The working principle of fiber ball filter, capsule extrusion fiber filter, pressure plate fiber filter and self-pressing fiber filter. The structural characteristics and existing problems are deeply analyzed from theory to practice. It is confirmed that the fiber filter has the advantages of high operating flow rate, strong sewage interception ability and good effluent quality.
Compared with granular filter materials. The specific surface area of the fiber filter material is larger, and there is a larger interface to adsorb and retain suspended matter.
At the same time, the fiber is relatively soft, which can realize the reasonable filtration method of density adjustment or gradually smaller filter pore size along the direction of water flow during filtration. Deep filtration has been realized to a great extent, and the water quality, sewage interception capacity and operating flow rate of the equipment have been greatly improved.
Fiber Ball Filter
Take a bundle of short fibers and tie or heat-melt them tightly in the center. Make the short fibers form a radial spherical structure. The individual characteristic of this kind of fiber ball is that the fiber in the center of the ball is dense. The closer to the edge of the ball, the looser the fiber, and the uneven distribution of porosity.
The fiber ball filter is filled with fiber balls in the container to form a bed. Because the individual fiber balls are relatively loose, the fiber filaments between the fiber balls in the bed can interpenetrate with each other. At this point the individual characteristics of the fiberballs are not important.
The bed forms a whole. The pressure on the fiber balls in the bed is the sum of the fluid resistance of the filtered water flow, the gravity of the fiber balls and the gravity of the suspended solids (if the water flow passes through the bed from top to bottom, the force in the filter layer along the flow direction is sequentially Increasing) .
Because the fiber ball has certain elasticity. Under pressure, the porosity of the filter layer and the filter pore size gradually change from large to small, and the specific surface area of the filter material gradually changes from small to large.
This is an ideal filtration method in which the filtration efficiency increases from low to high. Suspended solids with large diameters that are easy to filter out can be retained by the upper filter layer.
Suspensions with small diameters and difficult to filter out can be intercepted by the middle or lower filter layer. In the whole filter layer, mechanical sieving and contact flocculation are fully exerted. So as to achieve a higher filtration rate, interception capacity, and better effluent quality.
The disadvantages of this filter are:
- Because the fiber ball is a radial sphere, the fibers near the center of the ball are dense.
- Loosening cannot be achieved during backwashing, and it is difficult to completely remove the trapped dirt.
- The fiber balls are easy to lose when cleaning with air and water, and the fiber balls are easy to break when cleaning with mechanical agitation.
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Capsule Squeeze Fiber Filter
The long fiber bundle is hung in the equipment, the weight is hung under the fiber bundle, and several soft capsules are installed in the fiber layer. Before filtration, the capsules are filled with water, and the long fibers are squeezed horizontally so that the porosity of the fiber layer and the filter pore size are gradually distributed from large to small.
The characteristics of this section of the filter layer are similar to those of the fiber ball filter layer, and the filtering effect is also relatively close. The filtration rate of the filter is generally 20~40/h.
Under the condition of the same influent turbidity, the sewage interception capacity is 2~4 times that of the sand filter. When cleaning the filter, first drain the water in the capsule to loosen the long fiber bundle bed. Then wash with an air-water mixture.
Fillable and drainable soft capsules are installed in the hanging long fiber filter layer. The problems of compaction (filtering), loosening (cleaning) and fiber loss of the fiber layer are solved.
In the initial stage of filtration, the filter layer will generate a large thrust. The upper fibers close to the water outlet will be subjected to great longitudinal pressure.
Because the longitudinal stiffness of the fiber filament is extremely small, it must be bent under this pressure, so that the fiber layer moves up as a whole.
When the fiber layer intercepts a certain amount of suspended solids, the resistance of the bed head increases. This upward shift distance will be greater.
Since the center of the capsule is penetrated by a steel pipe (for water intake and drainage), the lower end of the capsule is connected to the lower end of the steel pipe (this is convenient for inserting into the fiber layer during installation). So the capsule will not move as the fiber layer moves up.
The outer wall of the capsule generates an upward pulling force under the friction of the fiber. On the other hand, lateral accumulation occurs due to the bending of the upper fibers under pressure. The increase in the occupied lateral space causes the upper capsule to be squeezed and its volume reduced. Force the volume of water in the lower part of the capsule to increase.
The lower part of the capsule is subject to both upward pulling force and internal outward pressure. This should be the main reason why the capsule is easy to break and mostly breaks from the lower part.
In addition, the compaction of the fiber by the capsule squeeze filter depends on the capsule. This takes up a larger filter area, and the filter linear velocity is highest at the point of maximum extrusion (ie, the point of minimum fiber porosity).
High filtration rate is not conducive to filter material adsorption and interception of suspended solids. Therefore, this part will not effectively trap suspended matter.
The upper fiber layer is compressed transversely and compacted. And the filtration area is large, so it should be this layer that plays a role in ensuring the water quality of the effluent.
Pressure Plate Fiber Filter
There are two structural forms of this filter.
One is that the pressure plate is on the upper part, and the other is that the pressure plate (also called thrust plate) is on the lower part.
Design idea: fix the long fiber bundles with the orifice plate. The other end is set on a pressure plate with a certain opening ratio (the pressure plate is designed as a double-layer structure or a floating bucket is added to make its overall density close to that of water).
In addition, a guiding and limiting device for the pressure plate is designed. In the initial stage of operation, it depends on the head resistance of the water flow on the pressure plate.
First, the fibers near the pressure plate are bent to increase the hydraulic head resistance and generate downward pressure.
Squeeze the lower fiber layer further. When the pressure plate is pressed down to an appropriate position, the limit device stops it from moving down. When backwashing, the overall density of the design of the pressure plate is close to that of water. It can be washed up by the water flow to stretch the fiber layer and realize the thorough cleaning of the fiber layer.
Self-Pressing Fiber Filter
Self-pressurized fiber filters have two structural forms.
One is that the water outlet hole plate is in the lower part. One is that the water outlet hole plate is on the upper part.
One end of the long fiber bundle is fixed on the water outlet hole plate, and the other end is connected with a component with extremely small mass that limits the relative position of the fiber bundle.
The so-called self-pressure means that it does not rely on other devices. The compression of the fiber layer is achieved only by the head resistance of the water flow on the fiber layer.
In fact, long-filament filaments have very little longitudinal stiffness. As long as the fibers are properly treated and a suitable packing density is maintained. Depending on the head resistance of the filter layer, the fiber layer can be compressed like a fiber ball filter.
When water flows through the fiber layer from top to bottom. Under the action of hydraulic head resistance, the fiber bears downward longitudinal pressure and the downward pressure on the fiber is greater as it goes down.
Due to the small longitudinal stiffness of the fiber, bending occurs when the longitudinal pressure is large enough. Then the fiber layer will move down as a whole, and the lowest fiber will be bent and compressed first.
This bending and compression process gradually moves up until the supporting force of the fiber layer is balanced with the head resistance of the fiber layer (the compression process takes 3 to 5 minutes). The longitudinal pressure on the fiber layer increases sequentially along the water flow direction.
Therefore, the degree of compression and bending of the fiber layer along the water flow direction also increases sequentially, and the porosity and filter pore size of the filter layer are distributed from large to small along the water flow direction. In this way, the ideal bed state for efficiently intercepting suspended matter is achieved.
Epilogue
Fiber filtration equipment has been used in China for more than ten years. With its incomparable advantages such as high operating flow rate, high sewage interception capacity and excellent effluent water quality, it has developed rapidly.
As an emerging filtration technology, there is still a lot of room for development in terms of filtration mechanism, material selection, equipment structure and field application. It needs to be further researched, developed and perfected.