SEWAGE DISPOSAL WORKS
THEIR
DESIGN AND CONSTRUCTION

By W. C. EASDALE, M.S.E. M.R.San.I.

AUTHOR OF “THE PRACTICAL MANAGEMENT
OF SEWAGE DISPOSAL WORKS”

155 ILLUSTRATIONS

London:
E. & F. N. SPON, Limited, 57 HAYMARKET

New York:
SPON & CHAMBERLAIN, 123 LIBERTY STREET

1910

PREFACE

In the course of the preparation of a series of articles for “Surveying and the Civil Engineer,” dealing with the numerous and varied types of appliances used in connection with Sewage Disposal Works, it occurred to the Author that it might be useful to many Engineers, and especially to Students, to have the whole series published in a permanent form for reference. At the same time, it appeared to afford an excellent opportunity to include full details of all the various methods of design and construction in general use, and thus provide a complete work dealing with the whole subject. The result is the present volume, which, it is hoped, will prove of value to those engaged in this branch of engineering. In any future editions that may be required, it will be the endeavour of the Author to omit any details which may have become obsolete, and to include particulars of any new methods of construction, systems or appliances, which may be brought into use from time to time, and he will therefore be glad to receive particulars of new appliances and systems as they are introduced.

W. C. EASDALE.

28 Victoria Street,
Westminster, S.W.
1910.

CONTENTS

SEWAGE DISPOSAL WORKS

THEIR DESIGN AND CONSTRUCTION

INTRODUCTION.

In approaching a subject upon which so much has already been written, it may be desirable to point out that the improvements which have taken place in recent years in connection with sewage disposal are so extensive and varied, and have developed at such a comparatively rapid rate, that most treatises now in existence are in many respects more or less out of date. It is true that further developments may be anticipated in the future, but these will probably be concerned more with additions and improvements in matters of detail than of principles, which are now to a great extent agreed upon. The time would thus appear to have arrived when it is desirable to describe in detail the various methods of construction now generally adopted in the design, of present-day sewage disposal works.

In order to avoid a repetition of the usual preliminary details to be found in all the existing literature devoted to this subject, it is assumed that readers are acquainted with the nature of the problem to be solved in the design of sewage disposal works, with the varying characteristics of the different liquids included under the term “sewage,” and with the engineering formulæ and requirements involved in the design of tanks, filters, and similar constructional works. The present volume will thus consist exclusively of descriptions, illustrated with drawings and photographs, of the various tanks, chambers, filters, beds, and other details of sewage works, including the numerous types of appliances required in connection therewith.

In other works dealing with this subject it has been customary to use as illustrations, drawings of works actually carried out by their authors or other engineers. While these are interesting and valuable to a certain extent, their application under other conditions is limited, and their usefulness is thus much reduced. In the present volume the illustrations of the various details of construction do not, as a rule, represent actual working drawings adapted to any particular set of conditions, but are shown in diagrammatic form for the purpose of serving as suggestions to engineers in search of ideas which they can adapt to meet the requirements of any particular scheme upon which they may be engaged. It will follow that the engineer must in all cases rely upon his own practical experience and judgment in deciding to adopt any of the various methods of design and construction illustrated and described in the following pages; and it may be found that a combination of several types, or even a combination of several details of different types, combined with practical experience and mature judgment, will frequently produce the most suitable and efficient scheme.

SCREENS.

On arrival at the disposal works, the first stage of the process through which the sewage passes is generally that of screening for the purpose of arresting the grosser solids in suspension. In a number of cases where the sewage is delivered by gravitation, there are no screens of any kind in use, reliance being placed upon the detritus chambers to perform the duty of arresting the floating solids, as well as the grit and other mineral matters, of such a specific gravity that they are readily deposited by simply reducing the velocity of the flow. Where the levels involve the use of pumping plant, screens are a necessity, and, as the Royal Commission on Sewage Disposal have expressed the opinion that all sewage should be screened, it will apparently be necessary to provide screens in all future schemes.

Fig. 1.—Simple Screen.

Simple Screens.—The simplest type of screen is in the form of a grating, consisting of vertical iron bars in a stout iron frame, arranged to fit into grooves cut in the side walls of the screen-chamber, or in channel-iron guides attached to the sides of the chamber. As a general rule the vertical bars are round in section, but some engineers prefer to use flat bars with their longer side parallel to the line of flow, while others even go so far as to use wedge-shaped bars with the thick end facing the flow of sewage. In the latter case, the idea is to facilitate the passage through the screen of those matters which are too small to be arrested on the front of the bars, but large enough to be caught between the bars, and thus possibly choke the intervening spaces. As all simple fixed screens must of necessity be cleaned by hand, they are usually arranged at an angle of about 60 degrees to the floor of the chamber, in order that the matters arrested may be more easily drawn up by a hand rake to the top of the screen. [Fig. 1] shows a screen of this type in plan and section, with a large scale detail of the round, flat and wedge-shaped bars previously described. It will be noticed that a narrow platform of boards is shown across the chamber, at the top of the screen, to receive the screenings, which are then thrown into a barrow for removal to their final destination. One important point to be remembered in the design of the chamber for screens of this type is, that the bottom of the screen should be placed in a sump some 12 inches or so below the invert of the incoming sewer, so as to provide space for the accumulation of a certain amount of sludge and screenings without choking the screen. This sump should be provided with a washout valve. It is advisable to have all screen-chambers in duplicate, so that one of them may be in use while the other is being cleaned. The spaces between the bars vary in width with the character of the sewage, but the distance most generally adopted is half an inch. The important point to be considered is, that while the screen should arrest all the larger suspended matters it is intended to intercept, it should allow a free passage to all others without becoming rapidly choked. Another important factor in the efficiency of a fixed screen is its width. The greater the width, the less will be the liability to choke, and consequently it will not require raking so frequently to keep it in proper order.

Rotary Screens.—Where the flow of sewage is sufficient for the purpose, and it is desired to reduce the necessary attention to the minimum, the self-cleansing rotary screen, manufactured by Messrs. John Smith and Co., may be adopted. This is illustrated in [Fig. 2], from which it will be seen that it consists of a revolving wire screen, extended between two rollers, one below and the other above the sewage level. The upper roller is rotated by means of a water wheel driven by the sewage. A rotary brush is fitted to the shaft and driven in the opposite direction to the screen roller, so that it brushes off the screenings into a trough, from which they are removed by hand.

Fig. 2.—Rotary Screen.

Screens for Deep Sewers.—In cases where the depth of the sewer makes it inconvenient to adopt a fixed screen, the double lifting screen, manufactured by Messrs. Adams Hydraulics Ltd., may be used, as shown in [Fig. 3]. This consists of duplicate screens, arranged to slide up and down in cast-iron guides attached to the walls of the chamber. These screens are raised and lowered by a chain, which passes over a drum revolved by hand. The main screen is in the form of a basket, with a hinged front, which falls to the floor of the chamber when this screen is lowered into position. When it is desired to clean out this basket screen, the other plain guard screen is lowered into position in front of the basket-screen, and the latter is then raised. As the chain by which the basket-screen is raised is attached to the top of the hinged front, the action of raising this screen first draws up the hinged front and this prevents the screenings falling out. After this screen has been emptied, it is again lowered into position, and the guard-screen raised to permit the sewage to flow direct into the basket-screen.

Fig. 3.—Double Lifting Screen.

Fig. 4.—Mechanical Screen.

Fig. 5.—Mechanical Screen.

Fig. 6.—Mechanical Screen.

Mechanical Screens.—In larger schemes, where power is available for the purpose, mechanically operated screens are frequently adopted, as they are not only self-cleansing but the screenings are delivered automatically at or above the ground level, and thus very little labour is involved in removing these matters. [Figures 4], [5], [6], [7] illustrate four examples of this type of screen, manufactured respectively by Messrs. Ham, Baker and Co., Ltd., Messrs. J. Blakeborough and Son, Ltd., Messrs. S. S. Stott and Co., and Messrs. J. Wolstenholme and Co. The general features of these screens are an inclined screen or strainer, fixed in the channel or catchpit through which the sewage flows to the tanks or to the pumps, and a raking apparatus with special shaped prongs, which travel in the spaces between the bars forming the screen and remove the refuse. The Stott screen includes a rake cleaning gear, consisting of a revolving steel comb, by means of which the screenings are removed from the prongs of the rake while they are in motion.

Fig. 6a.—Rake Cleaning Gear for Fig. 6.

Fig. 7.—Mechanical Screen.

In the case of the screen, manufactured by Messrs. Whitehead and Poole, illustrated in [Fig. 8], the bars are of tapered steel, and are so arranged that they can be removed and replaced if necessary. The special friction drive with which this machine is fitted, prevents the breaking of the chain should the rake prongs become caught in the screen. The rake-cleaning gear consists of two swing levers, which carry a cleaning comb and a balance weight to hold it in position over the dirt tray. As the rakes bring up the screenings and reach the delivery position, they pass through the cleaning comb, which is, at the same time, forced down by a catch on the chain engaging with flanged rollers on the end of the swing levers. In this way the rakes are effectively cleaned, and it is impossible for the rake prongs and the comb to foul each other.

In addition to the screens already described, mention may be made of the special drum-shaped screen invented by Mr. Baldwin Latham and the numerous types of mechanically-operated screens in use in Germany, all more or less elaborate in character. Further details of these are probably unnecessary, as the aim of the engineer engaged in the design of sewage disposal works should be to adopt those appliances which are of the simplest possible form consistent with the requirements of the case with which he is called upon to deal. Some engineers prefer to use screens specially designed by themselves to meet the requirements of each particular scheme, and while this method provides scope for the exercise of a considerable amount of ingenuity, it is liable to involve greater expense than would be incurred by the adoption and possible adaptation of one of the various types already on the market.

Fig. 8.—Mechanical Screen with Rake Cleaning Gear.

STORM-WATER OVERFLOW WEIRS.

The proper design of weirs for diverting the excess volume of sewage in times of storm has not in the past always received sufficient consideration. Too frequently it has been dealt with by rule of thumb. In the first place the position for the weir has not always been well chosen; but, as a result of the recommendations of the Royal Commission on Sewage Disposal, it will be necessary in the future to construct these weirs, in all cases which require the approval of the Local Government Board, after the screen. This is a wise precaution, as it prevents the possibility of a storm-water overflow coming into action as a result of want of attention to the screen. In this position the factor which has the greatest influence upon the proper working of such weirs is the rate of flow into the detritus tanks, i.e. the area of the inlets to these tanks. It is true that these may be regulated by the use of valves, but unless these valves, when once adjusted to the correct height, can be permanently locked in that position, it leaves them at the mercy of an unscrupulous workman, who may, if he wishes, close them entirely, and thus cause the entire flow of sewage to pass over the storm overflow weir in order to save himself the trouble of attending to the tanks and filters. It is probably with the intention of preventing the possibility of such mismanagement that the Local Government Board object to valves on the inlets to the detritus tanks. One method of preventing trouble is to use simple hand-stops, and provide the frames in both inlets but only one door, so that it is impossible for the man to close both inlets at the same time. The Local Government Board are also usually averse to the use of any type of movable weir, and prefer the simple fixed weir.

Diverting Plate.—Many ingenious devices have been adopted in the past for the purpose of ensuring the diversion of all the excess volume above a certain fixed quantity. One of these is shown in [Fig. 9], where it is assumed that all in excess of the volume which is taken by the sewer flowing four-fifths full is to be discharged over the overflow. In order to facilitate this result, an iron plate is fixed at the level of the weir (say four-fifths of the diameter of the sewer), over the whole of the outlet end of the chamber or man-hole, with a sharp edge on the side facing the flow, so that when the sewage in the chamber rises above this level, the excess volume above that flowing at a depth of four-fifths of the diameter of the sewer, is automatically diverted by the plate and caused to pass away over the weir. The invert of the chamber must naturally correspond with the diameter of the sewer.

Fig. 9.

Fixed Weirs.—Even this ingenious method of diversion is, however, not accurate, as no provision is made to counteract the effect of the increased head on the outlet from the chamber, due to the backing up of the sewage in passing over the weir. Where a fixed weir is alone permissible, the only really satisfactory method of securing the desired result, is to increase the width of the overflow weir to such an extent that the maximum depth of storm-water, which may possibly flow over the weir, is reduced to the minimum, say one inch, and thus the effect of this head on the normal outlet from this chamber (i.e. on the inlet to the detritus or sedimentation tanks) is also reduced to the minimum. This will necessitate careful consideration, and a special set of calculations in each case. Where it is found that the execution of the above suggestion involves the construction of a weir of abnormal and unpractical width, it will be found convenient to arrange the normal dry-weather outlet from this chamber in the form of a narrow vertical slot, which can be most easily provided in a simple door or stop in a grooved frame, fixed in the outlet from this chamber. [Fig. 10] shows an example of this slotted door, and when the correct width of the slot has been ascertained by actual experiment, the door should be bolted to the frame, so that it cannot be removed or altered by any unauthorised person. From the drawing it will be seen that it is not difficult to calculate the dimensions of the slot orifice, so that with the head due to the height of the storm overflow weir it shall discharge the desired volume (say three times the dry-weather flow), and if the width of the overflow weir is then calculated to take the excess volume with a depth of one inch of water over the weir, this extra one inch of head will have very little effect on the discharge through the slot outlet.

Fig. 10.

Fig. 11.—Floating Weir.

Fig. 12.—Swinging Siphon.

Movable Weirs.—If, however, it is desired to provide for an absolutely correct diversion of the storm-water, this can only be done by the use of a movable weir. There are two types of this form of weir on the market at present, both manufactured by Messrs. Adams Hydraulics, Ltd. [Fig. 11] shows a floating weir, circular in form, arranged by means of floats to rise and fall freely with the level of the sewage in the chamber. The joint between the fixed and moving portions of the apparatus consists of an air-lock, and is thus frictionless. The floats are adjusted to bring the lip of the weir at such a depth below the top water level, that the volume which can pass over the weir without raising it is the maximum volume which it is desired to pass to the tanks and filters. As soon as the flow of sewage exceeds this volume, it naturally causes the floats to raise the lip of the weir, and in this way the volume passing to the tanks and filters can never exceed the predetermined fixed volume, and all in excess must pass over the overflow weir. [Fig. 12] shows a swinging syphon, which has the same effect as the floating weir. In this case the syphon has both legs trapped, so that it acts as a continuous syphon, and it is pivoted on the top of the division wall to swing freely. To the inlet leg, on the sewer side of the division wall, is attached an adjustable float, of sufficient buoyancy to raise this leg of the syphon (and with it the outlet leg as well) as the sewage rises in the chamber. It will be seen that the difference in level between the lip of the inlet leg and the buoyancy point of the float, represents the head which controls the maximum rate of flow through the syphon, and that immediately this is exceeded the float rises, and with it the syphon leg, so that all the excess volume of sewage, above the fixed maximum rate of flow through the syphon, must of necessity pass over the storm-water overflow weir.

DETRITUS TANKS OR GRIT CHAMBERS.

The function of these tanks is to arrest all mineral matter, such as stones, sand, road-grit, and similar substances which cannot be decomposed in the subsequent stages of treatment, and would thus choke the tanks and filters. The essential factor in their operation is a reduction of the rate of flow of the sewage, so that all matters of a greater specific gravity than the water and the organic matters in suspension may be deposited by subsidence. At the same time the velocity should not be reduced to such an extent as to allow the organic matters in suspension to settle out, as these can be more suitably dealt with in the subsequent tanks provided for that purpose. From this it will be seen that considerable care is needed in designing these tanks if they are to have the desired effect. Further, it is very essential that every facility shall be provided for removing the matters which are deposited with as little trouble as possible.

Capacity of Detritus Tanks.—Too frequently there is very little evidence of design in these tanks, especially in the provision of suitable sludge outlets. Before all, there should be at least two detritus tanks in every case, so that one may remain in work while the other is being cleaned out, and, if the recommendations of the Royal Commission on Sewage Disposal are followed, each should have a capacity of not less than one-hundredth of the daily dry-weather flow. A simple form of detritus tank is shown in [Fig. 13]. The essential features are, a floor with a sharp fall towards the inlet end of the tank and a sludge outlet at its lowest point. In this case a sludge plug valve is shown. This is suitable for all cases where the sludge can be discharged to the sludge bed by gravitation. Where the levels do not permit of this, and it becomes necessary to raise the sludge, a chain-pump may be fixed in the detritus tank itself. As, however, this would involve a separate chain-pump for each detritus tank, as well as for each of the other subsequent tanks, it is usually found more convenient in such cases to construct a separate sludge-well provided with a chain-pump, and arranged at such a depth that the sludge from all the tanks will reach it by gravitation. This arrangement will be shown later in connection with the sedimentation tanks. The inlets to detritus tanks must be provided with valves, so that the flow of sewage may be shut off when it becomes necessary to empty the tanks. In order to prevent any misuse of these valves either in error, or wilfully, by closing both simultaneously and thus causing the whole of the sewage to pass over the storm-water overflow weir, the inlet valves should consist of grooved frames with one interchangeable door. By this means it is impossible for anyone to close both inlets at the same time.

Fig. 13.—Detritus Tanks.

Fig. 14.

Dortmund Type of Tank.—Where the volume of sewage is fairly large, and it would be convenient to have the sludge outlet at 2 feet to 3 feet below the level of the invert of the outfall sewer, the advantages of designing the detritus tanks on the lines of the Dortmund tank may be considered. An example of this kind of tank is shown in [Fig. 14]. Tanks of this type have the following special features:—Great depth—from 16 feet to 20 feet below water level—and the bottom in the form of an inverted cone, with an outlet at its apex connected to a cast-iron sludge delivery-pipe, which may be carried up either outside the tank, as shown in solid lines, or on the inside of the tank, as shown in dotted lines. In either case this pipe should be continued vertically up to, and finish with, an open end at the level of the top of the wall of the tank, so as to form a means for inspection and rodding in case the pipe should become choked. From this vertical pipe a right-angled branch is arranged at about 2 feet below the top-water level in the tank, and provided with a sluice-valve. Ordinarily this valve is closed. When it is desired to remove the sludge deposited in the cone-shaped bottom of the tank, the sluice-valve on the sludge outlet is opened and the sludge is forced up by the head of water, due to the difference in level between the top-water level in the tank and the invert of the sludge-outlet. It has been stated that this method of sludge removal is subject to difficulties, due to the consolidation of the sludge in the cone, to such an extent that it becomes of too thick a consistence to flow up the vertical pipe. In some cases a special mechanical contrivance is adopted, by means of which the sludge may be stirred up at the apex of the cone-shaped bottom while the sludge-valve is open. Again, in a special form of tank which has been brought into use in Germany, the sludge is stirred up by means of jets of water, under pressure from the main, forced through a ring-shaped perforated pipe laid near the apex of the cone. In both cases it is evidently assumed that the sludge will only be removed at long intervals, and in the author’s opinion the difficulties referred to above may be avoided by the application of the motto “Little and often,” as described in his book on the management of Sewage Disposal Works.

Fig. 15.—Sludge Scraper in Detritus Tank.

Special Apparatus for Sludge Removal.—A further type of detritus tank is illustrated in [Fig. 15], in which the tank is circular in form but has a flat bottom. The sludge is discharged by the same means as that shown in [Fig. 14], but a special scraping machine operated by hand is used to facilitate the removal of the sludge by drawing it towards the inlet to the sludge pipe, which is situated at the centre of the floor. The scraper, which is manufactured by Messrs. Ham, Baker and Co., is helical in form, and is attached to and rigidly supported by a framework mounted on the central shaft, which is rotated by suitable gearing fixed at the side of the tank over the sludge discharge inspection chamber, so that the operator may be able to regulate the rate of the sludge delivery. It will be noticed that the outlet from this tank is by means of cross-channels, described in detail later in connection with sedimentation tanks.

TANKS.

Under this heading are included a large number of tanks of various types and systems, for each of which some particular advantage is claimed in ordinary circumstances, or some peculiar suitability for special conditions. All are, however, ostensibly designed for the purpose of arresting the organic matters in suspension, in order to prepare the sewage for the subsequent stage of oxidation in contact beds, on percolating filters or on land.

Types and Capacities of Ordinary Tanks.—In addition to detritus tanks described in the preceding chapter, the Royal Commission on Sewage Disposal, in its fifth Report, has dealt with five different methods of tank treatment in detail. These are:—

1. Septic tanks, having a total capacity of about 24 hours’ dry-weather flow.

2. Continuous-flow settlement tanks without chemicals, having a total capacity of about 15 hours’ dry-weather flow.

3. Continuous-flow settlement tanks with chemicals, having a total capacity of about 8 hours’ dry-weather flow.

4. Quiescent settlement tanks without chemicals.

5. Quiescent settlement tanks with chemicals.

The two last-mentioned have each a total capacity of about 24 hours’ dry-weather flow.

Fig. 16.

Fig. 16.

In all these five types of tanks the method of construction is very similar, generally rectangular in plan and of a moderate depth. As a rule they are connected by means of a supply channel to the preceding detritus tanks, and the total capacity is divided up into a number of units varying with the size of the scheme. The Royal Commission suggest the following divisions:—

1. Septic tanks: 5 tanks, with an additional spare tank.

2. Continuous-flow settlement without chemicals: 6 tanks, with 2 additional spare tanks.

3. Continuous-flow settlement with chemicals: 6 tanks, with 2 additional spare tanks.

4. Quiescent-settlement without chemicals: 8 tanks, with 2 additional spare tanks.

5. Quiescent settlement with chemicals : 8 tanks, with 2 additional spare tanks.

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.

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.

Fig. 18.

Fig. 18.

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.

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.

Fig. 21.—Sludge Elevator.

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.

Fig. 22.

Fig. 23.

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.

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.

Fig. 26.

Fig. 27.

Fig. 28.

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.

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.

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.

Fig. 31.

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].

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.

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 b¹ and b², be so appointed that two-thirds of it flows from the outer compartment over b¹ and one-third over b² from the inner compartment, the only entrance to which is by the opening b³, [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 b⁴, [Fig. 35]. The overflows from the weirs b¹ and b², are, by the channels b⁵ and b⁶, 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 c¹, [Fig. 39], having openings c² in the lower edge. In the drawing the second section of the tank is shown divided in two parts by the wall and weirs c³, [Figs. 34] and [40], and they occupy together about seven-eighths of the circumference of the tanks. The weirs c³ 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 c², [Figs. 34] and [39]; and consequently the deposition of solids is accelerated by this flow, so that they collect in the lower part, c⁴, [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 e⁵ 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 c⁶, [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 e³, [Fig. 36], passed into the supplementary section E, which is fitted with colloiders, e¹, [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 e⁵, [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, e⁴, [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 c⁷, [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.

Fig. 34.

Fig. 35.

Fig. 36. Fig. 37.

Fig. 38.Fig. 39.

Fig. 40.Fig. 41.

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.

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.

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.

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.

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.

Fig. 47.—“Separator” Tank.

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 A¹ 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 A¹ through the walls on all sides into the outer tank B¹, 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 B¹ 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 S₁, 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.

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.

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.

Fig. 51.—Chemical Mixer.

Fig. 52.—Pneumatic 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.

Fig. 53.—Sludge Press.

Fig. 54.—Sludge Press.