The present invention relates to a filter band press for treating industrial sewage and sludge. The filter band press in accordance with the invention is especially suitable for treating sewage and sludge in the pulp and paper industry. The purpose of a preferred embodiment of the apparatus is to increase the dry substance content of the sludge coming from a mill to a value suitable for combustion, in other words to into the range of 35 - 40 1.

There are several known filter band presses of approximately the same construction, and the apparatus disclosed in U.S. Patent 4,604,195 is an example. Said apparatus comprises a top wire section and a bottom wire section with the pulp to be treated between the two sections being fed to the Fourdrinier part of the bottom wire section. There a part of the pulp is thickened due to gravity or due to a vacuum prevailing under the bottom wire section which transfers the pulp between wires. When the distance between the wires diminishes, water is pressed from the pulp being treated or from sludge or the like through the wire sections. Additionally, several nips are arranged in the press so that the liquid tends to be forced from the sludge or like.

The consistency of sludge entering the filter press from the thickeners and clarifiers has normally been in the range of 2 - 4 %. Since the filter press should be capable of in¬ creasing the consistency of the sludge to the above mentioned value of about 40 %, the capacity of filter reduces and the characteristics set for the filter increases, since it should be capable * of raising the consistency of the sludge about 35 %. The concept of a prethickener in combination with the

filter band press cannot be avoided, because it is possible easily to increase the capacity of the press by arranging a preliminary water extraction device for the press in the flow passage of the sludge or like, whereby the consistency of the sludge is increased.

Mills tend to supply sludge with a lower consistency to the press, that is, the infeed consistency may be as low as 1 - 2 % or even lower. If a simple water extraction device can be developped for easy installation in existing equipment, it is possible to operate with the old machinery, although the infeed consistency of the press would be reduced. When con¬ structing a completely new mill it would be possible to obtain savings in the devices used. With an increase in the infeed consistency, for example, from 1 % to 4 % a drop in the equipment required for treatment by a half is possible af¬ fording considerable economic savings.

There are existing devices which have been known to use prethickening apparatuses. For example, it is disclosed in U.S. Patent 4,153,550 that the sludge fed to the wire sections of the filter press may be prethickened, however, the appa¬ ratus is not described in detail in the patent. According to Fig. 1, the apparatus seems to be a rotatable wire-coated cylinder with sludge being fed from the end of the cylinder to the wire section. In another patent publication GB Patent 1504628, chemicals are fed, if required, to the material in a separate mixer, subsequently the material is fed to a drum sieve divided into chambers by annular barriers. The material thickened in the cylinder is discharged from the end of the cylinder to the flat part of the filter press. Such pre¬ thickening apparatus, as disclosed in the GB Specification, however, has not proved to be extremely successful, because as the sludge is fed to the first annular section of the thick¬ ening cylinder, the majority of the sludge flows over the thicker pulp portion sliding or circulating on the surface of

the cylinder to a second section, in which a similar pheno¬ menon takes place. In other words, the material thickened on the surface of the thickening cylinder does not properly allow new sludge on the thickening surface, but carries it along forming a bridge of sorts over the wall between the section into the next section.

The problems experienced in the past are solved by an appara¬ tus in accordance with the present invention whereby the material to be thickened is forced to flow continuously the new untreated sludge flowing directly to the thickening surface. Said apparatus is characterized in that a preliminary water extraction device includes a screw feeder with an outer rim on the thread of the screw with a filter surface wrapped about the outer rim and means for feeding the material to the preliminary water extraction device with additional means for receiving the prethickened material and also for receiving the filtrate. The apparatus is further characterized in that the combined rotational movement and horizontal movement effected by the screw make the sludge rotate effectively for preventing the accumulation of the sludge on the surface of the filter member. As a result, no fiber mat, through which the liquid penetrates poorly, is formed to block the discharge of the liquid or water.

The apparatus in accordance with the present invention in described in detail below, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic elevational view of a filter band press, for intensifying the filtering operation and including an apparatus embodying the present invention; and

Fig. 2 is a schematic view of a preferred embodiment cf an apparatus in accordance with the present invention.

A filter band press illustrated in Fig. 1 includes a bottom wire section 1 and a top wire section 2. The bottom wire section 1 has a bottom wire 3, a breast roll 4 at the inlet end of the press, support rolls 5, and press rolls 6 at the outlet end of the press. One of the press rolls 6 operates together with the breast roll 4 as a creasing roll. Top wire section 2 includes creasing rolls 7 , at the inlet and outlet ends, support rolls 8, intermediate the inlet and outlet ends, and press rolls 9 adjacent the outlet end. Material to be treated is fed to the flat part 10 of the bottom wire section 1 by a feed apparatus 11. Feed apparatus 11 is connected with a preliminary water extraction device in accordance with the present invention.

According to Fig. 2, the preliminary water extraction device 20 in accordance with the present invention, comprises in a preferred embodiment two axially extending screw threads 22, 23 each located on the same shaft 21, with the screw threads arranged so that when the shaft 21 is rotated each screw thread feeds the material toward the center of the shaft, that is, to the open section located between the two screw threads. A filter surface 24, such as a wire cloth, is attached to the ridge or outer rim of each screw threads 22, 23, with the filter surface completely, laterally enclosing the screws threads. The screw threads 22, 23 are arranged so that a free or open space is located between the adjacent ends of the screw threads for discharging material from the preliminary water extraction device 20 to the flat part 10 of the filter band press. The material flows from the preliminary water extraction device 20 through a duct 25, to the flat part 10 of the filter band press. The duct 25, as shown in Fig. 2, increases in width from the device 20 to the flat part 10 so that it extends for almost the entire width of the bottom wire 3. The cross-sectional area of the duct 25, however, remains substantially constant along its length so that the shape of the cross-section changes but the area does not. Moreover, the

area does not. Moreover, the preliminary water extraction device includes a filtrate vat 26 for the liquid filtered through the filter surface 24 and the liquid accumulating in the vat can be pumped for further treatment. A drive means 27 is provided at one end of the shaft 21. Preferably the drive means 27 is an electric motor with an adjustable rotational speed of, whereby the feed and, at the same time, the treat¬ ment speed of the preliminary water extraction device can be easily adjusted. Further, the preliminary water extraction device 20 includes feed devices 28 at the outer end of the screw threads 22, 23 for supplying the material to be treated, sludge or the like onto the screen surface 24 formed on the screw threads from the opposite ends of shaft 21.

The device operates so that the material flow to be treated, for example, the sewage sludge in the wood processing in¬ dustry, is divided into two partial flows directed from feed devices or nozzles 28, to the inlet ends of the screw threads 22, 23 on the shaft 21. Screw threads 22, 23 convey the material received from the nozzles 28 to the open space between the screw threads and then into the feed duct 25. As the material floats through the screw threads, liquid is removed, passing through the filter surface 24 due to gravity. At the same time, screw threads 22, 23 the continuous transfer of the material toward the discharge duct 25, so that the inlet material flows on a clean filter surface and is immedi¬ ately exposed to an efficient thickening effect. It has been found in tests that the described thickener arrangement is capable of raising the consistency of the material from the value of 0.5 - 1.0 % to 3 - 5 % on a filter surface of about one meter in length, that is in the axial direction of the shaft 21. Furthermore, it has been found that the material to be thickened does not form a mat on the filter surface, rather during thickening the material flocks and lumps roll along the threads of the screw and the filter surface without sticking into the surface.

As can be noted in the above description, it has been possible to eliminate or minimize the drawbacks of the prior art apparatuses by a simple apparatus effective in its operation. With the apparatus of the present invention, the consistency of the material being treated can be increased from 1 % to 5 %, in other words, the dry substance content of the material can be increased to five times the original content, whereby the filter band press is able to discharge liquid from the material so that the material exiting from the press fulfills the qualifications effective combustion. It should be noted, however, that while the above described apparatus is a pre¬ ferred embodiment of the present invention, and indicates only one potential use, there are other ways in which the invention can be employed in accordance with the accompanying claims. Accordingly, it is possible that the screw feeder is not a two-part but a one-part feeder with the material to be treated flowing only in one direction of the screw thread shaft with the discharge of the prethickened material taking place at the opposite end of the filter surface enclosed screw thread. Furthermore, it is possible to use a completely closed thread as the screw thread, in other words, a thread which extends from the shaft to the filter surface or a partially open thread with a clearance of a particular size formed between the thread and shaft.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such prin¬ ciples.