soil represented by these limits there are many soils in which sewage may be successfully disposed of 58by 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. ”

“ In the first case, nine lineal feet of hole would have to be drilled in order to get one cubic yard of rock, nearly double the amount required where the hole is five feet deep. Usually the distance between the holes is made equal to the depth of the holes, although in some rock the depth can, with advantage, be made greater than that distance. If the rock is very loose and seamy, deep holes may sometimes not be warranted, because the effect of the blasting is taken up by the loose rock in such a way that the value of the explosive is not realized. ”

“ 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. ”

“ In a great many small installations it may be excluded altogether, since all the hand work required is contributed by the householder at such times as the other work of the place may allow, without any additional cost. In other places, if a money value be placed on such labor, it may be expressed in terms of the cost of a hired man whose rate of wages, paid monthly, in addition to board, would be always less than if wages were paid to day laborers living at their own homes. ”

“ Happier still may he be when he realizes that he need have no ‘off years’, and he knows that the waters he admits to his fields at will are freighted with rich fertilizing elements usually far more valuable to the growing crops than any that he can purchase and apply at a costly rate—a cost that makes serious inroads upon the profits of the majority of farmers cultivating the worn-out or deteriorated soils in the older States 100year by year. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ Material left over has to be hauled away, and in order to leave the plant in an attractive dress, seeding or sodding the earth slopes is desirable. It is even desirable to plant shrubbery around the edges of the beds, partly as a screen and partly to minimize the offensive suggestions which seem to be inseparable from any plant dealing with sewage. The cost of these final improvements may be as little or as much as the owner and builder chooses, but it is urged that their value should not be overlooked. ”