The almost universal adoption at the present time of biological methods
of sewage purification by means of artificial filters, is due entirely
to the original experimental work of Mr. W. J. Dibdin, at the Barking
Outfall Works of the London County Council. These experiments were
carried out with a contact bed, and during the subsequent ten years
an enormous number of works were constructed upon this principle. At
the present time, however, it is a somewhat rare occurrence to find
contact beds proposed for sewage disposal schemes of any size. It
has been stated that the principle upon which they are operated is
unscientific, that they rapidly become clogged and useless, and that,
in any case, they are not capable of dealing with sewage at the same
rate as percolating filters, or of producing such a high degree of
purification. With regard to the first point it would be futile to
endeavour to explain what is and what is not scientific. This must be
left to the scientists. That contact beds have in many cases become
clogged and useless cannot be denied, but there is also very little
doubt that this unsatisfactory result has been due to one or more
of the following causes: (_a_) overwork, (_b_) improper methods of
operation, (_c_) the use of unsuitable material for filling the beds,
(_d_) insufficient sub-drainage. It was most unfortunate that for some
years the general idea of a contact bed was that it consisted of a
simple excavation in the ground, filled with coke or similar material,
into which the sewage was discharged, held up for two hours, and then
drawn off; a very simple but crude affair altogether. It is now known
that contact beds, like other systems, can only deal with limited
volumes of sewage, the actual amount depending upon the character
of the sewage and other factors; that there is a proper method of
operating the beds, and that it must be strictly adhered to if the best
results are to be produced; that unsatisfactory material is worse than
useless, and that very ample means of sub-drainage are absolutely
essential to the continued efficiency of the beds. It is probable that
if these essential factors had been properly understood and acted upon
from the outset, there would have been very few failures to record.
It has been stated that contact beds are obsolete, but there are
engineers who even now recommend this system, and consider it
satisfactory under some, if not under all conditions. In the opinion of
the author contact beds are not obsolete, and there are cases where the
conditions preclude the adoption of any other method of purification.
Under these circumstances, it is considered desirable to describe in
the following pages the details of design and construction which have
been found by experience to be necessary to ensure satisfactory results.
_General Principles of Design._—The first point to be decided before
commencing the design of a scheme of contact beds is whether single,
double, or triple contact is necessary to produce the desired degree of
purification, and this will depend upon the strength of the sewage and
the destination of the final effluent. Single contact alone will not be
sufficient, except in a very few cases where the sewage is weak (highly
diluted), and even then it will necessitate the use of fine-grade
material for filling the beds, and consequently a tank effluent of
exceptional quality as regards the matters in suspension in order that
the fine material may not be rapidly choked. Where a sufficient area of
land of a suitable character can be procured at a convenient level for
treating the effluent from the beds, single contact may be adopted with
material of medium-grade, but even in this case special attention must
be devoted to the preliminary process in tanks, so as to reduce the
amount of solids in suspension in the tank effluent to the minimum. As
a rule it will be found safer to adopt double contact, as the primary
beds may then be filled with coarse grade material, which will be less
liable to choke, and it will not be necessary to rely so much upon the
land or any other final process that may have been provided. In special
cases, and particularly where the sewage is strong, or an exceptionally
high degree of purification is essential, triple contact should be
adopted, but the tertiary beds may consist of a set of sand filters
similar in construction to those described on pages 185 to 188. In some
quarters the question of the grading of the material is considered of
slight importance, and very little difference has been made in the size
of the material for the primary and secondary beds, but in the author’s
opinion it is absolutely essential that each series of beds should be
filled with finer material than the preceding series, and the material
in the final stage of treatment should be as fine as possible, so long
as it does not contain any dust. In making these statements, it is
assumed that the question of sub-drainage will be dealt with on the
lines recommended later under that heading.
Another factor which has an important bearing upon the general design
of a scheme of contact beds, is the method of operation which is to
be adopted. It is generally assumed that all contact beds are worked
in what is known as eight-hour cycles: viz. 1 hour filling, 2 hours
standing full, 1 hour emptying, and 4 hours standing empty for rest and
aeration. There has, however, been a tendency in the past to overlook
the fact that the periods of standing full, and of emptying the beds,
are the only sections of the cycle which are, as a rule, under absolute
control. Unless special provision is made for the purpose, the time
taken to fill each bed depends entirely upon the rate of flow of the
sewage to the works, and the period of rest empty also depends upon the
frequency with which the beds are filled, and thus indirectly upon the
rate of flow of the sewage. For example, a set of four beds designed to
receive each three fillings per day in wet weather, should not receive
more than one filling per day in dry weather. Assuming that one-half
the total flow comes down in six hours, it will be found that it takes
six hours to fill two beds in the middle of the day, or three hours to
fill one bed. During the remaining eighteen hours the other two beds
are filled, one of them in say six hours and the other in twelve hours.
The times taken to fill the four beds in this scheme would therefore
be—No. 1, three hours; No. 2, three hours; No. 3, six hours; and No.
4, 12 hours. In each case the period of filling is thus much in excess
of the one hour prescribed under the eight hours cycle. The obvious
remedy is to subdivide the total area into a larger number of smaller
beds, but if this is carried to its logical conclusion it will be seen
that there must be 24 beds if the time taken to fill any one bed is
not to exceed one hour. While it is very desirable to arrange this
subdivision in order to secure the proper cycle of operations, the
number of schemes where it is economically practicable are few, and
recourse must be had to some other method of reaching the same end.
This has already been recognised by most engineers, and provision is
now usually made for a tank known under various names, such as dosing
tank, collecting tank, equalising tank or holding-up tank, in which the
tank effluent is stored until the volume accumulated is equal to the
capacity of one contact bed, and the latter is then filled within the
regulation time of one hour. If the necessity for making provision on
these lines to ensure the proper working cycle had been recognised in
the early days of contact beds, it would doubtless have prevented the
troubles which have arisen in many places.
From the preceding observations, it will be seen that it is very
necessary to come to a decision as to the method of operation to be
adopted, before designing any scheme of contact beds. If the method
of subdivision into a large number of small beds is preferred,
the planning of the separate series, and the probable cost of the
additional work involved, must be taken into consideration. On the
other hand, if a smaller number of larger beds with a suitable dosing
tank are preferred, the extra fall required for the latter must be
provided for, even if it involves the reduction of the depth which
would otherwise be available for the beds themselves.
[Illustration: FIG. 136.]
There is still another matter which has a considerable influence upon
the general design of a scheme of contact beds, viz. the slope of the
ground upon which they are to be constructed. If it has a fairly rapid
and even slope, the tanks and beds may be arranged close together, as
shown in Fig. 136. The only part that needs special care in this case
is the cross-wall between the primary and secondary beds, which will
need strengthening, especially in its lower half, in order to resist
the extra pressure it is required to take.
[Illustration: FIG. 137.]
Where the slope is not so great, a saving in the cost of excavation may
be effected by arranging the separate tiers of beds at some distance
apart, as indicated in Fig. 137, and connecting one with the other
by pipes. The aim to be attained in arranging the beds under these
conditions is to have the entire area of the floors on solid soil, with
the walls half in and half out of the ground.
Another set of conditions occasionally met with, is where the site of
the works is perfectly flat and the position of the outlet for the
final effluent involves the construction of the secondary beds either
wholly or partly below the surface. In such cases the primary beds will
come above ground, and it will then be found economical to arrange each
set of beds in two rows end to end, with a central combined supply
channel and effluent carrier, the latter being formed in the space
between the walls which support the former, as shown, Fig. 138. If
there is not sufficient head to allow of the supply channel being made
deep enough to serve as the dosing tank, the latter may be constructed
across the ends of the settling tanks, as suggested in Fig. 139, or in
any other convenient position. A dosing tank in this position lends
itself to the method of feeding the beds by means of closed pipes
instead of by open channels, whether in sets of four, with a central
chamber for the inlets and outlets illustrated in Fig. 139, or in
series as Figs. 136 and 137.
[Illustration: FIG. 138.]
There are doubtless other alternatives, or combinations of methods,
which may be adapted to meet the exigencies of peculiar conditions
of site and fall, but the foregoing details will probably suffice to
suggest ideas to those in need of them in designing schemes of contact
beds.
_General Construction._—The most important point to be borne in
mind in constructing contact beds is, that they must be absolutely
watertight. Should they leak in any way, the sewage may pass away
untreated; or it may find its way into adjacent beds, and thus prevent
these from being properly aerated during the periods of rest when
empty. It is therefore evident that they should be constructed in
a substantial manner. The floors are usually of concrete, and the
thickness of the floor will depend upon the nature of the subsoil.
If for any reason the floors have to be laid upon made-up ground,
provision should be made by means of piers or cross-walls, carried down
to the solid subsoil to support the floor, independent of the made-up
ground which is, in all cases, absolutely untrustworthy. The walls
of the beds may be constructed either of concrete throughout or of
brickwork in cement, and they should be of such a thickness that they
will withstand the pressure of the head of water which would result if
the beds were filled to the top of the walls.
It will not be found satisfactory to place reliance either upon
brickwork or upon concrete alone to form a watertight bed, and in both
cases the whole of the floors, as well as the walls, should be rendered
with cement mortar in the proportion of 2 parts of sharp clean sand to
1 part of Portland cement. No rendering should be done during frosty
weather or during excessive heat, as in both cases it will
usually be found defective, and it is better to stop the work
altogether for a time than have to patch it up afterwards.
[Illustration: FIG. 139.]
One safeguard which can be adopted to prevent difficulties later on,
is to insist upon testing all such beds with water to the full height
before any of the filtering material is placed in position. Any slight
defects which may appear can be made good then at very little expense.
If the defects are not discovered until after the beds are filled with
the filtering material, they can only be properly rectified by removing
the material, and this involves a considerable outlay. In order that
these tests may be carried out without friction, it is necessary to
stipulate clearly in the specification for the work that each bed is to
be tested with water to the full depth before any material is placed
in the bed, and that the contractor must take full responsibility for
making the beds absolutely watertight. It is, of course, understood
that the method of construction adopted by the engineer is such that,
if properly executed, the beds will be absolutely watertight; and,
in order to prevent any misunderstanding, an item should be included
in the quantities for the contractor to provide whatever sum he may
consider necessary to allow for making these tests. It may be thought
sufficient to state simply that the contractor should make all
absolutely watertight, and to leave it to him to provide the means for
doing so. It will, however, be found more satisfactory to all concerned
to provide all means both in specification and in the quantities for
attaining the desired results. It is not sufficient even to use the
word “watertight” alone in this connection, as the interpretation of
this word may be subject to differences of opinion which are obviated
by the addition of the word “absolutely.”
The foregoing observations refer not only to contact beds but to the
tanks and other portions of the work which are required to hold water.
_Methods of Distribution._—Much diversity of opinion exists with
reference to the question of distribution in filling contact beds. The
many methods which have been tried in various places, may be arranged
under four headings, viz. (_a_) above the surface of the material;
(_b_) at the surface level; (_c_) just below the surface; (_d_) at the
bottom of the bed. Some engineers hold the opinion that distribution
over a large area of the material has no value, and that it matters
little how the bed is filled so long as the liquid finds its way into
the interstices of the material with as little disturbance as possible.
The simplest method which fulfils these requirements is to allow the
sewage to flow over the surface at the inlet end of the bed, but this
soon causes the surface at this point to become clogged, and unless
it is cleaned at frequent intervals the solids very quickly become
washed down into the bed, and in the end this portion of the material
will have to be removed and washed before it can be used again. The
efforts to avoid this trouble have resulted in the numerous methods of
distribution referred, to above.
Taking them in the order given, the idea of discharging the sewage
above the surface level (_a_) by means of elevated troughs or pipes,
has been to imitate to some extent the method found necessary in the
case of percolating filters, and thus aerate the sewage before it
enters the bed. The difficulties which arise in this case are that some
extra fall is needed, and the provision of the troughs or pipes with
suitable supports is costly. It is also extremely difficult to arrange
the distribution by these means so that it shall be uniform over the
whole area, and unless this is done it cannot be of much advantage.
Distribution at the surface level (_b_) may be provided by means
of shallow grips in the material itself, and these have the great
advantage that if they become clogged to such an extent as to prevent
the sewage from freely passing into the bed, it is a small matter for
the manager to cut a fresh grip in another direction and leave the
first one to dry up when the sludge in it can be easily removed by
hand. Another method is to use rows of stoneware channels, or wooden
or iron troughs, with their edges set level with the surface of the
material, so that the sewage may flow over the edges or through holes
or notches in the sides. This is usually satisfactory, but it is not
an easy matter to maintain all the channels at the same level, and
after they have been in operation for a time it will be found that the
material immediately under the troughs or channels is badly clogged,
and can only be cleaned or renewed by removing the channels.
Sub-surface distribution (_c_) is arranged by means of perforated or
open-jointed pipes, laid below the surface of the material and thus
out of sight. The reasons for adopting this method are: that it avoids
the unsightliness caused by surface distribution; that the surface is
kept free from obstructions, and thus allows free aeration when the bed
is emptied; and last, but not least, it prevents any nuisance arising
from the evolution of obnoxious gases in the tank effluent whenever
it is over-septicised, a not infrequent occurrence in the case of
old-fashioned schemes, or in new works where the volume of sewage for
which the tanks were designed has not yet reached its maximum. This
method has the disadvantage that when the openings in and between the
pipes become choked, more labour is involved in cleaning them than in
the case of open channels or troughs on the surface.
Filling from the bottom (_d_) is assumed to possess all the advantages
and none of the disadvantages caused by the other methods. The
distribution is certainly uniform, as the liquid first fills the
sub-drains and then rises at the same level throughout the whole of the
material, forcing out any carbonic acid gas that may have accumulated
in the lower part of the bed. As the sewage does not appear on the
surface at all, there is no unsightliness and no trouble from bad
odours. On the other hand, it is evident that the solids in suspension
in the sewage or tank effluent are retained at the bottom of the bed,
especially in the under-drains, and thus they will appear in large
quantities in the effluent. Unless some special provision is made, by
means of an effluent settling tank or sand-filter, to arrest these
solids in suspension in the final effluent, they will be liable to
cause trouble in the stream, and will, in any case, seriously affect
the results of any analyses that may be made. The usual manner of
arranging this method of filling, is to cause the sewage to flow into
an open or covered chamber at the inlet to the bed, the walls of the
chamber being provided at the floor level with openings connected to
the sub-drains laid on the floor of the bed.
Whatever method of distribution is adopted, it is desirable that the
surface of the filter material shall be not less than 3 inches above
the highest level to which the sewage will rise, so that the liquid may
not be visible at the surface.
_Sub-Drainage._—Reference has already been made to the fact that
lack of ample under-drains has often been the cause of the failure
of contact beds in the past. The general practice for a long time
consisted in placing a layer of coarse material on the floor of the
bed, and providing a few rows of ordinary agricultural drain-pipes laid
with open joints. In some few cases special perforated pipes were used,
in others the pipes were partly embedded in the concrete floor. In the
opinion of the author, however, no drains in the form of pipes are
satisfactory, as they do not leave a space _at_ the floor level as a
free exit for the solids in the effluent. Where pipes are used it will
generally be found after a few months’ operation that these solids, in
the form of black sludge, have accumulated along the sides of the pipes
and among the material at the floor level, and when this once commences
the accumulation continues to take place, rising gradually in the bed
until the interstices are choked to such an extent that the liquid
capacity of the bed is reduced to a fraction only of its original
volume. The trouble was intensified by the comparatively small number
of the pipe drains usually found in the beds. It was evidently assumed
that the matters would travel laterally through the layer of coarse
material at the floor level. Unfortunately an additional impediment to
the free flow of these matters was caused in many cases by the want of
sufficient fall on the floor itself. Very little consideration will
show that a large area of floor requires a considerable slope in order
to produce the velocity of discharge necessary to remove matters in
suspension, yet it was seldom that a gradient of more than 1 in 200 was
provided, and in a few cases the surface of the floor was absolutely
flat. Under these conditions it is difficult to see how any other
result could be expected. It may be argued that it was not properly
understood in those days that the solids in suspension (converted
organic matters, the products of oxidation) must be removed if the
filtering material is to retain its working capacity, but this fact has
long been recognised in connection with percolating filters, which have
in most cases been constructed upon complete false floors, provided
with perforations, and with a suitable slope on the surface of the
actual floor.
There is very little doubt that the question of providing ample means
of sub-drainage deserves special consideration; and, in the author’s
opinion, the floors of all contact beds should be laid with much
greater fall to the outlet than in the past, and they should, in
addition, be covered entirely with a false floor of special floor-tiles
of the kind described in connection with percolating filters (pages 91
to 94). If the usual bottom layer of coarse material is then laid upon
the false floor, it will be found that the beds will maintain their
normal working capacity for a much greater length of time than in beds
constructed on the old style. Instead of arranging the slope on the
floor from the inlet end to the outlet end of the bed, it is preferable
to construct an effluent channel with a suitable fall down the centre
of the bed to the outlet, and arrange the floor with a cross-fall from
the sides to the centre channel, which may be covered by slabs of
concrete or stone laid upon the top of the floor tiles where they abut
upon the edges of the channel as suggested, Fig. 139_a_.
[Illustration: FIG. 139_a_.]
_Material for Filling Contact Beds._—The remarks made under the
heading “Filtering Material” for percolating filters (pages 101 to
103), apply with equal force to contact beds. In none of the many
cases which have come under the observation of the author has it been
possible to obtain such satisfactory results from other material as
from clinker, when used under the same conditions as to the strength
and volume of the sewage treated. It is true that excellent effluents
have been produced by beds filled with burnt clay, broken bricks or
stones, but it will usually be found that in such cases the material
is more or less clogged, and that the volume successfully treated
per day is considerably less than that which would be dealt with by
clinker. The reasons are the same as those already mentioned in pages
101 to 103, and there is no need to say more here than to recommend as
strongly as possible the use of hard-burnt vitrified furnace clinker as
already described for percolating filters.
The foregoing remarks apply more particularly to the material for
coarse and medium-grain contact beds. Clinker of the kind described is
equally suitable for fine-grain contact beds, but it is difficult to
break it to the required grade except at great cost, and a considerable
loss in bulk due to the production of fine dust. For fine-grain beds,
requiring material specified to pass a ¼-inch square mesh,
and to be retained on a ⅛-inch square mesh, clean coke-breeze from
gas-works (not ashes) will be found to be the most efficient. The
important point to be observed in preparing material for fine-grain
contact beds is that, while none of the particles should exceed
¼ inch in diameter, as many as possible of the finer particles,
⅛ inch and 3/16 inch in diameter, should be retained, but all dust
should be removed even if it necessitates washing the material for that
purpose. Even the finer particles down to 1/16 inch diameter might be
used, but it will be found extremely difficult to arrange the sifting
process to arrest these without retaining the dust as well, especially
if the material is at all damp, as the fine meshes required for the
purpose quickly become clogged with the dust, and the sieve or screen
is rendered useless.
Among other materials which may be adopted for fine-grain contact beds,
broken saggars in the pottery districts, or slag in the neighbourhood
of ironworks, are probably the next best, but only if they are properly
graded in the manner described above. Indeed the chief difficulty in
securing this fine-grade material is the preparation and grading,
particularly where large quantities are required and the situation
of the beds involves much handling of the material after it has been
sifted. It is, however, of such extreme importance to have it as fine
as possible, without including any dust, that the stipulations in the
specification with regard to this material should be made very clear
and definite, so that the contractor may make sufficient provision
in his prices to enable him to comply with the specification in its
strictest sense.
In addition to the bottom layer of floor-tiles and coarse material
prescribed in the section relating to sub-drainage, it will be
necessary to provide an additional layer above this, about 3 inches in
depth, of a medium grade, to support the very fine material and prevent
it being washed through the interstices in the coarse bottom layer, and
special attention must be devoted to the method of distribution at the
surface in order to avoid disturbance of the fine material.
[Illustration: FIG. 140.—AUTOMATIC APPARATUS FOR CONTACT BEDS.]
_Automatic Apparatus for Contact Beds._—Although contact beds can be
operated by hand, this involves continuous and regular supervision by
a man, and, unless he is under strict control, the proper cycle of
operation may not be adhered to, and the result will, in that case,
not be satisfactory. In the early days of contact beds, several types
of apparatus were designed for the purpose of operating these beds
automatically, so that the cost of manual labour would be reduced to
a great extent, and in addition, the possibility of mismanagement
avoided. Among these appliances, one of the most widely known is that
manufactured by Messrs. Adams Hydraulics, Limited; and shown in Fig.
140. In small installations, the low draught syphons described above in
connection with dosing apparatus are used to give alternate fillings
to pairs of contact beds. Where more than two beds are in use the
automatic air-lock feed is used. This consists of a cast-iron inverted
U-pipe, both legs of which are trapped, either in self-contained
castings or in separate chambers. The sewage flows through one, the
others being charged with air. When the sewage in the bed has reached
the proper height it overflows into a small chamber in which a dome is
fixed. This dome is connected by means of an air-pipe to the cast-iron
inlet feed, and as the sewage rises in the chamber round the dome, the
air contained therein is forced up into the feed which it fills, thus
forming an air-lock, which prevents the sewage from passing through and
stops the supply to the bed. At the same time that the feed is stopped
by the transfer of air in the manner described, the compression of
the air in another small dome in the same pit has forced a water seal
on the air-pipe leading from this dome to the feed in the next bed,
and thus liberates the air-lock in the feed of that bed and allows
the sewage to flow to this in rotation. The whole operation is then
repeated with each bed in rotation, the last bed in the series, when
full, starting the feed in the first bed again. These same feeds may
be arranged to hold up sewage in a collecting or dosing tank, so as to
ensure the accumulation of sufficient sewage to fill each bed at one
dose within a reasonable time.
[Illustration: FIG. 141.—CONTACT BED APPARATUS.]
In order to ensure that the sewage is held up in each bed for a
suitable period for contact, the outlet is provided with an automatic
syphon, which is arranged in such a way that the filling of the bed
alone can start it. When the bed is full, the sewage flows through a
pipe with an adjustable orifice into the timing pit. When this pit is
full, the compression of the air in a small dome placed in the pit, and
connected to the syphon by means of an air-pipe, releases the air in
the syphon and allows it to start at the end of the desired period
of contact. A special feature of these syphons is that they are so
arranged that after they have emptied the bed they continue in action
as syphons, taking the drainings from the bed, however small in volume
they may be, and stopping off only when the bed commences to fill
again, thus ensuring a thorough draining of the material. Another
special feature of this apparatus is that there are no moving parts,
the whole operation depending upon the transfer of air by means of the
head of liquid. It is important to note that by this apparatus the
period of contact in any bed can be arranged for any particular length
of time, quite irrespective of the rate of flow of the sewage to the
works, and independent of the filling and emptying of the other beds in
the series.
[Illustration: FIG. 142.—CONTACT BED APPARATUS.]
[Illustration: FIG. 143.—CONTACT BED APPARATUS.]
[Illustration: FIG. 143_a_.—CONTACT BED APPARATUS.]
Another apparatus which has been used largely in the past is that
supplied by the Septic Tank Company, Ltd. This apparatus is made in two
forms, illustrated in Figs. 141 and 142. The chief difference between
the two types is that in the one case, Fig. 141, the period of contact
is controlled by the filling of the next bed, and is thus dependent
upon the rate of flow of the sewage, while in the other case, Fig.
142, the period of contact is “timed” for from one to two hours, and
is independent of the rate of flow of the sewage. In both cases each
set of gear is built up on its own bed-plate, and comprises the inlet
and outlet valves and the connecting pipes to and from the same. The
valves are of the simple spindle-type, and are connected by rods to
rocking levers and actuated by buckets or floats, working in chambers
or pits which are in communication with the different beds. By means
of overflows from the beds to the buckets, and other devices, the
various portions of the gear are actuated in such a manner that they
automatically fill the several beds in each set in regular rotation,
hold them full for contact, and eventually discharge the treated liquid
to the effluent drain. Full details of the method of operation can be
obtained from the manufacturers, who claim that the gear will work
satisfactorily without attention other than the oiling of the bearings
and joints every few weeks.
Another type of automatic apparatus for contact beds is that
manufactured by Messrs. J. Blakeborough and Sons, as used in the Triple
Tank System of sewage treatment, Fig. 143. In this case the beds are
arranged in sets of three, and the filling of each is effected by the
overflow from the last bed filled, the discharge being effected by the
rising of the liquid in the last bed filled. The apparatus consists
broadly of a slide-valve controlling the outlet-pipe, and connected by
levers to floating cylinders located in separate chambers. One chamber
controls the opening and the other the closing of the valve. The outlet
is provided with a rising and falling arm, which is connected by a
lever to a balance float fixed in a chamber, and coupled by means of a
pipe to the float-chamber of another bed. The method of operation is us
follows:—The tank effluent flows by gravitation to bed A, the filtered
effluent thus rising in outlet chamber A, and also in the overflow
chamber which is connected by pipes to outlet chamber A. As soon as bed
A becomes full, the filtered effluent overflows into closing chamber
A, which is coupled by a pipe to opening chamber B, and the floating
cylinders in each case are raised, with the result that the valve of
bed A closes and the valve of bed B opens, the liquid thus commencing
to run on to bed B. The same action as above is repeated when bed B is
full, and bed C is to be filled, whilst bed C in turn is coupled to bed
B, so that the triple action is repeated over and over again so long as
sewage continues to flow to the beds. As bed A fills, the liquid rises
in outlet chamber A, and, this being coupled to float chamber C, the
liquid rises to a corresponding height in float chamber C. Some time
elapses before the liquid rises to such a height in the float chamber
as to sink the mouth of the outlet pipe below the surface of the liquid
in the outlet chamber, this space of time (which can be regulated as
desired) representing the length of time that the bed is allowed to
stand full before commencing to empty. When the liquid has risen to
a given height in float chamber C, the balance float is raised, this
action tipping the lever and lowering the rising and falling outlet
pipe in outlet chamber C, thus drawing off the effluent from the top
slowly, and without disturbing the whole contents of the bed. (_Note_:
Bed C is assumed to be full). After the liquid in the bed has been
drawn off, the rising and falling outlet pipe remains stationary at the
bottom of the chamber until the next action takes place, which is as
follows:—When bed A becomes full, it is allowed to stand full until
the liquid in bed B (now filling) has risen to a given height, when it
raises the balance float in the float chamber A in a similar manner as
described above, and thus empties the bed A, at the same time emptying
the float chamber C, in which is fixed the balance float connected to
the rising and falling outlet pipe of the bed C (now standing empty),
thus raising the outlet pipe and rendering the bed again ready for use.
[Illustration: FIG. 144.—CONTACT BED APPARATUS.]
[Illustration: FIG. 144_a_.—CONTACT BED APPARATUS.]
Messrs. Glenfield and Kennedy, Ltd., also manufacture apparatus for
operating contact beds, as shown in Fig. 144. This arrangement of
the apparatus delivers the sewage to six beds. There are two valve
boxes, A, internally divided into three compartments. Three sets of
tube valves, B, on each valve box, control the inlets to the several
compartments of the valve box and also to the beds, which are connected
up to the valve boxes with suitable pipes. As the sewage collects in
the measuring chambers, it raises the float C—the float chamber being
in communication—which, through the rack-and-pinion shown, turns the
shaft D. To a sleeve, E, over the shaft D, a hammer, F, is keyed, while
a stopper catch, G, is mounted freely. Keyed to the shaft D are lifting
levers H and K. The lever K lifts the hammer F to the vertical, and,
being free to rotate with the sleeve E, it falls and strikes on one
of the copper buffers, L, in the turning plate M, causing it to turn.
In like manner the stopper catch G is thrown over by the lifter H—a
little in advance of the hammer—and, resting on the stopper plate N,
drops into one of the notches, O, thus stopping the gear at the proper
place. The turning of plate N causes the roller lever P, keyed to the
vertical shaft Q, to rotate—through the agency of the mitre gearing
and horizontal shaft—thus actuating the lever R, and raising the tube
valve B. Two valves are operated simultaneously, one on each valve box.
The valves are held open by the levers P, until, the water being
run off, the weight of the float descending puts the gear in motion
again—by returning the hammer to its original side—and moves the
roller levers P off the end of the levers R, thus allowing the tube
valves B to close and the water to collect once again. The force of
the blow of the hammer as it strikes the buffer L can be regulated
within certain limits, for, on the outer end of the sleeve E carrying
the hammer, a lever, S is keyed, which, as it works in unison with the
hammer, and is attached to the piston of the swivel cataract adjustable
oil cylinder T, has the effect of cushioning the fall of the hammer.
Another type of syphonic apparatus for contact beds is manufactured
by Messrs. Burn Bros., as shown in Fig. 145. In this case the primary
filters are usually supplied with sewage from a collecting or dosing
tank in which two or more discharge syphons are fixed, or they may be
filled from a supply channel under certain circumstances. In the former
case a syphon discharges immediately the collecting tank is full. A
“Sequela” relief apparatus is attached to each syphon, and causes
these to discharge alternately or in rotation. The relief apparatus is
divided into three compartments, and depends for its working on the
transference of oil, of a special nature, from one compartment, A, to
another compartment, C, via compartment B, in stages corresponding
with the number of syphons under control, each relief apparatus at the
commencement being set a stage in advance of the one next to it. After
a syphon has discharged, the oil which has been transferred to the
compartment C in the relief apparatus is automatically returned to the
compartment A, and the apparatus is then ready for another series of
operations. Thus the oil, which is non-evaporative and non-freezing, is
used over again and again, and as it does not come in contact with the
sewage, it remains quite pure and serviceable for years. A discharge
syphon is fixed to each filter, and, in order to ensure a proper period
of contact of the sewage with the filtering material, each syphon
is provided with a “Horometer” relief apparatus. This apparatus can
be set to give a period of contact varying from twenty minutes to
twenty-four hours. The “Horometer,” like the “Sequela,” depends upon
the transference of oil from one compartment to another, but in this
case only two compartments are necessary, A and B. As the filter fills,
the oil is forced, by air pressure, to rise in a vertical pipe from
compartment A above the level of a regulating tap, which is set to pass
the oil into compartment B in the time determined upon for the contact
of the sewage in the filter, and as soon as the necessary quantity of
oil has been transferred through the tap, the syphon discharges. After
the syphon has discharged, the oil is automatically returned from
compartment B to compartment A, and the apparatus is again ready
for use. No watertight brick chambers are required in the filter in
connection with the apparatus, thus effecting considerable economy in
structural work. It is only necessary to construct a screen in dry
brickwork or perforated iron round the syphons to hold up the filtering
material.
[Illustration: FIG. 145.—CONTACT BED APPARATUS.]
[Illustration: FIG. 145_a_.—CONTACT BED APPARATUS.]
The syphons manufactured by Messrs. George Jennings, Ltd., actuated by
air-valves as described under the heading of “Dosing Apparatus,” can
also be adapted for filling and emptying contact beds.
[Illustration: FIG. 146.—CONTACT BED APPARATUS.]
The Enock apparatus for contact beds, Fig. 146, manufactured by
Messrs. A. G. Enock and Co., Ltd., is a simple device working on the
principle of the ball valve. A float, which takes the place of the
ball, is raised by liquid entering a pit, which pit is outside the bed
or tank which has to be emptied by the valve. The valve is attached to
a vertical rod in connection with a horizontal weighted lever, at the
other end of which the float is fixed. When a tank is full, it flows
into the float chamber, and the rise of the liquid in this pit lifts
the float and opens the valve, thereby allowing the contents of the
tank to escape. The float pit then slowly empties itself by means of a
small outlet pipe, and the valve closes so that the tank is ready to
receive more liquid. This apparatus can be arranged so as to fill a
number of beds in rotation, the inlet valve to each pit being either
opened or closed as required by the overflow of liquid from each
contact bed in turn. The outlet valves to the contact beds are similar
to those already described and if the first beds are filled in
rotation, no further connection between the apparatus in the lower beds
will be required, each valve working absolutely independently of the
others.
The chief advantage claimed for this type of apparatus is, that it
can be adjusted so as to suit any required level of liquid in any
particular bed.
_CAPACITY OF PERCOLATING FILTERS AND CONTACT BEDS._
Although this separate section is devoted to the question of
calculating the capacities of percolating filters and contact beds, it
is mainly for the purpose of stating that it is impossible to formulate
any rules which admit of general application. It may reasonably be
pointed out that this last statement is a truism, and affords no
assistance to those in search of information on the subject. There is,
however, so great a tendency in some quarters, to rely upon results
obtained in one place under certain conditions as a guide in designing
a scheme in another place under possibly totally different conditions,
that it is impossible to repeat the statement in question too often.
In the first place, long and practical experience is necessary to
enable an engineer to come to a decision as to what are the conditions
under which any particular scheme is to be carried out, and which
of them will have a bearing upon the methods to be adopted in the
design of the works. A careful study of the fifth report of the Royal
Commission on Sewage Disposal, will show that they assume over 70
different sets of conditions under which percolating filters and
contact beds may be adopted. The capacity of the filters required to
produce the desired results will depend upon the strength of the sewage
to be treated, the type of tank adopted for the preliminary process of
sedimentation, the grade of material to be used, the amount of fall
available, the final destination of the effluent, and other factors, all
of which again may be affected by other circumstances, which must of
necessity be taken into consideration. As the basis for calculating the
capacity of filters may vary between 15 and 200 gallons of the daily
dry-weather flow per cubic yard of material, it is evident that there
is a wide margin for error, and the only safe course to adopt is to
allow for the worst possible conditions and thus provide a large margin
of safety. Although the suggestions made in the fifth report of the
Royal Commission with regard to the provision to be made under various
sets of conditions may be taken as a guide, to some extent, it should
be borne in mind that the figures given represent the minimum which
should be allowed in each case, and the only really safe guide in these
matters is long practical experience of a large number of works under
the greatest possible variety of conditions.
In order, however, to provide a rough guide for the purpose of making
preliminary estimates, it may be stated here that, under ordinary
conditions, with sewage of average strength, a properly designed
preliminary process, suitable material of medium grade, and not less
than 4 feet deep for percolating filters, it should be possible to
produce an effluent which will not create a nuisance by providing—
(_a_) Percolating filters, at the rate of one cubic yard
for every 84 gallons of the daily dry-weather volume, or
(_b_) Contact beds, at the rate of one cubic yard in each
series for every 56 gallons of the daily dry-weather volume.
In other words, the ratio of the cubic capacity of filter material to
the daily dry-weather volume of the sewage for all ordinary purposes
may be taken as—
(_a_) 2 to 1 for percolating filters.
(_b_) 3 to 1 for contact beds.
TABLE, GIVING THE RATIO OF THE TOTAL CUBIC CAPACITY OF PERCOLATING
FILTERS AND CONTACT BEDS TO THE DAILY DRY-WEATHER VOLUME OF THE SEWAGE
UNDER VARYING CONDITIONS.
──────────────────────────┬──────────────────────────────────────
│ Percolating Filters
├──────────────────────────────────────
│ Strength of Sewage
├────────────┬────────────┬────────────
│ Strong │ Average │ Weak
├────────────┴────────────┴────────────
Preliminary Process. │ Grade of Material
(See pages 23, 29). ├──────┬─────┬──────┬─────┬──────┬─────
│Coarse│ │Coarse│ │Coarse│
│ or │ Fine│ or │ Fine│ or │ Fine
│medium│ │medium│ │medium│
──────────────────────────┼──────┼─────┼──────┼─────┼──────┼─────
Detritus tanks │11·20 │ — │ 6·72 │ — │ 4·20 │ —
│ │ │ │ │ │
Septic tanks │ 3·73 │ — │ 2·40 │ — │ 1·68 │ 1·68
│ │ │ │ │ │
Continuous flow settlement│ 3·73 │ — │ 2·40 │ — │ 1·68 │ 1·68
│ │ │ │ │ │
Quiescent settlement │ 3·36 │ 6·72│ 1·68 │ 2·40│ 1·29 │ 1·29
│ │ │ │ │ │
Continuous flow chemical }│ │ │ │ │ │
precipitation }│ 2·58 │ 3·36│ 1·68 │ 2·10│ 1·12 │ 0·96
│ │ │ │ │ │
Quiescent chemical }│ │ │ │ │ │
precipitation }│ 1·68 │ 2·58│ 1·29 │ 1·29│ 0·98 │ 0·84
──────────────────────────┴──────┴─────┴──────┴─────┴──────┴─────
──────────────────────────┬───────────────────────────────────────────
│ Contact Beds
├───────────────────────────────────────────
│ Strength of Sewage
├──────────────┬──────────────┬─────────────
│ Strong │ Average │ Weak
├──────────────┴──────────────┴─────────────
Preliminary Process. │ Number of Series
(See pages 23, 29). ├────┬────┬────┬────┬────┬────┬────┬────┬───
│ │ │ │ │ │ │ │ │
│ ×1 │ ×2 │ ×3 │ ×1 │ ×2 │ ×3 │ ×1 │ ×2 │ ×3
│ │ │ │ │ │ │ │ │
──────────────────────────┼────┼────┼────┼────┼────┼────┼────┼────┼───
Detritus tanks │ — │ — │6·72│ — │6·72│ — │ — │4·42│ —
│ │ │ │ │ │ │ │ │
Septic tanks │ — │ — │5·09│ — │4·42│ — │2·24│2·54│ —
│ │ │ │ │ │ │ │ │
Continuous flow settlement│ — │ — │5·09│ — │4·42│ — │2·24│2·54│ —
│ │ │ │ │ │ │ │ │
Quiescent settlement │ — │ — │3·81│ — │3·36│ — │1·68│ — │ —
│ │ │ │ │ │ │ │ │
Continuous flow chemical }│ │ │ │ │ │ │ │ │
precipitation }│ — │5·09│ — │ — │3·36│ — │1·26│ — │ —
│ │ │ │ │ │ │ │ │
Quiescent chemical }│ │ │ │ │ │ │ │ │
precipitation }│ — │3·90│ — │ — │2·54│ — │1·26│ — │ —
──────────────────────────┴────┴────┴────┴────┴────┴────┴────┴────┴───
KEY: ×1 = Single
×2 = Double
×3 = Triple
In the case of contact beds these figures give the cubic capacity of
each series, and they must be doubled or trebled respectively for
double and triple contact. Both percolating filters and contact beds,
constructed on this basis, would be capable of treating up to three
times the dry-weather flow in times of storm.
Having given the above method of calculation, in a form not usually
adopted in connection with sewage disposal, and bearing in mind the
misunderstandings which frequently arise in comparing the various
methods in use at the present time in different countries, it may be
useful to set out in this form the figures which, it is understood,
have been adopted by the Local Government Board, on the basis of
the fifth report of the Royal Commission on Sewage Disposal, as the
minimum which they consider suitable under varying conditions. In the
Table opposite, the figures given represent the ratio which the cubic
capacity of the filters bears to the daily dry-weather volume of the
sewage, whether it be in gallons, cubic feet, cubic metres, vedros, or
any other term of measurement.
_Examples._—1. A daily dry-weather volume of 10,000 gallons
of sewage, of average strength, is to be treated upon
percolating filters of medium sized material, after
preliminary treatment in septic tanks.
10,000 gallons × 2·40 = 24,000 gallons
= 3,840 cubic feet
= total cubic capacity of filters
2. A daily dry-weather volume of 3,000 cubic metres of weak
sewage is to be treated upon single contact beds, after
preliminary treatment in continuous flow settlement tanks.
3,000 cubic metres × 2·24 = 6,720 cubic metres
= total cubic capacity of beds
For the purpose of the above Table, the strength of the sewage is
estimated according to the amount of oxygen absorbed from permanganate
of potash in four hours, as indicated in the fifth report of the Royal
Commission as follows:—
Parts per 100,000
“Strong” sewage = oxygen absorbed 17 to 25
“Average” ” = ” ” 10 to 12
“Weak” ” = ” ” 7 to 8
A quick method of converting gallons into cubic feet is to multiply the
gallons by the reciprocal 0·16. This can be done rapidly (frequently by
mental calculation) by multiplying the gallons by 4, and the product
again by 4, and inserting the decimal point between the second and
third figures from the right-hand side, thus:—
24,000 × 4 = 96,000 × 4 = 384,000 = 3840·00
= cubic feet
This is useful in calculating the capacity of tanks, and for all similar
purposes.
_STORM-WATER TREATMENT._
In connection with Sewage Disposal Works, the term “storm-water” is
generally understood to mean the extra volume which reaches the works
in times of rainfall, in excess of three times, up to and including
six times, the average dry-weather flow; so that the volume of
storm-water for which provision should be made is equal to three times
(volumes) the daily dry-weather flow. Prior to the publication of the
fifth report of the Royal Commission, it was usual to provide a rough
straining filter for the storm-water, or to reserve a portion of the
land for the purpose of dealing with it by broad irrigation. In either
case the area of filter surface or land required was 1 superficial yard
for every 500 gallons of storm-water (D.W.F. × 3/500 = super-yards).
As the result of their investigations, the Royal Commission came to the
conclusion that “storm-water” filters, as generally constructed under
these conditions, were useless for the purpose for which they were
required, and this confirmed the views of most engineers of experience.
Where suitable land can be secured for the purpose, and arranged in
such a manner that it is reserved solely and entirely for treating the
storm-water, this method may still be adopted. If this is not possible,
stand-by tanks may be constructed for the purpose of receiving the
storm-water. These tanks are to be not less than two in number, and
should have a total capacity of not less than one-quarter of the
average daily dry-weather flow. The only overflow at the outfall works
from which storm-water may be discharged direct to the stream, or other
final effluent outlet, must be from these stand-by tanks, and it should
not come into operation until these tanks are full. Having regard to
these recommendations, it is necessary in every scheme to provide at
least two special storm-water stand-by tanks, with a total capacity of
¼ D.W.F.; and the drawing, Fig. 147, illustrates a simple method of
constructing these. In this case the inlets are in the form of weirs,
running the full width of the tank, so that if the channel leading to
these tanks is in communication with, and at the same level as, the
inlets to the detritus tanks, and the latter are provided with slotted
doors (see Fig. 10, page 19), the weir at the inlets to the stand-by
tanks will act as the actual storm-overflow, and, being of considerable
length, the maximum height to which the water will rise in passing over
this weir will be very small, and will thus have very little effect
upon the rate of flow to the sedimentation tanks.
As the only overflow discharging direct to the stream must be from
these tanks, and must only come into operation when they are full,
the outlet is also constructed in the form of a weir discharging into
a channel from which a pipe would be laid to the stream. A further
requirement in connection with these tanks is that they should always
be kept empty, ready to receive the excess of storm-water at any time.
From this it is evident, that these tanks must be emptied after each
heavy shower or storm which increases the rate of flow of sewage to
the works beyond three times the dry-weather flow. Unfortunately,
no directions are given as to the manner in which this is to be
accomplished. In the absence of any definite statement to the contrary,
it might be inferred that, after the overflow from these tanks has
ceased, their contents may also be discharged direct to the stream. As,
however, this would necessitate outlets at or near the bottom of the
tanks, there would appear to be a possibility of the suspended matters
deposited in the tanks being discharged to the stream—the very thing
the tanks are designed to prevent. With such an arrangement, also,
there will be a risk of the man in charge of the works, either wilfully
or by an oversight, leaving the outlets at the bottom of the tanks
open, and thus permitting the storm-water to pass direct to the stream
without the settlement which it is anticipated by the Royal Commission
(Fifth Report, page 233, par. 352) will be provided for all storm-water
arriving at the works.
[Illustration: FIG. 147.—STORM-WATER STAND-BY TANKS.]
It is obvious that these tanks must be emptied after every heavy shower
or storm, and that facilities must be provided both for drawing off the
supernatant water and for removing the deposit which will accumulate at
the bottom. In the author’s opinion, the only safe method is to provide
floating arm outlets for the supernatant water, and to discharge this
to land or to a special filter for further treatment, or, better still,
to pump it up to the detritus tanks to be treated again with the
ordinary sewage. In schemes where the whole of the sewage is pumped
at the works, the contents of these storm-water stand-by tanks should
certainly be discharged into the pump-well, as this would not involve
the provision of special pumping plant. With regard to the sludge from
these tanks, this should be drawn off by means of special outlets, and
dealt with on sludge draining beds in the manner previously described
(page 83).
The difficulties which frequently arise in designing suitable and
convenient methods of dealing with storm-water, render it desirable
that very careful consideration should be given to the question as to
whether it would not be more satisfactory, from the point of view of
both economy and efficiency, to omit the stand-by tanks, and increase
the capacity of the filters required to deal with the _ordinary_ sewage,
to such an extent that they will be capable of dealing with the
whole volume of sewage and storm-water combined up to six times the
dry-weather flow, and thus obviate the necessity for any storm-overflow
at all at the outfall works. If this idea were universally adopted,
it would necessitate greater care in the construction of any
storm-overflow required on the line of the outfall sewer itself before
it reaches the works, but there are (or should be if the sewers were
properly constructed) so few cases where the excess of flow, even
during the heaviest rainfall, ever reaches six times the dry-weather
flow, that the extra cost involved cannot be considered excessive if
the greater certainty of securing satisfactory results at all times is
taken into consideration.
_MEASURING APPARATUS._
In recent years the provision of proper means of measuring and
recording the flow of sewage at disposal works is becoming more
general, but there are still a very large number of works at which it
is impossible to obtain any trustworthy information as to the volume
of sewage treated. As long as all works smoothly, and there is no
trouble with the effluent, it is considered superfluous to trouble
about the quantity of the liquid which passes through. When, however,
difficulties arise, and it becomes necessary to investigate the cause
of the trouble, it is of the utmost importance to be in a position to
ascertain the daily volume of the sewage arriving at the works and the
variations in the rate of flow, as well as the quantities dealt with
by each separate tank and filter. It is also of great assistance, in
making investigations at such times, to have a definite record of the
volumes treated day by day during the preceding six months; indeed the
possession of a complete record of the daily flow of sewage over the
whole period during which a sewage works has been in operation, is a
valuable asset not only to those responsible for the works themselves
but also to the authorities who control the streams and watercourses,
and to investigators in search of information to be used for the
public benefit. The initial cost of suitable measuring and recording
appliances is not excessive, but, when once the works are completed
and in good working order, the local surveyor or manager has great
difficulty, and usually finds it impossible, to persuade his committee
that the outlay is justified. It is, therefore, desirable that
engineers should in all cases make provision for such apparatus in the
contract for the construction of the works, and thus make sure that it
will be available when required.
[Illustration: FIG. 148.—GAUGE WEIR PENSTOCKS.]
Various types of measuring appliances are in use. Among these the
simplest is the gauge weir penstock Fig. 148, manufactured by Messrs.
Adams Hydraulics, Ltd., from which the depth of water flowing over the
weir can be observed at any time. This, however, does not provide a
record of the volume flowing at all times. For this purpose the same
firm supply several types of recorders, among which the simplest is the
type shown in Fig. 149. This is supplied with 24-hour or 7-day
movement, and charts which register the volume in gallons per minute,
per hour, or per 24 hours.
[Illustration: FIG. 149.—RECORDING APPARATUS.]
Another measuring apparatus is that supplied by Messrs. George
Jennings, Ltd., Fig. 150. This consists briefly of a hollow copper
float attached to a brass rod, which is carried up through a hollow
column into the indicator box provided with a glass hinged door. The
brass rod is provided with pointers, which move up and down with the
rods and indicate the water level.
[Illustration: FIG. 149_a_.—VOLUME RECORDER.]
Messrs. George Kent, Ltd., supply a large number of various types of
recorders and measuring instruments, including the well-known “Venturi”
meter and others actuated by floats to indicate the discharge over a
weir. The Venturi meter type of measuring apparatus adapted for sewage,
is shown in Fig. 151. In this case the Venturi tube may form part of
the ordinary supply pipe, or it may be fitted in a chamber built in the
supply channel, the tube connected through the chamber with its ends
terminating in the channel at either side. This measuring apparatus may
be fitted with the following types of recorders. A diagram only,
counter only, or combined counter and diagram in one instrument. The
recorder may be placed in any convenient position within a thousand
feet of the meter tube.
[Illustration: FIG. 150.—MEASURING APPARATUS.]
[Illustration: FIG. 151.-THE “VENTURI” METER.]
[Illustration: FIG. 152.—MEASURING APPARATUS.]
Several types of measuring apparatus are manufactured by Messrs.
Glenfield and Kennedy. One of these is illustrated in Fig. 152. This
consists of an apparatus for recording the volume of water or sewage
flowing over a weir, a chart being revolved by clockwork, and the
volume indicated by a pen actuated by a float and cord working over
pulleys. This instrument can be fitted with cam and pen carriage to
show the rate of discharge on the chart (24 hours or 7 days), in
gallons of cubic feet per minute over a V-notch, rectangular weir, or
in an open channel of known dimensions. It is claimed that by simply
taking the area of the diagram, the total discharge for any period can
be ascertained much more quickly and more correctly than from a diagram
simply giving the height flowing over the weir.
A somewhat novel form of apparatus, recently introduced by Messrs.
Adams Hydraulics, Ltd., for measuring the flow of sewage in a channel,
consists of a water-wheel by means of which the velocity of the flow is
registered. In order to maintain the paddles of the wheel at a uniform
depth below the surface of the liquid, the wheel is carried upon a
shaft supported by two floats provided with vertical guide rods working
in brackets attached to the sides of the channel. By this means the
whole of the apparatus rises and falls with the liquid in the channel.
The wheel-shaft is provided with a bevel-toothed wheel, which engages
with another similar wheel attached to a flexible shaft, and this
drives a set of geared wheels similar to a flush-tank counter, which
thus record the number of revolutions made by the paddle-wheel.
_STERILISATION OF SEWAGE EFFLUENTS._
It has been recognised in many quarters that, although it is possible
by modern methods of sewage disposal to secure a high degree of
purification from a chemical point of view, it may be necessary in
certain cases to take steps to remove the large numbers of bacteria
present in all such effluents. Experiments have been made which
demonstrate that, when a pure culture of some specific organism is
added to a sewage in a sufficiently large quantity, it may pass through
the tanks and filters, and appear in the final effluent. This result
is not necessarily a conclusive proof that the bacteria in sewage
effluents are dangerous, as the experiments do not represent normal
practical conditions. On the other hand, it is true that, in ordinary
practice, sewage effluents contain large numbers of _B. coli_, which is
admittedly of intestinal derivation, and although this bacillus is not
a disease organism itself, its survival in an effluent is considered
an indication of the presence of sewage matter, and consequently
of the possibility of the survival of any pathogenic germs which
may be present in the crude sewage. On this basis, scientists argue
that sewage effluents are _potentially_ dangerous—that there is a
possibility of the pollution of drinking-water or shell-fish by the
bacteria present in sewage effluents. This being so, it is evident that
an additional process will, in some cases, be required to remove the
bacteria, and in a few cases sand filters have been provided for this
purpose. These, however, involve a comparatively high initial expense,
and a considerable annual outlay for maintenance, and in some quarters
it is considered that sterilisation may possibly be a means of securing
the desired result at less cost, and with a higher percentage of
removal of bacteria, and consequently with a higher degree of safety.
It is true that a number of scientists, in replying to a question which
was submitted to them on this subject by the Royal Commission on Sewage
Disposal, stated that in their opinion sterilisation was impracticable,
but this was in the year 1903, and there is good reason for assuming
that if the same question were put to the same men to-day, the replies
would in many cases be modified, if not entirely different. Practical
experiments in the sterilisation of sewage effluents have been few
and far between in this country, but in the United States of America
a large number of reliable experiments under varying conditions have
been made, and the results published. From these it is evident that
sterilisation is not only possible, but economically practicable.
Unfortunately, both in America and elsewhere, attempts were made to
sterilise crude sewage and tank effluents, and the results of these
experiments were so unsatisfactory, both in efficiency and cost, that
they gave the impression that sterilisation was impracticable.
In the opinion of the author, sterilisation should be restricted to the
destruction of living organisms, and should only be used in the case
of liquids with a high degree of chemical purity, and a low content
of matters in suspension. It is quite possible at the present time
to produce sewage effluents which comply with these conditions, by
means of properly designed, constructed and managed sewage disposal
works, as long as these include suitable effluent settling tanks
for the removal of the solids in suspension. Effluents which comply
with the provisional standard suggested by the Royal Commission, and
drinking-water supplies which are very slightly polluted, would be very
suitable.
A number of different methods of sterilisation have been tried, but so
far as the existing knowledge of the subject extends, the application
of chlorine in one form or another is generally admitted to be the most
efficient and economical process. The chlorine may be applied in the
form of a solution of chloride of lime, or as a hypochlorite of sodium,
or of magnesium. The solution of chloride of lime may be prepared by
the sewage works manager, but this necessitates a considerable amount
of care and knowledge in order to secure the correct strength at all
times, and it is, of course, essential that the chloride of lime itself
should always be of a known strength. The hypochlorite of sodium may be
produced chemically, and this can be purchased of known strength from
chemical manufacturers. For large volumes of sewage effluent, however,
it will probably be found most economical to utilise an electrically
produced hypochlorite of sodium, as this can be prepared on the works
as and when required, of a uniform strength, and at a comparatively low
cost.
In the case of the Digby process, briefly stated, this consists in
passing an electric current through a solution of sodium chloride, with
the result that the sodium chloride is broken up into its component
parts, and chlorine is liberated at the positive pole, while sodium is
deposited at the negative pole. The sodium and the chlorine are then
allowed to recombine in the form of hypochlorite, and this solution
is then ready for application to the liquid to be sterilised. Other
processes are similar in principle but vary in detail.
[Illustration: FIG. 153.—THE DIGBY MERIDIONISER.]
This process naturally involves the use of an electric current and
an apparatus for producing the hypochlorite. Of the latter there are
several on the market, and an illustration of the Digby Meridioniser,
which is manufactured and supplied by Messrs. Adams Hydraulics, Ltd.,
is given in Fig. 153. The particular feature of this apparatus consists
in the manner in which the re-combination of the anode and cathode
products are secured. Instead of the re-combination taking place in the
main body of the electrolyte, it can only take place in the special
porous compartment enclosing the electrodes. The re-combination may
take place in either the anode or the cathode compartment, the products
of the one compartment being conveyed to the other compartment. Thus
the caustic hydrate from the cathode cells flows by gravity into the
anode compartment, the two compartments being connected by a glass
pipe. Fresh water or water containing an excess of alkali is run into
the cathode cell. This process gives hypochlorite solution of low
saline content, the only salt present in the resultant liquor being
that due to diffusion, depending upon the porosity of the closely
covering compartment walls, or upon such a reaction as that covered
by the Blount hypothesis. A cross-section of this apparatus is shown
at Fig. 153, in which A is the positive lead; BB negative leads, C
outflow, D inflow, EE cathodes, F anode.
Another machine is that supplied by Messrs. Oxychlorides (1907),
Limited. This machine consists essentially of a graphite anode
of circular cross-section (except as to about 4 in. at the top,
which is left open for the free escape of gases evolved during the
electrolysis), and within it a metallic cathode of smaller circular
cross-section. The annular space between the anode and cathode is
filled with a solution of common salt, or with sea-water, through which
a current is passed from a low-potential dynamo. In either case the
ultimate result is to obtain some of the chlorine of the salt in an
active or available form, the only difference being that in the case of
using strong salt solution, a concentrated form of available chlorine
may be obtained, while with sea-water a weaker solution results. Where
sea-water is readily obtainable it is naturally more economical to
make use of it, and to employ the larger volume of less concentration;
while where sea-water is unobtainable and salt is expensive, or where
chlorides in the effluent are objected to, it is more advantageous to
prepare concentrated solutions, and to dilute them when required for
use. This machine is made in various sizes to suit varying conditions.
It was used by the Royal Commission on Sewage Disposal in connection
with their experiments at Guildford, referred to in their Report of
1908, pages 198 to 201.
Electrolytic hypochlorite of magnesium is being produced daily, by the
Borough of Poplar, for use as a disinfectant. The highly effective
qualities of this solution, as well as the low cost of production,
makes it a very valuable disinfectant, and for the results obtained
every credit is due to Dr. F. W. Alexander, the Medical Officer of
Health, who initiated the scheme, and to whose unbounded energy and
enthusiasm the success of the work is due. This solution is, of course,
equally suitable for the sterilisation of sewage effluents. The cost of
production of this solution, of an average strength of from 4·5 to 5
grammes of available chlorine per litre (·45 to ·5 per cent. solution),
is estimated at under one penny per gallon. The apparatus in use at
Poplar has been supplied by the Farringdon Engineering Co., and an
illustration of the plant is shown in Fig. 154.
[Illustration: FIG. 154.—ELECTROLYTIC HYPOCHLORITE OF MAGNESIUM
PLANT.]
It consists of four cells, each containing ten elements, consisting of
one positive and two negative plates. The positive plates consist of
thin platinum wire, wound upon slate slabs, and the negative electrodes
are of zinc. The four cells are placed one above the other, so that
the liquid passes through from one to the other by gravitation. The
feed-tank at the top contains a solution of sodium chloride and
magnesium chloride, and is fitted with a glass gauge to indicate the
amount of solution in the tank. From this tank the solution passes
through a small ball-valve cistern, so as to maintain a constant rate
of flow. The feed-tank is also provided with a plate, operated by a
chain carried over to the outside of the tank, by means of which the
liquid to be electrolysed can be stirred from time to time, so as to
secure a uniform strength of solution. The solution passes through
the four electrolysers in series, being subjected to the action of a
regulated current of 15 to 17 amperes at 230 to 250 volts, being 5·7
to 6·2 volts per cell. After the electrolysed solution leaves the last
cell it runs into a small tank, where a fixed amount of hydroxide of
magnesium is mixed with it, and it is claimed that by this means the
solution is rendered stable, a quality which should be of much value
where the solution has to be stored for any length of time.
The question as to what is the most suitable sterilising agent to use
under certain conditions, and in what proportion it should be added to
the sewage effluent, is a matter for the chemist and biologist. The
method of application is, however, the duty of the engineer. As in the
case of other chemicals, there are two ways in which it may be applied.
The solution may be added to the sewage effluent in equal doses of
varying strength, or in varying doses of equal strength. There is a
third method, involving the variation of the dose and the strength
of the solution, but while this is not impossible it is probably not
practical. The chief difficulty to be overcome is the variation of
both the rate of flow and the strength of the sewage, and the most
practical solution is to prepare the sterilising agent of a uniform
strength, and vary the doses in direct proportion to the flow of the
effluent, the minimum dose being sufficient for the maximum strength
of the sewage. This method was adopted by the author in the case of a
small scheme of sewage disposal, which he prepared for a place where
the only outlet for the final effluent was a discharge over an area of
chalk subsoil, from which the water supply of a large town is drawn. In
this case, he designed a simple apparatus which does not involve any
special appliances, and which would be quite satisfactory for ordinary
practical purposes. A new apparatus for the purpose in question has,
however, recently been introduced by Messrs. Nixon and Mannock. As
will be seen from the illustration, Fig. 155, it is based upon the
application of the Venturi principle, and involves the use of a
Venturi tube, as previously described under the heading of “Measuring
Apparatus.” In fact, the same Venturi tube can be utilised to serve
both for measuring the flow of the effluent, and for applying the
sterilising agent in direct proportion to the flow of the effluent.
[Illustration: FIG. 155.—NIXON AND MANNOCK’S APPARATUS FOR
INJECTING CHEMICAL SOLUTION.]
The apparatus consists of a cylinder C, the top of which is connected
by means of a pipe fitted with a three-way cock to the “Upstream”
end of the tube A. A similar connection is made from the bottom of
the cylinder to the “throat” B. A piston of the type used in the Kent
Standard Water Meters, and provided with a counterbalance weight,
works in the cylinder by means of the difference of the pressure on
the two sides of the Venturi tube. The chemical solution (e.g. a 5
per cent. solution of chlorine) is supplied to the underside of the
piston, and the pressure on the upper side of the piston being greater
than the pressure on the underside, the chemical is forced down by the
piston and injected through the injection tube and regulating valve
into the effluent at the “throat” of the Venturi tube. As the flow
of the effluent through the Venturi tube produces a difference of
pressure which varies as the square root of the velocity, the rate of
injection will also vary in the same proportion. The injection is thus
in exact proportion to the flow, and any variation of the flow will
automatically cause a corresponding variation in the rate of injection.
When the chemical re-agent is exhausted, the piston will be at the
bottom of the cylinder, and the pointer at zero. In order to recharge
the cylinder with the chemical, the three-way cocks must be reversed
by means of the hand lever, thereby cutting off pipes A and B, and
simultaneously connecting the top of the cylinder to the waste pipe,
and the bottom to the supply from the chemical storage tank, which is
fixed at such a height that the head will rapidly force the piston up
and re-fill the cylinder with the chemical. The three-way valves are
then reversed, and the apparatus is again in full working order. The
apparatus shown is applicable to the treatment of 1000 gallons per
hour, and will only need recharging once per day of 24 hours.
A feature of this apparatus is that it is self-starting, and should the
flow cease, the injection will also automatically stop, the static head
on both sides of the piston being equal. There is absolute immunity
from danger or over-injection of the chemical by this system, and
this is a valuable factor in the treatment of potable water. Where
absolutely necessary, the same firm can supply a de-chlorinating
apparatus. By means of the indicator, the works manager is constantly
informed of the exact amount of chemical injected, and the scale
readings can be compared with those of a Venturi Meter operated by the
same Venturi tube. This apparatus can be supplied of a larger size, and
provided with automatic recharging gear for larger installations.
Whatever method may be adopted for applying the sterilising agent, it
is essential in all cases to have a storage tank to receive the mixture
in order to provide time for the chemical to have full effect. So far
as can be ascertained at present, a storage capacity equal to one
hour’s flow of the liquid to be sterilised will be sufficient under
ordinary circumstances, but provision should be made for thoroughly
mixing the chemical with the effluent and for drawing off any deposit
which may occur in the tank without interfering with the normal working
of the plant.
Although the present volume is devoted entirely to the disposal of
sewage, it may be stated here that in the matter of sterilisation,
the suggestions that have been made apply with equal force to
drinking-water supplies. Where the water contains a considerable amount
of matter in suspension, it would be advisable to provide means for
ample storage and settlement before passing it through the sterilising
plant.
_Note._—An apparatus for the injection of chlorine
solutions for the purpose of sterilising sewage effluents
and drinking water has recently been brought out by the
Candy Filter Co., Ltd., and has for some months been in
practical operation, dealing with 200,000 gallons of river
water per day for an important municipal waterworks in
the country. In this case the installation includes a
de-chlorinating process, and it is stated that the results
of tests in actual work show that the sterilised water
contains neither _B. coli_ nor free chlorine.