Part 3

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.

_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.

_Hydrolytic Tank._—There are, however, several types specially designed to eliminate these troubles altogether, by separating the flow of sewage through the tank from that section in which the composition of the sludge takes place. Among these is the hydrolytic tank. This tank is already well known to most engineers, in the original form designed by Dr. W. Owen Travis, and adopted at Hampton-on-Thames, but a new and improved method of construction has recently been brought out. The principle of this tank may be described as the deposition and collection of the impurities in sewage by a process of physical de-solution, the matters being separated in the order of their grossness and specific gravity, namely (_a_) the removal of the grosser solids by means of screens; (_b_) the settling of the heavy inorganic solids in a detritus chamber; and (_c_) the separation of the lighter solids in suspension and in a colloidal state. Finally, means are provided whereby the deposit in the various chambers may be collected and removed with facility. It is impossible in the space available to describe in full the reasons for the various details of construction which have been adopted, but the accompanying illustrations, Figs. 34 to 42, which have been kindly furnished by Messrs. Shone and Ault, Civil Engineers, illustrate an example of the latest type of the tank. Fig. 34 is a plan section of the tank, and Figs. 35 to 42 are vertical sections on the lines indicated. The tank is by preference circular, as shown. The sewage is delivered from the pipe S through a screening chamber, in which the gross matters, such as rags and vegetable debris are retained on the screen A, and are from time to time removed by hand or mechanically. The sewage passes over the weir _a_ into the first section B, which occupies about one-eighth of the circumference and is divided into two parts by the diaphragm _b_, Fig. 35. The flow of the sewage through this first section B may, by the weirs _b_^1 and _b_^2, be so appointed that two-thirds of it flows from the outer compartment over _b_^1 and one-third over _b_^2 from the inner compartment, the only entrance to which is by the opening _b_^3, Fig. 35, in the bottom of the diaphragm _b_; so that the deposition of the solids by gravity is accelerated by the flow of the one-third of the sewage into the inner part of the compartment B. The solids collect in the conical bottom part _b_^4, Fig. 35. The overflows from the weirs _b_^1 and _b_^2, are, by the channels _b_^5 and _b_^6, directed to the downtake _c_, Figs. 34, 35, 38 and 39, which delivers the sewage near the bottom of the outer compartment C, which latter, with the inner compartment D, forms the second section of the tank. These two compartments are divided by the diaphragm _c_^1, Fig. 39, having openings _c_^2 in the lower edge. In the drawing the second section of the tank is shown divided in two parts by the wall and weirs _c_^3, Figs. 34 and 40, and they occupy together about seven-eighths of the circumference of the tanks. The weirs _c_^3 are so proportioned that 85 per cent. of the liquid passes directly through the outer compartment, and 15 per cent. indirectly through the inner compartment of the first portion of the second section of the tank, into the respective compartments of the second portion of that section. It should be noted that the only passages for the flow of liquids into the inner compartment are the openings _c_^2, Figs. 34 and 39; and consequently the deposition of solids is accelerated by this flow, so that they collect in the lower part, _c_^4, Fig. 35, of the inner compartment B. The flow through the second portion of this second section of the tank is governed by the weirs _e_^5 and _e_, Figs. 34 and 36, which weirs are shown of such proportion as to cause 70 per cent. of the liquid to flow directly through the outer compartment, and 30 per cent. indirectly through the inner compartment. The colloiders _c_^6, Figs. 34 and 35, are fixed vertically in the outer compartments to attract and absorb the solids in pseudo solution. It will thus be clear that 70 per cent. of the sewage flows in a direct manner through the outer compartment, and in doing so deposits practically the whole of its permanent and a considerable portion of its convertible solids. The effluent from the inner compartment D of the second section of the tank is, by the submerged channel _e_^3, Fig. 36, passed into the supplementary section E, which is fitted with colloiders, _e_^1, Figs. 34, 35, and 41. This effluent, which has become fouled by the disturbance caused by the evolution of gases in the inner compartment of the second section, is thus submitted to a further de-solution action by absorption and other processes. Finally the outflow from the outer compartment C, of the second section over the weir _e_^5, Figs. 34 and 36, and the outflow from the supplementary section E, over the weir _e_, are passed away from the tank by a common channel, _e_^4, Figs. 34 and 37, whence the effluent may, for further treatment, be led to filters or on the land. The overflows from the two weirs may, however, be led away from the tank by independent channels for separate treatment. The solids, collected in the form of sludge in the lower parts of the sections, can be drawn off periodically through the pipes _c_^7, Figs. 34, 35, 41, and 42, governed by valves into the central chamber F, Figs. 34 and 35, from which it may be led by the pipe _f_ to adjoining land, or elsewhere for further treatment. The lighter solids, that collect in the form of scum on the surface of the liquid in the tank, may be skimmed off or drawn into the channels _g_, Figs. 34, 38, and 40, and conducted to the central chamber F, and disposed of similarly to the sludge. The tank is, or may be, constructed of concrete, which may be reinforced as required according as it is wholly or partly above the ground. Its shape may be greatly varied according to local requirements and other considerations.

_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.

“_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.

“_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.

_The “Fieldhouse” Tank._—This is illustrated in Fig. 48 (from a drawing supplied by the patentee, Mr. J. Fieldhouse) from which it will be seen that the sewage enters the central chamber A^1 by the inlet pipe M, the end of which is turned down to deliver the sewage immediately over the inverted cone C. Between the inverted cone C and the side of this chamber an annular space E is provided, so that the solids which are deposited may find their way into the cone-shaped sludge chamber below, from which they are drawn off by means of valve D and sludge pipe F. The liquid passes from the central chamber A^1 through the walls on all sides into the outer tank B^1, by way of the oblique passages H, by which the liquid is deflected in a downward direction, and eventually flows over the outer circular weir K into the effluent channel L. The outer tank B^1 is divided into sections, each of which is provided with a sludge sump and sludge valve N. Scum-boards are provided both radially T, and in front of the weir J, and the latter may be lowered when it is desired to draw off the scum. This operation is performed by closing slides S_{1}, so as to cause the sewage to head up in the tank, and the scum of any section may then be drawn off by lowering the particular end board J next to the weir K, and allowing the scum to overflow into the effluent channel L and thence to the sludge bed. The special features of this tank are:—(_a_) the cone-shaped bottom of each section, to facilitate the withdrawal of the sludge without discharging the liquid contents; (_b_) the oblique passages H in the wall between the inner and outer tanks, for the purpose of deflecting the flow of the sewage in a downward direction, and thus assisting the deposition of the matters in suspension; (_c_) the removable scum boards in the outer tank, to allow of the removal of the scum; (_d_) the general design by which the sewage enters at the centre, and thence spreads in all directions until it flows in a thin film over a weir of comparatively enormous length, thereby causing a gradually increasing reduction in the velocity of the flow, and thus providing every facility for the deposition of a very large percentage of the matters in suspension.