Page:The New International Encyclopædia 1st ed. v. 18.djvu/30

* SEWAGE. 16 SEWAGE. tation, alone, would effect all the purification necessary, as well as recover fertilizing mate- rial of great value. Unfortunately, the process was only a partial one, and left the decanted liquid, or sewage effluent, in a condition which was likely to give rise to great offense. At the same time the precipitate, or sludge, as the solid matter is called, proved to be unavailable for plant food. The next step was to try to coax a given ai'ea of land to do more work than before. The means employed, interiiiittent fil- tration, was to apply the sewage at intervals, on specially prepared areas, called filter beds, with periods of rest between. The raising of crops was made quite secondary, or abandoned. In some cases the filter beds were supplementary to sewage farms, designed to receive the sew- age when it would flood the crops; in others, effluent from precipitation works was applied to the beds. Where suitable land is available intermittent filtration is all that could be desired, in degree of purification effected, but in many sections the proper sort of land (sandy and easily drained) cannot be had. The relatively high rates of application, as compared with sewage farming, clog the beds with the organic matter retained on and in the filtering material. Re- course to sedimentation, or to chemical precipi- tation, many times tried, revives the old sludge problem. In the early days of sewage disposal no one dreamed that of the various systems in use, including disposal in water, all but one of the practicable processes depend upon bacteria for their efficiency, and that this single exception, chemical precipitation, would one day be held up as opposed to nature. Such has proved to be the case. The theory of intermittent filtration, when it was at last established on a scientific basis, was that the bacteria involved were aero- bic, or require an abundance of oxygen for their life processes. On this account, the sewage, which passes continuously through the beds while in service, was shut off at more or less frequent but regular intervals, depending on the character of the filtering material. As the sewage drained out of the beds air was sucked in to take its place, thus affording a new air supply for the bacteria in the beds, which, be- tween dosings, could occupy themselves with the stored organic matter. In the latest filters, or so-called bacteria ieds, or contact beds, the germs are given a longer period to work on the sewage, while in some of the recent bacterial processes another class of microbes are enlisted in the service of man. In the bacteria beds there is a sequence of filling, standing full, emptying and finally resting, each C}'cle re- quiring from 8 to 24 hours, according to the periods of rest, which vary with local condi- tions. If one bed does not effect a sufficient de- gree of purification, a second and finer one, and even a third, may be employed. In case the sewage is held so long in a bed that the oxygen is exhausted, the aerobic bacteria give place to the anaerobic, or those thriving in the absence of oxygen. Anaerobic action may be secured by employing a receptacle containing no filtering material, known as the septic tank, through which the sewage flows slowly, but in which the suspended matters are retained by sedimentation, to be acted upon by the bacteria. The anaerobic action breaks down or liquefies the organic matter; the aerobic action nitrifies it, or converts it into .stable mineral compounds, available for higher forms of life. The septic effluent may be discharged on to filter beds, or into water not used for domestic suppl}-, if the latter is ample in volume; and the effluent from bacteria beds may be used. Dilution is the method of sewage disposal most commonly employed outside of England. As usually practiced it can scarcely be said to be a system of disposal, since the sewage is dis- charged into the nearest body of water with little regard to consequences. In Jlassachu- setts. New York, Xew Jersey, and Ohio, all new disposal schemes must be approved by a cen- tral body, which is the State Board of Health in all States but New Jersey, and the State Sewage Commission in that conunonwealth. In England all new disposal works involving loans must be approved by the Local Government Board. The stringent legislation against water pollu- tion renders the employment of dilution alone a less common practice there than in America. The first principle in disposal by dilution, indeed, in all sewage disposal, is never to endanger a public water supply; the second is not so to over- load the stream or other l>ody of water as to create a nuisance. The best example in the United States of disposal by dilution was furnished, first by the city of Boston, and afterwards by Boston and other near-by towns united to form the Metro- politan Sewerage District. The various communi- ties in the district have their individual sewer- age systems. These all discharge into one or the other of two large trunk or outlet sewers, leading to carefully selected points of discharge. At one of the outlets, located at Moon Island, the sewage is stored in reservoirs and discharged at ebb-tide. At the other, or Deer Island outlet, it is discharged continuously. Pumping is nec- essary for each outlet sewer. A third outlet sewer, which will also discharge continuously in Boston Harbor, was under way in 1001. The other two were built in 1884 and 1S0>5, respec- tively. At Milwaukee and Chicago huge pump- ing works and tunnels were built sevei-al years ago to pump lake water into rivers badly pol- luted by sewage, mitigating the nuisance by dilution. The Chicago flushing tunnel was put in use in 1880, and the one at IMilwaukee in 1888. The Chicago Drainage Canal (q.v.) is by far the most notable work ever undertaken for the disposal of sewage by dilution. The ca- pacity of the canal was based on a flow of 1 cubic foot of water per minute to each five in- habitants, some years hence, or 2160 gallons of diluted sewage per day for each future inhabi- tant. Broad Ireigation, or Sewage Farming, does not differ essentially from ordinary irrigation ( see Irrigation ), except for the fact that sewage is used instead of normal water, and that the sewage is applied the year round, or as nearly so and in as large quantities as the land and crops will permit. See Sewage Farming for detailed descriptions of methods and results. Sedimentation, alone, is so slow that very little work is accomplished thereby, or else the