Technical Field
The present invention relates to a cleaning method for a water treatment membrane.
Background Art
In a seawater desalination device provided with a reverse osmotic membrane,
seawater to be treated is first passed through the 5 treatment device filled with a hollow fiber
membrane or the like so that impurities such as solid matter are eliminated. The seawater
treated with the treatment device is pressurized with a high-pressure pump and brought
into contact with the reverse osmotic membrane so as to be separated into freshwater
which passes through the reverse osmotic membrane and concentrated seawater which
10 does not pass through the reverse osmotic membrane. The resulting freshwater is used
for applications such as drinking water.
One cause of decreases in the permeability of a reverse osmotic membrane is
clogging resulting from the attachment of scale including metal compounds such as iron
and manganese as well as organic matter including microorganisms contained in the
15 seawater and metabolic products thereof. A chemical cleaning line is typically installed
in a seawater desalination device provided with a reverse osmotic membrane for the
purpose of eliminating this clogging. When the amount of treated water of the reverse
osmotic membrane or the hollow fiber membrane has reduced, the operation is stopped
and chemical cleaning is performed using chemicals.
20 A known method for eliminating scale attached to a water treatment membrane
such as a reverse osmotic membrane or a hollow fiber membrane includes a method of
cleaning with a cleaning fluid containing a chemical such as hypochlorous acid, citric acid,
or hydrogen peroxide. In addition to the chemicals described above, Patent Document 1
proposes a cleaning fluid containing percarbonate and an iron salt. Since percarbonate
25 produces hydrogen peroxide in water, it reacts with the iron salt to yield an OH radical.
An effect of oxidatively decomposing organic matter using this OH radical is described.
Citation List
Patent Document
Patent Document 1: Japanese Patent No. 4384741B
30 Summary of Invention
Technical Problems
In order to avoid the oxidative degradation of a water treatment membrane, citric
acid, which has a mild detergency, is often used for the purpose of cleaning scale.
3
However, since the detergency of citric acid is low around room temperature, it is difficult
to sufficiently eliminate metal compounds in the scale attached to a water treatment
membrane with citric acid. In addition, organic matter such as insoluble polysaccharides
insoluble in acid is contained in the scale attached to the water treatment membrane. It is
therefore extremely difficult to sufficiently 5 eliminate scale containing organic matter with
citric acid.
To eliminate organic matter in scale, the use of a potent oxidizer such as
hypochlorous acid or hydrogen peroxide has also been proposed in the past. However,
due to concerns regarding the oxidative degradation of the water treatment membrane,
10 there are practically no cases in which this has been used on a practical level at an actual
water treatment plant. The cleaning fluid containing sodium percarbonate proposed in
Patent Document 1 produces hydrogen peroxide from sodium percarbonate, so the concern
regarding the oxidative degradation of the water treatment membrane is not eliminated.
Scale attached to a water treatment membrane used in an actual water treatment
15 plant contains not only organic matter, but also large quantities of metal compounds.
Since these metal compounds function as a cracking catalyst for hydrogen peroxide, active
oxygen having strong oxidative power is generated in large quantities, which induces the
oxidative degradation of the water treatment membrane or diminishes the membrane
performance.
20 As described above, conventional cleaning methods has a problem in that it is
difficult to select the type of chemical to be contained in the cleaning fluid since there is a
trade-off relationship between avoiding the oxidative degradation of the water treatment
membrane and achieving the detergency required to eliminate the attached scale. In
addition, there are also problems with conventional methods in which a cleaning fluid is
25 brought into contact with the water treatment membrane.
Cleaning is performed with a circulation cleaning method, wherein a feeder line
and an outlet line are connected to a vessel provided with a water treatment membrane on
the inside thereof, and a cleaning fluid is continuously fed from the feeder line so that the
circulating cleaning fluid is brought into contact with the water treatment membrane, after
30 which the cleaning fluid is continuously discharged from the outlet line. With this
method, the cleaning fluid easily flows to locally formed flow paths or channels which
have little resistance due to a small amount of scale attachment, but it is difficult for the
cleaning fluid to flow to sites where there is a large amount of scale attachment. Thus,
the cleaning effect of sufficiently eliminating scale is difficult to achieve.
4
The present invention was conceived in order to solve the problems described
above, and an object thereof is to provide a cleaning method for a water treatment
membrane which yields a sufficient cleaning effect and is capable of suppressing the
oxidative degradation of a water treatment membrane caused by active oxygen originating
5 from an oxidizer.
Solution to Problem
In order to solve the above-described problems, the present invention provides the
following means.
A first aspect of the present invention is a cleaning method for a water treatment
10 membrane provided with a primary surface for inflow of untreated water and a secondary
surface for outflow of treated water, the method including: a first cleaning step of bringing
a first cleaning fluid containing a metal eluent into contact with at least the primary surface
and eliminating metallic scale attached to the water treatment membrane; and a second
cleaning step of bringing a second cleaning fluid containing an oxidizing agent into contact
15 with at least the primary surface and eliminating organic scale attached to the water
treatment membrane.
With the cleaning method for a water treatment membrane according to the first
aspect, most metal scale is eliminated by the first cleaning step, so there is no concern that
a large amount of active oxygen originating from the oxidizer may be generated in the
20 second cleaning step. As a result, it is possible to suppress the oxidative degradation of
the water treatment membrane due to active oxygen and to achieve a sufficient cleaning
effect.
A second aspect of the present invention is the cleaning method for a water
treatment membrane according to the first aspect, wherein in the first cleaning step, after
25 the first cleaning fluid is passed in the forward direction from the primary surface to the
secondary surface, at least the primary surface is kept in a state immersed in the first
cleaning fluid.
With the cleaning method for a water treatment membrane according to the second
aspect, it is possible to supply a fresh first cleaning fluid which contains an appropriate
30 concentration of a metal eluent without containing an eluent from the scale to the primary
surface, where the amount of attached scale is large. Further, by maintaining an
immersed state, it is possible to deliver the first cleaning fluid to every corner of the water
5
treatment membrane and to sufficiently elute the metal scale attached to the primary and
secondary surfaces. As a result, the cleaning efficiency is further enhanced.
A third aspect of the present invention is the cleaning method for a water treatment
membrane according to the first or second aspect, wherein in the second cleaning step,
after the second cleaning fluid is passed in the reverse 5 direction from the secondary surface
to the primary surface, at least the primary surface is kept in a state immersed in the
second cleaning fluid.
With the cleaning method for a water treatment membrane according to the third
aspect, it is possible to wash the decomposition products of the organic scale to the
10 primary surface side so as to enable efficient cleaning. Further, by maintaining an
immersed state, it is possible to deliver the second cleaning fluid to every corner of the
water treatment membrane and to sufficiently elute the organic scale attached to the
primary and secondary surfaces. As a result, the cleaning efficiency is further enhanced.
A fourth aspect of the present invention is the cleaning method for a water
15 treatment membrane according to any one of the first through third aspects, wherein, prior
to the first cleaning step, the method further includes a preliminary cleaning step of
bringing a third cleaning fluid containing an oxidizer with a lower concentration than that
of the second cleaning fluid into contact with at least the primary surface so as to eliminate
organic scale attached to the water treatment membrane.
20 With the cleaning method for a water treatment membrane according to the fourth
aspect, the concentration of the oxidizer that is used is low, so a rinsing step is unnecessary
prior to transitioning to ordinary operation. That is, it is possible to transition to ordinary
operation immediately after the preliminary cleaning step is performed. Thus, it is
possible to reduce the rate of attachment of organic scale by frequently performing the
25 preliminary cleaning step as a type of routine maintenance.
A fifth aspect of the present invention is the cleaning method for a water treatment
membrane according to the fourth aspect, wherein an operation of performing water
treatment using the water treatment membrane and the preliminary cleaning step are
alternately repeated at least once.
30 With the cleaning method for a water treatment membrane according to the fifth
aspect, it is possible to transition to the ordinary operation for performing water treatment
without a rinsing step after a simple preliminary cleaning step is completed in a short
6
amount of time, which makes it possible to enhance operational efficiency. In addition,
by alternately repeating the operation and the preliminary cleaning step, it is possible to
extend the amount of time until the amount of attached organic scale reaches an amount
that would impede operations, which makes it possible to further enhance operational
efficiency. After running for a long period of time, 5 time can be taken to clean organic
scale that has attached in large quantities with the first and second cleaning steps.
A sixth aspect of the present invention is the cleaning method for a water treatment
membrane according to the fifth aspect, wherein organic scale attached to the water
treatment membrane gradually increases each time the operation and the preliminary
10 cleaning step are alternately repeated.
With the cleaning method for a water treatment membrane according to the sixth
aspect, when transitioning to the first and second cleaning steps, it is easy to leave behind a
small amount of metal scale in the water treatment membrane after the first cleaning step.
Using the small amount of remaining metal scale as a catalyst, active oxygen originating
15 from the oxidizer in the second cleaning step is generated by the action of the catalyst,
which makes it possible to even further enhance the organic scale cleaning effect.
A seventh aspect of the present invention is the cleaning method for a water
treatment membrane according to any one of the first through sixth aspects, wherein the
metal eluent is any one or more types selected from the group consisting of citric acid,
20 phosphonic acid, glycolic acid, ethylenediaminetetraacetic acid, formic acid, and oxalic
acid.
With the cleaning method for a water treatment membrane according to the seventh
aspect, it is easy to eliminate most metal scale while leaving behind a small amount of
metal scale in the water treatment membrane. Using the small amount of remaining
25 metal scale as a catalyst, active oxygen originating from the oxidizer is generated by the
effect of the catalyst in the second cleaning step, which makes it possible to even further
enhance the organic scale cleaning effect.
An eighth aspect of the present invention is the cleaning method for a water
treatment membrane according to any one of the first through seventh aspects, wherein the
30 oxidizer contained in the second cleaning fluid is any one or more types selected from the
group consisting of hydrogen peroxide, percarbonate, persulfate, hypochlorite,
permanganate, chlorine dioxide, and ozone.
7
With the cleaning method for a water treatment membrane according to the eighth
aspect, a small amount of metal scale remaining in the first cleaning step is used as a
catalyst, and active oxygen originating from the oxidizer can be generated by the action of
the catalyst. As a result, it is possible to even further enhance the organic scale cleaning
5 effect.
A ninth aspect of the present invention is the cleaning method for a water treatment
membrane according to any one of the first through eighth aspects, wherein, prior to the
first cleaning step, freshwater is passed through the water treatment membrane to eliminate
salt attached to the water treatment membrane.
10 With the cleaning method for a water treatment membrane according to the ninth
aspect, it is possible to prevent the reduction of the metal scale eliminating effect of the
metal eluent by allowing the metal eluent used in the first cleaning step to precipitate by
reacting with the salt.
A tenth aspect of the present invention is the cleaning method for a water treatment
15 membrane according to any one of the first through ninth aspects, wherein, after the first
cleaning step, a catalyst solution containing a metal salt functioning as a catalyst for
generating active oxygen from the oxidizer contained in the second cleaning fluid is
brought into contact with the water treatment membrane.
With the cleaning method for a water treatment membrane according to the tenth
20 aspect, even if nearly all of the metal scale is eliminated in the first cleaning step, a metal
component contained in the catalyst solution may be attached to the water treatment
membrane, and the metal component may be made to function as a catalyst in the second
cleaning step so as to generate active oxygen in the water treatment membrane. As a
result, it is possible to even further enhance the organic scale cleaning effect.
25 Advantageous Effects of Invention
With the cleaning method for a water treatment membrane according to the present
invention, it is possible to suppress the oxidative degradation of the water treatment
membrane due to active oxygen originating from an oxidizer, and to achieve a sufficient
cleaning effect.
30 Brief Description of Drawings
FIG. 1 is a cross-sectional schematic view of a reverse osmotic membrane module
in which a reverse osmotic membrane is provided in a vessel.
8
FIG. 2 is a cross-sectional schematic view of a reverse osmotic membrane
illustrating a state in which scale is eliminated stepwise by the cleaning method of a first
embodiment of the present invention.
FIG. 3 is a cross-sectional schematic view of a reverse osmotic membrane
illustrating a state in which scale is eliminated 5 stepwise by the cleaning method of a
second embodiment of the present invention.
Description of Embodiments
The cleaning method of the present invention can be applied to a known water
treatment membrane. The types and shapes of water treatment membranes to which the
10 cleaning method of the present invention can be applied are not particularly limited. For
example, the membrane may be a flat disc-shaped membrane, a hollow fiber membrane, a
spiral membrane, or a tubular membrane. The water treatment membrane preferably has
at least two surfaces including a front surface and a back surface, that is, a primary surface
(front surface) for inflow of untreated water to be treated and a secondary surface (back
15 surface) for outflow of treated water passing through the water treatment membrane. The
type of untreated water to be treated by the water treatment membrane is not particularly
limited, and examples thereof include seawater, river water, tap water and wastewater, rain
water, and industrial wastewater.
The cleaning method of the present invention exhibits an outstanding effect on a
20 water treatment membrane to which organic scale and metal scale are attached, so the
untreated water preferably contains organic substances and metal salts. In addition, since
the cleaning method of the present invention can efficiently clean a water treatment
membrane installed in a large-scale water treatment device in an online state without
eliminating the water treatment membrane to the outside of the device, the method is
25 suitable for the cleaning of a reverse osmotic membrane (RO membrane) installed in a
seawater desalination plant, for example.
The constituent material of the water treatment membrane to which the cleaning
method of the present invention is applied is not particularly limited, and examples thereof
include polyolefin, polysulfone, polyacrylonitrile, polyester, polycarbonate, polyamide,
30 polyvinyl chloride, polyvinyl alcohol, polyvinylidene chloride, polyvinyl fluoride,
polyvinylidene chloride, hexafluoropropylene, chlorotrifluoroethylene, tetrafluoroethylene,
cellulose acetate, silicone polymers, and ceramics.
An example of a water treatment membrane to which the cleaning method of the
present invention can be applied is an RO membrane module 1 illustrated in FIG. 1. In
9
the RO membrane module 1 illustrated in FIG. 1, a plurality of hollow fiber RO
membranes 2 (also simply called “hollow fiber membranes 2” hereafter) are folded back in
a U-shape, fixed with a resin while maintaining a state in which the end of each hollow
fiber membrane 2 is open, and further housed in a vessel (pressure-resistant container) 6.
Seawater SW is fed into the vessel 6 from a feeder 5 line 3 and brought into contact
and passed through a primary surface constituting the outer periphery of the hollow fiber
membranes 2. Desalinated permeated water FW is accumulated at both ends of each
hollow fiber membrane 2 from a secondary surface constituting the inner periphery of the
hollow fiber membrane 2 and is collected from a permeated water outlet line 4.
10 Concentrated water that has not passed into each hollow fiber membrane 2 is discharged
from a brine outlet line 5 to the outside of the vessel 6.
Embodiments of the cleaning method of the present invention will be described
hereinafter using a case in which an RO membrane 2 provided in the RO membrane
module 1 of FIG. 1 is used as an example.
15 As illustrated in FIG. 2(I), metal scale 11 containing metal ions contained in the
seawater and organic scale 12 containing organic matter are attached to a primary surface
2a of an RO membrane 2 after a desalination operation (run) has been performed.
In the drawing, each scale is depicted in two separate layers for convenience, but in
actuality, both scales are attached in a mixed state. In addition, there are also cases in
20 which the same scale is attached not only to the primary surface 2a, but also to the inside
and a secondary surface 2b of the RO membrane 2, but such cases are not illustrated.
Ordinarily, the amount of scale attached to the primary surface 2a is greater than the
amount of scale attached to the inside and the secondary surface 2b of the RO membrane
2.
25 First Embodiment
In the cleaning method of a first embodiment for cleaning an RO membrane 2
provided with a primary surface 2a for inflow of untreated water and a secondary surface
2b for outflow of permeated water, a first cleaning step and a second cleaning step are
performed in this order. There may be other steps between the first and second cleaning
30 steps, and there may be other steps before the first cleaning step or after the second
cleaning step.
10
The first cleaning step is a step in which a first cleaning fluid containing a metal
eluent is brought into contact with at least the primary surface 2a so as to eliminate the
metal scale 11 attached to the RO membrane 2. The first cleaning fluid preferably also
comes into contact with the inside and the secondary surface 2b of the RO membrane 2.
The second cleaning step is a step in 5 which a second cleaning fluid containing an
oxidizer is brought into contact with at least the primary surface 2a so as to eliminate the
organic scale 12 attached to the RO membrane 2. The second cleaning fluid preferably
also comes into contact with the inside and the secondary surface 2b of the RO membrane
2.
10 In this embodiment, it is important for the organic scale 12 to be eliminated using
an oxidizer after the metal scale 11 is first eliminated. If an oxidizer were to be brought
into contact with the RO membrane 2 in a state with a large amount of residual metal scale
11, a large amount of active oxygen (called active oxygen as a term including
oxygen-based radicals) may be generated from the oxidizer since the metal scale 11
15 functions as a catalyst. When a large amount of active oxygen is generated on the surface
and inside of the RO membrane 2, a problem arises in which the oxidative degradation of
the RO membrane 2 occurs. In this embodiment, the metal scale 11 is eliminated in
advance, which makes it possible to prevent active oxygen originating from the oxidizer
from oxidatively degrading the RO membrane 2 and to sufficiently eliminate both the
20 metal scale 11 and the organic scale 12.
Procedure of the first cleaning step
The procedure of this embodiment is such that concentrated water is first
discharged from the brine outlet line 5, and after the first cleaning fluid is passed in the
forward direction (in the filtering direction) from the primary surface 2a of the RO
25 membrane 2 to the secondary surface 2bof the RO membrane 2, at least the primary
surface 2a is kept in a state of being immersed in the first cleaning fluid.
By passing the first cleaning fluid through in the forward direction, it is possible to
supply a fresh first cleaning fluid which contains an appropriate concentration of a metal
eluent without containing an eluent from the scale to the primary surface 2a, where the
30 amount of attached scale 11 and 12 is large.
The first cleaning fluid may also be allowed to pass through in the reverse direction,
but the metal eluent may be consumed on the secondary surface 2b side or may be trapped
in the secondary surface 2b without being able to pass through the RO membrane 2. This
11
may prevent the supply of a sufficient amount of the metal eluent to the primary surface 2a
and may diminish the cleaning efficiency in comparison to the case of the forward
direction. When the first cleaning fluid is passed through in the reverse direction, a metal
eluent capable of passing through the RO membrane 2 is used.
After the first cleaning fluid is allowed 5 to pass through, the first cleaning fluid is
kept in a state in which the space on the primary surface 2a side inside the vessel 6 is filled
with the first cleaning fluid. This makes it possible to maintain a state in which at least
the primary surface 2a is immersed in the first cleaning fluid. By increasing the pressure
slightly in this state, some of the first cleaning fluid penetrates into the RO membrane 2
10 and leaks to the secondary surface 2b. As a result of this pressurization, the inside and
the secondary surface 2b of the RO membrane 2 may also be immersed at the same time as
the primary surface 2a. Alternatively, the space inside the membrane on the side where
water has accumulated may be filled by injecting the first cleaning fluid into the vessel 6
from the permeated water outlet line 4 so as to maintain a state in which the secondary
15 surface 2b of the RO membrane 2 is immersed in the first cleaning fluid.
The method for maintaining a state in which the RO membrane 2 is immersed in
the first cleaning fluid is not particularly limited. For example, the first cleaning fluid
may be fed from the feeder line 3 to fill the space on the primary surface 2a side inside the
vessel 6, and the circulation of the first cleaning fluid may be stopped by then stopping
20 feeding and sealing the vessel 6. Alternatively, the first cleaning fluid may be continued
to be fed even after the space on the primary surface 2a side inside the vessel 6 is filled
with the first cleaning fluid, and the first cleaning fluid may be discharged from the brine
outlet line 5 in the same amount as the fed amount so as to maintain a state in which the
RO membrane 2 is immersed in the first cleaning fluid while the first cleaning fluid is
25 circulated.
By maintaining a state in which at least one, and preferably both, of the primary
surface 2a and the secondary surface 2b are immersed in the first cleaning fluid, it is
possible to sufficiently elute the metal scale 11 attached to the primary surface 2a and the
secondary surface 2b and to thereby further enhance the elimination efficiency thereof.
30 The amount of time that the surfaces are kept in the immersed state is not
particularly limited and may, for example, be set to less than 24 hours and preferably from
around 1 to 6 hours. In addition, a standard for ending cleaning may be set by measuring
the turbidity, the concentration of the eluted metal scale 11, or the like of the first cleaning
fluid used for cleaning with a known method.
12
After being held for a prescribed amount of time, the eluted metal scale 11 is
discharged from the brine outlet line 5 or the like to the outside of the vessel 6 together
with the first cleaning fluid. A small amount of the metal scale 11 remains on the primary
surface 2a even after the first cleaning step, but this is ordinarily in a permissible amount
(FIG. 2(II)). In the event that an impermissibly large 5 amount of metal scale 11 remains, it
may be cleaned off to a permissible amount by repeating the first cleaning step two or
more times.
In this embodiment, the small amount of residual metal scale 11 is used as a
catalyst in the second cleaning step, which enhances the cleaning efficiency for the organic
10 scale 12.
Metal eluent
The metal eluent contained in the first cleaning fluid is not particularly limited as
long as it is a chemical capable of eluting metal ions contained in the water to be treated
(for example, seawater), such as iron, manganese, magnesium, and calcium constituting
15 the metal scale 11. The metal eluent is preferably a chemical which does not run the risk
of oxidatively degrading the RO membrane 2, inhibiting the operation of the RO
membrane module 1 after cleaning, or clogging the RO membrane 2 by transforming the
organic scale 12.
Examples of suitable metal eluents include citric acid, phosphonic acid, glycolic
20 acid, ethylenediaminetetraacetic acid, formic acid, and oxalic acid. The first cleaning
fluid preferably contains at least one type of a chemical selected from the group consisting
of the plurality of chemicals listed here.
One possible mechanism that allows these chemicals to elute the metal scale 11 is
that metal ions and the chemicals form a chelate bond so as to dissolve in the first cleaning
25 fluid.
The concentration of the metal eluent contained in the first cleaning fluid is not
particularly limited and may be set appropriately in accordance with the type of the metal
eluent that is used. The concentration range of the suitable metal eluents listed above is
preferably from 0.5 to 20 mass% and more preferably from 2 to 3 mass%. Here, the total
30 mass of the first cleaning fluid is 100 mass%.
When there is a possibility that the metal eluent that is used may react with salts
contained in the seawater to produce a precipitate, the salts attached to the RO membrane 2
13
are preferably eliminated in advance by passing freshwater through the RO membrane 2
prior to the first cleaning step. When the first cleaning fluid contains phosphonic acid,
for example, precipitation occurs as a result of the phosphonic acid reacting with calcium
or magnesium in the seawater, so these salts are preferably eliminated in advance. Here,
the salts are not components that are attached to 5 the RO membrane 2 and are therefore
differentiated from the metal scale.
The direction in which freshwater passes may be the forward direction (from the
primary surface 2a toward the secondary surface 2b) or the reverse direction (from the
secondary surface 2b toward the primary surface 2a). It is preferable for salts to not be
10 attached to the line for feeding freshwater, so freshwater is preferably fed to the RO
membrane 2 from the permeated water outlet line 4. When freshwater is fed to the RO
membrane 2 from the permeated water outlet line 4, the freshwater is inevitably passed in
the reverse direction. When passing freshwater in the reverse direction, the cleaning
efficiency can be further enhanced by performing so-called flushing cleaning, whereby air
15 is passed together with the freshwater.
In addition, by allowing air to pass through, the first cleaning fluid to be fed next
can reach every corner inside the water treatment membrane once the flushing water is
sufficiently discharged from inside the membrane, which makes it possible to further
enhance the cleaning efficiency.
20 Procedure of the second cleaning step
Next, a second cleaning fluid containing an oxidizer is injected into the secondary
surface 2b side of the RO membrane 2 inside the vessel 6 from the permeated water outlet
line 4, and the second cleaning fluid containing the oxidizer is passed through in the
reverse direction from the secondary surface 2b of the RO membrane 2 to the primary
25 surface 2a of the RO membrane 2. At least the primary surface 2a is then kept in a state
in which it is immersed in the second cleaning fluid.
By allowing the second cleaning fluid to pass through in the reverse direction, it is
possible to wash away the decomposition products of the organic scale 12 to the primary
surface 2a side so as to enable efficient cleaning. When the second cleaning fluid is
30 passed through in the reverse direction, an oxidizer capable of passing through the RO
membrane 2, such as hydrogen peroxide or chlorine monoxide is used.
The second cleaning fluid may also be allowed to pass through in the forward
direction. However, the decomposition products described above contain polymers
14
originating from microorganisms. For this reason, when the second cleaning fluid is fed
in the forward direction in the initial stages of the second cleaning step, the RO membrane
2 may be clogged by the decomposition products or the cleaning efficiency may be
diminished in comparison to the case of the reverse direction.
After the second cleaning fluid is allowed 5 to pass through, the second cleaning
fluid is kept in a state in which the inside of the vessel 6 on the primary surface 2a side is
filled with the second cleaning fluid, which makes it possible to maintain a state in which
at least the primary surface 2a is immersed in the second cleaning fluid. By applying the
pressure somewhat in the forward direction in this state, some of the second cleaning fluid
10 penetrates into the RO membrane 2 and leaks to the secondary surface 2b. As a result,
the inside and the secondary surface 2b of the RO membrane 2 may also be immersed at
the same time as the primary surface 2a. Alternatively, the space inside the membrane on
the side where water has accumulated may be filled by injecting the second cleaning fluid
from the permeated water outlet line 4 so as to maintain a state in which the secondary
15 surface 2b of the RO membrane 2 is immersed in the second cleaning fluid.
The method for maintaining a state in which the RO membrane 2 is immersed in
the second cleaning fluid is not particularly limited. For example, the second cleaning
fluid may be fed from the feeder line 3 to fill the space on the primary surface 2a side
inside the vessel 6, and the circulation of the second cleaning fluid may be stopped by then
20 stopping feeding and sealing the vessel 6. Alternatively, the second cleaning fluid may
be continued to be fed even after the space on the primary surface 2a side inside the vessel
6 is filled with the second cleaning fluid, and the second cleaning fluid may be discharged
from the brine outlet line 5 in the same amount as the fed amount so as to maintain a state
in which the RO membrane 2 is immersed in the second cleaning fluid while the second
25 cleaning fluid is circulated.
By maintaining a state in which at least one, and preferably both, of the primary
surface 2a and the secondary surface 2b are immersed in the second cleaning fluid, it is
possible to sufficiently elute the organic scale 12 attached to the primary surface 2a and the
secondary surface 2b and to thereby further enhance the elimination efficiency thereof.
30 The amount of time that the surfaces are kept in the immersed state is not
particularly limited and may, for example, be set to less than 24 hours and preferably from
around 1 to 6 hours. In addition, a standard for ending cleaning may be set by measuring
the turbidity, the concentration of the eluted or decomposed organic scale 12, the TOC
15
(Total Organic Carbon), the COD (Chemical Oxygen Demand), or the like of the second
cleaning fluid used for cleaning with a known method.
After being held for a prescribed amount of time, the eluted organic scale 12 is
discharged from the brine outlet line 5 or the like to the outside of the vessel 6 together
with the second cleaning fluid so as to obtain 5 an RO membrane 2 in which the metal scale
11 and organic scale 12 have been cleaned off (FIG. 2(III)). In the event that the organic
scale 12 remains, further cleaning may be performed by repeating the second cleaning step
a plurality of times.
Oxidizer
10 The oxidizer contained in the second cleaning fluid may be an oxidizer which
generates active oxygen when it comes into contact with a metal, or an oxidizer which
does not generate active oxygen even when it comes into contact with a metal. The
oxidizer is not particularly limited as long as it is a chemical capable of oxidatively
decomposing the organic scale 12.
15 From the perspective of enhancing cleaning efficiency, the metal scale 11
remaining in the RO membrane 2 is preferably used as a catalyst, and an oxidizer which
generates active oxygen in the RO membrane 2 is preferably used. Because active
oxygen oxidatively decomposes the organic scale 12 with high potency, it is possible to
enhance the cleaning efficiency in comparison to cases in which active oxygen is not used.
20 Here, in the event that an excessive amount of active oxygen is generated, the RO
membrane 2 may be oxidatively degraded. However, since the metal scale 11 is
eliminated in advance in this embodiment, the remaining metal component functioning as
a catalyst is minimal, and the generation of active oxygen in an amount great enough to
cause the oxidative degradation of the RO membrane 2 is prevented.
25 Examples of the oxidizer described above include hydrogen peroxide, percarbonate,
persulfate, hypochlorite, permanganate, chlorine dioxide, chlorine monoxide, and ozone.
Here, the cations constituting each salt are not particularly limited, and examples thereof
include inorganic cations such as sodium, potassium, lithium, calcium, magnesium,
beryllium, and ammonium. More specific examples of salts that are preferable as
30 oxidizers include sodium percarbonate, sodium persulfate, ammonium persulfate, sodium
hypochlorite, and potassium permanganate. The second cleaning fluid preferably
contains at least one type of an oxidizer selected from the group consisting of the plurality
of oxidizers listed here.
16
Of these, hydrogen peroxide, percarbonate, persulfate, and ozone, which generate
active oxygen without generating chlorine, are preferable, and percarbonate and hydrogen
peroxide, which generate OH radicals having a potent oxidizing power, are even more
preferable.
The concentration of the oxidizer 5 contained in the second cleaning fluid is not
particularly limited and may be set appropriately in accordance with the type of the
oxidizer that is used. The concentration range of the suitable metal oxidizers listed above
is preferably from 0.5 to 10 mass% and more preferably from 2 to 3 mass%. Here, the
total mass of the second cleaning fluid is 100 mass%.
10 Feeding the catalyst
When nearly all of the metal scale 11 is eliminated in the first cleaning step, the
amount of active oxygen generated in the RO membrane 2 in the second cleaning step may
be so small that the decomposition of the organic scale 12 due to active oxygen does not
sufficiently occur. When this problem is foreseen, a catalyst solution containing a metal
15 salt functioning as a catalyst is preferably brought into contact with the RO membrane 2
after the first cleaning step. An appropriate amount of a metal salt or metal ion as a
catalyst attaches to the RO membrane 2 in contact with the catalyst solution. As a result,
active oxygen is sufficiently generated from the oxidizer contained in the second cleaning
fluid, and the elimination efficiency of the organic scale 12 is thereby enhanced.
20 When a first cleaning fluid containing phosphonic acid is used in the first cleaning
step, there is a high probability that most of the metal scale 11 has been eliminated and that
no metal components capable of functioning as a catalyst remain in the RO membrane 2.
The reason for this is that phosphonic acid has a particularly potent effect of eliminating
the metal scale 11.
25 The method for bringing the catalyst solution into contact with the RO membrane 2
is not particularly limited, and an example thereof includes the same method as the method
used to bring the first or second cleaning fluid into contact with the RO membrane 2.
The catalyst solution may be fed alone to the RO membrane 2 before the second cleaning
fluid is fed to the RO membrane 2, or a mixed solution prepared by mixing the second
30 cleaning fluid and the catalyst solution may be fed to the RO membrane 2. However, the
generation of active oxygen is initiated at the point when the mixed solution is prepared
and before it reaches the RO membrane 2. Therefore, it is preferable to first bring the
catalyst solution alone into contact with the RO membrane 2 and to provide an appropriate
17
amount of a catalyst metal to the RO membrane 2 in advance in that the amount, location,
and timing of the generation of active oxygen are easily controlled.
The catalyst contained in the catalyst solution has a function of generating active
oxygen as a result of coming into contact with the oxidizer contained in the second
cleaning fluid. The type of the metal salt functioning as 5 the catalyst may be selected
appropriately in accordance with the type of the oxidizer. Examples thereof include
known metal salts including transition metals such as iron and manganese. More specific
examples of preferable metal salts include iron chloride and manganese chloride.
The concentration of the metal salt functioning as the catalyst in the catalyst
10 solution is not particularly limited and may be set, for example to 1 to 10 mass%. Here,
the total mass of the catalyst solution is 100 mass%.
The amount of time that the catalyst solution and the RO membrane 2 are in
contact with one another is not particularly limited and may be set to around 1 to 60
minutes as long as the concentration of the metal salt is within the range described above.
15 Rinsing step
With the objective of preventing the oxidizer and the metal eluent from remaining
in the RO membrane 2, a rinsing step of rinsing the RO membrane 2 is preferably
performed with a rinsing fluid such as seawater or freshwater not containing an oxidizer
after the second cleaning step.
20 The method of rinsing the RO membrane 2 is not particularly limited, and
examples thereof include a method of feeding seawater from the feeder line 3 so as to
come into contact with the primary surface 2a of the RO membrane 2 and maintaining a
state in which the RO membrane 2 is immersed in the seawater while continuously
discharging the seawater from the brine outlet line 5, and a method of flushing (reverse
25 cleaning) the RO membrane 2 by injecting freshwater from the permeated water outlet line
4 in the reverse direction.
The amounts of the oxidizer and the metal eluent contained in the discharged
rinsing fluid can be measured with known methods so as to assess whether to end the
rinsing step. After the rinsing step is complete, normal operation is performed.
30 Second Embodiment
Prior to the first step in the first embodiment described above, a preliminary
washing step may be performed, wherein a third cleaning fluid containing an oxidizer with
18
a lower concentration than that of the second cleaning fluid is brought into contact with at
least the primary surface 2a of the RO membrane 2 so as to eliminate the organic scale 12
attached to the RO membrane 2.
The detergency of the third cleaning fluid is weaker than that of the second
cleaning fluid, so the organic scale 12 is not completely 5 eliminated. However, since the
oxidizer concentration is comparatively low, a rinsing step is unnecessary prior to
transitioning to the normal operation for desalinating seawater. Thus, the amount of time
that the normal operation is stopped to perform the preliminary cleaning step is short, and
the process from the beginning to the end of the preliminary cleaning step is completed in
10 0.1 to 2 hours, for example.
The type of the oxidizer contained in the third cleaning fluid is not particularly
limited, and, for example, one or more types of the oxidizers listed as oxidizers that may
be contained in the second cleaning fluid may be applied. Specifically, sodium
percarbonate and hydrogen peroxide, for example, are preferably mixed at a molar ratio of
15 2:3 and used as the oxidizer of the third cleaning fluid.
The type of the oxidizer contained in the third cleaning fluid may be the same as or
different than the type of the oxidizer contained in the second cleaning fluid.
The concentration of the oxidizer contained in the third cleaning fluid is preferably
low enough for the rinsing step to be unnecessary and is preferably from 1 ppm to 100
20 ppm and more preferably from 5 to 10 ppm. When within this preferable range, the
elimination of the organic scale 12 can be executed smoothly, so there is no risk that the
decomposition products of the organic scale 12 will cause the clogging of the RO
membrane 2, and the rinsing step is also unnecessary.
The method for bringing the third cleaning fluid into contact with the RO
25 membrane 2 is not particularly limited and may be the same as the method used to bring
the first or second cleaning fluid into contact with the RO membrane 2. However, from
the perspective of ending the preliminary cleaning step in a short amount of time, it is
preferable to use a method of mixing the oxidizer in the seawater to be fed from the feeder
line 3 to prepare a third cleaning fluid containing seawater, feeding the third cleaning fluid
30 from the feeder line 3 in the same manner as in the normal operation, and bringing the
third cleaning fluid into contact with the RO membrane 2.
19
In the second embodiment, it is preferable to alternatively repeat the normal
operation and the preliminary cleaning step one or more times prior to transitioning from
the preliminary cleaning step to the first cleaning step. That is, it is preferable to
frequently perform simple cleaning with the preliminary cleaning step as a form of routine
5 maintenance.
Specifically, a cycle of performing normal operation for 8 hours, ending the
preliminary cleaning step in 1 hour, and once again performing normal operation for 8
hours may be repeated a plurality of times, for example. That is, this is a cycle of feeding
the third cleaning fluid to the RO membrane 2 from the feeder line 3 intermittently (for
10 example, every 8 hours).
By repeating the preliminary cleaning step, it is possible to reduce the rate of
increase of the amount of the organic scale 12 attached to the RO membrane 2. Such
reduction makes it possible to reduce the frequency with which the first and second
cleaning steps are performed. For example, when the preliminary cleaning step is
15 performed every 8 hours, the frequency with which the first and second cleaning steps are
performed can be set to about 1 time per 1 to 3 months.
As illustrated in FIG. 3, when ordinary operation and the preliminary cleaning step
are alternately repeated (FIGS. 3(IV) and (V), A1), the organic scale 12 is cleaned, and the
rate at which the organic scale is attached, which is originally comparatively high, is
20 reduced (FIG. 3(V)), while the rate at which the metal scale 11 is attached is reduced
negligibly. As a result, the metal scale 11 is attached gradually so that a considerable
amount of metal scale is accumulated (FIG. 3(VI), A2). If the second cleaning step were
to be performed without the first cleaning step being performed (skipped) in a state in
which a large amount of the metal scale 11 has accumulated in the RO membrane 2, a
25 large amount of active oxygen would be generated at once in the RO membrane 2, causing
the oxidative degradation or clogging of the RO membrane 2. However, in this
embodiment, the first cleaning step (FIG. 3(VII), A3) and the second cleaning step (FIGS.
3(VIII) and (IX)) are performed in this order in the same manner as in the first
embodiment described above after the preliminary cleaning step, so there is no concern
30 regarding the generation of a large amount of active oxygen. That is, when the
preliminary cleaning step is performed, the significance of performing the first cleaning
step prior to the second cleaning step is enhanced substantially.
As described above, after metal scale has accumulated in the water treatment
membrane each time the preliminary cleaning step is performed, the process transitions to
20
the first cleaning step so as to eliminate most of the metal scale with the metal eluent of the
first cleaning fluid, which makes it easy to allow an appropriately small amount of metal
scale to remain as a catalyst in the water treatment membrane. Using the metal scale
remaining in the water treatment membrane as a catalyst in the following second step,
active oxygen originating from the oxidizer of the second 5 cleaning fluid is generated by
the action of the catalyst, which makes it possible to even further enhance the organic scale
cleaning effect.
Known chemicals such as surfactants or pH adjusters for promoting cleaning may
be added to the first cleaning fluid, the second cleaning fluid, the third cleaning fluid, and
10 the catalyst solution described above as necessary.
Embodiments of the cleaning method for a water treatment membrane according to
the present invention were described above, but the present invention is not limited to the
aforementioned embodiments and may be changed as appropriate in a range that does not
deviate from the main intent of the present invention, or the components of the
15 embodiments described above may be replaced with known components as necessary.
Examples
Next, the present invention will be described in further detail using examples, but
the present invention is not limited by these examples.
Example 1
20 The RO membrane module 1 illustrated in FIG. 1, which was provided in a test
plant for desalinating seawater, was cleaned as follows.
First, after seawater was discharged from the brine outlet line 5, a first cleaning
fluid containing citric acid with a concentration of from 0.5 to 20 wt.% as a cleaning agent
(metal eluent) was fed from the feeder line 3 with a flow in the same forward direction as
25 the filtration direction, and the space on the primary surface 2a side of the RO membrane 2
inside the vessel 6 was filled. At this time, a fresh first cleaning fluid was allowed to pass
through the inlet part of the vessel 6, where there was a large amount of scale attachment.
After it was confirmed that the vessel 6 was full, immersion cleaning was performed for
less than 24 hours or from 1 to 6 hours (first cleaning step). After immersion cleaning,
30 the first cleaning fluid was discharged to the outside of the vessel.
Next, reverse cleaning was performed, wherein a second cleaning fluid containing
hydrogen peroxide (oxidizer) at a concentration of from 1 to 10 wt.% was injected from
21
the permeated water outlet line 4 with a flow in the opposite direction to the filtration
direction, and this was allowed to pass through the RO membrane 2. After the space on
the primary surface 2a side of the RO membrane 2 inside the vessel 6 was filled and it was
confirmed that the vessel 6 was full, immersion cleaning was performed for a certain
amount of time (second cleaning step). After 5 immersion cleaning, the second cleaning
fluid was discharged to the outside of the vessel.
As a result of performing the cleaning method described above, metal scale and
organic scale that had attached to the RO membrane 2 provided in the test plant were
sufficiently eliminated. In addition, in the second cleaning step, there was no generation
10 of active oxygen that would cause the oxidative degradation of the RO membrane 2, and
active oxygen was generated gradually by the catalytic action of the small amount of metal
scale remaining on the surface of the RO membrane 2, which yielded a sufficient cleaning
effect. The amount of active oxygen that was generated was assessed by the amount of
air bubbles of oxygen gas generated secondarily.
15 Comparative Example 1
The RO membrane module 1 was cleaned in the same manner as in Example 1 with
the exception that the second cleaning step was performed first and the first cleaning step
was performed thereafter. As a result, in the second cleaning step that was performed
first, a comparatively large amount of air bubbles were observed on the surface of the RO
20 membrane 2, so it was assessed that the generation of a relatively large amount of active
oxygen occurred to an extent that would cause concern regarding the oxidative degradation
of the RO membrane 2. In addition, the cleaning effect was inferior to that of Example 1.
Example 2
The RO membrane module 1 illustrated in FIG. 1, which was provided in a test
25 plant for desalinating seawater, was cleaned as follows.
First, after seawater was discharged from the brine outlet line 5, freshwater was
injected from the permeated water outlet line 4 with a flow in the opposite direction to the
filtration direction, and the freshwater was allowed to pass through the RO membrane 2 to
perform flushing reverse cleaning. Air was then blown in so as to discharge the flushing
30 water from the inside of the RO membrane 2 as much as possible, which created gaps in
the RO membrane 2 to allow the next first cleaning fluid to penetrate easily.
Next, a first cleaning fluid which contained phosphonic acid with a concentration
of from 0.5 to 20 wt.% as a cleaning agent (metal eluent) and had a pH level adjusted to 5
22
to 6 was fed from the feeder line 3 with a flow in the same forward direction as the
filtration direction, and gaps inside the RO membrane 2 and the space on the primary
surface 2a side of the RO membrane 2 inside the vessel 6 were filled. At this time, a
fresh first cleaning fluid was allowed to pass through the inlet part of the vessel 6 where
there was a large amount of scale attachment. After 5 it was confirmed that the vessel 6
was full, immersion cleaning was performed for less than 24 hours or from 1 to 6 hours
(first cycle of the first cleaning step). After immersion cleaning, the first cleaning fluid
was discharged to the outside of the vessel. The first cleaning fluid was then fed once
again, and the same immersion cleaning and discharge processes were performed (second
10 cycle of the first cleaning step).
Next, freshwater in which iron chloride with a concentration of from 10 to 30 wt.%
was dissolved (catalyst solution) was fed in the forward direction from the feeder line 3
and brought into contact with the primary surface 2a of the RO membrane 2 inside the
vessel 6, and the catalyst solution was discharged to the outside of the vessel in a state in
15 which iron ions remained on the RO membrane 2.
Next, a second cleaning fluid containing sodium percarbonate (oxidizer) at a
concentration of from 1 to 50 wt.% was charged from the feeder line 3 with a flow in the
same forward direction as the filtration direction, and after the space on the primary
surface 2a side of the RO membrane 2 inside the vessel 6 was filled and it was confirmed
20 that the vessel 6 was full, immersion cleaning was performed for a certain amount of time
(second cleaning step). After immersion cleaning, the second cleaning fluid was
discharged to the outside of the vessel.
As a result of the cleaning method described above, metal scale and organic scale
that had attached to the RO membrane 2 provided in the test plant were sufficiently
25 eliminated. In addition, in the second cleaning step, active oxygen was generated
gradually by the catalytic action of the small amount of iron ions remaining on the surface
of the RO membrane 2, which yielded a sufficient cleaning effect.
Industrial Applicability
The present invention can be applied extensively in the field of water treatment
30 membranes.
While the above has described embodiments of the present invention in detail with
reference to the drawings, each configuration of each embodiment and the combinations
thereof are merely examples, and additions, omissions, substitutions, and other changes
23
may be made without deviating from the spirit and scope of the present invention. The
present invention is not to be considered as being limited by the foregoing description but
is only limited by the scope of the appended claims.

We Claim:
1. A cleaning method for a water treatment membrane provided with a primary
surface for inflow of untreated water and a secondary surface for outflow of
treated water, the method comprising:
a first cleaning step of bringing a first cleaning fluid containing
phosphonic acid as a metal eluent into contact with at least the primary surface
and eliminating metallic scale attached to the water treatment membrane; and
a second cleaning step of bringing a second cleaning fluid containing
an oxidizing agent into contact with at least the primary surface and eliminating
organic scale attached to the water treatment membrane.
2. The cleaning method for a water treatment membrane according to claim 1,
wherein in the first cleaning step, after the first cleaning fluid is passed in the
forward direction from the primary surface to the secondary surface, at least the
primary surface is kept in a state immersed in the first cleaning fluid.
3. The cleaning method for a water treatment membrane according to claim 1 or 2,
wherein in the second cleaning step, after the second cleaning fluid is passed in
the reversed direction from the secondary surface to the primary surface, at least
the primary surface is kept in a state immersed in the second cleaning fluid.
4. The cleaning method for a water treatment membrane according to any one of
claims 1 to 3, wherein, prior to the first cleaning step, the method further
including a preliminary cleaning step of bringing a third cleaning fluid
containing an oxidizer with a lower concentration than that of the second
cleaning fluid into contact with at least the primary surface so as to eliminate
organic scale attached to the water treatment membrane.
5. The cleaning method for a water treatment membrane according to claim 4,
wherein an operation of performing water treatment using the water treatment
membrane and the preliminary cleaning step are alternately repeated at least
once.
6. The cleaning method for a water treatment membrane according to claim 5,
wherein organic scale attached to the water treatment membrane gradually
increases each time the operation and the preliminary cleaning step are
alternately repeated.
25
7. The cleaning method for a water treatment membrane according to any one of
claims 1 to 6, wherein any one or more types selected from the group consisting
of citric acid, glycolic acid, ethylenediaminetetraacetic acid, formic acid, and
oxalic acid are further contained as the metal eluent.
8. The cleaning method for a water treatment membrane according to any one of
claims 1 to 7, wherein the oxidizer contained in the second cleaning fluid is any
one or more types selected from the group consisting of hydrogen peroxide,
percarbonate, persulfate, hypochlorite, permanganate, chlorine dioxide, and
ozone.
9. The cleaning method for a water treatment membrane according to any one of
claims 1 to 8, wherein, prior to the first cleaning step, freshwater is passed
through the water treatment membrane to eliminate salt attached to the water
treatment membrane.
10. The cleaning method for a water treatment membrane according to any one of
claims 1 to 9, wherein, after the first cleaning step, a catalyst solution
containing a metal salt functioning as a catalyst for generating active oxygen
from the oxidizer contained in the second cleaning fluid is brought into contact
with the water treatment membrane.