Fig. 57.—Sludge-drying Apparatus.
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.
Fig. 58.—Sludge-Draining Bed.
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.