In the early days of sewage disposal no one dreamed that of the various systems in use, including disposal in water, all but one of the practicable processes depend upon bacteria for their efficiency; and that this single exception, chemical precipitation, would one day be held up as opposed to nature. Such has proved to he the case. The theory of intermittent filtration, when it was at last established on a scientific basis, was that the bacteria involved were Nero bic, or require an abundance of oxygen for their life processes. On this account, the sewage, which passes continuously through the beds while in service, was shut off at more or less frequent but regular intervals, depending on the character of the filtering material. As the sewage drained out of the beds air was sucked in to take its place, thus affording a new air supply for the bacteria in the beds, which, be tween closings, could occupy themselves with the stored organic matter. In the latest filters, or so-called bacteria beds, or contact beds, the germs are given a longer period to work on the sewage, while in some of the recent bacterial processes another class of microbe,s are enlisted in the service of man. In the bacteria beds
there is a sequence of filling, standing full, emptying and finally resting, each cycle re quiring from 8 to 24 hours, according to the periods of rest, which vary with local condi tions. If one bed does not effect a sufficient de gree of purification, a second and finer one, and even a third, may be employed. In case the sewage is held so long in a bed that the oxygen is exhausted, the aerobic bacteria give place to the anaerobic, or those thriving in the absence of oxygen. Anaerobic action may be secured by employing a receptacle containing no filtering material. known as the septic tank, through which the sewage flows slowly, but in which the suspended matters are retained by sedimentation, to be acted upon by the bacteria.
The anaerobic action breaks down or liquefies the organic matter; the aerobic action nitrifies it, or converts it into stable mineral compounds, available for higher forms of life. The septic effluent may be discharged on to filter beds, or into water not used for domestic supply, if the latter is ample in volume; and the effluent from bacteria beds may be used.