Sewerage and Sewage Treatment

by Harold E. Babbitt

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.

CHAPTER XVIII
ACTIVATED SLUDGE

265. The Process.—In the treatment of sewage by the activated sludge process the sewage enters an aëration tank after it has been screened and grit has been removed. As it enters the aëration tank it is mixed with about 30 per cent of its volume of activated sludge. The sewage passes through the aëration tank in about two to four hours during which time air is blown through it in finely divided bubbles. The effluent from the aëration tank passes to a sedimentation tank where it remains for one-half an hour to an hour to allow the sedimentation of the activated sludge. The supernatant liquid from the sedimentation tank is passed to the point of final disposal. A portion of the sludge removed from the tank is returned to the influent of the aëration tank. The remainder may be sent to any or all of the following: the sludge drying process, the reaëration tanks, or to some point for final disposal. Sections of the activated sludge plant at Houston, Texas, are shown in Fig. 177.

The biological changes in the process occur in the aëration tank. These changes are dependent on the aërobic organisms which are intensively cultivated in the activated sludge. When placed in intimate contact with fresh sewage, brought about by the agitation caused by the rising air, and in the presence of an abundance of oxygen, the organic matter is partially oxidized. The putrefactive stage of the organic cycle is avoided. Colloids and bacteria are partially removed probably by the agitation effected in the presence of activated sludge but the exact action which takes place is not well understood.

Fig. 177.—Activated Sludge Plant at Houston, Texas.
Eng. News, Vol. 77, p. 236.