The expressions “contact bed system” or “oxidation bed system” are in so far inappropriate as they describe only portions of the process but not the whole. The term “contact bed” describes the first stage, and the term “oxidation bed” portion of the second stage only.
Term most suitable.
The term which seems most suitable of all is “artificial self-purification in contact beds,” as it includes every phase of this lengthy process applied in an artificial form; the term “natural self-purification” being applied to land treatment of sewage, as it is the only method in which the self-purifying powers are employed under natural conditions.
Working operations.
(b) Explanation of Process.—The cycle of operations commences with the filling of the bed, and during the same the sewage comes gradually in contact with the filling material. When the bed is full, the inflow is stopped and the sewage allowed to remain in contact with the material for some time. The bed is then emptied, and a period of rest is given it before the filling is commenced again.
Purification of sewage in full bed due to absorbing powers of filling material and only to a small extent due to activity of micro-organisms.
It has been held, that while the sewage is in the contact bed it undergoes a very rapid process of decomposition by bacteria, but it must be evident, that as the sewage—including filling—remains only for about two hours in the bed, the micro-organisms would have to work at an express rate. This fact alone is apt to make this theory very doubtful, but apart from it, it has been proved by experiments that the by far greater amount of purification—whilst the sewage is in the beds—is due to the absorbing powers of the filling material, which are derived from the surface attraction of its component particles.
Retention of suspended matters by bed.
Absorbing powers of filling material.
The filling material retains in its upper layers the suspended matters, which it strains out of the sewage in a purely mechanical manner, much after the fashion of a screen, and when the bed is filled its absorbing powers come into play, which cause the removal of the dissolved matters out of the liquid and their retention on the surface of the particles. This latter process is probably a chemico-physical one assisted by the micro-organic life in the sewage.
Decomposition of organic substances by micro-organisms when bed is empty.