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Sewage applied 6 times a week till July 23, afterwards 24 times a week.
Surface raked about 1 inch deep each week, beginning October 15.
Emuent clear or nearly so.

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Sewage applied 24 times a week.
Surface raked about 1 inch deep each week till April 17, afterwards about 3 inches deep each week.
Experiments interrupted by high water in the river January 25, 26, March 14-16, 23-27, April 14-16.
Efluent clear and practically colorless.

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Sewage applied 24 times a week till October 31, afterwards 6 times a week.
Burface raked about 3 inches deep each week.
Effluent clear and practically colorless.

[blocks in formation]

Anguillula, .
Rotatorian ova,

-1-1

3.3
6.6

50 100

FILTRATION OF WATER.

The experiments upon the intermittent filtration of water described in the special report upon the Purification of Sewage and Water (pages 601 and 695) have been continued with most satisfactory results.

The prevalence of an excessive amount of typhoid fever among the users of some of the public water supplies of the State, and the conclusive evidence that, in some cases at least, the water was the means of carrying the germs of that disease, * have given a new impetus to the study of water filtration.

Our attention has been specially directed toward the bacteria. In sewage purification the removal of the organic matters capable of putrefaction is often of the greatest importance; but in the filtration of sewage-polluted water — water capable of causing disease if it is not filtered — the removal of the germs of disease is the all-important point. The effluents from the tanks filtering water have invariably been well purified as far as chemical constituents are concerned ; color has been much reduced or entirely removed, according to the rate of filtration, the amount of water which had previously passed through the filter and other conditions; and the organic matters have been so reduced as to be unobjectionable in amount.

As a result of a very careful study of the conditions of taking samples and of determinations of the numbers of the various species of bacteria in the effluent of Filter Tank No. 8 and in the applied water, the results of which will be found in the report of the biological work, which follows, it was found that if any bacteria passed the number must be very small, - at least as low as one in a thousand.

Annual Report, 1890, page 525; also a Report upon the Sanitary Condition of the Water Supply of Lowell, Mass., by Prof. W. T. Sedgwick, to the Lowell Water Board, Lowell, Mass., 1891.

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