JP4800461B2 - Backwashing method in filtration equipment - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a backwashing method in a filtration apparatus that performs a filtration treatment after performing a coagulation treatment with a flocculant added.
[0002]
[Prior art]
In the case of producing purified water or industrial water using river water or the like as raw water, or in wastewater treatment or the like, coagulation sedimentation treatment and filtration treatment are widely used for separation of suspended substances. That is, in the coagulation sedimentation treatment, first, an aluminum-based coagulant or the like is added to and mixed with the raw water to make the suspended substances in the raw water coarse. And this coarse floc is introduce | transduced into a sedimentation basin, and most of suspended solids are removed by carrying out a sedimentation process. Next, the supernatant water (coagulation precipitation treated water) obtained in this sedimentation basin is further treated with a filtration device to further remove the remaining fine suspension. In this way, clear treated water is obtained.
[0003]
Here, in the filtration device, when the filter medium captures a predetermined amount of suspended solids, the filtration resistance increases and the outflow of flocs increases, so backwashing for regenerating the filter medium is performed. This backwashing is performed by backflowing backwashing water (usually treated water) through a filtration device and removing the trapped suspended matter.
[0004]
However, immediately after the backwashing, suspended substances (fine flocs) in the treated water increase, and sufficient filtration cannot be performed in many cases.
[0005]
This is because when aluminum flocculant is used in normal filtration treatment, aluminum hydroxide is thinly adhered to the surface of the filter medium, which contributes to the removal of fine flocculent flocs. Since the kimono is removed, the treated water immediately after backwashing is considered to deteriorate.
[0006]
Therefore, it has been proposed to perform a backwash by adding a flocculant to the backwash water. By such backwashing, a flocculant is supplied into the filter medium during backwashing, and adhesion of, for example, aluminum hydroxide on the filter medium surface is promoted. Accordingly, it is considered that fine floc can be sufficiently removed even immediately after back washing.
[0007]
[Problems to be solved by the invention]
However, even when a flocculant is added to the backwash water, it has been found that the amount of suspended solids in the treated water immediately after the end of backwash cannot be improved so much. This is because the conditions for attaching the flocculant to the entire filter medium are not sufficiently considered.
[0008]
This invention is made | formed in view of the said subject, and it aims at providing the backwashing method of the filtration apparatus which can perform a favorable filtration process immediately after backwashing.
[0009]
[Means for Solving the Problems]
The present invention, after the aggregation treatment with added Atsumarizai, in backwashing method in the filtration apparatus for performing a filtration treatment, at its backwashing process, poly aluminum chloride is added to a 40-80 mg / l, that the pH had rows backwash with the backwash water is controlled within a range of 5.0 ± 0.5, a film of aluminum hydroxide formed in filter material surface to adjust the zeta potential of the filter medium surface It is characterized by.
[0010]
Thus, according to the present invention, polyaluminum chloride (referred to as PAC) is added to the backwash water so as to be 40 to 80 mg / l, and the pH of the backwash water is within the range of 5.0 ± 0.5. And backwash. Thereby, the flocculant adheres to the surface of the filter medium of the entire filter layer, and the surface state of the filter medium can be made suitable for filtration, for example, the redox potential of the filter medium surface can be about 0 to −10 mV. Therefore, in the filtration after backwashing, the outflow of fine floc can be prevented from the beginning, and treated water with good water quality can be obtained. In addition, although what contains 10% of PAC as Al ₂ O ₃ is usually marketed, 40-80 mg / l in the present invention means that 40-80 mg of PAC having the above-mentioned concentration per liter of backwash water. In this case, the added amount of Al is 2.12 to 4.24 mg / l.
[0011]
In addition, it is preferable that the amount of the backwash water to be passed is 1.5 to 2.5 times the void volume of the filter medium in the filtration device. With such an amount of water, a suitable filter medium can be coated with the flocculant, and consumption of the flocculant and the acid can be maintained appropriately.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention (hereinafter referred to as embodiments) will be described with reference to the drawings.
[0013]
FIG. 1 is a diagram showing the overall configuration of the coagulation / separation apparatus according to the present embodiment. Raw water such as river water and lake water is first introduced into the mixing tank 10. The mixing tank 10 is supplied with the flocculant from the flocculant storage tank 12 by the flocculant pump 14. The flocculant is preferably an inorganic aluminum flocculant, and particularly preferably PAC (polyaluminum chloride). The mixing tank 10 is provided with a stirrer 16 so that the raw water and the flocculant are rapidly stirred. In this mixing tank 10, the coagulant-mixed water in which the coagulant is mixed flows into the coagulation tank 18. A slow stirrer 20 is disposed in the coagulation tank 18, and the coagulant admixture water is slowly stirred to coalesce and coarsen the coagulation flocs.
[0014]
Next, the coagulant-mixed water after slow stirring from the coagulation tank 18 flows into the inclined plate settling tank 22. The inclined plate settling tank 22 is divided into an inlet side and an outlet side by a partition plate 22a, and a settling portion 22b having a deep tank depth is formed on the inlet side. And the lower part of this sedimentation part 22b is the sludge storage part 22c which stores sedimentation sludge. A large number of inclined plates 22d are arranged on the outlet side to form inclined plate precipitation portions 22e. The flocculant-mixed water flows into the precipitation portion 22b, where it is precipitated, and then passes under the partition plate 22a and passes upward through the inclined plate precipitation portion 22e. Then, when passing through the inclined plate 22d of the inclined plate settling portion 22e, further precipitation processing is performed, and sludge is precipitated toward the tank bottom. Since the tank bottom of the inclined plate sedimentation part 22e is inclined deeper toward the sludge storage part 22c, the sediment sludge moves to the sludge storage part 22c by gravity. Then, the supernatant that has passed through the inclined plate settling portion 22e is discharged from the inclined plate settling tank 22. In addition, the sludge settled in the sludge storage part 22c of the inclined plate sedimentation tank 22 is suitably extracted and disposed of separately.
[0015]
By such a coagulation sedimentation process, a considerable part of the suspended solids is removed from the sedimentation water from the inclined plate sedimentation tank 22. This precipitated treated water is introduced into the filter 24. The filter 24 is a gravitational rapid filter having two filtration layers of an anthracite filtration layer 24b and a sand filtration layer 24a. In some cases, additional flocculant or coagulant aid may be injected into the precipitation-treated water, stirred with a line mixer, and supplied to the filter 24. The filter is a pressure filter. Also good.
[0016]
A filter medium other than anthracite may be used as the filter medium for the filter layer 24b, or the filter layer 24b itself may be omitted. Moreover, it is also suitable for the filtration layer 24a to use garnet or the like instead of sand, or to make the sand and garnet multilayer.
[0017]
Then, the filtered water of the filter 24 is distributed in the treated water tank 26 and then distributed.
[0018]
The treated water in the treated water tank 26 can be supplied to the bottom of the filter 24 by the backwash pump 28. Therefore, the treated water is supplied to the filter 24 in an upward flow so that the filtration layer in the filter 24 can be backwashed.
[0019]
That is, as the water flow continues, the suspended substance trapped in the filtration layer gradually increases, the filter medium is saturated, and the filter 24 cannot capture the suspended substance any more. This can be detected by an increase in filtration resistance, an increase in treated water turbidity, or the like. Therefore, the filter medium is regenerated by washing before the filter medium is completely saturated.
[0020]
The timing of this cleaning is performed by setting a timer based on empirically obtained time or setting a filtration resistance by a differential pressure gauge. Further, the backwashing is performed by combining backwashing using filtered water or backwashing with surface washing or air washing.
[0021]
Here, in this embodiment, the flocculant from the flocculant storage tank 30 is added by the flocculant pump 32 to the backwash water line from the backwash pump 28 to the filter 24. As this flocculant, PAC may be used similarly to the flocculant storage tank 12 described above, and the flocculant from the flocculant storage tank 12 may be added. Further, an acid (acid solution such as sulfuric acid) from the acid storage tank 34 is added by the acid pump 36.
[0022]
Thus, in this embodiment, in addition to normal backwashing, backwashing with backwashing water containing a flocculant is performed, thereby forming a flocculant coating on the filter medium surface. That is, a predetermined amount of backwash water adjusted to a predetermined pH and added with a predetermined amount of flocculant is passed through the filter 24, thereby forming a film of aluminum hydroxide on the surface of the filter medium.
[0023]
In particular, in this embodiment, a pH meter 38 and a flow meter 40 are provided in the backwash line, and the detection result is supplied to the controller 42. Then, the controller 42 controls the driving of the flocculant pump 32 and the acid pump 36 based on these detection results, and controls the pH of the backwash water and the addition amount of the flocculant to be a predetermined amount. Control the amount of backwash water to which the agent and acid are added.
[0024]
By passing a predetermined amount of backwash water adjusted to such a predetermined pH and added with a predetermined amount of flocculant through the filter 24, an appropriate flocculant film is formed on the surface of the filter medium. Is controlled so that the zeta potential of the filter medium of the filtration layer 24a is 0 to -10 mV. That is, when the aluminum hydroxide adheres to the surface of the filter medium, the zeta potential of the filter medium increases. Further, since the zeta potential is changed by the addition of acid, the amount of acid added is also adjusted. Thereby, the zeta potential on the surface of the filter medium is controlled to 0 to -10 mV.
[0025]
Here, the zeta potential of the filter medium indicates an adhesion state of aluminum hydroxide to the filter medium surface. Therefore, by setting the zeta potential of the filter medium to a predetermined value (0 to -10 mV), the surface state of the filter medium can be adjusted to a state in which fine flocs can be easily captured. And by attaching the aluminum hydroxide to the surface of the filter medium with the backwash water adjusted as described above, in the filtration after the backwash, the fine floc is prevented from flowing out from the beginning, and the water quality is good. Treated water can be obtained.
[0026]
Moreover, in this embodiment, backwashing is performed in the following procedure. (I) First, backwashing is performed by allowing the filtered water to flow back as it is. For example, LV (empty linear velocity) = 40 m / h × 8 minutes. As described above, air backwashing may be combined. (Ii) Next, backwashing with backwashing water to which a flocculant (PAC) and an acid are added is performed. LV = 40 m / h × 1 minute. (Iii) Next, it rests in a stationary state for 30 seconds.
[0027]
In this way, backwashing is performed in which backwashing with backwashing water to which an aggregating agent and an acid as a zeta potential adjusting agent are added is performed for a short period of time, followed by a rest period. As a result, the flocculant and the filter medium are sufficiently mixed, a time for contact is obtained, and the aluminum hydroxide is sufficiently adhered to the surface of the filter medium.
[0028]
In particular, the backwash water to which the flocculant and acid are added according to the present embodiment uses PAC (a liquid containing 10% of the active ingredient Al ₂ O ₃ ) as the flocculant, and the addition amount is 40 to 80 mg / L. . The pH of the backwash water is 5.0 ± 0.5, and this is the total amount of water, which is 1.5 to 2.5 times the void volume of the filter medium in the filter 24. Through such circulation of the backwash water to the filter medium, a process for adjusting the zeta potential of the filter medium as described above to 0 to -10 mV can be performed. That is, by adjusting the concentration of PAC to this extent, an appropriate aluminum hydroxide film can be formed on the filter medium surface. Even if a high-concentration flocculant is passed through, the diffusion efficiency of the flocculant is deteriorated and an effective film formation cannot be performed. If the concentration is low, the amount of water passing is increased and a sufficient film formation cannot be performed. Further, even if the backwash water is increased, there is no adverse effect on the zeta potential control of the filter medium. However, meaningless backwashing is performed, and energy and coagulant are wasted. Therefore, it is preferable to control within the above range.
[0029]
As described above, in this embodiment, the amount of flocculant added to the backwash water is adjusted so that the zeta potential of the filter medium is 0 to −10 mV. This zeta potential indicates the electrical potential that exists across the solid-liquid interface and indicates the surface charge for suspended matter in water. Normally, suspended substances (viscosity components, algae, etc.) contained in river water are negatively charged, and each suspended substance is negatively charged. It has become. The aggregating agent first neutralizes this potential to weaken the repulsive force, and then agglomerates or aggregates. Therefore, it is desirable that the zeta potential of the aggregate floc is closer to the neutral point, that is, zero. Usually, the zeta potential of suspended substances in raw water is −20 mV or less, and the zeta potential of aggregated floc is −10 mV or more.
[0030]
Here, in the coagulation sedimentation / rapid filtration method generally used in water purification treatment, it is known that minute floc flows out from the rapid filter, particularly immediately after the start of filtration.
It is known that the zeta potential of this minute floc is as low as −15 mV or less and aggregation is not sufficiently performed.
[0031]
The filter medium is negatively charged as it is in the suspended substance in water. By setting the zeta potential of the filter medium to −10 mV or more, the adhesion of the flocculant hydroxide to the filter medium surface can be made sufficient. And by forming this deposit | attachment, the capture | acquisition ability of a floc can be improved and the raise of the suspended solid concentration in a treated water can be prevented. It is not preferable to set the zeta potential to 0 mV or more because it is not economical and the aluminum concentration in the cleaning wastewater becomes high.
[0032]
In this embodiment, PAC (a liquid containing 10% of the active ingredient Al ₂ O ₃ ) is used as the flocculant, and the amount added is 40 to 80 mg / L. The pH of the backwash water is 5.0 ± 0.5, and this is the total amount of water, which is 1.5 to 2.5 times the void volume of the filter medium in the filter 24. By such circulation of the backwash water to the filter medium, the zeta potential of the filter medium is adjusted to 0 to -10 mV.
[0033]
【Example】
Experiments were performed using the apparatus of FIG.
[0034]
"Experimental conditions"
・ Raw water flow rate: 3000m ³ / D
Mixing tank: residence time 4 minutes, G value 250 to 400 s ⁻¹
・ Sedimentation basin: Sedimentation basin with upward flow type inclined plate, residence time 40 minutes, rising speed 5 cm / min
・ Filtration pond specifications: φ5000mm × 5000mm × H4000mm (filtration area 25m ² )
-Filtration rate (LV): 5 m / h (120 m / d)
Filter medium: Silica sand Specific gravity 2.5m, porosity 45%, effective diameter 0.6mm, uniformity coefficient 1.4, filter layer height 600mm
・ Water flow time: 48 hours (Washing starts with a timer)
-Raw water turbidity: 8-30 degrees-Raw water pH: 7.2-7.5
-Flocculant: PAC 10-30 mg / l
・ Target treatment water turbidity: less than 0.1 degree ・ Washing condition: water backwash LV = 40 m / h × 8 minutes Water volume = 25 m ² × 40 m / h × (8 ÷ 60) = 133.3 m ³ / times PAC + sulfuric acid Water backwash LV = 40 m / h × 1 minute water amount = 25 m ² × 40 m / h × (1 ÷ 60) = 16.7 m ³ / filter medium void volume = 25 m ² × 0.6 m × 0.45 = 6.75 m ³
As 2.5 times the amount of water, 6.75 m ³ × 2.5 = 16.9 m ³ / times backwash water: filtered water is used. Adjust the pH to 5.0 ± 0.5.
PAC: Adjusted to be 40-80 mg / L.
[0035]
"Experimental result"
Add 10 mg / L of PAC as a flocculant to raw water with a turbidity of 8 degrees, add sulfuric acid as an acid in order to adjust the pH to 7.0, and mix using the stirrer 16 in the mixing tank 10. The mixture was gently stirred at 18 and subjected to precipitation treatment in an inclined plate settling tank (may be without an inclined plate) 22 and then supplied to a filter 24 to obtain treated water.
[0036]
By the way, if the filter 24 passes water to some extent, the gap between the filter media is saturated due to turbidity, and the water cannot be continued. The time until the filtration cannot be continued depends on the turbidity concentration in the inflow water of the filter, the water flow rate, and the like, and is usually about 24 to 72 hours. In this experiment, backwashing was performed every 48 hours. .
[0037]
The backwash condition, Mizugyakuarai the LV = 40m / h × 8 minutes, Mizugyakuarai by (i) PAC60mg / L and 75% pH 5 to ± 0.5 flocculant added water was 20 mL / ^{m 3} inject sulfate Was performed at LV = 40 m / h × 1 min (2.5 times the pore volume of the filter medium 6.75 m ³ ). In addition, after the water backwashing which added this flocculant, the stationary time was set to about 30 second, and adhesion to the filter medium surface of the flocculant was accelerated | stimulated.
[0038]
By such treatment, it was confirmed that the treated water was not deteriorated immediately after backwashing and the target turbidity could be maintained continuously.
[0039]
【The invention's effect】
As described above, according to the present invention, the addition amount of the flocculant and the pH adjuster to the backwash water is controlled to a preset value. Thereby, the flocculant adheres to the surface of the filter medium, and the surface state of the filter medium can be made suitable for filtration. Therefore, in the filtration after backwashing, the outflow of fine floc can be prevented from the beginning, and treated water with good water quality can be obtained.
[0040]
In particular, the coagulant is polyaluminum chloride (referred to as PAC), and this PAC (10% Al ₂ O ₃ ) is added to the backwash water so as to be 40 to 80 mg / l. Control within the range of 0 ± 0.5, and the adjusted backwash water flow rate is 1.5 to 2.5 times the void volume of the filter medium in the filter device. Coagulant coating can be performed.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a configuration of an apparatus according to an embodiment.
[Explanation of symbols]
10 Mixing tank, 18 Coagulation tank, 22 Inclined plate sedimentation tank, 24 Filter, 30 Coagulant storage tank, 32 Coagulant pump, 38 pH meter, 40 Flow meter, 42 Controller.

Claims (4)

In the backwashing method in the filtration apparatus that performs the filtration treatment after the aggregation treatment with the addition of the flocculant,
In the backwashing step, polyaluminum chloride is added so as to be 40-80 mg / l, and backwashing is performed using backwashing water whose pH is controlled within the range of 5.0 ± 0.5, A backwashing method in a filtration device, wherein a film of aluminum hydroxide is formed on the surface of a filter medium to adjust a zeta potential on the surface of the filter medium. The method of claim 1, wherein
A backwashing method in a filtration device, wherein the amount of water flow of the backwashing water is 1.5 to 2.5 times the void volume of the filter medium in the filtration device. The method according to claim 1 or 2, wherein
The zeta potential is a backwash method in which the zeta potential is in the range of 0 to -10 mV. In the method of Claims 1-3,
After completion of the backwash by the backwash water, suspended in a stationary state, backwash method in the filtration apparatus characterized by contacting the backwash water of the coagulant and filter media.

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