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increased. The quantity of sewage applied in each period varied but four per cent.

The facts previously cited as to the behavior of tank No. I upon the resumption of intermittent filtration after a period of continuous filtration indicate that impurities stored in the soil when the conditions are unfavorable for purification are, as soon as the proper conditions obtain, rapidly removed in the form of nitrates, and this without any period of rest.

This fact is of particular interest as effecting the disposal of sewage by filtration in winter. During short periods of severe winter weather the surface of filtering areas can be easily kept free from ice by the continuous application of sewage, which may be followed, as the weather moderates, by intermittent applications. The soil, storing in an inoffensive form, during the continuous filtration such impurities as cannot be oxidized, and subsequently when the conditions for intermittent filtration are favorable these stored impurities are oxidized and removed in addition to the oxidation of the sewage applied from day to day.

Experiments with intermittent filtration through gravel stones as large as beans, from which all particles of soil and sand have been washed out, have shown that when sewage is applied at the rate of 126,600 gallons per acre per day for a period of three months 98.5 per cent. of the organic matter was removed and the stones were as clean as at the beginning.

Practically then, it may be said that the soil can purify sewage for an indefinite time.

It is said that sewage has been applied to the Craigentinny meadows, near Edinburgh for the last 200 years.

Quantity and Concentration of Sewage. From the above Table and the conditions essential to purification it seems fair to infer that the capacity of a soil filter to remove a certain amount of organic matter from sewage depends upon the degree of dilution. For instance, if the organic wastes from each person be mixed with 26 gallons of water the organic matter in the

sewage will be more readily removed than if mixed with 100 gallons. The degree of dilution for German, English and American cities, as indicated by the statistics of water consumption, is about 26, 35 and 100 gallons respectively. Excessive dilution must seriously interfere with the aëration of the filter upon which in a large measure depends its activity.

In like manner the dilution of sewage by storm water is a serious objection in filtration over limited areas where the application should be regular and substantially uniform in quantity. The experiments at Lawrence have demonstrated that :

"The preparation required to render filtering areas effective appears to be the introduction by the sewage of the particular organisms fitted to aid in this work, and their accumulation with a proper food supply, and other favorable conditions by which they become in time adapted to accomplish the most complete purification with the quantity of sewage received Any change in quantity or mode of application may disorganize this working colony and prevent the best results, until there is time for a re-adjustment adapted to the new conditions."

Influence of Area. In the purification of sewage by intermittent filtration the object sought is ordinarily the application of the sewage to limited areas in the immediate vicinity of urban districts, where the value of land practically prohibits its application over areas sufficiently broad to favor its utilization in agriculture. In broad irrigation and utilization of the sewage, the areas available and the comparatively small quantity of the effluent make the process one much less likely to be disturbed by lack of proper management.

Experiments made at Paris upon the action of growing plants upon sewage during irrigation by M. Marie'-Davy gave the following results: Out of a supply of 5,000 to 6,000 cubic meters (tons) per hectare (2.5 acres) per month, only one thirtieth of the water supplied reached the subterranean drains. Vegetation consequently acts as a powerful upward drainage. The plants absorb the useful elements of the sewage and yield to the atmosphere, by evaporation, nearly the whole of the liquid which has served to convey them. Thus purification and agriculture utilization perfect each other.

It is said that at Dantzig during the hardest frost the sewage sinks beneath the surface coating of ice and snow and filters through the soil without causing any injury to the plants or trouble to the contractor.

Sewage Disposal at Pullman.-The sewage disposal works at Pullman offer a good opportunity for observing the success which may attend intermittent downward filtration in northern latitudes under somewhat unfavorable conditions.

The writer made a somewhat thorough examination of these disposal works during the past winter and also chemical analyses of the crude and purified sewage with especial reference to determining the feasibility of disposing of sewage by intermittent downward filtration in winter in this climate.*

Pullman is sewered by the Separate System. The population is about 11,000, the sewage from dwellings averages from 120 to 130 gallons a day per capita. The average number of gallons of sewage pumped per day in 1890 was 1,800,000. The balance of the sewage being the discharge from factories and possibly some ground water.

The sewage is pumped from the collecting well through a twenty inch cast-iron pipe to a sewage farm about three miles south of the city. At the farm end of this pipe the sewage goes into a receiving tank made of boiler iron which is set a few feet above the surface of the ground. Through the center of this tank there is a screen in an oblique position through the meshes of which substances more than half an inch in diameter cannot pass. The sewage passes through this screen and thence into the distributing pipes.

One hundred and forty acres of land have been thoroughly piped and underdrained for the reception and purification of sewage. Hydrants are placed at suitable intervals so that the distribution can be conveniently effected.

Cox,

*I am indebted to Mr. Duane Doty, Editor of the Pullman Journal, Mr. Assistant Manager, Mr. C. W. Campbell, Superintendent of the sewage farm, and Mr. Chas H. O'Neil, Assistant Engineer of the sewage pumping station, for information and assistance.

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