620 BACTERIOLOGY OF THE SOIL, WATER, AND AIR 



drains are designed to remove the purified water and they have little 

 or nothing to do with the actual process or purification. The sand 

 layer per se has little action in the purifying process; it does not 

 strain out bacteria, because the spaces between the sand grains are 

 very great compared with the size of the organisms. The sand does 

 support upon its upper surface, however, a thin, delicate continuous 

 layer of microorganisms, the Schmutzdecke, through which the 

 water (or sewage) passes. This layer is so compact and so closely 

 matted together that all suspended matter (including both pathogenic 

 and non-pathogenic bacteria) in the supernatant water is strained 

 out, and the dissolved organic substances pass with the raw or unfil- 

 tered water through the bodies of the microorganisms which collec- 

 tively comprise the Schmutzdecke. During this passage the dissolved 

 organic matter undergoes the same general degradation to nitrates 

 and other fully-mineralized products of microbic digestion that 

 organic substances in the upper layers of the soil undergo; the puri- 

 fication of water by sand filtration is, therefore, a catabolic phase in 

 the nitrogen cycle, brought about by bacterial activity precisely as 

 the mineralization of organic substances in the upper layers of the 

 soil is a catabolic phase of the nitrogen cycle. The final products in 

 each case are normally nitrates and other inorganic salts. 



The efficiency of the purification of water or of sewage by the 

 method of sand filtration is therefore to be measured chemically and 

 bacteriologically. Chemically a complete transformation of complex 

 organic compounds (ordinarily determined as albuminoid and "free 

 ammonia") to nitrates is an indication that the digestive power of the 

 filter is at par. Bacteriologically a disaupearance of all bacteria 

 derived from human or animal excrement and a great reduction of 

 the total numbers of bacteria in the filtered water as compared with 

 the unfiltered water is evidence of the bacterial efficiency of the 

 filter. 



The chief source of danger in potable waters is bacterial contamina- 

 tion from human sources. A simple inspection of water frequently 

 fails to detect contamination, and even a chemical examination may 

 not suffice to reveal pollution. Millions of typhoid bacilli may be 

 introduced into a liter of water without inducing changes that could 

 be detected visually or chemically. The bacteriological examination 

 of water, therefore, is from ten to one hundred times more delicate 

 than the chemical examination as a means of detecting contamination 

 of water with human or animal waste. 



