JPS6333960B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an improvement in a continuous compression dewatering method in which sludge is held and compressed between endless belt-shaped continuously running filter cloths for dewatering, and particularly to a method for improving the peelability of a dewatered cake from the continuously running filter cloths. Excess activated sludge discharged during industrial wastewater treatment and sewage treatment must be disposed of by disposal, incineration, etc., but if it can be dehydrated to the lowest possible water content,
This reduces the transport weight and molten volume, and also greatly reduces the energy required for incineration. However, conventional dehydration methods such as belt press, centrifugal dehydration, and vacuum dehydration are labor intensive and have a limit to the moisture content that can be achieved.
% or less is difficult. In continuous compression dewatering, in which water-containing sludge is sandwiched between two endless belt-shaped filter cloths that are continuously operated and passed between rows of pressing rollers, the sludge that has been spread out relatively thinly is repeatedly subjected to a corresponding amount of time. Since it is possible to apply a strong clamping pressure, the dehydration effect is good and thorough, which is advantageous in reducing the moisture content of the dehydrated cake.
However, in the case of advanced dewatering of activated sludge, a problem arises in that the peelability of the dehydrated cake from the filter cloth becomes poor. In other words, the moisture content of dehydrated cake is around 70%, and in urban sewage
When continuous, strong, repeated compression is applied to reduce the reduction to around 60%, the dehydrated cake is around 1 mm thick and has a small weight, so the cake transfers to the lower filter cloth due to its own weight, resulting in normal peeling. This phenomenon hardly occurs, and the cake is tightly adsorbed on commonly used filter cloths. As a result, a lot of dehydrated cake remains attached to the upper filter cloth, and in this case, the dehydrated cake attached to the upper and lower filter cloths is irregularly attached to the upper and lower filter cloths in the form of isolated islands, so it is difficult to remove the dehydrated cake from the lower filter. Unlike cloth transfer cake, it cannot be satisfactorily scraped off with a scraper or the like. This deterioration in the peelability of the dehydrated cake is a major impediment to lowering the water content of the sludge dehydrated cake by continuous compression dewatering, and for this reason, advanced dewatering of activated sludge is hardly practical in conventional methods. It was deemed impossible. The present invention provides a solution to the problem of poor peelability of a dehydrated cake due to high dehydration in continuous press dewatering of activated sludge. Through research and experiments on roller compression dewatering of hydrous sludge, it was discovered that when a filter cloth with specific properties is used, the compressed sludge cake will recall surrounding water during depressurization in the latter half of the squeeze. The method of the invention exploits this phenomenon in relation to the strippability of the sludge dewatering cake. In other words, in order to give priority to the adhesion of the sludge dewatering cake to one of the two continuously running endless belt-shaped filter cloths that hold and compress the water-containing sludge, the adhesion of the dewatering cake to the other filter cloth is compared. Moisture recall is applied to reduce the target size. As mentioned above, the activated sludge cake that is being squeezed between the upper and lower filter cloths is contained in the upper and lower filter cloths after passing through the maximum squeeze point, that is, the nip point, of the pressing section. This causes a phenomenon that brings back moisture. As a result of research into this phenomenon, much of the above-mentioned recalled water is
It was confirmed that it was concentrated on the surface of the cake, between the cake and the filter cloth, or on the surface of the filter cloth, and almost never entered the inside of the cake. Therefore, by taking advantage of this fact and particularly increasing the amount of moisture recalled from the upper filter cloth, it is possible to retain the moisture necessary for peeling on the surface of the cake or between the cake and the filter cloth. By reducing the amount of water in the lower filter cloth, it becomes possible to make the cake adhere to the lower filter cloth. Therefore, in the present invention, filter cloths having different specific properties are selected and used in combination in order to make the adhesion of cake to the upper filter cloth and the lower filter cloth different. Hereinafter, the present invention will be explained specifically and in detail with reference to the accompanying drawings. FIG. 1 shows an example of a continuous compression dewatering apparatus to which the method of the present invention is applied. Squeezing rollers 2 are arranged around the rotating drum 1, and an endless belt-shaped upper filter cloth 3, a lower filter cloth 4, and a water-permeable back-up belt 5 are successively stacked and sandwiched between them. It is wound around various rollers and is continuously operated at the same speed as the rotating drum 1 rotates. During the operation of the filter cloth and belt, the water-containing sludge is continuously fed from the supply section 6 onto the lower filter cloth 4, and then is held between the two filter cloths when they are closed, while passing between the rotating drum 1 and the pressing roller 2. It is repeatedly subjected to strong clamping and compression, and is sent out as a dehydrated cake at the end of the clamping section. 7 is a scraper for scraping the sludge dewatering cake from the filter cloth. The continuous compression dewatering device to which the present invention is applied may be of a different type from the above, for example, a type in which pairs of squeeze rollers are arranged in a linear manner, and the above examples are not intended to limit the present invention. The optimal combination of the upper filter cloth 3 and the lower filter cloth 4 is as follows. That is, the upper filter cloth is a web made of fine denier polyamide fibers of 6 denier or less (1 denier is 50 mg per 450 m).
A felt structure is used in which the base fabric is needle-punched and integrated into a felt structure. The base fabric used must have dimensional stability during use. Therefore, synthetic fibers suitable for this purpose can be used, or natural fibers can be woven and then treated with a resin or the like. The web fibers of the felt structure are important; they are made of hydrophilic polyamide fibers, which have the highest water content among synthetic fibers, and their fiber properties make it easy to peel off the cake. It should have a dense structure that prevents it from digging into the inside. The preferred weight of the web is 50g to 1000g per square meter. In addition, like the base fabric, the felt after needle punching can be resin coated on the back side or treated with resin as a whole in order to increase dimensional stability during actual use. As the lower filter cloth 4, a smooth filter cloth is desirable so that the cake can be easily scraped off with the scraper 7. From this point of view, synthetic multifilament, monofilament, or composite woven fabrics with a relatively smooth surface structure are used, but other fibrous tissue filter fabrics can also be used. be. FIG. 2 schematically shows the pressing process of the method of the present invention in an enlarged manner, with one press nip representative. The continuous press dewatering device illustrated in FIG. 1 has a large number of press rollers 2 arranged in a row to perform press dewatering repeatedly. This corresponds to the pair replaced with 2. Therefore, the left half of FIG. 2 can be understood as showing the action of the left-most pressing roller, and the right half showing the action of the right-most pressing roller. 8 indicates a cake. The press nip part is 4
The situation can be divided into two areas, , and , and the status of each area will be explained below. In the lower part of the figure, the total pressure distribution curve a, the water pressure distribution curve c inside the cake, and the water pressure distribution curve f inside the filter cloth 3 on the felt structure are shown superimposed, and the cake thickness curve t is shown separately. ing. () Section The activated sludge cake 8 and the upper filter cloth 3 of the felt structure are compressed between the rotating drum 1 having a diaphragm peripheral surface and the pressing roller 2, but the activated sludge cake has a higher moisture content than the felt. Since the rate is high, it becomes saturated at the end point s of the first zone, while the felt is not saturated in this phase, but water pressure is naturally generated because the water squeezed out in the first zone flows backwards and enters the second zone. Water flows out of the system from the felt upper filter cloth 3 along the diaphragm surface of the rotating drum 1, the lower filter cloth 4, the backup belt 5, and the surface of the pressing roller 2. () Section The water pressure in the activated sludge cake rises rapidly, and water moves from the activated sludge cake to the felt.
In most cases the felt also reaches saturation in this area. The water pressure created by the applied press load also forces water towards the second zone. In this area, water is forced out of the activated sludge cake, where the water pressure is highest, to the felt, to the lower filter cloth, and then out of the system. () 1st zone Even if the total pressure curve a begins to fall after passing through the nip center n, the activated sludge cake continues to be pressurized until the water pressure decrease in the cake internal water pressure distribution curve c exceeds that, and reaches the minimum water content point m. reach In the middle of this zone, the water pressure in the felt passes through the zero point and the felt attempts to return to its unsaturated state. () Zone The felt expands in this zone, increasing the degree of unsaturation and creating negative pressure, but the negative pressure in the activated sludge cake is greater than that in the felt, so water flows from the felt to the activated sludge. Moving on to the cake. Furthermore, since the activated sludge cake has a smaller capillary diameter than the felt, water also migrates from the felt to the activated sludge cake due to capillary force. When the felt and cake are about to separate at the end of the second zone, the water film on the contact surface between them splits and adheres to both surfaces, so that the activated sludge cake 8 is easily peeled off from the felt filter cloth 3. . In this way, the activated sludge cake 8 transfers and adheres to the lower filter cloth 4 in the form of a continuous film, but it is easy to scrape off with the scraper 7, and the use of the smooth surface of the lower filter cloth 4 makes it easy to remove the cake from it. Helps make it easier. Next, the implementation results are shown. () Combination of upper and lower filter cloths (A) Example A

【table】

【table】 (B) Example B

【table】 (C) Comparative example C

【table】 () Results of implementation

[Table] The sample sludge is activated sludge from food industry wastewater with TS=
1.55%, SS=1.35%, and chemical injection conditions were 0.5% polymer and 10.0% FeCl ₃ per dry solid (DS). As described above, in the present invention, when water-containing sludge is sandwiched between two endless belt-shaped filter cloths that are continuously operated and dewatered by applying squeezing force during the operation process, polyamide fibers are attached to one of the filter cloths. By using a combination of the felt structure used for the web and a filter cloth woven from synthetic fibers for the other filter cloth, the cake can be completely adhered only to the filter cloth woven from synthetic fibers to create a felt structure. Because the cake is not attached to the filter cloth, the peelability of the dehydrated cake is extremely good, and as a result, it is possible to dehydrate activated sludge with a low water content, which is much lower than that of various conventional dewatering methods. 70
%, and in the case of urban sewage sludge, the cake thickness is 1 mm with a sludge cake moisture content of around 60%.
It has the effect of being able to make a dehydrated cake before and after.

[Brief explanation of the drawing]

Fig. 1 is a schematic side view showing an example of a continuous compression dewatering apparatus to which the method of the present invention is applied, and Fig. 2 is an enlarged model of the compression process of the method of the present invention, representative of one press nip section. FIG. 1... Rotating drum, 2... Pressing roller, 3...
Upper filter cloth, 4... Lower filter cloth, 5... Backup belt, 6... Supply section, 7... Scraper, 8... Cake, ,,,... Area, a... Total pressure distribution curve, c ...Cake internal water pressure distribution curve, f...
Felt internal water pressure distribution curve, s... end point of the area, n... nip center, m... minimum water content point,
t...Cake thickness curve.

Claims (1)

[Claims] 1 When dewatering water-containing sludge by sandwiching water-containing sludge between two continuously running endless belt-shaped filter cloths and applying squeezing force during the running process, a synthetic fiber woven cloth is used as one filter cloth, and the other By using a felt structure having a web of polyamide fibers as the cloth, a mutual relationship was created in which the return water from the latter filter cloth after compression reduces the adhesion of compressed sludge to the filter cloth and preferentially adheres to the former filter cloth. A continuous compression dewatering method for water-containing sludge, characterized by: