Methods of Applying the Water
In distributing the water or sewage over the soil in the case of a single house, no elaborate methods are required. In the case of large farms supplied with sewage from a considerable population, elaborate systems of piping or open-channel conduits are required, and the problem of working out and adjusting the necessary sizes and grades becomes a complicated matter for which engineering knowledge and experience are required. But for the small flow which comes from individual houses and from the small area involved, no such elaborate preparations are required. The essence of the distribution consists in carrying the water onto the field to be irrigated at such a low velocity that no surface soil or valuable manures are washed away; and in adjusting the volume of the flow and the requirements of the soil, there are three characteristic conditions which require different treatments.
In the first place, the area may be practically level and the crop raised may be either grass or grain. In such a case the sewage should be led onto the field which may properly be enclosed on four sides with a low, that is, six to twelve inches, earth-dike, and at each irrigation the field may be flooded about two inches deep. The next irrigation would probably not be required for a week, so that this method requires a number of beds to be worked one after another and, except where the soil is very dense, so much so that percolation is very slow, this method is not suitable because of the slow rate at which the sewage is delivered.
The second method of distribution, and one more suitable for the conditions under discussion, is to lay out the field in parallel beds from three to six feet wide and from forty to one hundred feet long. These beds are separated by furrows into which the sewage is discharged. If the grade of these furrows is properly adjusted to the porosity of the soil, that is, made about six inches in one hundred feet for open, sandy loam, and about two inches in one hundred feet for fine, clay loam, the soil will absorb the needed moisture as the sewage flows over it and there should be no ponding or excess of water at any point of the field. By dividing the field into three parts, or in arranging the flow of sewage so that it enters only two or three furrows at a time, the flow can be so changed from day to day as to furnish all parts of the area with the irrigating water, and at the same time not overload and choke the soil particles. On the beds may be planted and grown whatever vegetables are desired. A good basis for determining the area and length of furrows required is to provide a length of thirty feet of furrow for each person of the household. The total length thus obtained should not, however, be made continuous, but should be arranged in three parts, or in multiples of three, so that one-third of the total length only may be used on any one day, the other parts serving for other days, so that a rotation is practised.
Fig. 48.—Distribution of Sewage and Arrangement of Check Levees on a Hillside.
The third condition involves the application of the sewage to a steep slope, and this may be treated in either one of two ways. The sewage may be led to the top of the hill and allowed to flow, for a short distance only, over the surface on which, presumably, grass is to be grown. If the length of the furrow is more than about a dozen feet, the flowing stream acquires enough velocity to wash the surface and to form gullies. To prevent this, a secondary ditch or small bank is thrown up to arrest the flow. The water is led out again from behind this ditch or bank at intervals, to repeat the process further down the hill (see Fig. [48]). If the slope of the ground is moderate, so that there is no tendency of the water to form gullies, the water may be let out of the ditch at intervals and allowed to distribute itself over the field, as shown in Fig. [49]. The water thus overflowing should be collected in a drain at the lower end of the slope, and will be found suitably purified for discharge into any running stream not used for drinking purposes. The occasional use of a shovel or hoe may be needed to change the flow of the water over the field if it is found that any tendency exists for definite channels to be formed.
Fig. 49.—Distribution of Sewage on a Hillside of Moderate Slope.
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.
In the case of orchards, irrigation is practised by flooding the ground around the tree, being careful, however, to throw up a mound of earth around the tree so that no water comes within two feet of the tree itself. Fig. [50] shows a Western method of forming square beds, each bed about twenty feet on a side, with one tree at the centre. Furrows are also used to distribute the water, a common practice followed being to being to have the furrow always under the extreme edge of the foliage, thus discharging the water in the vicinity of the tender rootlets of the tree. Usually the furrow system is carried only in one direction, so that the application of water by this method is not so complete as by the flooding method. But for small volumes of water constantly applied, it is probably more satisfactory. Fig. [51] shows a grain field irrigated by the furrow method.
Fig. 50.—Square Beds for Orchards According to Some Western Practice.
In all cases where irrigation is practised, stress is laid by those experienced in the matter on the necessity of cultivation of the soil in connection with the irrigation. Apparently, there is a tendency for the surface layers, with the application of water, to cake or crust over the lower strata, thus depriving the soil of the necessary air. In order to break up this crust, the soil must be continually worked, either by a hoe or rake or some sort of horse cultivator. Where the ground is shaded, as in the case of land covered with grass or grain, the tendency to crust is not so marked, but on cultivated land where root crops are grown the cultivator must be used regularly after each irrigation. Where the sewage is carried onto the field in furrows, the soil in the furrow should be hoed at frequent intervals, not only to break up the crust which deprives the soil of the air, but in order to open the particles of soil for the reception of the irrigating-water.
Fig. 51.—Grain Field in Spring, in Process of Irrigation.
It must also be remembered, as has been pointed out before, that the success of any method of applying sewage to soil depends upon the frequent change from bed to bed, the actual time interval depending on the character of the soil. If the soil is fine, the same area may be used for a week at a time, and then given two weeks’ rest. If the soil is more open, this interval should be reduced, and with very coarse particles it may be found desirable to shift the flow from one bed to another after an interval of a few hours only. Experience and careful observation on the moisture-carrying capacity of the bed is the best guide to the operation of sewage irrigation.
Whether or not this method of disposing of the sewage of a single house is to be selected depends largely upon the slope of the ground from the house toward the garden. It is not desirable to have sewage exposed to the air in the immediate vicinity of a dwelling-house. Rarely would any odors be generated to such an extent as to be offensive to the occupants of the house, since the sewage sinks into the ground before putrefaction of the organic matter sets in and the exposed material left on the surface of the ground is of too attenuated a type to become offensive even if it does putrefy before drying. There is, however, the danger of odors being formed where distribution is imperfect and where pools are allowed to form in the furrow. There is also the danger of the transmission of disease germs from the sewage-irrigated field to the occupants of the house through the agency of flies. Health statistics of English farms show this danger to be a very remote one, since the health of the workman on those farms is as good or better than the average throughout England. But the possibility of infection exists and must not be overlooked.
No method of disposal requires so much and such constant care, although the results show in the improved yield from the farm. This method of broad irrigation is emphatically not the method to be used except where labor is adequate for proper soil cultivation and where this labor can be given constantly and ungrudgingly. Finally, it must be pointed out that care should always be exercised to prevent irrigating sewage coming in direct contact with any of the soil produce. Certainly sewage should not be used to sprinkle over lettuce or celery or strawberries, even if the yield is thereby increased. Undoubtedly any disease germs thus distributed over the fruits and vegetables would, through the antiseptic action of the sunlight and air, soon be destroyed, but the very method of irrigation is repulsive, and the danger, while slight, is sufficient to forbid that method of fertilizing. No statistics, however, are available to show that cows eating sewage-irrigated grass are adversely affected in health, and for years the practice of thus pasturing cows has been carried on in England. For human beings, however, vegetables grown in soil that is separated from the sewage by a foot or more is the safer as well as more æsthetic arrangement.
CHAPTER VII
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. 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. Again, in the southern part of the country labor may be had for $1.25 a day, whereas in the central portion of the United States it is necessary to pay $1.75 a day, and in the extreme West from $2.00 to $3.00 a day for common labor. Often, too, the working day is of different length in different parts of the country. In the estimates which follow, labor is assumed to cost $1.60 for eight hours’ work, that is, at the rate of twenty cents an hour. If, in adapting the estimates of this chapter to any particular installation, the question of labor may be neglected because of the fact that the householder will himself do all the required work, then the item of labor cost may be eliminated. If other units than those here assumed are suitable for the particular locality where any plant is to be built, then the labor item must be modified accordingly.
Material.—The cost of material always varies very greatly in different parts of the country. This is partly because of different freight and other transportation rates between the factories where material is made up and the particular place where that material is to be used; and partly because the profits made by the middleman increase as the material gets further and further away from the centres of civilization. Thus, in a large city six-inch sewer pipe may be sold in such large quantities that the freight rate is low and the dealer is satisfied with a small profit on each foot of pipe. In the country districts the dealer sells but little, and feels that he must have a larger profit to compensate him for the expense of keeping the material on hand. Thus, six-inch sewer pipe may be had at prices ranging from six cents up to sixteen cents per running foot, depending on the store from which it is bought.
It is evident, therefore, that it will not be possible to name any unit price which will be generally applicable, and it will be necessary for any intending builder to secure from local firms the unit prices from which his own individual estimate may be made up. The following discussion, however, will indicate the items comprising the necessary estimate, and will furnish an example by which the estimate sheet can be prepared.
Laying Sewers and Drains.—The main drain from the house to the sewage disposal plant is of five- or six-inch pipe generally, the former being sufficiently large and a little cheaper than the six-inch pipe. The latter has the advantage of size and consequent greater freedom from clogging. The cost of five-inch pipe at a store in a village of any considerable size should be ten cents per foot, and the cost of six-inch pipe twelve cents per foot.
This pipe weighs twelve and fifteen pounds per foot respectively, and, with an ordinary wagon, fifty feet of six-inch pipe, weighing about eight hundred pounds, is a load; if four trips a day are possible from the residence to the store and if the cost of the team is estimated at $4 a day, each trip will cost $1, and each foot of pipe will cost two cents more for being hauled from the store to the grounds.
In laying the pipe, cement and sand are necessary for joints. For both kinds of pipe there is required about one cubic foot of mortar for each fifty joints, the mortar being sufficient to fill the joints and to make a collar or ring outside. In order to make this cubic foot of mortar, half a bag of cement and half a cubic foot of sand will be required. The cost of the cement out of the village store is about fifty cents a bag, although in a small place it may be seventy-five cents, or even one dollar. If one were buying cement in large quantities, a price as low as thirty cents a bag might be had. If the cement is delivered in cloth bags, a rebate of ten cents a bag is usually given if the bag is returned in good condition.
The cost of sand is usually dependent upon the cost of hauling. It will require forty minutes to shovel one yard of sand into a wagon, or at twenty cents an hour it would cost about fifteen cents. 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. If four loads of sand can be delivered per day, with a cost of fifty cents per hour for team and driver, the sand will cost $1.35 per yard on the grounds, this amount being increased or decreased if the number of trips per day must be made less or more.
Excavation.—The cost of excavation depends on the character of the material and on the amount of water present, the cost of pumping or bailing the latter, if in large quantity, adding materially to the cost of shovelling. The material through which the trenches are driven may vary from a sand which can be shovelled without loosening, to solid rock which must be blasted, an intermediate condition of soil being known as hardpan and its excavation costing nearly as much time and effort as rock itself. If the soil is sand, into which a shovel or spade can be pushed without any picking of the material, the cost, as already stated, will be about fifteen cents a cubic yard for shovelling, and if the excavation is in trench and not more than six feet deep, the entire trench can be excavated for seven and a half cents a lineal foot. It is very unusual, however, to have conditions so favorable that such a low price can be counted on. If the material requires picking, instead of fifteen cents a cubic yard it will cost thirty cents a cubic yard, and a trench two feet wide and six feet deep will cost fifteen cents a running foot instead of seven and a half cents. If care is not taken at the start to throw the dirt well back, it will be necessary to re-handle the dirt from the bottom of the trench, throwing it back on the pile, and this will add from five to ten cents a cubic yard, depending on what proportion of the entire excavation has to be re-handled. In the excavation for a tank, it is quite possible that the entire material may have to be re-handled and the cost thus be increased by fifteen cents a cubic yard. If the ground is very hard, as when boulders and clay are intermixed, it may require twice as much time for loosening as for shovelling, in which case the cost of digging the trench will be forty-five cents a cubic yard, or twenty-two and a half cents per lineal foot, with five or ten cents added if the material has to be re-handled.
If the material is a loose sand or gravel, the trench will probably require sheeting, that is, boards or planks on each side of the open trench with braces between, in order to prevent caving of the banks. If new lumber has to be purchased for this purpose and its cost added to the cost of excavation, an additional sum per cubic yard or per lineal foot will be added, somewhat in proportion to the total amount of excavation to be done. Finally, if the soil through which the trench is being dug contains water, it may be necessary to have one or two men continuously pumping during all the time that the excavation is going on, and this also will add to the cost per cubic yard or per lineal foot of the trench.
Refilling may be done by hand or may be done by a drag scraper at the end of a rope, so that the team of horses may be on one side of the trench and draw into it from the other side the excavated material. This costs only five cents per cubic yard. If the dirt is thrown back by hand, the cost will be that of shovelling, namely, about fifteen cents per cubic yard. If the dirt has to be tamped in the trench, the cost will then be that of another man, and backfilling will often add thirty cents a cubic yard to the cost of excavation.
As a summary, it may be said that excavation alone in earth may cost from fifteen cents to forty-five cents a cubic yard, and that backfilling may add to this from five to thirty cents a cubic yard, the entire cost, therefore, varying from twenty cents to seventy-five cents a cubic yard for excavation and backfilling together. Nor is it possible to be more definite in explaining the proper price to put on excavation since the character of the material and the nature of the excavation are of such importance in fixing that cost. If the excavation is for a tank, it is often possible to rig a derrick with a long arm on the side of the excavation and, by means of a bucket, transfer the excavated material from the hole to the bank cheaper than by repeated shovelling or by carrying out the dirt in a wheelbarrow. Wheelbarrow work is always expensive, the cost of transporting earth in a wheelbarrow a distance of fifty feet being about ten cents a cubic yard. Sometimes a horse may be used to great advantage to lift the bucket and operate the derrick in place of a hand-worked windlass, although the use of the horse is hardly worth while unless the excavation is more than ten feet deep.
The excavation for the trenches of a sub-surface irrigation system cannot be estimated on the same basis as for a larger trench. More time is required proportionally in trimming and grading the sides and bottom, so that the cost per cubic yard is much increased. Thus, while such trenches contain about one cubic foot of earth per lineal foot, and on the basis of twenty-seven cents per yard would cost only one cent per lineal foot to dig, it is probable that, under ordinary conditions, this amount would be doubled.
The cost of underdrains must be made up from the cost of the pipe used and the cost of the necessary excavation. In the bottom of artificial filter beds, the latter amounts to little or nothing. In natural filter beds, the trenches are deeper and the cost of the underdrainage depends largely on this excavation cost. The cost of the pipe varies from two to ten cents per foot, depending on the kind of pipe used and its unit cost.
Rock Excavation.—If the trench or the place for the tank is to be in rock, the cost of excavation is much increased. The rock must be drilled and blasting powder or dynamite used to loosen the material so that it can be thrown out later by hand. In ordinary rock, a man will drill from six inches to twelve inches of hole per hour, that is, the hole will cost from ten to twenty cents per lineal foot. The depth of the hole determines the amount of rock loosened per charge. If the holes are three feet deep, about one-third of a cubic yard is loosened per hour, while if the holes are five feet deep, one cubic yard of rock is loosened per hour. This indicates at once the economical advantage of deep holes compared with shallow ones. 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. Shallower holes, more frequently blasted, utilize the explosive gases more completely.
The kind of explosive which may be used varies from slow, low-power black powder to rapid, high-power nitro-glycerine, the many forms of dynamite and high-grade powder in use being combinations of nitro-glycerine with some absorbent. 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. The cost of forty-per-cent dynamite is about twenty cents per pound, and the cost of powder is about twelve cents per pound. On the average, it may be assumed that it will require one pound of the former and one and a half pounds of the latter per cubic yard of ordinary rock excavated. The cost of lifting the blasted material out of the trench will be at about the same rate as that of earth.
Concrete.—The walls of tanks made of concrete depend for their cost upon the cost of the material and the cost of the labor involved. It is usually more economical to use gravel as the basis of the concrete if any is available, and in order that the product may be of good quality it is always best to screen this gravel, separating it into sand and stone. The proper size of screen for this operation should be not greater than one-half-inch mesh. The stone and sand can then be re-combined with the cement in the proportion of one part of cement to two and a half parts of sand to five parts of stone, this mixture making a very strong and impervious combination. The cost of this mixture depends chiefly on the length of haul for the gravel and on the natural grading of the material. If the proportions required for concrete exist naturally in the bank or stream bed from which the gravel is to be obtained, there is little or no waste involved in screening, and the only cost is that of handling the material twice. If, on the other hand, the amount of stone is inadequate, it may be necessary to waste a good deal of the fine sand and enough material has to be shovelled to produce the required amount of coarse media. Assuming that the cost of shovelling the material from the stream bed is fifteen cents a cubic yard, and that the haul is two miles, so that four trips a day are made, then the gravel can be delivered where it is to be converted into concrete at a cost of one dollar for hauling and thirty cents for shovelling, while if the haul is only one mile, so that eight trips a day can be made, the cost will be eighty cents per cubic yard. If any waste of gravel is necessary, these costs will be increased correspondingly.
At the site of the proposed plant the sand and gravel must be mixed with the cement and carried to place. It has been found by experience when the mixing is done thoroughly and by hand, and when the resulting concrete can be shovelled directly behind the forms, that the cost of the mixing and placing is about one dollar per cubic yard of concrete. If the concrete has to be wheeled into place this cost will be added to.
In order to make a cubic yard of concrete, it is necessary to have nearly one cubic yard of the coarse material, whether this be rounded stones from a gravel bank or angular stones from a stone-crusher. Seven-eighths of a cubic yard of stone may be safely considered as necessary for a cubic yard of concrete. To this must be added three-eighths of a cubic yard of sand for a one to two and a half to five mixture. When this amount of stone and sand have been thoroughly mixed together, four and a half bags of cement should be added. 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. In order to determine whether this latter combination is permissible on any particular piece of work, a test may be made by thoroughly mixing the materials together in the proportions named and testing the volume of this mixture (B) in a box of measured dimensions. Then the same volumes mixed together in the former proportions (mixture A), and tested in the same box will show the relative value by occupying either more or less space than the other mixture (B). If less, mixture A is a better one, and should be used; if more, then the latter mixture, B, is the better one.
The amount of water required for mixing concrete depends upon the temperature of the outside air as well as upon the personal ideas of the person in charge of the mixing. Some builders like wet concrete and some like dry concrete. It should be noticed, however, that wet concrete is cheaper because it requires little or no tamping. Wet concrete, however, should be spaded, that is, a spade forced down into the mixture, particularly against the forms, so that particles of air caught between the stones may escape, and so that there may be no pockets between the stones into which the liquid cement mixture does not penetrate. It is generally considered that about fifteen per cent of the volume of concrete is the necessary volume of water for the mixture. This amounts to thirty gallons, or a barrel of water, to a yard of concrete, although the sizes of barrels vary, and a cement barrel would not be large enough, and a road-oil barrel would be too large.
The cost of forms depends, again, on the cost of material and on the cost of labor. Rough lumber varies in price from twenty to forty dollars a thousand feet, board measure, delivered on the grounds, and the cost of framing and placing it varies from eight to twenty dollars per thousand, depending on the skill of the carpenters and on their daily wages. In order to estimate the cost of the lumber required for building false work, it is best to determine exactly the amount of lumber required, and get the price from a lumber yard on that quantity. Ordinarily, it is safe to say that a carpenter in building forms will be able to saw and nail in place 250 board feet per day, so that, knowing the amount of lumber to be used and the wages of the carpenter, it will be easy to determine the cost of the forms as first set up. They may be taken down and removed for the purpose of re-assembling in another place for about half the cost of placing originally, and by carefully arranging to build the forms in panels or sections, they may be removed by a carpenter at the rate of 4,000 or 5,000 board feet per day.
Valves.—In connection with a sewage-disposal plant, valves are essential at many points. At the entrance to the several parts of the settling tank, flap valves are suitable to admit or keep out sewage from the several compartments. Gate valves are used on the by-pass lines and on connecting lines between the tank and the filter beds in order to be of service when it is occasionally necessary to clean the beds. More simple valves may be used in manholes where a diversion of the flow is required and where perfect and complete water-tightness is not essential. These valves may be made of plank, sliding up and down in grooves left in the concrete walls for that purpose. Sludge valves may be made to fit in the bottom of the tank, and depend for their water-tightness on the weight of the valve itself with the aid of a rubber packing which is placed between the valve and its setting. The cost of these various kinds of valves cannot be given exactly, since their cost depends upon freight and profit of the various commission men through whom the valves are ordered, but, generally speaking, they will be found to differ but little from the costs given in the following table:
| TABLE | |
|---|---|
| Flap valve as shown in Fig. [16] | $5.00 |
| Gate valve (iron bearings) for 6–inch pipe (Fig. [13]) | 20.00 |
| Gate valve (bronze bearings) for 6–inch pipe | 30.00 |
| Sludge valve as shown in Fig. [10] | 4.00 |
| Iron slide valve as shown in Fig. [11] | 15.00 |
Dosing Devices.—Dosing devices referred to in Chapter III are usually purchased directly from the manufacturer, and while their cost varies a little, depending upon the cost of freight, an ordinary single automatic siphon may be estimated at $15, the difference in price varying a little with the different makes of siphon. If an alternate discharge is required, then two siphons must be installed, by means of which alternate intermittency is secured, the variation, however, being only from one to the other and back again to the first. If a plural alternate discharge is to be used, the cost may be estimated roughly for a 6–inch siphon at from $50 to $75 for each unit, this price including the necessary piping but not the cost of setting.
Filling Material.—Artificial sand filters require a sand of uniform size and one free from dirt. These two requirements add very materially to the cost of sand, since it is almost impossible to find a natural sand which fulfils the necessary requirements. A few sections of the country are fortunate in having sand in the vicinity suitable for filtration purposes without any washing or screening. Such parts of the country, however, are limited to those where sand has been deposited by glacial action, and is essentially silicious in character. It is hopeless to expect to find suitable sand in the centre of New York State, for example, and even with washing and screening, sand in this locality is far from being desirable. It will be found, further, that after this undesirable sand is washed and screened the cost of the final product is so great that it is usually cheaper to use broken stone either as a filter or as a contact bed.
Washing sand in small quantities is done by throwing the sand into a channel through which water is passing, the sand being retained by a series of low partitions in the channel. If the water enters the box or channel through a pipe at the bottom, frequent entrance holes being provided along the sides and bottom of this pipe, the sand is kept in a state of suspension, the dirt more readily washed out, and a much smaller amount of water used. The cost of shovelling the sand into the washer and again out of the washer, about thirty cents per cubic yard, must be added to the original cost of the sand. The cost of water, if pumped by hand or by steam, will be about ten cents per cubic yard of sand cleaned, making the total cost about forty cents per cubic yard. If only three-fourths of the unwashed sand is available for use, then the cost of the final product is a little less than fifty cents per cubic yard. The sand before being brought to the washer will have been sifted at an additional cost of perhaps thirty cents per cubic yard. Hauling from the bank to the washer, or from the washer to the site of the disposal works, or both, if the water supply requires the washer to be placed at some distance from the sand bank, will add from fifty cents to one dollar a yard to the costs already indicated. It may generally be assumed that it will be impossible to put sand into an artificial filter for less than $1.50 a cubic yard, and it may easily cost $2.50 a yard if the sand bank is at considerable distance from the site of the works.
Broken stone in most parts of the country can now be bought from a stone-crushing plant. If road construction has been in progress in the vicinity, the contractor for the work has been obliged to open a quarry and set up a crushing-and-screening plant, and it will generally be possible to buy broken stone from such a contractor at about fifty cents per cubic yard. The cost of hauling and the cost of shovelling into the beds must be added to determine the cost of the stone in place. Sometimes it is cheaper to bring the stone from a distance by rail, such stone costing about $1.25 at the railroad station. Then the cost of hauling and shovelling must be added. It will be noticed that the cost of stone does not differ materially from the cost of sand, and since the amount of stone needed is only about one-quarter of the sand needed, it is generally cheaper to build a stone bed. The purification, it will be remembered, however, is decidedly inferior.
Finishing.—There is always some slight expense necessary in finishing and cleaning up after any piece of construction work. 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.
The following table is given as a guide and help in putting together the various items that make up the total cost of a sewage-disposal plant. Each line should be carefully considered, and if the item mentioned is to be used or paid for, the amount in the last column should be filled out.
| TABLE OF ITEMS ON WHICH TO BASE ESTIMATE OF COST OF SEWAGE-DISPOSAL PLANT | ||||
|---|---|---|---|---|
| No. of Units | Cost | |||
| Excavation and Refilling | ||||
| Trenches in sandy soil, shallow depth | at | per cu. yd. | ||
| Trenches in stiff soil, shallow depth | at | per cu. yd. | ||
| Trenches in sandy soil, deep cut | at | per cu. yd. | ||
| Trenches in stiff soil, deep cut | at | per cu. yd. | ||
| Tank, depth and soil duly considered | at | per cu. yd. | ||
| Beds, depth and soil duly considered | at | per cu. yd. | ||
| Embankments between filter beds (additional cost) | at | per cu. yd. | ||
| Trenches for sub-surface lines | at | per lin. ft. | ||
| Trenches for underdrains | at | per lin. ft. | ||
| Trenches for sludge disposal | at | per lin. ft. | ||
| Surfacing and Finishing | ||||
| Surface soil placed | at | per sq. yd. | ||
| Gravel in walks | at | per sq. yd. | ||
| Flowers and shrubbery | Total amount | |||
| Concrete Work | ||||
| Manholes on pipe lines | at | each | ||
| Settling tank, bottom, sides and roof | at | per cu.yd. | ||
| Dosing tank, in addition to settling tank | at | per cu. yd. | ||
| Manholes on sub-surface lines | at | per cu. yd. | ||
| Concrete in contact beds or filters | at | per cu. yd. | ||
| Concrete in protection wall at end of outfall | at | per cu. yd. | ||
| Pipe Lines | ||||
| 5– or 6–inch tile pipe (laid), house to disposal plant | at | per lin. ft. | ||
| 6–inch pipe used in disposal plant, laid | at | per lin. ft. | ||
| 6–inch pipe from plant to outfall | at | per lin. ft. | ||
| 6–inch pipe for sludge line | at | per lin. ft. | ||
| 6–inch pipe for main underdrain | at | per lin. ft. | ||
| 3–inch agricultural tile in sub-surface disposal | at | per lin. ft. | ||
| 4–inch agricultural tile in underdrains | at | per lin. ft. | ||
| 6–inch specials, bends, tees, etc., in addition to cost of straight pipe | at | each | ||
| 3–inch specials, bends, tees, etc., in addition to cost of straight pipe | at | each | ||
| Cast-iron pipe | at | per lb. | ||
| Cast-iron specials, bends, etc. | at | per lb. | ||
| Wooden troughs for surface distribution, in place | at | per 1000 ft. B.M. | ||
| Filter Material | ||||
| Gravel filling around sub-surface tile, placed | at | per cu. yd. | ||
| Sand filling for artificial filter beds | at | per cu. yd. | ||
| Broken stone for contact beds or for filters | at | per cu. yd. | ||
| Broken stone for sludge beds | at | per cu. yd. | ||
| Valves | ||||
| Gate valves, Fig. 13 | at | each | ||
| Flap valves, Fig. 16 | at | each | ||
| Slide valves, Fig. 11 | at | each | ||
| Wooden slide valves | at | each | ||
| Sludge valves, Fig. 10 | at | each | ||
| Tools | ||||
| Shovels, long or short handled | at | each | ||
| Picks | at | each | ||
| Wheelbarrows, wooden or steel | at | each | ||
| Sieves for screening sand and gravel | at | each | ||
| Saws, hammers, and axes | Total amount | |||
| Lumber | ||||
| For sheeting and bracing; rough lumber | at | per 1000 ft. B.M. | ||
| For forms for concrete work, sized | at | per 1000 ft. B.M. | ||
| For runways, staging and mixing boards, plank | at | per 1000 ft. B.M. | ||
| Hardware | ||||
| Nails for forms, staging, etc. | at | per 100 lbs. | ||
| Bolts or wire for concrete forms | at | per lb. | ||
| Iron Work | ||||
| Manhole frames and covers | at | per lb. | ||
| Steps for manholes | at | per lb. | ||
| Siphons | ||||
| Flushing siphons for dosing tank | at | complete | ||
| Timed siphons for emptying contact beds | at | complete | ||
| ————— | ————— | |||
| Total | ||||
Cost of Maintenance.—As to the cost of maintenance, very little that is definite can be said. Sub-surface irrigation plants should require no expenditure except for the occasional cleaning of the sedimentation tank. If this is emptied three times a year, the labor needed would amount to about a half-day’s time on each occasion for a family of ordinary size. 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 none of the installations is this excessive in comparison with the benefits received, and it should not be considered a burden to expend this amount of time in maintaining so important a part of the household economy as the disposal of the household wastes in a sanitary manner. It must not be forgotten, however, that no sewage-disposal plant is exempt from occasional break-down or accidents, and that there must be a constant supervision exercised. This supervision should not require much more time than above suggested, but should be exercised for the purpose of correcting irregular flows or distribution before the value of the plant is utterly destroyed.