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.

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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.

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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.

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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 8s. per million gallons of sewage treated, or about 0·8d. 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 ? 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.

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