The rate of working this tank is much slower than the process at Champaign Illinois.
At Southwold England, there is another good illustration of this method. This has been called an open septic tank and is used in conjunction with two beds, an anaerobic, and an aerobic bed, the latter containing polarite.
This plant is interesting from the fact that the effluent from the anaerobic bed is distributed over the aerobic bed by means of a revolving sprinkler which prevents the liquid from passing unequally through the large grain, porous material. The action upon these beds is continuous.
From a report of this system the analysis of gas found in three samples are as follows:—
| Oxygen | Nitrogen | Carbon dioxide | Sulphuretted hydrogen | |
|---|---|---|---|---|
| Septic tank | 0.25% | 28.46% | 70.03% | 1.26% |
| Anaerobic bed | 7.26% | 39.10% | 52.37% | 0.91% |
| Aerobic bed | 20.62% | 78.63% | 0.75% | 0.00% |
Contact Bed
The contact bed as distinguished from the bacteria bed may be said to be made up of fine material, while the bacteria beds are built of coarse material. The latter are used for taking out the rougher solids, and the former for taking out the more finely divided material, and the organic matter in solution. While this distinction is not commonly made it seems to be growing into usage.
In the process of purification by means of contact beds the sewage is applied intermittently by distributing pipes or troughs so as to slowly fill the beds which are filled from a depth of from three to six feet with any kind of hard, porous, jagged material. The change which takes place is due to the action of certain bacteria in the presence of air. Although this process is not a new one, the method by which the results were obtained was not fully understood until the more recent discoveries in the science of bacteriology were made.
The beds are made water tight to a depth of about four feet, the bottom being channeled or tiled to drain the effluent either to a secondary bed or into the effluent channel.
Many eminent men have advocated special material as coal, coke, clinker slag, sand, and gravel, and even glass for filling material. The results do not differ materially and Prof. L. P. Kennicutt in an article in the Journal of the Association of Engineering Societies, February 1900 makes the following statement, “The material should be more or less porous so as to have a larger water absorbing area and have a jagged surface on which the gelatinized micro-organisms can be easily retained.”