Chapter XXI. The dosing tank should have a capacity sufficient to cover

the bed to a depth of about 1 to 3 inches at one dose, and the siphon should discharge at a rate of about one second-foot for each 5,000 square feet of filter area. A dose should disappear within 20 minutes to half an hour after it is applied to the filter. With the rate stated and four applications per day to a depth of 1 inch at each dose, the rate per acre per day will be 109,000 gallons.

The filtration of sewage through sand in a manner similar to the _rapid sand filtration_ of water is being attempted at the Great Lakes Naval Training Station. No results of this treatment have been published and the practical success of the method has not been assured.

=259. Cost of Filtration.=—Only comparative figures can be given in stating the costs of filtration, as most data available are based on pre-war conditions, and are therefore unreliable for present conditions. The variations from the figures given may be very large but in general the relative costs have not changed. The figures given in Table 90 are suggestive of the relative costs of the different forms of filtration.

TABLE 90

RELATIVE COSTS OF DIFFERENT METHODS OF SEWAGE TREATMENT

Costs in Dollars per Million Gallons per Day ─────────────────────────┬───────────────┬──────────────┬────────────── Form of Treatment │First Cost[166]│Operation and │ Total │ │ Maintenance │ ─────────────────────────┼───────────────┼──────────────┼────────────── Coarse screens │ │ │ 0.20 Fine screens │ │ │ 3.00 Plain sedimentation │ 7.00│ 3.00│ 10.00 Chemical precipitation │ │ │ 22.00[167] Septic tank │ 7.00│ 1.00│ 8.00 Imhoff tank │ 10.00│ 1.00│ 11.00 Contact bed │ 8.00│ 2.00│ 10.00 Trickling filter │ 4.00│ 2.00│ 6.00 Intermittent sand filter │ 15.00│ 10.00│ 25.00 Activated sludge │ 6.50│ 8.50│ 15.00[168] ─────────────────────────┴───────────────┴──────────────┴──────────────

IRRIGATION

=260. The Process.=—Broad irrigation is the discharge of sewage upon the surface of the ground, from which a part of the sewage evaporates and through which the remainder percolates, ultimately to escape in surface drainage channels. Sewage farming is broad irrigation practiced with the object of raising crops. Broad irrigation can be accomplished successfully without the growing of crops, but it is seldom attempted as some return and sometimes even a profit can be obtained from the crops raised. Broad irrigation and sewage farming differ from intermittent sand filtration in the intensity of the application of the sewage, the method of preparing the area on which the sewage is to be treated, and the care in operation. In broad irrigation and intermittent sand filtration the paramount consideration is successful disposal of the sewage. In sewage farming the paramount consideration is the growing of crops. The growing of crops may be combined with irrigation and filtration, however, but the crop should be sacrificed to the successful disposal of the sewage.

The change which occurs in the characteristics of the sewage due to its filtration through the ground is the same as occurs in aërobic filtration. The effect on the crops is mainly that of an irrigant, as the manurial value of the sewage is small.

=261. Status.=—The disposal of sewage by broad irrigation was practiced in England previous to the development of any of the more intensive biologic methods of treatment. It was considered the only safe and sanitary method for the disposal of sewage, and as a result, areas irrigated by sewage were common throughout England. Crops were grown on these areas as a minor consideration, and sewage farming gained some of its popularity from the apparent success of these disposal areas. The success of sewage farms is due more to generous irrigation in dry years than to fertilization by sewage.

The sewage farms of Paris and Berlin are frequently cited as examples of the successful and remunerative disposal of sewage by farming in connection with broad irrigation. Kinnicutt, Winslow, and Pratt[169] state:

The Berlin Sewage farms offer examples of broad irrigation under better conditions ... of 21,008 acres receiving sewage, 16,657 acres were farmed by the city, 3,956 acres were leased to farmers, and only 395 acres were unproductive. The contributing population at this time was 2,064,000 and the average amount of sewage treated was 77,000,000 gallons, giving a daily rate of treatment of about 3,700 gallons per acre of prepared land. The soil is sandy and of excellent quality. A quarter of the area operated by the authorities is devoted to pasturage, and about a third to the cultivation of cereals, of which winter rye and oats are the most important. Potatoes and beets are grown in considerable amounts and a wide variety of other crops in smaller proportions.... Even fish ponds are made to yield a part of the revenue, and the drains on some of the farms have been successfully stocked with breed trout.

The cost of the Berlin farms to March 31, 1910, was $17,470,000, somewhat more than half being the purchase price of the land. The expenses for this year amounted to $1,300,385 for maintenance, and $741,818 for interest charges. The receipts were $1,240,773 and there was an estimated increase of $122,593 in value of live stock and other property.

The conditions at Berlin are quoted at length to indicate the success which can accompany broad irrigation, and as an example of what is being done abroad, where the rainfall is light and the soil is suitable.

In the United States success in sewage farming has not been marked. This may be due partially to the relative weakness of American sewages, to the cost of labor, to lack of satisfactory irrigation areas, and to inattention to details. An attempt was made to grow crops on the sand filters at Brockton, Mass., but it was finally abandoned as the interests of the crops and the successful treatment of the sewage could not both be satisfied. At Pullman, Illinois,[170] in 1880, there was commenced probably the most extensive attempt at sewage farming in eastern United States. The farm was a failure from the start, because of the clay soil, and it was subsequently abandoned. Sewage farming, mainly as a subsidiary consideration to the filtration of sewage, is practiced in a few cities in the eastern portion of the United States to-day. Among the cities mentioned by Metcalf and Eddy[171] are Danbury, Conn., and Fostoria, Ohio. In the western portion of the United States where water is scarce and the ground is porous, sewage has been used as an irrigant with some success. Such use of sewage cannot be considered as a method of treatment since the prime consideration is the growing of crops. In this process all sewage not used as an irrigant is discharged without treatment into water courses. According to Metcalf and Eddy there were 35 cities in California in 1914 that were operating sewage farms. Among these are Pasadena, Fresno, and Pomona. Other farms, notably the pioneer farm at Cheyenne, Wyo., have been abandoned because of the local nuisance created and the lack of financial success.

=262. Preparation and Operation.=—A porous sandy soil on a good slope and with good underdrainage is most suitable for broad irrigation. Impervious clay or gumbo soils are unsuitable and should not be used. They become clogged at the surface, forming pools of putrefying sewage, or in hot weather form cracks which may permit untreated sewage to escape into the underdrains.

The sewage may be distributed to the irrigated area in any one of five ways which are known as: flooding, surface irrigation, ridge and furrow irrigation, filtration, and subsurface irrigation. In flooding, sewage is applied to a level area surrounded by low dikes. The depth of the dose may be from 1 inch to 2 feet. In surface irrigation the sewage is allowed to overflow from a ditch over the surface of the ground into which it sinks or over which it flows into another ditch placed lower down. This ditch conducts it to a point of disposal or to another area requiring irrigation. Ridge and furrow irrigation consists in plowing a field into ridges and furrows and filling the furrows with sewage while crops are grown on or between the ridges. In filtration the sewage is distributed in any desired fashion on the surface and is collected by a system of underdrains after it has filtered through the soil. In subsurface irrigation the sewage is applied to the land through a system of open-joint pipes laid immediately below the surface, similarly to a system of underdrains. Combinations of and modifications to these methods are sometimes made. Underdrains may be used in connection with any of these forms of distribution.

The preparation of the ground consists in: the construction of ditches or dikes to permit of any of the above described methods of application, grading of the surface to prevent pooling, the laying of underdrains, and the grubbing and clearing of the land. The main carriers may be excavated in open earth or earth lined with an impervious material. The distribution of the sewage from the main carriers to groups of laterals may be controlled by hand-operated stop planks. If the soil has a tendency to become waterlogged it may be relieved by installing underdrains at depths of 3 to 6 feet, and 40 to 100 feet apart. The tile underdrains may discharge into open ditches excavated for the purpose which serve also to drain the land. Drains should be used where the ground water is within 4 feet of the surface, and the open ditches should be cut below the drains to keep the ground water out of them. Four or 6–inch open-joint farm tile may be used for underdrains. The porosity of the soil will be increased by cultivation. Where particular care is taken in the cultivation of the soil so that sewage can be applied at a high rate, broad irrigation merges into the more intensive intermittent filtration through sand.

Before being turned on to the land, sewage should be screened and heavy-settling particles should be removed. The rate of application may be increased as the intensity of the preliminary treatment is increased. The rate at which sewage may be applied is dependent also on the character of the soil, and may vary between 4,000 and 30,000 gallons per acre per day, although higher rates have been used with the effluent from treatment plants and on favorable soil. The sewage should be applied intermittently in doses, the time between doses varying between one day and two or three weeks or more, dependent on the weather and the condition of the soil. The methods of dosing vary as widely as the rates. The dose may be applied continuously for one or two weeks with correspondingly long rests, or it may be applied with frequent intermittency alternated with short rests, interspersed with long rest periods at longer intervals of time. When applying the sewage to the land the rate of application of the dose is about 10,000 to 150,000 gallons per acre per day. The area under irrigation at any one time may be as much as 10 to 15 acres. The rate of the application of the sewage is also dependent on the weather and may vary widely between seasons. It is obvious that a rain-soaked pasture cannot receive a large dose of sewage without danger of undue flooding. One of the principal difficulties with the treatment or disposal of sewage by broad irrigation is that the greatest load of sewage must be cared for in wet seasons when the ground is least able to absorb the additional moisture.

=263. Sanitary Aspects.=—A well-operated sewage farm should cause no offense to the eye or nose, and is not a danger to the public health. In Berlin, a portion of the sewage farms are laid out as city parks. The liquid in the drainage ditches or underdrains may be clear, odorless, and colorless, high in nitrates and non-putrescible. Where the farm has been improperly managed or overdosed the condition may be serious from both esthetic and health considerations. Sewage may be spread out to pollute the atmosphere and to supply breeding places for flying insects which will spread the filth for long distances surrounding the farm. The character of the crop is also a sanitary consideration.

=264. The Crop.=—From a sanitary viewpoint no crops which come in contact with the sewage should be cultivated on a sewage farm. Such products as lettuce, strawberries, asparagus, potatoes, radishes, etc., should not be grown. Grains, fruits, and nuts are grown successfully and as they do not come in contact with the sewage there is no sanitary objection to their cultivation in this manner. Italian rye grass and other forms of hay are grown with the best success as they will stand a large amount of water without injury. The raising of stock is also advisable for sewage farms where hay and grain are cultivated. The stock should be fed with the fodder raised on the irrigated lands and should not be allowed to graze on the crops during the time that they are being irrigated. This is due as much to the danger of injury to the distributing ditches and the formation of bogs by the trampling of the cattle, as to the danger to the health of the cattle.