| BACTERIA IN WATER APPLIED TO AND EFFLUENT FROM LAWRENCE FILTER. | ||||||
|---|---|---|---|---|---|---|
| RAW WATER. | ||||||
| 1893. | 1894. | 1895. | 1896. | 1897. | 1898. | |
| January | 7,700 | 18,700 | 7,500 | 13,314 | 6,519 | |
| February | 7,600 | 15,040 | 12,600 | 13,113 | 4,653 | |
| March | 6,500 | 20,770 | 5,900 | 12,055 | 3,748 | |
| April | 11,200 | 8,420 | 3,800 | 6,904 | 2,320 | |
| May | 6,000 | 7,000 | 9,600 | 4,625 | 2,050 | |
| June | 8,300 | 9,000 | 6,400 | 4,650 | 6,775 | |
| July | 2,400 | 10,000 | 3,900 | 6,240 | 2,840 | |
| August | 3,100 | 5,000 | 2,700 | 10,700 | 8,575 | |
| September | 57,500 | 6,500 | 5,000 | 12,300 | 27,300 | 6,100 |
| October | 22,200 | 25,300 | 19,000 | 5,300 | 13,200 | 5,120 |
| November | 10,800 | 16,600 | 8,700 | 5,600 | 6,644 | 4,310 |
| December | 8,100 | 23,800 | 6,700 | 9,695 | 5,581 | 5,200 |
| Average | 24,650 | 10,417 | 11,111 | 7,108 | 10,360 | 4,850 |
| EFFLUENT. | ||||||
| January | 129 | 206 | 166 | 91 | 39 | |
| February | 244 | 283 | 315 | 79 | 45 | |
| March | 455 | 405 | 133 | 67 | 34 | |
| April | 281 | 84 | 40 | 47 | 21 | |
| May | 134 | 68 | 56 | 35 | 48 | |
| June | 110 | 68 | 22 | 56 | 50 | |
| July | 25 | 50 | 39 | 106 | 22 | |
| August | 36 | 38 | 146 | 72 | 28 | |
| September | 6,850 | 42 | 40 | 37 | 98 | 67 |
| October | 1,216 | 116 | 60 | 30 | 33 | 28 |
| November | 161 | 175 | 64 | 37 | 27 | 122 |
| December | 111 | 364 | 84 | 67 | 24 | |
| Average | 2,084 | 176 | 121 | 91 | 61 | 46 |
| Average efficiency | 91.55 | 98.31 | 98.91 | 98.72 | 99.41 | 98.95 |
CHEMNITZ WATER-WORKS.
The only other place which I have found where anything approaching intermittent filtration of water is systematically employed is Chemnitz, Germany. The method there used bears the same relation to intermittent filtration as does broad irrigation of sewage to the corresponding method of sewage treatment; that is, the principles involved are mainly the same, but a much larger filtering area is used, and the processes take place at a lower rate and under less close control.
Fig. 17.—Plan of Area used for Intermittent Filtration at Chemnitz.
The water-works were built about twenty years ago by placing thirty-nine wells along the Zwönitz River, connected by siphon pipes, with a pumping-station which forced the water to an elevated reservoir near the city (Fig. 17). The wells are built of masonry, 5 or 6 feet in diameter and 10 or 12 feet deep, and are on the rather low bank of the river. The material, with the exception of the surface soil, and loam about 3 feet deep, is a somewhat mixed gravel with an effective size of probably from 0.25 to 0.50 mm., so that water is able to pass through it freely. The wells are, on an average, about 120 feet apart, and the line is seven eighths of a mile long.
It was found that in dry times the ground-water level in the entire neighborhood was lowered some feet below the level of the river without either furnishing water enough or stopping the flow of the river below. The channel of the river was so silted that, notwithstanding the porous material, the water could not penetrate it to go toward the wells.
A dam was now built across the river near the pumping-station, and a canal was dug from above the dam, crossing the line of wells and running parallel to it on the back side for about half a mile. Later a similar canal was dug back of the remaining upper wells. Owing to the difference in level in the river above and below, the canals can be emptied and filled at pleasure. They are built with carefully prepared sand bottoms, and the sand sides are protected by an open paving, to allow the percolation of as much water as possible, and the sand is cleaned by scraping, as is usual with ordinary sand filters, once a year or oftener.
The yield from the wells was much increased by these canals, but the water of the river is polluted to an extent which would ordinarily quite prevent even the thought of its being used for water-supply, and it was found that the water going into the ground from the canals, and passing through the always saturated gravel to the wells, without coming in contact with air at any point, after a time contained iron and had an objectionable odor.
To avoid this disagreeable result the meadow below the pumping-station was laid out as an irrigation field (Fig. 16). The water from above the dam was taken by a canal on the opposite side of the river through a sedimentation pond (which, however, is not now believed to be necessary and is not always used), and then under the river by a siphon to a slightly elevated point on the meadow, from which it is distributed by a system of open ditches, exactly as in sewage irrigation. The area irrigated is not exactly defined and varies somewhat from time to time; the rate of filtration may be roughly estimated at from 100,000 to 150,000 gallons per acre daily, although limited portions may occasionally get five times these quantities for a single day. The water passes through the three feet of soil and loam, and afterward through an average of six feet of drained coarse sand or gravel in which it meets air, and afterward filters laterally through the saturated gravel to the wells. The water so obtained is invariably of good quality in every way, colorless, free from odor and from bacteria. The surface of the irrigated land is covered with grass and has fruit-trees (mostly apple) at intervals over its entire area.