The table above contains the results obtained in the various experiments, and from these it would appear as if on an average, with a velocity of 1 inch per minute, 25 per cent. of the total sludge would be destroyed or liquefied in a septic tank. Generally speaking, therefore, the following figures will be somewhat near the mark, where the plant is worked systematically and carefully supervised. Per cent. Suspended matters remaining in tank 35 “ “ destroyed or liquefied in tank 25 “ “ escaping in effluent 40 --- Total 100 These figures mean that 35 per cent. of the total suspended matters will have to be dealt with as sludge, 25 per cent. will be destroyed or liquefied in the septic tank, and the remaining 40 per cent. will be deposited on and in the contact beds. It has already been pointed out that it is claimed that the septic tank sludge is denser and contains less moisture than ordinary sludge, and that about half of it is mineral matter. As previously stated, at Manchester a reduction of about 16 per cent. in the dissolved matter has been observed in the open septic tank. (_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: Per cent. 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.]