Part 4

_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.