. A treatise on hygiene and public health . this operation is rendered increase of pressure, too, drives the impurities to a greater depthinto the sand; and there is liability, in filtering under too great a head,when the bed has become somewhat clogged, especially if the water inthe clear-water well is allowed to fall below the level of the sand, thatthe water will force its way through the sand where the clogging materialoffers the least resistance, and thus pass downward irregularly and inactual streamlets. In some places the filter-beds are cleaned by«forcing the filtered wat
. A treatise on hygiene and public health . this operation is rendered increase of pressure, too, drives the impurities to a greater depthinto the sand; and there is liability, in filtering under too great a head,when the bed has become somewhat clogged, especially if the water inthe clear-water well is allowed to fall below the level of the sand, thatthe water will force its way through the sand where the clogging materialoffers the least resistance, and thus pass downward irregularly and inactual streamlets. In some places the filter-beds are cleaned by«forcing the filtered waterbackward through the filtering material, and stirring up at the same timethe upper part of the sand-layer. The dirty water is allowed to overflowand to run to waste. This method presupposes an abundance of waterand the ability to command the necessary pressure, and is hardly to berecommended. It is practised at Ziirich, in Switzerland, where the waterfiltered is practically spring water rendered turbid by clay or other min-eral Object and Hesidts of Filtration on the JLarge Scale. Having considered the method of filtration in common use, we maynow profitably inquire more closely into the object which it aims to ac-complish, and the results which are actually obtained. Filtration, in its strict sense, is simply a mechanical operation, and 272 OJf DRINEII^G-WATEE AND PUBLIC WATER-SUPPLIES. directs itself to the removal of such substances as are carried in suspensionin the water. The suspended matter, which by its presence in our water-supplies makes filtration desirable, is somewhat various in the suspended matter will settle quite readily b}^ virtue of thecomparatively high specific gravity of the particles, as will be the case ofthe mineral matter consisting of sand, mica, etc. Such substances arereadily removed by filtration; but we have seen that it is generally moreeconomical to subject the water to a process of sedimentation first, andsettlin
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Keywords: ., bookcentury1800, bookdecade1870, booksubjecthygiene, bookyear1879
