Sewage Disposal Works: Their Design and Construction

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:—

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

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:—

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.

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.

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.

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. 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.

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.

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.

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.

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.

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.

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.

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.

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 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.