JP7586657B2 - Method for operating water treatment device and water treatment device - Google Patents

Description

The present invention relates to a method for operating a water treatment device and a water treatment device.

Water treatment devices that have a reverse osmosis membrane (RO membrane) are known as devices that remove impurities from the water to be treated. In these devices, the water to be treated (raw water) is supplied to the RO membrane at a specified supply pressure, and the RO membrane separates the water to be treated (permeated water) and concentrated water. This makes it possible to obtain treated water (permeated water) from which impurities have been removed.

Water treatment equipment with RO membranes is required to operate stably for a long period of time, and to achieve this, it is important to prevent silica in the raw water from precipitating on the surface of the RO membrane to cause scale formation. In response to this, methods for suppressing the formation of silica scale have been known, including a method of adjusting the pH of the concentrated water to 6 or less to delay silica precipitation even if the silica concentration in the concentrated water is equal to or higher than the silica solubility (see, for example, Patent Document 1), and a method of increasing the silica solubility by adjusting the pH of the raw water to 9.5 or more (see, for example, Patent Document 2).

Patent No. 3187629 Patent No. 4496795

However, in all of the above methods, additional measures must be taken, such as adjusting the pH of the permeate or installing another RO membrane downstream. Especially in small and medium-sized water treatment equipment, there are limitations to taking such additional measures due to installation space and costs.

The object of the present invention is to provide a method for operating a water treatment device and a water treatment device that suppresses clogging of the reverse osmosis membrane due to silica scale and enables stable operation over a long period of time.

In order to achieve the above-mentioned object, a method for operating a water treatment apparatus of the present invention includes a turbidity clarifier that removes suspended solids contained in water to be treated, a reverse osmosis membrane device that is provided downstream of the turbidity clarifier and separates the water to be treated that has passed through the turbidity clarifier into permeate and concentrated water, and a pump that is provided upstream of the reverse osmosis membrane device and pressurizes the water to be treated that is supplied to the reverse osmosis membrane device, the reverse osmosis membrane device having a polyamide-based reverse osmosis membrane with a corrected flux of 1.40 m/d/MPa at 25°C or more, the method including: The method includes a step of performing separation using a reverse osmosis membrane device while adding a scale inhibitor consisting of 2-phosphonobutane-1,2,4-tricarboxylic acid and a mixture of acrylic acid and a terpolymer of (meth)acrylic acid/2-acrylamide-2-methylpropanesulfonic acid/substituted (meth)acrylamide so that the silica concentration in the concentrated water does not become equal to or higher than the silica solubility at a previously measured water temperature, and storing the permeated water in a treated water tank; and a step of flushing the primary side of the reverse osmosis membrane device with the water to be treated that has passed through the clarifier after stopping the separation using the reverse osmosis membrane device.

The water treatment device of the present invention includes a turbidity eliminator that removes suspended solids contained in the water to be treated, a reverse osmosis membrane device that is provided downstream of the turbidity eliminator and separates the water to be treated that has passed through the turbidity eliminator into permeate and concentrated water, the reverse osmosis membrane device having a polyamide-based reverse osmosis membrane with a correction flux of 1.40 m/d/MPa at 25°C or higher, a pump that is provided upstream of the reverse osmosis membrane device and pressurizes the water to be treated that is supplied to the reverse osmosis membrane device, a chemical injection device that is provided upstream of the reverse osmosis membrane device and adds a scale inhibitor consisting of 2-phosphonobutane-1,2,4-tricarboxylic acid and a mixture of acrylic acid and a terpolymer of (meth)acrylic acid/2-acrylamido-2-methylpropanesulfonic acid/substituted (meth)acrylamide to the water to be treated before or after passing through the turbidity eliminator, and a control device that executes the operating method described above.

This water treatment device and operating method can suppress the generation of silica scale without requiring installation space for additional measures or incurring extra costs.

As described above, the present invention makes it possible to suppress clogging of the reverse osmosis membrane due to silica scale, enabling stable operation over a long period of time.

1 is a schematic diagram showing a configuration of a water treatment device according to an embodiment of the present invention. 1 is a graph showing changes in flux retention over time in an example and a comparative example.

The following describes an embodiment of the present invention with reference to the drawings.

Figure 1 is a schematic diagram showing the configuration of a water treatment device according to one embodiment of the present invention.

The water treatment device 10 of this embodiment is a device that removes impurities contained in raw water (water to be treated) to produce treated water, and includes a turbidity removal device 11, a reverse osmosis membrane (RO membrane) device 12, and a treated water tank 13. The turbidity removal device 11 has a turbidity removal membrane that captures and removes suspended matter contained in the raw water. A microfiltration membrane (MF membrane) is used as the turbidity removal membrane, but is not limited to this as long as it can remove suspended matter, and for example, an ultrafiltration membrane (UF membrane) may be used. The RO membrane device 12 is provided downstream of the turbidity removal device 11 and separates the turbidity removal water from which suspended matter has been removed by the turbidity removal device 11 into concentrated water containing impurities and permeate from which impurities have been removed, and has an RO membrane. Specifically, the RO membrane device 12 is made of an RO membrane module and has an RO membrane element housed in a cylindrical vessel (pressure vessel).

The water treatment device 10 of this embodiment is assumed to be used as a small- to medium-sized water treatment device, and the installation space and the feed water pressure of the raw water may be limited. Therefore, as the RO membrane module of the RO membrane device 12, a single RO membrane element is housed in a single vessel. In addition, as the RO membrane of the RO membrane element, a polyamide-based RO membrane for ultra-low pressure is used, which can obtain a sufficient amount of permeated water at a low operating pressure. Specifically, a polyamide-based RO membrane is used, which has a corrected flux (corrected permeation flux) of 1.40 m/d/MPa at 25°C or more when a sodium chloride aqueous solution with a concentration of 500 mg/L is permeated at an operating pressure of 0.7 MPa at a temperature of 25°C. The "corrected flux (corrected permeation flux)" is in accordance with JIS K 3802:2015 (membrane terminology), and here means the permeated water amount ( ^m3 /d) per unit pressure (1 MPa) and unit membrane area (1 ^m2 ) converted to a value at a water temperature of 25°C. As such a membrane element, for example, an RO membrane element (product number: OFR-670) manufactured by Organo Corporation can be used. The treated water tank 13 temporarily stores the permeated water (treated water) that is separated by the RO membrane device 12 and supplied to the point-of-use.

The water treatment device 10 also has multiple lines L1 to L5 through which raw water, treated water, etc. flow. That is, the device has a raw water line L1 that supplies raw water to the clarifier 11, a water supply line L2 that supplies the clarifier 11 with the clarifier 11 with the RO membrane device 12, a permeate line L3 that passes through the permeate from the RO membrane device 12 and supplies it to the treated water tank 13, a drain line L4 that passes through the concentrated water from the RO membrane device 12 and discharges it to the outside, and a water supply line L5 that sends the treated water in the treated water tank 13 to a use point. The clarifier 11 is supplied with raw water from which residual free chlorine has been removed by an activated carbon filter (not shown) provided upstream of the raw water line L1.

Furthermore, the water treatment device 10 has a pressure pump 14 provided in the water supply line L2, a feedwater conductivity meter 15 also provided in the water supply line L2, a concentrated water conductivity meter 16 provided in the drainage line L4, and a water level sensor 17 provided in the treated water tank 13. The pressure pump 14, together with a feedwater pump (not shown) provided in the raw water line L1, has the function of pressurizing the decontaminated water supplied to the RO membrane device 12, in other words, functions as a pressure adjustment means for adjusting the water supply pressure (operation pressure) to the RO membrane device 12. The feedwater conductivity meter 15 and the concentrated water conductivity meter 16 detect the conductivity of the decontaminated water flowing through the water supply line L2 and the concentrated water flowing through the drainage line L4, respectively, and are used to determine the timing to end the flushing process described later. The water level sensor 17 detects the water level in the treated water tank 13 and is used to determine the timing to end and resume the water collection operation mode described later. In addition, the permeate line L3 and the drain line L4 are each provided with a three-way valve V1 and a flow rate control valve V2. The permeate line L3 is connected to the blow line L6 via the three-way valve V1, and the three-way valve V1 can be switched between a first position in which the permeate from the RO membrane device 12 is supplied to the treated water tank 13 and a second position in which the permeate is discharged to the outside through the blow line L6. Note that, for example, when the RO membrane 12 is an RO membrane for ultra-low pressure as described above, a sufficient amount of permeate can be obtained even at a low operating pressure, so the pressure pump 14 may be omitted and the decontaminated water supplied to the RO membrane device 12 may be pressurized only by the feed water pump.

In addition, the water treatment device 10 has a control device 18 that controls the operation of the water treatment device 10. Specifically, the control device 18 executes a flushing process for flushing the primary side of the RO membrane device 12 for a certain period of time when the water treatment device 10 transitions from the water sampling process performed during normal operation to the standby process. These three processes are described below.

The water sampling process is started when the water level in the treated water tank 13 detected by the water level sensor 17 falls below a predetermined lower limit level, or when a request to sample water is received from a use point. In the water sampling process, the pressure pump 14 is operated, the three-way valve V1 is switched to the first position, and the opening of the flow control valve V2 is adjusted, so that separation is performed by the RO membrane device 12, and the permeate (treated water) is stored in the treated water tank 13.

At this time, the separation by the RO membrane device 12 is performed under conditions in which the silica concentration of the concentrated water does not exceed the silica solubility at a pre-measured water temperature in order to prevent silica from precipitating on the membrane surface of the RO membrane and causing scale to adhere. Specifically, a recovery rate (the ratio of the flow rate of the permeated water to the sum of the flow rate of the permeated water and the flow rate of the concentrated water) is set based on the silica concentration of the clarified water measured in advance, so that the silica concentration of the concentrated water does not exceed the silica solubility at a pre-measured water temperature, and the flow rate of the concentrated water flowing through the drain line L4 is adjusted to achieve the set recovery rate. Here, the flow rate of the concentrated water is adjusted by a flow rate control valve V2 provided on the drain line L4, and the set flow rate is determined based on the set value of the recovery rate and the flow rate of the permeated water estimated from the set value of the operating pressure of the RO membrane device 12. It is known that the silica solubility S (mg/L) under neutral conditions is expressed by the following formula (1), where the water temperature is T (K).
logS=a+bT ^-1 +cT ² +dlogT (1)
Here, a = -3.697, b = -485.24, c = -2.268 x 10 ^-6 and d = 3.608.

In this embodiment, the clarified water from which suspended solids have been removed by the clarifier 11 is supplied to the RO membrane device 12 at a sufficient membrane surface linear velocity by the pressure pump 14, and the RO membrane device 12 performs the water sampling process under conditions in which the silica concentration of the concentrated water does not exceed the silica solubility at the water temperature measured in advance. Therefore, silica scale should not be generated on the membrane surface of the RO membrane, and stable operation should be possible even over a long period of operation. However, in reality, the inventors have verified that the transmembrane pressure difference (pressure difference between the primary and secondary sides of the membrane) of the RO membrane may increase during the standby process after the water sampling process is stopped, and as a result, as shown in the comparative example described below, the flux retention rate may decrease with operation time. This is thought to be due to the use of an RO membrane for extremely low pressures, but in this embodiment, in order to suppress such an increase in transmembrane pressure difference, a flushing process is performed immediately after the above-mentioned water sampling process is stopped until the standby process is started. The water sampling process is stopped when the water level in the treated water tank 13 detected by the water level sensor 17 reaches or exceeds a predetermined upper water level, or when there is no longer a request to sample water from the point of use.

In the flushing process, with the pressure pump 14 operating, the three-way valve V1 is switched to the second position and the flow rate control valve V2 is fully opened, so that the decontaminated water supplied to the primary side of the RO membrane of the RO membrane device 12 is discharged to the outside through the drainage line L4 without passing through the RO membrane. In this way, by flushing the primary side of the RO membrane device 12 with decontaminated water immediately after the separation by the RO membrane device 12 is stopped and the water collection process is stopped, it is possible to suppress the decrease in flux retention rate over operation time, as shown in the examples described below.

After the flushing process is performed for a certain period of time, the pressurizing pump 14 is stopped, the flushing process ends, and the standby process begins. The standby process continues until the water level in the treated water tank 13 detected by the water level sensor 17 falls below a predetermined lower limit water level, or until a water sampling request is received from the point of use. When the standby process ends, the pressurizing pump 14 is operated and the opening of the flow rate control valve V2 is adjusted, so that the water that has accumulated on the secondary side of the RO membrane during the standby process is discharged to the outside through the blow line L6. The three-way valve V1 is then switched to the first position, and the water sampling process is resumed, in which the permeate is stored in the treated water tank 13.

The time for which the flushing process is performed is not particularly limited and is typically 2 minutes to 12 hours, but from the viewpoint of reducing operating costs, it is preferable that the time be as short as possible. Therefore, during the flushing process, the conductivity of the decontaminated water and the conductivity of the flushing wastewater may be detected, and a decision as to whether or not to terminate the flushing process may be made based on these detected conductivities. Specifically, the detection value by the feedwater conductivity meter 15 may be compared with the detection value by the concentrated water conductivity meter 16, and the flushing process may be terminated, for example, when the latter (conductivity of the flushing wastewater) falls within a range of ±5% of the former (conductivity of the decontaminated water). Note that a resistivity meter may be installed instead of the conductivity meters 15 and 16, and a similar decision may be made based on the detected resistivity.

In this embodiment, the RO membrane module of the RO membrane device 12 is a single vessel containing a single RO membrane element, but the number of RO membrane elements in the entire RO membrane device 12 is not limited to one and may be multiple. That is, the RO membrane module of the RO membrane device 12 may contain multiple RO membrane elements in a single vessel. The RO membrane device 12 may also be an RO membrane module unit consisting of multiple vessels, and each vessel may contain a single RO membrane element or multiple RO membrane elements. However, the flushing process of this embodiment is more effective when the RO membrane device 12 has a single RO membrane element, which tends to increase the recovery rate per element, compared to when the RO membrane device 12 has multiple RO membrane elements as a whole.

In addition, in this embodiment, in order to more reliably suppress the precipitation of scale components such as silica and calcium on the membrane surface of the RO membrane, a scale inhibitor may be added to the raw water or the clarified water by a chemical injection pump. The type of scale inhibitor is not limited to a specific one, and examples thereof include phosphonic acid compounds such as phosphonic acids and salts thereof, such as 1-hydroxyethylidene-1,1-diphosphonic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid, ethylenediaminetetramethylenephosphonic acid, and nitrilotrimethylphosphonic acid; phosphoric acid compounds such as orthophosphates and polymerized phosphates; maleic acid compounds such as polymaleic acid and maleic acid copolymers; and acrylic acid polymers. Examples of the acrylic acid polymers include copolymers such as poly(meth)acrylic acid, maleic acid/(meth)acrylic acid, (meth)acrylic acid/sulfonic acid, and (meth)acrylic acid/nonionic group-containing monomers, (meth)acrylic acid/sulfonic acid/nonionic group-containing monomers, (meth)acrylic acid/acrylamide-alkylsulfonic acid/substituted (meth)acrylamide, and (meth)acrylic acid/acrylamide-arylsulfonic acid/substituted (meth)acrylamide terpolymers. Examples of (meth)acrylic acids constituting the terpolymer include methacrylic acid, acrylic acid, and (meth)acrylates such as their sodium salts. Examples of acrylamide-alkylsulfonic acids constituting the terpolymer include 2-acrylamido-2-methylpropanesulfonic acid and its salts. Examples of substituted (meth)acrylamides constituting the terpolymer include t-butylacrylamide, t-octylacrylamide, and dimethylacrylamide.

Among these, it is preferable to use one that contains at least one of a phosphonic acid compound and an acrylic acid polymer. In order to simultaneously suppress scale derived from calcium and silica, it is particularly preferable to use a scale inhibitor that is made of 2-phosphonobutane-1,2,4-tricarboxylic acid and a mixture of acrylic acid and a terpolymer of (meth)acrylic acid/2-acrylamido-2-methylpropanesulfonic acid/substituted (meth)acrylamide.

Commercially available scale inhibitors for RO membranes include the "Orpersion" series manufactured by Organo Corporation, the "Flocon (registered trademark)" series manufactured by BWA Water Additives, the "PermaTreat (registered trademark)" series manufactured by Nalco, the "Hypersperse (registered trademark)" series manufactured by General Electric Company, and the "Kuriverter (registered trademark)" series manufactured by Kurita Water Industries Ltd.

(Example)
Next, the effects of the present invention will be described with reference to specific examples.

In this example, the water treatment device shown in FIG. 1 was used, and a 2-minute flushing process was performed when moving from the water sampling process to the standby process. The water sampling process and the standby process were repeated for 100 days, and the above-mentioned corrected flux was measured every 10 hours. Groundwater was used as the raw water, and the water temperature was 17°C and the silica concentration was 49 mg/L. In addition, an RO membrane element (product number: OFR-670) manufactured by Organo Corporation, which had a corrected flux of 1.46 m/d/MPa and at 25°C before the start of measurement, was used as the RO membrane element of the RO membrane device, the operating pressure of the RO membrane device was 0.75 MPa, and the recovery rate was 40%. Therefore, from the above formula (1), the silica solubility of the concentrated water was 99 mg/L. In addition, a mixture of 2-phosphonobutane-1,2,4-tricarboxylic acid and a terpolymer of acrylic acid and (meth)acrylic acid/2-acrylamido-2-methylpropanesulfonic acid/substituted (meth)acrylamide was used as the scale inhibitor. Such a scale inhibitor was then added to the clarified water so that the concentration in the concentrated water was 30 mg/L. When moving from the standby process to the water sampling process, a three-minute blowing process (a process in which water that had accumulated on the secondary side of the RO membrane during the standby process is discharged to the outside) was carried out. The total time for the water sampling process per day was approximately 6 to 12 hours.

As a comparative example, measurements were performed under the same conditions as in the examples, except that the flushing process was not performed when transitioning from the water sampling process to the waiting process.

Figure 2 is a graph showing the measurement results in the examples and comparative examples. The flux retention rate shown on the vertical axis of the graph is a relative value when the corrected flux at the start of operation is set to 100%.

As is clear from Figure 2, in the comparative example, even though the water sampling process was performed under conditions in which the silica concentration of the concentrated water did not exceed the silica solubility at the water temperature measured in advance, it was confirmed that the flux retention rate gradually decreased with operation time. The ultra-low pressure RO membrane used in the comparative example has a tendency to have a high concentration near the membrane surface because the amount of permeated water is large relative to the membrane surface flow rate, and this is the reason for the decrease in flux retention rate in the comparative example, which is thought to be due to silica precipitation on the membrane surface of the RO membrane, which causes the membrane surface to become clogged.

On the other hand, in the working examples, even though an RO membrane for ultra-low pressure was used as in the comparative examples, no significant decrease in flux retention was observed, and it was confirmed that stable operation could be continued. This is thought to be because the primary side of the RO membrane was flushed with clarified water every time the water sampling process was stopped, which mitigated the increase in concentration near the membrane surface of the RO membrane and suppressed clogging of the membrane surface due to the generation of silica scale.

REFERENCE SIGNS LIST 1 Water treatment device 11 Turbidity elimination device 12 Reverse osmosis membrane (RO membrane) device 13 Treated water tank 14 Pressure pump 15, 16 Conductivity meter 17 Water level sensor L1 Raw water line L2 Water supply line L3 Permeate water line L4 Drain line L5 Water supply line V1 Three-way valve V2 Flow rate control valve

Claims (5)

A method for operating a water treatment system comprising: a turbidity eliminator that removes suspended solids contained in water to be treated; a reverse osmosis membrane device that is provided downstream of the turbidity eliminator and separates the water to be treated that has passed through the turbidity eliminator into permeate and concentrated water; and a pump that is provided upstream of the reverse osmosis membrane device and pressurizes the water to be treated that is supplied to the reverse osmosis membrane device, wherein the reverse osmosis membrane device has a polyamide-based reverse osmosis membrane with a corrected flux of 1.40 m/d/MPa or more at 25°C, comprising:
a step of performing the separation using the reverse osmosis membrane device while adding a scale inhibitor consisting of 2-phosphonobutane-1,2,4-tricarboxylic acid and a mixture of acrylic acid and a terpolymer of (meth)acrylic acid/2-acrylamide-2-methylpropanesulfonic acid/substituted (meth)acrylamide to the water to be treated before or after passing through the clarification device, so that the silica concentration of the concentrated water does not become equal to or exceeds the silica solubility at a water temperature measured in advance, and storing the permeated water in a treated water tank;
and flushing a primary side of the reverse osmosis membrane device with the water to be treated that has passed through the clarifier after the separation by the reverse osmosis membrane device is stopped. The method for operating a water treatment device according to claim 1, wherein the reverse osmosis membrane device is a reverse osmosis membrane module having a single reverse osmosis membrane element housed in a pressure vessel. detecting the conductivity of the water to be treated and the conductivity of the concentrated water that have passed through the clarifier while flushing the primary side of the reverse osmosis membrane device;
The method for operating a water treatment apparatus according to claim 1 , further comprising the step of determining whether or not to terminate the flushing based on the detected electrical conductivity. The method for operating a water treatment device according to any one of claims 1 to 3, wherein the turbidity removal device has a microfiltration membrane. A clarifier for removing suspended solids contained in the water to be treated;
A reverse osmosis membrane device is provided downstream of the clarifier and separates the water to be treated that has passed through the clarifier into permeate and concentrated water, the reverse osmosis membrane device having a polyamide-based reverse osmosis membrane with a corrected flux of 1.40 m/d/MPa at 25°C or more;
a pump provided upstream of the reverse osmosis membrane device and configured to pressurize the water to be treated that is supplied to the reverse osmosis membrane device;
A chemical injection device is provided upstream of the reverse osmosis membrane device and adds a scale inhibitor consisting of 2-phosphonobutane-1,2,4-tricarboxylic acid and a mixture of acrylic acid and a terpolymer of (meth)acrylic acid/2-acrylamide-2-methylpropanesulfonic acid/substituted (meth)acrylamide to the water to be treated before or after passing through the clarification device;
A water treatment device comprising: a control device that executes the operating method according to any one of claims 1 to 4.

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