FORM 2

THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
[See section 10, Rule 13]
FILTRATION MEMBRANE CLEANING APPARATUS, FILTRATION MEMBRANE
CLEANING METHOD AND WATER TREATMENT SYSTEM;
MITSUBISHI ELECTRIC CORPORATION, A CORPORATION ORGANISED AND
EXISTING UNDER THE LAWS OF JAPAN, WHOSE ADDRESS IS 7-3,
MARUNOUCHI 2-CHOME, CHIYODA-KU, TOKYO 1008310, JAPAN
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE
INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED

2

SPECIFICATION

Technical Field
[0001] The present application relates to a filtration membrane cleaning
apparatus, a filtration membrane cleaning method, and a water treatment
system.
Background Art
[0002] Membrane separation using a filtration membrane is a water treatment
method for removing suspended substances and bacteria from wastewater
containing organic substances, such as clean water, sewage and industrial
wastewater. The filtration membrane has pores smaller in diameter than the
suspended substances and bacteria, and the suspended substances and
bacteria can be stably removed from raw water such as the wastewater, but
the pores may become clogged due to adhesion of the suspended substances if
the treatment is continued. When the pores are clogged, transmembrane
differential pressure increases and the amount of filtered water decreases,
resulting in a decrease in the treatment capacity. Therefore, it is necessary to
periodically carry out cleaning called backwashing in which cleaning agent is
caused to flow in the direction opposite to the filtration direction to remove the
suspended substances adhering to the inside or surface of the filtration
membrane.
[0003] Ozone that has high oxidizing power and is easy for the post-treatment
is considered to be suitable as the cleaning agent, but ozone water in which
ozone is dissolved in water is used to prevent changes in the pore structure due
to drying of the membrane. At this time, in order to obtain a sufficient
cleaning effect, it is necessary to produce ozone water in which ozone is
dissolved to a level near the saturated solubility, but an undissolved ozone gas
as a by-product has been wastefully discharged as an exhaust ozone gas.
3
[0004] Therefore, a technique has been proposed in which the exhaust ozone
gas is used to reduce the volume of the excess sludge in the reaction tank or to
clean the surface of the primary side of the filtration membrane (membrane
surface cleaning) to effectively utilize ozone (for example, refer to Patent
Document 1 or 2).
Citation List
Patent Document
[0005] Patent Document 1: Japanese Patent Application Laid-open No.
2001-70761 (Paragraphs 0013 to 0014, 0017, FIG. 1)
Patent Document 2: Japanese Patent No. 6271109 (Paragraphs 0021 to 0034,
FIG. 3 to FIG. 6)

Summary of Invention
Problems to be solved by Invention
[0006] Indeed, the exhaust ozone gas has an oxidizing effect and may
contribute to the volume reduction of the excess sludge. However, when the
exhaust ozone gas was actually injected into a reaction tank, it was found that
the transmembrane differential pressure was increased or the time required
for the backwashing was increased in spite of the volume reduction of the
excess sludge. In other words, it was found that the charging of the exhaust
ozone gas into the reaction tank does not necessarily lead to the improvement
of the cleaning efficiency of the filtration membrane.
[0007] It is an object of the present application to solve the problems described
above and to provide a filtration membrane cleaning apparatus, a filtration
membrane cleaning method, and a water treatment system which can
effectively use ozone and efficiently clean the filtration membrane.
Means for solving Problems
[0008] A filtration membrane cleaning apparatus disclosed in the present
application includes an accumulation unit to generate ozone water by
dissolving an ozone gas into water and accumulate the generated ozone water
4
and an exhaust ozone gas of a by-product, a backwashing mechanism to
perform backwashing on a filtration unit installed in a biological reaction tank
for filtering raw water, the backwashing mechanism causing the ozone water
taken out from the accumulation unit to flow from a secondary side of a
filtration membrane to a primary side thereof, a membrane surface cleaning
mechanism to perform membrane surface cleaning by discharging bubbles
containing ozone in the biological reaction tank and causing the discharged
bubbles to flow along a surface on the primary side of the filtration membrane,
a controller to control operation of the backwashing mechanism and operation
of the membrane surface cleaning mechanism so as to perform the membrane
surface cleaning by the membrane surface cleaning mechanism prior to the
backwashing by the backwashing mechanism, and an exhaust ozone gas
dilution unit to dilute the exhaust ozone gas taken out from the accumulation
unit for supplying the membrane surface cleaning mechanism with the diluted
exhaust ozone gas as a gas to be discharged as the bubbles.
[0009] Further, a filtration membrane cleaning method disclosed in the
present application includes an accumulation step in which ozone water is
generated by dissolving an ozone gas into water and the generated ozone
water and an exhaust ozone gas of a by-product are accumulated, a membrane
surface cleaning step in which bubbles containing the ozone gas are caused to
flow along a surface on a primary side of a filtration membrane for a filtration
unit that performs a filtration process of raw water; and a backwashing step in
which the ozone water accumulated in the accumulation step is taken out and
the ozone water taken out is caused to flow from a secondary side of the
filtration membrane toward the primary side thereof, wherein in the
membrane surface cleaning step, the exhaust ozone gas accumulated in the
accumulation step is caused to flow as the bubbles after being diluted.
Effect of Invention
[0010] According to the filtration membrane cleaning apparatus and the
filtration membrane cleaning method disclosed in the present application,
5
since bubbles for cleaning the membrane surface are generated by diluting the
discharged ozone gas, the filtration membrane can be efficiently cleaned.
Brief Description of Drawings
[0011] FIG. 1 is a schematic diagram showing a configuration of a filtration
membrane cleaning apparatus and a water treatment system equipped with
the filtration membrane cleaning apparatus according to Embodiment 1.
FIG. 2 is a flow chart showing a filtration membrane cleaning method
according to Embodiment 1.
FIG. 3A to FIG. 3B show cross-sectional schematic diagrams for each of
states in which clogging in filtration membranes is classified into three
respective states.
FIG. 4 is a diagram showing a relationship between a distance from a
discharge portion and an ozone concentration in bubbles when the bubbles
containing an ozone gas is discharged in water.
FIG. 5 is a schematic diagram showing a configuration of a filtration
membrane cleaning apparatus and a water treatment system equipped with
the filtration membrane cleaning apparatus according to Embodiment 2.
FIG. 6 is a schematic diagram showing a configuration of a filtration
membrane cleaning apparatus according to Embodiment 3.
FIG. 7 is a flow chart showing a filtration membrane cleaning method
according to Embodiment 3.
FIG. 8 is a schematic diagram showing a configuration of a filtration
membrane cleaning apparatus and a water treatment system equipped with
the filtration membrane cleaning apparatus according to Embodiment 4.
FIG. 9 is a flow chart showing a filtration membrane cleaning method
according to Embodiment 4.
FIG. 10 is a schematic diagram showing a piping configuration when
switching of processes is controlled for each of a plurality of filters in the water
treatment system according to Embodiment 5.
6
FIG. 11 is a block diagram showing a hardware configuration of a control
unit in the filtration membrane cleaning apparatus according to each
embodiment.
Modes for carrying out Invention
[0012] Embodiment 1
FIG. 1 to FIG. 3 illustrate a filtration membrane cleaning apparatus, a water
treatment system equipped with the filtration membrane cleaning apparatus,
and a filtration membrane cleaning method, according to Embodiment 1. FIG.
1 is a schematic diagram showing a configuration of the filtration membrane
cleaning apparatus and the water treatment system equipped with the
filtration membrane cleaning apparatus, and FIG. 2 is a flowchart showing a
part relating to the cleaning of a filtration membrane in the operation of the
water treatment system equipped with the filtration membrane cleaning
apparatus, that is, the filtration membrane cleaning method. FIG. 3A to FIG.
3B are cross-sectional schematic diagrams of filtration membrane portions
showing states for each of categories in which clogging in filtration membranes
is classified into three categories. FIG. 4 is a diagram showing a relationship
between a distance from a discharge portion and an ozone concentration in
bubbles when the bubbles of an exhaust ozone gas and a diluted exhaust ozone
gas are discharged in the water containing sludge. FIG. 1 shows an
opening/closing state of valves in a process of performing membrane surface
cleaning.
[0013] As shown in FIG. 1, in the water treatment system 1, a filtration unit 5
is installed in a biological reaction tank 4 for storing raw water such as clean
water, sewage, industrial wastewater containing organic substances which is a
subject to be treated. In the filtration unit 5, a filtration membrane 51 (refer
to FIG. 3) for separating and cleaning suspended substances from biological
reaction tank mixed water Wr accumulated in the biological reaction tank 4 is
used because it has smaller pores in diameter than the suspended substances.
The filtration membrane 51 is an organic hollow fiber membrane made of a
material having ozone resistance, such as polyvinylidene fluoride (PVDF) or
7
polytetrafluoroethylene (PTFE). Note that, the side of the filtration
membrane 51 in contact with the biological reaction tank mixed water Wr, i.e.,
the upstream side in the filtration process, is referred to as a primary side, and
the side in contact with the cleaned water (filtered water), i.e., the downstream
side in the filtration process, is referred to as a secondary side.
[0014] A raw water pipe 71 for supplying the raw water to the primary side of
the filtration is connected to the biological reaction tank 4, and a filtered water
pipe 72 for taking out filtered water after cleaning treatment is connected to a
secondary side portion (for example, a frame not shown) of the filtration unit 5.
A filtration pump 44 for sucking the filtered water from the filtration unit 5 is
disposed at a tip end of the filtered water pipe 72. At a tip end of the filtration
pump 44, a three-way valve 45 for switching between sending the sucked
filtered water to either a filtered water transfer pipe 73 for transferring the
sucked filtered water to a filtered water tank (not shown) or a filtered water
supply pipe 24 for supplying the filtered water to the filtration membrane
treatment apparatus 10 is provided.
[0015] Further, an air diffuser 41 is provided under the lower portion of the
filtration unit 5 in the biological reaction tank 4 for forming an upward flow of
the biological reaction tank mixed water Wr flowing upward along the surface
of the membrane on the primary side of the filtration membrane 5 by an air lift
action. The air diffuser 41 is connected to a blower 42 serving as an air
supply source for aeration through an aeration pipe 43.
[0016] The filtration membrane cleaning apparatus 10 is an apparatus for
cleaning the filtration membrane 51 by flowing ozone water, which is cleaning
water, through the filtration membrane 51 in the filtration unit 5 in the
reverse direction of the filtration and is basically provided with an ozone gas
generating device 11 and an ozone water generation tank 12 for generating the
ozone water using the ozone gas supplied from the ozone gas generating device
11.
[0017] The ozone gas generating device 11 includes a raw material gas supply
unit (not shown) and an ozone gas generating unit (not shown) for generating
the ozone gas using oxygen supplied from the raw material gas supply unit as
8
a raw material. As the raw material gas supply unit, for example, an oxygen
generator using a liquid oxygen cylinder, vacuum pressure swing adsorption
(VPSA) or the like is used, but it is not particularly limited thereto as long as it
is equipment capable of supplying oxygen or the like. As the ozone gas
generating unit, for example, a discharge type ozone generating device can be
used.
[0018] The ozone water generation tank 12 is not limited in shape and, for
example, has a tank vertically long in shape and stores therein the filtered
water supplied from the filtered water supply pipe 24 via the three-way valve
45. A liquid phase region Pw occupied by the ozone water in which ozone is
dissolved is formed in the lower and middle portions of the ozone water
generation tank 12, and a vapor phase region occupied by the exhaust ozone
gas generated as a secondary product accompanied in the generation of the
ozone water is formed in the upper portion thereof.
[0019] A connection port with a circulation pipe 22 for circulating water (ozone
water) in the liquid phase region Pw is provided in each of an intermediate
portion in the vertical direction of the ozone water generation tank 12 and a
tank bottom portion where the liquid phase region Pw is formed. In the
circulation pipe 22, a circulation pump 15 is provided so that water in the tank
is sucked from a connection port provided in the tank bottom portion and is
returned from a connection port provided in the intermediate portion.
Further, for example, a gas suction device 14 for sucking the ozone gas
supplied from the ozone gas generating device 11 by the Venturi effect is
disposed, and the ozone water is generated by dissolving the ozone gas in the
water flowing through the circulation pipe 22.
[0020] Note that, in the present embodiment, the inlet and the outlet of the
circulation pipe 22 are respectively connected to the tank bottom portion of the
ozone water generation tank 12 and the liquid phase region Pw in the
intermediate portion thereof, but the present invention is not limited to
thereto. The inlet may be in any of the liquid phase regions Pw, and the
outlet may be in the tank bottom portion or in the vapor phase region Pg
9
portion, as long as the position is such that the ozone water may flow into the
circulation pipe 22 and return to the ozone water generation tank 12.
[0021] In the component for generating the ozone water, the use of the gas
suction device 14 is not a limitation. For example, as disclosed in Patent
Document 2, the air diffuser for blowing the ozone gas into the liquid phase
region Pw may be provided, and the effect of the present application can be
obtained if a system is such that the exhaust ozone gas is generated when the
ozone gas is brought into contact with water to generate ozone water.
[0022] In contrast, in the present embodiment, since the ozone in the ozone gas
generating device 11 is sucked and dissolved due to the negative pressure
generated by the flow of the fluid in the gas suction device 14, it is not
necessary to supply a pressurized ozone gas from the ozone gas generating
device 11. However, when bubbling of the ozone gas is performed in the ozone
water generation tank 12, an ozone generating device having structure for
generating a necessary pressure is required.
[0023] A three-way valve 16 whose other part is connected to the ozone water
supply pipe 25 is provided between the circulation pump 15 and the gas
suction device 14, and thus it is configured such that switching between the
circulation for generating ozone water and the supply of ozone water to the
filtration unit 5 can be performed. In FIG. 1, the three-way valve 16 is
oriented to communicate the filtration pump 15 with the gas suction device 14
and thus a path is set such that the water (ozone water) in the liquid phase
region Pw circulates the ozone water in the ozone water generation tank 12
and the circulation pipe 22. When the three-way valve 16 is switched such
that the circulation pump 15 communicates with the ozone water supply pipe
25, ozone water can be supplied to the secondary side of the filtration unit 5 for
backwashing.
[0024] In addition to the above-described configuration, the water treatment
system 1 and the filtration membrane cleaning apparatus 10 are provided
with a control unit (not shown) for controlling the operation, and these are the
basic configuration of the filtration membrane cleaning apparatus 10 and the
water treatment system 1 equipped with the filtration membrane cleaning
10
apparatus 10. In the filtration membrane cleaning apparatus 10 according to
each embodiment of the present application, in addition to the basic
configuration, an exhaust ozone gas pipe 23 for introducing the exhaust ozone
gas generated in the ozone water generation tank 12 into the biological
reaction tank 4 is connected. Specifically, the exhaust ozone gas pipe 23 is
connected to a connection port provided at the top of the tank in which the
vapor phase region Pg is formed in the ozone water generation tank 12.
In the exhaust ozone gas pipe 23, an on-off valve 17 to control opening/closing,
and a buffer tank 13 are disposed. The capacity of the buffer tank 13 or the
capacity of the ozone water generation tank 12 is appropriately designed
according to the amount of the exhaust ozone gas necessary for the membrane
surface cleaning described later or the amount of the ozone water necessary for
the backwashing.
[0025] In the filtration membrane cleaning apparatus 1 according to
Embodiment 1, a tip opening 23x of the exhaust ozone gas pipe 23 is located
directly below the filtration unit 5 in the biological reaction tank 4, and it is
configured such that air from the blower 42 is mixed in the middle of the pipe.
Specifically, a branch portion for communicating with a branch part 46 of the
aeration pipe 43 is provided between the on-off valve 17 and the tip opening
23x of the exhaust ozone gas pipe 23. The air from the blower 42 can be added
to the exhaust ozone gas by opening an on-off valve 18 provided at the branch
portion.
[0026] Next, referring to the flowchart of FIG. 2, operation based on the basic
configuration will be described first. In the process of filtering the raw water
(step S10), as shown in FIG. 1, the raw water is introduced into the biological
reaction tank 4 from the raw water pipe 71, and by suction of the filtration
pump 44, the biological reaction tank mixed water Wr in the biological reaction
tank 4 is subjected to solid-liquid separation into suspended substances and
filtered water in the filtration unit 5. Note that, in the schematic diagrams
including FIG. 1 according to each embodiment, an opening/closing state of the
valves in a membrane surface cleaning process by the exhaust ozone gas is
shown unless otherwise noted.
11
[0027] The biological reaction tank mixed water Wr flowing into the primary
side is treated with organic sludge containing a complex of microorganisms
called activated sludge, and the cleaned water is separated from the activated
sludge and suspended substances by the filtration membrane 51 and passes to
the secondary side as filtered water. The filtered water having passed to the
secondary side is transferred to the filtered water tank via the filtered water
pipe 72, the filtration pump 44, the three-way valve 45, and the filtered water
transfer pipe 73. While the three-way valve 16 is set in a circulating state
during the process of treating the raw water, the on-off valves 17 and 18 are in
a closed state unlike FIG. 1, so that the flow between the filtration unit 5 and
the filtration membrane cleaning apparatus 10 is cut off.
[0028] When an end condition of the process of treating the raw water is
reached ("Yes" in step S20), the supply of the raw water and the operation of
the filtration pump 44 are stopped, and the filtration process is ended. Note
that, it is desirable that the end condition of the filtration process be controlled
by, for example, transmembrane differential pressure indicating the degree of
clogging of the filtration membrane 51. However, as long as the property of
the raw water do not change extremely, for example, an appropriate index
such as the filtration time or the flow rate of the filtered water discharged from
the filtration pump 44 may be set.
[0029] After completion of the filtration process, the process proceeds to a
process of cleaning the filtration membrane 51 using ozone. In a method of
cleaning the filtration membrane using ozone water, an ozone water
generation process (steps S30 to S90) for generating ozone water and an ozone
water backwashing process (steps S100 to S120) using the generated ozone
water are performed. The method of cleaning the filtration membrane
according to the present application is characterized in that the membrane
surface cleaning process (steps S50 to S70) in which the exhaust ozone gas is
diluted and utilized is performed during the ozone water generation process.
Each process will be described below.
[0030] In the ozone water generation process, the circulation pump 15 is
activated to circulate the water in the liquid phase region Pw of the ozone
12
water generation tank 12 within the circulation pipe 22, and the ozone gas
generated in the ozone gas generating device 11 is dissolved in the gas suction
device 14 to start the generation of ozone water (step S30). At this time, the
ozone water in which the ozone is dissolved is returned to the ozone water
generation tank 12 with bubbles of a non-dissolved exhaust ozone gas
contained in the ozone water. At this time, dissolved ozone concentration Co
in the liquid phase region Pw in the ozone water generation tank 12 increases,
and tank internal pressure Pt increases due to the inflow of the exhaust ozone
gas.
[0031] As the first stage in this process, it is determined whether or not a
necessary amount of the exhaust ozone gas generated as a secondary product
accompanying the generation of the ozone water has been secured for cleaning
the membrane surface (step S40). Strictly speaking, it is necessary to confirm
the tank internal pressure Pt indicating a predetermined amount of the
exhaust ozone gas and the ozone gas concentration in the exhaust ozone gas
after understanding the volume of the gas phase region Pg, but either one of
these may be used, or the ozone gas supply time or other indices may be used.
Here, a case where the supply time of the ozone gas exceeds a predetermined
time is set as an end condition, and thus, as the end condition, the ozone gas
supply continues until the elapsed time reaches the predetermined time
(during "No" in step S40).
[0032] When the supply time reaches the predetermined time ("Yes" in step
S40), it is determined that the end condition is satisfied, the circulation pump
15 is stopped, the supply of the exhaust ozone gas from the gas suction device
14 is stopped, and the process proceeds to the membrane surface cleaning
process using the exhaust ozone gas (steps S50 to S70). In the membrane
surface cleaning process, the on-off valve 17 is opened, and the exhaust ozone
gas accumulated in the vapor phase region Pg of the ozone water generation
tank 12 (and the buffer tank 13) is guided to the exhaust ozone gas pipe 23.
Further, the on-off valve 18 is opened, the air from the blower 42 is guided to
the exhaust ozone gas pipe 23, and the exhaust ozone gas is diluted and
13
discharged from the tip opening 23x (step S50). This process is continued
until a second predetermined time has elapsed (during "No" in step S60).
[0033] At this time, an on-off valve (not shown) provided between the aeration
pipe 43 and the air diffuser 41 is closed, and only the diluted exhaust ozone gas
is discharged from the tip opening 23x directly under the filtration unit 5.
The diluted ozone gas discharged directly under the filtration unit 5 becomes
bubbles and flows along a primary side surface 51f1 (refer to FIG. 3) which is
the primary surface of the filtration membrane 51 in the filtration unit 5.
[0034] The bubbles of the diluted exhaust ozone gas flowing along the primary
side surface 51f1 of the filtration membrane 51 peel off and remove the
suspended substances adhering to the membrane surface of the filtration
membrane 51 by the physical action of the shear force and the chemical action
of the ozone gas as the oxidizing gas, thereby cleaning the membrane surface
of the filtration membrane 51. Note that, this cleaning also has an effect of
weakening the adhesion of the adhering substances that cannot be peeled off
or removed, to the filtration membrane 51. Thus, the diluted exhaust ozone
gas supplied to the primary side acts as an aeration gas for cleaning the
membrane surface. Therefore, the exhaust ozone gas is consumed in the
filtration unit 5, and the contamination of the filtration membrane 51 is
reduced.
[0035] Here, the necessity of diluting the exhaust ozone gas flowing along the
membrane surface will be described. As shown in FIG. 3A to FIG. 3C, the
clogging (fouling) of the filtration membrane 51 is mainly classified into three
categories in accordance with the states of the accumulated organic substances
90.
Category 1: As shown in FIG. 3A, a membrane clogging substance 90a
(activated sludge or suspended particles) sufficiently larger than the diameter
of the pore 51p (membrane pore diameter) of the filtration membrane 51 such
as the activated sludge deposits on the membrane surface (primary side
surface 51f1) to form a cake layer.
Category 2: As shown in FIG. 3B, a membrane clogging substance 90e having a
size substantially equal to the membrane pore diameter enters the (primary
14
side) inlet or the inside of the pore 51p and causes the membrane fouling in
such a manner as to clog the pore 51p.
Category 3: As shown in FIG. 3C, a membrane clogging substance 90f
sufficiently smaller than the membrane pore diameter passes through the pore
51p, but a part of the membrane clogging substance 90f gradually clogs the
pore 51p by being adsorbed and adhered to the inner wall surface of the pore
51p.
[0036] The membrane fouling of Category 1 caused by the cake layer adhering
to the membrane surface is mainly classified as reversible membrane fouling,
and the membrane fouling occurring at the inlet and inside of the pore 51p,
such as complete clogging of Category 2 or standard clogging of Category 3, is
classified as irreversible membrane fouling. In the case of the irreversible
membrane fouling, since it cannot be restored by physical cleaning such as
membrane aeration (corresponding to membrane surface cleaning) or
filtration stopping, it can only be restored by chemical cleaning. In contrast,
in the case of the reversible fouling of Category 1, it is restored even by
physical cleaning.
[0037] When the ozone gas and the activated sludge come into contact with
each other, the cell walls of the bacteria contained in the activated sludge are
destroyed and the soluble organic substances 90, etc., contained in the bacteria
are released into the liquid phase. This is known as a re-biodegradation
(dissolution) technology of the activated sludge by ozone.
When a high concentration of the ozone gas is continuously supplied from the
lower part of the membrane, the activated sludge is dissolved and the
concentration of organic substances in the liquid phase around the filtration
membrane 51 increases. As a result, the soluble and colloidal components
released from the sludge tend to accumulate on the membrane surface,
forming the state shown in FIG. 3B or FIG. 3C, or a gel layer on the membrane
surface that is not the cake layer made of the activated sludge, but a physically
irreversible membrane fouling by the dissolved organic substances 90 tends to
be formed. That is, if the exhaust ozone gas is continuingly supplied, the
irreversible membrane fouling occurs, and to the contrary, frequency of
15
chemical cleaning with ozone water, sodium hypochlorite, or the like increases,
which may increase the operation cost.
[0038] In contrast, in Embodiment 1 and the subsequent embodiments, the
diluted exhaust ozone gas with low concentration is used for cleaning the
membrane surface instead of the exhaust ozone gas with high concentration.
Therefore, the generation of the membrane clogging substance 90e or the
membrane clogging substance 90f due to the re-biodegradation (dissolution) of
the activated sludge more than necessary is suppressed, and the irreversible
membrane fouling is made less likely to progress, thereby suppressing the
increase in the operation cost of the chemical cleaning.
[0039] Therefore, as the effects of ozone gas concentration in the bubbles used
for membrane cleaning, a chemical cleaning effect and the possibility of the
occurrence of the irreversible membrane fouling (as non-inducibility) were
evaluated. The results are shown in Table 1.
[0040]
[Table 1]
Table 1 Ozone gas concentration of bubbles in the membrane surface cleaning
and its effect on the sludge
[0041] As shown in Table 1, the chemical cleaning effect can be sufficiently
achieved (○) when the ozone concentration was maintained at or above C2 (30
g/m3), was insufficient (Δ) when the ozone concentration is in the range of C1
(10 g/m3) to less than C2, and was not different from air aeration when the
ozone concentration was less than C1 (×). When the ozone concentration was
kept at or below C3 (150 g/m3), the irreversible membrane fouling did not occur
(○), but occurred when the concentration exceeded C3, and when the
concentration exceeded C4 (200 g/m3), clogging due to the irreversible
membrane fouling became significant (×).
Concentration(g/m3
) 0 to C1 C1 to C2 C2 to C3 C3 to C4
Larger
than C4
Chemical cleaning effect  ∆ ○ ○ ○
Fouling non-inducibility ○ ○ ○ ∆ 
16
[0042] Namely, in order to prevent the irreversible membrane fouling, it can be
understood that the ozone concentration in the exhaust ozone gas used for
cleaning the membrane surface should be adjusted to C3 (150 g/m3 : standard
state) or less. Meanwhile, as shown in FIG. 4, the ozone concentration in the
bubbles that are discharged in the biological reaction tank 4 for cleaning the
membrane surface and flow along the membrane surface decreases as the
increase in the distance from the discharge portion. Therefore, in order to
maintain the chemical cleaning effect of the ozone gas over the entire surface
of the membrane surface (within effective area for cleaning membrane surface),
it is desirable to adjust the concentration at the time of the discharge so that
the concentration at the most distant position can be equal to or greater than
C2 (30 g/m3).
[0043] The dissolved ozone concentration Co of the ozone water for the
backwashing is preferably 30 mg/L, and the concentration of the ozone gas
used to generate the ozone water in the concentration range is 200 g/m3.
When the ozone gas is dissolved in water, the ozone concentration of the
exhaust ozone gas secondarily generated depends on the ozone concentration
Co of the water to be dissolved, and as the dissolved ozone concentration Co
increases, the ozone gas concentration of the exhaust ozone gas generated also
increases. However, although varying with temperature and pressure, when
the dissolved ozone concentration Co of the ozone water in the liquid phase
region Pw reaches a required range, the concentration of the exhaust ozone
gas accumulated in the vapor phase region Pg actually ranges from 100 g/m3 to
200 g/m3.
[0044] Therefore, even if the concentration of the exhaust ozone gas is not
strictly measured, the concentration can be adjusted to a desired concentration
by controlling the dilution ratio. Therefore, in place of the concentration
control, the exhaust ozone gas is diluted to 1.4 times or more, and thus
efficient membrane surface cleaning is possible. At this time, if the dilution
ratio is kept at three times or less, an ozone gas concentration of 30 g/m3 or
more can be obtained, so that the chemical cleaning effect can also be reliably
obtained.
17
[0045] Further, supplying a diluted exhaust ozone gas with low concentration
instead of the exhaust ozone gas with high concentration for cleaning the
membrane surface has the following advantages. When the bubbles
containing the ozone gas are discharged directly under the filtration unit 5,
the bubbles move upward along the membrane surface. During the upward
movement, the ozone gas reacts with the activated sludge in the biological
reaction tank mixed water Wr at the interface between the gas phase and the
liquid phase, dissolves partly while the ozone gas is consumed, and diffuses in
the biological reaction tank 4 from the high concentration side to the low
concentration side according to Fick's law. In this case, since the ascending
speed increases as the bubble diameter increases, the ascending speed of the
bubble becomes larger when the exhaust ozone gas is discharged by being
diluted with air than when the exhaust ozone gas is discharged as it is.
[0046] Since the ozone gas in the bubble ascends while being consumed as
described above, the larger the ascending speed of the bubble, the smaller the
concentration difference due to the difference in the height of the membrane.
Therefore, as shown in FIG. 4, the change of the dissolved ozone concentration
Co with respect to the height is smaller when the exhaust ozone gas is
discharged by being diluted than when the exhaust ozone gas is discharged as
it is. That is, when the diluted exhaust ozone gas is used, the bias of the
dissolved ozone concentration distribution with respect to the height direction
of the membrane is reduced, and the membrane surface can be cleaned
relatively uniformly.
[0047] As the surface of the membrane is cleaned, the resistance to the water
passing through the membrane decreases. Therefore, if the cleaning of the
membrane surface is uneven, in the backwashing described later, it is
considered that the flow of the ozone water is biased to the portion of the
membrane surface with a high degree of cleaning, and the flow of ozone water
to the portion of the membrane surface with a low degree of cleaning is
insufficient. That is, the uniformity of the degree of cleaning of the
membrane surface also contributes to the efficiency of the backwashing.
Therefore, the use of the diluted exhaust ozone gas for cleaning the membrane
18
surface is important not only for preventing the irreversible membrane fouling
but also for improving the efficiency of the backwashing.
[0048] Back to the description of the processes again. When the membrane
surface cleaning is continued for the second predetermined time ("Yes" in step
S60), the on-off valves 17 and 18 are closed, the exhaust ozone gas for cleaning
the membrane surface is stopped, and the membrane surface cleaning process
using the (diluted) exhaust ozone gas is finished (step S70). At this time, the
blower 42 is also stopped. Then, as the second stage, the circulation pump 15
is operated, and the generation of ozone water using the gas suction device 14
is restarted (step S80).
[0049] The second stage in the generation of ozone water is to increase the
dissolved ozone concentration Co in the ozone water to the concentration
required for the backwashing. Therefore, strictly speaking, it is desirable to
measure the dissolved ozone concentration Co, but also in this case, in addition
to the measurement value of the dissolved ozone concentration Co described
later, for example, the supply time of the ozone gas, the tank internal pressure,
or the like may be used. Therefore, whether or not the resumed ozone water
generation continues for a third predetermined time is set as an end condition,
and the ozone water generation continues until the end condition is satisfied
(during "No" in step S90). The condition that the dissolved ozone
concentration Co becomes a necessary concentration may be, for example, the
ozone gas supply time, the tank internal pressure, and the like, in addition to
the measurement value of the dissolved ozone concentration Co described
later.
[0050] If it is determined that the end condition is satisfied and the desired
dissolved ozone concentration Co is obtained ("Yes" in step S90), the ozone
water backwashing process is started. First, the three-way valve 45 is
switched so that the stopped filtration pump 44 communicates with the
filtered water pipe 72, and the valve on the side of the filtered water transfer
pipe 73 is closed. The three-way valve 16 is switched so that the circulation
pump 15 communicates with the ozone water supply pipe 25, thereby forming
a flow path between the liquid phase region Pw of the ozone water generation
19
tank 12 and the secondary side of the filtration unit 5. The ozone water in the
liquid phase region Pw of the ozone water generation tank 12 is supplied to the
secondary side of the filtration unit 5, and the backwashing is performed by
flowing the ozone water from the secondary side toward the primary side, that
is, in the direction opposite to the filtration with respect to the filtration
membrane 51 (step S100). The backwashing is continued until the end
condition is satisfied (during "No" in step S110).
[0051] The driving force for transferring the ozone water to the filtration unit 5
and causing the ozone water to flow from the secondary side to the primary
side of the filtration membrane 51 is the gas pressure (tank internal pressure
Pt) in the vapor phase region Pg including the buffer tank 13. From the
viewpoint of the cleaning effect of the filtration membrane 51 or the prevention
of ozone liberation again in the ozone water during the transfer, it is preferable
to supply the ozone water to the secondary side at a supply pressure as high as
possible, but if the pressure applied to the filtration membrane 51 is too high,
there is a possibility that the filtration membrane 51 is damaged when the
pressure exceeds the withstand pressure of the filtration membrane 51.
Therefore, it is desirable to control the pressure applied to the filtration
membrane 51 within an appropriate range by providing a pressure adjusting
valve (not shown), for example. The control pressure is preferably 50 kPa or
less from the viewpoint of a balance between the cleaning effect by ozone water
and the prevention of breakage of the filtration membrane.
[0052] The time required for cleaning the filtration membrane 51 may be
about 30 minutes, depending on the size of the filtration membrane 51 or the
degree of contamination. Therefore, for example, assuming that the end
condition is a duration of 30 minutes, if the cleaning continues for 30 minutes
("Yes" in step S110), the backwashing process ends (step S120) and the process
returns to the raw water filtration process. Specifically, the three-way valve
16 is switched so that the circulation pump 15 communicates with the gas
suction device 14, and the supply of the ozone water to the filtration unit 5 is
stopped. With the state above, the filtration pump 44 is activated, and the
water filtered by the filtration unit 5 is supplied to the ozone water generation
20
tank 12. When a predetermined amount of water is stored in the ozone water
generation tank 12, the three-way valve 45 is switched so that the filtration
pump 44 communicates with the filtered water transfer pipe 73, and the
supply of the filtered water is stopped.
[0053] The aeration from the air diffuser 41 by the blower 42 and the supply of
the raw water to the biological reaction tank 4 are restarted, and thus the
filtration is restarted (step S10). Thus, by repeating the filtration process,
the ozone water generation process including the membrane surface cleaning
process using the diluted exhaust ozone gas, and the ozone water backwashing
process, the effective use of ozone is achieved, the filtration membrane is
efficiently cleaned, and stable water treatment can be possible.
[0054] The dissolved ozone concentration Co required for the backwashing is
set to a predetermined value near the saturated solubility. Since the higher
the dissolved ozone concentration Co is, the higher the cleaning effect on the
filtration membrane 51 is, it is preferable that a set concentration value be as
close to the saturated solubility as possible. However, since the saturated
solubility varies depending on the temperature, pH of the solvent, and
atmospheric pressure, a constant value of 30 mg/L or more may be set as the
set concentration value in order to keep the dissolved ozone concentration Co
constant for each cleaning. Further, a supply time of the ozone gas with
which the dissolved ozone concentration Co becomes the set concentration
value may be determined in advance, and the supply time described above
may be set in the actual ozone water generation process instead of determining
the set concentration value.
[0055] Embodiment 2
In Embodiment 1, an example has been described in which the tip opening of
the exhaust ozone gas pipe is placed directly under the filtration unit to
perform the membrane surface cleaning. In Embodiment 2, an example in
which the exhaust ozone gas pipe is connected to the air diffuser for the
aeration and the diluted exhaust ozone gas for cleaning the membrane surface
is discharged from the air diffuser will be described. FIG. 5 is a schematic
diagram showing the configuration of a filtration membrane cleaning
21
apparatus according to Embodiment 2 and a water treatment system equipped
therewith and shows an opening/closing state of the valves in the process of
performing the membrane surface cleaning as in FIG. 1 of Embodiment 1.
Except for the part ahead of the branch part of the exhaust ozone gas pipe, the
present embodiment is the same as Embodiment 1, and description of the
same parts will be omitted. The operation is basically the same as that of
Embodiment 1, and FIG. 2 used for the description of Embodiment 1 is
referred to, and the description of similar parts is omitted.
[0056] As shown in FIG. 5, in the filtration membrane cleaning apparatus 10
according to Embodiment 2 and the water treatment system 1 equipped with
the filtration membrane cleaning apparatus 10, the exhaust ozone gas pipe 23
is connected to the air diffuser 41. As a result, the exhaust ozone gas supplied
from the ozone water generation tank 12 is diluted by the aeration air supplied
from the blower 42 in the air diffuser 41 and can be discharged from the air
diffuser 41 toward the filtration unit 5. With this configuration, bubbles
similar to those in the aeration can flow along the membrane surface.
That is, in the case of Embodiment 1, it is necessary to make up the tip
opening 23x in a shape for generating bubbles for cleaning the membrane
surface, but this can be omitted in Embodiment 2.
[0057] Further, it is possible to omit the installation of the branch part 46 that
has the on-off valve 18 and is used in Embodiment 1, and the operation of the
on-off valve 18 is also unnecessary. For example, in the membrane surface
cleaning process (steps S50 to S70) using the exhaust ozone gas, which was
described in Embodiment 1, the membrane surface cleaning can be started
only by operating the on-off valve 17. Specifically, in step S50, when the
on-off valve 17 is opened and the exhaust ozone gas flows into the exhaust
ozone gas pipe 23, the exhaust ozone gas is diluted by the aeration air in the
air diffuser 41, and the diluted exhaust ozone gas can be discharged from the
air diffuser 41. In step S70, the on-off valve 17 is closed to stop the exhaust
ozone gas for cleaning the membrane surface, and the membrane surface
cleaning process using the diluted exhaust ozone gas is ended. Note that the
blower 42 is stopped before the backwashing.
22
[0058] As described above, in Embodiment 2, the number of components can be
reduced and the operation control is simplified as compared with Embodiment
1. Note that, the description is made such that, in FIG. 5, the exhaust ozone
gas pipe 23 and the aeration pipe 43 are connected to different connection
ports of the air diffuser 41, and the exhaust ozone gas is diluted by air in the
air diffuser 41. However, this is not a limitation. The exhaust ozone gas
may be mixed with air before entering the air diffuser 41 and may enter the air
diffuser 41 in a diluted state.
[0059] Embodiment 3
In the above-described Embodiments 1 and 2, an example has been described
in which whether or not the exhaust ozone gas for cleaning the membrane
surface or the ozone water for backwashing has been secured is determined by
time management. In Embodiment 3, a configuration example will be
described in which the determination is made on the basis of the dissolved
ozone concentration in the ozone water or the internal pressure of the buffer
tank. FIG. 6 and FIG. 7 show a filtration membrane cleaning apparatus
according to Embodiment 3, a configuration of a water treatment system
equipped therewith, and a filtration membrane cleaning method, and FIG. 6 is
a schematic diagram showing a part of the configuration of the filtration
membrane cleaning apparatus, and FIG. 7 is a flowchart showing a part
related to the cleaning of the filtration membrane, that is, a filtration
membrane cleaning method within the operation of the water treatment
system equipped with the filtration membrane cleaning apparatus. The
exhaust ozone gas pipe and the other pipe ends are the same as in
Embodiment 1 or Embodiment 2, and the description of the same parts is
omitted. In addition, regarding the operation, description of the same parts
as those in FIG. 2 used in the description of Embodiment 1 will be omitted.
[0060] In the filtration membrane cleaning apparatus 10 according to
Embodiment 3, as shown in FIG. 6, a pressure gauge 81 for measuring the
tank internal pressure Pt is provided in the buffer tank 13. In order to
measure the dissolved ozone concentration Co of the ozone water in the liquid
phase region Pw, a dissolved ozone concentration meter 82 is provided
23
upstream to the gas suction device 14 in the circulation pipe 22. The
measured value of the tank internal pressure Pt measured by the pressure
gauge 81 and the measured value of the dissolved ozone concentration Co
measured by the dissolved ozone concentration gauge 82 are output to a
control unit 30. Note that, in order to clarify the relationship between the
measurement values and the control, in Embodiment 3, the control unit not
shown in Embodiments 1 and 2 is referred to as the control unit 30.
[0061] The operation will be described referring to FIG. 7. In step S42 of
Embodiment 3, instead of step S40 described in Embodiment 1, an end
condition for terminating the generation of ozone water for securing the
exhaust ozone gas as the first stage is whether or not the dissolved ozone
concentration Co has increased to a first concentration threshold ThC1 or more.
That is, as the first stage, the generation of the ozone water is continued until
the dissolved ozone concentration Co increases to the first concentration
threshold ThC1 or more. Then, when the dissolved ozone concentration Co
reaches the first concentration threshold ThC1, the ozone water generation in
the first stage is stopped, and the process proceeds to the membrane surface
cleaning process (step S50).
[0062] The purpose of the ozone water generation in the first stage is to secure
the amount of the exhaust ozone gas necessary for cleaning the membrane
surface, and it is necessary to confirm the amount of the exhaust ozone gas and
the ozone gas concentration. However, if the characteristic of the gas suction
device 14 including the flow rate of the circulation pump 15, the characteristic
of the ozone gas generating device 11, and the capacity and the amount of
water of the buffer tank 13 and the ozone water generation tank 12 are known,
the above values can be confirmed on the basis of the dissolved ozone
concentration Co. Therefore, it is possible to perform more accurate control
than in the case of simple time management.
[0063] Meanwhile, in step S62 instead of step S60 described in Embodiment 1,
as an end condition for terminating the membrane surface cleaning process, it
is determined on whether or not the tank internal pressure Pt has decreased to
or below an internal pressure lower limit value Thp. That is, the membrane
24
surface cleaning is continued until the tank internal pressure Pt decreases to
the internal pressure lower limit value Thp. Then, when the tank internal
pressure Pt falls to the internal pressure lower limit value Thp, the supply of
the exhaust ozone gas is stopped, and ozone water generation is restarted (step
S80).
[0064] In the membrane surface cleaning process, it is necessary to confirm the
amount of the exhaust ozone gas, which is the base of the diluted exhaust
ozone gas caused to flow along the membrane surface. On the basis of the
data of the tank capacity and the like, the equivalent volume at atmospheric
pressure of the exhaust ozone gas supplied to the filtration unit 5 can be easily
confirmed from the change in the tank internal pressure Pt. That is, a
predetermined volume of the exhaust ozone gas can be used for cleaning the
membrane surface. Further, for example, in the case of the simple time
management, if the tank internal pressure Pt falls below the pressure
required for bubbling in the filtering unit 5 within the time, it is assumed that
only the air containing no ozone is bubbled for the remaining time. However,
if pressure control is implemented, such a situation can be reliably avoided.
[0065] In step S92 instead of step S90 described in Embodiment 1, as an end
condition for terminating the generation of ozone water in the second stage, it
is determined whether or not the dissolved ozone concentration Co has
increased to a second concentration threshold ThC2 or more, which is larger
than the first concentration threshold ThC1. That is, when the dissolved
ozone concentration Co increases to the second concentration threshold ThC2 or
more, which is larger than the first concentration threshold ThC1, the ozone
water generation in the second stage is stopped, and the process proceeds to
the backwashing process (step S100).
[0066] The purpose of the ozone water generation in the second stage is to
secure the dissolved ozone concentration Co of the ozone water necessary for
the backwashing, and accurate control is possible by measuring the dissolved
ozone concentration Co as in the present embodiment.
25
[0067] In other words, by measuring the dissolved ozone concentration Co, it is
possible to more reliably generate the exhaust ozone gas suitable for cleaning
the membrane surface and the ozone water suitable for backwashing.
Further, by measuring the tank internal pressure Pt, a suitable amount of the
exhaust ozone gas can be used for cleaning the membrane surface.
[0068] Embodiment 4
In each of the above-described Embodiments 1 to 3, an example has been
described in which whether or not the filtration process or the backwashing
should be ended is determined on the basis of time. In Embodiment 4, an
example in which determination is made on the basis of transmembrane
differential pressure of the filtration membrane will be described. FIG. 8 and
FIG. 9 show a filtration membrane cleaning apparatus according to
Embodiment 4, a configuration of a water treatment system equipped
therewith, and a filtration membrane cleaning method. FIG. 8 is a schematic
diagram showing a configuration of the water treatment system including the
filtration membrane cleaning apparatus, and FIG. 9 is a flowchart showing a
part related to the cleaning of the filtration membrane, that is, a filtration
membrane cleaning method within the operation of the water treatment
system equipped with the filtration membrane cleaning apparatus. Other
than the measurement of the transmembrane differential pressure and the
operation control using a measurement value of the transmembrane
differential pressure, Embodiment 4 is the same as Embodiment 3, and the
description of the same parts is omitted.
[0069] In the water treatment system 1 according to Embodiment 4, as shown
in FIG. 8, a pressure gauge 83 is provided in the filtered water pipe 72 to
measure the transmembrane differential pressure. The pressure in the
filtered water pipe 72 measured by the pressure gauge 83 is output to the
control unit 30. The pressure gauge 83 does not directly measure the
transmembrane differential pressure but measures the pressure on the
secondary side of the filter membrane 51, which can be evaluated as the
transmembrane differential pressure since the primary side can be regarded
as atmospheric pressure. Further, since the filtered water pipe 72
26
communicates with the ozone water supply pipe 25, the pressure applied to the
ozone water supply pipe 25 during the backwashing, that is, the
transmembrane differential pressure, can also be measured by the pressure
gauge 83.
[0070] The operation will be described referring to FIG. 9. In step S22 of
Embodiment 4, instead of step S20 described in Embodiment 1, an end
condition for terminating the filtration process is whether or not the
differential pressure (transmembrane differential pressure ΔP) applied to the
filtration membrane 51 during filtration exceeds an upper limit value ThΔM.
That is, the filtration process is continued until the transmembrane
differential pressure ΔP exceeds the upper limit value ThΔM. When the
transmembrane differential pressure ΔP exceeds the upper limit value ThΔM,
the process proceeds to the ozone water generation process (step S30).
[0071] In the filtration process of removing organic substances from the
biological reaction tank mixed water Wr on the primary side and passing only
the water to the secondary side, as the filtration progresses, the activated
sludge or the like accumulates on the surface of the primary side of the
filtration membrane 51 to form the cake layer as described referring to FIG. 3.
With the formation of the cake layer, a pressure loss when the water passes
from the primary side to the secondary side of the filtration membrane 51, that
is, the transmembrane differential pressure ΔP increases. Since an increase
in the transmembrane differential pressure ΔP leads to an increase in damage
to the filtration membrane 51 in addition to a decrease in the throughput per
hour, it is important to control the transmembrane differential pressure ΔP.
By measuring the transmembrane differential pressure ΔP, as in the present
embodiment, precise control for water treatment efficiency and facility
management is possible.
[0072] Further, as for an end condition for terminating the backwashing, in
step S112 instead of step S110 described in Embodiment 1, the condition is
whether the differential pressure applied to the filtration membrane 51 during
the backwashing (transmembrane differential pressure ΔP), where ozone
water passes from the secondary side to the primary side, has been reduced to
27
the lower limit value ThΔW or less. In other words, the backwashing is
continued until the transmembrane differential pressure ΔP is reduced to the
lower limit value ThΔW or less.
Then, when the transmembrane differential pressure ΔP is reduced to the
lower limit value ThΔW or less, the backwashing process is terminated (step
S120), and the process proceeds to the filtration process (step S10).
[0073] The purpose of the backwashing is to remove the organic substances
accumulated in the cake layer or the pores 51p of the filtration membrane 51
due to the filtration process and to clean the filtration membrane 51.
Therefore, contrary to the filtration process, the key is how much the
transmembrane differential pressure ΔP is reduced, and measuring the
transmembrane differential pressure ΔP, as in the present embodiment,
enables accurate control.
[0074] Although in both steps S22 and S112 the pressure is referred to as the
transmembrane differential pressure ΔP, the transmembrane differential
pressure ΔP in the filtration process is a differential pressure associated with
the pressure loss when the flow is from the primary side to the secondary side,
and the pressure on the secondary side is negative. In contrast, the
transmembrane differential pressure ΔP in the backwashing process is a
differential pressure associated with the pressure loss when the flow is from
the secondary side to the primary side, and the pressure on the secondary side
is positive. Therefore, the upper limit value ThΔM and the lower limit value
ThΔW are opposite in polarity and simple comparison is not possible, but a
value obtained by dividing the absolute value of the upper limit value ThΔM by
the flow rate at the time of filtration is larger than a value obtained by
dividing the absolute value of the lower limit value ThΔW by the flow rate per
hour at the time of backwashing. Accordingly, by measuring the
transmembrane differential pressure ΔP and determining whether or not to
end the processes on the basis of the measured values, it is possible to
accurately control the end timing of the filtration process and the end timing of
the backwashing.
28
[0075] Note that, in the above description, a configuration is shown in which
the components or the operation for measuring the transmembrane
differential pressure are added with respect to Embodiment 3. However, this
is not a limitation and may be a configuration in which the above-described
components or the operation for measuring the transmembrane differential
pressure is added to the configuration described in Embodiment 1 or
Embodiment 2.
[0076] Embodiment 5
In each of the above-described Embodiments 1 to 4, a mode of switching from
the filtration process to the membrane surface cleaning process and then the
backwashing process has been described by focusing on one filtration unit. In
Embodiment 5, a focus is placed on the fact that the filtration unit is composed
of a plurality of filters, and a mode of coordinating the switching of processes
in accordance with the situation of each filter will be described. FIG. 10 is for
the description of a filtration membrane cleaning apparatus according to
Embodiment 5, a configuration of a water treatment system equipped
therewith, and a filtration membrane cleaning method and is a schematic
diagram showing a configuration when switching timing of the process is
controlled to shift with respect to each of the plurality of filters. In the figure,
the part related to the generation or storage of the ozone water in the filtration
membrane cleaning apparatus is omitted, and only the piping portion in the
vicinity of the filtration unit is shown.
[0077] Since each of Embodiments 1 to 4 described above focuses on a single
filtration unit 5, the filtration process, the membrane surface cleaning process,
and the backwashing process are described in the flowchart such that each
process is performed when no other process is performed. However, as in
Embodiment 5, there is a case where one filtration membrane cleaning
apparatus 10 is connected to the filtration unit 5 including a plurality of filters
5a to 5d that are independently controllable. In this case, in the mode where
the membrane cleaning is performed in such a way that the time is shifted for
each of the filters 5a to 5d, in addition to the filtration process, the membrane
29
surface cleaning process, and the backwashing process, there may be a case in
which the ozone water generation process is also simultaneously performed.
[0078] Therefore, among the flowcharts used in the description of the
operation in each of Embodiments 1 to 4, the contents of the operation for each
process are the same, but the order of operation between the processes will be
parallel. Meanwhile the order of the filtration process, the membrane surface
cleaning process, and the backwashing process for one filter is the same as
described in the flowcharts used in the description of the operation in each of
Embodiments 1 to 4 except for the part for generating ozone water.
[0079] In the water treatment system 1 according to Embodiment 5, as shown
in FIG. 10, four filters 5a to 5d and air diffusers 41a to 41d each located
directly below each of the filters 5a to 5d are installed in combination in the
biological reaction tank 4. Therefore, in the filtered water pipe 72 for drawing
out the treated water after the filtration, branch pipes 72a to 72d branched
through on-off valves 65a to 65d are connected to the filters 5a to 5d,
respectively. In the ozone water supply pipe 25 for supplying ozone water for
the backwashing, the branch pipes 25a to 25d branched through on-off valves
66a to 66d are connected to the branch pipes 72a to 72d, respectively.
Pressure gauges 83a to 83d for measuring the transmembrane differential
pressure ΔP for each of the filters 5a to 5d are connected to the branch pipes
72a to 72d, respectively.
[0080] Similarly, in the aeration pipe 43 for supplying the aeration air, branch
pipes 43a to 43d branched through on-off valves 47a to 47d are connected to
the air diffusers 41a to 41d, respectively. In the exhaust ozone gas pipe 23 for
supplying the exhaust ozone gas for cleaning the membrane surface, branch
pipes 23a to 23d branched through on-off valves 17a to 17d are connected to
the branch pipes 43a to 43d, respectively.
[0081] Although not shown in the drawings, it is assumed that the three-way
valves 16 and 45 described in the configurations in Embodiments 1 to 4 are
replaced with, for example, a combination of a branched pipe and on-off valves
for switching between opening and closing for each of the branched parts in
Embodiment 5. For example, taking the three-way valve 16 as an example,
30
the three-way valve is replaced with a T-shaped pipe branching from the
circulation pump 15 to the gas suction device 14 and the ozone water supply
pipe 25, and the above-described on-off valves 66a to 66d are provided on the
ozone water supply pipe 25 side. Then, the mode described as "switching
only" between the circulation in the circulation pipe 22 and the supply to the
ozone water supply pipe 25 is replaced with a mode in which the circulation is
basic and the ozone water supply can be performed "in parallel".
[0082] The operation will be described on the basis of the above-described
configuration. In FIG. 10, regarding the filter 5a and the filter 5b, the on-off
valves 47a and 47b are open (white) and the on-off valves 17a and 17b are
closed (black), and the aeration air is discharged from the air diffuser 41a
directly below. Since the on-off valves 65a and 65b are open and the on-off
valves 66a and 66b are closed, the secondary side becomes negative pressure
by the suction pump 44, and the biological reaction tank mixed water Wr flows
from the primary side to the secondary side. That is, in the filters 5a and 5b,
a filtration process of filtering the biological reaction tank mixed water Wr is
performed.
[0083] In contrast, regarding the filter 5c, both the on-off valve 47c and the
on-off valve 17c are open, and the diluted exhaust ozone gas in which the
exhaust ozone gas and the air are mixed is discharged from the air diffuser 41a
directly below the filter 5c. Since both the on-off valve 65c and the on-off
valve 66c are closed, there is no flow between the primary side and the
secondary side. In other words, in the filter 5c, the membrane surface
cleaning is performed with the diluted exhaust ozone gas.
[0084] And regarding the filter 5d, both the on-off valve 47d and the on-off
valve 17d are closed and, and bubbles are not discharged from the air diffuser
41a directly below. Since the on-off valve 65d is closed and the on-off valve
66d is open, ozone water from the ozone water generation tank 12 is supplied
to the secondary side of the filter 5d. That is, in the filter 5d, the back
washing with ozone water is performed.
[0085] Here, in the filters 5a and 5b in which the filtration process is being
performed, the transmembrane differential pressure ΔP during the filtration is
31
evaluated on the basis of the respective measurement values of the pressure
gauges 83a and 84b, and for example, it is determined whether or not the
filtration process should be continued as described in step S22 of Embodiment
4. Upon completion of the filtration process, the process proceeds to the
membrane surface cleaning process.
[0086] Similarly, in the filter 5d in which the backwashing process is being
performed, the transmembrane differential pressure ΔP during the
backwashing is evaluated on the basis of the measurement value of the
pressure gauge 83d, and for example, it is determined whether or not the
backwashing process should be continued as described in step S112. When
the backwashing process is ended, the process proceeds to the filtration
process. Further, in the filter 5c in which the membrane surface cleaning is
performed, for example, as described in step S60 of Embodiment 1, it is
determined whether or not the membrane surface cleaning process should be
continued on the basis of the duration. Then, the membrane surface cleaning
process is ended, the process proceeds to the backwashing process.
[0087] Note that, while each of the filters 5a to 5d individually repeats the
above-described processes, basically, the ozone water generation is
continuously performed. Specifically, while the circulation pump 15 is in
operation, water or ozone water circulates in the circulation pipe 22 regardless
of the state of the communication with the ozone water supply pipe 25. In
other words, it is assumed that ozone water is appropriately generated so as
not to be short of the exhaust ozone gas necessary for the membrane surface
cleaning or the ozone water necessary for the backwashing, in any one of the
filters 5a to 5d. Further, while the filtration pump 44 is in operation, the
filtered water after the treatment is controlled so as to be transferred to the
filtered water tank through the filtered water transfer pipe 73 regardless of
whether or not the filtered water is supplied to the ozone water generation
tank 12.
[0088] This configuration allows for efficient water treatment by performing
the cleaning process including the surface cleaning and the backwashing in an
optimal cycle in accordance with properties of the filters 5a to 5d, such as when
32
each of the filters 5a to 5d has different cake accumulation or different
filtration performance. Of course, when the properties of the filters 5a to 5d
are the same and the state of the biological reaction tank mixed water Wr to be
treated is also stable, each process can also be controlled by the time
management. In the case of the time management, for example, if any one of
the plurality of filters 5a to 5d is set to necessarily perform the filtration
process, the processing for the biological reaction tank mixed water Wr can be
continuously performed without delay.
[0089] That is, as shown in Embodiment 5, if the processes can be set
individually for each of the plurality of filters 5a to 5c, stable water treatment
can be implemented. Note that, in Embodiment 5, the example is shown in
which four filters 5a to 5d are used, but it is needless to say that the
embodiment can be applied to filters having two or more filters, not limited to
four.
[0090] Note that, when the control unit 30, a control unit for the water
treatment system (not shown), or a control unit in which they are integrated is
referred to as hardware 3, for example, as shown in FIG. 11, the hardware 3 is
configured with a processor 31 and a storage device 32. Although not shown,
the storage device includes a volatile storage device such as a random access
memory and a nonvolatile auxiliary storage device such as a flash memory.
Instead of the flash memory, a hard disk of an auxiliary storage device may be
provided. The processor 31 executes a program input from the storage device
32. In this case, the program is input from the auxiliary storage device to the
processor 31 via the volatile storage device. The processor 31 may output
data such as the calculation result to the volatile storage device of the storage
device 32 or may store the data in the auxiliary storage device via the volatile
storage device.
[0091] As exemplified in each embodiment, by adopting the membrane surface
cleaning using the diluted exhaust ozone gas in the present application, it is
possible to enhance the efficiency of ozone gas utilization with a simple
configuration without increasing the size of the apparatus or adding
equipment. In particular, for the purpose of cleaning the membrane surface,
33
not the exhaust ozone gas itself but the ozone gas after diluted is discharged
into the biological reaction tank 4 as bubbles, so that the irreversible
membrane fouling does not occur and the degree of contamination of the
membrane can be reliably reduced.
[0092] Further, since the exhaust ozone gas used for the membrane surface
cleaning is consumed in the biological reaction tank 4, and regarding the ozone
water used for the backwashing, ozone therein is consumed before flowing out
to the primary side of the filtration, a device for treating the excess ozone gas,
such as a reducing device using a catalyst, activated carbon, or the like, is not
required, and it is possible to prevent an increase in the size of the apparatus
and to prevent an increase in the cost. Furthermore, since, prior to the
backwashing, the degree of contamination of the filtration membrane is
reduced by the exhaust ozone gas, the time required for the backwashing can
be shortened, and the amount of ozone gas generated necessary for the
cleaning can be reduced.
[0093] Note that, although various exemplary embodiments and examples are
described in the present application, various features, aspects, and functions
described in one or more embodiments are not inherent in a particular
embodiment, and can be applicable alone or in their various combinations to
each embodiment. Accordingly, countless variations that are not illustrated
are envisaged within the scope of the art disclosed herein. For example, the
case where at least one component is modified, added or omitted, and the case
where at least one component is extracted and combined with a component in
another embodiment are included.
[0094] As described above, according to the filtration membrane cleaning
apparatus 10 of each embodiment, the apparatus is configured to include an
accumulation unit (gas suction device 14, ozone water generation tank 12,
buffer tank 13) to generate the ozone water by dissolving the ozone gas into
water and accumulate the generated ozone water and the ozone gas of a
by-product; a backwashing mechanism (ozone water generation tank 12,
circulation pump 15) to perform the backwashing on the filtration unit 5
installed in the biological reaction tank 4 for filtering the raw water, the
34
backwashing mechanism causing the ozone water taken out from the
accumulation unit to flow from the secondary side of the filtration membrane
51 to the primary side thereof; a membrane surface cleaning mechanism
(ozone water generation tank 12, buffer tank 13, air diffuser 41) to perform the
membrane surface cleaning by discharging bubbles containing ozone in the
biological reaction tank 4 and causing the discharged bubbles to flow along the
surface on the primary side (primary side surface 51f1) of the filtration
membrane 51; the control unit 30 to control the operation of the backwashing
mechanism and the operation of the membrane surface cleaning mechanism so
as to perform the membrane surface cleaning by the membrane surface
cleaning mechanism prior to the backwashing by the backwashing
mechanism; and an exhaust ozone gas dilution unit (blower 42) to dilute the
exhaust ozone gas taken out from the accumulation unit for supplying the
membrane surface cleaning mechanism with the diluted exhaust ozone gas as
a gas to be discharged as the bubbles. Thus, the ozone is effectively utilized,
and the filtration membrane is efficiently cleaned without causing the
irreversible membrane fouling.
[0095] In particular, since the exhaust ozone gas dilution unit dilutes the
exhaust ozone gas taken out from the accumulation unit to 1.4 times or more
or to the ozone concentration of 150 g/m3 or less, the irreversible membrane
fouling is reliably prevented and the filtration membrane can be efficiently
cleaned.
[0096] If the membrane surface cleaning mechanism is configured to be
connected to the air diffuser 41 for aerating the raw water stored in the
biological reaction tank 4 and discharge the bubbles from the air diffuser 41,
an additional constituent member can be omitted.
[0097] Further, since the pressure gauge 83 to measure the differential
pressure (transmembrane differential pressure ΔP) between the primary side
and the secondary side when the fluid is caused to flow between the primary
side of the filtration membrane 51 and the secondary side thereof is provided,
and the control unit 30 determines at least whether or not to start the
backwashing or whether or not to end the backwashing in accordance with the
35
measured differential pressure, efficient filtration processing can be
implemented.
[0098] And in a water treatment system including the biological reaction tank
4 to store the raw water for the filtration process, a filtration unit 5 installed in
the biological reaction tank 4 for filtering the raw water, and the filtration
membrane cleaning apparatus 10 described above for cleaning the filtration
membrane 51 of the filtration unit 5, the filtration membrane 51 is efficiently
maintained and the filtration treatment capacity is improved.
[0099] The filtration unit 5 includes the plurality of filters 5a to 5d that each
independently perform the filtration processing of the raw water, and each of
the plurality of filters includes the pressure gauge 83(83a to 83d) for
measuring the differential pressure (transmembrane differential pressure ΔP)
between the primary side and the secondary side when the fluid is caused to
flow between the primary side of the filtration membrane 51 of each filter and
the secondary side thereof, and the control unit 30 can efficiently process the
filtration membrane 51 by prioritizes a filter having a higher rate of increase
in the differential pressure among the plurality of filters for the membrane
surface cleaning and the backwashing.
[0100] Further, the filtration membrane cleaning method according to each of
the embodiments is configured to include an accumulation step (steps S30 and
S80) in which the ozone gas is dissolved in water to generate the ozone water
and the generated ozone water and the exhaust ozone gas of the by-product are
accumulated; a membrane surface cleaning step (step S50) in which the
bubbles containing the ozone gas are caused to flow along the surface of the
primary side (primary side surface 51f1) of the filtration membrane 51 for the
filtration unit 5 that performs the filtration process of the raw water; and a
backwashing step (step S100) in which the ozone water accumulated in the
accumulation step is taken out and the ozone water taken out is caused to flow
from the secondary side of the filtration membrane 51 toward the primary side
thereof, wherein in the membrane surface cleaning step, the exhaust ozone gas
accumulated in the accumulation step is caused to flow as the babbles after
being diluted. Thus, the ozone can be effectively utilized, and the filtration
36
membrane can be efficiently cleaned without causing the irreversible
membrane fouling.
[0101] In particular, in the membrane surface cleaning step, the accumulated
exhaust ozone gas is diluted to 1.4 times or more, or to the ozone concentration
of 150 g/m3 or less, so that the irreversible membrane fouling is reliably
prevented and the filtration membrane can be efficiently cleaned.
Description of Reference Numerals and Signs
[0102]
1 : water treatment system, 4 : biological reaction tank, 5 : filtration unit, 5a to
5d : filter, 10 : filtration membrane cleaning apparatus, 11 : ozone gas
generating device, 12 : ozone water generation tank, 13 : buffer tank, 14 : gas
suction device, 15 : circulation pump, 16 : three-way valve (for ozone water), 17,
17a to 17d : on-off valve (for exhaust ozone gas), 18 : on-off valve (for dilution),
22 : circulation pipe, 23 : exhaust gas pipe, 23a to 23d : branch pipe, 23 x : tip
opening, 24 : filtered water supply pipe, 25 : ozone water supply pipe, 25a to
25d : branch pipe, 30: control unit, 41, 41a to 41d : air diffuser, 42 ; blower, 43 :
aeration pipe, 43a to 43d : branch pipe, 44 ; filtration pump, 45 : three-way
valve (for filtered water), 51 : filtration membrane. 51f1 : primary side surface,
51p : pore, 65a to 65d : on-off valve (for switching of filtered water), 66a to 66d :
on-off valve (for switching of ozone water), 71 : raw water pipe, 72 : filtered
water pipe, 72a to 72d : branch pipe, 73 : filtered water transfer pipe, 81 :
pressure gauge, 82 : dissolved ozone concentration meter, 83, 83a to 83d :
pressure gauge (for transmembrane differential pressure), 90 : organic
substance, Co : dissolved ozone concentration, Pg : gas phase region, Pt : tank
internal pressure, Pw : liquid phase region, Thc1 : first concentration threshold
(of dissolved ozone concentration when determining to start membrane surface
cleaning), Thc2 : second concentration threshold (of dissolved ozone
concentration when determining to start backwashing), Thp : internal
pressure lower limit value (of buffer tank pressure when determining to end
membrane surface cleaning), Th∆M : upper limit value (of transmembrane
differential pressure during filtration), Th∆W : lower limit value (of
37
transmembrane differential pressure when determining to end backwashing),
Wr : biological reaction tank mixed water, ∆P : transmembrane differential
pressure
38
WE CLAIM:
1. A filtration membrane cleaning apparatus comprising:
an accumulation unit to generate ozone water by dissolving an ozone
gas into water and accumulate the generated ozone water and an exhaust
ozone gas of a by-product;
a backwashing mechanism to perform backwashing on a filtration unit
installed in a biological reaction tank for filtering raw water, the backwashing
mechanism causing the ozone water taken out from the accumulation unit to
flow from a secondary side of a filtration membrane to a primary side thereof;
a membrane surface cleaning mechanism to perform membrane
surface cleaning by discharging bubbles containing ozone in the biological
reaction tank and causing the discharged bubbles to flow along a surface on
the primary side of the filtration membrane;
a controller to control operation of the backwashing mechanism and
operation of the membrane surface cleaning mechanism so as to perform the
membrane surface cleaning by the membrane surface cleaning mechanism
prior to the backwashing by the backwashing mechanism; and
an exhaust ozone gas dilution unit to dilute the exhaust ozone gas
taken out from the accumulation unit for supplying the membrane surface
cleaning mechanism with the diluted exhaust ozone gas as a gas to be
discharged as the bubbles.
2. The filtration membrane cleaning apparatus according to claim 1, wherein
the exhaust ozone gas dilution unit dilutes the exhaust ozone gas taken out
from the accumulation unit to 1.4 times or more or to an ozone concentration of
150 g/m3 or less.
3. The filtration membrane cleaning apparatus according to claim 1 or 2,
wherein the membrane surface cleaning mechanism is connected to an air
39
diffuser for aerating the raw water stored in the biological reaction tank and
discharges the bubbles from the air diffuser.
4. The filtration membrane cleaning apparatus according to any one of claims
1 to 3, further comprising:
a pressure gauge to measure a differential pressure between the
primary side and the secondary side when fluid is caused to flow between the
primary side of the filtration membrane and the secondary side thereof,
wherein
the control unit determines at least whether or not to start the
backwashing or whether or not to end the backwashing in accordance with the
measured differential pressure.
5. A water treatment system, comprising:
a biological reaction tank to store raw water;
a filtration unit disposed in the biological reaction tank for filtering the
raw water; and
a filtration membrane cleaning apparatus according to any one of
claims 1 to 4 for cleaning the filtration membrane of the filtration unit.
6. The water treatment system according to claim 5, wherein
the filtration unit includes a plurality of filters that each
independently perform a filtration process of the raw water, and each of the
plurality of filters includes a pressure gauge that measures a differential
pressure between a primary side of a filtration membrane of each filter and a
secondary side thereof when the fluid is caused to flow between the primary
side and the secondary side; and
the control unit prioritizes a filter having a higher rate of increase in
the differential pressure among the plurality of filters for the membrane
surface cleaning and the backwashing.
7. A filtration membrane cleaning method comprising steps of:
40
an accumulation step in which ozone water is generated by dissolving
an ozone gas into water and the generated ozone water and an exhaust ozone
gas of a by-product are accumulated;
a membrane surface cleaning step in which bubbles containing the
ozone gas are caused to flow along a surface on a primary side of a filtration
membrane for a filtration unit that performs a filtration process of raw water;
and
a backwashing step in which the ozone water accumulated in the
accumulation step is taken out and the ozone water taken out is caused to flow
from a secondary side of the filtration membrane toward the primary side
thereof, wherein
in the membrane surface cleaning step, the exhaust ozone gas
accumulated in the accumulation step is caused to flow as the bubbles after
being diluted.
8. The filtration membrane cleaning method according to claim 7, wherein, in
the membrane surface cleaning step, the accumulated exhaust ozone gas is
diluted to 1.4 times or more or to an ozone concentration of 150 g/m3 or less.