The term mechanical filters is used to designate a general class of filters differing in many respects quite radically from the sand filters previously described. They had their origin in the United States, and consisted originally of iron or wooden cylinders filled with sand through which the water was forced at rates of one to two hundred million gallons per acre daily, or from fifty to one hundred times the rates usually employed with sand filters. These filters were first used in paper-mills to remove from the large volumes of water required the comparatively large particles, which would otherwise affect the appearance and texture of the paper; and in their earlier forms they were entirely inadequate to remove the finer particles, such as the bacteria, and the clay particles which constitute the turbidity of river waters. Various improvements in construction have since been made, and, in connection with the use of coagulants, much more satisfactory results can now be obtained with filters of this class; and their use has been extended from manufacturing operations to municipal supplies, in many cases with most satisfactory results.
The information gathered in regard to the conditions essential to the successful design and operation of these filters in the last few years is very great, and may be briefly reviewed.
PROVIDENCE EXPERIMENTS.[39]
The first data of importance were secured from a series of experiments conducted by Mr. Edmund B. Weston of Providence, R. I., in 1893 and 1894, upon the Pawtuxet river water used by that city. The experimental filter was 30 inches in diameter, and had a layer of sand 2 feet 10 inches deep. The sand was washed by the use of a reverse current, the sand being stirred by a revolving rake at the same time. The amount of coagulant employed was about 0.7 of a grain per gallon. The raw water was practically free from turbidity, and the filter was operated to remove color and bacteria.
The removal of color, as stated in Mr. Weston’s report, amounted to from 70 to 90 per cent. The experiments extended over a period of ten months. The rate of filtration employed was about 128 million gallons per acre daily. The bacterial results of the first six months’ operations were rejected by Mr. Weston on account of defective methods of manipulation.
During the period from November 17, 1893, to January 30, 1894, the average bacterial efficiency of filtration was about 95 per cent, and the manipulation was considered to be in every respect satisfactory. The efficiency was occasionally below 90 per cent, but for four selected weeks was as high as 98.6 per cent. The average amount of sulphate of alumina used, as calculated from Mr. Weston’s tables, was two thirds of a grain per gallon. The highest efficiency followed the application of a solution of caustic soda to the filtering material. The first day following this treatment the bacterial efficiency was above 99 per cent. Afterwards it decreased until January 30, when the experiments were stopped. The high bacterial efficiency following the use of caustic soda was of such short duration as to suggest very grave doubts as to its practical value. It is extremely unfortunate that the experiments stopped only a week after this experiment, and the results were never repeated. I consider that the average bacterial efficiency of about 95 per cent obtained for the period of October 17 to January 30, when the manipulation was considered to be in every way satisfactory, more nearly represents what can be obtained under these conditions than the results for certain periods, particularly after the use of the caustic soda.
LOUISVILLE EXPERIMENTS.[40]
These experiments were inaugurated by the Louisville Water Company in connection with the manufacturers of certain patented filters. Mr. Charles Hermany, Chief Engineer of the Company, had general charge of the experiments. Mr. George W. Fuller was Chief Chemist and Bacteriologist and had direct charge of the work and has made a most elaborate report upon the same. In these examinations many devices were investigated; but the two which particularly deserve our attention are the filters known as the Warren Filter and the Jewell Filter.
These filters were operated for two periods, namely, from October 18, 1895, to July 30, 1896, and from April 5 to July 24, 1897. The investigations were directed toward the clarification of the river water from the mud, and to the removal of bacteria. The water was substantially free from color. The character of the water at this point was such that in its best condition at least three fourths of a grain of sulphate of alumina were necessary for its coagulation, and with this and with larger quantities of coagulant fair bacterial purification was nearly always obtained. The problem studied therefore was principally that of clarification from mud. The average efficiencies, as shown by the total averages, (page 248,) were as follows: Warren filter, bacterial efficiency, 96.7 per cent; Jewell filter, 96.0 per cent.