-----+----------------------------------------------- | Suspended Matters in Sewage. |-----------+-----------+------------+---------- | | | | No. | Remaining | Destroyed | Leaving | | in | and | Tank | Total. | Tank. | Liquefied | in | | | in Tank. | Effluents. | | | | | -----+-----------+-----------+------------+---------- | | | | | per cent. | per cent. | per cent. | 1 | 17 | 39 | 44 | 100 | | | | 2 | 41 | 22 | 37 | 100 | | | | 3 | 23 | 33 | 44 | 100 | | | | 4 | .. | .. | .. | .. | | | | 5 | average | 28 | .. | .. | say | | | 6 | | .. | .. | .. -----+-----------+-----------+------------+----------
[Sidenote: Septic tanks reduce the sludge difficulty to some extent, but do not altogether remove it.]
(_e_) _Destruction and Liquefaction of Sludge in Septic Tanks._—It was formerly maintained that the employment of a septic tank did away with all sludge difficulties, and one sees even now advertisements to that effect, that there is “no sludge” with a septic tank; but experience everywhere does not bear out this contention. On the contrary, there must be sludge with a septic tank, and the only question one has to consider is, to what extent does a septic tank reduce the quantity of sludge?
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.
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.
