JPWO2008096585A1 - Filtration apparatus and water treatment method - Google Patents

Abstract

Filtration unit (A) for removing at least suspended particles in water in order to provide a practical filtration device capable of obtaining high quality clarified water continuously and at low cost by efficiently using a filtration unit Filtration to obtain clarified water, wherein a plurality of filtration units are connected in parallel, and a refiltration unit (B) for removing suspended particles in water again is connected to the downstream side of the plurality of filtration units (A) as necessary. Each of the filtration units (A) is a clarified water line for taking out the filtered water of the filtration unit (A) as clarified water, and the filtered water of the filtration unit (A) to the refiltration unit (B). A re-filtration unit supply line is provided, and the re-filtration unit (B) is a filtration device having a merging line that joins the filtered water of the re-filtration unit (B) to the clarified water line.

Description

  The present invention relates to a filtration apparatus and a water treatment method for removing suspended substances and the like in raw water to obtain clarified water, and more specifically, stable water quality even when raw water quality and filtration performance fluctuate. The present invention relates to a filtration apparatus capable of obtaining high-purity filtered water at a low cost and an operation method thereof.

  Water purification technology for producing drinking water and irrigation water from river water and other natural water has long been popularized and developed mainly by chemical means such as coagulation sedimentation and pressurized flotation, and physical means by sand filtration. . Sand filtration is roughly classified into gravity filtration that obtains clarified water through a sand tank by gravity and pressure filtration that performs filtration by applying pressure with a pump, and is appropriately selected according to the raw water quality and location conditions.

  In recent years, so-called seawater desalination, in which seawater is desalted to produce drinking water and irrigation water in response to further serious water shortages, has been put into practical use. Seawater desalination has been put to practical use mainly in the Middle East region, where water resources are extremely small and oil heat resources are extremely abundant. Even if there is no heat source nearby, fresh water can be obtained from seawater with high efficiency. Recently, improvement in reliability and cost reduction have progressed as a result of technological advances in reverse osmosis, and many reverse osmosis seawater desalination plants have begun to be built even in the Middle East, where heat sources are abundant.

  Normally, when seawater is passed directly through a reverse osmosis membrane, the membrane surface is damaged due to the invasion of suspended substances or organisms contained in the seawater, and the membrane performance (water permeability performance, blocking performance) decreases due to adhesion to the membrane surface. Therefore, attention should be paid to the quality of the seawater supplied to the reverse osmosis membrane. In other words, the conventional water purification technology is also required for desalination of seawater using reverse osmosis, and the use of coagulating sedimentation and pressurized levitation as needed, and clear water from which suspended solids and microorganisms have been removed by sand filtration. It is common to supply to a reverse osmosis membrane. Recently, in place of sand filtration, microfiltration membranes having submicron pores and ultrafiltration membranes having a separation performance of 0.01 micron level are being adopted.

  Here, in both sand filtration and membrane filtration, impurities such as suspended substances accumulate on the surface and inside of the filter media as filtration progresses, making it impossible to obtain a filtration flow rate within the allowable pressure range. Depending on the situation, impurities begin to leak from the filter media. In order to avoid this situation, a cleaning process of the filter medium is required. Specifically, “backwashing” in which clear water such as filtered water is passed at high speed in the opposite direction to filtration is generally applied. However, if the impurities clogged in the gap of the filter medium are removed by the cleaning process, the gap of the filter medium increases. For this reason, since the impurities are likely to leak for a certain period of time immediately after backwashing, that is, until impurities accumulate between the filter media and the gap becomes small, the quality of the filtrate water obtained does not satisfy the standard value. In the meantime, it is common to drain the water as clear water without sampling.

In the case of normal water treatment, the target water quality is that the turbidity after filtration (as defined in JIS K0101-1998) is less than 0.1 (Non-patent Documents 1 and 2). Some filtered water was not generated so much, and drainage time after backwashing was at most within one hour. On the other hand, in the case of pretreatment in seawater desalination, it is necessary to set SDI (Silt Density Index, defined by ASTM D4189-95), which is a general standard for the quality of water supplied to a reverse osmosis membrane, to about 3 to 4. . However, there are many cases where the SDI cannot satisfy this standard even when the turbidity is 0.1, which is a standard for clean water. For this reason, depending on the water quality of seawater, filtration conditions, etc., it may become difficult to satisfy the supply water quality of a reverse osmosis membrane within the drainage time after backwashing within 1 hour. Therefore, in the case of one-stage filtration as shown in FIG. 3, water immediately after backwashing with poor water quality as exemplified in Non-Patent Document 3 and Non-Patent Document 4 is supplied to the reverse osmosis membrane, or the drainage time is reduced. It is necessary to stop using until the water quality is improved for a long time, or to adopt a large-scale seawater desalination plant in Okinawa (Non-Patent Documents 5 and 6) or two-stage filtration as exemplified in Patent Document 1 and FIG. There have been increased replacement costs for reverse osmosis membranes, increased operating costs due to a decrease in the amount of filtered water, and increased equipment costs and operating costs due to two-stage filtration.
Hideto Yuda, Environmental Technology, No. 26, 576-579 (1997) Council for Health and Welfare, Review of Water Quality Standards Review, Base 49/25, (2003) Norihito Tanbo, Junichi Ogasawara, Water Purification Technology, Gihodo Publishing (1985), page 80 H. B. Din et al., IDA (International Desalination Association), World Congress Singapore, SP03-144 (2005) Hiroshi Iwahori et al., IDA (International Desalination Association), World Congress Singapore, SP05-209 (2005) Masaaki Ando et al., IDA (International Desalination Association), World Congress Bahamas, SP03-080 (2003) Japanese Patent Laid-Open No. 06-304559

  An object of the present invention is to efficiently remove suspended substances and the like in raw water, and in particular, to produce clear water having sufficiently high water quality as supply water for a semipermeable membrane unit at a low cost.

The present invention for solving the above-described problems is characterized by the following (1) to (15).
(1) A plurality of filtration units (A) that remove at least suspended particles in water are connected in parallel, and suspended particles in water are removed again on the downstream side of the plurality of filtration units (A) as necessary. Each of the filtration units (A) is a filtration device that connects the refiltration unit (B) to obtain clarified water, and each filtration unit (A) is a clarified water line for taking out the filtrate of the filtration unit (A) as clarified water; The refiltration unit supply line which supplies the filtration water of this filtration unit (A) to the refiltration unit (B) is provided, and the refiltration unit (B) uses the filtered water of the refiltration unit (B) as the clarified water. A filtration device having a merge line that joins the line.
(2) The total allowable treated water amount of the refiltration unit (B) is smaller than the total allowable treated water amount of the filtration unit (A), and the total allowable treated water amount of the refiltration unit (B) The filtration device according to (1), wherein the filtration device is equal to or greater than the unit allowable processing water amount of the unit (A).
(3) The filtration unit (A) and the refiltration unit (B) are both sand filtration units, and the particle size of the sand of the refiltration unit (B) is larger than the particle size of the sand of the filtration unit (A). The filtration device according to (1) or (2), which is small.
(4) The filtration device according to (1) or (2), wherein the filtration unit (A) is sand filtration, and the refiltration unit (B) is a microfiltration membrane or an ultrafiltration membrane.
(5) The filtration according to any one of (1) to (4), wherein the refiltration unit (B) has a drainage line for discharging the filtered water of the refiltration unit (B) to the outside of the system. apparatus.
(6) Branching from the refiltration unit supply line, or in parallel with the refiltration unit supply line, the drainage line for discharging the filtrate of the filtration unit (A) out of the system (1 )-(5).
(7) A water treatment method for obtaining clear water using the filtration device according to any one of (1) to (6), wherein suspended particles in water are removed by the plurality of filtration units (A). Each of the filtered water is obtained, and when each of the filtered water does not satisfy the water quality standard value, the filtered water that does not satisfy the water quality standard value is supplied to the refiltration unit (B), and suspended particles in the water are further supplied. The water treatment method which removes and joins the filtrate obtained by this filtration unit (B) with the filtrate of the filtration unit (A) which satisfies the said water quality reference value.
(8) A part of the plurality of filtration units (A) is intermittently washed, and the filtered water of the washed filtration unit (A) is re-filtered for a certain time immediately after the washing. ), And the filtered water of the other filtration unit (A) is taken out as clarified water.
(9) The water treatment method according to (7) or (8), wherein when the filtrate of the refiltration unit (B) does not satisfy a water quality reference value, the filtrate is drained out of the system.
(10) The supply water according to any one of (7) to (9), wherein when the supply water to the refiltration unit (B) does not satisfy another water quality reference value, the supply water is drained out of the system. Water treatment method.
(11) The water treatment method according to any one of (7) to (10), wherein the water quality reference value is SDI or turbidity.
(12) A water treatment method for obtaining clarified water using the filtration device according to any one of (1) to (6), wherein a part of the plurality of filtration units (A) is intermittently washed. For a certain time immediately after the washing, the filtered water of the filtration unit (A) that has performed the washing is supplied to the refiltration unit (B), and the filtered water of the other filtration units (A) is taken out as clear water. Water treatment method.
(13) A flocculant is added to the supply water of the filtration unit (A), and a flocculant of a different type from the supply water of the filtration unit (A) is added to the supply water of the refiltration unit (B). The water treatment method according to any one of (7) to (12).
(14) The water treatment method according to any one of (7) to (13), wherein the clear water obtained by the filtration device is further desalted.
(15) A water treatment device comprising a semipermeable membrane unit for desalting the clarified water of the filtration device on the downstream side of the filtration device according to any one of (1) to (6).

  According to the present invention, a plurality of filtration units (A) for removing suspended particles in water such as seawater and river water are connected in parallel, and downstream of the plurality of filtration units (A). Since the re-filtration unit (B) that removes suspended particles in the water again is connected accordingly, the filtered water of the filtration unit (A) can be taken out as clear water, and the water quality is sufficient. For example, the filtrate of the filtration unit (A) immediately after washing can be supplied to the re-filtration unit and processed, and the filtrate can be taken out as clarified water. As a result, the recovery rate of clarified water is increased, and the scale of equipment can be minimized, so that clarified water with excellent water quality can be stably obtained at low cost as a pretreatment for a semipermeable membrane at low cost. It becomes possible.

It is a schematic flowchart which shows one embodiment of the filtration apparatus which concerns on this invention. It is a schematic flowchart which shows the other aspect of the filtration apparatus which concerns on this invention. It is a schematic flowchart which shows the one aspect | mode of the conventional filtration apparatus (1 step | paragraph process). It is a schematic flowchart which shows the one aspect | mode of the conventional filtration apparatus (two-stage process). It is a figure which shows the time-dependent change of SDI in a reference example.

Explanation of symbols

1: Raw water 2: pH adjustment means 3: Flocculant addition means 4: Supply pump of filtration unit (A) 5: Filtration unit (filtration unit (A))
6: Filtration unit (re-filtration unit (B))
7: Supply pump of refiltration unit (B) 8: Supply valve of filtration unit (A) 9: Clear water valve 10: Backwash drain valve 11 of filtration unit (A) 11: Filtration water discharge valve 12: Refiltration unit ( B) supply valve 13: refiltered clarified water valve 14: backwash drain valve 15 of the refiltration unit (B) 15: refiltered water drain valve 16: filtrate drainage line 17: drainage line 18: clarified water tank 19: filtration Backwash water supply valve 20 of unit (A): Backwash water supply valve 21 of refiltration unit (B): Backwash pump 22 of filtration unit (A) 22: Backwash pump 23 of refiltration unit (B): Waste water Pit 24: Membrane filtration unit 90: Clear water line 91: Merge line

  First, a basic flow of a filtration apparatus according to the present invention and a water treatment method using the apparatus will be described with reference to the schematic diagram shown in FIG.

  In the filtration apparatus shown in FIG. 1, raw water 1 is supplied to filtration units 5 a to 5 d (filtration unit (A)) by a supply pump 4 through pH adjusting means 2 and coagulant adding means 3. The water treated by the filtration units 5a to 5d is stored in the clarified water tank 18 through the clarified water lines 90a to 90d provided with the clarified water valves 9a to 9d when the filtered water quality is good according to the filtered water quality. It has become. Moreover, when the quality of the filtrate water obtained from either of the filtration units 5a to 5d is not good, that is, when the filtrate water obtained from any of the filtration units 5a to 5d does not satisfy a predetermined water quality standard value, Close any of the clarified water valves 9a to 9d corresponding to the filtered water not satisfying the water quality standard value, and open any of the filtered water discharge valves 11a to 11d and the supply valve 12 to the filtration unit 6 instead. The filtered water that does not satisfy the water quality standard among the filtered water of the units 5a to 5d is supplied to the filtration unit 6 (refiltration unit (B)), and the suspended particles in the water are further removed by the filtration unit 6 to improve the water quality. It is possible. The filtered water of the filtration unit 6 is combined with the clarified water lines 90 a to 90 d through the merging line 91 provided with the refiltered clarified water valve 13 as clarified water, and is connected to the clarified water tank 18.

  The filtration units 5a to 5d and the filtration unit 6 include backwash pumps 21 and 22 so that backwashing is possible. Further, the backwash drainage is sent to the wastewater pit 23 through the drainage line 17 by opening the backwash drainage valves 10 a to 10 d and 14. Although not essential for the application of the present invention, the filtered water quality of the filtration unit 6 is not good due to the raw water quality and the configuration of the filtration units 5a to 5d. Is not satisfied, the refiltered clarified water valve 13 is closed and the refiltered water drain valve 15 is opened to discharge the filtered water from the filtration unit 6 out of the system, thereby preventing the water quality in the clarified water tank 18 from being deteriorated. It can be done. In addition, when the water quality of the filtration unit 6 satisfies the reference value, the water is sent to the clarified water tank 18 through the merging line 91.

  Furthermore, in the apparatus shown in FIG. 1, the filtered water immediately after backwashing is reused, for example, when the water quality is extremely poor, such as immediately after backwashing of the filtration unit (A), or when chemicals are used for backwashing of the filtration unit (A). When it is not suitable for supply to the filtration unit (B), the filtered water of the filtration units 5a to 5d can be discharged out of the system and the supply of water to the filtration unit 6 can be stopped. Specifically, it has the filtrate drainage line 16 of the filtration units 5a-5d. The filtration water quality of the filtration units 5a to 5d is observed with an online monitor as described later, and when the quality of the raw water quality is deteriorated or the filtration units 5a to 5d are troubled and the good quality cannot be expected even through the filtration unit 6 or filtration. When the unit 6 is damaged, the water flow to the filtration unit 6 is temporarily stopped to protect the filtration unit 6. As shown in FIG. 1, the filtrate drain line 16 may be provided in parallel with being completely separated from the supply line to the filtration unit 6 in addition to being branched from the supply line to the filtration unit 6.

  Here, the most realistic method for determining whether or not to stop the supply of water to the filtration unit 6 is to monitor the filtered water quality of the filtration unit (A) online. Examples of the online monitor include a turbidity meter, a fine particle counter, an SDI meter, a TOC meter, and an oil content meter. For the purpose of the present invention, a turbidity meter and an SDI meter are preferable. The turbidimeter applied to the present invention is preferably a high-precision turbidimeter with a measurement limit value smaller than 0.1, which is the water purification level, and for example, a scattered light laser turbidimeter is suitable. On the other hand, since the SDI meter requires time in units of minutes, when an SDI meter is used, it is preferable to use another online monitor.

  By the way, since the filtration unit (A) is clogged with time due to flocs generated by suspended substances and flocculants contained in the raw water, it is necessary to carry out backwash from time to time to remove clogging of the filter medium. There is. Here, the timing of backwashing is generally carried out by setting an interval in advance and periodically, or in the case of constant flow operation, when the filtration pressure exceeds a set upper limit. In addition, as clogging progresses, suspended matter and other substances gradually pass through the filter media, so-called breakthrough, and deterioration of filtered water quality is observed. Filtered water quality is monitored and water quality is set. Backwashing can also be performed when the value is exceeded.

  However, the filtration unit (A) after backwashing removes clogged suspended matter, etc., but suspended matter remains in the entire filter medium due to backwashing or suspended while the filtration resistance is reduced. It is not preferable to send the filtered water as clarified water to the clarified water tank 18 until the suspended matter leaks or the filter medium is clogged again to a certain extent and the filtration performance is improved. . Therefore, in the conventional technology that does not include the refiltration unit (B) as illustrated in FIG. 3, the water is drained through the filtration unit drain valves 11 a to 11 d for a while after the backwashing is finished and the filtration is resumed. . In the case of a general water purification treatment, this drainage is about 1 hour at most, but it is necessary to further improve the water quality in order to use it as the feed water for the reverse osmosis membrane.

  Therefore, as a result of intensive studies by the present inventors for the purpose of seawater desalination, in many cases where the filtration unit is appropriately designed and operating conditions are set, the filtered water obtained from the filtration unit (A) has an SDI of 4 It takes about 2 to 3 hours to reach below, and about 5 hours to reach below 3 and during that time the SDI of the clear water finally obtained by performing refiltration by the refiltration unit (B) is After the backwashing of the filtration unit (A), it should be surely 4 or less. Furthermore, when the SDI of the filtered water obtained from the filtration unit (A) becomes about 3, the SDI after passing through the refiltration unit (B). Turned out not to change.

  Furthermore, when the present inventors followed the water quality change over time of the raw water, the filtered water of the filtration unit (A), and the filtered water of the refiltration unit (B), the raw water was obtained by filtering with the filtration unit (A). In the filtered water, the suspended solid concentration decreases with time after backwashing, and reaches a certain concentration within 5 hours, but the soluble organic substance concentration exhibits almost the same removal performance immediately after backwashing and after the lapse of time. In addition, in the filtered water after further refiltering the filtered water of the filtration unit (A), the refiltration unit (B) until the suspended solid concentration in the filtered water of the filtration unit (A) reaches a steady state. The suspended solids concentration of the filtered water in the filtration unit (A) also decreases, but when the suspended solids concentration of the filtered water in the filtration unit (A) becomes steady, the suspended solids concentration that can be reduced by the refiltration unit (B) also reaches the steady state. Soluble organic Concentration revealed that express the same removal performance regardless of the elapsed time from the backwash of the filtration unit (A).

  From these results, if the filtered water quality of the filtration unit (A) after the suspended solid concentration reaches a nearly steady state after about 5 hours of backwashing is good, then the treatment by the refiltration unit (B) A conclusion was reached that it was unnecessary.

  In view of these, only the filtered water after backwashing in which the water quality is not sufficiently good in the filtration unit (A) is treated in the refiltration unit (B) until the water quality is good, and other than that of the filtration unit (A) It was concluded that it is efficient to obtain filtered water directly as clear water.

  In addition, when performing the above-mentioned washing with a plurality of filtration units (A), it is not carried out for all filtration units at the same time, but only for some filtration units, and the filtration units used for washing are sequentially switched. It is preferable to continue. By carrying out like this, the remaining filtration unit (A) can perform the normal driving | operation for obtaining clarified water, and as a whole filtration apparatus, clarified water can be obtained continuously stably.

  In addition, the necessity of the treatment in the refiltration unit (B) is as follows. The filtered water quality of the filtration units 5a to 5d (A) is monitored by the online water quality monitor as described above. However, in order to obtain a stable amount of water from multiple units, set the unit to pass through the filtration unit (B) for a certain period of time after backwashing the filtration unit (A). It is also preferable to keep it. In this way, it is easy to set an operation sequence such as switching to another unit.

  By the way, as a filtration unit applicable to this invention, it does not restrict | limit in particular, According to raw | natural water quality, such as general sand filtration (slow filtration, rapid filtration), activated carbon filtration, fiber filtration, membrane filtration, etc. It can be used as appropriate. In addition, as an auxiliary treatment before and after that, coagulation, precipitation, flotation, adsorption and the like can be used together, and can be appropriately selected according to the impurities contained in the raw water. In particular, it is preferable to remove in advance components that are difficult to remove with a filtration unit (A), such as a soluble polymer and oil, and for this purpose, a method of removing by adding an aggregating material and allowing it to settle or float Is effective.

  Moreover, in order to optimize the separation size of filtration, the particle size of sand, the pore size of the membrane and the like can be selected as appropriate. However, in the filtration unit (A), the filtration water quality for obtaining the desired water quality is basically available. In the refiltration unit (B), since it is required to remove what has leaked in the filtration unit (A), the filtration size of the re-pass unit (B) is smaller than the filtration size of the filtration unit (A). It is preferable. Specifically, for example, when limited to sand filtration, it is preferable that the particle size of the sand of the re-pass unit (B) is smaller than the particle size of the sand of the filtration unit (A). In addition, when there is no coarse filtration treatment that can remove relatively large suspended substances in the previous stage of the filtration unit (A), trapping of suspended substances is not limited to the surface when the filtration unit (A) is so-called multi-layer filtration. Since so-called depth filtration is performed, an increase in filtration pressure can be suppressed, which is preferable. On the other hand, in the refiltration unit (B), since the water which has already passed through the filtration unit (A) is supplied, in the case of sand filtration, single layer filtration is preferable. Further, when membrane filtration is applied, there is no particular limitation, but the object of the present invention is achieved when the filtration size (that is, the pore diameter) of the refiltration unit (B) is made smaller than the filtration size of the filtration unit (A). Is preferable. In general, membrane filtration can remove impurities with high accuracy without variation compared to other filtration methods, but is often inferior to sand filtration due to the strength of the membrane and recoverability against backwashing. Therefore, even when membrane filtration is applied, it is preferable to apply sand filtration to the filtration unit (A) and apply membrane filtration to the refiltration unit (B).

  FIG. 2 illustrates a flow when sand filtration is applied to the filtration unit (A) and membrane filtration is applied to the refiltration unit (B). In this flow, except that the membrane filtration unit 24 is arranged as the refiltration unit (B), and the filtration unit backwash drain valve 14 and the piping provided therewith in FIG. It is the same. In addition, membrane filtration can be performed by closing the re-filtered water drain valve 15 during filtration to “total filtration” for filtering the entire supply water, or filtering a part and concentrating suspended substances in the rest. It is also possible to use “cross flow filtration” in which the drained water is continuously drained from the drain valve 15. By adopting such a configuration, it is possible to draw out the excellent points of both types.

  The filter medium applicable to the present invention is not particularly limited. In sand filtration, dredged sand, anthracite, garnet, pumice, etc. are generally used. As a typical size, anthracite having an average particle size of about 1 to 3 mm and dredged sand having an average particle size of about 0.3 to 1 mm are often used in the case of multiple layers, but depending on the quality of raw water It is also possible to use pumice with a large particle size or finer garnet in combination. In the case of membrane filtration, in order to reduce the SDI which is the object of the present invention, a so-called microfiltration membrane having an average pore diameter of about 0.1 to 1 μm or an ultrafiltration of about 0.001 to 0.1 microns. It is preferred to use a membrane.

  In addition, when using a microfiltration membrane or an ultrafiltration membrane, the shape is not particularly limited, such as a flat membrane, a tube membrane, and a hollow fiber membrane, but the membrane area per volume can be increased. Therefore, a hollow fiber membrane is preferable. As a material, an inorganic material, a polymer material, or a composite material thereof can be used. However, in the case of a hollow fiber membrane, it is difficult to impart sufficient strength to the membrane, and damage due to material fatigue due to rocking is also possible. Since it is likely to occur, application of a polymer film is effective. Examples of the material for these hollow fiber membranes include polyacrylonitrile, polyimide, polyethersulfone, polyphenylene sulfide sulfone, polyvinylidene fluoride, polytetrafluoroethylene, polypropylene, and polyethylene. Examples of the pore structure include a porous symmetric membrane having a uniform pore structure, an asymmetric membrane having a dense portion on the surface, and a composite membrane composed of two or more kinds of materials.

  According to the results of intensive studies by the present inventors, a hollow fiber membrane made of polyvinylidene fluoride is superior from the viewpoint of mechanical strength, resistance to dirt, and chemical stability, and the supply water is groundwater or deep seawater. In the case of surface water such as surface seawater, river water, and lake water, an asymmetric membrane or a composite membrane having a dense portion on the surface of the supply water is excellent. ing. On the other hand, as a module structure, as a method of pressure filtration of supply water, a spiral type in which a flat membrane is wound in a laver shape, a pressure type in which a tube membrane or a hollow fiber membrane is housed in a container, a module that sucks the filtration side As the structure, an immersion type can be mentioned.

Furthermore, although there is no restriction | limiting in the processing capacity of a filtration unit (A) and a refiltration unit (B) single-piece | unit, and operating conditions from the main point of this invention, the refiltration unit (B) whole is a filtration unit (A). In view of the fact that the total allowable treated water amount of the refiltration unit (B) is smaller than the total allowable treated water amount of the filtration unit (A), the refiltration unit It is preferable that the total allowable treated water amount of (B) is equal to or larger than the single allowable treated water amount of the filtration unit (A). Further, in order for the refiltration unit (B) to be in time for the backup of the filtration unit (A), the base number of the filtration unit (A) is Na, the backwash interval is Ta ₁ , and the backwash + drainage time is Ta. _{2. When} the supply time Ta _{3 to} the refiltration unit (B), the base number of the refiltration unit (B) alone is Nb, the backwash interval is Tb ₁ , and the backwash + drainage time is Tb ₂
Tb ₂ / Tb ₁ ≦ 1- (Ta ₂ + Ta ₃ ) / Ta ₁ × Na / Nb
It is good to do.

  By the way, when the flocculant is added before the filtration unit (A) as described above, the kind of the flocculant is not particularly limited, and inorganic systems such as polyaluminum chloride, sulfate band, iron (III) chloride, and the like. It is possible to select from a wide range of flocculants and cationic, anionic and nonionic polymer flocculants. However, depending on the flocculant, it is preferable to adjust the pH at which the flocculent is easy to flocculate, for example, iron (III) chloride. In general, the pH is 5 or more and less than 8, preferably 7 or less. On the other hand, a flocculant can also be added in front of the refiltration unit (B). In this case, since a substance that could not be aggregated and removed by the filtration unit (A) is contained, it is effective to add a coagulant having a different type from the filtration unit (A), for example, a different charge.

The filtration apparatus of the present invention is not particularly limited to drinking water production, industrial water production, wastewater treatment, lake purification, etc., and is suitable for purifying various raw waters. Furthermore, the treated water obtained by applying the filtration device of the present invention can be used as it is according to the water quality standard, but a semipermeable membrane such as a reverse osmosis membrane is provided downstream of the filtration device of the present invention. It is also preferable to provide a unit and desalinate the clarified water obtained by the filtration device. If used as feed water for reverse osmosis membranes, etc., very clear water can be produced with high recovery and low cost. In particular, by applying the filtration device of the present invention to pretreatment of a large plant that produces fresh water from seawater, it is possible to supply low-cost, stable, clear seawater to the reverse osmosis membrane, greatly contributing to the reduction of seawater desalination costs. I can do it.
Here, the reverse osmosis membrane to be used is required to highly remove salt from seawater, but polymer materials such as cellulose acetate polymer, polyamide, polyester, polyimide, and vinyl polymer can be used. . In addition, the membrane structure has a dense layer on at least one side of the membrane, and on the asymmetric membrane having fine pores gradually increasing from the dense layer to the inside of the membrane or the other side, or on the dense layer of the asymmetric membrane. Either a composite membrane having a very thin separation functional layer formed of another material may be used.

  However, among them, a composite membrane having a high pressure resistance, high water permeability, and high solute removal performance and having an excellent potential and using polyamide as a separation functional layer is preferable. In particular, when seawater is used as raw water, it is necessary to apply a pressure equal to or higher than the osmotic pressure to the supply water side, and an operating pressure of at least 5 MPa is often applied substantially. In order to maintain high water permeability and blocking performance against this pressure, a structure in which polyamide is used as a functional layer and is held by a support made of a porous membrane or nonwoven fabric is suitable. Moreover, as the polyamide semipermeable membrane, a composite semipermeable membrane having a separation functional layer of a crosslinked polyamide obtained by polycondensation reaction of a polyfunctional amine and a polyfunctional acid halide on a support is suitable.

<Reference example>
Seawater (total solute concentration: 3.4 wt., Water temperature: 25 ° C., pH = 7.5) near Toray Industries, Inc. factory adjusted to pH = 6.2 with sulfuric acid (sulfuric acid consumption 26 mg / liter), flow rate After adding 15 mg / liter of iron (III) chloride at 250 liter / h, the mixture was stirred in a tank with a volume of 20 L, and then a pressure levitation device (width 200 mm × length 300 mm × height 2000 mm, levitation LV = 4 m / h, The pressure water pressure was 0.5 MPa and the circulating water volume was 30 liters / h). The treated water was supplied to the sand filtration unit (A) at the same flow rate of 250 liter / h and filtered. This filtration unit is a vertical type with a diameter of 20 cm, and 500 mm of sand (effective diameter 0.6 mm, uniformity coefficient 1.4) is used as a filter medium, and anthracite (effective diameter 1.4 mm, uniformity coefficient 1.4) thereon. 500 mm is laminated. The sand filter was operated at a backwash interval of 24 hours (23 hours of filtration operation + 1 hour of backwash time).

  Furthermore, the re-filtration unit (B) having the same structure as that of the filtration unit (A) except that the treated water of the filtration unit (A) is a single layer of cinnabar sand (effective diameter 0.35 mm, uniformity coefficient 1.4) as a filter medium. ), The whole amount was filtered at a backwash interval of 48 hours (47 hours of filtration operation + 1 hour of backwashing).

When SDI of each filtered water was measured immediately after backwashing of the filtration unit (A), it was as shown in FIG. Note that the SDI, using SDI measuring apparatus manufactured by Millipore Corporation, according to the method of ASTM D4189-95, of 15 minutes measured (i.e., _{SDI 15)} was carried out. Moreover, in the figure, the wavy arrow means the approximate estimation line of SDI, and the practice arrow means the timing when each filter unit is backwashed.

<Example 1>
From the above reference example, the flow shown in FIG. 1 is configured using four filtration units (A) (5a-5d) and one refiltration unit (B) (6), and operates in the sequence shown in Table 1. Then, the treated water up to 5 hours after backwashing the four filtration units (A) will be treated in the re-filtration unit (B), and clarified water with SDI ≦ 3 can always be produced at 23 m ³ / day. calculated. In addition, the total operation time including backwashing of the filtration unit (A) and the refiltration unit (B) is 116.5 hours / group, and the use amounts of the flocculant iron chloride (III) and sulfuric acid are 345 g / day, respectively. 600 g / day. Moreover, the ratio of the clarified water production amount to the water intake amount was 100%.

In the table, B, F, FB, R, and F1 to F4 are B = back washing, F = filtration, FB = filtration to re-filtration, R = pause, F1 to F4 = filtration of filtration units 5a-5d The operation state of water re-filtration is shown.

<Comparative Example 1>
As shown in FIG. 3, the flow which does not provide a refiltration unit (B) was assumed. From the reference example, when the refiltration unit (B) is not provided, the water quality of 5 hours in 24 hours is SDI> 3 with one filtration unit, and 1 hour of backwash is subtracted, and 18 hours, SDI ≦ 3 It became possible to produce clear water. That is, considering the same number of filtration units as in Example 1 (A + B = 5 units) in order to make the equipment costs the same, the clarified water is 22.5 m ³ / day, and the 5 filtration units (A) are backwashed. The total operating time is 120 hours / group, and the usage of the flocculant iron (III) chloride and sulfuric acid is 430 g / day and 750 g / day, respectively, and the amount of water produced is slightly less than in Example 1. As a result, the operation time and chemical consumption were large (that is, the operation cost was high). Moreover, the ratio of the clarified water production amount to the water intake amounted to 78%. Table 2 shows the operation sequence.

In the table, B, F, FB, R, and F1 to F4 indicate operating states of B = back washing, F = filtration, FB = filtration → refiltration, and R = pause.

Claims (15)

A plurality of filtration units (A) for removing at least suspended particles in water are connected in parallel, and a refiltration unit for removing suspended particles in water again if necessary on the downstream side of the plurality of filtration units (A). (B) is a filtration device for obtaining clarified water, wherein each filtration unit (A) comprises a clarified water line for taking out the filtrate of the filtration unit (A) as clarified water, and the filtration unit. It has a refiltration unit supply line that supplies the filtered water of (A) to the refiltration unit (B), and the refiltration unit (B) joins the filtered water of the refiltration unit (B) to the clarified water line. Filtration device that has a merging line. The total allowable treated water amount of the refiltration unit (B) is smaller than the total allowable treated water amount of the filtration unit (A), and the total allowable treated water amount of the refiltration unit (B) is equal to the filtration unit (A The filtration device according to claim 1, which is equal to or greater than a single permissible amount of treated water. The filtration unit (A) and the refiltration unit (B) are both sand filtration units, and the particle size of the sand of the refiltration unit (B) is smaller than the particle size of the sand of the filtration unit (A). Item 3. The filtration device according to item 1 or 2. The filtration device according to claim 1 or 2, wherein the filtration unit (A) is sand filtration, and the refiltration unit (B) is a microfiltration membrane or an ultrafiltration membrane. The said refiltration unit (B) is a filtration apparatus in any one of Claims 1-4 which has the waste_water | drain line which discharges | emits the filtered water of a refiltration unit (B) out of the system. It has a drainage line which branches off from the refiltration unit supply line, or in parallel with the refiltration unit supply line, and discharges filtrate of filtration unit (A) out of the system. The filtration apparatus in any one. A water treatment method for obtaining clarified water using the filtration device according to claim 1, wherein suspended particles in water are removed by the plurality of filtration units (A) to obtain filtered water, respectively. When each of the filtered water does not satisfy the water quality standard value, filtered water that does not satisfy the water quality standard value is supplied to the refiltration unit (B) to further remove suspended particles in the water. The water treatment method which joins the filtrate obtained by (B) with the filtrate of the filtration unit (A) which satisfies the said water quality reference value. A part of the plurality of filtration units (A) is intermittently washed, and the filtered water of the washed filtration unit (A) is supplied to the re-filtration unit (B) for a certain time immediately after the washing. And the water treatment method of Claim 7 which takes out the filtrate water of other filtration units (A) as clarified water. The water treatment method according to claim 7 or 8, wherein when the filtered water of the refiltration unit (B) does not satisfy a water quality reference value, the filtered water is drained out of the system. The water treatment method according to claim 7, wherein when the supply water to the refiltration unit (B) does not satisfy another water quality reference value, the supply water is drained out of the system. The water treatment method according to claim 7, wherein the water quality reference value is SDI or turbidity. A water treatment method for obtaining clarified water using the filtration device according to any one of claims 1 to 6, wherein a part of the plurality of filtration units (A) is intermittently washed, and immediately after the washing. A water treatment method in which the filtered water of the filtration unit (A) that has been washed is supplied to the re-filtration unit (B) for a certain period of time, and the filtered water of the other filtration units (A) is taken out as clarified water. The flocculant is added to the feed water of the filtration unit (A), and a flocculant of a different type from the feed water of the filtration unit (A) is added to the feed water of the refiltration unit (B). The water treatment method according to any one of 7-12. The water treatment method according to any one of claims 7 to 13, wherein the clear water obtained by the filtration device is further desalted. The water treatment apparatus by which the semipermeable membrane unit which desalinates the clarified water of this filtration apparatus is provided in the downstream of the filtration apparatus in any one of Claims 1-6.

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