Chapter II: Part 2

_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 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.

“_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.

_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.

_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.

_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.

_Sludge Presses._—When it is desired to reduce the liquid content of the sludge as far as possible, the general practice is to make use of sludge presses for this purpose. Several types are illustrated in Figs. 53, 54 and 55, manufactured by Messrs. Manlove, Alliott and Co., Ltd., Messrs. Goddard, Massey and Warner, and Messrs. S. H. Johnson and Co., Ltd. All are based upon the principle of compressing the liquid sludge under high-pressure between iron plates which support cloth or other filtering material, through which the liquid passes into grooves on the faces of the plates, and thence by way of conduits in the plates themselves to the floor below. The several makes have different methods of opening and closing the plates, and the presses are made of various sizes for operation by hand or by power. Fig. 56 shows a complete sludge-pressing plant as designed by Messrs. S. H. Johnson and Co., Ltd. The description of the details of this plant is as follows.

The sewage enters the works by the channel A, and passes first through the bar screen B. The screening is necessary to remove anything that would tend to produce obstruction in the inlets to the press chambers and be liable to cause breakage of the press plates. The sewage next meets with the milk of lime from the lime mixer C, with which it is mixed by flowing along the gravitation mixer D. The pneumatic lime mixer produces lime milk of a constant strength, and the flow is adjusted in proportion to the requirements of the sewage. Should it be necessary to add sulphate of alumina to the sewage, this is produced by the pneumatic alumina mixer E, and is added to the sewage after the latter has been thoroughly mixed with lime. Air for working the pneumatic lime and alumina mixers is provided by the blowing engine R. The treated sewage then passes further along the zigzag channel into the precipitating tanks F, the ends of two of which are shown in the drawing. It is advisable to have two or more tanks, so as to allow sufficient time for precipitation. The usual capacity of the precipitation tanks is equal to 6 hours’ flow of the sewage, and they may continue running, overflowing continuously, for a considerable time, but not so long as will produce putrefactive decomposition and thereby cause a nuisance. The precipitation tanks, which are cleared out alternately, are provided with hinged flap valves G and underground stoneware pipes to convey the sludge into the liming sump H, the top water being first decanted off. In the liming sump the sludge is limed with milk of lime from the lime mixer I, which is also worked by the blowing engine R above referred to. From the liming sump the sludge passes into the sludge tank J, by means of the pair of automatic rams K. The automatic rams work alternately, one filling by means of vacuum, whilst the other is being discharged by means of compressed air. As soon as the one is emptied and the other filled, the action is reversed, and so on, each filling and emptying alternately, thereby keeping up a continuous discharge. By being drawn into the rams, and thence forced into the sludge tank, the sludge becomes thoroughly mixed with the lime. This liming of sludge causes a considerable further deposition and concentration of the sludge, and after standing all night the supernatant water is decanted off by the skimmer L. The sludge, now ready for pressing, is allowed to run by gravitation into the automatic rams K previously referred to, and thence discharged into the sludge presses M by means of compressed air, the compressed air being supplied by the air-compressor N, which also acts as a vacuum pump for drawing the sludge from the sludge pump into the automatic rams. The solid portion of the sludge is retained in the chambers of the sludge presses by the filter cloths, the effluent being discharged into the trough at the side of each press, and thence by down pipes and gullies into the effluent channel O, being treated again in the gravitation mixer, and finally flowing away with the effluent from the precipitating tanks. The press chambers are known to be filled with solid sludge cakes, when effluent ceases to flow from the outlets of the chambers. The presses are then opened and the cakes discharged into a tipping truck Q, by which they are removed to the final disposal site.

_Hydro-extractor for Sludge._—An entirely different method has been adopted in the special apparatus in use at Hanover and other towns in Germany, the Schaefer-ter-Mer centrifugal sludge de-hydrating apparatus manufactured by the Hanoversche-Maschinenbau A.-G., vormals Georg Egestorff, and illustrated in Fig. 57. In this apparatus the centrifugal force resulting from the rapid rotation of the drum into which the liquid sludge is fed, is utilised to throw out the solid matters from the centre towards the circumference, where they are caught in the outer part of the drum of the machine. The drum revolves continuously, but at regular intervals it is opened automatically in sections for a brief period, so that the dry sludge is thrown outwards against the fixed casing and thus becomes broken up and falls to the bottom, and thence to an endless-band transporter by which it is discharged outside the building. At the moment when the sections of the outer casing of the drum of the machine are opened to allow the dry sludge to be thrown out, the wet sludge is prevented from passing into these sections by the automatic closing of the inner slide door, which is opened as soon as the outer slide is closed. The water extracted falls into an annular channel below, from which it flows, by way of a pipe, back to the settling tanks to be treated again. The result of a series of special tests of this apparatus showed that the liquid contents of the sludge was reduced from 92 per cent. to 50 per cent. The installation at Hanover has now been in operation since June 1908, dealing with a daily volume of 6·6 million gallons of sewage from a population of 280,000. From particulars supplied by the town authorities, it appears that the total cost of operating the complete plant, including the settling tanks and the sludge treatment apparatus, amounts to about 8_s._ per million gallons of sewage treated, or about 0·8_d._ per head of population per annum.

Messrs. Manlove, Alliott and Co., Ltd., have now entered into an arrangement with the above-mentioned firm to take up the control of the patents and the sole manufacture and sale of the Schaefer-ter-Mer Sludge-Drying Apparatus in Great Britain and the Colonies.

_Sludge Draining Beds._—Although the methods of disposal of sludge must vary in different localities according to the means available for the purpose, and most of them involve very little, if any, constructional work, it may be desirable to describe the various points which should be taken into consideration in the construction of suitable draining beds, as these should be included in the original design of any scheme in which they are to be used. Their chief function is to provide means for removing the maximum amount of the liquid contents of the sludge in the minimum of time, and it is obvious that this desideratum can only be secured by spreading out the liquid sludge in thin layers upon material through which the liquid may readily pass without carrying with it any of the sludge. The first of these requirements necessitates the provision of an ample area of draining surface, and the second involves the use of a suitably graded material provided with ample means of drainage. The beds themselves may be simple excavations in the ground, as shown in Fig. 58, or may be constructed of brickwork or concrete, but in either case it is absolutely essential that the floor should be covered with tiles, or other means of sub-drainage, leading to a free outlet, which should be connected to the screen chamber, detritus tanks, pump well, or some other point at the inlet to the works, so that it may be treated over again with the crude sewage. Whatever material is used for filling the bed, the lower portion which is placed on the floor and over the drainage tiles should be of large size, 2 inches to 3 inches in diameter. The next layer should be 1½ inches to ½ inch in diameter, and the top layer 6 inches to 9 inches in depth, should be fine material ¼ inch to ⅛ inch in diameter. In the author’s opinion, coke-breeze will probably be found to be the best material for the top layer, and it would be a good precaution to provide beforehand a quantity of this material in reserve to replace what is lost in removing the dried sludge from the surface of the beds.

It will be found advisable in operating these beds to discharge the sludge from the tanks in small quantities at frequent intervals, rather than in large quantities at long intervals, and it is very important that each layer of dried sludge should be removed before the next layer is delivered to the bed. It cannot be too strongly urged that sludge disposal needs as much care and attention as any other stage of the process of sewage disposal, and if this is available, and ample area of draining beds is provided, there should be no difficulty in solving this usually troublesome problem.

_PERCOLATING FILTERS._

In approaching the subject of the design of filters for the purpose of oxidising the organic matters—in solution and in suspension—contained in the liquid which leaves the preliminary process in tanks, the first consideration is the question of site. Where the slope of the ground permits of the construction of the filters on or above the ground level, much expense for excavation may be avoided, so long as the base of the filter can be laid on solid ground. In cases where the site of the works is comparatively flat, it is impossible to avoid excavation, and other means must be adopted to keep the actual cost as low as possible consistent with efficiency.

_General Design._—Taking the latter case first, it should be observed that some engineers consider it desirable to construct retaining walls under all circumstances, but the author does not agree with this idea. In the first place the walls do not, of themselves, have any influence on the efficiency of the filters in producing a satisfactory effluent, and if a filter can be constructed without them there is no reason why they should not be omitted. This applies especially where the filters have to be constructed entirely below the surface of the ground. The chief point to be considered is that the effluent shall have a free outlet, with facilities for inspection. In the case of all filters, the best method of securing a free outlet for the effluent is to provide the floor with a suitable slope from the centre to all sides. When the floor is at some depth below ground this requirement necessitates an effluent channel on all sides of the filter. Two methods of carrying this into effect are illustrated, Figs. 59 and 60. Fig. 59 shows one retaining wall for the filter and an additional retaining wall for the surrounding earth, carried up to the surface with the effluent channel between the two walls. In Fig. 60 the arrangement is similar, but the outer wall is a dwarf wall to form the effluent-channel, and the surrounding earth is cut back to a slope of natural repose, the earth bank usually being sown with grass or covered with turf. It is essential that the outer wall shall be carried up above the toe of the earth bank, in order to prevent soil being washed down into the effluent-channel, but a surface-water drain should be laid to take any water that may accumulate at the back of this wall.

These methods of construction would, however, only be adopted by those who consider it essential to provide means for lateral aeration to the filter by constructing the retaining wall for the filter itself of pigeon-hole brickwork. In the author’s opinion, however, lateral aeration on these lines is altogether ineffective. It can only affect the filter material for a distance of a foot or two from the wall. It is true that in some cases horizontal perforated ventilating pipes have been provided, radiating from the centre of the filter to the outer wall and terminating with open ends on the outside. The effect of these is, however, dependent almost entirely upon the temperature of the atmosphere and the direction of the wind, and even if they do induce air currents into the body of the filter the air will pass along the line of least resistance, and therefore find its way through those interstices which are open and not through those which are choked in any way and thus most in need of aeration. The author believes that the aeration of a filter is most effectively secured by the action of the sewage itself, as it falls from the surface to the floor drawing in the air from the top, and that if this is not effective, no amount of lateral openings will produce the desired result. If this contention is correct, there is no need to incur the additional expense involved in the construction of retaining walls, and the filters may be designed on the lines indicated in Fig. 61, where the filtering material fills the whole of the natural excavation, and no walls are required. It is true that this method involves an increase in the amount of filtering material beyond what is actually in use, and the cost of this must be set against the cost of the wall in the alternative method shown in Fig. 62, as the cost of the excavation in either case is about the same. Much will depend upon the cost of the filtering material in different localities.

In both these methods it will be noticed that there is no outer effluent channel, and that the floor slopes from the circumference to the centre, where an effluent receiving chamber is constructed, from which an effluent discharge pipe leads to the next stage in the process. This is not quite so satisfactory as the arrangement shown in Figs. 59 and 60, but it is the most convenient under the circumstances.

When it is possible to place the filter floor on or within 2 feet of the surface of the ground, the method illustrated in Fig. 60 is the design most commonly adopted. Sometimes the arrangement of the floor shown in Figs. 61 and 62 is preferred. There is, however, still another type of floor which is applicable to this case. This is illustrated in Fig. 63, from which it will be seen that the whole of the floor slopes in one direction, and the effluent is thus discharged over one-half of the circumference of the filter, with the result that the effluent channel is only one-half the length of that required in the case of Fig. 60.

_Floors of Filters._—In all cases the floors for percolating filters should be of a substantial character. They are usually constructed of cement concrete. The thickness of the concrete will depend upon the nature of the subsoil, but in any case it should be increased at the centre in order to provide a safe foundation for the revolving sprinkler. The surface of the floor should be smooth, so as to facilitate the flow of the effluent, and any suspended solids it may contain, to the outlet. From the preceding illustrations it will be noted that the slope of the floor may be in three directions: (_a_) from the centre to the circumference, (_b_) from the circumference to the centre, or (_c_) from one side of the filter to the other, a uniform slope in one direction along the diameter of the filter. It has been suggested by some that the floor should consist of a series of alternate V-shaped ridges and furrows, with slopes at right angles to the general slope of the floor, but this has the disadvantage that it causes difficulties in arranging the floor tiles and placing the filtering material in position, and further it increases the cost of construction without providing any real compensating advantages.

_Sub-drainage._—It has been recommended that the floors should be laid with a gradient of about 1 in 100, but the author believes that it would tend to assist the free discharge of the suspended solids in the effluent if this gradient were increased to 1 in 75, or even 1 in 50, and that if this were done, there would be no need to make any special provision for access to the under drains for cleaning purposes. Another important factor in securing a free discharge of the suspended solids, is the use of proper floor tiles or sub-drains. The old idea of laying rows of agricultural pipes with open joints, or even of perforated pipes, on the floor has been proved to be useless. In some cases it is considered sufficient to have rows of floor tiles, laid at any distance apart up to 10 feet at the circumference. There is, however, very little doubt that the only correct method is to support the whole of the filtering material on a complete false floor, so that the suspended solids which are carried down with the effluent, and thus reach the bottom of the filter at all points of its area, may fall freely into an open space, from which they will be carried away with the least possible obstruction by the flow of the liquid. It is certain that these suspended solids have a great tendency to adhere to any object with which they may come in contact, and that this can only be avoided by providing a free space immediately above the floor and over its whole area. The nearest approach to this acme of perfection is a complete false floor, with openings too small to allow the filtering material to pass through, but large enough to give a free passage to the suspended solids. One of the simplest methods is to form the false floor of bricks, laid flat on rows of bricks on edge. There are, however, several floor tiles on the market specially designed for this work. A number of these are illustrated in the accompanying figures. The Ames-Crosta tile, Fig. 64, has a simple flat top with corrugated edges supported on four short legs. When placed close together, the corrugations form apertures through which the liquid passes, and the flat tops form a table which provides every facility for placing the filtering material in position. A somewhat similar floor tile is the “Newham” shown in Fig. 65. In this case, however, each tile has only two legs, and thus there is less obstruction to the flow of the effluent. The “Stiff” floor tile, Fig. 66, is of much the same type as the two last described. Another material which has been adapted for this purpose is a single layer of the “Dibdin” type of slate slabs and blocks, as shown previously in connection with the preliminary treatment of sewage in tanks, Fig. 49. This method also provides a complete flat false floor, and has the additional advantage of occupying the least possible space. The slabs themselves may be split to any suitable thickness, and the supporting blocks may be cut to any size, as little as 1 inch in thickness if desired. The slabs vary in shape and size, but may be secured of larger area than tiles, so that the necessary supporting blocks are less in number than the feet of the tiles, and thus offer less obstruction to the effluent. The apertures for the passage of the liquid are provided by the holes formed by the irregular edges of the slabs, where they abut on one another.

There are, however, floor tiles of other shapes, such as the Candy tile, Fig. 67, for which it is claimed that the effective depth of the filter may be calculated from the floor, and that the height of the V-shaped opening under the tile provides greater drainage and ventilation openings than other tiles. Other drainage tiles are semicircular in section. The Naylor tile, Fig. 68, and the Albion floor tile, Fig. 69, designed and used by Mr. E. E. Ryder, Surveyor to the Bushey Urban District Council, both have openings at the floor level. The “Mansfield” floor tile, Fig. 70, is the type adopted for the extensive filters at the Birmingham Tame and Rea district sewage farm by Mr. J. D. Watson, Engineer to the Drainage Board.

In some cases, instead of providing a free space above the floor, drainage channels have been formed in the floor, with rebates in the sides to receive flat perforated tiles, as Fig. 71. This method of drainage certainly leaves the floor clear to receive the filtering material, but it has the disadvantage that it does not provide a free space under the whole of the material; and as the suspended solids in the effluent must of necessity travel some distance before they reach a channel, they are, to a great extent, arrested by the material which comes immediately on the floor, however large the separate pieces may be, and thus tend to choke the interstices.

Whatever method of sub-drainage may be adopted, it is advisable to continue the tiles, or channels, right through to the effluent channel or chamber in straight lines, so that the effluent may have a clear way throughout its course under the filter, until it reaches the open effluent channel or pipe.

_Walls of Filters._—The great diversity of methods adopted to retain in position the material of filters constructed above the level of the ground, must be extremely perplexing to anyone who investigates the subject. They vary from walls of such thickness and substantial material that they would be quite suitable for resisting the pressure of a head of water equal to the depth of the filter, down to no walls at all. In many cases walls 9 inches thick throughout and 6 feet in height are found to be quite satisfactory, while in a large number of cases the material itself, when it consists of clinker, is found standing in an almost vertical position with perfect success. The question naturally arises, if it is possible to construct filters satisfactorily without any retaining walls, why incur unnecessary expense in providing walls? The answer is that clinker is not universally adopted as a filtering material, and that the strength of circular walls is not sufficiently taken into consideration. The thickness of the wall will naturally depend upon its height, but assuming this to be 6 feet, and that the wall is in all cases carried down to a solid foundation, the extreme thickness that would be needed is probably 18 inches at the bottom and 9 inches at the top, as Fig. 72. As a general rule, however, it will probably be found quite sufficient to construct the wall of brickwork in cement, with the lower half 14 inches and the upper half 9 inches in thickness. In a large number of the most recently constructed filters, the walls have been omitted altogether when clinker has been used for the filter. Large pieces of this material have been selected, and carefully packed in the form of a dry rubble wall with perfect success, even when laid with a batter on the outside of as little as 1 in 6 or even 1 in 8. It is obvious that this method of construction needs special care and supervision, especially in providing a rough kind of bond between the different layers, if it is to be successful. A rough idea of this method is given in Fig. 73. As a kind of happy medium between this system and a wall to the full height of the filter, a dwarf wall has been adopted in a number of cases, and notably by Messrs. Willcox and Raikes, Civil Engineers, who have also designed a special form of coping, made in fire-clay or terra-cotta, as shown in Fig. 74. Again, in several places where suitable local stone, or stones and bricks resulting from the demolition of old houses or walls, was available at a cheap rate, these materials have been utilised to form dry rubble walls, similar in construction to the clinker walls illustrated in Fig. 73.

Occasionally the walls of filters are carried up several feet above the surface of the filter with the idea of forming a screen to protect the revolving sprinklers from the influence of wind. It is, however, obvious that these measures can only have an effect upon the outer ends of the revolving arms, and, as the action of the revolving sprinklers is only affected by wind when the head of sewage by which they are driven is at its minimum and the force of the wind is simultaneously at its maximum, the necessity for windscreens seldom arises. In actual practice, with an average head of sewage, no real difficulty is caused by wind.

So far, it has been assumed that the walls and floors of the filters can be constructed upon solid ground. Unfortunately this is not always possible. If the subsoil is of an unsubstantial nature it is advisable to lay the floor of a suitable thickness and diameter, so that the wall may be built upon the floor itself, and thus distribute the load over as large an area as possible. In some cases the levels necessitate the raising of the filter floor _above_ the surface of the surrounding ground. Under these circumstances the footings of the walls should be carried down to solid ground, and no reliance whatever should be placed upon made-up ground even to carry the floor alone. The outer edge of the floor should be supported by the footings of the wall, and the remainder carried upon piers or cross-walls of brickwork, concrete or masonry, extended down to solid ground. The piers or cross-walls should be sufficient in number to support the floor with safety. A smaller number may be used if steel joists are provided between them to take the weight of the floor and filter, or the whole floor may be of a properly designed reinforced concrete construction, supported at the circumference on the footings of the outer wall and at the centre on a substantial pier of concrete carried down to solid ground, with intermediate piers if the diameter of the filter is excessive.