JP5614644B2 - Membrane filtration method - Google Patents

Description

  The present invention relates to a membrane filtration method for filtering treated water using a filtration membrane, and particularly to a membrane filtration method that can be suitably used when filtering treated water in a water purification plant or the like.

  Conventionally, in various water treatment fields such as water treatment, sewage treatment, industrial water treatment, and industrial wastewater treatment, water to be treated is filtered to remove suspended solids (SS) and clean A membrane filtration method is used as a method for obtaining filtered water. In this membrane filtration method, generally, a filtration membrane in which fine pores are formed after agglomerating (flocculating) suspended substances in the water to be treated using an aggregating agent such as aluminum sulfate or polyaluminum chloride is used. The treated water is filtered.

  Here, when the water to be treated is continuously filtered by the membrane filtration method, the filtration membrane is clogged (fouling) with the continuation of filtration and the membrane differential pressure increases, and the filtration membrane exhibits the desired filtration performance. It may not be possible. For this reason, when treating the water to be treated using the membrane filtration method, the filtration membrane is periodically backwashed (hereinafter referred to as “backwashing”), and the suspended matter adhering to the filtration membrane is fouled. The ring-causing substances are regularly removed.

  However, even if backwashing is performed, fouling-causing substances cannot be completely removed from the filtration membrane, so when membrane filtration of treated water is performed for a long time, even if backwashing is carried out regularly, The filtration membrane is gradually clogged (that is, the filtration membrane is clogged), and the membrane differential pressure gradually increases. When the filtration membrane is blocked to a level at which filtration cannot be continued, it is necessary to perform cleaning of the filtration membrane using chemicals (CIP: chemical cleaning). In addition, if the clogging of the filtration membrane is not eliminated even after the chemical cleaning, the filtration membrane itself needs to be replaced with a new filtration membrane.

  Therefore, by suppressing the progress of clogging of the filtration membrane and reducing the rate of increase in the membrane differential pressure, long-term membrane filtration is realized at low cost by reducing the frequency of chemical cleaning of the filtration membrane and the frequency of replacement of the filtration membrane. As a method for this, a method of filtering the water to be treated after adjusting the pH of the water to be treated after adding the flocculant to a predetermined pH according to the quality of the water to be treated has been proposed (for example, see Patent Document 1). ).

JP 2008-221168 A

  However, the membrane filtration method described in Patent Document 1 has room for improvement in that the frequency of chemical cleaning and replacement of the filtration membrane is further reduced to further reduce the operating cost required for filtering the water to be treated. It was.

  Therefore, an object of the present invention is to provide a membrane filtration method capable of suppressing the progress of clogging of the filtration membrane and reducing the frequency of chemical cleaning of the filtration membrane and the replacement frequency of the filtration membrane.

  In order to achieve the above-mentioned object, the present inventors have conducted intensive research and conducted membrane filtration of water to be treated using a polyaluminum chloride solution (PAC solution) having a predetermined basicity as a flocculant. The inventors have found that it is possible to reduce the frequency of chemical cleaning and replacement of the filtration membrane by suppressing the progress of membrane clogging (that is, the chemical cost and the operating cost can be reduced), and the present invention has been completed.

That is, this invention aims to solve the above-mentioned problems advantageously, and the membrane filtration method of the present invention comprises water to be treated and a polyaluminum chloride solution having a basicity of 65% to 75%. Are mixed in a rapid agitation tank, the suspended substances in the water to be treated are aggregated to form fine flocs, and then the fine flocs are coarsened in a slow agitation tank to obtain an agglomerated treated water. And a membrane filtration step of subjecting the agglomerated treated water to dead-end filtration with a filtration membrane to obtain filtrate water, and a backwashing step of backwashing the filtration membrane at a predetermined interval, wherein the filtration membrane is a porous ceramic. Is a monolithic ceramic membrane having a plurality of water passage through holes, and the agglomerated treated water flows into the water passage through holes and is filtered by a separation layer located on the surface of the water passage through holes. It is characterized by that. Thus, if the water to be treated is pretreated with a polyaluminum chloride solution having a basicity of 65% or more and 75% or less , and the resulting agglomerated treated water is subjected to membrane filtration, a fouling causative substance adhering to the filtration membrane is obtained. The peelability (removability) at the time of backwashing can be improved. Therefore, if the water to be treated is filtered according to the membrane filtration method of the present invention, the progress of clogging of the filtration membrane can be suppressed, and the chemical cleaning frequency and replacement frequency of the filtration membrane can be reduced.

In addition, when the agglomerated treated water obtained by mixing a polyaluminum chloride solution having a basicity of 65% or more and 75% or less and the water to be treated is subjected to membrane filtration, the fouling causative substance adhering to the filtration membrane is a filtration membrane. In the case of a ceramic product, the releasability at the time of backwashing is particularly good. Therefore, if the filter membrane is made of ceramic, the progress of the clogging of the filter membrane can be further suppressed. In addition, when the filter membrane is made of ceramic, the sedimentation of the fouling-causing substances separated from the filter membrane at the time of backwashing becomes better, and the wastewater generated during backwashing (backwash wastewater) can be easily treated at low cost. it can.

Furthermore, when the ceramic filtration membrane has a so-called monolithic shape in which a plurality of water passage through holes are provided inside the porous ceramic, flocs in the coagulation treated water contact each other in the water passage through holes during filtration of the coagulation treated water. Further, since the densification and coarsening are further performed, the peelability of the fouling-causing substance at the time of backwashing is further improved, and the progress of clogging of the filtration membrane can be further suppressed. Further, the sedimentation property of the fouling-causing substance peeled from the filter membrane during backwashing is further improved, and the backwash wastewater can be easily treated at a lower cost. In addition, flocs in the agglomerated water are remarkably densified and coarsened in the water passage through holes when a polyaluminum chloride solution having a basicity of 65% to 75% is used as the PAC solution.
The “basicity of the polyaluminum chloride solution” in the present invention can be determined in accordance with the standard JWWA K 154 of the Japan Water Works Association (JWWA). Further, in the present invention, the “predetermined interval” for performing the backwashing can be determined by a known method according to the performance of the filtration membrane, for example, it may be a predetermined time interval, or may be filtered. The interval may be determined according to the magnitude of the membrane differential pressure, or may be determined according to the quality of the filtered water.

  And it is preferable that the membrane filtration method of this invention distribute | circulates backwash water to a filtration membrane by the pressure of 150 kPa or more in the said backwashing process. This is because when the backwash water is circulated through the filtration membrane at a pressure of 150 kPa or more, the fouling-causing substance is satisfactorily peeled from the filtration membrane, and the progression of the filtration membrane can be further suppressed.

  According to the membrane filtration method of the present invention, the progress of clogging of the filtration membrane can be suppressed, and the chemical cleaning frequency and replacement frequency of the filtration membrane can be reduced.

It is explanatory drawing which shows the structure of the membrane filtration system which can be applied when filtering to-be-processed water using the typical membrane filtration method according to this invention. It is explanatory drawing which cuts off a part of ceramic membrane and shows the structure of the ceramic membrane which can be used with the membrane filtration method according to this invention. It is a graph which shows the relationship between the filtration time at the time of carrying out the membrane filtration of the to-be-processed water using the method which concerns on an Example and a comparative example, and the membrane differential pressure immediately after backwashing.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The membrane filtration method of the present invention is characterized in that a polyaluminum chloride solution (hereinafter referred to as “PAC solution”) having a basicity of 65% or more and 75% or less is used as a flocculant.

  Here, an example of the membrane filtration method of this invention is not specifically limited, For example, it can implement using a membrane filtration system as shown in FIG. And an example of the membrane filtration method of the present invention can be used, for example, when water to be treated is filtered to produce tap water at a water purification plant, such as river water or lake water. The membrane filtration method of the present invention is not particularly limited, and can be used for sewage treatment using a membrane separation activated sludge method (MBR).

  A membrane filtration system 10 shown in FIG. 1 includes a flocculation / mixing tank 1 for mixing treated water and a PAC solution having a predetermined basicity to obtain flocculated water, and a flocculation / mixing tank 1 through a flocculated water pump 4. A membrane filtration unit 5 that obtains filtered water by filtering the supplied agglomerated treated water through a filtration membrane (not shown), and backwash water used for backwashing (backwashing) of the filtration membrane of the membrane filtration unit 5 are stored. And a backwash tank 6.

Here, the agglomeration mixing tank 1 includes a rapid stirring tank 2 for mixing the water to be treated and a PAC solution having a basicity of 65% to 75% under rapid stirring conditions, and the water to be treated mixed in the rapid stirring tank 2. And a slow stirring tank 3 for mixing a mixture of the PAC solution and the PAC solution under slow stirring conditions.

The rapid stirring tank 2 includes a stirrer 21 for stirring the inside of the tank, and a PAC pump 22 for adding a PAC solution having a basicity of 65% to 75% to the water to be treated in the rapid stirring tank 2. In the rapid stirring tank 2, the water to be treated and the PAC solution are immediately mixed by rapid stirring, and suspended substances in the water to be treated are aggregated to form fine flocs.

A PAC solution having a basicity of 65% or more and 75% or less added to the water to be treated in the rapid stirring tank 2 can be produced by, for example, a method described in JP-A-2009-203125. Specifically, the PAC solution has an Al ₂ O ₃ concentration of 5 to 17% by mass, a Cl / Al ₂ O ₃ (molar ratio) of 1.80 to 3.60, SO ₄ / Al ₂ O _{3, for example.} After adding an alkali metal and / or alkaline earth metal compound to a basic aluminum chloride solution having a (molar ratio) of 0 to 0.35 and a basicity of 40 to 63% at a temperature of 85 ° C. or lower, It can be produced by aging at a temperature of 65 to 85 ° C. for 0.5 to 2 hours.

  The slow stirring tank 3 includes a stirrer 31 for stirring the inside of the tank. The slow stirring tank 3 includes fine flocs obtained by mixing the water to be treated and the PAC solution in the rapid stirring tank 2. The treated water containing And in the slow stirring tank 3, the to-be-processed water containing the fine floc which is a mixture of to-be-processed water and a PAC solution is mixed on the slow stirring conditions of GT value = 23000-210000, for example, Underwater flocs become coarse.

That is, in the agglomeration mixing tank 1 composed of the rapid stirring tank 2 and the slow stirring tank 3, suspended substances in the water to be treated are aggregated by the aggregating action of the PAC solution, and agglomerated treated water containing coarse floc is obtained. .

  The agglomerated water pump 4 is a pump that feeds the agglomerated water from the slow stirring tank 3 of the agglomeration mixing tank 1 to the membrane filtration unit 5. In the membrane filtration system 10, an agglomerated treated water valve 51 that can be freely opened and closed is provided between the agglomerated treated water pump 4 and the membrane filtration unit 5.

The membrane filtration unit 5 includes a container having an inflow port through which the flocculated treated water flows in and an outflow port through which the filtered water flows out, on the primary side (the side into which the flocculated treated water flows) and the secondary side (the filtered water flows out). In this membrane filtration unit 5, the entire amount of the coagulated treated water that has flowed into the container is filtered (dead-end filtration). Then, the filtrate obtained by filtering the flocculated water with the membrane filtration unit 5 is sent to a filtrate water tank (not shown) via an open / close filtered water valve 52 .

Here, the filtration membrane used in the membrane filtration unit 5 is not particularly limited , and examples thereof include a microfiltration membrane or an ultrafiltration membrane made of an inorganic material. Specifically , a ceramic membrane can be used as the filtration membrane.

  The backwash water tank 6 is connected to the secondary side of the membrane filtration unit 5 (the outlet of the container constituting the membrane filtration unit 5) via a freely openable and reverse backwash valve 61. Backwash water used for backwashing the filtration membrane of the membrane filtration unit 5 is stored. And in this membrane filtration system 10, when backwashing the filtration membrane of the membrane filtration unit 5, by supplying pressurized air to the backwash water tank 6 and pushing out the backwash water in the backwash water tank 6, High-pressure backwash water is sent from the backwash water tank 6 to the secondary side of the membrane filtration unit 5 via the backwash valve 61. In addition, when the membrane of the membrane filtration unit 5 is backwashed, the backwash waste water flowing out from the primary side of the membrane filtration unit 5 (the inlet of the container constituting the membrane filtration unit 5) It passes through a pipe branched from the filtration unit 5 and is sent to a wastewater treatment facility (not shown) through an openable / closable backwash drainage valve 62. Incidentally, the backwash water is not particularly limited, and filtered water obtained by filtering the agglomerated treated water with the membrane filtration unit 5 can be used.

  In the membrane filtration system 10, the water to be treated is filtered while backwashing the filtration membrane of the membrane filtration unit 5 at predetermined intervals as follows. The backwash interval of the filtration membrane may be a predetermined time interval, may be determined according to the size of the membrane differential pressure of the filtration membrane, or may be determined according to the quality of the filtrate water. It is also good.

  First, when the water to be treated is filtered, the water to be treated and a predetermined base are opened with the flocculation water valve 51 and the filtration water valve 52 open and the backwash valve 61 and the backwash drain valve 62 are closed. The agglomerated water obtained by mixing the PAC solution with a certain degree (pretreatment step) is sent from the agglomeration mixing tank 1 to the primary side of the membrane filtration unit 5 by the agglomeration water pump 4, and the membrane of the membrane filtration unit 5 (Membrane filtration step).

  Then, when a certain filtration time has elapsed, or due to adhesion of fouling-causing substances such as flocks contained in the flocculated water to the filtration membrane, the membrane differential pressure of the membrane filtration unit 5 increases to a predetermined pressure. In this case, for example, the filtration membrane is backwashed by the following procedure (backwashing step). Specifically, first, the coagulation water valve 51 and the filtration water valve 52 are closed and the coagulation water pump 4 is stopped, and the backwash valve 61 is opened, and the backwash water pressurized with pressurized air is supplied. It sends to the secondary side of the membrane filtration unit 5 from the backwash water tank 6, and hold | maintains in the state which pressurized the filtration membrane (pressurization process). Next, the filtration membrane is backwashed by opening the backwash drain valve 62 and discharging backwash water from the backwash drain valve 62 (blow process). In addition, the backwash water (backwash drainage) containing the fouling causative substances such as flocks attached to the surface of the filtration membrane discharged in the blow process is discharged through a backwash drain valve 62 (not shown). ) And processed.

Here, in the membrane filtration method using this membrane filtration system 10, the suspended water in the treated water is aggregated using a PAC solution having a basicity of 65% or more and 75% or less. If the water has a pH of about 6 to 8.5, such as river water, the suspended matter can be sufficiently aggregated without adjusting the pH of the water to be treated, and the chemical cost required for pH adjustment can be reduced. At the same time, elution of aluminum from the PAC solution can be suppressed. Moreover, since organic matter is taken into the floc and removed together with suspended substances in the water to be treated, clean filtered water having a low concentration of organic matter can be obtained.

In addition, when a suspended substance in the water to be treated is aggregated using a PAC solution having a basicity of 65% or more and 75% or less , a floc having good peelability from the filtration membrane is formed. Therefore, in the membrane filtration method using the membrane filtration system 10, even if the floc adheres to the filtration membrane and becomes a fouling-causing substance, it can be easily peeled and removed from the filtration membrane by backwashing. Therefore, the backwashing at a predetermined interval can suppress the progress of clogging of the filtration membrane due to the accumulation of fouling-causing substances, and the frequency of chemical cleaning of the filtration membrane and the replacement frequency of the filtration membrane can be reduced. In addition, although it is not clear that flocs having good peelability are formed, it is presumed that the neutralization of charges is progressing for flocs formed using a PAC solution having a basicity of 65% to 75%. Has been. A PAC solution having a basicity of 65% to 75% contains a large amount of solid aluminum, and most of the aluminum in the PAC solution has a molecular weight cut-off of 3 kDa or more, and the crystallization of aluminum in the solution proceeds. It is speculated that this is because a PAC solution having a basicity of 65% or more and 75% or less has high charge neutralization power.

  In addition, the membrane filtration method of this invention is not limited to the said example, A change can be suitably added to the membrane filtration method of this invention.

  Specifically, in the membrane filtration method of the present invention, the filtration membrane of the filtration membrane unit 5 is made of a ceramic filtration membrane, such as alumina, titania, zirconia, silica, mullite, spinel, or a mixture thereof. It is preferable to use a ceramic membrane. Specifically, the membrane of the membrane filtration unit 5 is a monolith type having a lotus root-like hole (water passage through hole) 71 into which water to be treated flows, as shown in FIG. The ceramic film 7 is preferably used. In the membrane filtration unit 5 in which the monolithic ceramic membrane 7 is accommodated inside the container, the agglomerated water that has flowed into the holes 71 of the ceramic membrane 7 is separated from the separation layers (dense The membrane surface) and flows out of the outer surface 72. That is, in the membrane filtration unit 5 using this ceramic membrane 7, the lotus root-like hole 71 side of the ceramic membrane 7 is the primary side, and the rear side from the separation layer is the secondary side.

In this way, if a filter membrane made of ceramic is used as the filter membrane, fouling-causing substances such as flocks are better peeled off from the filter membrane during backwashing, so when using a filter membrane made of organic material This is because the progress of clogging of the filtration membrane can be further suppressed, and the chemical cleaning frequency of the filtration membrane and the replacement frequency of the filtration membrane can be further reduced. In addition, when a ceramic filter membrane is used, the sedimentation of fouling-causing substances such as flocks peeled off from the filter membrane during backwashing becomes better, so the installation area of the settling tank used for the treatment of backwash wastewater, etc. This is because the backwash waste water treatment can be carried out easily and at low cost. The reason why the fouling-causing substance is improved in peelability and sedimentation when a ceramic filter membrane is used is not clear, but it was formed using a PAC solution having a basicity of 65% or more and 75% or less . It is presumed that this is because the flocs with neutralization of charge and the surface of the ceramic film have a low affinity and the dense flocs easily peel off.

  In addition, if a filtration membrane made of a monolithic ceramic membrane is used, flocs in the coagulated water are efficiently in contact with each other in the hole (water passage through hole) 71, so that the contact efficiency between the flocs is improved and coarse. In addition, the formation of dense flocs can be promoted.

Therefore, in the membrane filtration method of the present invention, if a monolithic ceramic membrane is used as the filtration membrane, the releasability of the fouling-causing substance during backwashing is further improved, so that the progression of the filtration membrane blockage is further suppressed, The frequency of chemical cleaning of the filtration membrane and the replacement frequency of the filtration membrane can be further reduced. Moreover, the sedimentation property of the fouling-causing substances such as flocks peeled off from the filter membrane during backwashing can be improved, and the backwash wastewater treatment can be carried out easily and at low cost .

  Furthermore, in the membrane filtration method of the present invention, the pressure of backwash water when backwashing the filtration membrane by flowing backwash water from the secondary side to the primary side of the membrane filtration unit 5 is 150 kPa or more, preferably 300 to 500 kPa. It is preferable that This is because, when pressurized water having a pressure of 150 kPa or more is circulated through the filtration membrane as backwash water, the fouling-causing substances are favorably separated from the filtration membrane. In addition, the water flow conditions of the backwash water can be controlled by adjusting the flow rate of the pressurized air supplied to the backwash water tank 6.

  Moreover, it is preferable that the membrane filtration method of this invention supplies the aggregation process water to the membrane filtration unit 5 by a dead end system, and filters. This is because when the agglomerated treated water is filtered by the dead end method, the fouling-causing substance adheres to the filtration membrane in a state where it can be easily peeled off, and the fouling-causing substance is easily peelable during backwashing. The filtration conditions of the flocculated water in the membrane filtration unit 5 can be controlled by adjusting the discharge flow rate, the discharge pressure, etc. of the flocculated water pump 4.

In the membrane filtration method of the present invention, the PAC solution has a basicity of 65% to 75% and an SO ₄ concentration of 1 to 4% by mass when the Al ₂ O ₃ concentration is 10.2% by mass. More preferably, it is preferable to use a PAC solution of 1.5 to 3.5% by mass. If the basicity of the PAC solution is less than 65%, residual aluminum tends to increase, such being undesirable. In addition, when the SO ₄ concentration is less than 1% by mass, the stability of the PAC solution is increased, but the cohesiveness tends to deteriorate. When the SO ₄ concentration exceeds 4% by mass, the stability of the PAC solution tends to decrease. . Note that the basicity of PAC solution, more preferably 72% or less 68% or more. If the basicity is 68% or more and 72% or less, the effect of the present invention is best obtained and the residual aluminum can be reduced. Further, the Al ₂ O ₃ concentration in the PAC solution is preferably in the range of 9 to 11% by mass, and more preferably in the range of about 10% by mass.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to the following Example at all.

(Example)
Using the membrane filtration system shown in FIG. 1, a monolith-type ceramic filtration membrane (pore diameter: 0.1 μm, outer diameter: φ30 mm, length: 1000 mm) is shown in FIG. ) To obtain filtered water.
In addition, in the water to be treated, a PAC solution having a basicity of 70% (Al ₂ O ₃ : 10.2% by mass, Cl: 7.0% by mass, manufactured according to Example 1 of JP 2009-203125A, SO ₄ : 2.8% by mass, Ca: 0.4% by mass, Na: 1.2% by mass, basicity: 70%) at a ratio of 25 mg to 1 L of water to be treated. The stirring speed was 83 rpm (residence time: 3 minutes), and the stirring speed in the slow stirring tank was 60 rpm (residence time: 6 minutes). Incidentally, pH adjustment of to-be-processed water was not implemented. Moreover, the flow rate of the coagulated treated water to the membrane filtration unit is 3.0 m ³ / m ² · day, and backwashing is performed with backwashing water at a pressure of 500 kPa every 180 minutes (pressurization process: 10 seconds, blow process: 5 seconds).
And the membrane differential pressure immediately after backwashing was measured and evaluated. The results are shown in FIG. Moreover, the property of to-be-processed water and filtered water was evaluated according to the water supply test method (VI-1-3, VI-1-9, VI-1-21, VI-3-7). The results are shown in Table 1. Furthermore, the sedimentation of flocs in backwash wastewater was evaluated. The results are shown in Table 2.
The floc sedimentation was evaluated by measuring the time-dependent change in the sludge interface height when the backwash wastewater was placed in a graduated cylinder with a capacity of 2 L, and the sludge interface height at 0 minutes was set as 100%. It was done by doing.

(Comparative example)
The treated water was filtered in the same manner as in Example except that the basicity of the PAC solution added to the treated water was set to 54% to obtain filtered water.
And the membrane differential pressure immediately after backwashing, the property of to-be-processed water and filtered water, and the sedimentation property of the floc in backwashing waste_water | drain were evaluated similarly to the Example. The results are shown in FIG.

  As shown in FIG. 3, the membrane differential pressure immediately after backwashing after 30 days from the start of filtration of the water to be treated is 21.3 kPa in the example and 48.0 kPa in the comparative example. Therefore, it can be seen that the increase in the membrane differential pressure is suppressed in the example as compared with the comparative example, and the progression of the blocking of the filtration membrane is suppressed. A slight increase in the membrane pressure difference is presumed to be due to an unavoidable filtration membrane blockage such as the occurrence of scaling. Moreover, it turns out from Table 1 that the filtered water of an Example has little residual Al. Furthermore, it can be seen from Table 2 that the flocs in the backwash waste water of the examples have very good sedimentation properties.

  According to the membrane filtration method of the present invention, the progress of clogging of the filtration membrane can be suppressed, and the chemical cleaning frequency and replacement frequency of the filtration membrane can be reduced.

DESCRIPTION OF SYMBOLS 1 Coagulation mixing tank 2 Rapid stirring tank 3 Slow stirring tank 4 Coagulation water pump 5 Membrane filtration unit 6 Backwash water tank 7 Ceramic membrane 10 Membrane filtration system 21 Stirrer 22 PAC pump 31 Stirrer 51 Aggregation water valve 52 Filtration water Valve 61 Backwash valve 62 Backwash drain valve 71 Hole 72 Outer surface

Claims (2)

The water to be treated and a polyaluminum chloride solution having a basicity of 65% or more and 75% or less are mixed in a rapid stirring tank, and suspended substances in the water to be treated are aggregated to form fine flocs. A pretreatment step of coarsening the fine flocs in a stirring tank to obtain agglomerated treated water;
A membrane filtration step of dead-end filtering the agglomerated treated water with a filtration membrane to obtain filtered water;
A backwashing step of backwashing the filtration membrane at predetermined intervals;
Including
The filtration membrane is a monolithic ceramic membrane in which a plurality of water passage through holes are provided inside a porous ceramic,
The membrane filtration method, wherein the agglomerated treated water flows into the water flow through hole and is filtered by a separation layer located on a surface of the water flow through hole.   2. The membrane filtration method according to claim 1, wherein in the backwashing step, backwash water is circulated through the filtration membrane at a pressure of 150 kPa or more.