Another good example of the septic tank method is the one in use at Exeter, England. This plant has been in operation since 1896. It was designed for 98000 gallons capacity. The sewage is allowed to remain in the tank nearly 24 hours. The tank is used in conjunction with five beds in which further purification is used. The beds are filled with coke breeze and one is always resting for a week at a time. The tank is made nearly air tight by an arched roof and any gasses collecting may be burned at a vent. 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:— +---------------------------------------------------------------------+ ¦ ¦ ¦ ¦ Carbon ¦Sulphuretted ¦ ¦ ¦ Oxygen ¦ Nitrogen ¦ dioxide ¦ 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.