(_f_) _Formation of Gas in Septic Tank._—It was at one time suggested that the gases formed in septic tanks during anaerobic fermentation might be utilised for lighting or heating purposes, but anyone well acquainted with the subject will admit that such a use is outside the range of practical possibilities.
At Manchester, 100 gallons of sewage evolved in twenty-four hours about a cubic foot of gas, which on an average contained:
Marsh gas, CH{ 4} 73 Carbon dioxide, CO{ 2} 6 Hydrogen, H 5 Nitrogen, N (by difference) 16 --- Total 100
At this rate 1 million gallons of sewage will evolve 10,000 cubic feet of gas, or 0·2 tons of gas, in twenty-four hours.
[Sidenote: Septic tank effluent more suitable for nitrification.]
(_g_) _Mixing Action of Septic Tank._—There is one advantage possessed by a septic tank which cannot be disputed, and that is the mixing action going on within it. The fresh sewage on its arrival becomes mixed with stale sewage, and, owing to the rising of lumps of sludge from the bottom, and other causes, the contents of the tank become of a more uniform composition, which must entail a corresponding advantage for the subsequent contact bed treatment.
[Sidenote: The septic tank effluent is so far as bacterial purity is concerned practically raw sewage.]
(_h_) _Micro-organisms in Effluent from Septic Tank._—Although the available number of experiments on the micro-organisms contained in the effluent from a septic tank is not large, yet they support the conclusion which one would form by analogous reasoning, that so far as the bacterial flora is concerned the effluent is practically raw sewage.
[Sidenote: Continuous contact beds still in an experimental stage.]
It is necessary to make at this point a few short observations on the artificial self-purification in continuous contact beds.
This method of artificial purification has frequently been called “continuous filtration,” but it will be much better to reserve the term “filtration” for the percolation of water through fine material, such as sand, and to call the continuous flow of sewage through coarser material continuous contact bed treatment, as the processes going on during the same are more analogous to those going on in an intermittent contact bed than to those taking place in a waterworks filter.