_Rate of Flow and Deposition of Suspended Matters._
-----+------------+-----------------------+
| | Rate of Flow. |
| +-----------------------+
| | | |
No. | Name of | Length of | Velocity |
| Town. | Sojourn | of Flow |
| | of Sewage | per |
| | in Tank. | minute. |
| | | |
-----+------------+-----------+-----------+
| | | |
| | days. | inches. |
1 | Exeter | 1·0 | 0·52 |
| | | |
2 | Manchester | 0·44 | 5·58 |
| | | |
3 | “ | 0·56 | 4·44 |
| | | |
4 | Leeds | 0·5 | 1·68 } |
| | | |
5 | “ | 1·0 | 0·84 } |
| | | |
6 | “ | 2·0 | 0·42 } |
-----+------------+-----------+-----------+
-----+-----------------------------------------------
| 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.
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.]
(_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.