Page:The New International Encyclopædia 1st ed. v. 18.djvu/31

* SEWAGE. tanks must be made so large that their expense is prohibitive. Small settling tanUs are oc- casionally used to remove some of the heavier and rapidly subsiding particles, and when com- bined with screens for the retention of coarse floating matter they appreciably lighten the work of filter beds, or diminisli water pollution. Chemical Precipitation is little more than ac- celerated sedimentation, although under certain conditions some of the dissolved organic matter is removed. A chemical with the jjowcr of pre- cipitating, or throwing down, the susi)ended mat- ters is admitted to and mixed with the sewage by simple means, after which the sewage passes to the settling or precipitating tanks, which are generally rectangular and not very deep. The tanks are operated on either the continuous or intermittent plan. If continuous, the several tanks are connected by weirs, so arranged that the sewage has to How the length of each tank before it is admitted to the next. The clarified sewage, or tank effluent, flows out in a thin sheet from the top of the last tank. "hen the solid' matter, or sludge, has aceunuilated in the bottom of the tanks to such an extent as to make its removal necessary, valves are opened in the out- let pipes and the effluent is drawn down to a point above the sludge level. To avoid disturb- ing the sludge a hinged pipe is used, the upper end of which floats at and falls with the surface level of the liquid in the tank. In the intermit- tent system each tank is filled, stands full the required period, then has its effluent decanted as described. The sludge is either pumped to filter presses or is run on to drainage beds, the object in either ease being to reduce the water contents. Where presses are used the re- sulting sludge cakes, as they are called, are some two inches thick and thirty inches in diameter, and still retain a large percentage of moisture. The final disposal of the sludge is often no easy task. It was originally supposed that it would sell readily, but as a rule managers of sewage works are fortunate if they can get farmers to remove it as a gift. Sometimes it is used to fill in land. In England sludge is not infrequently burned in refuse destructors, or garbage furnaces, with other town refuse. Another means of sludge disposal, available for seaboard cities, is dump- ing it at sea. The London County Council em- ploys a fleet of sea-going vessels for this pur- pose, having a capacity of 1000 long tons, or 2.'240,000 pounds of sludge each. Chemical pre- cipitation will remove about .50 per cent, of the total organic matter in sewage and nearly all the matter in suspension. The chemical most commonly used is lime, and next to it stands sulphate of alumina. The two are frequently used together. The first chemical treatment plant for town sewage seems to have been put in use at Man- chester, England, in 1844. The use of lime was suggested by Dr. Thos. Clark, of Aberdeen, who, during the same year, invented the lime process for softening water. (See Water Purifica- tion.) In the United States a small chemical precipitation plant was installed at the Brighton Beach Hotel, on Long Island, N. Y., in 1880. and the first town plant to treat sewage with chemi- cals was at Long Branch, N. J., where the works were put in operation in 1886. From 1887 to isnn several additional chemical plants were built, the most notable one being installed at 17 SEWAGE. Worcester, Mass., in the latter year. During the year I'JOO the Worcester precipitation plant treated an average of 1.3,000,000 gallons a day, using 12S0 pounds of lime for 1,000,000 gallons of sewage as a precipitant, or an average of 8.61 grains per gallon. The cost of treatment per 1,000,000 gallons was $11.70, of which $0.48, or more than half, was for sludge pressing and allied work. The average purification etl'ected, as indicated by the reduction of albuminoid am- monia, was 53.18 per cent, of the suspended or- ganic matter. In 1900 the city of Providence, R. I., opened a still larger chemical precipitation plant. It includes 20 precipitation tanks, ca- pable of holding collectively U.LSS.OOO gallons; a sludge well, sludge ejectors for lifting the sludge, five sludge storage reservoirs, and six- teen filter presses; besides which there are a large chemical storage building, a chemical lab- oratory, and various other accessories. The tanks are operated on the continuous flow plan. The sludge is forced from the sludge reservoirs to presses by compressed air. The sewers of Providence are on the combined system and pro- vision is nuide for wasting some of the combined storm water and sewage of heavy rains. Intermittent Filtration marks a new era in sewage disposal. The principles involved in this and the later and more rapid bacterial processes have already been stated. The amount of sewage which can be treated on one acre of intermittent filter beds ranges fi-om 20,000 to 100,000 gallons a day, according to the character of the material. ' Within these limits ordinary sewage may be brought to a high degree of purity. The best material for this process is a fairly coarse, angular sand, but with proper dosing either fine or very coarse sand may be used. Loamy earth is not suited tor intermittent filtration, on account of the low rates which must be employed; clayey soils are out of the question. Crops may be grown on intermittent filtration areas, providing they are made second- ary to the purification of the sewage. Bacteria Beds are largely an English out- growth, since 1891. of the Massachusetts work on intermittent filtration. There have been various modifications of these beds, such as the use of coal._ burnt clay, and coke, for filtering material; placing the beds in tiers, or in terraces; and aim- ing to use the anaerobic and aerobic bacteria to- gether, or the latter alone. But the essential fea- ture of bacteria beds is the retention of the sewage a longer time in the beds than is possible with intermittent filtration, after which there is a resting period, similar to that in the older process, but shorter. The bacteria beds were evolved in England because of the scarcity of sandy land suitable for intermittent filtration. It being necessary to transport sand or some other filtering material, and make it up into wholly artificial beds, it was imperative that the more expensive beds should treat the sewage at a higher rate. This was found to be possible, but the purification not being sufficiently com- plete for all conditions, a second, or even a third bed was added where necessary. There are many claimants for the introduction of bacteria beds, but it appears that the first and most practical early work was that begun in 1892 at the Barking chemical precipitation plant of the London sewerage system by W. ,T. Dilidin, chemist to the London County Council, aided by