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Yearly Averages of Analyses of Effluent of Filler No. 6.

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YEAR.

Yearly Averages of Analyses of Effluent of Filler No. 9 A.

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Yearly Averages of Analyses of Effluent of Filler No. 10.

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ASHES OR CINDERS AS FILTERING MATERIALS.

Towards the close of the year two filters were put in operation, one containing 4.5 feet in depth of coal ashes and the other contain ing the same depth of cinders; that is, coal ashes from which the finer material has been removed. Cinder filters are in use in many places in England for rapid filtration of sewage and are known as "roughing" filters, and the material is valuable for filtration purposes wherever land of a suitable character cannot be found, or for preliminary filtration before disposal upon land; but the first systematic experiment in this country upon their use for the complete purification of sewage by intermittent filtration was made by Prof. Chas. MacMillan of Princeton University, to whom we are indebted for the suggestion.

The two filters at the station are known as filters Nos. 80 and 81. Filter No. 80 is of ashes and receives sewage at the rate of 70,000 gallons per acre daily. The effluent of the filter has been clear and bright from the beginning of the experiment, but nitrification did not begin until after the close of the year. Filter No. 81 is of cinders and has received sewage at the rate of 550,000 gallons per acre daily. (For details see page 504.)

AVERAGE PURIFICATION OF LAWRENCE SEWAGE BY THE SEVERAL FILTERS IN 1896.

The following table gives the average results obtained by the filters receiving Lawrence sewage during 1896. The qualitative efficiency of the filters is shown by the percentage removal of organic matters of the applied sewage as indicated by the albuminoid ammonia and oxygen consumed.

NUMBER OF FILTER.

Average Per Cent. of Albuminoid Ammonia, Oxygen consumed and of Bacteria removed from the Sewage by the Several Filters, with Average Rates of Filtration, 1896.

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WORK OF THE FILTERS FOR 1896.

Filters Nos. 1 to 10, inclusive, are each one two-hundredth of an acre in area and are out of doors; filters Nos. 12 to 21 A, inclusive, are one twenty-thousandth of an acre in area and within the buildings; Filter No. 59 is one ten-thousandth of an acre in area, Filter No. 66, one three-thousandth of an acre in area, and Filter No. 67, one six-thousandth of an acre in area. These three filters are also

within the buildings.

Filter No. 1.

This filter, containing 60 inches in depth of coarse sand of an effective size of 0.48 millimeter, was in good condition at the beginning of the year and nitrification was active. On Nov. 15, 1895, the filter was covered with canvas stretched over a frame and this cover was removed on March 12. The surface was spaded over 6 inches deep on April 6 and again on October 23. During the last week of October following this last spading no sewage was applied to the filter.

The physical condition of the filter was excellent throughout the year, the applied sewage passed readily from the surface and the character of the effluent was satisfactory, as shown by the following table, giving the monthly averages of the weekly analyses:

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Sewage applied, 300 gallons six times a week, except from October 23 to November 6, when filter was allowed to rest. March 1 to 12, experiment interrupted by freshet. Surface raked 1 inch deep each week, except twice in January, once in December and during period of rest. Surface spaded 6 inches deep April 6 and October 23 March 12, canvas cover removed. June 13 to August 31, a trap 18 inches high was attached to effluent pipe. During March, 6 inches of snow removed; during Decem ber, 6 inches of snow and 4 inches of ice removed.

Filter No. 2.

This filter contains 60 inches in depth of fine sand of an effective size of 0.08 millimeter, with two trenches, 1 foot wide and 2 feet deep, of medium fine sand of an effective size of 0.19 millimeter. It has been in good condition throughout the year and has given a clear, bright, well-purified effluent.

The following table gives the monthly averages of the analyses of the effluent, and the notes below give the management of the filter during the year:

Effluent of Filter No. 2.

[Parts per 100,000.]

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Sewage applied, 200 gallons six times a week, except from October 23 to November 6, when filter was allowed to rest. March 1 to 9, experiment interrupted by freshet. Surface of trenches raked 1 inch deep each week, except twice in January, once in February and once in December. February 6, surface of trenches dug over to a depth of 4 inches. March 21 and October 27, sand removed from trenches; sides of trenches scraped and raked, and bottom of trenches dug over to a depth of from 3 to 5 inches; sand replaced in trenches. August 1, cut grass and weeds on surface; weight of grass and weeds 170 pounds 8 ounces. During January, 9 inches of snow removed from entire surface and 4 inches of ice from surface of trenches; during February, 14 inches of snow removed from entire surface and 23 inches of ice from trenches; during March, 6 inches of snow removed from surface; during December, 74 inches of snow removed from surface and 38 inches of ice from trenches.

Filter No. 4.

This filter contains 60 inches in depth of fine river silt of an effective size of 0.04 millimeter, with two circular trenches, about 14 inches wide and 12 inches deep, of coarse sand of an effective size

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