CHEMICAL METHOD 349 



suspended material, including bacteria, is removed by the sand 

 which becomes more efficient as used, due to the rapid formation 

 of a mat of finely divided sediment, in which protozoa often multi- 

 ply, and assist biologically in removing many bacteria. In time the 

 mat becomes very thick and the filtration although effective is 

 unduly slow. The water is then allowed to subside below the 

 surface and about half an inch of the sand removed, after which 

 filtration is resumed. The sand removed is washed to. free it from 

 collected impurities and is later replaced on the bed after succes- 

 sive scrapings have reduced the filter to about one foot in thickness. 



The filters are usually divided into units of convenient size,, 

 about half an acre, so that one unit may be cleaned without inter- 

 ruption of the system. The slow sand filter removes about 99 

 per cent, of the bacteria, about one-third of the coloring matter 

 and its long effective use has established the fact that it has a favor- 

 able effect upon the health of the community where used. 



Chemical Method. The chemical disinfection of water on a 

 large scale is now almost exclusively effected with substances 

 yielding chlorin, chief of which are bleaching powder (chlorid of 

 lime), sodium hypochlorite and free chlorin. The action of these 

 substances is essentially similar and dependent upon the quantitative 

 active chlorin which they contain. They are usually added in 

 quantities sufficient to give from 0.5 to 1 part of active chlorin 

 per million parts of water. 



The use of bleaching powder in the purification of waters is 

 cheap, reliable, harmless and easy of application, which makes it 

 an attractive method, but when used on impure waters containing 

 organic matter it gives rise to amins, chloramins and other com- 

 pounds of unknown composition which impart to the water unpleas- 

 ant flavors. 



Alum is often used either alone or in connection with the mechan- 

 ical sand filter, and if used under controlled conditions is very 

 effective and leaves no undesirable constituents in the water. 

 The quantity should be accurately determined for each water as 

 it varies with the turbidity and quantity of calcium carbonate 

 contained in the water. 



Potassium permanganate is often used in the disinfecting of 

 small quantities of waters, but its effectiveness cannot be depended 

 upon except against the cholera spirillum. Moreover, the disagree- 

 able taste and the color imparted to the water are a serious drawback. 



Chlorazene, the new disinfectant suggested by Dakin, has much 

 which commends itself for use in the disinfection of small quantities 

 of water, as in the concentration of 1 : 300,000 it will sterilize ordi- 

 narily heavily contaminated water in thirty minutes. Such a 

 concentration imparts a very slight taste to the water but is per- 

 fectly palatable. It is non-toxic and if used for only short intervals 



