The general features of construction are:—

Substantial walls in brickwork, concrete, plain or reinforced, and

of a suitable thickness to withstand with safety the pressures they

are required to resist; sloping floors provided with suitable outlets

for both liquid and solid contents at the bottom, and specially

arranged inlets and outlets at the top. In connection with the

floors, sufficient care has not always been devoted in the past to

the consideration of the most convenient method to adopt, in view

of the necessity of removing the sludge. In some cases, the floors

have been laid with a slope towards the outlet end, and, as the

greatest accumulation of deposit takes place at the inlet end, great

difficulties have been experienced in removing the sludge. There is

very little doubt that if suitable arrangements are made, by means of

which the accumulation of solids deposited at the inlet ends of tanks

can be removed without drawing off the total contents of the tank, much

labour will be saved. With this end in view, the design illustrated in

Fig. 16 is suggested as a model which may be adopted exactly as shown,

or, with some modifications, adapted to meet the special requirements

of particular cases. It will be noticed that a submerged weir wall is

introduced at some distance (which will vary with the method upon which

the tank is operated and with the character of the sewage) from the

inlet end of the tank, so as to retain the larger portion of the solids

in this separate compartment. The floor of this section is laid with a

comparatively steep gradient leading to the sludge outlet. A separate

outlet, fitted with a floating arm, may be provided for drawing off the

top water down to the level of the top of the weir wall. Below this

level, in ordinary circumstances, only the contents of the separate

compartment at the inlet end of the tank will be drawn off in removing

the sludge. A valve is provided at the bottom of the weir wall, so that

the entire contents of the tank may be drawn off should it be found

necessary at long intervals. An alternative to the submerged weir wall

is shown in Fig. 17, in the form of a division wall carried up to the

top of the tank, with orifices below the top water level through

which the sewage passes when the tank is in use. These apertures are

provided with valves, so that they may be closed when the solids in

the compartment at the inlet end of the tank are drawn off, and thus

obviate the necessity for emptying the whole of the tank.

[Illustration: FIG. 17.]

From observations which have been made in various places, it has been

found that although the actual capacity of the tanks corresponded to

anything from 12 up to 24 hours of the daily dry weather flow, the

period during which the sewage remained in the tank, or rather the

time taken for the sewage to pass through the tank, was much less than

it was anticipated would be the case. In one instance, it was noticed

that the sewage passed through a tank of a capacity equal to 15 hours’

dry weather flow in 4 hours, and, although it is obvious that the

same efficiency of sedimentation could not be secured by passing the

sewage at the same rate through a tank of a capacity of 4 hours’ flow,

it would seem that the full effect of the larger tank was not brought

into play. A possible explanation is that the form of the tank and the

arrangement of the inlet and outlet were such that the flow of sewage

through the tank was more or less in a direct line from the inlet to

the outlet, and this, if correct, would lead to the conclusion that

there is room for improvement in the design of the tank, in order to

cause the sewage in its passage to be spread out over the whole area of

the tank. With this end in view the author has specially designed the

arrangement illustrated, Fig. 18, as a suitable method of preventing

the sewage passing direct from the inlet to the outlet. It will be

noticed that the sewage enters the first compartment about 3 feet below

the top water level, and by means of three cross walls is made to flow

down to within a short distance of the floor in one compartment, and up

to within a short distance of the top water level in the next, and

that this occurs twice in the total length of the tank. By sloping the

floor from the centre both ways, i.e. to the inlet and outlet ends,

and providing sludge outlets at the lowest points in each case, every

facility is made for removing the deposit and for emptying each half

of the tank whenever it may be found necessary. Further, by arranging

the sludge outlets in pockets or sumps, situated below the level of

the lowest point of the floor itself, it is possible to draw off the

sludge in small quantities at frequent intervals without emptying the

tank itself. The chief factors in causing the sewage to be uniformly

spread out over the whole area of the tank are, however, the valves

or penstocks on the inlet and outlet pipes, and on the pipes in

the central cross wall. By suitable adjustment of these penstocks,

partially closing those through which the sewage has a tendency to flow

most freely and opening the others, there should be no difficulty in

securing a uniform distribution of the sewage. In any case the actual

direction of the flow of the sewage is, by means of these penstocks,

entirely under control. The inlets to the tank being submerged below

the water level in the supply channel, will secure a more uniform rate

of flow through all the inlet pipes than if they were placed at the top

water level, and the valves on these pipes provide facilities for any

further regulation that may be required. The most important point to

be observed, however, is that the rate of flow from the outlets of the

tank should be uniform. In order that this may be secured, these pipes

are submerged on the inside of the tank, but have their outlets set at

the top water level, so that the actual discharge may be visible, and

thus render it possible to regulate the rate of flow from each pipe by

means of the penstocks provided for the purpose. Further, the openings

in the middle cross wall may be adjusted to control the direction of

the flow through the tank by means of the penstocks, which also serve

to shut off either half of the tank when the other is emptied.

[Illustration: FIG. 18.]

[Illustration: FIG. 18.]

[Illustration: FIG. 19.]

Another method of ensuring uniformity of flow over the whole area of a

tank, is to arrange it in the form of a wedge, with the inlet at the

narrow end and the outlet in the form of a weir at the wide end. This

form of tank is shown, Fig. 134, page 183, for settling out the humus

in filter effluents. The same tank, with a greater depth, would be

equally suitable as an ordinary sedimentation tank for sewage, and

several could be arranged in such a way that three or four would form

a half-circle, i.e. the angle between the two side walls of each tank

would be 60 degrees or 45 degrees.

The principles embodied in the preceding suggestions can be applied to

most types of rectangular tanks.

_Sludge Well._—In connection with the actual method of conducting

the sludge from these tanks to the sludge disposal area, the remarks

made under the heading of detritus tanks will apply. A convenient

arrangement for a sludge well, where a number of tanks are involved,

is shown in Fig. 19, which is self explanatory. For small schemes a

chain-pump operated by hand may be used to raise the sludge from the

well. In larger schemes where power is available, sludge elevators of

the bucket type, as shown in Figs. 20, 21 and 21A, are very

convenient.

_Roofs over Tanks._—With regard to the question of roofs over

tanks, it is now generally admitted that these have very little, if

any, effect upon the working of the tank, and they may therefore

be dismissed in a few words. Under certain circumstances it may be

desirable for sentimental reasons to cover sewage tanks, and in such

cases the general practice is to form concrete arches covered with

earth and sown with grass. Reinforced concrete construction may

sometimes be found very suitable, while, in other cases, galvanized

corrugated-iron roofs, supported on an iron framework carried on

the walls of the tanks, are preferred. In very small installations,

1½-inch or 2-inch creosoted deal boards, laid loose, but fitting close

together with their ends supported in a rebate in the top of the wall,

make a very good cover, as they are easily removed whenever it becomes

necessary to inspect or gain access to the tank.

[Illustration: FIG. 20.—SLUDGE ELEVATOR.]

_Details of Inlets and Outlets._—Among the most important points to

be considered in designing sewage tanks is the arrangement of the

inlets and outlets, as upon these depends to a very great extent the

efficiency of the process. In order to afford a means of selecting the

most suitable arrangement for any particular case a number of different

methods are illustrated.

[Illustration: FIG. 21.—SLUDGE ELEVATOR.]

[Illustration: FIG. 21A.—SLUDGE ELEVATOR.]

Fig. 22 shows the simplest form of trapped inlet and outlet, consisting

of cast-iron Tee junction pipes, the junction being built into the

wall of the tank and fitted with a valve or penstock. The lower end of

the trapped pipe is generally about 3 feet below the top water level,

but in special cases may be much deeper. The upper end of this pipe

terminates at some distance (e.g. about twice the diameter of the

inlet junction) above the top water level, and the top is left open or

fitted with a blank flange for purposes of inspection. Where a roof is

provided over the tank, it is desirable to continue this pipe up and

through the roof, so that it may still be available for inspection. In

large tanks, or any tanks having a width of more than 6 feet, several

of these inlet and outlet pipes should be provided, one for about every

6 feet of width, in order to spread the sewage as much as possible over

the whole area of the tank. A valve should be provided on the inlet

pipe. This is essential in order that the flow of sewage to the tank may

be shut off whenever it needs attention or has to be emptied. Where

there are several tanks with their outlets discharging into a common

channel, it will be found desirable to have valves on the outlets as

well as on the inlets. A slight fall should always be allowed from the

invert of the inlet to the invert of the outlet pipe, and again from

the latter to the tank effluent channel or pipe leading to the filters.

[Illustration: FIG. 22.]

[Illustration: FIG. 23.]

[Illustration: FIG. 24.]

In Fig. 23 a somewhat similar arrangement is shown, but instead of Tee

junctions the inlets and outlets are formed of easy bends, which may be

in cast-iron or glazed stone ware as indicated. The observations made

above in connection with Fig. 22 apply generally to Fig. 23.

Fig. 24 is a plan of Fig. 23, to show a number of inlets and outlets to

one tank.

In Fig. 25 the trapped inlet and outlet is formed by means of a cross

wall carried up to the top of the tank with openings at the floor level

in the form of arches. It is considered by some engineers that this

method is a more substantial form of construction, and that it assists

to a great extent in spreading the flow of the sewage over the whole

area of the tank.

In Fig. 26 both the inlet and outlet is in the form of a weir, running

the full width of the tank, and it is probable that this is the most

efficient means of ensuring that the flow of sewage shall spread over

the whole area of the tank. The trapping of the inlet and outlet in

this case is obtained by the use of scum boards or plates, as shown.

When more than one tank of this type is required, it becomes necessary

to provide a separate feed channel or carrier in addition to the

channel immediately in front of the inlet weir, in order to arrange

means for shutting one or more tanks out of work when required.

[Illustration: FIG. 25.]

The method of arranging the inlets and outlets shown in Fig. 27,

consists of constructing extra deep sewage carriers and tank effluent

channels, and making the connections from these to the tank at the

desired depth below the top water level in the tank. It is true that

these deep channels always stand full of sewage or tank effluent while

the tank is in operation, but it is assumed that the passage into the

tank of all solid matters in suspension is facilitated, especially

during the minimum flow of sewage. It is essential that both channels

should be well dished towards the tank on either side, so as to avoid

all corners where solids may lodge, and render it easy to clean out the

channels when the tank is emptied.

[Illustration: FIG. 26.]

[Illustration: FIG. 27.]

[Illustration: FIG. 28.]

[Illustration: FIG. 29.—TYPE OF FLOATING ARM.]

The various types of inlets and outlets described above are more

particularly suitable for tanks which come under the terms “septic” and

“continuous-flow sedimentation without chemicals.” It is not necessary

that the inlets and outlets should both be of the same type. Various

combinations may be adopted, according to the requirements of each case

and the judgment of the engineer. Similar methods may be utilised for

“continuous-flow sedimentation tanks with chemicals,” but they need

the addition of floating arms for the purpose of drawing off the top

water before the sludge is removed. The type of inlet and outlet more

generally in use for chemical precipitation processes is shown in Fig.

28, as in these cases there is no need to preserve a scum on the

surface. The connection between the sewage carrier and the tank is

usually in the form of a sluice gate, and simple wooden boxes are

provided round the inlet and outlet in order to divert the flow towards

the bottom of the tank. It is also found desirable in some cases to

provide scum-boards for the purpose of arresting the grease, which

naturally rises to the surface, and must not be allowed to pass away

with the effluent. The floating arm outlet is essential, particularly

for tanks which are designed for “quiescent sedimentation with or

without chemicals,” and the usual form of outlet into a channel a few

inches only below the inlet level is not needed, as tanks of this type

are filled and allowed to stand full for a certain period, and the

contents are then drawn off through the floating arm. The function of

this appliance is to draw off the whole of the clear liquid contents,

from a point a few inches below the surface, at a slow rate, and

without disturbing the sludge at the bottom.

[Illustration: FIG. 30.—DECANTING VALVE.]

A type of floating arm is shown in detail in Fig. 29. In order to

prevent any possibility of these arms drawing off sludge by an

oversight, when approaching the floor of the tank, the chain attached

to the float should be arranged to check the fall of the arm at a

point which will be above the level of the sludge, or, if there is

any possibility of the chain being tampered with by unauthorised

persons, the fall of the arm may be arrested with certainty by means

of a bracket, built into and projecting from the wall of the tank, or

by means of a short pier of brickwork and concrete, built up on the

floor of the tank under the arm to the required level. Another method

of drawing off the top water from tanks has been introduced by Messrs.

Willcox and Raikes, Civil Engineers, and is manufactured by Messrs.

Adams Hydraulics, Ltd. As will be seen from the illustration, Fig.

30, it consists of a cast-iron stand-pipe, in sections, each of which

makes a tight joint with the one below it. A spindle, working in a

screwed nut in a bracket or pillar at the top, passes through crossbar

guides inside the stand-pipe sections. This spindle has projections at

irregular intervals, arranged in such a manner that as the spindle is

screwed up it lifts the top section first, then the second, and lastly

the third, and thus makes it possible to draw off the supernatant

water in three layers, each of which may if desired be discharged in

different directions. Finally, the sludge may be drawn off through the

same outlet to the sludge-disposal area.

[Illustration: FIG. 31.]

As the distance which the sewage travels in “continuous flow settlement

tanks with chemicals” is frequently an important factor in securing the

maximum efficiency, it may be found economical to arrange the tanks in

the form shown in Fig. 31, where each tank has a division wall, carried

through from the inlet end to within a few feet of the opposite end,

so that the sewage travels a distance equal to twice the length of the

tank before passing to the outlet. This arrangement requires only one

carrier, but this must be provided with suitable sluice-gates opposite

to each tank, in addition to similar gates on the inlet and outlet from

each tank.

The Dortmund type of tank, described under the heading of detritus

tanks, may also be adapted for sedimentation tanks, but the outlet

should be arranged in such a manner as to reduce the velocity of the

flow at this point to the minimum. This is usually secured in by

causing the liquid to flow over a weir formed by the circular wall of

the tank, or by a number of weirs consisting of cast-iron channels laid

transversely across the top of the tank. In either case it becomes

necessary to form a circular effluent channel round the top of the

tank, to receive the effluent after it has passed over the weirs.

These two arrangements are illustrated in Figs. 32 and 33, the former

showing the circular weir wall, and the latter the transverse cast-iron

channels. Both edges of each of these channels act as weirs, so that

the total effective length of weir is thus greatly increased. The

inlets, conical bottoms, and sludge outlets for these two tanks, would

be similar to those shown in connection with this form of detritus tank

(Fig. 14). Mr. S. R. Lowcock, M.Inst. C.E., has stated that in his

experience an excellent effluent can be obtained by drawing off the

liquid at one point, and at about two feet below the top water level. A

method of accomplishing this is shown in dotted lines on Fig. 32.

[Illustration: FIG. 32.]

_Special Types of Tanks._—One of the troubles which frequently arises

in the operation of all types of natural sedimentation or septic tanks

is a nuisance from smell, due to offensive gases given off by the

effluent. These are the result of the decomposition under anaerobic

conditions of the organic matter deposited in the tanks. It is possible

to arrange them in such a way, that the conditions which cause the

trouble may to a great extent be avoided even in the ordinary types of

tanks.

[Illustration: FIG. 33.]

_Hydrolytic Tank._—There are, however, several types specially

designed to eliminate these troubles altogether, by separating the flow

of sewage through the tank from that section in which the composition

of the sludge takes place. Among these is the hydrolytic tank. This

tank is already well known to most engineers, in the original form

designed by Dr. W. Owen Travis, and adopted at Hampton-on-Thames, but

a new and improved method of construction has recently been brought

out. The principle of this tank may be described as the deposition and

collection of the impurities in sewage by a process of physical

de-solution, the matters being separated in the order of their

grossness and specific gravity, namely (_a_) the removal of the grosser

solids by means of screens; (_b_) the settling of the heavy inorganic

solids in a detritus chamber; and (_c_) the separation of the lighter

solids in suspension and in a colloidal state. Finally, means are

provided whereby the deposit in the various chambers may be collected

and removed with facility. It is impossible in the space available to

describe in full the reasons for the various details of construction

which have been adopted, but the accompanying illustrations, Figs. 34

to 42, which have been kindly furnished by Messrs. Shone and Ault,

Civil Engineers, illustrate an example of the latest type of the tank.

Fig. 34 is a plan section of the tank, and Figs. 35 to 42 are vertical

sections on the lines indicated. The tank is by preference circular,

as shown. The sewage is delivered from the pipe S through a screening

chamber, in which the gross matters, such as rags and vegetable debris

are retained on the screen A, and are from time to time removed by hand

or mechanically. The sewage passes over the weir _a_ into the first

section B, which occupies about one-eighth of the circumference and is

divided into two parts by the diaphragm _b_, Fig. 35. The flow of the

sewage through this first section B may, by the weirs _b_^1 and _b_^2,

be so appointed that two-thirds of it flows from the outer compartment

over _b_^1 and one-third over _b_^2 from the inner compartment, the

only entrance to which is by the opening _b_^3, Fig. 35, in the bottom

of the diaphragm _b_; so that the deposition of the solids by gravity

is accelerated by the flow of the one-third of the sewage into the

inner part of the compartment B. The solids collect in the conical

bottom part _b_^4, Fig. 35. The overflows from the weirs _b_^1 and

_b_^2, are, by the channels _b_^5 and _b_^6, directed to the downtake

_c_, Figs. 34, 35, 38 and 39, which delivers the sewage near the bottom

of the outer compartment C, which latter, with the inner compartment

D, forms the second section of the tank. These two compartments are

divided by the diaphragm _c_^1, Fig. 39, having openings _c_^2 in the

lower edge. In the drawing the second section of the tank is shown

divided in two parts by the wall and weirs _c_^3, Figs. 34 and 40,

and they occupy together about seven-eighths of the circumference of

the tanks. The weirs _c_^3 are so proportioned that 85 per cent. of

the liquid passes directly through the outer compartment, and 15 per

cent. indirectly through the inner compartment of the first portion of

the second section of the tank, into the respective compartments of

the second portion of that section. It should be noted that the only

passages for the flow of liquids into the inner compartment are the

openings _c_^2, Figs. 34 and 39; and consequently the deposition of

solids is accelerated by this flow, so that they collect in the lower

part, _c_^4, Fig. 35, of the inner compartment B. The flow through the

second portion of this second section of the tank is governed by the

weirs _e_^5 and _e_, Figs. 34 and 36, which weirs are shown of such

proportion as to cause 70 per cent. of the liquid to flow directly

through the outer compartment, and 30 per cent. indirectly through

the inner compartment. The colloiders _c_^6, Figs. 34 and 35, are

fixed vertically in the outer compartments to attract and absorb the

solids in pseudo solution. It will thus be clear that 70 per cent. of

the sewage flows in a direct manner through the outer compartment,

and in doing so deposits practically the whole of its permanent and

a considerable portion of its convertible solids. The effluent from

the inner compartment D of the second section of the tank is, by the

submerged channel _e_^3, Fig. 36, passed into the supplementary section

E, which is fitted with colloiders, _e_^1, Figs. 34, 35, and 41. This

effluent, which has become fouled by the disturbance caused by the

evolution of gases in the inner compartment of the second section, is

thus submitted to a further de-solution action by absorption and other

processes. Finally the outflow from the outer compartment C, of the

second section over the weir _e_^5, Figs. 34 and 36, and the outflow

from the supplementary section E, over the weir _e_, are passed away

from the tank by a common channel, _e_^4, Figs. 34 and 37, whence

the effluent may, for further treatment, be led to filters or on the

land. The overflows from the two weirs may, however, be led away from

the tank by independent channels for separate treatment. The solids,

collected in the form of sludge in the lower parts of the sections,

can be drawn off periodically through the pipes _c_^7, Figs. 34, 35,

41, and 42, governed by valves into the central chamber F, Figs. 34

and 35, from which it may be led by the pipe _f_ to adjoining land,

or elsewhere for further treatment. The lighter solids, that collect

in the form of scum on the surface of the liquid in the tank, may be

skimmed off or drawn into the channels _g_, Figs. 34, 38, and 40, and

conducted to the central chamber F, and disposed of similarly to the

sludge. The tank is, or may be, constructed of concrete, which may be

reinforced as required according as it is wholly or partly above the

ground. Its shape may be greatly varied according to local requirements

and other considerations.

[Illustration: FIG. 34.]

[Illustration: FIG. 35.]

[Illustration: FIG. 36.]

[Illustration: FIG. 37.]

[Illustration: FIG. 38.]

[Illustration: FIG. 39.]

[Illustration: FIG. 40.]

[Illustration: FIG. 41.]

[Illustration: FIG. 42. HYDROLYTIC TANK.]

_Imhof Tank._—A somewhat similar tank has been introduced in Germany,

and is known as the Imhof tank; but in this case the whole of the

sewage is passed through direct to the outlet, and none is allowed to

flow through the portion in which the decomposition of the sludge takes

place. These tanks are known in Germany as “Emscherbrunnen,” from the

district in which they were first introduced. The present type has been

designed by Dr. Imhof, the engineer to the Emschergenossenschaft at

Essen, in Germany, and is shown, Fig. 43. It should be noted that the

arrangements may be varied in special cases. Where the daily flow is

considerable, at least two such tanks are recommended, and the inlets

and outlets are so arranged that the direction of the flow may be

reversed at regular intervals, in order that both tanks may receive

an equal proportion of the solid matters. The method of removal of

the sludge is usually arranged on the same lines as that previously

described in connection with the Dortmund type of detritus tank. It

is, however, evident that difficulties are occasionally experienced in

drawing off the sludge when it has been allowed to remain in the tanks

for long periods untouched, as it is suggested that a connection from

the water supply service may be carried down to the bottom of these

tanks, to permit of a jet of water under pressure being directed upon

the sludge in order to stir it up and thus facilitate its withdrawal.

[Illustration: FIG. 43.—IMHOF TANK.]

_Skegness Tank._—With the same end in view—the separation of

the process of sludge liquefaction from the bulk of the sewage

flow—Messrs. Elliott and Brown, Civil Engineers, devised an ingenious

arrangement of tanks for the scheme of sewage disposal which they

carried out at Skegness. In this installation the sewage first enters

a settling tank on the Dortmund principle, from which it overflows at

the top into a dosing tank which gives intermittent discharges to the

filters. The usual sludge delivering pipe from the settling tank is

connected into the _bottom_ of a separate sludge liquefying tank, the

floor of which is some four or five feet below the top water-level

of the settling tank. The upper part of the sludge liquefying tank

is also connected to the dosing tank in such a way, that when the

latter discharges it draws off several inches depth of the supernatant

water from the top of the sludge liquefying tank at each discharge.

The result of this operation is, that each time the dosing tank is

discharged an artificial difference in level is created between the top

water levels in the settling tank and the sludge liquefying tank—the

latter being the lower of the two—and as they are in direct

communication through the sludge pipe, the extra head in the settling

tank causes a movement to take place through the sludge pipe, and thus

forces some sludge up into the sludge liquefying tank, where it remains

for any desired period for liquefaction without unduly fouling the tank

liquor delivered to the filters.

[Illustration:

=_Candy-Whittaker Patent Bacterial Sewage Purification Tank_=

_Sectional Elevation_

FIG. 44.]

_Candy-Whittaker Bacterial Tank._—Somewhat similar in form to some of

the previously described tanks, the Candy-Whittaker bacterial tank is

circular in plan and provided with a deep inner cone, which divides it

into two compartments as shown, Fig. 44. The sewage enters the outer

compartment through a pipe, by means of which it is evenly distributed.

The outlet is through submerged effluent troughs situated inside the

cone, so that the sewage must flow down to within a short distance

of the bottom of the tank in order to pass under the bottom of the

cone and reach the outlet troughs. In consequence of this method of

construction, the bulk of the solids in suspension are deposited in

a circular =V=-shaped gutter or sump, from which the sludge is

removed by the pressure due to the head of water forcing it up a sludge

pipe similar to that previously described in connection with the

Dortmund type of tank. In the Candy tank, however, the inlet end of the

sludge pipe has a returned end with a swivel joint, which is rotated by

means of a vertical spindle operated by a crank handle at the side of

the tank, working through suitable gearing. It is claimed that any scum

which may be formed on the surface by floating solids, or by sludge

freed from the bottom of the tank by gases produced by fermentation, is

retained in the outer compartment, and thus prevented from passing away

at the outlet with the clarified sewage.

_Non-septic Cylinder._—The troubles due to foul-smelling gases arising

from the over-septicisation of sewage in tanks, are very liable to

occur in small installations for country houses, where the daily volume

varies periodically, and may drop to a mere dribble when the family

is away and only one or two servants are left in the house. To meet

the requirements of these cases, an arrangement has been designed

by Messrs. Adamsez, Ltd., which consists of a deep glazed fir-eclay

cylinder, provided with special inlet and outlet pipes. In consequence

of the small diameter of the cylinder, the sewage passes direct through

to the outlet in a very short space of time, but leaves the solids in

suspension in the cylinder, where they undergo decomposition without

affecting to any great extent the character of the fresh sewage on its

way to the filter. This tank is shown in connection with a small filter

and special distributing apparatus in Fig. 45, and is known as the

“Non-septic” cylinder. The sewage, as it leaves this cylinder, is well

suited for further oxidation in properly constructed filters, or on

suitable land without any possibility of causing a nuisance from smell.

[Illustration: FIG. 45.—“NON-SEPTIC” CYLINDER WITH SMALL FILTER.]

“_Kessel._”—In addition to those already described, other ingenious

devices have been designed with the same end in view, viz. the

prevention of nuisance from smell. Two of these, introduced by the

Septic Tank Co., are based upon the theory that it is desirable to

separate the solids in sewage from the liquid at the earliest possible

moment after they enter the sewer. These are illustrated in Figs. 46

and 47. The former shows what is known as the “Kessel,” its name in

Germany, where it was first used. Briefly described, it consists of a

vacuum chamber, in which the sewage rises, by reason of the pressure

of the atmosphere, to a height of about 25 feet, and then flows down

again through a vertical tube, emerging from the apparatus at a level a

few inches below the level of the invert of the incoming sewer. It is

claimed that the deposition of the solids in suspension, due to their

specific gravity being slightly greater than that of the liquid

sewage, is greatly assisted by taking place in vacuum, and that a

high percentage of the suspended solids is removed. The bottom of the

“Kessel” is in the form of an inverted cone, to the apex of which

a sludge pipe is connected, with its outlet end delivering into a

separate sludge well. The deposit which takes place in the “Kessel”

is drawn off at frequent intervals, before it has had time to become

foul, and the capacity of the “Kessel” is so small by comparison with

the daily flow of sewage, that the latter passes out very slightly

altered in character from the state in which it entered. The apparatus

is provided with various arrangements, for ensuring its continuity of

action, for producing the necessary vacuum, and for facilitating the

removal of the sludge. Other advantages claimed for the system are

that it is constructed _above_ the level of the sewer, so that costly

construction below ground is avoided, and that only a few inches of

fall are lost between the inlet and the outlet.

[Illustration: FIG. 46.—“KESSEL” TANK.]

“_Separator._”—The second apparatus shown in Fig. 47 is of an entirely

different character, and is aptly designated by the term “Separator.”

It consists of a number of comparatively shallow settling tanks, each

provided at the top with a metal grating, the separate bars of which

are in the form of narrow channels, with open ends discharging into a

common effluent carrier. The edges of these channels are accurately

planed to form weirs, over which the liquid portion of the sewage flows

in an extremely thin film. These channels are provided with adjusting

set-screws, so that they may all be set at exactly the same level, and

thus ensure a uniform depth of flow over the edges of the whole of the

channels in each tank. The combined length of the channels in each tank

form a weir of comparatively enormous width, so that the velocity with

which the sewage approaches the edges of the channels is extremely low,

with the result that a high percentage of the matters in suspension

are arrested in the tank and are slowly deposited to form sludge. The

bottom of each separate compartment of these tanks is in the form of a

sump provided with a sludge valve connected to a common sludge delivery

pipe, leading to the sludge disposal area by gravity if the latter is

at a lower level or to a sludge well if the tanks are below ground. In

order to prevent the decomposition of the sludge from proceeding so far

as to cause a nuisance from smell, the deposit in the tanks is drawn

off at frequent intervals.

[Illustration: FIG. 47.—“SEPARATOR” TANK.]

[Illustration: FIG. 48.—THE “FIELDHOUSE” TANK.]

_The “Fieldhouse” Tank._—This is illustrated in Fig. 48 (from a

drawing supplied by the patentee, Mr. J. Fieldhouse) from which it

will be seen that the sewage enters the central chamber A^1 by the

inlet pipe M, the end of which is turned down to deliver the sewage

immediately over the inverted cone C. Between the inverted cone C and

the side of this chamber an annular space E is provided, so that the

solids which are deposited may find their way into the cone-shaped

sludge chamber below, from which they are drawn off by means of valve

D and sludge pipe F. The liquid passes from the central chamber A^1

through the walls on all sides into the outer tank B^1, by way of the

oblique passages H, by which the liquid is deflected in a downward

direction, and eventually flows over the outer circular weir K into

the effluent channel L. The outer tank B^1 is divided into sections,

each of which is provided with a sludge sump and sludge valve N.

Scum-boards are provided both radially T, and in front of the weir J,

and the latter may be lowered when it is desired to draw off the scum.

This operation is performed by closing slides S_{1}, so as to cause the

sewage to head up in the tank, and the scum of any section may then

be drawn off by lowering the particular end board J next to the weir

K, and allowing the scum to overflow into the effluent channel L and

thence to the sludge bed. The special features of this tank are:—(_a_)

the cone-shaped bottom of each section, to facilitate the withdrawal of

the sludge without discharging the liquid contents; (_b_) the oblique

passages H in the wall between the inner and outer tanks, for the

purpose of deflecting the flow of the sewage in a downward direction,

and thus assisting the deposition of the matters in suspension; (_c_)

the removable scum boards in the outer tank, to allow of the removal of

the scum; (_d_) the general design by which the sewage enters at the

centre, and thence spreads in all directions until it flows in a thin

film over a weir of comparatively enormous length, thereby causing a

gradually increasing reduction in the velocity of the flow, and thus

providing every facility for the deposition of a very large percentage

of the matters in suspension.

[Illustration: FIG. 49.—SLATE BEDS IN COURSE OF CONSTRUCTION.]

_Slate Beds._—From the foregoing it will be gathered that there is

a growing tendency to reduce the process of putrefaction in tanks

under anaerobic conditions to the minimum, consistent with the removal

of solids. If this theory is carried to its logical conclusion, it

would appear to point to the elimination of all anaerobic conditions.

That this is not generally done is probably due to the fact that a

preliminary process of putrefaction to some extent, is, by many,

considered essential in the removal of solids in sewage. On the other

hand, there are some who are not of this opinion. Mr. W. J. Dibdin has

always contended that putrefaction is not necessary, and his system of

slate beds is designed as a preliminary process in which the conditions

are purely aerobic. Fig. 49 shows details of this system, from which it

will be seen that it consists essentially of a watertight tank filled

with superimposed layers of plates, usually about 2 inches apart. In

order to prevent any misunderstandings, it should be noted that the

description “slate beds” has arisen through the adoption of thin slate

slabs, with distance pieces of slate blocks, as the most economical

method of construction. No special value is ascribed to the slate

itself, beyond its cheapness in the particular form required and its

durability, it being practically everlasting. The essence of the system

is the use of horizontal plates to receive and retain the deposit of

solid matters in suspension in the sewage, so that they are decomposed

or digested, after the settled liquid has been drawn off, by aerobic

bacteria and other higher forms of life, including worms, all of which

thrive only in the presence of air. The beds are filled with the raw

sewage, which is then allowed to remain for a period of about two hours

for quiescent settlement, after which the liquid is slowly drawn off.

It is true that during the period of standing full the solids in the

sewage are not actually in the presence of air, but it is claimed that

a certain amount of air is retained on the under side of the plates,

and the oxygen thus available, in addition to the oxygen present in the

raw sewage, is sufficient to prevent the setting up of putrefaction

during the comparatively short period of standing full. As the liquid

is drawn off, air enters freely between all the layers, so that the

deposited solids are then immediately brought into close contact with

air, from which the aerobic bacteria and other organisms can draw the

oxygen they need for their life functions. The result is that the

ultimate residue of solids is of quite a different character from

sludge of the ordinary type. It is of a granular nature, which rapidly

dries on a properly constructed draining bed, and, when dry, resembles

ordinary peaty mould. Independent information as to the actual amount

of ultimate solid residue resulting from this system is not yet

available, but it is generally admitted that, when properly operated,

putrefaction does not occur at any stage of the process, and that there

is an entire absence of nuisance from smell throughout the works. When

new, these slate beds have a liquid capacity of over 80 per cent. of

the gross capacity of the beds, but it is usual, in calculating the

size of the beds required for a particular volume of sewage, to allow

for a normal working capacity of 66 per cent. of the gross capacity,

and to provide for one filling per day in dry weather. These beds are

generally constructed with a working depth of 3—4 feet, but they may be

as little as 1 foot in depth where it is necessary to reduce the total

fall required for the works to the minimum. The residue of the solids

after treatment in these beds passes out in the effluent, and it is

understood that it has not been found necessary to wash out the beds or

remove the deposit on the slates themselves, even after several years

of operation with strong sewage. In designing beds for this system, the

chief points to be borne in mind are that the constructional work shall

be absolutely watertight, and that the fall on the floor shall be

sufficient to allow the solid residue to pass freely to the outlet with

the effluent. The beds may be operated by hand by means of penstocks

on the inlets and outlets, or automatically by means of special

apparatus of the type which will be described later in connection with

contact-beds. It is, however, important that the liquid shall not be

discharged from the beds at too rapid a rate.

_SLUDGE DISPOSAL._

_Sludge Removal._—In connection with the discharge of sludge from

tanks of any kind, there are several appliances adapted to meet the

requirements of particular cases. Where the sludge-disposal area is

at a lower level than the bottom of the tank, a simple sludge-plug or

penstock on the inlet to the sludge-pipe may be used, or a sluice valve

may be inserted on the sludge-pipe after it leaves the tanks. Where the

sludge-disposal area is 2 feet or more below the level of the surface

of the sewage in the tank, and the floor of the latter is provided

with a suitable sump in which the sludge may accumulate, the method of

withdrawing the sludge by utilising the pressure of the head of liquid

in the tank, as described in connection with the Dortmund type of

detritus tank, may be adopted with advantage.

In cases where it is necessary to raise the sludge to the disposal

area, a hand-operated chain-pump may be used for small schemes, or for

large volumes, and where power is available, sludge elevators of the

bucket type, as shown on pages 40 to 42, and manufactured by Messrs.

S. S. Stott and Co., Messrs. Ham, Baker and Co., Ltd., and Messrs.

Adams Hydraulics, Ltd., will be found convenient. These appliances

are usually erected in special sludge wells, to which the sludge is

delivered by gravity. In the case of long tanks, in which the floors

are comparatively flat, and especially where the sludge is allowed

to accumulate until it has become consolidated to a great extent,

difficulties are experienced in causing the sludge to flow to the

outlet by gravity. This usually involves the employment of men to

descend into the tank and force the sludge towards the outlet by means

of squeegees, a slow and laborious process.

[Illustration: FIG. 50.—CHEMICAL MIXER.]

_Chemical Mixers._—The methods adopted for adding the necessary

chemicals to sewage for chemical precipitation are various. Where

alumina-ferric is used, the simplest method is to place blocks of the

precipitant in wire cages placed in the inlet channel so that the flow

of the sewage itself dissolves the block as required. It has been found

that this method is not economical in some cases, and the precipitant

is dissolved beforehand in a suitable mixer in order that it may be

added to the sewage in the form of a solution. This applies specially

to the lime process, and several forms of these mixing machines are

shown in Figs. 50, 51 and 52, made by Messrs. Goddard, Massey and

Warner, Messrs. Manlove, Alliott and Co., Ltd., and Messrs. S. H.

Johnson and Co., Ltd. These may be driven by power or by the flow

of the sewage itself, but the most important point which requires

attention is that the strength of the solution shall vary with the

strength of the sewage, either by varying the rate of flow of a

solution of uniform strength, or by varying the strength of a solution

flowing at a uniform rate.

[Illustration: FIG. 51.—CHEMICAL MIXER.]

_Sewage Mixers._—Even after the chemical solution has been added to

the sewage, it is necessary to make sure that it is thoroughly mixed

with the sewage. The simplest method of doing this is by means of

baffle-plates fixed in the channel leading to the tanks. Other methods

are by paddle-wheels driven by the sewage itself; by allowing the

sewage to drop in a chamber on to a projecting pier or stone; by using

power to drive (_a_) a plunger moving up and down in a sump, (_b_) a

vertical shaft to which horizontal paddles are attached to rotate in

the sewage channel, (_c_) to operate a device similar to the well-known

mechanical egg-whisk, (_d_) or to force compressed air through a

perforated pipe laid in the sewage channel. Indeed, there is no end to

the various mechanical devices which are used for this purpose.

[Illustration: FIG. 52.—PNEUMATIC CHEMICAL MIXER.]

[Illustration: FIG. 53.—SLUDGE PRESS.]

[Illustration: FIG. 54.—SLUDGE PRESS.]

_Sludge Presses._—When it is desired to reduce the liquid content of

the sludge as far as possible, the general practice is to make use of

sludge presses for this purpose. Several types are illustrated in Figs.

53, 54 and 55, manufactured by Messrs. Manlove, Alliott and Co., Ltd.,

Messrs. Goddard, Massey and Warner, and Messrs. S. H. Johnson and Co.,

Ltd. All are based upon the principle of compressing the liquid sludge

under high-pressure between iron plates which support cloth or other

filtering material, through which the liquid passes into grooves on

the faces of the plates, and thence by way of conduits in the plates

themselves to the floor below. The several makes have different methods

of opening and closing the plates, and the presses are made of various

sizes for operation by hand or by power. Fig. 56 shows a complete

sludge-pressing plant as designed by Messrs. S. H. Johnson and Co.,

Ltd. The description of the details of this plant is as follows.

[Illustration: FIG. 55.—SLUDGE PRESS.]

The sewage enters the works by the channel A, and passes first through

the bar screen B. The screening is necessary to remove anything that

would tend to produce obstruction in the inlets to the press chambers

and be liable to cause breakage of the press plates. The sewage next

meets with the milk of lime from the lime mixer C, with which it is

mixed by flowing along the gravitation mixer D. The pneumatic lime

mixer produces lime milk of a constant strength, and the flow is

adjusted in proportion to the requirements of the sewage. Should it be

necessary to add sulphate of alumina to the sewage, this is produced

by the pneumatic alumina mixer E, and is added to the sewage after

the latter has been thoroughly mixed with lime. Air for working the

pneumatic lime and alumina mixers is provided by the blowing engine

R. The treated sewage then passes further along the zigzag channel

into the precipitating tanks F, the ends of two of which are shown

in the drawing. It is advisable to have two or more tanks, so as to

allow sufficient time for precipitation. The usual capacity of the

precipitation tanks is equal to 6 hours’ flow of the sewage, and they

may continue running, overflowing continuously, for a considerable

time, but not so long as will produce putrefactive decomposition and

thereby cause a nuisance. The precipitation tanks, which are cleared

out alternately, are provided with hinged flap valves G and underground

stoneware pipes to convey the sludge into the liming sump H, the top

water being first decanted off. In the liming sump the sludge is limed

with milk of lime from the lime mixer I, which is also worked by the

blowing engine R above referred to. From the liming sump the sludge

passes into the sludge tank J, by means of the pair of automatic

rams K. The automatic rams work alternately, one filling by means of

vacuum, whilst the other is being discharged by means of compressed

air. As soon as the one is emptied and the other filled, the action is

reversed, and so on, each filling and emptying alternately, thereby

keeping up a continuous discharge. By being drawn into the rams, and

thence forced into the sludge tank, the sludge becomes thoroughly mixed

with the lime. This liming of sludge causes a considerable further

deposition and concentration of the sludge, and after standing all

night the supernatant water is decanted off by the skimmer L. The

sludge, now ready for pressing, is allowed to run by gravitation into

the automatic rams K previously referred to, and thence discharged into

the sludge presses M by means of compressed air, the compressed air

being supplied by the air-compressor N, which also acts as a vacuum

pump for drawing the sludge from the sludge pump into the automatic

rams. The solid portion of the sludge is retained in the chambers of

the sludge presses by the filter cloths, the effluent being discharged

into the trough at the side of each press, and thence by down pipes

and gullies into the effluent channel O, being treated again in the

gravitation mixer, and finally flowing away with the effluent from the

precipitating tanks. The press chambers are known to be filled with

solid sludge cakes, when effluent ceases to flow from the outlets of

the chambers. The presses are then opened and the cakes discharged into

a tipping truck Q, by which they are removed to the final disposal site.

[Illustration: FIG. 56.—COMPLETE SLUDGE-PRESSING PLANT.]

[Illustration: FIG. 57.—SLUDGE-DRYING APPARATUS.]

[Illustration: FIG. 57.—SLUDGE-DRYING APPARATUS.]

_Hydro-extractor for Sludge._—An entirely different method has been

adopted in the special apparatus in use at Hanover and other towns in

Germany, the Schaefer-ter-Mer centrifugal sludge de-hydrating apparatus

manufactured by the Hanoversche-Maschinenbau A.-G., vormals Georg

Egestorff, and illustrated in Fig. 57. In this apparatus the

centrifugal force resulting from the rapid rotation of the drum into

which the liquid sludge is fed, is utilised to throw out the solid

matters from the centre towards the circumference, where they are

caught in the outer part of the drum of the machine. The drum revolves

continuously, but at regular intervals it is opened automatically in

sections for a brief period, so that the dry sludge is thrown outwards

against the fixed casing and thus becomes broken up and falls to the

bottom, and thence to an endless-band transporter by which it is

discharged outside the building. At the moment when the sections of

the outer casing of the drum of the machine are opened to allow the

dry sludge to be thrown out, the wet sludge is prevented from passing

into these sections by the automatic closing of the inner slide door,

which is opened as soon as the outer slide is closed. The water

extracted falls into an annular channel below, from which it flows,

by way of a pipe, back to the settling tanks to be treated again. The

result of a series of special tests of this apparatus showed that the

liquid contents of the sludge was reduced from 92 per cent. to 50 per

cent. The installation at Hanover has now been in operation since June

1908, dealing with a daily volume of 6·6 million gallons of sewage

from a population of 280,000. From particulars supplied by the town

authorities, it appears that the total cost of operating the complete

plant, including the settling tanks and the sludge treatment apparatus,

amounts to about 8_s._ per million gallons of sewage treated, or about

0·8_d._ per head of population per annum.

Messrs. Manlove, Alliott and Co., Ltd., have now entered into an

arrangement with the above-mentioned firm to take up the control of

the patents and the sole manufacture and sale of the Schaefer-ter-Mer

Sludge-Drying Apparatus in Great Britain and the Colonies.

[Illustration: FIG. 58.—SLUDGE-DRAINING BED.]

_Sludge Draining Beds._—Although the methods of disposal of sludge

must vary in different localities according to the means available

for the purpose, and most of them involve very little, if any,

constructional work, it may be desirable to describe the various

points which should be taken into consideration in the construction of

suitable draining beds, as these should be included in the original

design of any scheme in which they are to be used. Their chief function

is to provide means for removing the maximum amount of the liquid

contents of the sludge in the minimum of time, and it is obvious that

this desideratum can only be secured by spreading out the liquid

sludge in thin layers upon material through which the liquid may

readily pass without carrying with it any of the sludge. The first

of these requirements necessitates the provision of an ample area of

draining surface, and the second involves the use of a suitably graded

material provided with ample means of drainage. The beds themselves

may be simple excavations in the ground, as shown in Fig. 58, or may

be constructed of brickwork or concrete, but in either case it is

absolutely essential that the floor should be covered with tiles, or

other means of sub-drainage, leading to a free outlet, which should be

connected to the screen chamber, detritus tanks, pump well, or some

other point at the inlet to the works, so that it may be treated over

again with the crude sewage. Whatever material is used for filling

the bed, the lower portion which is placed on the floor and over the

drainage tiles should be of large size, 2 inches to 3 inches in

diameter. The next layer should be 1½ inches to ½ inch in diameter, and

the top layer 6 inches to 9 inches in depth, should be fine material ¼

inch to ⅛ inch in diameter. In the author’s opinion, coke-breeze will

probably be found to be the best material for the top layer, and it

would be a good precaution to provide beforehand a quantity of this

material in reserve to replace what is lost in removing the dried

sludge from the surface of the beds.

It will be found advisable in operating these beds to discharge the

sludge from the tanks in small quantities at frequent intervals, rather

than in large quantities at long intervals, and it is very important

that each layer of dried sludge should be removed before the next layer

is delivered to the bed. It cannot be too strongly urged that sludge

disposal needs as much care and attention as any other stage of the

process of sewage disposal, and if this is available, and ample area

of draining beds is provided, there should be no difficulty in solving

this usually troublesome problem.

_PERCOLATING FILTERS._

In approaching the subject of the design of filters for the purpose of

oxidising the organic matters—in solution and in suspension—contained

in the liquid which leaves the preliminary process in tanks, the first

consideration is the question of site. Where the slope of the ground

permits of the construction of the filters on or above the ground

level, much expense for excavation may be avoided, so long as the base

of the filter can be laid on solid ground. In cases where the site of

the works is comparatively flat, it is impossible to avoid excavation,

and other means must be adopted to keep the actual cost as low as

possible consistent with efficiency.

_General Design._—Taking the latter case first, it should be observed

that some engineers consider it desirable to construct retaining walls

under all circumstances, but the author does not agree with this idea.

In the first place the walls do not, of themselves, have any influence

on the efficiency of the filters in producing a satisfactory effluent,

and if a filter can be constructed without them there is no reason why

they should not be omitted. This applies especially where the filters

have to be constructed entirely below the surface of the ground. The

chief point to be considered is that the effluent shall have a free

outlet, with facilities for inspection. In the case of all filters, the

best method of securing a free outlet for the effluent is to provide

the floor with a suitable slope from the centre to all sides. When the

floor is at some depth below ground this requirement necessitates an

effluent channel on all sides of the filter. Two methods of carrying

this into effect are illustrated, Figs. 59 and 60. Fig. 59 shows one

retaining wall for the filter and an additional retaining wall for

the surrounding earth, carried up to the surface with the effluent

channel between the two walls. In Fig. 60 the arrangement is similar,

but the outer wall is a dwarf wall to form the effluent-channel, and

the surrounding earth is cut back to a slope of natural repose, the

earth bank usually being sown with grass or covered with turf. It is

essential that the outer wall shall be carried up above the toe of

the earth bank, in order to prevent soil being washed down into the

effluent-channel, but a surface-water drain should be laid to take any

water that may accumulate at the back of this wall.

These methods of construction would, however, only be adopted by those

who consider it essential to provide means for lateral aeration to

the filter by constructing the retaining wall for the filter itself

of pigeon-hole brickwork. In the author’s opinion, however, lateral

aeration on these lines is altogether ineffective. It can only affect

the filter material for a distance of a foot or two from the wall. It

is true that in some cases horizontal perforated ventilating pipes

have been provided, radiating from the centre of the filter to the

outer wall and terminating with open ends on the outside. The effect

of these is, however, dependent almost entirely upon the temperature

of the atmosphere and the direction of the wind, and even if they do

induce air currents into the body of the filter the air will pass along

the line of least resistance, and therefore find its way through those

interstices which are open and not through those which are choked in

any way and thus most in need of aeration. The author believes that the

aeration of a filter is most effectively secured by the action of the

sewage itself, as it falls from the surface to the floor drawing in the

air from the top, and that if this is not effective, no amount of

lateral openings will produce the desired result. If this contention is

correct, there is no need to incur the additional expense involved in

the construction of retaining walls, and the filters may be designed on

the lines indicated in Fig. 61, where the filtering material fills

the whole of the natural excavation, and no walls are required. It is

true that this method involves an increase in the amount of filtering

material beyond what is actually in use, and the cost of this must be

set against the cost of the wall in the alternative method shown in

Fig. 62, as the cost of the excavation in either case is about the

same. Much will depend upon the cost of the filtering material in

different localities.

[Illustration: FIG. 59.]

[Illustration: FIG. 60.]

[Illustration: FIG. 61.]

[Illustration: FIG. 62.]

In both these methods it will be noticed that there is no outer

effluent channel, and that the floor slopes from the circumference

to the centre, where an effluent receiving chamber is constructed,

from which an effluent discharge pipe leads to the next stage in the

process. This is not quite so satisfactory as the arrangement shown in

Figs. 59 and 60, but it is the most convenient under the circumstances.

When it is possible to place the filter floor on or within 2 feet of

the surface of the ground, the method illustrated in Fig. 60 is the

design most commonly adopted. Sometimes the arrangement of the floor

shown in Figs. 61 and 62 is preferred. There is, however, still another

type of floor which is applicable to this case. This is illustrated in

Fig. 63, from which it will be seen that the whole of the floor slopes

in one direction, and the effluent is thus discharged over one-half

of the circumference of the filter, with the result that the effluent

channel is only one-half the length of that required in the case of

Fig. 60.

_Floors of Filters._—In all cases the floors for percolating filters

should be of a substantial character. They are usually constructed

of cement concrete. The thickness of the concrete will depend upon

the nature of the subsoil, but in any case it should be increased at

the centre in order to provide a safe foundation for the revolving

sprinkler. The surface of the floor should be smooth, so as to

facilitate the flow of the effluent, and any suspended solids it may

contain, to the outlet. From the preceding illustrations it will be

noted that the slope of the floor may be in three directions: (_a_)

from the centre to the circumference, (_b_) from the circumference

to the centre, or (_c_) from one side of the filter to the other, a

uniform slope in one direction along the diameter of the filter. It

has been suggested by some that the floor should consist of a series

of alternate V-shaped ridges and furrows, with slopes at right angles

to the general slope of the floor, but this has the disadvantage that

it causes difficulties in arranging the floor tiles and placing the

filtering material in position, and further it increases the cost of

construction without providing any real compensating advantages.

[Illustration: FIG. 63.]

[Illustration: FIG. 64.—FLOOR TILE.]

[Illustration: FIG. 65.—“NEWHAM” FLOOR TILE.]

_Sub-drainage._—It has been recommended that the floors should be laid

with a gradient of about 1 in 100, but the author believes that it

would tend to assist the free discharge of the suspended solids in the

effluent if this gradient were increased to 1 in 75, or even 1 in 50,

and that if this were done, there would be no need to make any special

provision for access to the under drains for cleaning purposes. Another

important factor in securing a free discharge of the suspended solids,

is the use of proper floor tiles or sub-drains. The old idea of laying

rows of agricultural pipes with open joints, or even of perforated

pipes, on the floor has been proved to be useless. In some cases it is

considered sufficient to have rows of floor tiles, laid at any distance

apart up to 10 feet at the circumference. There is, however, very

little doubt that the only correct method is to support the whole of

the filtering material on a complete false floor, so that the suspended

solids which are carried down with the effluent, and thus reach the

bottom of the filter at all points of its area, may fall freely into

an open space, from which they will be carried away with the least

possible obstruction by the flow of the liquid. It is certain that

these suspended solids have a great tendency to adhere to any object

with which they may come in contact, and that this can only be avoided

by providing a free space immediately above the floor and over its

whole area. The nearest approach to this acme of perfection is a

complete false floor, with openings too small to allow the filtering

material to pass through, but large enough to give a free passage

to the suspended solids. One of the simplest methods is to form the

false floor of bricks, laid flat on rows of bricks on edge. There are,

however, several floor tiles on the market specially designed for this

work. A number of these are illustrated in the accompanying figures.

The Ames-Crosta tile, Fig. 64, has a simple flat top with corrugated

edges supported on four short legs. When placed close together, the

corrugations form apertures through which the liquid passes, and the

flat tops form a table which provides every facility for placing the

filtering material in position. A somewhat similar floor tile is

the “Newham” shown in Fig. 65. In this case, however, each tile has

only two legs, and thus there is less obstruction to the flow of the

effluent. The “Stiff” floor tile, Fig. 66, is of much the same type as

the two last described. Another material which has been adapted for

this purpose is a single layer of the “Dibdin” type of slate slabs

and blocks, as shown previously in connection with the preliminary

treatment of sewage in tanks, Fig. 49. This method also provides

a complete flat false floor, and has the additional advantage of

occupying the least possible space. The slabs themselves may be split

to any suitable thickness, and the supporting blocks may be cut to any

size, as little as 1 inch in thickness if desired. The slabs vary in

shape and size, but may be secured of larger area than tiles, so that

the necessary supporting blocks are less in number than the feet of the

tiles, and thus offer less obstruction to the effluent. The apertures

for the passage of the liquid are provided by the holes formed by the

irregular edges of the slabs, where they abut on one another.

[Illustration: FIG. 66.—“STIFF” FLOOR TILE.]

There are, however, floor tiles of other shapes, such as the Candy

tile, Fig. 67, for which it is claimed that the effective depth of

the filter may be calculated from the floor, and that the height

of the V-shaped opening under the tile provides greater drainage

and ventilation openings than other tiles. Other drainage tiles are

semicircular in section. The Naylor tile, Fig. 68, and the Albion

floor tile, Fig. 69, designed and used by Mr. E. E. Ryder, Surveyor

to the Bushey Urban District Council, both have openings at the floor

level. The “Mansfield” floor tile, Fig. 70, is the type adopted for the

extensive filters at the Birmingham Tame and Rea district sewage farm

by Mr. J. D. Watson, Engineer to the Drainage Board.

[Illustration: FIG. 67.—CANDY FLOOR TILE.]

[Illustration: FIG. 68.—NAYLOR FLOOR TILE.]

In some cases, instead of providing a free space above the floor,

drainage channels have been formed in the floor, with rebates in the

sides to receive flat perforated tiles, as Fig. 71. This method of

drainage certainly leaves the floor clear to receive the filtering

material, but it has the disadvantage that it does not provide a free

space under the whole of the material; and as the suspended solids in

the effluent must of necessity travel some distance before they reach

a channel, they are, to a great extent, arrested by the material which

comes immediately on the floor, however large the separate pieces may

be, and thus tend to choke the interstices.

[Illustration: FIG. 69.—FLOOR TILE.]

[Illustration: FIG. 70.—“MANSFIELD” FLOOR TILE.]

Whatever method of sub-drainage may be adopted, it is advisable to

continue the tiles, or channels, right through to the effluent channel

or chamber in straight lines, so that the effluent may have a clear

way throughout its course under the filter, until it reaches the open

effluent channel or pipe.

[Illustration: FIG. 71.]

_Walls of Filters._—The great diversity of methods adopted to retain

in position the material of filters constructed above the level of

the ground, must be extremely perplexing to anyone who investigates

the subject. They vary from walls of such thickness and substantial

material that they would be quite suitable for resisting the pressure

of a head of water equal to the depth of the filter, down to no walls

at all. In many cases walls 9 inches thick throughout and 6 feet in

height are found to be quite satisfactory, while in a large number

of cases the material itself, when it consists of clinker, is found

standing in an almost vertical position with perfect success. The

question naturally arises, if it is possible to construct filters

satisfactorily without any retaining walls, why incur unnecessary

expense in providing walls? The answer is that clinker is not

universally adopted as a filtering material, and that the strength

of circular walls is not sufficiently taken into consideration. The

thickness of the wall will naturally depend upon its height, but

assuming this to be 6 feet, and that the wall is in all cases carried

down to a solid foundation, the extreme thickness that would be needed

is probably 18 inches at the bottom and 9 inches at the top, as Fig.

72. As a general rule, however, it will probably be found quite

sufficient to construct the wall of brickwork in cement, with the lower

half 14 inches and the upper half 9 inches in thickness. In a large

number of the most recently constructed filters, the walls have been

omitted altogether when clinker has been used for the filter. Large

pieces of this material have been selected, and carefully packed in

the form of a dry rubble wall with perfect success, even when laid

with a batter on the outside of as little as 1 in 6 or even 1 in 8.

It is obvious that this method of construction needs special care and

supervision, especially in providing a rough kind of bond between the

different layers, if it is to be successful. A rough idea of this

method is given in Fig. 73. As a kind of happy medium between this

system and a wall to the full height of the filter, a dwarf wall has

been adopted in a number of cases, and notably by Messrs. Willcox and

Raikes, Civil Engineers, who have also designed a special form of

coping, made in fire-clay or terra-cotta, as shown in Fig. 74. Again,

in several places where suitable local stone, or stones and bricks

resulting from the demolition of old houses or walls, was available at

a cheap rate, these materials have been utilised to form dry rubble

walls, similar in construction to the clinker walls illustrated in Fig.

73.

[Illustration: FIG. 72.]

[Illustration: FIG. 73.]

Occasionally the walls of filters are carried up several feet above

the surface of the filter with the idea of forming a screen to protect

the revolving sprinklers from the influence of wind. It is, however,

obvious that these measures can only have an effect upon the outer ends

of the revolving arms, and, as the action of the revolving sprinklers

is only affected by wind when the head of sewage by which they are

driven is at its minimum and the force of the wind is simultaneously

at its maximum, the necessity for windscreens seldom arises. In actual

practice, with an average head of sewage, no real difficulty is caused

by wind.

[Illustration: FIG. 74.]

So far, it has been assumed that the walls and floors of the filters

can be constructed upon solid ground. Unfortunately this is not always

possible. If the subsoil is of an unsubstantial nature it is advisable

to lay the floor of a suitable thickness and diameter, so that the wall

may be built upon the floor itself, and thus distribute the load over

as large an area as possible. In some cases the levels necessitate the

raising of the filter floor _above_ the surface of the surrounding

ground. Under these circumstances the footings of the walls should

be carried down to solid ground, and no reliance whatever should be

placed upon made-up ground even to carry the floor alone. The outer

edge of the floor should be supported by the footings of the wall, and

the remainder carried upon piers or cross-walls of brickwork, concrete

or masonry, extended down to solid ground. The piers or cross-walls

should be sufficient in number to support the floor with safety. A

smaller number may be used if steel joists are provided between them

to take the weight of the floor and filter, or the whole floor may be

of a properly designed reinforced concrete construction, supported at

the circumference on the footings of the outer wall and at the centre

on a substantial pier of concrete carried down to solid ground, with

intermediate piers if the diameter of the filter is excessive.

_Planning of Filters._—The preceding notes with regard to floors,

sub-drainage and walls, apply generally to all filters, whether

they are for revolving or travelling distributors or for sprays or

fixed troughs. The most suitable plan for travelling distributors is

naturally rectangular, but where more than one such filter is required

it will be found economical to arrange them in pairs, with a central

supply channel feeding two distributors, one on each side. This applies

equally to other types of fixed distributors, but care should be taken

to divide the total area into reasonable units. In all except the

smallest schemes (which should consist of not less than two units) it

will be found that three, or multiples of three, units form a very

convenient method of arrangement. This suggestion with regard to

the subdivision of the total area applies to filters for revolving

distributors, so far as their number is concerned, but there are

several methods of arranging the area of the filters themselves. When

the filters are separated from one another they may be placed in

regular order of one kind or another if the site is uniformly flat, or

irregularly to suit the contour of the ground if the site is uneven.

It has been considered an objection to the use of circular filters

that even if they are placed close together a considerable amount of

space is rendered useless. There may be some justification for this

objection in cases where the area of the site is limited. On the other

hand, the objection has been overcome by arranging the total filter

area on one common floor, fixing the revolving distributors so that the

circumferences of the areas they cover meet where possible, and leaving

the spaces not covered by the distributors free of material to provide

some lateral aeration for whatever it is worth, or as an alternative to

provide a convenient position for chambers to receive the effluent from

the contiguous filters. This arrangement is illustrated in Fig. 75. If

other arrangements are made for the discharge of the effluent, then the

spaces in question may be filled with material and utilised as filters

by fixing smaller suitable distributors, as Fig. 77. This has been done

at Kingston-on-Thames in converting existing rectangular contact beds

into percolating filters, while at Darwen, Lancs, the intermediate

spaces not covered by the revolving distributors are utilised as

filters and the distribution effected by means of fixed sprays.

[Illustration: FIG. 75.]

[Illustration: FIG. 76.]

[Illustration: FIG. 77.]

In other cases the filters for revolving distributors have been

constructed octagonal in plan instead of circular, as shown in Fig.

76. By constructing suitable retaining walls the whole scheme presents

a very good appearance, and the intervening spaces can be utilised

for effluent chambers or as filters with smaller distributors, as

previously described, and practically the whole area is thus utilised.

_Filtering Material._—On this subject there exists a great diversity

of opinion. Some engineers are satisfied to use any kind of material

which will not disintegrate rapidly, while others pin their faith to

one particular kind. Again, the grading of the material is a matter

upon which it is seldom possible to find two engineers in complete

agreement. The opinion is frequently expressed that true economy

consists in utilising local material as far as possible, sometimes even

to the extent of adopting a local product, even though it is admittedly

not so good as some other material which may cost slightly more for

carriage from a distance.

Undoubtedly the first consideration is to secure a material which will

not disintegrate, but this is not the only essential qualification. The

author has had many opportunities of observing the results obtained

from various materials, and, for dealing with an average sewage, he

has never seen a better material than hard-burnt vitrified furnace

clinker. This material, of the proper kind, is practically equal to

stone or gravel in its ability to withstand the various influences

which tend to cause disintegration, but it possesses the advantage over

stone and gravel of having numerous cavities, which apparently form

the most suitable means of assisting the development of the bacterial

gelatinous growth, which appears to be the essential factor in causing

the deposition of the organic matters in suspension and in solution in

the tank effluent. It would seem as if the smooth surfaces of gravel or

broken stone cannot retain this growth, and that it is washed away as

soon as it begins to form. It is true that excellent effluents are

obtained from filters of gravel or stone, but, so far as the author

is aware, only by providing a larger cubic capacity of filter than

would be required if proper clinker were used. It is thus questionable

whether the lower cost of local stone or gravel does, in fact, result

in ultimate economy, if a smaller quantity of the right kind of

clinker, at a slightly higher price per cubic yard, will secure the

same result.

It will be noticed that stress is laid upon the necessity of using the

_right kind_ of clinker. This is intentional, as the word “clinker”

appears to cover a large variety of material. House refuse, cinders,

and over-burnt bricks, as well as the products of refuse destructors,

are all considered as “clinker,” especially in cases where a contractor

finds he has taken a very low price for filter material in making up

his tender. In the author’s opinion, the only kind of clinker, indeed

the only kind of material, which should be used for percolating filters

is the extremely hard clinker from boiler furnaces, more or less

vitrified throughout, and not only of irregular shape, with a rough

surface, but possessing numerous cavities on all sides. Clinker of

this type is occasionally to be obtained from destructor furnaces, but

it depends upon the character of the original refuse, and probably to

some extent upon the method of stoking. In any case it necessitates the

exercise of experienced judgment and discrimination in selection, and

in some cases destructor clinker is so soft, and so evidently certain

to undergo rapid disintegration, that it should be rejected at all

costs.

Among other materials which are used for percolating filters in various

parts of the country are coal, broken saggars, stone of various kinds,

including granite, gravel, broken bricks, coke, cinders, coke-breeze,

and slag from ironworks, but in the author’s opinion none of these are

so satisfactory as the right kind of clinker described above.

_Grading of Filtering Material._—There is probably as much, if not

more, diversity of opinion on this point as in the matter of the kind

of material most suitable for filters. The sizes in actual use vary

from ⅛ inch to 3 inches and even 6 inches. Some engineers stipulate for

a uniform grade throughout the filter, others prefer to have different

grades at different depths, while still others are satisfied to allow

small and large pieces to be mixed together indiscriminately. In the

author’s opinion the last mentioned method is the least satisfactory of

all. One of the essential factors in obtaining the maximum efficiency

from percolating filters is a free passage for air to enter into all

parts of the filter. This can only be secured if the interstices

between the pieces of material are kept clear at all times, but when

small and large pieces are mixed together, the small have a natural

tendency to fall into the spaces between the large pieces and thus

choke them. Even if this were not of importance, the usual methods

of filling a filter do not permit of the uniform distribution of the

finer particles among the larger, so that the usual result is that some

portions of the filter consist almost entirely of fine and others of

coarse material, and the results must of necessity be unequal.

On the face of it, the division of a filter into layers, each

consisting of a different grade of material ranging from coarse at the

bottom to fine at the top, would appear to be an excellent idea, but

it is only those who have attempted to put this idea into practice

who appreciate the extreme difficulty of carrying it out. With the

exception of very small filters, where constant supervision and an

unlimited amount of labour is available, it will be found impracticable

under ordinary circumstances.

There still remains the first-mentioned alternative, viz. to have

the material as nearly as possible of a uniform grade throughout the

filter. Whether the grade should be fine, medium or coarse, will depend

upon the strength and character of the liquid to be treated, but in

either case the best practice from all points of view is, in the

opinion of the author, to provide all of a uniform grade. In making

this statement, it is assumed that there will be a layer about 6 inches

deep of coarser material all over the floor, as this is necessary to

prevent the finer grade above it being washed through in the effluent

or choking the apertures in the sub-drains. In some cases also a

modification is adopted in providing the top layer, about 6 inches to

12 inches in depth, of finer material than the bulk of the filter, in

order to arrest any suspended solids that may be present in the tank

liquor, upon the surface of the filter, from which they can be more

readily removed than if they were allowed to enter the filter. On the

other hand, some prefer to allow these solids to enter the filter,

but to provide ample clear interstices and sub-drainage, so that

these solids after treatment in the filter may be washed out as humus

or “converted products” in the effluent, from which they are easily

removed by settlement in suitable tanks to be described later.

_Methods of Distribution._—The question of distribution is the most

important factor in the successful operation of percolating filters.

At first it was considered sufficient simply to spread the liquid as

evenly as possible over the surface of the filter by any convenient

method. Gradually improvements in methods were introduced with varying

results, and at the present time a large number of different appliances

are to be found in actual use, some producing jets, others a fine

spray, and others again what is termed a “thin film.” All of these have

their advocates, and all undoubtedly can be made to give satisfactory

results. One of the essential requirements of any method is that it

shall give even distribution per unit of superficial area of the

filter. Whether the jet or the spray or the thin film is the most

efficient in this respect is a matter of opinion.

On this question of distribution, one of the most important points

to be taken into consideration in forming an opinion as to the best

method to adopt is, what happens after the liquid is discharged on

to the surface of the filter? What happens beneath the surface, in

the body of the filter? The generally-accepted theory is that the

liquid trickles slowly over the surface of the separate pieces of the

material, dropping from one to the other and ultimately falling to

the floor of the filter, and thence flowing to the effluent drain. No

definite investigations into this question appear to have been made

until the year 1909, when a series of extremely interesting experiments

were carried out by Mr. W. Gavin Taylor, M. Am. Soc. C.E., Resident

Engineer, Sewage Disposal Works, Waterbury, Conn., U.S.A. The results

of these tests were published in the “Engineering Record” of June 5,

1909, and illustrated by excellent graphic tables, which show that,

“Percolating liquids were more equally diffused in the fine material

than in the coarse; that the plotted curves of diffusion approximate

parabolas having their apexes at the surface of the filter material;

and that, in general, about one-half of the total lateral movement

taking place within a 6 foot depth was effected within the uppermost

foot of material.” These observations apply to the effect produced by

single drops applied at the surface. Further tests made to ascertain

the effect of distribution on several adjacent points at the surface of

the filter demonstrated that, “The liquid from each point of

application spread out through the material in the normal cone of

diffusion, until its cone was intersected by that from an adjacent

point of application; that there the two or many liquid films united,

as tributary sources, into minute streams which interrupted, to a

considerable degree, the continuance of diffusion and tended to descend

through the remaining material at a higher velocity and along lines of

least resistance. The streaming tendency increased rapidly as the rates

of application were made greater.” Among the final conclusions drawn

from these tests the most important is that, “The desideratum in the

application of sewage to percolating filters is to attain perfection

in aerial distribution, and that a high efficiency in sub-surface

distribution is fostered by a slow continuous rate of application

rather than by an intermittent application at a higher rate.” This last

sentence fully confirms the author’s own opinion with regard to the use

of dosing tanks which are dealt with later under the heading “Methods

of Feeding Percolating Filters.”

In the following pages a number of the many appliances which have been

introduced for distributing tank effluents upon percolating filters are

illustrated and described in detail, the various types being grouped

under separate headings.

[Illustration:

_Sectional Elevation on Line A.B._

_Sectional Plan on Line C.D._