at the bottom of the tank, where it is protected from the stream of sewage which is passing through the tank (Fig. 45a). The gases which are formed by the action of the anaerobic bacteria on the organic matter in the Effluent Influent Sediment .ation. Influent Sediment- Effluent 1 sewage pass up through the smaller, inner cylinder, and escape into the air. This prevents them from coming in contact with the sewage flowing through the tank. Sprinkling Filters.Sprinkling filters are the outcome of experiments with gravel filters at the Lawrence Experiment Station, but the experiments of Mr. Joseph Corbett, borough engineer of Salford, England, have brought this method of sewage purification into prominence in England. The filters are constructed of five feet of crushed stone resting on six inches of gravel, that is, gravel underdrains. The crushed stone employed varies in size from 0.25 to 2 inches in diameter. Sludge Digestion FIG. 45a.-Emscher tank. The sewage is applied intermittently to the surface of the filter in the form of a spray. This gives opportunity for the sewage to become aërated. Satisfactory results are obtained when the sewage contains about 80 per cent. of the amount of air required to saturate it. Sewage may be applied to filters of this character at the rate of 2,000,000 gallons per acre per day and obtain an effluent that is not putrescible. The effluent will contain only about 10 per cent. of the applied bacteria, and will show a reduction of the organic matter and its conversion into 1 Special Report Mass. State Board of Health, 1890, part 2, pp. 549 and 565. nitrates and nitrites through the agency of the nitrifying bacteria. During 1904-05 Mr. George A. Johnson conducted extensive investigations upon the serviceability of different modern methods of sewage purification at Columbus, Ohio. The methods of purification tested in these investigations were: sedimentation, precipitation, septic tank, filtration through sand filters, contact filters, and sprinkling filters. The process which is recommended to the city of Columbus as being the most satisfactory is that of the sprinkling filters, preceded by the preliminary clarification of the sewage in settling basins: "1. Preliminary clarification of the sewage in basins holding on an average about an eight-hour flow and operated on the basis of the septic treatment. "2. Purification of the septic effluent to a non-putrescible state by sprinkling filters at an average rate of 2,000,000 gallons per acre daily. "3. Final clarification of the effluent of the sprinkling filters in basins holding an average flow of about two hours." At the Sewage Experiment Station in Philadelphia the efficiency of the sprinkling filter in the purification of sewage was studied in detail. With filters exposed to the weather and receiving sewage "partially settled" the maximum rate obtained was 250,000,000 gallons per acre per day. With a filter protected from the weather a rate of three and one-tenth million gallons was used. The filters were constructed of trap-rock, one to three inches in size, in beds ranging from six to nine feet in depth. The bacterial efficiency of the sprinkling filters, when operated at a rate between 2,500,000 and 3,000,000 gallons per acre per day, was 400,000 per c. c., representing a reduction of 86 per cent. The degree of purification, which it is aimed to secure in the modern systems of sewage purification, is such that the effluent shall not be putrescible and give rise to no perceptible pollution of the body of water into which the effluent is allowed to flow. The amount of reduction of the organic matter in the sewage may vary in the effluent of different systems, depending upon the extent to which it is diluted by the body of water into which it is discharged and the use of such water at points near by. As the sprinkling-filter effluent contains large numbers of the sewage bacteria, it is at times advisable to treat this effluent with hypochlorites in order to disinfect it. Since 1859 the city of Birmingham, England, has been studying in a practical way various methods of sewage purification, beginning with sedimentation, chemical precipitation, filtration, septic tank treatment, and finally purification by means of sprinkling filters. They have found that the latter method is far cheaper and more satisfactory than any other method heretofore employed, and they are now displacing all other methods for the sprinkling filters. Removal of Sewage by Liernur System.-The satisfactory disposal of sewage is influenced directly by the fact whether the removal is by the separate or combined system. In the separate system of removal two sets of pipes are provided, one set for the household sewage, and another for the storm-water. This simplifies the purification process to a considerable extent. When the separate system of removal is employed some provision must be made for either flushing out the pipes carrying the household sewage, such as a flushing tank at the head of each branch sewer, or one of the methods of "air removal" may be employed. The method of air removal in more common use is what is known as the Liernur system. In this system there are two sets of pipes, the one set containing air only, and by the production of a partial vacuum in this set of pipes the sewage is extracted from the drain pipes. The system is so arranged that the discharges from each house are delivered into an air-tight metal reservoir, from which they are in turn drawn by suction into larger collecting tanks, these lat 1 ter delivering the matter into a stream or into the disposal works. The main collecting tanks, receiving the sewage from the different branches, are at the lowest point of the district drained. A powerful suction pump produces a vacuum in the system, and once a day the entire system is exhausted. Each branch drain and reservoir is extracted in turn by closing off the rest of the system by means of valves; and finally the main collecting tanks are extracted. The entire operation is the work of one man, who makes the tour of the system, his only labor being the opening and closing of valves. There is nothing to give offence to the senses in this system, as all the tanks are underground and the valves are operated from connections at the surface of the ground. Owing to the manner in which the material is collected, its conversion into fertilizer is commercially possible, so as to yield a considerable revenue. At Trouville, France, where the population during the season reaches 20,000, it is estimated that the receipts from the sale of the poudrette will furnish a material income over and above the operating expenses. This system of air extraction is especially adapted for places in which the sewers lie too low to discharge directly into streams. The Liernur system of extraction has been in operation for more than twenty-five years in Amsterdam and Leyden, and its success in these older installations has led to its introduction into other cities. The latest application of the method has recently been completed at the watering-place Trouville, France. Commercial Value of Sewage.- Many scientists have attempted, by chemical analysis, to demonstrate the commercial value of the constituents of sewage as fertilizer. One investigator has estimated the yearly solid and liquid excreta of an adult person to yield 7.44 kilograms, an amount sufficient to fertilize about 365 kilograms of wheat, rye, or oats, or about 410 kilograms. of barley; equivalent to 34 kilograms of Peruvian guana. Several scientists have estimated the manurial value of London sewage to range from 3 to 5 cents per ton, having an annual value of 14,000,000 to 20,000,000 dollars. It does not matter, however, what the intrinsic value may be of the manurial constituents of sewage, the nitrogen, phosphoric acid, and potash salts, so long as the conditions affecting supply and demand can neither be controlled nor regulated, its commercial value must remain very small-indeed, so small that sewage is much more likely to become a source of expense than one of revenue to any community. This has been the experience almost everywhere where attempts have been made to utilize sewage as fertilizer. The compressed sludge, the product of precipitation works, accumulates far more rapidly than it can be disposed of as fertilizer, and it frequently becomes a troublesome matter to dispose of it economically and satisfactorily. Even in such instances where sewage is utilized to irrigate cultivated fields it has at times been found detrimental to the crops raised, principally on account of the excessive quantities applied keeping the land constantly in a water-logged condition. This is said to have been the experience at Pullman, Ill. |