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. [Illustration: 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.