Part 2

_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.

_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.

_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.

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.

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.