[Sidenote: Septic tanks only used in combination with contact beds.]
Although it has never been claimed, and is further not open to doubt,
that a septic tank alone and unaided by subsequent treatment in
intermittent or continuous contact beds does not sufficiently purify
the sewage, in these remarks the work of the septic tank only will be
considered, as the treatment in contact beds will be dealt with
separately.
[Sidenote: Septic tank a suitable name.]
(_a_) _Name of Septic Tank._—A good many names have been suggested by
different observers—such as “anaerobic fermentation tank,” “putrefying
tank,” “liquefying tank,” “cess-pit,” etc.—but there appears to be no
reason why the name “septic tank” should not be adhered to, as it
describes sufficiently correctly the work done by the tank, which is
chiefly of a septic nature.
(_b_) _Covered or Open Septic Tank._—Before dealing with the processes
taking place in a septic tank, it will not be out of place to consider
here, shortly, whether a closed septic tank confers advantages over an
open septic tank sufficiently great to justify the considerably
greater expenditure necessitated by its construction.
It is well known, that at Exeter in the first experimental
installation of this process, the septic tank was covered in by an
arched roof; but subsequent experiments made elsewhere do not seem to
support the theory then advanced, that such a tank should be a closed
one. This is chiefly due to the thick skin which, after a few months’
work, forms on the surface of closed or open tanks, and which
according to locality and season may reach a thickness of from 1 to 2
feet; it is maintained then that this cheap natural cover does away
with the expensive artificial cover.
In the report on the treatment of the Manchester sewage, by Messrs.
Baldwin Latham, Percy F. Frankland and W. H. Perkin, it is stated on
page 54, amongst the conclusions and recommendations, as follows:—“The
anaerobic or septic process is found to take place as effectively in
an open tank as in a closed one.” This conclusion does not appear to
have been modified by the experiments made subsequent to the issue of
this report.
In the Leeds experiment a similar result was obtained.
[Sidenote: Closed septic tanks possess generally speaking no
advantages over open ones.]
Whilst it would, therefore, appear to be correct to say, generally,
that closed septic tanks afford no material advantages over open ones,
so far as the purification of the sewage is concerned, they may become
necessary in special cases, when the smells emanating from open ones
might create nuisances in crowded neighbourhoods.
The following remarks refer, therefore, equally to open as well as to
closed septic tanks, and no distinction will be made between them.
[Sidenote: The work in the septic tank is chiefly done by obligatory
anaerobes.]
(_c_) _Explanation of Process._—Although the processes taking place in
septic tanks are at present but imperfectly understood, they may be
said to be in the main due to anaerobic micro-organisms, i.e. due to
such micro-organisms which carry on their life’s work in the absence
of oxygen. They split up or peptonise the organic compounds in the
absence of air, and the group of changes brought about by them has
been termed “anaerobic fermentation” or “putrefaction.” During the
same, it is claimed that a considerable amount of the sludge retained
in the tank is liquefied or destroyed, and that the rest becomes so
changed as to be denser than ordinary sludge, and to contain less
moisture.
[Sidenote: Dissolved matters entering and leaving the septic tank.]
Concerning the amount and nature of the dissolved matters entering and
leaving an open septic tank, the following is taken from the
Manchester report for the year ending March 27, 1901:—
[Sidenote: Manchester observations.]
“A series of determinations have been made of the amount of
dissolved matter entering and leaving the tank, by evaporating
known volumes of the sewage and effluent after filtration through
paper and weighing the solid residue.
“An average of six determinations (confirmed by similar
observations in connection with the closed septic tank) gave the
following results:—
----------------------------------+--------------------------------
Raw Sewage. | Open Septic Tank Effluent.
Dissolved matter, grains | Dissolved matter, grains
per gallon. | per gallon.
------------+--------------+-------+----------+-------------+-------
| Organic and | | | Organic and |
Mineral. | Volatile. | Total.| Mineral. | Volatile. | Total.
-----------+--------------+-------+----------+-------------+-------
33·0 | 33·0 | 66·0 | 30·8 | 25·0 | 55·8
| | | | |
| | | Reduction, in per cent.
| | | 6·67 | 24·24 | 15·45
-----------+--------------+-------+----------+-------------+-------
“A certain amount of loss of ammonia, as ammonium carbonate, will
take place on evaporation in both cases, and this will probably
be greater with septic tank effluent.
“An examination of the residue obtained by evaporating large
quantities of open septic tank effluent (filtered through paper),
shows that the mineral matters largely consist of iron oxide,
from the decomposition of organic compounds of iron, and calcium
sulphate. Among the volatile constituents have been detected
ammonium carbonate, mercaptan-like compounds of very offensive
smell, acetic and butyric acids. No evidence of the presence of
amines could be found in the residue on evaporation, but by
distilling large volumes of the liquid and carefully analysing
the platinum salts obtained from the distillate, the presence of
amines is indicated.
“Research in this direction is being continued; careful
comparison especially will be made of the products obtained by
evaporation and distillation of crude sewage and septic tank
effluent respectively.
“The evidence, however, points to a breaking down of albuminoid
and cellulose matter in the septic tank into simpler and to some
extent volatile compounds. The reactions are probably hydrolytic
in character, ammonia, amines, carbonic acid, water, and possibly
alcohol, being produced.
“A further quantity of organic matter also disappears as methane,
nitrogen and hydrogen.”
[Sidenote: Must aerobic fermentation in all cases be preceded by
anaerobic fermentation?]
It will be clear from the foregoing, that the changes going on in a
septic tank are entirely different from those brought about in contact
beds, and the question whether a septic tank is a necessity for the
subsequent contact bed treatment, or whether it is a distinct
disadvantage, can only be definitely settled when we know whether
aerobic fermentation, i.e. decomposition, must in all cases be
preceded by anaerobic fermentation, i.e. putrefaction, and to what
extent, or whether such a succession of changes is not necessary.
[Sidenote: At Manchester contact beds accustomed to septic tank
effluent did not at once purify raw sewage.]
It is interesting to note in connection with this point, that during
the Manchester experiments it was established that contact beds, which
have become accustomed to septic tank effluent, will not at once
purify comparatively fresh sewage.
(_d_) _Velocity of Flow through Tank._—The velocity of flow through
the septic tank is of great importance, as on it depends the size of
the installation.
It seems to have become a habit to express this velocity by the length
of the sojourn of the sewage in the septic tank—for instance, “the
flow of sewage through the tank was such that it would fill it in
twenty-four hours”; but as all tanks vary in size, and as in
consequence the distance which has to be traversed by the sewage from
the entrance to the exit in twenty-four hours is different in nearly
every case, such a habit is, to say the very least, very misleading.
It will not be disputed that the deposition of the suspended solids in
sewage is dependent on the rate of movement of the liquid, and that in
a quickly moving liquid there will be less deposition than in a very
slowly travelling liquid.
-----------------+----------+---------+-----------+-------------
Town. | Length. | Width. | Depth. | Contents.
-----------------+----------+---------+-----------+-------------
| | | |
| feet. | feet. | ft. in. | gallons.
Manchester tanks | 300 | 100 | 6 0 | 1,125,000
| | | |
Leeds “ | 100 | 60 | 7 7 | 250,000
-----------------+----------+---------+-----------+-------------
Bearing this in mind it will not be without interest to examine the
velocities employed during the Manchester and Leeds experiments. The
tanks employed in these have the dimensions given in the table on the
preceding page.
Now assuming that each tank is to be filled once in twenty-four hours
we obtain the following velocities:
Manchester (300′ 0″ × 12″)/1440 = 2″·5 per minute.
Leeds (100′ 0″ × 12″)/1440 = 0″·84 per minute.
Which means that in the Manchester experiments the velocity would have
been three times as large as in Leeds; and it is clear that if the
sewage of both towns was identical the results, so far as the
retention of the suspended matters in the tanks are concerned, could
not have been identical. As a matter of fact, considerably greater
velocities have been used in the Manchester experiments, as will be
shown later on.
[Sidenote: Rate of flow through septic tanks should not be expressed
by the length of sojourn in tank but by some linear measurement in a
stated time.]
It will be clear from this, that it is most misleading and erroneous
to express the rate of flow by the length of the sojourn of the sewage
in the tank, and that the velocity should in each case be expressed by
some linear measurement in a stated time—probably inches per minute.
The next point to consider is the velocity to be employed in septic
tanks; and here it is not without interest to refer to the various
experiments enumerated with their results in the next table.
The difference in the results obtained, so far as the suspended
matters are concerned, is probably due to the different character of
the various sewages experimented with; but so low a velocity as 0·52
inch, as used in the Exeter experiments, does not appear to be
necessary.
In the Leeds experiments, it was found that the filling of the tank
once in twenty-four hours gave the best results; and as the velocity
then was 0·84 inch per second it will be somewhat near the mark to
recommend generally a velocity of 1 inch per minute. On the assumption
that the sewage shall remain twenty-four hours in the tank, this gives
a length of tank of 120 feet, which is a very suitable one.