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.
Fig. 55.—Sludge Press.
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.
Fig. 56.—Complete Sludge-pressing Plant.