WATER AND SEWAGE 699 
remembered that bacteria of the soil effect a mineralization of orj^anic 
substances, and the purification of water and of sewage, which is 
grossly polluted water, is ordinarily accomplished by a direct applica- 
tion of the same natural process. 
For convenience in operation, filters are constructed which are 
essentially water-tight basins (to pre^'ent the entrance of extraneous, 
unpurified water) containing underdrains covered with a layer of 
sand of uniform size, from 2 to 4 feet in thickness.^ The underdrains 
are designed to remove the purified water and they have little or 
nothing to do with the actual process of 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 micro- 
organisms, 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 unfiltered water through the 
bodies of the microorganisms which collectively comprise the Schmutz- 
decke. 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 purification 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 inor- 
ganic salts.- 
The ever increasing cost of land around cities has led to the use of so- 
called mechanical filters as substitutes for the slow sand filters. INIechan- 
ical filters differ from slow sand, or "biological" filters in that an 
artificial "Schmutzdecke" is produced upon the surface of the sand by 
the addition of alum to the water before it reaches the filter itself. 
The slightly alkaline reaction of the water causes A1(0H)3 to form from 
the alum. This settles upon the sand surface as a dense layer nearly 
impervious to bacteria. Mechanical filters may be operated at rates 
varying from 100,000,000 to 200,000,000 gallons of effluent per day, 
whereas slow sand filters rarely yield over 5,000,000 gallons per day. 
Mechanical filters, which have their name from the mechanical device 
for washing the sand when it has become clogged, and fails to deliver the 
requisite amount of filtered water, do not digest the organic matter in 
suspension and in solution in the water, hence they do not contribute 
1 The details of structure and operation of filters designed for the purification of 
water and sewage are beyond the scope of this volume. 
2 Chick, H.: Study of Process of Nitrification with Reference to Purification of 
Sewage, Proc. Roy. See, Series B., 1906, 77, 241. 
