JPH0356085B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD The present invention relates to a precoat filtration method in which a filter aid is once agglomerated and then precoated, followed by a filtration step of the stock solution. Prior Art In the conventional precoat filtration method, a commercially available filter aid is precoated as it is and the filtration process of the stock solution is started. The purpose of this precoating is to "improve the particle trapping effect" by improving the filtrate flow path formed by the filter aid, and is based on a physical action. Therefore, the particle trapping effect is affected by the particle size distribution, porosity, etc. specific to the filter aid, and high filtration efficiency cannot be obtained depending on the nature of the stock solution, for example, when it is colloidal. In such cases, in order to obtain a large amount of filtrate per unit time, it is necessary to use a filter aid with a coarse particle size distribution, but this does not result in a satisfactory clear solution; This resulted in a drawback that the desired amount of filtrate could not be obtained. Moreover, the pre-coat layer is relatively unstable and tends to crack or collapse during the pre-coating or filtration process, resulting in impossibility of filtration or extremely poor clarity of the filtrate. It was not always easy to continue the filtration operation. Problems of the Invention The present invention eliminates the deficiencies of the conventional precoating method as described above, and also (1) improves the effect of removing fine particles in the filtration stock solution, and (2) adds an adsorbent substance to the conventional filter aid. By doing so, it is an object of the present invention to provide a filtration method that also makes it possible to remove and recover soluble substances in the filtration stock solution, such as trace components such as phosphorus components, oils, nitrogen components, and heavy metals. Means for Solving the Problems Now, the filtration method of the present invention will be explained in detail. Filter aid is a porous powder that forms a precoat layer on the filtration surface of vacuum drum filters and pressurized leaf filters that have filter cloth or wire mesh attached as a filter medium, making it possible to improve the filtrate flow path three-dimensionally. Filter aids include diatomaceous earth, perlite,
There are cellulose, wood flour, activated carbon, pulverized carbon, etc.
Depending on the application, there are also mixtures of these products. Such filter aids are suspended in a precoat tank containing a relatively clear liquid such as water or filtrate. The concentration varies depending on the size of the tank and the type and size of the filter, but should be in the range of 0.1 to 30% by weight. Stir with the stirrer in the pre-coat tank, and add the flocculant to this. The flocculant is an inorganic flocculant such as polyaluminum chloride, aluminum sulfate, or iron salt, or a polymer flocculant such as polyacrylamide, starch derivative, chitosan, etc. The concentration of the flocculant added depends on the type of flocculant and the fine particle group in the filtration stock solution. Although it varies depending on the amount and nature of the amount, it is generally in the range of 0.1 mg/-10 g/. After stirring to uniformly mix the flocculant (usually after several minutes), precoating begins. In the case of a vacuum drum type pre-coat filter, the vacuum pump is driven to rotate a drum wrapped with wire mesh or filter cloth around the filter medium, and the filter aid suspension is poured from the pre-coat tank into the filter tank and the filter aid is coated on the surface of the filter medium. A layer is formed to complete the precoat. The thickness of the precoat layer is arbitrary depending on the desired continuous operation time, but it is usually 30 to 100 mm.
Before and after. In general, it is advantageous to apply thicker layers because the continuous operation time can be extended. Next, pour the stock solution into the filtration tank and start filtration. Particles in the stock solution are captured on the surface of the precoat layer to form a filter cake, which is scraped off with a scraper along with the surface of the precoat layer, and filtration is always performed using a fresh precoat surface. In a pressurized precoat filter, the liquid is pumped and filtered. There are various types of filter media, such as those with filter cloth or wire mesh similar to vacuum drum pre-coated filters, sheet-shaped ones, ceramics, ring-shaped ones, etc. Depending on the shape of the filter media, leaf filters,
There are filter presses, candle filters, etc. In the precoating method, the precoat tank and the filtrate are connected in a closed circuit, and the filter aid suspension is pumped through a pump to perform circulation filtration. The thickness of the precoat is 1
~30mm. Next, switch the filter to filtration of the stock solution and start filtration. To assist in understanding the method of the present invention, a system diagram of an example of a sludge filtration device using a filter aid is shown in the accompanying drawings. Note that this filtration device itself is known. In the figure, 1 is a slurry pump that pumps up the filtration solution mixed with sludge and sends it to a sand removal tank (desander) 2. The undiluted solution from which sand has been removed in the sand removal tank 2 is further sent to a sedimentation tank (desilter) 3. The stock solution, which has been precipitated to some extent in the settling tank 3, is pumped up by the second slurry pump 4 and sent to the head tank 5. The stock solution is sent from the head tank 5 to the precoat filter 6. On the other hand, a mixture of the filter aid and filtrate or water in the filter aid tank 8 is supplied to the filtration tank below the precoat filter 6 by a third slurry pump 9. The addition of the flocculant according to the invention takes place in this step. In the illustrated example, the precoat filter 6 is a vacuum drum filter, and the inside of the drum is evacuated by a vacuum pump 12. After a precoat of 30 to 100 mm has been completed with this precoat filter 6 as described above, the stock solution is supplied from the head tank 5 to perform clarifying filtration. In the figure, numeral 7 denotes an advancing knife or scraper, which scrapes away filter slag deposited on the pre-coated filter surface, thereby always performing filtration with a fresh filter surface. In the figure, 11 is a filtrate tank, which is a tank for collecting the filtrate filtered by the precoat filter 6. The filtrate thus obtained is supplied to a desired location by a filtrate pump 10. A product obtained by adding an adsorptive substance to the above-mentioned powder or granular filter aid is called a composite product, and hereinafter abbreviated as composite filter aid. This adsorbent substance can adsorb trace amounts of soluble substances in the filtration stock solution, such as phosphorus components, oils, nitrogen components, heavy metals, etc., and includes activated carbon, zeolite, calcium salts, white carbon, etc.
It is usually used in a range of 0.1 to 10% by weight. Examples Next, the present invention will be explained in more detail with reference to Examples. Example Using dredged bottom sludge from Kasumigaura as a filtration stock solution, a comparison experiment was conducted between the conventional precoat method and the precoat method of the present invention using a vacuum drum type precoat filter with a filtration area of 2.0 m ² . The results were as shown in the table below. Experiment No. in the table.
1 and 3 are the results obtained by the method of the present invention, and 2 and 4 are the results obtained by the conventional precoating method.

[Table] (Notes) Experimental method: 60 kg of diatomaceous earth filter aid was placed in a 400 capacity pre-coat tank and stirred.
3: 400g of polyaluminum chloride (PAC)
(concentration 1000 mg/) was added and stirred, and after 5 minutes precoating was started to form a 60 mm precoat layer, followed by dehydration filtration of bottom mud with a concentration of 100 g/. The cutting progress speed of the scraper is 0.05mm/
The filtration was performed at a drum rotation speed of 1 r.pm, and the filtration was terminated when the precoat layer reached 10 mm, and the extent to which the sludge particles had penetrated into the precoat layer was observed. A filter cloth was used as the filter medium, and although it was clean in Experiment Nos. 1 and 3, it was dirty in Experiments No. 2 and 4, and clogging with sludge was observed. The effects of the present invention are partially described in the Examples, but can be summarized as follows. (1) Compared to conventional methods, the precoat is more stable, and in the case of vacuum drum type precoat filters, there is no fear of collapse, and filtration operations can be maintained without interruption. (2) According to the method of the present invention, the precoat of the vacuum drum type precoat filter has a strong effect of capturing fine particles in the stock solution on the surface, as in the embodiment, and prevents the penetration of fine particles unlike the conventional precoat. Because there is no
There is no clogging on the filter media surface and the filtration speed is fast. Cleaning of filter media can also be significantly reduced. (3) Compared to the conventional method of adding a flocculant to a stock solution and performing flocculation treatment, the flocculation treatment solution of the method of the present invention can be reused, so the amount of flocculant can be reduced. (4) Trace soluble substances can be removed and recovered by the precoat filtration method using the composite filter aid of the present invention. Activated carbon, which is an adsorbent substance, is particularly effective for removing oil and heavy metals, zeolite is particularly effective for nitrogen components, calcium carbonate is particularly effective for phosphorus components, and white carbon is commonly used for substances with a low concentration of soluble substances. can. As is well known, the importance of "countermeasures against eutrophication of the water environment" has only increased in recent years. Nitrogen and It can contribute to the removal of phosphorus. The removal and recovery of heavy metals can also contribute to the treatment of metal plating wastewater, the recovery of trace metals in water, for example, the removal and recovery of mercury and cadmium, and the removal of heavy metal components in dredged sludge.

[Brief explanation of drawings]

The drawing is a system diagram of one embodiment of a filtration device used in the filtration method of the present invention. 1...Slurry pump, 2...Sand removal tank, 3...
... Sedimentation tank, 4 ... Slurry pump, 5 ... Head tank, 6 ... Precoat filter, 7 ... Advancing knife (scraper), 8 ... Filter aid tank, 9 ... Slurry pump, 10 ... Filtrate pump , 11...filtrate tank, 12...vacuum pump.

Claims (1)

[Claims] 1. Adding a single or composite filter aid powder having an average particle size of 3 to 150 μm to water or a relatively clear filtrate, and then stirring and mixing to suspend the filter aid. ,
A flocculant to obtain the desired concentration in the range of 0.1 mg/~10 g/ is added to this, and after stirring, a pre-coat filter layer is formed to a predetermined thickness using a pressure differential pre-coat filter, and the thus formed pre-coat A filtration method for precoating an agglomerated filter aid, characterized in that the filtration surface of a filter layer is cut with a scraper or the like so that a fresh precoated filtration surface always appears, and the undiluted solution is filtered using a pressure difference. 2. The filtration method according to claim 1, wherein the filter aid powder is made of diatomaceous earth, perlite, cellulose, carbon, or a mixture thereof. 3. Claim No. 3, wherein the composite product of powder and granular material for filter aid consists of powder and granular material for filter aid containing 0.1 to 10% by weight of an adsorbent substance such as activated carbon, zeolite, calcium salt, white carbon, etc. The filtration method according to item 1 or 2. 4. Any one of claims 1 to 3, wherein the flocculant is an inorganic flocculant such as polyaluminum chloride, aluminum sulfate, or iron salt, or a polymeric flocculant such as polyacrylamide, a starch derivative, or chitosan. Filtration method as described.