Besides the area devoted to broad irrigation there are fifteen filter beds having a total area of 91⁄2 acres specially prepared for intermittent downward filtration, to which the sewage is applied during a considerable portion of the winter, and whenever it cannot conveniently be applied to the surface under broad irrigation. These filter beds are underdrained by drains 122 feet apart which converge at a man-hole.

The soil of the filter beds and of the entire farm is the rich alluvial prairie soil underlaid by a yellowish clay subsoil and poorly adapted for purifying large quantities of sewage, especially when the temperature is low. Nevertheless the results obtained here are very flattering.

I am informed that it is usual to apply the sewage of one day to about three of the beds, which are then allowed to rest for three days at least. About half of the filter beds are cropped each year with quickly maturing crops, such as plants to be transplanted. While the crops are growing no sewage is applied. Plate XIX is reproduced from a photograph of the filter beds taken at the time the examination was made, March 12th, 1891. The weather had been severe for about ten days previous and the temperature at 7 A. M. was 12° above zero. The beds are arranged, on ground slightly inclined, at different levels. Sewage is admitted to any of the high level beds at will through gate chambers, one of which is shown in the foreground. On the day previous the bed at the right was flooded with sewage to the depth of about 10 inches. This had nearly all disappeared beneath the surface, a thin sheet of ice one-fourth to one-half inch thick had formed on the impounded sewage, which, as the sewage sank beneath the surface was broken into small fragments. There is no accumulation of ice which interferes with the filtration of the sewage. The temperature of the sewage when applied to the beds, as taken in the carrier after being conveyed three miles underground at the season when the subsurface temperature as shown by observations is about at its lowest, was 51° F. The temperature of the effluent as taken in the man-hole to which the subsoil drains converge was 38° F.

The latent heat given up by the sewage during this fall in temperature quickly melts the ice that may have accumulated on the filter beds and the sewage sinks rapidly beneath the surface.

Experiments previously detailed show that between these temperatures nitrification is comparatively active.

There was no offensive odor at the beds, with the exception of that coming from a deposit of sludge near the gate chamber, and this did not extend for any great distance. I am informed that it is usual to keep this sludge spaded beneath the surface. The surface of the beds are occasionally turned over with a plow to assist in keeping them from becoming sodden so as to exclude the air.

A sample of the crude sewage was taken at the end of the carrier for analysis and also a sample of the purified effluent was taken from the man-hole to which the tile drains converge. The effluent was clear and sparkling and not unpleasant to the taste. The table below contains the results of an analysis of the samples collected as above stated.

ANALYSIS OF CRUDE AND PURIFIED SEWAGE FROM THE PULLMAN SEWAGE FARM.

The degree to which the purification has proceeded (assuming that there has been no concentration or dilution of sewage

from evaporation, subsoil water or other causes) is indicated by

Much more favorable results may be expected, of course, during the warmer weather. The intention was to ascertain the conditions at about the most unfavorable season. As previously stated the soil is not adapted to purify large quantities of sewage having too much finely divided organic matter in its upper layers which interferes with aëration and also too retentive a subsoil.

Plate XX is reproduced from a photograph of the area devoted to broad irrigation and crops. Sewage is applied to this area whenever the weather in spring and fall is favorable and it can be applied without interfering with the crops. In general, however, no sewage is applied to growing crops. The sewage is distributed to this area through a system of vitrified pipes having hydrants at convenient intervals from which the sewage is allowed to flow over the surface of the ground. Sewage is also applied to meadow lands adjoining this area in the spring.

The principal crops raised upon the sewage farm are early potatoes, cabbages, beets, onions, celery, cauliflower, parsnips and squashes. Mr. Campbell, the superintendent, informs me that the gross receipts from the farm (140 acres) was about $12,000 last year.

The experiments of the Massachusetts State Board of Health and the results obtained at Pullman under conditions more than usually unfavorable indicate that the disposal and purification of sewage on the land in this climate is entirely practicable in a great majority of cases.