97% of water, as most sludge does. In this tank, however,
some sludge is likely to adhere to the sides of the cone, which
must be cleaned occasionally by hand or by a revolving
scraper. In the Candy tank* the bottom is flat and the sides
circular and vertical; and both sides and bottom are cleaned

TANKS.

by a squeegee revolving on a central vertical shaft, the sludge being forced into and through a pipe at the bottom by a hydrostatic head of 18 inches as just described. In the upward-flow tanks the sewage rises at the rate of .005 to .01 foot per second; and as the precipitant falls at an average rate of .02 to .03 foot per second, it slowly reaches the bottom of the tank. Experience seems to show, however, that not

* See Engineering News, December 28, 1899.

quite so large a percentage of organic matter is removed in these as in horizontal-flow tanks. Upward-flow tanks are particularly adapted to localities where the available space is small

With whatever style of tank, the sludge should be removed at short intervals, since it is liable to decay and affect the purity of the effluent, gives off foul gases, and even rises in flaky masses to float and putrefy on the surface. In a few instances the treated sewage has been run directly onto land divided into beds by high embankments, where the liquid matter drains off and the sludge, when dry, is raked up. For this purpose large areas are necessary, as the soil quickly becomes water-soaked unless given long periods of rest.

The sludge from precipitation-tanks being only concentrated filth, the difficulties of disposal have been merely focussed upon a smaller volume of matter, which must still be disposed of in some way. There is manurial matter of value in this, but no process has yet been found by which it can be utilized at a profit, and disposal of the sludge remains the problem of this method of treatment. Glasgow sludge contains 4.63% of organic matter, 5.60% of mineral matter, and 89.77% of water. The table of analyses of English sludges on page 394 is taken from Robinson's "Sewerage and Sewage Disposal."

In a few places a small amount of sludge is removed by the farmers, but this cannot be relied upon as a method of disposal. London maintains six or more sludge ships, each carrying 1000 tons, which carry 300,000,000 gallons of sludge daily fifty miles to sea and dump it there. In several places in this country the sludge is run onto "sludge-beds" of porous soil, ashes, burnt clay, or sand, through which the liquid soaks, leaving a dry deposit like heavy, coarse brown paper which can be burned; or is used for filling low lands, as at Sheepshead Bay. The drying sludge gives off consider

TABLE NO. 29.

DR. WALLACE'S ANALYSES OF SEWAGE SLUDGE (AIR-DRIED).

Lime...

Magnesia

Oxide of iron
Alumina

Sand, etc.

7.62 8.53 8.30
2.18 11.19 12.74
.18
.90 1.37
6.20 2.70 3.20 1.98
6.75
2.68 2.58 2.97
33.50 41.13 37.93 29.50

10.53

13.77 6.64

5.71

8.42

13.11 15.25

22.71

14.50 1.06

16.90

20.27 9.16

6.65

9.68

17.51 20.16

31.09

1.66

2.II

ΙΟΙ.22

5.07
2.01 4.14 2.66
3.49 3.89 4.13 5.80
7.04
21.80 10.23 37.83
42.00 31.60
99.96 100.62 100.06 100.58 99.16 100.26

able odor, and a better plan in many cases is to prepare deep furrows, run the sludge into these, and cast a heavy earth covering over them from other parallel furrows prepared for the next batch of sludge.

Where there is not the land or other facilities for using sludge-beds (and much land is needed, since each bed, after an application of sludge, requires a long rest), the sludge is generally pressed into cakes by squeezing out the diluting water and reducing the amount of this from about 95% to from 50% to 75%. The cakes are formed by filter-presses, composed of a number of circular or square iron cells (in East Orange 36, Columbian Exposition 60, and Worcester 125), the faces of which are grooved and recessed, which rest vertically face to face in a simple frame and slide away from each other on horizontal guides. Between each two cells is a canvas bag. Through these cells passes a central feedpassage through which the sludge is forced into the canvaslined cells, the water being expelled through the canvas by a pressure in the feed-pipe of about 100 pounds per square inch. In Worcester the cakes thus formed are 36 inches in diameter and inch thick. They give off little odor. They will burn without other fuel if containing no more than 70% to 73% of water.

The fluid forced out by the press is treated again, either combined with the crude sewage or in separate tanks. To Το enable the water to separate more readily from the sludge, milk of lime is generally added to this to "cut the slime."

In Worcester one part of sludge is obtained from ninety of sewage, there being one ton of solid matter to 750,000 gallons of sewage, 34% of this being organic matter. With a lime precipitant there will be about .4 lb. of sludge per capita daily. This can be burned, used for filing in, or buried in pits. The first method is the best, an ordinary garbage-cremator being used. It has been suggested that

the addition of peroxide of manganese to sludge will supply oxygen and prevent putrefactive action; but this has not been tried on a practical scale and the expense would probably be prohibitive.

ART. 92. COST OF PRECIPITATION.

The Glasgow plant, to treat 10,000,000 gallons daily, cost $335,000 exclusive of site. The cost of the treatment is $17 per million gallons, or 14 cents per capita annually. London, to handle 250,000,000 gallons daily, paid $4,066,448 for a plant, including $662,322 for six sludge-ships. The precipitation expenses are $2.98 per 1,000,000 gallons, sludge disposal $1.66. The New Rochelle plant, to treat 750,000 gallons daily, cost about $19,000. The East Orange, for 1,500,000 gallons daily, cost $75,000; maintenance 60 cents. per capita annually, exclusive of interest; lime 95 cents per barrel, alum 1 cents per pound. Round Lake in 1892 paid 3 cents per pound for perchloride of iron. The White Plains plant, for 400,000 gallons daily, cost $50,049; maintenance $12 per day for 250,000 gallons. At Chautauqua the cost of the plant was $16,500; that of chemicals (lime at 83.2 cents per barrel and alum at 2.15 cents per pound) was, in 1893, .04 cent per capita per day; total maintenance 57 cents per capita per year. At the Columbian Exposition $8.80 per ton was paid for lime, $13.40 for copperas, and $20.40 for alum. Worcester pays about $7.00 per ton for lime, $25 for sulphate of alumina (not used there now). In the Powers system of chemical treatment used in the 26th Ward, Brooklyn, the total cost is about $35 per 1,000,000 gallons; capacity 4,500,000 gallons per day; cost of plant $204,852.64. The cost of pressing sludge into cakes is about 50 to 75 cents per ton of cake which is 50% moisture.