JP2010201335A - Water treatment system and water treatment method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a water treatment system and a water treatment method which can reduce a bad influence on a membrane separator in real time by taking a state of a sand filter into consideration by detecting a filtrate of the sand filter. <P>SOLUTION: In the water treatment system including the sand filter 1 for sand-filtering raw water and the membrane separator 4 for membrane-separating the filtrate of the sand filtration, a coagulant supply device 20 for adding a coagulant is provided on the raw water side of the sand filter 1, a detection part 10 for detecting the coagulant is provided in a passage of the filtrate of the sand filter 1, and a controller 15 is provided which controls the coagulant supply device 20 based on signals from the detection part 10 so that the concentration of the coagulant becomes a predetermined value or less. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a water treatment system and a water treatment method including a sand filtration device that performs sand filtration after adding a flocculant to raw water, and a membrane separation device that performs membrane separation of the filtrate after sand filtration. The present invention relates to a technology for automatically controlling the amount.

  So-called water treatment systems that produce reverse osmosis membranes, nanofiltration membranes, ultrafiltration membranes, agricultural water, industrial water, domestic water, etc. using microfiltration membranes are very effective in separating components of fluid mixtures. is there. A typical assumption is that a pressurized fluid mixture is brought into contact with the membrane surface, and due to the difference in chemical potential, one or more components of the fluid mixture permeate the membrane and mass transport through the membrane. Separation is possible due to the difference in speed. Such a fresh water generation system using a separation membrane is treated in a water state suitable for the separation membrane by pretreatment in order to exhibit separation performance.

  For pretreatment, what is called membrane pretreatment using a separation membrane with a larger pore size and sand filtration are generally known, but sand filtration is still used because of the simplicity of cost and equipment principle. Has been. The sand filter uses a flocculant to agglomerate suspended substances and microorganisms in the raw water. However, the amount of flocculant added is temporarily different from the raw water quality due to seasonal fluctuations, temperature fluctuations, typhoons, etc. Since it changes according to changes in seawater conditions, etc., it is necessary to adjust the amount added.

  In the situation where the coagulant is insufficient, the coagulation effect becomes insufficient, and the substance that should be supplemented by the coagulant and the sand filtration device flows into the subsequent membrane separation device, and decreases the separation performance, or It becomes a factor which reduces the lifetime of a separation membrane. On the other hand, when the amount of the flocculant added is excessive, the flocculant itself flows into the membrane separation device, which causes a decrease in separation performance. That is, as for the addition amount of the flocculant, an addition amount that is not excessive or insufficient with respect to the changing raw water quality is desired. However, the amount of the present flocculant added is often adjusted depending on the quality of the final permeate and the experience value of the plant operator.

  With respect to the above problem, Patent Document 1 below describes a method of controlling the amount of addition by measuring the ultraviolet absorbance in the filtrate after membrane separation. In this document, after the admixture floc formation pond, after passing through the circulation tank and after the membrane separator, the ultraviolet absorbance is measured, and based on the result, the amount of the flocculant added to the admixture floc formation pond can be adjusted. Proposed.

  Patent Document 2 proposes a method of measuring turbidity upstream of a sand filter and downstream of a flocculant injection part and a sedimentation basin.

JP-A-8-117747 Japanese Patent Laid-Open No. 5-168819

However, in the content of the invention described in Patent Document 1, the following problems occur because the ultraviolet absorptiometer is detected after passing through the circulation tank and the membrane separation device. That is, since the detection unit and the part where the result is reflected are far apart and there is stagnation of treated water in the tank, there is a large time lag between the timing at which feedback is required and the detection timing. That is, even when an excessive flocculant is actually added, the behavior is detected later in time and process. In addition, since a detection portion exists after the membrane separation device, organic substances and inflow flocculants necessary for detection pass through the membrane separation device. That is, since the detection system is unavoidable for the passage of substances harmful to the membrane separation apparatus, the invention content includes factors that shorten the life of the separation membrane.

  Moreover, in the method of detecting turbidity on the upstream side of the sand filter device as in the method of Patent Document 2, it is impossible to detect this for the aggregate that temporarily leaks from the sand filter device. was there. For example, sand filtration equipment regularly has a washing process called backwashing that removes sediment deposited in the sand filtration tank by flowing sand filtration permeate in the direction opposite to the normal flow direction. In some cases, the flocculant or the agglomerate may flow out. In addition, since the sand filtration tank can not be used during backwashing, usually a plurality of sand filtration tanks are provided and pretreatment is performed using another sand filtration tank during backwashing, Even when switching sand filtration tanks, flocculants and agglomerates may flow out, and these structural problems adversely affect the membrane separation device, shortening the separation performance and the life of the separation membrane element. End up.

  Accordingly, an object of the present invention is to provide a water treatment system and a water treatment method that can reduce the adverse effects on the membrane separation device in real time by taking into account the state of the sand filtration device by detecting the filtrate of the sand filtration device. Is to provide.

The above object can be achieved by the present invention as follows.
That is, the water treatment system of the present invention is a water treatment system comprising a sand filtration device that performs sand filtration of raw water and a membrane separation device that performs membrane separation of the filtrate after sand filtration, on the raw water side of the sand filtration device. A flocculant supply device for adding the flocculant is provided, and a detection unit for detecting the flocculant is provided in the flow path of the filtrate of the sand filtration device, and the concentration of the flocculant is predetermined based on a signal from the detection unit. A control device for controlling the flocculant supply device so as to be equal to or lower than the value is provided. In particular, the control of the concentration of the flocculant is preferably controlled so that the concentration flowing into the membrane separation device is a predetermined value or less.

  According to the water treatment system of the present invention, since the flocculant in the filtrate of the sand filter is detected by the detection unit, the supply amount of the flocculant can be controlled in consideration of the state of the sand filter. Compared with the case of detection on the upstream side of the filtration device, the adverse effect on the membrane separation device can be reduced more reliably. In addition, the supply amount of the flocculant can be controlled in real time as compared with the case of detection on the downstream side of the membrane separation apparatus. As a result, it is possible to provide a water treatment system that can reduce the adverse effects on the membrane separation device in real time by taking into account the state of the sand filtration device.

  In the above, the flow path switching valve operated by the operation signal and the branch flow for branching the flow path through the flow path switching valve to the flow path downstream of the position where the detection unit of the filtrate flow path is provided. It is preferable that the control device performs control to switch the flow path switching valve to the branch flow path side when the concentration of the flocculant exceeds a predetermined value based on a signal from the detection unit. .

  According to this water treatment system, control is performed to switch the flow path switching valve to the branch flow path side so that the concentration of the flocculant does not exceed a predetermined value. Even when the flocculant (including those contained in the flocculent) leaks, it is possible to more reliably prevent the flocculant from flowing into the membrane separation device by discharging the filtrate from the branch channel. Can do.

  The detection unit includes a light receiving unit that receives light from a light source, and a bypass path that branches from the flow path of the filtrate is provided between the light source and the light receiving unit along the bypass path. It is preferable to form an optical path with a distance of 10 cm to 3 m.

  When a normal pipe is used to form an optical path perpendicular to the flow path, a sufficient length of the optical path cannot be secured and sufficient sensitivity is difficult to obtain. According to the above water treatment system, along the bypass path Since the optical path is formed, a sufficiently long optical path can be secured, and the detection sensitivity of the flocculant is improved.

  Moreover, it is preferable that the said flocculant is ferric chloride and the said detection part detects a flocculant using the light of wavelength 450nm or less. When using ferric chloride as a flocculant, detection suitable for the absorption spectrum of ferric chloride is possible by using light having a wavelength of 450 nm or less.

  On the other hand, the water treatment method of the present invention is a water treatment method in which the filtrate is subjected to membrane separation after sand filtration of raw water, and a flocculant is added to the raw water side of the sand filtration, and the sand filtration filter is used. The addition amount of the flocculant is controlled based on a signal obtained by detecting the flocculant in the liquid flow path so that the concentration of the flocculant becomes a predetermined value or less.

  According to the water treatment method of the present invention, in order to detect the flocculant in the filtrate after sand filtration, the supply amount of the flocculant can be controlled in consideration of the state of the sand filtration device. Compared with the case of detection on the upstream side, the adverse effect on membrane separation can be reduced more reliably. In addition, the supply amount of the flocculant can be controlled in real time as compared with the case of detection on the downstream side of the membrane separation. As a result, it is possible to provide a water treatment method that can reduce the adverse effect on membrane separation in real time in consideration of the state of sand filtration.

The schematic block diagram which shows an example of the water treatment system of this invention Sectional drawing of the principal part which shows an example of the principal part of the water treatment system shown in FIG. The flowchart which shows an example of control of the water treatment system of this invention The schematic block diagram which shows the other example of the water treatment system of this invention

  As shown in FIG. 1, the water treatment system of the present invention includes a sand filtration device 1 that performs sand filtration of raw water and a membrane separation device 4 that performs membrane separation of the filtrate after sand filtration. In the present embodiment, a seawater desalination plant will be described as an example. In the seawater desalination plant, a water storage tank 2 is provided between the sand filtration device 1 and the membrane separation device 4, and the filtrate of the water storage tank 2 is provided. Is guided to the high-pressure pump 3 and supplied to the membrane separation device 4 in a pressurized state. In the seawater desalination plant, a reverse osmosis membrane is generally used, and the filtrate that has been pressurized and passed through the reverse osmosis membrane is separated into permeated water and concentrated water.

  The sand filtration apparatus 1 is a filtration apparatus that uses particulate matter such as sand as a filter medium, and supplements suspended substances inside the particulate substance that is a filter medium. As the sand filter 1, both a slow sand filter and a quick sand filter can be used.

  For example, a slow sand filter is provided with a water collecting device at a lower part of a 2.5 to 3.5 m depth filtration basin, and a gravel layer (for example, thickness 40-60 cm) and a sand layer (for example, effective diameter 0). .3 to 0.45 mm, depth 70 to 90 cm), and filtration by gravity is performed. Purification is mainly biological.

  In the rapid sand filter, filtration is performed at a higher speed, but on the lower water collecting device, a gravel layer (for example, a thickness of 30 to 40 cm) and a sand layer (for example, an effective diameter of 0.45 to 0.7 mm, a depth). 60-70 cm) and filtering by gravity.

  These sand filtration apparatuses 1 perform backwashing (backwashing) of the filter medium when the degree of clarity and pressure loss exceed a certain value. Immediately after backwashing, there is a tendency for the amount of agglomerated material or agglomerated material to flow out of the filter medium temporarily.

  The water storage tank 2 has a function of storing the filtrate from the sand filtration device 1. The sand filter 1 and the water storage tank 2 are connected by a flow path such as a pipe, and a pump or the like for suction or transportation can be provided between them.

  The high-pressure pump 3 applies a predetermined pressure or more to the raw water of the membrane separation device 4, and the pressure of the pump is appropriately set according to the type of separation membrane of the membrane separation device 4. The water tank 2 and the membrane separation device 4 are connected by a flow path such as a pipe, and the high-pressure pump 3 is provided in the flow path.

  As the membrane separation device 4, a reverse osmosis membrane is generally used in a seawater desalination plant, but pretreatment with a separation membrane such as a microfiltration membrane and / or an ultrafiltration membrane may be further performed. Further, instead of the reverse osmosis membrane, a membrane separation device 4 using a separation membrane such as a microfiltration membrane and / or an ultrafiltration membrane may be used.

  The material which comprises a reverse osmosis membrane is not specifically limited, For example, various polymer materials, such as a cellulose acetate, polyvinyl alcohol, polyamide, polyester, can be used. Of these, a polyamide-based reverse osmosis membrane is preferable, and an aromatic polyamide-based composite membrane is particularly preferable.

  The membrane forms of reverse osmosis membranes include hollow fibers, flat membranes, tubular membranes, etc. Flat membranes can be used by incorporating them into spiral and frame and plate modules, and a plurality of hollow fibers are bundled. Can be used in a module.

  As the microfiltration membrane, a polymer organic membrane such as polyolefin, polysulfone, polypropylene, polyethylene, polystyrene, polyacrylonitrile, cellulose acetate or the like can be used. The pore diameter of the microfiltration membrane is preferably 0.01 μm or more and 10 μm or less.

  In addition, as the ultrafiltration membrane, a polymer organic membrane such as polysulfone, polypropylene, polystyrene, polyacrylonitrile, cellulose acetate, and polyethylene can be used. The pore size of the ultrafiltration membrane is preferably a fractional molecular weight of 20000 or more and a pore size of 0.01 μm or less.

  As shown in FIG. 1, the water treatment system of the present invention is provided with a flocculant supply device 20 for adding a flocculant to the raw water side of the sand filtration device 1. The flocculant supply device 20 includes a storage tank 21 for storing the flocculant solution and a pump 22 whose flow rate can be adjusted. Instead of the pump 22, it is also possible to use a valve or the like whose flow rate can be adjusted by the opening degree.

  In the illustrated example, the flocculant supply device 20 that supplies the flocculant to the pipe that supplies the raw water is shown. However, the flocculant supply device 20 is supplied directly to the sand filtration device 1 or mixed for mixing the raw water and the flocculant. A tank may be provided to supply the flocculant therein.

  As the flocculant stored in the storage tank 21, in addition to the flocculant stock solution, an aqueous solution or suspension thereof can be used. However, when controlling the supply amount, it is preferable to use a dilute solution of the flocculant.

  As the aggregating agent, inorganic aggregating agents such as iron chloride, sulfuric acid band, polyaluminum chloride, aluminum oxide and iron sulfate, or polymer aggregating agents such as polyacrylamide and polyacrylic acid cationized polymer can be used. When the coagulant is ferric chloride, it is possible to optimize the addition amount of the coagulant that is currently most commonly used, and the versatility is enhanced.

  The supply amount adjustment mechanism of the flocculant supply device 20 is preferably a mechanism capable of adjusting the flow rate by an electrical operation signal, and is preferably a constant flow pump capable of adjusting the flow rate by the operation signal.

  The water treatment system of the present invention includes a detection unit 10 that detects flocculant in the flow path of the filtrate of the sand filtration device 1, and based on a signal from the detection unit 10, the concentration of the flocculant becomes a predetermined value or less. Thus, a control device 15 for controlling the flocculant supply device 20 is provided. The flocculant detection unit 10 is installed between the sand filtration device 1 and the water storage tank 2, but it is desirable that the flocculant detection unit 10 be installed as close to the sand filtration device 1 as possible from the viewpoint of quickly feeding back the leakage of the flocculant. In addition, as a flow path of the filtrate which provides the detection part 10, the flow path after sand filtration inside the sand filtration apparatus 1 is also included.

  The detection unit 10 can be used as long as it can detect the presence or concentration of the flocculant according to the type of flocculant. For example, any of optical sensors using absorbance, chemical sensors for detecting pH, etc., electrical sensors for detecting electrical resistance, etc. may be used. Of these, optical sensors are preferably used.

  As shown in FIG. 2, the detection unit 10 using the optical sensors preferably has a light receiving unit 12 that receives light from the light source 11, and the detection unit 10 branches off from the flow path of the filtrate. It is preferable that a bypass path 14 is provided and the optical path 13 is formed along the bypass path 14. Specifically, a structure in which the light source 11 and the light receiving unit 12 are arranged to face each other so as to sandwich the optical path 13 is exemplified.

  Since the detection part 10 is provided in the bypass path branched from the piping between the sand filtration apparatus 1 and the water storage tank 2, and is passing the part of the optical path 13, it becomes in-line real-time detection. As for the optical path 13, when the detection system is an optical detection type, a structure that blocks light other than the light from the light source 11 is desirable.

  As a light source, since it is necessary to overlap with the absorption spectrum of the flocculant, when using ferric chloride, it is necessary to be a light source having a wavelength of 450 nm or less. A light source that emits ultraviolet rays can be used in principle, but from the viewpoint of strength, a mercury lamp, UV-LED, or ultraviolet laser is preferable.

  The light receiving unit 12 only needs to have a function of converting received light into an electric signal. For example, a photodiode or a photomultiplier tube can be used. preferable. The light source 11 and the light receiving unit 12 may be arranged so as to be in direct contact with the liquid. However, it is preferable that a window is provided in the optical path portion and the liquid is not contacted with the liquid.

  In this case, the optical path securing window needs to be made of a material that transmits ultraviolet rays, and for example, quartz glass or the like is used. Since the addition amount of the flocculant is generally several ppm, the optical path length of a normal spectrophotometer, that is, it is difficult to detect at 1 cm, so the optical path length is increased in order to increase the detection sensitivity. It is necessary. In the present invention, the optical path length is preferably 10 cm or more, and more preferably 100 cm or more. However, if the optical path length is increased in order to increase the sensitivity, it becomes difficult to construct the optical system this time. Therefore, the distance between the light source 11 and the light receiving unit 12 is preferably 10 cm to 3 m.

  The control device 15 controls the flocculant supply device 20 based on a signal from the detection unit 10 so that the concentration of the flocculant becomes a predetermined value or less. In the illustrated example, the signal output from the detection unit 10 is sent to the control device 15 that performs feedback control of the flocculant addition amount, and the supply amount is controlled in conjunction with the pump 22 of the flocculant supply device 20.

  The control by the control device 15 may process a signal (including data) from the detection unit 10 via a computer, and the control method includes on / off operation, proportional operation, integration operation, differentiation operation, a combination of these, For example, any of PID control and the like may be used. These controls can also be performed using a commercially available controller.

  For example, by setting the concentration of the flocculant as a measurement value and a target value, and using a commercially available controller that can output the operation signal of the pump 22 based on the deviation, the target value is set to a minute concentration. The flocculant supply device 20 can be controlled so that the concentration of the flocculant becomes a predetermined value or less. As the minute concentration to be set, it is preferable to set an upper limit concentration (for example, 0.1 ppm) in a range that does not affect the downstream separation membrane. In such a controller, PID control is generally performed.

  On the other hand, in the present invention, since feedback control is performed according to the concentration of the flocculant in the filtrate after passing through the sand filter 1, it is preferable to perform control in consideration of a time lag while passing through the sand filter 1. . Further, the supply amount of the flocculant is preferably controlled so as to be as large as possible as long as it does not leak from the sand filtration device 1 in order to suitably aggregate the suspended matter of the raw water. An example of such control is shown in FIG. Hereinafter, this control will be described.

  In this control, as shown in FIG. 3, first, the initial value of the supply amount of the flocculant is determined in advance according to the turbidity of the raw water. In addition, a variable corresponding to the counter is reset (N = 0, M = 3).

  The detection unit 10 always detects the concentration of the flocculant, calculates the average value of the concentration detected at regular time intervals, and sets it as input data (sampling data) at regular time intervals. The control device 15 is configured to wait for the input of this data and start the calculation by the input. After inputting the data, 1 is first added to the variable M. Next, it is determined whether or not the input data is equal to or less than a set value (for example, 0.1 ppm) of the concentration of the flocculant. If it is equal to or less, 1 is added to the variable N. Next, it is determined whether or not the variable N is 5 or more. If the subroutine does not exceed the set value of the concentration of the flocculant, the process from the data input to the addition of the variable N is repeated until the variable N becomes 5 or more. Is called.

  When the variable N becomes 5 or more, the corresponding operation signal is changed so that the subroutine is exited and the supply amount of the flocculant is increased. In the illustrated example, the supply amount of the flocculant is increased by 5%. The reason why the variable N is set to 5 or more is to exit the subroutine because the increase in the supply amount of the flocculant is reflected in the detected concentration and there is a time lag. Further, if the detected concentration does not exceed the set value during a certain waiting time, it is considered that the supply amount of the flocculant is insufficient.

  On the other hand, when the set value of the concentration of the flocculant is exceeded, it is further determined whether or not the variable M is 3 or more. The corresponding operation signal is changed. At this time, the variables N = 0 and M = 3 are reset. In the illustrated example, the supply amount of the flocculant is reduced by 10%. The reason why the variable M is set to 3 or more is to exit the subroutine because there is a time lag even though the decrease in the supply amount of the flocculant is reflected in the detected concentration.

  That is, even after reducing the supply amount of the flocculant, since the detected concentration does not decrease while passing through the sand filtration device 1, it is better to maintain the supply amount of the flocculant during this period. Considering the period, the variable M is 3 or more as a condition for reducing the supply amount. However, since the variable M is added every time data is input, if the density average value continues below the set value, the variable M is always 3 or more, and the density average value is set to the set value first. If exceeded, the supply amount is immediately reduced.

  By controlling as described above, it is possible to control the supply amount of the flocculant to be substantially maximized while bringing the concentration of the flocculant leaking from the sand filtration device 1 close to zero. That is, it is possible to provide a water treatment system that minimizes the influence on the membrane separation device while adding the maximum concentration at which the flocculant does not leak as an appropriate concentration.

  Next, a more preferred embodiment of the present invention will be described with reference to FIG. In this embodiment, as shown in FIG. 4, a flow path switching valve 25 that operates in response to an operation signal and a flow path switching valve on the flow path downstream of the position where the filtrate flow path detection unit 10 is provided. And a branch flow path 26 for branching the flow path via 25, and the control device 15 determines the flow path switching valve 25 when the concentration of the flocculant exceeds a predetermined value based on a signal from the detection unit 10. Is switched to the branch channel side.

  The predetermined value when the control for switching the flow path switching valve 25 is performed may be the same as or different from the predetermined value when the flocculant supply device 20 is controlled. For example, when both are made the same, the switching of the flow path switching valve 25 may frequently occur. For example, when the latter predetermined value is doubled the predetermined value of the former, the switching frequency of the flow path switching valve 25 is changed. Can be reduced.

  The filtrate branched from the branch flow path 26 can be discarded, but can also be recycled into the raw water flow path of the sand filtration apparatus 1 or the sand filtration apparatus 1.

  This embodiment is particularly effective in preventing flocculants and aggregates leaking after backwashing the sand filtration device 1 from flowing downstream. In addition, the control of the flocculant supply device 20 may not be able to cope with a change in the amount of suspended material in the raw water, but even in that case, the flow path switching valve 25 can be set immediately in response to the increase of the flocculant. It is possible to switch to the branch channel side.

  As described in detail above, the water treatment system of the present invention minimizes the influence on the membrane separation device while maximizing the coagulation effect, and has an optimum addition amount that varies depending on the season, temperature, weather, etc. It can be controlled without depending on the experience of the operator.

  On the other hand, the water treatment method of the present invention is a water treatment method in which the filtrate is subjected to membrane separation after sand filtration of raw water, and a flocculant is added to the raw water side of the sand filtration, and the sand filtration filter is used. The addition amount of the flocculant is controlled based on a signal obtained by detecting the flocculant in the liquid flow path so that the concentration of the flocculant becomes a predetermined value or less. The water treatment method of the present invention can be preferably implemented using the water treatment system of the present invention described above.

DESCRIPTION OF SYMBOLS 1 Sand filtration apparatus 4 Membrane separation apparatus 10 Detection part 11 Light source 12 Light reception part 13 Optical path 14 Bypass path 15 Control apparatus 20 Flocculant supply apparatus 25 Flow path switching valve 26 Branch flow path

Claims (5)

In a water treatment system comprising a sand filtration device that performs sand filtration of raw water and a membrane separation device that performs membrane separation of the filtrate after sand filtration,
While providing a flocculant supply device for adding a flocculant to the raw water side of the sand filtration device,
A detection unit that detects flocculant is provided in the flow path of the filtrate of the sand filtration device, and the flocculant supply device is controlled based on a signal from the detection unit so that the concentration of the flocculant becomes a predetermined value or less. The water treatment system characterized by providing the control apparatus which performs.   A flow path switching valve that operates according to an operation signal and a branch flow path that branches the flow path through the flow path switching valve are provided in the flow path rather than the position where the detection section of the filtrate flow path is provided, The water treatment system according to claim 1, wherein the control device performs control to switch the flow path switching valve to the branch flow path side when the concentration of the flocculant exceeds a predetermined value based on a signal from the detection unit.   The detection unit includes a light receiving unit that receives light from a light source, and a bypass path that branches from the flow path of the filtrate is provided. A distance between the light source and the light receiving unit is 10 cm to the bypass path. The water treatment system according to claim 1 or 2, wherein an optical path of 3 m is formed.   The water treatment system according to any one of claims 1 to 3, wherein the flocculant is ferric chloride, and the detection unit detects the flocculant using light having a wavelength of 450 nm or less. In the water treatment method for membrane separation of filtrate after sand filtration of raw water,
The flocculant is added to the raw water side of the sand filtration, and the flocculant has a concentration equal to or lower than a predetermined value based on a signal obtained by detecting the flocculant in the flow path of the filtrate of the sand filtration. The water treatment method characterized by controlling the addition amount.

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