Decomposition of organic matter by bacterial agency is not a complete
method of treating sewage, as will be shown later under "Septic tanks."
It is sufficient to observe here that in all practical methods of
treatment aeration plays a vital part. The air or the sewage, or both,
must be in a finely divided state, as when sewage percolates through
the interstices of a porous, air-filled soil. The principle involved
was clearly stated 30 years ago by Hiram F. Mills, a member of the
Massachusetts State Board of Health. In discussing the intermittent
filtration of sewage through gravel stones too coarse to arrest even the
coarsest particles in the sewage Mr. Mills said: "The slow movement of the
sewage in thin films over the surface of the stones, with air in contact,
caused a removal for some months of 97 per cent of the organic nitrogenous
matter, as well as 99 per cent of the bacteria."
[Illustration: BPR-RE 1387
Fig. 6.--A typical nuisance. (Board of Health, Milwaukee.) A yard like
this is an eyesore, a fire menace, a breeding place for mosquitoes and
vermin, a refuge for rats and mice, a source of noxious odors and foul
drainage, and a violation of every sanitation code.]