JP7637562B2 - Membrane filtration system, membrane filtration method, chemical addition backwash device, and control device for chemical addition backwash device - Google Patents

Description

The present invention relates to a membrane filtration system that performs membrane filtration of water to be treated using a filtration membrane such as a turbidity removing membrane, a membrane filtration method, a chemical addition backwash device used in the membrane filtration system, and a control device for the chemical addition backwash device.

In recent years, the demand for water has increased, and there is an increasing demand for the recovery and reuse of wastewater. In wastewater recovery, which recovers and reuses wastewater, there is a method in which after biological treatment, coagulation, and solid-liquid separation treatment, membrane filtration treatment of biologically treated water containing suspended matter and solid-liquid separated water is performed using a filtration membrane such as a turbidity removal membrane. In this method, the biological treatment, coagulation, and solid-liquid separation treatment may become unstable due to changes in the properties of the water to be treated, and membrane fouling (blockage) may occur in the membrane filtration treatment. In this case, for example, backwashing is performed by passing backwash water from the secondary side to the primary side of the membrane, and physical cleaning is performed to physically peel off the deposits on the membrane surface or inside the membrane. In order to eliminate membrane fouling that cannot be eliminated by backwashing, chemically enhanced backwashing (CEB: Chemical Enhanced Backwash) may be performed, in which a chemical-added backwash solution in which an oxidizing agent such as sodium hypochlorite, or a chemical such as an alkali or acid is added to the backwash water is used.

When the properties of the water to be treated fluctuate, it is difficult to stably carry out the biological treatment, coagulation, and solid-liquid separation treatments that precede the membrane filtration treatment. When biological treatment becomes unstable, if the chemical recovery effect of backwashing with added chemicals in the membrane filtration treatment is insufficient, membrane fouling will occur mainly due to organic matter. When coagulation and solid-liquid separation treatments become unstable, if the chemical recovery effect of backwashing with added chemicals in the membrane filtration treatment is insufficient, membrane fouling will occur mainly due to metal-based coagulants that have become small particles. If the cause of membrane fouling is different, the type of chemical used for backwashing with added chemicals will differ.

When membrane fouling occurs, it becomes difficult to obtain treated water, and the wastewater recovery system must be shut down or large-scale chemical cleaning of the membranes or the purchase of new membranes must be required. Therefore, there is a need for a method that can determine optimal cleaning conditions early on and perform chemical backwashing to suppress membrane fouling, allowing stable operation even if the properties of the water to be treated fluctuate.

Patent Document 1 describes a membrane treatment device having a membrane module that performs membrane filtration of water to be treated that contains organic matter, in which at least one series of mini membrane modules that have the same membrane material and shape as the membrane module and have a membrane area of 1/2 to 1/100 of the membrane module are installed side by side, and the mini membrane module is configured to be able to supply water to be treated that is the same as that of the membrane module.

However, the membrane treatment device described in Patent Document 1 aims to determine the conditions for chemical cleaning of the membrane, not for backwashing with the addition of chemicals.

Patent Document 2 describes a wastewater recovery system that includes a membrane filtration means for membrane filtration of wastewater containing organic matter, and a cleaning means for periodically performing chemical cleaning (backwashing with chemical addition) of the membrane of the membrane filtration means using at least two types of chemicals, including one acid and one alkali, and that selects whether or not to use the same chemical in the next chemical cleaning as in the previous chemical cleaning by referring to a preset reference transmembrane pressure ΔPx, calculated from the transmembrane pressure difference (ΔP0) before the chemical cleaning and the transmembrane pressure difference (ΔP1) after the chemical cleaning.

However, in the wastewater recovery system described in Patent Document 2, the conditions for the next chemical addition backwash are selected based on the transmembrane pressure difference in the device, so it may not be possible to determine optimal conditions for chemical addition backwash early. Determining the chemical conditions for chemical addition backwash based on the transmembrane pressure difference before and after chemical cleaning in the device, as in Patent Document 2, not only does it not allow for early chemical selection, but it also requires the device to be stopped for cleaning. Furthermore, there is a disadvantage in that the volume of the membrane module built into the device is large, which increases the amount of chemicals used and the amount of wastewater.

Patent No. 3656908 Patent No. 6693804

The object of the present invention is to provide a membrane filtration system that can determine optimal cleaning conditions early on and perform chemical addition backwashing to suppress membrane fouling even if the properties of the water being treated fluctuate in a membrane filtration process that uses a filtration membrane, a membrane filtration method, a chemical addition backwashing device used in the membrane filtration system, and a control device for the chemical addition backwashing device, thereby enabling stable operation.

The present invention provides a main membrane filtration device that filters water to be treated using a main filtration membrane, a sub-membrane filtration device that filters water to be treated branched off from the water to be treated of the main membrane filtration device using a sub-filtration membrane, a main chemical addition backwashing means for performing chemical addition backwashing of the main filtration membrane, a sub-chemical addition backwashing means for performing chemical addition backwashing of the sub-filtration membrane, a sub-filtration resistance detection means for detecting a filtration resistance value of the sub-filtration membrane, and a warning system based on filtration resistance values detected by the sub-filtration resistance detection means before and after the chemical addition backwashing performed by the sub -chemical addition backwashing means. and an output means for outputting at least one of information on the chemical addition backwashing of the main filtration membrane, execution of chemical addition backwashing of the main filtration membrane, execution of chemical cleaning of the main filtration membrane, change of cleaning conditions for the main filtration membrane, and proposal for improving operating conditions of the main filtration membrane , and a control means for controlling conditions for the chemical addition backwashing of the main filtration membrane performed by the main chemical addition backwashing means based on filtration resistance values before and after the chemical addition backwashing detected by the sub-filtration resistance detection means, wherein water passes through the sub-membrane filtration device at a higher flux than the main membrane filtration device.

The present invention includes a main membrane filtration process for filtering water to be treated using a main filtration membrane, a sub-membrane filtration process for filtering water to be treated branched from the water to be treated in the main membrane filtration process using a sub-filtration membrane, a main chemical addition backwash process for performing chemical addition backwashing of the main filtration membrane, a sub-chemical addition backwash process for performing chemical addition backwashing of the sub-filtration membrane, and an output process for outputting at least one of an alarm, execution of chemical addition backwashing of the main filtration membrane, execution of chemical cleaning of the main filtration membrane, change of cleaning conditions of the main filtration membrane, and a proposal for improving the operating conditions of the main filtration membrane, based on filtration resistance values before and after the chemical addition backwashing performed in the sub-chemical addition backwashing process, in which water is passed through at a higher flux in the sub-membrane filtration process than in the main membrane filtration process, and the conditions of the chemical addition backwashing of the main filtration membrane performed in the main chemical addition backwashing process are controlled based on the filtration resistance values before and after the chemical addition backwashing . This is a membrane filtration method.

The present invention is a chemical addition backwash device that performs chemical addition backwashing of a main membrane filtration device that filters water to be treated using a main filtration membrane, and includes a sub-membrane filtration device that uses a sub-filtration membrane to filter water to be treated branched off from the water to be treated of the main membrane filtration device, a main chemical addition backwashing means for performing chemical addition backwashing of the main filtration membrane, a sub-chemical addition backwashing means for performing chemical addition backwashing of the sub-filtration membrane, a sub-filtration resistance detection means for detecting a filtration resistance value of the sub-filtration membrane, and a control means for controlling conditions for chemical addition backwashing of the main filtration membrane performed by the main chemical addition backwashing means based on the filtration resistance values before and after the chemical addition backwashing detected by the sub-filtration resistance detection means, and the chemical addition backwashing device passes water through the sub-membrane filtration device at a higher flux than the main membrane filtration device .

The present invention is a control device for a chemical addition backwash device that performs chemical addition backwash of a main membrane filtration device that filters water to be treated using a main filtration membrane, the chemical addition backwash device comprising: a sub-membrane filtration device that uses a sub-filtration membrane to filter water to be treated branched off from the water to be treated of the main membrane filtration device; a sub-chemical addition backwash means for performing chemical addition backwash of the sub-filtration membrane; and a sub-filtration resistance detection means for detecting a filtration resistance value of the sub-filtration membrane, and a control means for controlling the conditions for chemical addition backwash of the main filtration membrane based on the filtration resistance values before and after the chemical addition backwash detected by the sub-filtration resistance detection means, and the control device for a chemical addition backwash device that passes water through the sub-membrane filtration device at a higher flux than the main membrane filtration device .

In the present invention, in a membrane filtration process using a filtration membrane, even if the properties of the water being treated fluctuate, optimal cleaning conditions can be determined early and chemical addition backwashing can be performed to suppress membrane fouling, thereby providing a membrane filtration system that can operate stably, a membrane filtration method, a chemical addition backwashing device used in the membrane filtration system, and a control device for the chemical addition backwashing device.

FIG. 1 is a schematic configuration diagram illustrating an example of a membrane filtration system according to an embodiment of the present invention. 1 is a flowchart illustrating an example of a membrane filtration method according to an embodiment of the present invention. 1 is a graph showing an example of filtration resistance (/m) versus filtration amount (m ³ /m ² ) when backwashing is performed in a membrane filtration device. 1 is a graph showing an example of filtration resistance (/m) versus filtration volume (m ³ /m ² ) when backwashing and chemical-addition backwashing were performed in a membrane filtration device. 1 is a graph showing the transmembrane pressure difference (MPa) versus the number of filtration days when cleaning condition 1 was performed with chemical addition backwashing in the main membrane filtration apparatus of Example 1. 1 is a graph showing the filtration resistance (/m) versus the filtration amount (m ³ /m ² ) when cleaning condition 1 was performed with chemical addition backwashing in the sub-membrane filtration apparatus of Example 1. 1 is a graph showing the transmembrane pressure difference (MPa) versus the number of filtration days when cleaning condition 1+cleaning condition 2 were performed with chemical addition backwashing in the main membrane filtration apparatus of Example 2. 1 is a graph showing the filtration resistance (/m) versus the filtration amount (m ³ /m ² ) when cleaning condition 1+cleaning condition 2 were performed in the sub-membrane filtration apparatus of Example 2 with chemical addition backwashing.

The following describes an embodiment of the present invention. This embodiment is an example of implementing the present invention, and the present invention is not limited to this embodiment.

An example of a membrane filtration system according to an embodiment of the present invention is outlined in FIG. 1, and its configuration will be described.

The membrane filtration system 1 includes a main membrane filtration device 10 that filters the water to be treated using a main filtration membrane, a sub-membrane filtration device 12 that filters the water to be treated branched off from the water to be treated of the main membrane filtration device 10 using a sub-filtration membrane, a main backwash piping 32 as a main chemical addition backwash means for performing chemical addition backwashing of the main filtration membrane of the main membrane filtration device 10, a sub-backwash piping 42 as a sub-chemical addition backwash means for performing chemical addition backwashing of the sub-filtration membrane of the sub-membrane filtration device 12, a flowmeter 25 and a pressure gauge 26 as sub-filtration resistance detection means for detecting the filtration resistance value of the sub-filtration membrane of the sub-membrane filtration device 12, and an output unit 14 as output means for performing a predetermined output based on the filtration resistance value before and after the chemical addition backwashing performed by the sub-chemical addition backwashing means detected by the flowmeter 25 and the pressure gauge 26. The main backwash piping 32 may function as a main backwash means for backwashing the main filtration membrane of the main membrane filtration device 10, and the sub-backwash piping 42 may function as a sub-backwash means for backwashing the sub-filtration membrane of the sub-membrane filtration device 12.

In the membrane filtration system 1 of FIG. 1, the main treated water pipe 28 is connected to the treated water inlet of the main membrane filtration device 10, the main treated water pipe 30 is connected to the main treated water outlet, and the main backwash liquid inlet is connected to the main backwash liquid pipe 32. The sub treated water pipe 38 branched from the main treated water pipe 28 is connected to the treated water inlet of the sub membrane filtration device 12, the sub treated water pipe 40 is connected to the sub treated water outlet, and the sub backwash liquid inlet is connected to the sub backwash liquid pipe 42. The chemical outlet of the acid tank 18, which is an example of the first chemical tank, and the main backwash pipe 32 are connected via the acid pipe 34, which is the first chemical pipe, through the pump 22. The chemical outlet of the oxidizer tank 20, which is an example of the second chemical tank, and the main backwash pipe 32 are connected via the oxidizer pipe 36, which is the second chemical pipe, through the pump 24. An acid pipe 44 branching off from the acid pipe 34 is connected to the sub-backwash pipe 42, and an oxidant pipe 46 branching off from the oxidant pipe 36 is connected to the sub-backwash pipe 42. A flowmeter 25 and a pressure gauge 26 are installed in the sub-treated water pipe 38. An output unit 14 is installed in the pressure gauge 26.

The membrane filtration system 1 may include a control device 16 as a control means. The control device 16 may be connected to the output unit 14, the pump 22, the pump 24, the flow meter 25, the pressure gauge 26, etc., by wired or wireless electrical connection or the like so as to be able to communicate with them. The membrane filtration system 1 may further include a communication device as a communication means. The communication device is connected to the control device 16, and is connected to a server, for example, via the Internet. The server is provided with at least a recording unit or a calculation unit. The recording unit stores, for example, data measured by the sub-filtration resistance detection means. The calculation unit performs calculations using the data on the sub-filtration resistance, and may output an alarm, the execution of chemical addition backwashing, the execution of chemical cleaning, etc., or the data stored in the recording unit may be used together to calculate the execution history of chemical addition backwashing and chemical cleaning, the amount of chemical used, and the cost of chemicals, and may be stored or output. The output may be sent to the control device 16 again via the Internet, and an alarm may be issued to the output unit 14, chemical addition backwashing and chemical cleaning may be performed, or the cleaning conditions may be changed.

The membrane filtration method and operation of the membrane filtration system 1 according to this embodiment will be described.

The treated water to be treated is sent to the main membrane filtration device 10 through the main treated water piping 28, where the main filtration membrane is used to perform membrane filtration of the treated water (main membrane filtration process). The obtained main treated water is discharged through the main treated water piping 30.

In the membrane filtration system 1, the main membrane filtration device 10 is cleaned, for example, periodically, when the main membrane filtration device 10 is continuously operated. The main filtration membrane is cleaned, for example, by backflowing at least a portion of the main treated water from the secondary side to the primary side of the main membrane filtration device 10 through the main backwash piping 32 as main backwash water (main backwash process). The main backwash wastewater is discharged from the primary side of the main membrane filtration device 10.

If the fouling of the main filtration membrane is not eliminated even after the main backwash is performed, a chemical-added backwash is performed with a main chemical-added backwash liquid obtained by adding a chemical to the main backwash water (main chemical-added backwash process). In the main chemical-added backwash process, for example, acid is supplied from the acid tank 18 to the main backwash water in the main backwash pipe 32 through the acid pipe 34 by the pump 22, and acid washing is performed, or an oxidant is supplied from the oxidant tank 20 to the main backwash water in the main backwash pipe 32 through the oxidant pipe 36 by the pump 24, and oxidant washing is performed. In addition to the oxidant, an alkali may be used. The main chemical-added backwash effluent is discharged from the primary side of the main membrane filtration device 10. In the main chemical-added backwash process, an immersion process may be performed in which the main filtration membrane is immersed in the main chemical-added backwash liquid for a predetermined time. In the main chemical-added backwash process, the main chemical-added backwash effluent discharged from the primary side of the main membrane filtration device 10 may be circulated to the secondary side of the main membrane filtration device 10.

As mentioned above, membrane fouling (blockage) may occur during membrane filtration treatment in wastewater recovery, etc. The main causes of membrane fouling in membrane filtration treatment are organic matter resulting from the upstream biological treatment process, and metal particles derived from inorganic coagulants resulting from the upstream coagulation and solid-liquid separation treatment processes. When the properties of the water to be treated fluctuate, the main cause is that the biological treatment, coagulation, and solid-liquid separation treatments in the upstream stages of membrane filtration become unstable, and the amount of organic matter, metal particles, etc. flowing into the main membrane filtration device 10 changes, so the main cause of fouling also changes. If the main cause of membrane fouling is different, the type of chemical used for chemical addition backwashing will be different.

In the membrane filtration system 1, a sub-membrane filtration process is provided by a sub-membrane filtration device 12 equipped with a sub-backwashing means and a sub-chemical addition backwashing means, in addition to the main membrane filtration process by the main membrane filtration device 10. Based on the filtration resistance value in the sub-membrane filtration process detected by the flowmeter 25 and pressure gauge 26, which are sub-filtration resistance detection means, an output according to the status of the chemical addition backwashing in the sub-membrane filtration device 12 is issued, for example, when the difference in the filtration resistance value before and after the chemical addition backwashing exceeds a predetermined threshold value, an alarm or other output is issued. As a result, even if the properties of the water to be treated fluctuate, the optimal cleaning conditions can be determined early in the sub-membrane filtration device 12, and chemical addition backwashing can be performed under those cleaning conditions in the main membrane filtration device 10 to suppress membrane fouling, enabling stable operation.

By providing a sub-membrane filtration device 12 in addition to the main membrane filtration device 10, the optimal cleaning conditions for chemical addition backwashing can be determined without stopping the main membrane filtration device 10, maximizing the operating time of the main membrane filtration device 10.

An example of a method for determining optimal cleaning conditions for chemical addition backwashing using the sub-membrane filtration device 12 is described below.

The treated water branched off from the main treated water pipe 28 is sent to the sub-membrane filtration device 12 through the sub-treated water pipe 38, where the sub-membrane filtration device 12 performs membrane filtration of the treated water using a sub-filtration membrane (sub-membrane filtration process). The resulting sub-treated water is discharged through the sub-treated water pipe 40.

In this way, in the membrane filtration system 1, when the main membrane filtration device 10 is continuously operated, the treated water branched off from the main treated water pipe 28 is also passed through the sub-membrane filtration device 12. The sub-filtration membrane of the sub-membrane filtration device 12 is also washed, for example, periodically. The washing of the sub-filtration membrane is performed, for example, by backflowing at least a portion of the sub-treated water as sub-backwash water from the secondary side of the sub-membrane filtration device 12 through the sub-backwash pipe 42 as sub-backwash water (sub-backwash process), as in the case of the main filtration membrane. The sub-backwash drainage is discharged from the primary side of the sub-membrane filtration device 12. The sub-backwash process may be performed periodically, or may be performed based on the filtration resistance value detected by the flowmeter 25 and the pressure gauge 26, for example, when the filtration resistance value detected by the flowmeter 25 and the pressure gauge 26 exceeds a predetermined backwash reference value.

If the fouling of the sub-filtration membrane is not eliminated even after the sub-backwashing is performed, a chemical-added backwashing is performed with a sub-chemical-added backwashing solution in which a chemical has been added to the sub-backwashing water (sub-chemical-added backwashing process). In the sub-chemical-added backwashing process, for example, acid is supplied from the acid tank 18 to the sub-backwashing water in the sub-backwashing pipe 42 through the acid pipe 34 and the acid pipe 44 by the pump 22, and acid washing is performed, or an oxidant is supplied from the oxidant tank 20 to the sub-backwashing water in the sub-backwashing pipe 42 through the oxidant pipe 36 and the oxidant pipe 46 by the pump 24, and oxidant washing is performed. An alkali may be used in addition to the oxidant. The sub-chemical-added backwashing wastewater is discharged from the primary side of the sub-membrane filtration device 12. In the sub-chemical-added backwashing process, a soaking process may be performed in which the sub-filtration membrane is soaked in the sub-chemical-added backwashing solution for a predetermined time. In addition, in the sub-chemical addition backwash process, the sub-chemical addition backwash effluent discharged from the primary side of the sub-membrane filtration device 12 may be circulated to the secondary side of the sub-membrane filtration device 12.

In the main backwash process, the main backwash pipe 32 functions as the main backwash means, and in the sub-backwash process, the sub-backwash pipe 42 functions as the sub-backwash means. In the main chemical addition backwash process, the acid tank 18, the pump 22, the acid pipe 34, etc. function as the first chemical supply means, for example, the acid supply means, the oxidizer tank 20, the pump 24, the oxidizer pipe 36, etc. function as the second chemical supply means, for example, the oxidizer supply means, and in addition to the acid supply means and the oxidizer supply means, the main backwash pipe 32 functions as the main chemical addition backwash means. In the sub-chemical addition backwash process, the acid tank 18, the pump 22, the acid pipe 34, the acid pipe 44, etc. function as the first chemical supply means, for example, the acid supply means, the oxidizer tank 20, the pump 24, the oxidizer pipe 36, the oxidizer pipe 46, etc. function as the second chemical supply means, for example, the oxidizer supply means, and in addition to the acid supply means and the oxidizer supply means, the sub-backwash pipe 42 functions as the sub-chemical addition backwash means.

Here, before and after the start of the sub-chemical addition backwashing process, the water to be treated is passed through the sub-membrane filtration device 12, and the filtration resistance value is measured from the flow rate and pressure before and after the start of the sub-chemical addition backwashing process by the flowmeter 25 and pressure gauge 26, which are sub-filtration resistance detection means. Based on the filtration resistance value calculated from the decrease in flow rate and the increase in pressure detected by the flowmeter 25 and pressure gauge 26, for example, when ΔR ₁ (R ₀ -R _{1 ) calculated from the filtration resistance value before the chemical addition backwashing (R 0 )} and the filtration resistance value after the chemical addition backwashing when the sub-chemical addition backwashing is performed exceeds a predetermined threshold value, such as a CEB reference value, _a predetermined output such as an alarm is performed (output process). If the temperature of the water to be treated fluctuates and it is desired to eliminate the temperature dependency of the filtration resistance value, it is desirable to further install a thermometer in the sub-treated water piping 38 and calculate the filtration resistance value of the sub-filtration membrane taking into account the viscosity based on the measured water temperature of the water to be treated.

This allows early detection of fouling of the sub-filtration membrane in the sub-membrane filtration device 12. When fouling of the sub-filtration membrane is detected, the optimal conditions for chemical addition backwashing are determined in the sub-membrane filtration device 12, and chemical addition backwashing is performed in the main membrane filtration device 10 under the optimal cleaning conditions determined in the sub-membrane filtration device 12. This makes it possible to suppress membrane fouling even if the properties of the water to be treated fluctuate, and enables stable operation of the main membrane filtration device 10. By determining the optimal conditions for chemical addition backwashing in the sub-membrane filtration device 12 in advance, it is not necessary to stop operation of the main membrane filtration device 10, and the main membrane filtration process can be continued. This allows the operating time of the main membrane filtration device 10 to be maximized.

The sub-filtration membrane in the sub-membrane filtration device 12 has, for example, the same membrane material, membrane shape, and membrane pore size as the main filtration membrane in the main membrane filtration device 10, but has a smaller membrane area than the main filtration membrane. If the optimal cleaning conditions for chemical addition backwashing are determined in the sub-membrane filtration device 12, which is smaller than the main membrane filtration device 10, the amount of chemical liquid consumed to determine the optimal conditions for chemical addition backwashing can be minimized.

In the sub-membrane filtration device 12 (sub-membrane filtration process), it is preferable to pass water at a higher flux than in the main membrane filtration device 10 (main membrane filtration process). This allows the occurrence of fouling of the sub-filtration membrane in the sub-membrane filtration device 12 to be detected earlier. If optimal chemical addition backwash conditions are determined early in the sub-membrane filtration device 12 when the occurrence of fouling of the sub-filtration membrane is detected, and chemical addition backwash is performed in the main membrane filtration device 10 under the optimal cleaning conditions determined in the sub-membrane filtration device 12, membrane fouling can be suppressed even if the properties of the water to be treated fluctuate, and stable operation of the main membrane filtration device 10 becomes possible.

In the example of the membrane treatment device described in Patent Document 1, water is passed through the device and the mini membrane module at the same flux, and the membrane is chemically cleaned only after the transmembrane pressure difference in the mini membrane module exceeds 150 kPa. This makes it difficult to detect the occurrence of fouling at an early stage and to respond quickly to changes in the properties of the water being treated.

The flux in the sub-membrane filtration device 12 (sub-membrane filtration process) is, for example, 2 to 10 times, and preferably 4 to 6 times, the flux in the main membrane filtration device 10 (main membrane filtration process). If the flux in the sub-membrane filtration device 12 is less than twice the flux in the main membrane filtration device 10, it may not be possible to detect the occurrence of fouling in the sub-filtration membrane at an earlier stage, whereas if it exceeds 10 times, the occurrence of fouling may be over-evaluated, resulting in a reduced prediction accuracy.

In addition, for example, the conditions of the main backwash of the main filtration membrane and the conditions of the main chemical addition backwash performed in the main membrane filtration device 10 may be controlled based on the filtration resistance values before and after the sub backwash in the sub membrane filtration device 12 and the filtration resistance values before and after the sub chemical addition backwash detected by the pressure gauge 26. For example, the backwash conditions of the main membrane filtration process, such as the frequency of the main backwash and the conditions of the chemical addition backwash, such as the frequency of the main chemical addition backwash, the chemical concentration of the main chemical addition backwash solution used in the main chemical addition backwash, and the type of chemical used in the main chemical addition backwash, may be controlled based on the tendency of the resistance increase immediately after the backwash in the sub membrane filtration device 12 and the resistance increase immediately after the chemical addition backwash. These controls may be performed manually or automatically.

An example of a method for determining more specific conditions for chemical addition backwashing is described below with reference to the flowchart shown in Figure 2.

In the sub-membrane filtration device 12, after the membrane filtration process is performed for a predetermined time on the water to be treated branched from the main water to be treated pipe 28 of the main membrane filtration device 10, for example, sub-backwashing is performed periodically, and if the fouling of the sub-filtration membrane is not eliminated even after the sub-backwashing, chemical addition backwashing is performed. The filtration resistance value (R ₀ ) before the chemical addition backwashing when the chemical addition backwashing is performed is measured (S10), and after the chemical addition backwashing is performed under cleaning condition 1 (for example, an oxidizing agent is used as the chemical) (S12), the filtration resistance value (R ₁ ) after the chemical addition backwashing is measured, and for example, the control device 16 calculates ΔR ₁ (R ₀ -R ₁ ), which is the difference between the filtration resistance value before the chemical addition backwashing (R ₀ ) and the filtration resistance value after the chemical addition backwashing (R ₁ ) (S14). The control device 16 refers to the CEB chemical condition change standard ΔR _ref , which is a preset standard filtration resistance increase degree, for the calculated ΔR ₁ .

When ΔR ₁ ≦CEB chemical condition change standard ΔR _ref , it is determined that the filtration resistance has been sufficiently reduced by the first evaluated cleaning condition 1 (i.e., chemical addition backwashing is sufficient), so the control device 16 issues an instruction to perform chemical addition backwashing of the main membrane filtration device 10 under cleaning condition 1, and chemical addition backwashing is performed in the main membrane filtration device 10 under cleaning condition 1 (S20). In this case, the process may end without performing chemical addition backwashing under the second cleaning condition 2 in the sub-membrane filtration device 12.

When ΔR ₁ > CEB chemical condition change standard ΔR _ref , cleaning condition 2, which is a different chemical condition from the previous one, is used in the next chemical addition backwash. For example, after chemical addition backwash is performed under cleaning condition 2 (e.g., acid is used as the chemical) (S16), the filtration resistance value (R ₂ ) after chemical addition backwash is measured, and ΔR ₂ (R ₀ -R 2 ), which is the difference between the filtration resistance value (R ₀ ) before chemical addition backwash and the filtration resistance value (R ₂ ) after chemical addition backwash, is calculated by, for example, the control device ₁₆ (S18). For example, the control device 16 refers to the CEB chemical condition change standard ΔR _ref for the calculated ΔR ₂ .

When ΔR2 _≦ CEB chemical condition change standard _ΔRref , it is determined that the filtration resistance has been sufficiently reduced by the second evaluated cleaning condition 2 (i.e., chemical addition backwashing is sufficient), so the control device 16 issues an instruction to perform chemical addition backwashing of the main membrane filtration device 10 under cleaning condition 2, and chemical addition backwashing is performed in the main membrane filtration device 10 under cleaning condition 2 (S22). In this case, the process may end without performing chemical addition backwashing under the next cleaning condition in the sub-membrane filtration device 12.

If ΔR ₂ > CEB chemical condition change standard ΔR _ref , the next chemical addition backwash will use cleaning conditions that are different from the previous chemical conditions, and thereafter, the same flow as S10 to S14 will be repeated until ΔR _N ≦ CEB chemical condition change standard ΔR _ref (N is an integer greater than or equal to 1).

From cleaning condition 2 onwards, different cleaning conditions may be used. For example, a different chemical may be used than in cleaning condition 1, or the same chemical as in cleaning condition 1 may be used at a different chemical concentration, or a different cleaning concentration or cleaning time may be used.

When ΔR _N ≦CEB chemical condition change reference ΔR _ref , the control device 16 may control the cleaning conditions for the chemical addition backwash of the main membrane filtration device 10. For example, the control device 16 may control the pump 22 and the pump 24.

If it is believed that fouling caused by organic matter is more prevalent than fouling caused by inorganic matter based on the properties of the water to be treated, then, for example, in cleaning condition 1, a condition in which an oxidizing agent + alkali that is effective in removing organic matter is used as the chemical for chemical-added backwashing is evaluated first, and then, in cleaning condition 2, a condition in which an acid that is effective in removing inorganic matter is used as the chemical for chemical-added backwashing is evaluated. If it is believed that fouling caused by inorganic matter is more prevalent than fouling caused by organic matter based on the properties of the water to be treated, then, for example, in cleaning condition 1, a condition in which an acid that is effective in removing inorganic matter is used as the chemical for chemical-added backwashing is evaluated first, and then, in cleaning condition 2, a condition in which an oxidizing agent + alkali that is effective in removing organic matter is used as the chemical for chemical-added backwashing is evaluated.

In addition, the filtration resistance increase degree (ΔR _cake , ΔR _backwash , ΔR _CEB ) may be calculated from the filtration resistance value of the chemical-added backwash of the sub-membrane filtration device 12, and a warning that may be considered based on the membrane clogging behavior or a proposal for operational improvement may be output to the output unit 14. Figure 3 shows an example of the filtration resistance (/m) relative to the filtration volume (m ³ /m ² ) when backwashing is performed in a membrane filtration device. Figure 4 is a graph showing an example of the filtration resistance (/m) relative to the filtration volume (m ³ /m ² ) when backwashing and chemical-added backwashing are performed in a membrane filtration device.

The filtration resistance increase degree ΔR _cake is the filtration resistance that increases in one membrane filtration process, and is the slope of the resistance generated by cake attached to the membrane surface. Cake resistance is a reversible membrane blockage that is recovered by backwashing. The filtration resistance increase degree ΔR _backwash is the slope of the filtration resistance that is difficult to recover by backwashing. The filtration resistance increase degree ΔR _CEB is a filtration resistance that is difficult to recover even by backwashing with the addition of chemicals, and is an irreversible membrane blockage.

For example, when ΔR _cake becomes higher than normal, an alarm is issued to the output unit 14, and suggestions for improving the operating conditions are presented. Examples of the alarm include "abnormal cake resistance." When the cake resistance becomes abnormal, it is considered that the concentration of suspended solids in the raw water for membrane filtration is increased, and therefore examples of suggestions for improving the operating conditions include "check the turbidity of the pre-treated water and adjust the operation,""reduce the filtration time by half," and so on.

For example, when ΔR _backwash becomes higher than normal, an alarm is issued to the output unit 14, and a proposal for improving the operating conditions is presented. Examples of the alarm include "abnormal backwash recovery". When the backwash recovery is abnormal, it is considered that the organic matter treatment in the upstream stage (e.g., biological treatment or coagulation sedimentation) is defective, and therefore, examples of the proposal for improving the operating conditions include "optimization of the coagulation treatment in the upstream stage (reconsider the amount of coagulant added, change the pH, etc.)".

For example, when ΔR _CEB becomes higher than normal, an alarm is issued to the output unit 14, and suggestions for improving the operating conditions are presented. Examples of the alarm include "abnormality in backwash recovery after chemical addition." Examples of suggestions for improving the operating conditions include "change the type of CEB chemical solution,""increase in CEB concentration," and so on.

The control device 16 stores data on the pressure and flow rate measured when the treated water is passed through the sub-membrane filtration device 12, calculates data on the increase in filtration resistance immediately after backwashing and the increase in filtration resistance immediately after chemical addition backwashing, and may automatically control the conditions for chemical addition backwashing of the main membrane filtration device 10 and the sub-membrane filtration device 12 based on this (e.g., frequency of chemical addition backwashing, type of chemical, chemical concentration, chemical addition backwashing time, etc.).

A pressure gauge can be installed in the main treated water pipe 28 of the main membrane filtration device 10 to measure the filtration resistance before and after backwashing and before and after chemical addition backwashing, and the increase in transmembrane pressure difference can be confirmed as a cross-check.

The output from the output unit 14 may be an audiovisual alarm such as a display or sound indicating the status of the chemical addition backwash in the sub-membrane filtration device 12, as well as a response method for responding to the status of the chemical addition backwash, improvement suggestions, etc. The output unit 14 may be, for example, anything that can display and output information, and is not particularly limited, but examples include a display device that is a display means such as a liquid crystal display or an organic EL display, and an audio output device that is an audio output means such as a speaker. For example, the method of issuing an alarm may be to display on a touch panel of a control panel, to notify an operator via Internet communication, or to notify a monitoring room, etc.

The output unit 14 may output an alarm or the like when the difference in the filtration resistance value before and after the chemical addition backwash exceeds, for example, a predetermined threshold value. The alarm threshold value may be preset to one value, or may be changed in stages or proportionately.

The control device 16 is composed of, for example, a microcomputer and electronic circuits, which are configured to include a calculation means such as a CPU that calculates a program, and storage means such as a ROM and RAM that store the program and the calculation results. The control device 16 has a function to control the conditions of the chemical addition backwash of the main filtration membrane performed in the main membrane filtration device 10 based on the filtration resistance values before and after the chemical addition backwash performed in the sub-membrane filtration device 12, detected by the sub-filtration resistance detection means. The control device 16 has a function to control the amount of chemicals added to the main chemical addition backwash solution, for example by adjusting the on/off and flow rate of the pumps 22 and 24.

The water to be treated is water containing suspended solids, organic matter, inorganic matter such as metal particles, and the like, and is not particularly limited. Examples include biologically treated water obtained by biologically treating wastewater containing organic matter discharged from semiconductor manufacturing plants, food factories, etc. (biological treatment process), solid-liquid separation treated water obtained by coagulation and solid-liquid separation treatment (coagulation/solid-liquid separation treatment process), and solid-liquid separation treated water obtained by coagulation and solid-liquid separation treatment of the biologically treated water after it has been biologically treated. Water that has been treated through other processes before or after the biological treatment process or the coagulation/solid-liquid separation treatment process may also be used.

There are no particular limitations on the biological treatment, so long as it uses microorganisms to treat the water to be treated. There are no particular limitations on the coagulation and solid-liquid separation treatment, so long as it uses a coagulant, such as an inorganic coagulant, such as polyaluminum chloride, or a polymer coagulant, to perform coagulation treatment and solid-liquid separation treatment of the water to be treated.

The membrane filtration system and membrane filtration method according to this embodiment are suitable for use when the properties of the water to be treated are such that the content of organic matter as indicated by TOC varies in the range of 1 to 50 mg/L, or the content of metal particles such as iron, aluminum, manganese, and other inorganic matter such as dissolved metals as indicated by ICP analysis varies in the range of 0.01 to 20 mg/L.

The main filtration membrane and the sub-filtration membrane are, for example, turbidity removing membranes such as ultrafiltration membranes (UF membranes) or microfiltration membranes (MF membranes). The shape of the membrane is, for example, a hollow fiber membrane.

There are no particular limitations on the acid used for chemical backwashing, but examples include inorganic acids such as hydrochloric acid and sulfuric acid, and organic acids such as oxalic acid and citric acid. From the standpoint of recovery during chemical backwashing, it is preferable to use organic acids such as oxalic acid and citric acid.

In acid cleaning, it is preferable to add acid so that the pH of the backwash solution containing the chemicals to which the acid has been added is in the range of 1 to 3, and from the standpoint of cost-effectiveness, it is even more preferable to add acid so that the pH is in the range of 2 to 3.

There are no particular limitations on the oxidizing agent used in chemical backwashing, but examples include chlorine-based oxidizing agents such as sodium hypochlorite.

There are no particular restrictions on the alkali used for chemical backwashing, but it is preferable to use relatively inexpensive sodium hydroxide, sodium hypochlorite, etc.

When cleaning using an oxidizing agent + alkali, it is preferable to add alkali so that the pH of the backwash solution with added alkali is in the range of 10 to 13, and from the standpoint of cost-effectiveness, etc., it is even more preferable to add alkali so that the pH is in the range of 11 to 12.

In the chemical backwash, chemicals other than acids, alkalis, and oxidizing agents, such as membrane cleaners such as surfactants, may be used as necessary. Examples of membrane cleaners include the ULTRASIL series manufactured by ECOLABO.

The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

Example 1
Using the membrane filtration system 1 shown in Fig. 1, the coagulation and solid-liquid separation treated water was passed through under the conditions shown in Table 1, and backwashing and chemical addition backwashing were performed. Water was passed through the main membrane filtration device at a flux of 1.2 (m/d), and through the sub-membrane filtration device at a flux of 7 (m/d) (5.83 times the flux of the main membrane filtration device).

Possible cleaning conditions for chemical backwashing are: cleaning condition 1: sodium hypochlorite, an oxidizing agent, plus sodium hydroxide, an alkali (effective against fouling caused by organic matter); cleaning condition 2: oxalic acid, an acid (effective against fouling caused by inorganic matter).

[Cleaning condition 1: Sodium hypochlorite + sodium hydroxide]
In cleaning condition 1, an aqueous solution of sodium hypochlorite + sodium hydroxide (sodium hypochlorite concentration: 900 mg/L, sodium hydroxide concentration: 280 mg/L) was used as the chemical-added backwashing solution. Figure 5 shows the transmembrane pressure difference (MPa) versus the number of filtration days when cleaning condition 1 was performed with chemical-added backwashing in the main membrane filtration apparatus. Figure 6 shows the filtration resistance (/m) versus the filtration volume ( ^m3 / ^m2 ) when cleaning condition 1 was performed with chemical-added backwashing in the sub-membrane filtration apparatus of Example 1.

As shown in FIG. 6, the measurement results of the filtration resistance value of the sub-membrane filtration device showed that the filtration resistance increase degree ΔR _CEB increased when backwashing with the addition of chemicals of sodium hypochlorite + sodium hydroxide was performed, and it was determined that cleaning by backwashing with the addition of chemicals was insufficient.

As shown in Figure 5, the measurement results of the filtration resistance value of the main membrane filtration device where chemical-added backwashing was performed under cleaning condition 1 also confirmed an increase in the transmembrane pressure difference, confirming that the cleaning effect of chemical-added backwashing was insufficient.

[Cleaning condition 1: sodium hypochlorite + sodium hydroxide + cleaning condition 2: oxalic acid]
Therefore, in addition to cleaning condition 1 using sodium hypochlorite, which is effective against fouling caused by organic matter, chemical addition backwashing was also carried out under cleaning condition 2 using oxalic acid, which is effective against fouling caused by inorganic matter.

In cleaning condition 2, an aqueous solution of oxalic acid (oxalic acid concentration: 0.2 wt%) was used as the chemical-added backwashing solution. Fig. 7 shows the transmembrane pressure difference (MPa) versus the number of filtration days when cleaning condition 1 + cleaning condition 2 were performed with chemical-added backwashing in the main membrane filtration apparatus of Example 2. Fig. 8 shows the filtration resistance (/m) versus the filtration volume ( ^m3 / ^m2 ) when cleaning condition 1 + cleaning condition 2 were performed with chemical-added backwashing in the sub-membrane filtration apparatus of Example 2.

As shown in FIG. 8, it was confirmed from the measurement results of the filtration resistance value of the sub-membrane filtration device that the increase in the filtration resistance increase degree ΔR _CEB was suppressed, and therefore it was determined that the cleaning was sufficient.

As shown in Figure 7, there was almost no increase in the filtration resistance value of the main membrane filtration device when chemical addition backwashing was performed under cleaning condition 1 + cleaning condition 2, and stable operation was possible.

By using cleaning condition 1 and cleaning condition 2 in combination in the main membrane filtration device based on the measurement results of the filtration resistance value of the sub-membrane filtration device, membrane fouling was suppressed and stable operation was achieved for six months.

In this way, in membrane filtration processes using filtration membranes, even if the properties of the water being treated fluctuate, optimal cleaning conditions can be determined early on and backwashing with the addition of chemicals can be performed to suppress membrane fouling, making stable operation possible.

1 Membrane filtration system, 10 Main membrane filtration device, 12 Sub-membrane filtration device, 14 Output section, 16 Control device, 18 Acid tank, 20 Oxidizer tank, 22, 24 Pump, 25 Flow meter, 26 Pressure gauge, 28 Main treated water pipe, 30 Main treated water pipe, 32 Main backwash pipe, 34, 44 Acid pipe, 36, 46 Oxidizer pipe, 38 Sub treated water pipe, 40 Sub treated water pipe, 42 Sub backwash pipe.

Claims (4)

A main membrane filtration device that filters the water to be treated using a main filtration membrane;
A sub-membrane filtration device that filters the water to be treated branched from the water to be treated of the main membrane filtration device using a sub-filtration membrane;
A main chemical addition backwashing means for performing chemical addition backwashing of the main filtration membrane;
A sub-chemical addition backwashing means for performing chemical addition backwashing of the sub-filtration membrane;
A sub-filtration resistance detection means for detecting a filtration resistance value of the sub-filtration membrane;
an output means for outputting at least one of a warning, execution of chemical addition backwashing of the main filtration membrane, execution of chemical cleaning of the main filtration membrane, change of cleaning conditions of the main filtration membrane, and a proposal for improving operating conditions of the main filtration membrane, based on the filtration resistance values before and after the chemical addition backwashing performed by the sub-chemical addition backwashing means detected by the sub-filtration resistance detection means ;
A control means for controlling conditions of the chemical addition backwashing of the main filtration membrane performed by the main chemical addition backwashing means based on the filtration resistance values before and after the chemical addition backwashing detected by the sub filtration resistance detection means;
Equipped with
A membrane filtration system , characterized in that the sub-membrane filtration device passes water at a higher flux than the main membrane filtration device . a main membrane filtration process for filtering the water to be treated using a main filtration membrane;
A sub-membrane filtration process of filtering the water to be treated branched from the water to be treated in the main membrane filtration process using a sub-filtration membrane;
A main chemical addition backwashing process for adding chemicals to the main filtration membrane and backwashing the main filtration membrane;
A sub-chemical addition backwashing process for performing chemical addition backwashing of the sub-filtration membrane;
an output process for outputting at least one of an alarm, execution of chemical addition backwashing of the main filtration membrane, execution of chemical cleaning of the main filtration membrane, change of cleaning conditions of the main filtration membrane, and a proposal for improving operating conditions of the main filtration membrane, based on the filtration resistance values before and after the chemical addition backwashing performed in the sub-chemical addition backwashing process;
Including,
In the sub-membrane filtration process, water is passed through at a higher flux than in the main membrane filtration process,
A membrane filtration method , characterized in that conditions for chemical addition backwashing of the main filtration membrane performed in the main chemical addition backwashing process are controlled based on filtration resistance values before and after the chemical addition backwashing . A chemical addition backwashing device that adds chemicals to a main membrane filtration device that filters water to be treated using a main filtration membrane,
A sub-membrane filtration device that filters the water to be treated branched from the water to be treated of the main membrane filtration device using a sub-filtration membrane;
A main chemical addition backwashing means for performing chemical addition backwashing of the main filtration membrane;
A sub-chemical addition backwashing means for performing chemical addition backwashing of the sub-filtration membrane;
A sub-filtration resistance detection means for detecting a filtration resistance value of the sub-filtration membrane;
A control means for controlling conditions of the chemical addition backwashing of the main filtration membrane performed by the main chemical addition backwashing means based on the filtration resistance values before and after the chemical addition backwashing detected by the sub filtration resistance detection means;
Equipped with
A chemical addition backwash device , characterized in that water is passed through the sub-membrane filtration device at a higher flux than that of the main membrane filtration device . A control device for a chemical addition backwashing device that performs chemical addition backwashing of a main membrane filtration device that filters water to be treated using a main filtration membrane, the chemical addition backwashing device comprising: a sub-membrane filtration device that uses a sub-filtration membrane to filter water to be treated that is branched off from the water to be treated of the main membrane filtration device; a sub-chemical addition backwashing means for performing chemical addition backwashing of the sub-filtration membrane; and a sub-filtration resistance detection means for detecting a filtration resistance value of the sub-filtration membrane.
A control device for a chemical addition backwash device, comprising: a control means for controlling the conditions of the chemical addition backwash of the main filtration membrane based on the filtration resistance values before and after the chemical addition backwash detected by the sub-filtration resistance detection means, and characterized in that water is passed through the sub-membrane filtration device at a higher flux than that of the main membrane filtration device .

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