[Sidenote: Enumeration of more important experiments.]
A great many experiments have been made during the last ten years with
artificial processes for the self-purification of sewage, and amongst
the more important the following may be mentioned:
London experiments.
Sutton „
Exeter „
Manchester „
Leeds „
Sheffield „
Leicester „
York „
Hamburg „
[Sidenote: Experiments have not been conducted on uniform lines.]
A casual observer might, therefore, consider himself justified in
thinking that all these experiments had added a great deal to our
knowledge of the intricate changes taking place in these processes,
but such a conclusion would not be justified in reality. For beyond
settling questions of local importance by chemical analysis, the
experiments, owing to a variety of causes, have not materially
enhanced the stores of our information, indeed not unfrequently the
results obtained are apparently contradictory and bewildering.
An experiment must be looked upon as a question addressed to nature,
and the answer will depend on the way the question has been put. If
this way differs in every case it must be clear that the answer, too,
will differ in every case, and it is this absence of uniformity which
greatly reduces the general value of these experiments.
These remarks must not be misunderstood to convey the impression as if
the experiments had not been conducted with care and skill! Far from
it! Some of them have been made with the greatest skill and care and
with the very evident desire to arrive at correct conclusions, and it
is only when they are placed side by side with other experiments, with
a view to deducing from them general conclusions concerning the
processes at work, that great difficulties are experienced. The result
of each experiment is governed by a large number of factors, which by
slightly different manipulations may attain in this ever-fluctuating
process different weights, so that the results may be contradictory,
and it is only by arranging these factors on a common basis, as it
were, and by addressing the questions to nature in the same systematic
and uniform way, that good general results may be expected.
It is well known, for instance, that in some cases septic tanks have
not given good results, whilst in others they have worked very well;
again, continuous filtration has failed in some experiments, whilst in
others, notably in the York experiments, it has given good results.
If, therefore, in future the mistake of the past is to be avoided, it
will be necessary to settle on a common line of action in all
experiments.
[Sidenote: Attempt to evolve general theory.]
In spite of all the difficulties which beset such a task, an attempt
will be made in the following observations to evolve some general
theory concerning the processes at work in the artificial
self-purification of sewage. Such a theory, it is quite clear, cannot
be complete in the present state of our knowledge, and it is sincerely
hoped that the many and serious gaps will be filled up by later
investigations.
For convenience of reference the different forms of the process, such
as are now employed, shall be dealt with separately, commencing with
contact or oxidation beds.
2. ARTIFICIAL SELF-PURIFICATION OF SEWAGE IN INTERMITTENT CONTACT
BEDS.
At the outset it may not be out of place to make a few remarks
concerning the various names given to this form of application. The
term “intermittent contact bed” is here used to distinguish this kind
of bed from the “continuous contact bed,” frequently called
“continuous filtration.”
[Sidenote: Names of process misleading.]
(_a_) _Name of Process._—This process has frequently been called
“biological process,” “bacteriological process,” “contact bed system”
or “oxidation bed system,” but all these terms do not appear to define
it sufficiently, as they do not cover the whole, but only phases or
stages in the same; hence, they do not seem appropriate.
[Sidenote: Biological process.]
The name “biological process” is decidedly misleading, for besides
biological agencies there are also at work physical (mechanical) and
chemical ones.
[Sidenote: Bacteriological process.]
The term “bacteriological or bacterial process” is likewise erroneous,
for besides bacteria a number of other micro-organisms participate in
it—such as yeast fungi, mould fungi, algæ, protozoa, and even higher
forms of life, such as earthworms and insects.
[Sidenote: Contact bed system.]
[Sidenote: Oxidation bed system.]
The expressions “contact bed system” or “oxidation bed system” are in
so far inappropriate as they describe only portions of the process but
not the whole. The term “contact bed” describes the first stage, and
the term “oxidation bed” portion of the second stage only.
[Sidenote: Term most suitable.]
The term which seems most suitable of all is “artificial
self-purification in contact beds,” as it includes every phase of this
lengthy process applied in an artificial form; the term “natural
self-purification” being applied to land treatment of sewage, as it is
the only method in which the self-purifying powers are employed under
natural conditions.
[Sidenote: Working operations.]
(_b_) _Explanation of Process._—The cycle of operations commences with
the filling of the bed, and during the same the sewage comes gradually
in contact with the filling material. When the bed is full, the inflow
is stopped and the sewage allowed to remain in contact with the
material for some time. The bed is then emptied, and a period of rest
is given it before the filling is commenced again.
[Sidenote: Purification of sewage in full bed due to absorbing powers
of filling material and only to a small extent due to activity of
micro-organisms.]
It has been held, that while the sewage is in the contact bed it
undergoes a very rapid process of decomposition by bacteria, but it
must be evident, that as the sewage—including filling—remains only for
about two hours in the bed, the micro-organisms would have to work at
an express rate. This fact alone is apt to make this theory very
doubtful, but apart from it, it has been proved by experiments that
the by far greater amount of purification—whilst the sewage is in the
beds—is due to the absorbing powers of the filling material, which are
derived from the surface attraction of its component particles.
[Sidenote: Retention of suspended matters by bed.]
[Sidenote: Absorbing powers of filling material.]
The filling material retains in its upper layers the suspended
matters, which it strains out of the sewage in a purely mechanical
manner, much after the fashion of a screen, and when the bed is filled
its absorbing powers come into play, which cause the removal of the
dissolved matters out of the liquid and their retention on the surface
of the particles. This latter process is probably a chemico-physical
one assisted by the micro-organic life in the sewage.
[Sidenote: Decomposition of organic substances by micro-organisms when
bed is empty.]
It is only after the bed has been emptied that the real activity of
the vast number of micro-organisms commences, which is directed
towards converting the organic substances into mineral ones. This
process of splitting up, decomposing, disintegrating and mineralising
organic waste products is an exceedingly complex one, which ever
fluctuates according to the prevailing conditions, and which does not
come to an end until finally stable mineral forms are reached. In the
presence of a plentiful supply of oxygen, the process proceeds as a
rule at a more rapid rate, and the intermediate forms produced are
less complex than in the comparative or total absence of this gas;
hence the progress of the process is largely determined by it. The
amount of oxygen necessary for bacterial activity is partly
abstracted, and with extraordinary energy, from the atmospheric air in
the pores of the filling material, and a portion of the substances
formed, such as carbonic acid and nitrogen—in gas form—escape into the
atmosphere, whilst the remaining portions are washed out of the bed
with other products, such as nitric acid, by the effluent.
Further remarks upon this process of mineralisation have been made in
connection with the subject of natural self-purification of sewage,
and these may be referred to here.
[Sidenote: Effluent from bed practically raw sewage as far as its
bacterial contents are concerned.]
The effect of the bed upon the bacterial flora of sewage is, as was to
be expected, but very slight, and it is on record now that, as far as
the micro-organic life is concerned, the effluent is to all intents
and purposes raw sewage.
[Sidenote: Silting up of bed.]
Some of the substances contained in raw sewage remain in the bed, no
matter how carefully the sewage has been previously strained, and
these, in combination with the slimy surface coating of the component
particles, the accumulation of mineralised substances in the pores,
the consolidation of the bed, the disintegration of the filling
material, and the liquid retained, lead gradually but surely to the
silting or sludging up of the bed.
[Sidenote: Theoretical original water capacity of bed.]
(_c_) _Water Capacity of Bed and Silting up._—The theoretical water
capacity of the bed, previous to commencing operations, is the
aggregate of the cubical space occupied by the pores or small passages
between the particles forming the filling material, and the pores of
the filling material itself; but in practice a certain amount of this
space is occupied by air, which it is impossible to dislodge
altogether in filling. The aggregate of the cubical space of the pores
may be called the pore-volume.
It is difficult to lay down general rules as to what the original
water capacity of a bed should be expressed in per cent. of the space
occupied by the filling material, but speaking within fairly wide
limits the following is somewhat near the truth.
[Sidenote: Original water capacity with spherical particles of uniform
size.]
When the particles forming the filling material are fairly spherical
and of equal size, the original water capacity of a bed amounts to
about 38 per cent. of the space occupied by the filling material; but
as in practice it is difficult to obtain spherical particles of
uniform size, the original water capacity is found to range from 35 to
45 per cent. of this space.
[Sidenote: Original water capacity with particles of different sizes.]
When, however, the particles are of materially different sizes, and
when the smaller ones fill up the spaces between the larger ones, the
original water capacity may sink down to as low as from 5 to 10 per
cent. of the space occupied by the filling material.
[Sidenote: Size of particles of filling material does, under certain
conditions, not affect original water capacity of bed.]
It has been further demonstrated that the water capacity of a bed is
not affected by the size of the particles, provided the latter are
spherical and of uniform size. In other words, the water capacity of
two beds filled with material of different sizes is the same, provided
the particles are spherical and of uniform size throughout each bed.
[Sidenote: Silting up of bed during regular work.]
[Sidenote: Rapid initial decrease of capacity.]
[Sidenote: Consolidation of bed.]
This original water capacity is, however, not maintained in regular
work, as has been pointed out already. Basing the observations on
regular work only, the original capacity decreases at first, after a
new bed has been started or after an old reconstructed bed has been
taken in hand, rapidly for some time and afterwards more slowly.
Graphically expressed, this decrease is not represented by a straight
line but approaches more nearly a parabolic curve. This initial rapid
decrease is chiefly due to the consolidation of the bed.
[Sidenote: Disintegration of filling material.]
In connection with the movements in the bed tending towards its
consolidation, it is also clear that the continual filling and
emptying operations cause the smaller particles to be washed out of
their original position and to be placed in the larger passages
between the filling material, and if this process is assisted by the
gradual disintegration of the particles composing the filling
material, it is clear that the pores must become smaller and smaller
in time, i.e. choked.
From these observations it follows that the filling material should be
a hard substance, which will only to a limited extent be subject to
this crumbling away process.
But besides these there are, as has already been pointed out, other
silting up agencies at work.
[Sidenote: Water-retentive power of filling material.]
Of the total quantity of sewage which has entered the bed a small
portion will always remain in it owing to the water retaining power of
the material. This power has sometimes been called “minimum water
capacity,” but as this name is liable to be misunderstood, it is
better to adopt here the term “water-retentive power” of material.
The quantity of the sewage retained by the bed varies with the
material and pore-volume, and is due to adhesion and capillary
attraction. The greater the pore-volume, and the greater the
percentage of fine pores, the greater is the quantity thus retained.
Clean gravel retains about 12 per cent. and fine sand about 84 per
cent. of its water capacity—i.e. expressed per cubic yard of filling
material, one cubic yard of clean gravel will retain about 10 gallons
and one cubic yard of fine sand about 70 gallons of water.
Through draining a bed for several hours through evaporation and other
atmospheric influence, a portion of the sewage retained is lost, but
the quantity so lost will vary continually with the circumstances
under which the bed is worked.
The water-retentive power of the filling material does not decrease
with the working of the bed, but increases, which in a large measure
is probably due to the slimy coat which forms round the surface of the
component particles, and to which reference is made in the following
paragraph.
[Sidenote: Slimy surface coating of component particles.]
A further silting-up agency is the slimy surface coating of the
particles of the filling material. This accumulation is well known to
all who have had to do with intermittent contact beds, and has been
described as spongy bacterial growth. The Manchester report for the
year ending March 27, 1901, contains on page 62 the following passage:
“This (spongy bacterial growth) is at once the cause of increased
efficiency in the bed and loss of capacity. On examining the material
of a contact bed in active condition, every piece is seen to be coated
over with a slimy growth. If this is removed it soon dries to a stiff
jelly, which can be cut with a knife. Under the microscope masses of
bacteria and zoogloea will be found to be present.”
[Sidenote: Accumulations of decomposed substances in the pores.]
In addition to this slimy surface-coating of the particles, there are
also found in the pores, especially in the upper layers of the filling
material—and in fine beds more so than in a coarse bed—accumulations
which are “akin to humus or garden soil.” They contain to a limited
extent only putrescible substances, and appear to be the remains of
organic matter decomposed by the activity of micro-organisms.
[Sidenote: Periods of rest will not permanently restore portion of the
original water capacity of the bed.]
It was formerly maintained with considerable persistency that periods
of rest would permanently restore to a systematically worked bed a
portion of its lost water capacity, but such a contention has been
proved to be wrong. It is quite true that immediately after periods of
rest an increase of the water capacity is very noticeable, which is
probably due to drying up processes within the bed during the rest,
but such an increase is not permanent and is lost again more or less
quickly; it is therefore only temporary and not permanent.
Where, however, a bed has not been systematically worked, i.e. where
it has been worked at a greater rate than is suitable, and where in
consequence of this a large quantity of undecomposed substances is
stored in it, a period of rest may permanently restore a portion of
the lost capacity; but this is due to the mineralisation of these
undecomposed organic substances during the rest.
It follows from these remarks, as has been stated above, that when the
organic substances are regularly decomposed during systematic work a
period of rest cannot materially affect the water capacity, and that
where a considerable permanent restoration of the water capacity takes
place the bed has not been properly worked.
[Sidenote: Decrease of capacity is accompanied to some extent by
increase of efficiency and _vice versa_.]
It would, however, be incorrect to assume that the silting up of the
bed affects its efficiency besides reducing the capacity. On the
contrary! To some extent decrease of capacity is accompanied by
increase of efficiency and _vice versa_!
[Sidenote: Higher capacity of beds in summer than in winter.]
At this point it ought to be stated that in the Manchester experiments
(see page 61 of the report for the year ending 27th March, 1901) a
higher average capacity is maintained during the summer than during
the winter, which is no doubt due to the greater activity of the
micro-organisms during the warm weather of the year.
[Sidenote: Raking of beds not advantageous.]
The raking of the surface does not materially affect the capacity of
the bed, and it is better to scrape off the matters retained on the
surface than to rake them into the body of the bed.
[Sidenote: Renovation of filling material either partially or wholly.]
It will be clear from these observations that, no matter how carefully
the bed has been worked, sooner or later a time will come when the
decrease of capacity becomes so pronounced as to render it impossible
any longer to treat the daily flow of sewage with the available plant;
and when this point has been reached a renovation, either partially or
wholly, of the filling material becomes an inevitable necessity.
[Sidenote: Minimum capacity of beds to be provided for.]
To provide for this at the outset, and thus avoid the difficulties of
reduced capacity, it seems advisable to lay down, when designing the
works, a minimum capacity, which will just allow the daily volume of
sewage to be treated by the plant, and which when reached will
necessitate the cleansing of the bed.
The idea, formerly frequently expressed, that the filling material
when rationally worked need not be renewed or renovated, can no longer
be maintained and is outside the reach of practical possibilities.
[Sidenote: Underdrainage of intermittent contact beds.]
At this place a word or two about the under drainage of intermittent
contact beds may not be out of place. It is of the greatest importance
that all drains should work well, and that the entrance of the sewage
into them should not lead to disturbance in the filling material,
especially should the tearing of portions of the filling material into
the drain pipes be avoided.
By carefully arranging the position, number, size and fall of the
master drains and branch drains, it is possible to reduce the
resistance so as to allow of a fairly even flow of sewage through all
the drains, and to prevent a great rush of water through the drains
near the outlet end.
[Sidenote: The presence of lime is of no consequence.]
In passing it may not be out of place to point out that the view,
formerly expressed, that an admixture of lime in some form would prove
advantageous to the purification of sewage, is not supported by the
experience gained.
[Sidenote: Absorbing effect increases with the time of contact.]
(_d_) _Absorbing Powers of Filling Material._—The absorbing effect of
any filling material seems to increase with the time of contact.
[Sidenote: Absorbing powers increase until bed has become ripe.]
It ought further to be pointed out that the absorbing powers of the
filling material gradually increase until the bed has become ripe.
This fact was formerly stated to be due to the development of the
proper micro-organisms within the bed, but it would seem to be
chiefly due to the slimy surface coating of the particles of the
filling material, or spongy bacterial growth, as it has frequently
been called, which does not only assist mechanical filtration but also
possesses high powers of absorbing oxygen.
[Sidenote: Absorbing powers soon cease in the absence of
micro-organisms and air.]
But it cannot be open to doubt that the absorbing powers of the
filling material are dependent in some way or other on the presence of
micro-organisms, for Dunbar has shown that in the absence of
micro-organisms and without periods of aeration these powers soon
cease.
[Sidenote: Oxygen is absorbed from air in the pores with great
energy.]
(_e_) _Consumption of Oxygen by the Filling Material._—The oxygen
necessary for the proper work of an intermittent contact bed is
abstracted with great energy from the atmospheric air, with which the
pores become filled during periods of rest. Through diffusion, and
through the vacuum created by the processes of absorption, further
quantities of oxygen are taken from the atmospheric air, even under
difficult conditions, and, as pointed out in the Manchester report for
the year ending 27th March, 1901, “there is, therefore, little need to
force air into a bed.”
[Sidenote: The oxygen taken up during aeration is not imparted to the
sewage at the next filling and does not escape in the effluent.]
The oxygen thus taken up is not imparted in gas form to the sewage
during the next filling, and the effluents from intermittent contact
beds are not saturated with oxygen. Dunbar states that the effluents
of a satisfactorily worked bed frequently only contain one cubic
centimetre of free oxygen per litre. Clowes reports a similar result
in his third report on the London experiments.
[Sidenote: The greatest quantity of oxygen is consumed during the
oxidation of the products formed by micro-organisms.]
There can be no doubt that by far the greatest quantity of oxygen is
consumed during the process of oxidation of the products formed by
micro-organisms from putrescible organic substances.
[Sidenote: Consumption of oxygen and formation of carbonic acid not
solely due to biological agencies.]
(_f_) _Formation of Carbonic Acid._—Dunbar has shown by his
experiments that the consumption of oxygen and the formation of
carbonic acid is not solely due to biological agencies, but is to some
extent the result of physico-chemical processes.
[Sidenote: More free carbonic acid contained in the effluent than in
the raw sewage.]
[Sidenote: By far the greatest portion of carbonic acid escapes into
the air.]
He further reports that in his experiments the effluents contained on
an average 100 milligram per litre more free carbonic acid than the
raw sewage, and that the quantity contained in the effluents
represents only a small portion of the total amount of carbonic acid
formed during the whole process. The by far greatest portion of
carbonic acid escapes into the air. Concerning the air in the pores of
the filling material during periods of aeration, Dunbar states that it
contains sometimes not less than from 6 to 10 per cent. carbonic acid.
[Sidenote: Nitrogen escapes in gas form into the air.]
(_g_) _Nitrogen._—It is quite clear from all experiments that a
considerable amount of the total nitrogen contained in raw sewage is
abstracted by the filling material of intermittent contact beds, and
it is interesting to ascertain what becomes of it! Does it accumulate
in the bed? In that case, one has a right to assume that the
satisfactory work of the bed would gradually cease! As this is,
however, not the case, and as, further, the sludge formed in the bed,
whether it be fairly fresh or very stale, only contains a very small
amount of total nitrogen, we must surmise that the nitrogen after its
retention by the bed escapes in gas form—like the carbonic acid—into
the atmosphere.
[Sidenote: The presence of nitric acid is not an unfailing guide for
determining the satisfactory character of the effluent.]
(_h_) _The Formation of Nitric Acid._—Concerning the presence of
nitric acid in the effluents from intermittent contact beds, Dunbar is
of opinion that it offers certain means for forming an opinion of the
processes taking place in the same, but that it is only in a
subordinate sense an indication of the degree of purification attained
and must not be taken as an unfailing guide for determining the
satisfactory character of the effluent.
[Sidenote: Nitrifying bacteria always present in town’s sewage.]
[Sidenote: Nitric acid is formed very rapidly, but only during periods
of rest.]
Nitrifying bacteria are always present in ordinary town’s sewage, but
it would appear that other micro-organisms besides Winogradsky’s
bacteria assist in the process of nitrification. Nitric acid is formed
very rapidly, but only during periods of rest, and besides aeration
other less powerful influences are at work.
[Sidenote: Reduction of nitric acid when bed is filled from bottom
with an upward flow.]
It is further interesting to note that, according to Dunbar, the
greater portion of nitric acid which has been formed during periods of
aeration becomes completely reduced in a very short time, when the bed
is filled with an upward flow from the bottom, and that only a small
portion remains in the form of nitrous acid.