Water Purification

WATER PURIFICATION. The partial purification of water probably dates back to the earliest times. No doubt, when a water looked very turbid, it was strained through any convenient material, or the suspended matters were allowed to settle out on standing. We know, too, that mariners found that dirty ill-tasting water collected at one port became often sweet and clean before the next port was reached. It is also not improbable that boiling water to purify it may have been an ancient precaution. Ages ago, everyone must have recognized the potent force of heat, and our ancestors possibly had their suspicions that dirty water caused disease and hence used heat as the most likely method of destroy ing impurities. Probably too, all sorts of things may have been added to water with the object, if not of purifying it, of at least rendering it more potable. Certainly as far back as 1612 the im portance of keeping filthy matters out of drinking water was well known, for in an indenture, relating to the New river, the following words occur : "Wee doe by these presents for us our heirs and Successors straightly charge and Comaund all pson and psons whatsoever That they or anie of them doe not hereafter cast or putt into the said new river anie earth rubbish soy le gravell stones dogges Catts or anie Cattle Carrion or anie unwholesome or uncleane thing nor shall wash nor clense anie clothes wooll or other thinge in the said river . . ." ". . . nor shall make or convey anie sincke, ditch Tanhowse dying howse or seege into the said river or to have anie fall into the same." As time went on sand came to be recognized as an effective filtering material, judged, no doubt, at first, by the clarified con dition of the filtered product. Sand filtration, as a practicable proposition, dates back to 1829 and the honour seemingly be longs to London (Chelsea Water Co.). Later, the purification of water by sand filtration became an established procedure and the method has been copied all over the world and is still talked about as the London system of water purification. Quite apart from the bacterial "findings," it came to be recognized that an adequately filtered water was safe as regards disease. Then Koch showed why safety was secured, by proving that filtration re moved 98 per cent of the bacteria. By this time bacteria had

been assigned a definite role in the causation of disease.

The London rate of filtration is very slow, namely, about 2 gallons per sq. ft. per hour, or about 4 inches vertical drop in the same time. Approximately, it is represented by observing the progress of the end of the minute hand of a watch as it hourly completes its circular journey. Fine sand (about 3 ft. in depth) was nearly always used, supported on a graded gravel substratum.

Efficacy of Sand.

At first, sanitarians wondered why sand could be so effective when the separate particles were so gross in size compared with bacteria. It came, however, eventually to be recognized that the sand particles gradually matured so that each grain was surrounded with a furry yet slimy coating. In addi tion, the particles at, or near, the surface were observed to be enveloped and covered over with a heavy coating of slimy mate rial which eventually became so dense as practically to stop the passage of water through it. When this occurred it became necessary to empty the filter, scrape off the surface layers of dirty sand and wash it very thoroughly before replacement. When a bed was re-started the results at first were apt to be unsatis factory, and the usual custom was to run the filtrate to waste for several hours, or days. Even when the filtered water was allowed to pass into consumption, many authorities advocated a specially low rate of filtration being maintained for several days. This fil tration has greatly reduced water-borne epidemic disease.

The increasing difficulty of securing supplies near at hand gradually led the great towns to invade the uplands and moorlands. Then new troubles arose, for although very pure and soft, such waters were often peaty and highly coloured and some of them were acid and acted upon lead. Experience showed that waters of this kind were best treated by filtering them through mechani cal filters at the rapid rate of about 5o gallons per sq. ft. per hour, using a coagulant (sulphate of alumina, dose usually about 1-2 grains per gallon) to remove the colour and render the filtered water attractive in appearance. These mechanical filters are easily and expeditiously cleaned by a reversal of the flow of water.