THE SEPTIC TANK.

THE SEPTIC TANK.

Its Use In The Treatment of Sewage.

In these days when it is very properly being rendered a criminal offence to discharge sewage and polluting matter from factories, gas works, and the like into lakes and rivers, it is gratifying to read of the good results obtained by septic tanks—whose successful use in England has excited great interest among sanitary engineers. The Exeter tank is most frequently quoted. Its capacity is 53,800 gallons; it is closed, but in other parts of England the open tank is employed with equally good results. The septic tank, therefore, has come to occupy a distinct place in sanitation. Its effects are to render it easier to act upon the sewage by nitrifying bacteria. The composition of the sewage is also equalised by such a tank; since it reduces the amount of sludge by changing into gaseous products, or liquefying a large part of the suspended matter at the bottom of the tank—this suspended matter having been already somewhat reduced and partially purified by being passed slowly through the tank. It also removes from the sew age a portion of the solids held in solution, as well as the suspended matter itself; and increases the amount of free ammonia, while it decreases the albuminoid ammonia. The action of the septic tank, however, depends altogether upon its contents. If these are solid fecal matter, fermentation takes place, and the matter is destroyed, a homogeneous and slightly turbid fluid resulting, in which whatever suspended matter there may be is in the form of almost invisible filaments—caused by the action of those bacteria, which, if Pasteur’s opinion is correct, live and act only in vessels in which no oxygen is present. Domestic sewage, therefore, presents little or no difficulty when treated in septic tanks. But it must not be forgotten that whatever organic matter is removed depends altogether on the nature of the sewage and the construction of the tank. The percentage of removal will be the greater in proportion to the strength of the sewage, the weather—warm weather to be preferred to cold—and the amount of alkalinity present, acid sewage from manufacturing waste being more difficult to treat—the amount removed being not more than thirty per cent., as against about fifty per cent, in the case of domestic sewage, this calculation being founded on the amount of albuminoid ammonia in the latter and the use of a properly built tank. It is not the case, however, that the solid matter in sewage is so perfectly lique tied or changed into gas that there remains only a trifling and negligible quantity of ash or mineral part of animal or vegetable substances contained in it. If that were so, the whole problem of sewage disposal would be virtually solved. In all septic tanks (which should IK* carefully operated and equally carefully constructed) part of the organic matter is left over in a solid state—the amount depending altogether on the species of solid matter in the sewage, the nature of that sewage, and its flow through the tank, whether swift or slow, or regular or irregular in its distribution. As has been said, if fecal matter only forms the mass of the sewage, then the completeness of its decomposition is assured. If, however. much cellulose, or fatty matter is present in it the bacteria cannot easily act upon it, and there remains a comparatively large amount of solid, tinliquefied matter, iust as in the case when the sewage is acid or full of trade waste. At a temperature of seventy degrees Fahr., also, the decomposition is more complete than when it is only fifty degrees. As to the conversion of the sludge into soluble or gaseous substances: In the case of the ordinary sewage of a town, which receives surface drainage, and under the most favorable conditions, English experience seems to be that only forty per cent, of it is so destroyed, and only twenty-five per cent., if the sewage contains much acid or trade refuse. Sometimes, also, for some reason or another, the greater part of the arrested matter forms a crust on the top of the liquid, so that over seventy-five per cent, may remain unchanged in the tank. This occurs more frequently in this country than elsewhere, and is probably due to there not being sufficient mineral, or other heavy matter to carry the suspended matter down to the bottom of the tank, or to keep it there, when, by the action of the gas in the sludge, it rises to the top. The septic tank can be used with intermittent filtration beds, contact beds, and percolating filters. The sewage taken from these tanks is no longer crude, as the suspended matter has been virtually totally removed—at most seventy per cent, of organic matter has disappeared—wherefore, there is no need of catch-basins and screens, nor of constant ly raking over the suspended matter, which forms a coating on the sand almost impervious to water, while in winter such beds will not treat crude sewage with complete success, AS, likewise, the sewage from a septic tank varies but little during the twentyfour hours, the area of filtration can easily be reduced one-half. As to the bad smell arising from such tanks and the trouble caused by the removal of the sludge: The former can be regulated by the rate at which the sewage flows through the tank, as on that depends the septic action. If these tanks are intelligently cared for and operated, there is little, if any bad smell given off from ordinary town sewage. Where they are in use in England very little trouble has arisen from their use. The sludge, of course, must be cleaned out frequently, as a large amount of the total suspended matter of the sewage that passes through the tanks is left behind in the shape of a permanent deposit. That, however, can be deodorised, if necessary, and so removed without causing any inconvenience. The effluent from the tanks, also, may be diverted into any river, stream, or lake, without any risk of the water being contaminated thereby.

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