Technical Field
The present invention relates to a suspended-matter
removing method and a suspended-matter removing apparatus.
The present invention particularly relates to a suspendedmatter
removing method and a suspended-5 matter removing
apparatus that are used in a seawater desalination plant and
a water treatment plant.
Background Art
In recent years, as the seawater desalination market has
10 been expanding due to global water shortage, seawater
desalination plants are being constructed. As a technology
for seawater desalination, there is known a method for
producing fresh water by removing salt in seawater with a
reverse osmosis membrane (RO membrane). A filtration
15 apparatus using an RO membrane performs removal of suspended
matters as a pretreatment.
In order to remove suspended matters, in general, a
flocculant is continuously injected into the seawater to
flocculate the suspended matters. As the flocculant, iron
20 salt is used. This metal reacts with an alkaline component
in the water to generate metal hydroxide.
The metal hydroxide acts as a binder, and collision and
contact of suspended matters in the seawater cause
conglomeration, generating flocs. An injection amount of the
25 flocculant is increased and decreased in accordance with an
3
amount of suspended matters in the seawater. For example,
when iron salt is used as the flocculant, the iron salt is
injected so as to be 0.5 to10 ppm as iron in the seawater.
Other methods for separating suspended matters include
filter filtration, centrifugation, and filtration using 5 a
solid filter material. A method using a solid filter material
is advantageous in that it is inexpensive as compared with
filter filtration or centrifugation, and easy to maintain.
For the solid filter material, those sized to have a diameter
10 of 300 to 2500 μm are typically used. When suspended matters
to be removed are small, the flocculant is added to water to
be treated to form flocs thereby to increase the size of an
object to be removed, and then the filtration is performed.
Here again, the flocculant is continuously injected to the
15 water to be treated (see PTL 1).
Continuous injection of the flocculant causes growth of
the flocs, which makes it easier to capture the flocs with a
downstream filter. However, the filter itself must be washed
regularly to discharge flocs that have been deposited inside,
20 to outside of the system. The flocs deposited in the filter
are discharged from inside of the filter by backwashing.
Citation List
Patent Literature
PTL 1 Japanese Unexamined Patent Application, Publication No.
25 2000-202460
4
Summary of Invention
Technical Problem
Washing-waste water discharged from backwashing has a
high turbidity, and adversely affects the environment if
discharged as it is. Therefore, the washing-waste 5 te water is
subject to solid-liquid separation with a dehydrator or the
like, and a remaining solid content is disposed as sludge
outside the system. Treatment of the sludge requires a sludge
treatment facility. The method of continuously injecting a
10 flocculant has a high environmental load.
When a large amount of a flocculant is used in
filtration using a solid filter material, flocs are captured
at a filter layer, and a differential pressure of the filter
layer is increased. An increase in the differential pressure
15 makes it difficult for the water to be treated to pass,
deteriorating removal efficiency. In order to reduce the
differential pressure, the filter layer must be backwashed.
The filter immediately after backwashing has a low removal
rate (capture rate) of suspended matters, and requires long
20 time (e.g., five hours or more) until the water quality of
filtrate becomes stable, causing deterioration of water
quality of the filtrate.
Although various mechanisms are considered as a
suspended-matter removal mechanism by filtration using a
25 solid filter material, for example, screening, removal by an
5
interception effect of sedimentation or the like in a
stagnant pool in a void or a gap, or adhesion/adsorption
(electrostatic, intermolecular force, or cohesion), they have
not been fully elucidated at present. Thus, there are
problems in improvement of a removal rate, and 5 in
stabilization of load fluctuation or water quality of
filtrate at starting.
When paying attention to the suspended-matter removal by
interception among the removal mechanisms, a passage becomes
10 smaller as a particle diameter of the solid filter material
is smaller, enabling removal of smaller suspended matters.
Moreover, using a smaller solid filter material increases a
specific surface area of the solid filter material, which can
increase a removal rate of fine suspended matters that can be
15 captured on a surface of the solid filter material by
Brownian luck.
However, when a small solid filter material is used, a
pressure loss of the filter is large, and power of a water
feed pump rises, increasing an operation amount. Moreover,
20 since an operation pressure is high, a container that stores
the solid filter material is required to have a higher
pressure resistance, increasing cost for the apparatus. In
other words, making a solid filter material smaller to
improve a removal rate is in a trade-off relation with the
25 cost.
6
The present invention has been made in view of the above
circumstances, and it is an object of the present invention
to provide a suspended-matter removing method and a
suspended-matter removing apparatus, that require no sludge
treatment facility, and inexpensively 5 ensively provide filtrate
satisfying a desired water quality standard, while
suppressing an increase in a differential pressure in a
filter layer.
Solution to Problem
10 The inventors, as a result of intensive study, have
obtained new knowledge that suspended matters of 0.1 to 10 μm
are not easily removed by a conventional filtration method
using a solid filter material, even when the solid filter
material is made smaller. Based on this, the inventors have
15 invented a suspended-matter removing method and a suspendedmatter
removing apparatus for removing suspended matters of
0.1 to 10 μm.
The present invention provides a suspended-matter
removing method including the steps of, by feeding a
20 protrusion element to a filter layer formed by filling a
solid filter material, adding a protrusion to a surface of
the solid filter material; after feeding of the protrusion
element in the step of adding a protrusion, determining
whether or not a protrusion satisfying a preset standard has
7
been added to the surface of the solid filter material, and
when it is determined that the protrusion has been added,
reducing a feeding amount of the protrusion element as
compared with when adding the protrusion; and passing water
5 to be treated containing suspended matters through the filter
layer having the solid filter material added with the
protrusion in a state in which the feeding amount of the
protrusion element is reduced.
In the invention above, the protrusion is added to the
10 surface of the solid filter material thereby to cause a
microscopic change in a flow of the water to be treated in
the filter layer, causing suspended matters having a size of
0.1 μm or more to 10 μm or less to be captured. This makes
it possible to improve water quality of filtrate even when
15 the water to be treated includes many suspended matters
having a size of 0.1 μm or more to 10 μm or less. A
fluctuation of water quality (load fluctuation) of the water
to be treated is allowed, and the water quality of the
filtrate can be stabilized.
20 In the invention above, since the protrusion element is
fed to the filter layer so as to add a protrusion to the
surface of the solid filter material, the protrusion can be
stably added in a short time. The filter layer formed by
filling the solid filter material added with the protrusion
25 can stably remove (capture) suspended matters at a high
8
removal rate (capture rate) from an initial stage of the step
of removing suspended matters from the water to be treated.
This can shorten a starting time of the filtration apparatus
as compared with a conventional one.
Reducing feeding of the protrusion 5 sion element enables
suppression of sludge generation. Whereas, even though the
amount is small, continuation of the feeding of the
protrusion element allows a protrusion to be additionally
formed even when the protrusion is stripped off, or water
10 quality of the water to be treated is deteriorated, providing
stabilization of the water quality of the filtrate.
In the invention above, suspended matters are removed
from the water to be treated with the feeding amount of the
protrusion element reduced, which can reduce sludge15
generation amount as compared with when the protrusion
element is continuously fed. This suppresses an increase in
a differential pressure in the filter layer, allowing a
backwashing interval to be prolonged.
In one aspect of the invention above, it is preferable
20 to stop feeding of the protrusion element in the step of
reducing the feeding amount of the protrusion element.
Stopping the feeding of the protrusion element enables
suppression of sludge generation, eliminating necessity of a
sludge treatment facility.
25 In one aspect of the invention above, there may be
9
further included a step of passing the water to be treated
through the filter layer in parallel with the step of adding
the protrusion. This makes it possible to add a protrusion
as required while filtering the water to be treated.
In one aspect of the invention above, a step 5 of
measuring a differential pressure between a first side of the
filter layer and a second side of the filter layer may be
included, to feed the protrusion element within a range where
the measured differential pressure is less than a
10 predetermined value, in the step of adding the protrusion.
Excessively forming protrusion to narrow a passage of
water to be treated allows an interception effect to be
enhanced, as with when a solid filter material with a small
diameter is used. However, according to one aspect of the
15 invention above, the protrusion can capture suspended matters
having a size of 0.1 μm or more to 10 μm or less, without
narrowing the passage to an extent allowing the enhancement
of the interception effect. Keeping the differential
pressure in the filter layer, which is generated by adding of
20 the protrusion, at less than the predetermined value, enables
a lower initial differential pressure, and a longer
maintenance interval.
In one aspect of the invention above, there may be
included a step of directly or indirectly measuring an amount
25 of a protrusion element contained in filtrate that has come
10
out from the filter layer in the step of adding the
protrusion, and it may be determined that the protrusion has
been added to the surface of the solid filter material when
the measured amount of the protrusion element becomes equal
5 to or less than a preset threshold value.
When the protrusion element is fed to the filter layer,
the protrusion element adheres to the surface of the solid
filter material to form a protrusion. In the step of adding
the projection, a decrease in an amount of the protrusion
10 element contained in the filtrate serves as an index
indicating that the protrusion element has adhered to the
surface of the solid filter material. Thus, according to the
aspect described above, it is possible to add a protrusion
required to capture suspended matters having a size of 0.1 μm
15 or more to 10 μm or less.
In one aspect of the invention above, a total feeding
amount of the protrusion element to the filter layer in the
step of adding the protrusion may be counted, and it may be
determined that the protrusion has been added to the surface
20 of the solid filter material when the counted total feeding
amount reaches a preset threshold value.
Presetting a total feeding amount of the protrusion
element to the filter layer allows desired protrusion to be
easily added.
25 In one aspect of the invention above, it is preferable
11
to include a step of inspecting water quality of the filtrate
that has come out from the filter layer in the step of
passing the water to be treated. When an inspection value of
the filtrate exceeds a preset threshold value, it is
determined that the protrusion satisfying a preset 5 et standard
has not been added to the surface of the solid filter
material, and the step of adding the protrusion is performed.
When the inspection value of the filtrate is equal to or less
than the preset threshold value, it is determined that the
10 protrusion satisfying the preset standard has been added to
the surface of the solid filter material, and the feeding
amount of the protrusion element is reduced as compared with
when adding the protrusion.
Since the protrusion element forms a protrusion by
15 adhering to the surface of the solid filter material, the
protrusion may be stripped off. When the protrusion is
stripped off, the stripped protrusion also becomes a
suspended matter, deteriorating water quality. Additionally,
when the protrusion is stripped off, a removal rate of
20 suspended matters in the filter layer is also lowered,
deteriorating water quality of the filtrate. According to
the aspect described above, since the protrusion is added in
accordance with the water quality of the filtrate, the water
quality of the filtrate can be more stable.
25 In one aspect of the invention above, in the step of
12
passing the water to be treated, it is preferable to pass the
water to be treated through a coarse-particle separation part
to make it to be water to be primarily treatedby mainly
separating suspended matters larger than 10 μm contained in
the water to be treated, and then pass the water 5 ter to be
primarily treated through the filter layer to remove
suspended matters having a size of 0.1 μm or more to 10 μm or
less.
Water to be treated containing many suspended matters
10 with a large particle diameter may cause clogging in an early
stage, due to an interception effect. According to the
aspect described above, since the coarse-particle separation
part roughly removes suspended matters having a large
particle diameter, a filtering part can remove suspended
15 matters having a size of 0.1 μm or more to 10 μm or less with
less influence of suspended matters having a large particle
diameter. Thus, the water quality of the filtrate that has
come out from the filtering part can be stabilized, the
differential pressure in the filter layer becomes less likely
20 to be generated, and a backwashing interval can be prolonged.
In one aspect of the invention above, a height of the
protrusion is preferably 4 μm or more. This allows the
protrusion to capture suspended matters having a size of 10
μm or less. When the height of the protrusion is too low, a
25 microscopic turbulence of a flow becomes less likely to be
13
generated, and suspended-matter particles are not transported
to the solid filter material, making it difficult for
suspended-matter particles to adhere.
In one aspect of the invention above, an average
particle diameter of the solid filter material is 5 s preferably
300 μm or more to 2500 μm or less. This can realize the
filter layer capable of providing an interception effect
while suppressing the differential pressure of the filter
layer.
10 In one aspect of the invention above, the protrusion
element can be made of kaolin. In one aspect of the invention
above, the protrusion element can be made of iron chloride.
In one aspect of the invention above, the protrusion element
can be made of high-molecular polymer.
15 Making the protrusion element of the above-described
materials makes it possible to inexpensively form a
protrusion to the surface of the solid filter material.
Making the protrusion element of the above-described
materials realizes the filter layer that can capture
20 suspended-matter particles having a size of 0.1 μm or more to
10 μm or less, while hardly increasing the differential
pressure of the filter layer.
When the protrusion element is made of iron chloride, in
the step of reducing the feeding amount of the protrusion
25 element, the feeding amount of the protrusion element is
14
preferably reduced such that content of the protrusion
element is less than 0.5 ppm as iron, in solution that passes
the filter layer.
Although an amount of iron chloride that is injected in
expectation of a flocculation effect is generally 1 ppm 5 or
more as iron, sludge generation can be suppressed even with a
less injection amount than the amount in which the
flocculation effect is expected, in one aspect of the
invention above. This is because a protrusion is formed to
10 the surface of the solid filter material, and the protrusion
removes suspended matters. In one aspect of the invention
above, even though the amount is small, continuation of
feeding of the protrusion element allows a protrusion to be
additionally formed even when the protrusion is stripped off,
15 or water quality of the water to be treated is deteriorated,
providing stabilization of the water quality of the filtrate.
The present invention provides a suspended-matter
removing apparatus that includes a filtering part having a
filter layer formed by filling a solid filter material; a
20 water-to-be-treated feeding part that feeds water to be
treated to a first side of the filtering part to pass the
water to be treated through the filter layer; a protrusionelement
feeding part that feeds a protrusion element to the
first side of the filtering part; a determination part that,
25 based on a preset standard, determines whether or not a
15
protrusion has been added to a surface of the solid filter
material; and a control part that, when the determination
part determines that the protrusion has been added, controls
the protrusion-element feeding part to reduce feeding amount
of the protrusion element as compared with when it 5 is
determined that the protrusion has not been added.
In one aspect of the invention above, the control part
may also control the protrusion-element feeding part to stop
feeding of the protrusion element when the determination part
10 determines that the protrusion has been added.
In one aspect of the invention above, there is included
a differential-pressure measurement part that measures a
differential pressure between the first side and a second
side of the filtering part, and the control part can control
15 a feeding amount of the protrusion element from the
protrusion-element feeding part such that the differential
pressure measured by the differential-pressure measurement
part becomes less than a predetermined value.
Advantageous Effects of Invention
20 A suspended-matter removing method and a suspendedmatter
removing apparatus according to the present invention
perform filtration of water to be treated with a filter layer
formed by filling a solid filter material that is added with
a protrusion, thereby to inexpensively provide filtrate
25 satisfying a water quality standard without necessity of a
16
sludge treatment facility, while suppressing an increase in a
differential pressure in the filter layer.
Brief Description of Drawings
Fig. 1 is a schematic block diagram of a suspended-matter
5 removing apparatus according to a first embodiment.
Fig. 2 is a schematic block diagram of a suspended-matter
removing apparatus according to a second embodiment.
Fig. 3 is a schematic block diagram of a suspended-matter
removing apparatus according to a third embodiment.
10 Fig. 4 is a schematic view explaining a passage width d0.
Fig. 5 is a graph showing a simulation result in Study 1.
Fig. 6 is a schematic view explaining a flow of water to be
treated.
Fig. 7 is a view showing a simulation result in Study 2.
15 Fig. 8 is a view showing a simulation result in Study 2.
Fig. 9 is a view showing a simulation result in Study 2.
Fig. 10 is a graph showing a simulation result in Study 3.
Fig. 11 is a graph showing a measurement result of a
differential pressure of a filter layer in Study 4.
20 Fig. 12 is a graph showing a measurement result of an SDI of
Tests A and B in Study 4.
Fig. 13 is a graph showing a measurement result of a
differential pressure of a filtering part (filter layer) in
Study 5.
25 Fig. 14 is a graph showing a measurement result of an SDI of
17
filtrate that has come out from the filtering part (filter
layer) in Study 5.
Fig. 15 is a graph showing a measurement result of
differential pressures of a coarse-particle separation part
5 and a filtering part (filter layer) in Study 6.
Fig. 16 is a graph showing a measurement result of an SDI of
filtrate that has come out from the filtering part (filter
layer) in Studies 6, 7, and 8.
Fig. 17 is a graph showing a measurement result of
10 differential pressures of a coarse-particle separation part
and a filtering part (filter layer) in Study 7.
Fig. 18 is a graph showing a measurement result of
differential pressures of a coarse-particle separation part
and a filtering part (filter layer) in Study 8.
15 Description of Embodiments
One embodiment of a suspended-matter removing method and
a suspended-matter removing apparatus according to the
present invention is now described below with reference to
drawings.
20 First Embodiment
Fig. 1 is a schematic block diagram of a suspendedmatter
removing apparatus according to the embodiment. The
suspended-matter removing apparatus 1 includes a filtering
part 2, a water-to-be-treated feeding part 3, a protrusion18
element feeding part 4, a determination part 5, and a control
part 6.
The filtering part 2 has at least one filter layer 2a, a
first opening 2b provided on a first side of the filter layer
2a, and a second opening 2c provided on a second side of 5 the
filter layer. The first opening 2b and the second opening 2c
are inflow/outflow ports for liquid, of the filtering part 2.
The first opening 2b is connected with a first passage 7.
The second opening 2c is connected with a second passage 8.
10 The filter layer 2a is formed by filling a solid filter
material in the filtering part. A filling amount of the
solid filter material is appropriately set. One filter layer
2a is formed by a solid filter material made of one kind of
material. A plurality of the filter layers 2a may be
15 laminated in the filtering part. For example, a sand filter
layer filled with sand and an anthracite filter layer formed
by filling anthracite may be laminated. Solid filter
materials made of different materials have different surface
conditions. Combination of filter layers formed by different
20 materials enables removal of suspended-matters with a wide
range of sizes.
A solid filter material to be used is granular or
fibrous. For example, the solid filter material is made of
sand, anthracite, crushed activated carbon, fiber bundle, and
25 the like. Since crushed activated carbon has an effect of
19
removing chlorine, using crushed activated carbon as the
solid filter material enables removal of chlorine contained
in water to be treated, in the filtering part. This can
prevent deterioration in an RO membrane, even when the RO
5 membrane is provided at a subsequent stage.
An average particle diameter of the solid filter
material is selected from 300 μm or more to 2500 μm or less.
A definition of "the average particle diameter of the solid
filter material" is based on AWWA B100-01 and JIS8801.
10 The water-to-be-treated feeding part 3 can feed water to
be treated to the first side of the filtering part 2, to pass
the water to be treated through the filter layer 2a. In this
embodiment, the water-to-be-treated feeding part 3 is
configured by a water-to-be-treated tank 3a and a first
15 feeding means 3b. The water-to-be-treated feeding part 3 is
connected to the first opening 2b of the filtering part 2 via
the first passage 7. The water-to-be-treated tank 3a is a
container that stores the water to be treated. The stored
water to be treated is seawater, dirty water, industrial
20 wastewater, or the like. The first feeding means 3b is a
pump or the like. The first feeding means 3b can feed the
water to be treated stored in the water-to-be-treated tank
3a, to filtering part 2 via the first passage 7.
The protrusion-element feeding part 4 can feed a
25 protrusion element to the first side of the filtering part 2.
20
In this embodiment, the protrusion-element feeding part 4 is
configured by a protrusion element tank 4a and a second
feeding means 4b. The protrusion-element feeding part 4 is
connected to the first opening 2b of the filtering part 2 via
the first passage 7, at a downstream side of the water-to-5 betreated
feeding part 3. The protrusion element tank 4a is a
container that stores the protrusion element. The second
feeding means 4b is a pump or the like. The second feeding
means 4b can feed the protrusion element stored in the
10 protrusion element tank 4a, to the filtering part 2 via the
first passage 7.
The protrusion element is made of iron chloride, iron
sulfate, polyaluminum chloride (PAC), aluminum sulfate,
mineral, high-molecular polymer (cationic high-molecular
15 polymer, anionic high-molecular polymer, and nonionic highmolecular
polymer), inorganic pigment, and the like. The
mineral is, for example, kaolin. For the cationic highmolecular
polymer, polyacrylic ester-based, polymethacrylic
acid ester-based, and polyacrylamide-based are suitable. As
20 the anionic high-molecular polymer, polyacrylamide-based and
polyacrylic acid-based are preferable. As the nonionic highmolecular
polymer, polyacrylic ester-based, polymethacrylic
acid ester-based, and polyacrylamide-based are preferable.
The inorganic pigment is, for example, calcium carbonate,
25 talc, and titanium oxide. The protrusion element may be
21
powder or liquid. In this embodiment, the protrusion element
is stored in the protrusion element tank in a solution state
prepared at a predetermined concentration (protrusion forming
liquid).
For 5 example, iron chloride becomes iron hydroxide in the
water, and a microfloc of the iron hydroxide adheres to the
surface of the solid filter material, to form a protrusion.
The microfloc may involve minute particles in the water. For
example, kaolin physically adheres to the surface of the
10 solid filter material, to form a protrusion. For example,
high-molecular polymer acts as an adhesive for bonding
particles contained in the water to the solid filter
material, and adheres to the surface of the solid filter
material along with the particles, to form a protrusion.
15 The protrusion element that is fed to the filter layer
may be one or more kinds. For example, when kaolin and highmolecular
polymer are fed to the filter layer, the kaolin
physically adheres to the surface of the solid filter
material, and particles contained in the water and the kaolin
20 adhere to the surface of the solid filter material through an
adhesive effect of the high-molecular polymer, to form a
protrusion.
The determination part 5 can determine, based on a
preset standard, whether or not a protrusion satisfying the
25 preset standard has been added to the surface of the solid
22
filter material. In this embodiment, the determination part
5 includes a counting means (not shown) that counts a total
feeding amount of the protrusion element. For example, the
counting means is connected to the second feeding means 4b.
For example, the counting means can receive a power-5 supply
ON/OFF signal of the second feeding means 4b, and count a
total feeding amount of the protrusion element based on a
time when the power supply of the second feeding means 4b is
ON, and a concentration of the protrusion element in the
10 protrusion forming liquid. The determination part 5 can
determine, when the counted total feeding amount of the
protrusion element reaches a preset threshold value, that a
protrusion satisfying the preset standard has been added to
the surface of the solid filter material. The determination
15 part 5 may be incorporated into the second feeding means 4b
or the control part 6.
The control part 6 can control the feeding amount of the
protrusion element from the protrusion-element feeding part 4
so as to reduce the feeding amount of the protrusion element
20 when the determination part 5 determines that a protrusion
satisfying the preset standard has been added (abbreviated as
a protrusion has been added). The control part 6 can control
the feeding amount of the protrusion element from the
protrusion-element feeding part so as to feed the protrusion
25 element to add a protrusion to the surface of the solid
23
filter material when the determination part 5 determines that
a protrusion satisfying the preset standard has been not
added (hereinafter abbreviated as a protrusion has not been
added). The feeding amount of the protrusion element
required for adding a protrusion to the surface of 5 the solid
filter material has been appropriately set in accordance with
a kind of the protrusion element. "Reduce the feeding amount
of the protrusion element" means decreasing the feeding
amount of the protrusion element as compared with when adding
10 the protrusion.
When protrusion elements, such as iron chloride and
high-molecular polymer, capable of providing a flocculation
effect are used, the feeding amount of the protrusion element
is set to be reduced to an amount with which at least a
15 flocculation effect cannot be expected. "Reduce the feeding
amount of the protrusion element" includes stopping of the
feeding amount of the protrusion element.
The control part 6 is, for example, configured by a CPU
(Central Processing Unit), a RAM (Random Access Memory), a
20 ROM (Read Only Memory), a computer-readable storage medium,
and the like. Then, a series of processing for achieving
various functions is, as an example, stored in a form of a
program in a storage medium or the like, and the CPU reads
the program into the RAM or the like to execute information
25 processing and arithmetic processing, thereby to achieve the
24
various functions. It should be noted that, the program may
be applied with a form such as a form that is previously
installed in a ROM or another storage medium, a form provided
in a state being stored in a computer-readable storage
medium, or a form that is delivered via a wired 5 or wireless
communication means. The computer-readable storage medium is
a magnetic disk, a magneto-optical disk, a CD-ROM, a DVD-ROM,
a semiconductor memory, or the like.
The suspended-matter removing apparatus 1 preferably
10 includes a water-quality inspection means 9 that inspects
water quality of filtrate that has come out from the second
side of the filtering part. The water-quality inspection
means 9 is, for example, an SDI (Silt Density Index)
measuring device, a turbidimeter, a TOC meter, an SS meter, a
15 UV meter, a COD meter, and the like. In Fig. 1, the waterquality
inspection means 9 is connected to the second passage
and the determination part 5. The water-quality inspection
means 9 can inspect the water quality of the filtrate
discharged from the filtering part 2 to the second passage,
20 and output an inspection result to the determination part 5.
The determination part 5 can determine that a protrusion has
not been added when the inspection value obtained from the
water-quality inspection means 9 exceeds a preset threshold
value, and determine that the protrusion has been added when
25 the inspection value becomes equal to or less than the
25
threshold value. The threshold value is appropriately set in
accordance with an item of water quality to be inspected.
The suspended-matter removing apparatus 1 may include,
at a downstream side of the filtering part 2, a reverseosmosis-
membrane treatment part 10, an 5 electrodialysis part
(not shown), an evaporator (not shown) or the like. The
reverse-osmosis-membrane treatment part 10 is, for example, a
reverse-osmosis-membrane treatment apparatus having a
plurality of reverse-osmosis-membrane elements in a
10 container. The reverse-osmosis-membrane treatment apparatus
can divide the water to be treated (filtrate) that has passed
through the filtering part 2, into fresh water and
concentrated water containing ions, salt or the like, with a
reverse osmosis membrane (RO membrane).
15 The suspended-matter removing apparatus 1 may include a
backwashing means (not shown) for backwashing the filter
layer 2a. The backwashing means is provided to the filtering
part 2 such that washing liquid flows from the second side
toward the first side of the filtering part 2a. The washing
20 liquid is supplied to the filtering part 2 by a liquid
supplying means such as a pump.
Next, a suspended-matter removing method according to
the embodiment is described. The suspended-matter removing
method according to the embodiment includes the following
25 steps (S1) to (S3).
26
(S1) A step of adding a protrusion
(S2) A step of reducing a feeding amount of the
protrusion element as compared with when adding a protrusion
(S3) A step of passing water to be treated containing
suspended matters, through the filter layer having a 5 solid
filter material added with the protrusion
In the step of adding a protrusion (S1), a protrusion
element is fed to the filter layer 2a, to add a protrusion to
the surface of the solid filter material.
10 The protrusion element is made of iron chloride, iron
sulfate, polyaluminum chloride (PAC), aluminum sulfate,
mineral, high-molecular polymer (cationic high-molecular
polymer, anionic high-molecular polymer, and nonionic highmolecular
polymer), inorganic pigment, and the like. The
15 mineral is, for example, kaolin. For the cationic highmolecular
polymer, polyacrylic ester-based, polymethacrylic
acid ester-based, and polyacrylamide-based are suitable. As
the anionic high-molecular polymer, polyacrylamide-based and
polyacrylic acid-based are preferable. As the nonionic high20
molecular polymer, polyacrylic ester-based, polymethacrylic
acid ester-based, and polyacrylamide-based are preferable.
The inorganic pigment is, for example, calcium carbonate,
talc, and titanium oxide.
The protrusion element adheres to the surface of the
25 solid filter material to form a protrusion itself, or bonds
27
particles in water to the solid filter material. For example,
iron chloride becomes iron hydroxide in the water, and a
microfloc of the iron hydroxide adheres to the surface of the
solid filter material, to form a protrusion. The microfloc
may involve minute particles in the water. For 5 example,
kaolin physically adheres to the surface of the solid filter
material, to form a protrusion. For example, high-molecular
polymer acts as an adhesive for bonding particles contained
in the water to the solid filter material, and adheres to the
10 surface of the solid filter material along with the
particles, to form a protrusion.
The protrusion element that is fed to the filter layer
may be one or more kinds. For example, when kaolin and highmolecular
polymer are fed to the filter layer, the kaolin
15 physically adheres to the surface of the solid filter
material, and particles contained in the water and the kaolin
adhere to the surface of the solid filter material through an
adhesive effect of the high-molecular polymer, to form a
protrusion.
20 The protrusion element may be powder or suspension
containing minute particles. In this embodiment, the
protrusion element is fed in a solution state containing the
protrusion element (protrusion forming liquid). A solvent of
the protrusion forming liquid is industrial water, seawater,
25 clear water or the like. When the protrusion element is made
28
of high-molecular polymer, the protrusion forming liquid is
preferably prepared with solution containing particles (e.g.
seawater).
A concentration of the protrusion element in the
protrusion forming liquid is set such that a p5 redetermined
amount of the protrusion element is fed when the protrusion
forming liquid is passed through the filter layer 2a. The
feeding amount of the protrusion element may be appropriately
set in accordance with a kind of the protrusion element and a
10 component of the water to be treated.
A protrusion is added by passing the protrusion forming
liquid through from the first side to the second side of the
filter layer 2a. This allows a protrusion to be added to the
surface of the solid filter material. A filtering speed of
15 the protrusion forming liquid is preferably same as a
filtering speed of the water to be treated. The filtering
speed can be adjusted by the first feeding means 3b or the
second feeding means 4b. When the filtering speed is
adjusted by the first feeding means 3b, the water to be
20 treated is passed through the filter layer 2a, in parallel
with the step of adding a protrusion (S1).
After the protrusion element is fed to the filter layer
2a to add a protrusion to the surface of the solid filter
material, the feeding amount of the protrusion element is
25 reduced as compared with when the protrusion is added (S2).
29
Based on a preset standard, it is determined whether or
not a protrusion has been added to the surface of the solid
filter material. "Standard" can be set by performing a
preliminary test or the like. In the preliminary test, the
water quality of the filtrate is inspected, for example, 5 by
passing the protrusion forming liquid containing the
protrusion element with an optional concentration through the
filter layer. The feeding amount of the protrusion element,
at a time when the inspection value becomes a desired value,
10 is set to be a threshold value (standard) of the feeding
amount of the protrusion element for adding a required amount
of the protrusion to the solid filter material.
In the step (S2), a total feeding amount of the
protrusion element to the filter layer 2a in the step of
15 adding a protrusion (S1) is counted, and it is determined
that a protrusion satisfying a preset standard has been added
to the surface of the solid filter material when the counted
total feeding amount reaches a preset threshold value. When
it is determined that the protrusion has been added, the
20 feeding amount of the protrusion element is reduced. The
extent of the reduction of the feeding amount of the
protrusion element may be appropriately set in accordance
with a kind of the protrusion element. When there is used a
protrusion application that can provide a flocculation effect
25 in accordance with a feeding amount, the feeding amount of
30
the protrusion element after being reduced is an amount with
which the flocculation effect cannot be expected even if
added to the water to be treated. For example, when the
protrusion element is made of iron chloride, it is reduced to
about less than 0.5 ppm as iron (Fe) with respect to 5 an
amount of solution to be passed through the filter layer 2a.
In the step (S2), the feeding amount of the protrusion
element may be set to be zero, by stopping the feeding of the
protrusion element.
10 Water to be treated containing suspended matters is
passed through the filter layer 2a (S3), with the feeding
amount of the protrusion element reduced (or stopped). Here,
a protrusion has been added to the surface of the solid
filter material filled in the filter layer 2a.
15 In the step of passing the water to be treated
containing suspended matters (S3), it is preferable to
inspect water quality of the filtrate that has come out from
the filter layer 2a. When an inspection value of the
filtrate exceeds a preset threshold value, the protrusion
20 element is again fed to the filter layer to add a protrusion
to the surface of the solid filter material (S2’). Then, the
feeding of the protrusion element is reduced (or stopped)
when the inspection value of the filtrate becomes equal to or
less than the preset threshold value (S3').
25 In (S3), "water-quality inspection" is performed with an
31
SDI measuring device, a turbidimeter, a TOC meter, an SS
meter, a UV meter, a COD meter and the like. The threshold
value is set in accordance with an inspection method. For
example, when the inspection method is an SDI, the threshold
5 value may be SDI<4 or the like.
When the protrusion element is fed to the filter layer
filled with the solid filter material, the protrusion element
comes into contact with the solid filter material to add a
protrusion to the surface of the solid filter material. At a
10 removal of suspended matters from the water to be treated,
passing the protrusion element through the filter layer at an
early stage allows the protrusion to be added to the surface
of the solid filter material in a short time. The filter
layer formed by filling the solid filter material added with
15 the protrusion can stably remove suspended matters at a high
removal rate from an initial stage of the step of removing
suspended matters from the water to be treated. This can
shorten a starting time of the suspended-matter removing
apparatus as compared with conventional ones. Additionally,
20 since the filter layer filled with the solid filter material
added with the protrusion can capture suspended matters of
0.1 μm or more to 10 μm or less, it is possible to improve
the water quality of the filtrate even when the water to be
treated includes many suspended matters having a size of 0.1
25 μm or more to 10 μm or less. Namely, it makes it possible to
32
cope with fluctuation in water quality of the water to be
treated. Adding a protrusion to the surface of the solid
filter material of 300 μm or more to 2500 μm or less provides
a suspended-matter removal effect more than an interception
5 effect.
Reducing the feeding amount of the protrusion element
enables suppression of sludge generation. This suppresses an
increase in a differential pressure in the filter layer,
which can prolong a backwashing interval and eliminate
10 necessity of a sludge treatment facility.
Even when the feeding of the protrusion element is
stopped, water quality of the filtrate in the step (S3) can
be stabilized until the protrusion is stripped off, as long
as the protrusion has once been added to the surface of the
15 solid filter material. The protrusion can be replenished by
continuing the feeding of the protrusion element, even though
the amount is small. Therefore, even if the protrusion is
stripped off, stability of the water quality of the filtrate
can be maintained. Moreover, when the feeding of the
20 protrusion element is stopped, an amount of protrusionelement
usage can be lowered, enabling reduction of treatment
cost.
Inspecting the water quality of the filtrate in the step
(S3) allows a protrusion to be added again to the surface of
25 the solid filter material when the water quality of the
33
filtrate is degraded. This can stabilize the water quality
of the filtrate even more.
Although, in the step of adding a protrusion (S1) in the
embodiment, a protrusion is added after the solid filter
material fills the filtering part, a similar effect can 5 be
obtained by forming the filter layer by filling the filtering
part with the solid filter material, that has been added with
a protrusion in another container.
{Second Embodiment}
10 Fig. 2 is a schematic block diagram of a suspendedmatter
removing apparatus according to the embodiment. The
suspended-matter removing apparatus 11 includes a filtering
part 2, a water-to-be-treated feeding part 3, a protrusionelement
feeding part 4, a differential-pressure measurement
15 part 12, a determination part 15, and a control part 16. The
filtering part 2, the water-to-be-treated feeding part 3, and
the protrusion-element feeding part 4 have a same
configuration as the first embodiment. The suspended-matter
removing apparatus 11 may include a water-quality inspection
20 means 9, as with the first embodiment.
The differential-pressure measurement part 12 can
measure a differential pressure between a first side (first
opening side) and a second side (second opening side) of a
filter layer 2a (the filtering part 2). In this embodiment,
25 the differential-pressure measurement part 12 is connected to
34
the first side and the second side of the filtering part 2.
The differential-pressure measurement part 12 is, for
example, a water pressure meter. The water pressure meter
detects pressures on the first side and the second side of
the 5 filtering part 2, to measure the differential pressure.
The determination part 15 can determine, based on a
preset standard, whether or not a protrusion has been added
to a surface of a solid filter material. In this embodiment,
the determination part 15 includes a protrusion-element10
amount measurement means (not shown) that directly or
indirectly measures an amount of the protrusion element
contained in the filtrate that has come out from the second
side (second opening side) of the filtering part 2. The
protrusion-element-amount measurement means may be sufficient
15 if it can directly or indirectly measure the amount of the
protrusion element. For example, when the protrusion element
is made of iron chloride, a water-quality analyzer capable of
monitoring an iron concentration can be used as the
protrusion-element-amount measurement means, to directly
20 measure the protrusion element. For example, using an SDI
measuring device as the protrusion-element-amount measurement
means enables indirect measurement of the protrusion element.
For example, when the protrusion element is made of kaolin,
using a turbidimeter as the protrusion-element-amount
25 measurement means enables indirect measurement of the
35
protrusion element.
When the protrusion element is indirectly measured, the
protrusion-element-amount measurement means can also serve as
the water-quality inspection means. In this embodiment, the
protrusion-element-amount measurement means is an 5 SDI
measuring device, which also serves as the water-quality
inspection means.
The determination part 15 can determine that a
protrusion has been added to the surface of the solid filter
10 material when a measured value of the protrusion-elementamount
measurement means becomes equal to or less than a
preset threshold value. The determination part 15 may also
determine that a protrusion has been added to the surface of
the solid filter material, when it is confirmed that the
15 measured value becomes equal to or less than a preset
threshold value and has been maintained in the state for a
certain time. The determination part 15 may be incorporated
into the control part 16.
The control part 16 is connected to the differential20
pressure measurement part 12, the determination part 15, and
a second feeding means 4b. The control part 16 can control a
feeding amount of the protrusion element from the protrusionelement
feeding part 4 such that the differential pressure
measured by the differential-pressure measurement part 12
25 becomes less than a predetermined value. The control part 16
36
receives a differential pressure value measured by the
differential-pressure measurement part 12, and automatically
controls the feeding amount of the protrusion element from
the protrusion-element feeding part 4 such that the
differential pressure is maintained at less 5 than the
predetermined value.
The control part 16 can control the protrusion-element
feeding part 4 to feed the protrusion element to add a
protrusion to the surface of the solid filter material when
10 the determination part 15 determines that a protrusion has
not been added, and to reduce the feeding amount of the
protrusion element when the determination part 15 determines
that a protrusion has been added.
The suspended-matter removing apparatus 11 may include,
15 at a downstream side of the filtering part 2, a reverseosmosis-
membrane treatment part 10, an electrodialysis part
(not shown), an evaporator (not shown) or the like. The
suspended-matter removing apparatus 11 may include a
backwashing means (not shown) for backwashing the filter
20 layer 2a.
The suspended-matter removing method according to the
embodiment includes the following steps (S11) to (S14):
(S11) A step of adding a protrusion
(S12) A step of measuring the differential pressure
25 between the first side of the filter layer and the second
37
side of the filter layer
(S13) A step of reducing a feeding amount of the
protrusion element as compared with when adding a protrusion
(S14) A step of passing water to be treated containing
suspended matters, through the filter layer having a 5 solid
filter material added with the protrusion
In the step of adding a protrusion (S11), the protrusion
element is fed to the filter layer 2a to add a protrusion to
the surface of the solid filter material. A procedure for
10 feeding the protrusion element to the filter layer 2a is same
as that of the first embodiment.
In this embodiment, while the protrusion element is
being fed to the filter layer 2a, the differential pressure
between the first side and the second side of the filter
15 layer 2a is measured (S12). In the step of adding a
protrusion (S11), the protrusion element is fed to the filter
layer 2a in a range that the differential pressure measured
at (S12) is less than a predetermined value. When the
measured differential pressure becomes equal to or more than
20 the predetermined value, the feeding of the protrusion
element is immediately stopped. The "predetermined value" may
be set based on an allowable pressure of the filtering part,
or may previously be set by performing a preliminary test or
the like. In the preliminary test, the differential pressure
25 of the filter layer is measured, and water quality of
38
filtrate is inspected, for example, by passing the protrusion
forming liquid containing the protrusion element with an
optional concentration through the filter layer. The
differential pressure of the filter layer when an inspection
value of the filtrate becomes a desired value may be 5 set to
be a predetermined value.
In the step (S13), an amount of the protrusion element
contained in the filtrate that has come out from the filter
layer 2a in the step of adding a protrusion (S11), is
10 directly or indirectly measured. When the measured amount of
the protrusion element becomes equal to or less than a preset
threshold value, it is determined that a protrusion has been
added to the surface of the solid filter material. When it
is determined that the protrusion has been added, the feeding
15 amount of the protrusion element is reduced (or stopped), as
with the step (S2) in the first embodiment.
Water to be treated containing suspended matters is
passed through the filter layer 2a (S14), with the feeding
amount of the protrusion element reduced (or stopped), as
20 with the step (S3) in the first embodiment.
In the step of passing the water to be treated
containing suspended matters (S14), it is preferable to
inspect the water quality of the filtrate that has come out
from the filter layer, as with the step (S3) in the first
25 embodiment.
39
According to the embodiment, measuring the differential
pressure between the first side and the second side of the
filter layer enables reliable suppression of an increase in
the differential pressure due to formation of a protrusion.
According to the embodiment, 5 measuring the amount of the
protrusion element in the filtrate that comes out when the
protrusion element is fed enables confirmation that the
protrusion element has not come out to the filtrate.
Thereby, in an indirect way, it can be confirmed that a
10 protrusion has been formed on the surface of the solid filter
material.
Third Embodiment
Fig. 3 is a schematic block diagram of a suspendedmatter
removing apparatus according to the embodiment. The
15 suspended-matter removing apparatus 21 has a same
configuration as that of the first embodiment except for
including a coarse-particle separation part 22.
The coarse-particle separation part 22 is provided
between a water-to-be-treated feeding part 3 and a filtering
20 part 2, in a preceding stage of a protrusion-element feeding
part 4. The coarse-particle separation part 22 mainly
separates suspended matters larger than 10 μm contained in
water to be treated. The coarse-particle separation part 22
is a sand filtration apparatus, a floatation-separation
25 apparatus, or the like. When the coarse-particle separation
40
part 22 is a sand filtration apparatus, the water to be
treated may be passed without addition of a flocculant. When
the coarse-particle separation part 22 is a floatationseparation
apparatus, solid-liquid separation is performed by
bonding/floating SS (sludge or floating matter) with a 5 large
amount of bubbles (micro-air) generated from water to be
treated mixed with saturated pressurized water.
In this embodiment, by passing water to be treated
through the coarse-particle separation part 22, suspended
10 matters larger than 10 μm are mainly separated from the water
to be treated, to make it water to be primarily treated.
Then, the water to be primarily treated is guided to the
filter layer, and suspended matters having a size of 0.1 μm
or more to 10 μm or less are removed.
15 The protrusion element can be fed to the filter layer
2a, at a same time as the guiding of the water to be
primarily treated to the filter layer. The protrusion
element may be fed to the filter layer 2a before the guiding
of the water to be primarily treated to the filter layer 2a.
20 In either case, a protrusion is added to the surface of the
solid filter material in accordance with the first embodiment
or the second embodiment, and then the feeding amount of the
protrusion element is reduced (or stopped).
According to the embodiment, by separating the rough
25 removal of suspended matters with a large particle diameter
41
in the water to be treated, and the removal of suspended
matters with a medium particle diameter of 0.1 μm or more to
10 μm or less, an increase in a differential pressure due to
clogging or the like in the filter layer can be suppressed.
This makes it possible to stabilize the water quality of 5 the
filtrate of the filter layer, and reduce a backwashing
frequency of the filter layer.
Next, a basis for the first to third embodiments and a
working effect are described.
10 (Study 1)
A study was made, through a simulation, regarding a
relationship between a capture rate and a size of suspended
matters captured in a filter layer (captured-particle
diameter) at a time when water to be treated containing
15 suspended matters is passed through a filter layer formed by
filling a solid filter material. A balance equation in the
filtration, in consideration of diffusion by Brownian motion
and an interception effect, was made for execution of the
simulation. A passage width d0 is equivalent to a diameter
20 of a small circle that is in a region surrounded by three
solid filter materials in contact with each other, and is in
contact with the three solid filter materials (see Fig. 4).
Diffusion of suspended matters due to turbulence of a flow
generated by unevenness on a surface is not considered. The
25 solid filter materials had a spherical shape, and particle
42
diameters of 100μm, 300μm (a minimum diameter of sand used
industrially for sand filtration), and 1200μm (a maximum
diameter of sand used industrially for sand filtration). A
filtering speed was 25 m/h (equivalent to cross-sectional
porosity of 50% of a sand filter column at a 5 superficial
velocity 12.5 m/h). In this simulation, the passage width d0
was same as the particle diameter of the solid filter
material.
A simulation result is shown in Fig. 5. In this figure,
10 the horizontal axis is the captured-particle diameter (μm),
and the vertical axis is the capture rate (%). According to
Fig. 5, as the solid filter material is smaller, the capture
rate of suspended matters having a size about 10 μm became
higher. However, it was confirmed that suspended matters
15 having a size of 0.1 μm to 5 μm can be hardly captured, even
when there was used a solid filter material having a size of
a minimum diameter of sand used industrially for sand
filtration.
A result of (Study 1) above shows that filtration using
20 the solid filter material can hardly remove suspended matters
of 0.1 μm or more to 10 μm or less. This result suggests
that, conventionally, as water to be treated contained more
suspended matters of 0.1 μm or more to 10 μm or less, water
quality of the filtrate was further degraded, even when a
25 same solid filter material was used for the filtration.
43
Thus, the inventors have concluded that, it is possible
to cope with load fluctuation and stabilize the water quality
of the filtrate, by removing suspended matters having a size
of 0.1 μm or more to 10 μm or less. In conventional
filtration using a solid filter material, the reason 5 why
suspended matters having a size of 0.1 μm or more to 10 μm or
less are not removed is considered as follows.
Fig. 6 shows a schematic view of a flow of water to be
treated when the water to be treated is passed through the
10 filter layer formed by filling a solid filter material. In
this figure, a symbol S represents a solid filter material,
and lines F extending in a vertical direction in the figure
represent stream lines of the water to be treated. The water
to be treated flowing in the filter layer is typically in a
15 laminar flow state as shown in Fig. 6. It is known that, in
the laminar flow state, a flow rate of the water to be
treated becomes lower as closer to a surface of the solid
filter material, and there is a region where the flow rate
becomes substantially zero (blocking-layer region) on the
20 surface of the solid filter material.
When the water to be treated is passed through the
filter layer formed by filling the solid filter material,
coarse suspended matters contained in the water to be treated
cannot be passed through a gap of the solid filter material,
25 and are captured. Even among suspended matters having a size
44
capable of being passed through a gap of the solid filter
material of the solid filter material, relatively larger
suspended matters may come out from the laminar flow by the
law of inertia, and collide with the solid filter material to
be captured. In the suspended matters contained in the 5 water
to be treated, fine suspended matters (colloidal particles
with a diameter of less than 0.1 μm) may be captured by the
solid filter material due to diffusion by Brownian motion.
Whereas, among the suspended matters contained in the
10 water to be treated, medium sized suspended matters
(particles with a diameter of 0.1 μm or more to 10 μm or
less) cannot come out of the laminar flow by the law of
inertia or the like, and are passed through the filter layer
with the laminar flow.
15 Based on the consideration above, a study was made
regarding a method for intentionally removing medium sized
suspended matters (particles with a particle diameter of 0.1
μm or more to 10 μm or less) from the laminar flow.
(Study 2)
20 A study was made, through a simulation, regarding a
behavior of suspended matters when water to be treated
containing suspended matters is passed through a filter layer
formed by filling a solid filter material added with a
protrusion. The simulation was performed by using the
25 Lattice Boltzmann Method (method for analyzing a fluid flow
45
by using the molecular kinetic theory, and movement of
suspended matters by using a motion equation). Diffusion by
Brownian motion is not considered. A passage width d0 was
600 μm, which was equivalent to a diameter of the solid
filter material, a length of the passage was 1.5 mm, 5 and a
flow rate was 25 m/h (equivalent to cross-sectional porosity
of 50% of a sand filter column at a superficial velocity 12.5
m/h). It was assumed that there was a protrusion with a
height of 60 μm and a width of 60 μm on a surface of the
10 solid filter material, and particle diameters of suspended
matters were 1 μm (suspended matter S1) and 5 μm (suspended
matter S2). In this condition, there is no interception
effect from the sizes of suspended matters, the size of
protrusion, and the passage width.
15 A simulation result is shown in Figs. 7 to 9. In Figs.
7 to 9, a vertical direction in the figure is a passage width
d0, and the water to be treated flows from left to right in
the figure. Fig. 7 is a view showing a flow of suspended
matters. Fig. 8 is a view illustrating a state of protrusions
20 in an early stage of passing of the water to be treated, and
Fig. 9 is a view illustrating a state of protrusions in a
late stage of passing of the water to be treated.
According to Fig. 7, it could be confirmed that a
presence of protrusions C caused a microscopic change in a
25 flow direction of suspended matters M. Accordingly, it was
46
confirmed that medium sized suspended matters came out of a
laminar flow, and the medium sized suspended matters out of
the laminar flow became easy to enter a blocking region, so
that a capture rate of the medium sized suspended matters
5 could be increased.
According to Figs. 8 and 9, it was confirmed that the
suspended matters M adhered to the protrusions C when the
water to be treated was passed through the filter layer
formed by filling the solid filter material formed with a
10 protrusion on a surface. A position where the suspended
matters M adhered was a corner facing an upstream side of a
passing direction of the water to be treated. It was
confirmed that suspended matters adhered to protrusions in
the early stage of passing water (Fig. 8), and other
15 suspended matters adhered around the suspended matters, that
had adhered to the protrusions in the early stage of passing
water, as a core, in the late stage of passing water (Fig.
9), so that the protrusions grown.
Although not illustrated, when the water to be treated
20 was passed through a filter layer filled with a solid filter
material not formed with a protrusion on a surface, no
suspended matter adhered to the surface of the solid filter
material.
A result of (Study 2) above suggests that, by feeding
25 the protrusion element to the filter layer to add a
47
protrusion satisfying a preset standard, suspended matters
contained in water to be treated adhere to the protrusion,
and thereby the protrusion can be grown, even when the
feeding amount of the protrusion element is reduced or
5 stopped afterward.
(Study 3)
A study was made, by using the Lattice Boltzmann Method,
regarding a minimum size of a protrusion required for
adhesion of suspended matters of 0.45 μm (an average pore
10 diameter of a filter for an SDI measurement) to 10 μm in
seawater, on a surface of the solid filter material.
Diffusion by Brownian motion is not considered. The
protrusion is rectangular, and a vertical length from the
surface of the solid filter material to the highest portion
15 of the protrusion was defined as a height. Particle
diameters of suspended matters were 0.45 μm, 2 μm, 5 μm, and
10 μm, and a calculation was performed for each of the
particle diameters. A passage width d0 was 600 μm, which was
equivalent to a diameter of the solid filter material, a
20 length of the passage was 1200 μm, and a flow rate was 0.006
m/s (a value equivalent to cross-sectional porosity of 50% of
a sand filter column at a superficial velocity 10.8 m/h). A
simulation result is shown in Fig. 10. In this figure, the
horizontal axis is the captured-particle diameter (μm), and
25 the vertical axis is the height of a protrusion (μm).
48
According to Fig. 10, as a size of the protrusion is
larger, small suspended matters could be captured more.
Placing a rectangular body (protrusion) of 4 μm enabled
removal of suspended matters of 10 μm. According to Fig. 10,
removal of suspended matters of 0.45 μm required a 5 rectangle
(protrusion) with a height of 40 μm.
(Study 4)

Protrusion forming liquid containing a protrusion
10 element was passed through a filter layer formed by filling a
solid filter material for three hours, to add a protrusion to
a surface of the solid filter material. Then, passing of the
protrusion forming liquid was stopped, and in that state,
water to be treated was passed through the filter layer for
15 three hours. A filtering speed was 10 m/h.
A filter column (column diameter 5 cm) was formed in a
three-layered structure of an anthracite filter layer, a sand
filter layer, and a gravel filter layer. The anthracite
filter layer, the sand filter layer, and the gravel filter
20 layer are sequentially arranged from an upstream side of the
passing direction of the water to be treated. The anthracite
filter layer is a filter layer formed by filling anthracite
with an average particle diameter of 700 μm. A length of the
anthracite filter layer is 200 mm. The sand filter layer is
25 a filter layer formed by filling sand with an average
49
particle diameter of 475 μm. A length of the sand filter
layer is 500 mm. The gravel filter layer is a filter layer
formed by filling gravel with an average particle diameter of
2000 μm. A length of the gravel filter layer is 100 mm.
The protrusion element was made of iron chloride (5 FeCl3:
Wako Pure Chemical Industries, Ltd.). Iron chloride reacts
with an alkaline component in water to generate iron
hydroxide, as formula (1) below. This iron hydroxide was
presumed to adhere to the filter material to form a
10 protrusion.
FeCl3 + 3HCO3
- = Fe(OH)3 + 3CO2 + 3Cl- ∙∙∙ (1)
Seawater was used as the water to be treated. An SDI of
the seawater before passing was 6.14. Protrusion forming
liquid containing the protrusion element was prepared, and
15 the protrusion forming liquid was passed through the filter
layer along with the water to be treated. A concentration of
the protrusion element in the protrusion forming liquid was
set so as to cause an Fe-concentration of 1 ppm with respect
to an amount of passing water.
20 During the passing of the water to be treated, a
differential pressure of the filter layer was measured by a
differential-pressure measuring device. Additionally, an Feconcentration
and an SDI of liquid (filtrate) that has passed
the filter layer were continuously measured. The Fe25
concentration was measured by a 2,4,6-tris-2-pyridyl-1,3,5-
50
triazine absorptiometric method (abbreviated as TPTZ
absorptiometric method) described in JIS B8224.
The SDI is obtained by the following formula (2) based
on a time required for filtration/collection at 206 kPa, by
using a filter with a diameter of 47 mm and an average 5 verage pore
diameter of 0.45 μm.
SDITm = (1 - Δt1/Δt2) × 100/Tm ∙∙∙ (2)
Δt1: A time (sec) required for filtration/collection of
initial 500 ml.
10 Δt2: A time (sec) required for filtration/collection of
500 ml after Tm minutes.
Tm: A time from the t1 filtration/collection starting
time to the t2 filtration/collection starting time (normally
15 minutes).
15 An upper limit of the SDI index is 6.67. Since the SDI
is decreased, it is suggested that a ratio of suspendedmatter
particles larger than 0.45 μm is decreased.

For comparison, only seawater was passed without passing
20 of the protrusion forming liquid through the filter layer,
and the measurement was performed as with Test A.
Fig. 11 shows a measurement result of a differential
pressure of the filter layer. In this figure, the horizontal
axis is an elapsed time (h), and the vertical axis is the
25 differential pressure (kPa) of the filter layer. According
51
to Fig. 11, by passing the protrusion forming liquid
containing iron hydroxide, the differential pressure of the
filter layer was slightly increased in Test A, but an
increase in the differential pressure was not observed after
the passing of the protrusion forming liquid was stopped. 5 In
Test B (a case without passing of protrusion forming liquid),
a change in a differential pressure of the filter layer was
hardly observed within the same period of time.
Fig. 12 shows a measurement result of an SDI of Tests A
10 and B. In this figure, the horizontal axis is an elapsed
time (h), and the vertical axis is the SDI (-).
According to Fig. 12, the SDI of the filtrate was
decreased to about 4 after two to three hours of passing in
Test A. Even after the passing of the protrusion forming
15 liquid was stopped, the SDI of the filtrate was maintained at
about 4.
Although not shown in Fig. 12, an Fe-concentration of
the filtrate reached 1 μg/L (detection lower limit) after two
hours of the passing in Test A. This shows that the iron
20 hydroxide contained in the protrusion forming liquid remains
in the filter layer. After the passing of the protrusion
forming liquid was stopped, the Fe-concentration of the
filtrate was maintained at 1 μg/L. Accordingly, it could be
confirmed that the iron hydroxide remaining in the filter
25 layer was not stripped off by subsequent water passing.
52
It was confirmed that, it is possible to add a
protrusion required to stabilize water quality of the
filtrate to the surface of the solid filter material, by
passing the protrusion forming liquid for three hours so as
to cause an Fe-concentration of 1 ppm with respect 5 pect to the
water to be treated. It is presumed that a suspended-matter
removal ability can be maintained unless iron hydroxide comes
out from the filter layer.
According to Fig. 12, the SDI of the filtrate remained
10 high at 5.21 when only the water to be treated was passed
through without passing of the protrusion forming liquid, as
with Test B. In Test B, it is presumed that, although
suspended matters were removed with mainly an interception
effect and diffusion by Brownian motion, medium suspended
15 matters (0.1 μm to 10 μm) could not be removed, preventing a
sufficient decrease of the SDI. It is presumed that the SDI
was kept high because medium suspended matters have not been
removed.
A result of this Study shows that, after passing of the
20 protrusion forming liquid through the filter layer, the water
quality of the filtrate can be improved quickly in two to
three hours. Even after the passing of the protrusion
forming liquid was stopped, the water quality of the filtrate
was stable.
25 In sand filtration using a typical flocculant, the
53
flocculant is continuously added. The flocculant and sludge
formed by suspended matters contained in the water to be
treated cause clogging of a filter layer, increasing a
differential pressure along with the continuation of the
filtration. Thus, in general, the filter layer 5 yer must be
washed in a washing speed in which a developing rate of air
washing (washing by collision between filter materials, using
air bubbling) and the filter water becomes 30%. Whereas, in
the present filtration method, which injects protrusion
10 forming liquid to add a protrusion to a surface of a solid
filter material, it is only capturing suspended matters
contained in water to be treated, reducing a washing
frequency of a solid-filter-material layer without increasing
a differential pressure.
15 (Study 5)
A suspended-mater removal test was performed by using a
suspended-matter removing apparatus provided with a coarseparticle
separation part (column diameter 5 cm) and a
filtering part (column diameter 5 cm).
20 A sand filtration apparatus was used as the coarseparticle
separation part. The sand filtration apparatus has
a sand filter layer (length 1200 mm) formed by filling sand
with an average particle diameter of 350 μm, and a gravel
filter layer (length 100 mm) formed by filling gravel with an
25 average particle diameter of 2000 μm. The sand filter layer
54
is on an upstream side of the gravel filter layer in a
passing direction of water to be treated.
The filtering part has a filter layer. The filter layer
is configured by an anthracite filter layer (length 200 mm)
formed by filling anthracite with an average 5 age particle
diameter of 700 μm, a sand filter layer (length 1000 mm)
formed by filling sand with an average particle diameter of
350 μm, and a gravel filter layer (length 100 mm) formed by
filling gravel with an average particle diameter of 2000 μm.
10 The anthracite filter layer, the sand filter layer, and the
gravel filter layer are arranged in this order from the
upstream side in the passing direction of the water to be
treated.
Water to be treated was passed through the coarse15
particle separation part by a water-to-be-treated feeding
part. Then, filtrate (primarily treated water) that had come
out from the coarse-particle separation part was passed
through the filtering part. The primarily treated water
before entering the filtering part was added with protrusion
20 forming liquid, and the protrusion forming liquid and the
primarily treated water were passed in same time. After
three hours from the start of passing, the passing of the
protrusion forming liquid was stopped. The water to be
primarily treated continued to be passed for three hours even
25 after the passing of the protrusion forming liquid was
55
stopped.
Differential pressures of the coarse-particle separation
part and the filtering part were measured by a differentialpressure
measuring device, during the passing of the water to
be treated and the primarily treated water. Additionally, 5 tionally, an
SDI of liquid (filtrate) that had passed the filtering part
was continuously measured. A filtering speed was 10 m/h.
The protrusion element was made of iron chloride
(FeCl3), and the protrusion forming liquid was fed so as to
10 cause an Fe-concentration of 1 ppm with respect to the
primarily treated water. An SDI of seawater before passing
is 6.28.
Fig. 13 shows a measurement result of differential
pressures of the coarse-particle separation part and the
15 filtering part (filter layer). In this figure, the horizontal
axis is an elapsed time (h), and the vertical axis is the
differential pressure (kPa). According to Fig. 13, during
the passing of the water to be treated, a change in the
differential pressure of the filtering part was hardly
20 observed at the coarse-particle separation part. According
to Fig. 13, while the differential pressure of the filtering
part was slightly increased during the passing of the
protrusion forming liquid, an increase in the differential
pressure was not observed during the passing of only the
25 primarily treated water after the passing of the protrusion
56
forming liquid was stopped.
Fig. 14 shows an SDI measurement result of the filtrate
that has come out from the filtering part. In this figure,
the horizontal axis is an elapsed time (h), and the vertical
axis is the SDI (-). According to Fig. 14, although 5 ugh the SDI
of seawater before passing was 6 or more, the SDI of the
filtrate of the filtering part was decreased to less than 4
after two to three hours of passing of the protrusion forming
liquid. The SDI of the filtrate of the filtering part could
10 be maintained at less than 4, even after the passing of the
protrusion forming liquid was stopped. While a standard of a
turbidity concentration required for feed water to an RO
(reverse osmosis) membrane is generally SDI<4, the filtrate
of two to three hours of passing satisfied the water quality
15 standard.
Based on the results of Studies 1 to 3, it is presumed
that the coarse-particle separation part mainly captures
suspended matters smaller than 0.1 μm, and suspended matters
larger than 10 μm. Since the SID has been decreased by
20 passing the primarily treated water from which coarse
particles are removed through the filtering part (filtering
layer), the filter layer seems to capture medium sized
suspended matters of 0.1 μm or more to 10 μm or less.
25
57
(Study 6)
A suspended-mater removal test was performed by using a
suspended-matter removing apparatus provided with a coarseparticle
separation part (column diameter 5 cm) and a
filtering part (column diameter 5 cm). A 5 sand filtration
apparatus was used as the coarse-particle separation part.
The sand filtration apparatus has a sand filter layer (length
800 mm) formed by filling sand with an average particle
diameter of 350 μm, and a gravel filter layer (length 100 mm)
10 formed by filling gravel with an average particle diameter of
2000 μm. The sand filter layer is on an upstream side of the
gravel filter layer in a passing direction of water to be
treated.
The filtering part has a filter layer. The filter layer
15 is configured by an anthracite filter layer (length 200 mm)
formed by filling anthracite with an average particle
diameter of 700 μm, a sand filter layer (length 600 mm)
formed by filling sand with an average particle diameter of
350 μm, and a gravel filter layer (length 100 mm) formed by
20 filling gravel with an average particle diameter of 2000 μm.
The anthracite filter layer, the sand filter layer, and the
gravel filter layer are arranged in this order from the
upstream side in the passing direction of the water to be
treated.
25 Water to be treated was passed through the coarse58
particle separation part by a water-to-be-treated feeding
part. Then, filtrate (primarily treated water) that had come
out from the coarse-particle separation part was passed
through the filtering part. The primarily treated water
before entering the filtering part was added with 5 h protrusion
forming liquid, and the protrusion forming liquid and the
primarily treated water were passed in same time. After
three hours from the start of passing, the passing of the
protrusion forming liquid was stopped. The primarily treated
10 water continued to be passed through for three hours even
after the passing of the protrusion forming liquid was
stopped.
Differential pressures of the coarse-particle separation
part and the filtering part were measured by a differential15
pressure measuring device, during the passing of the water to
be treated and the primarily treated water. Additionally, an
SDI of liquid (filtrate) that had passed the filtering part
was continuously measured. A filtering speed was 10 m/h.
The protrusion element was made of kaolin. As the
20 kaolin, powder with an average particle diameter of 10 to 15
μm was used (made by Takehara Kagaku Kogyo Co., Ltd.). The
protrusion forming liquid was fed to cause a kaolin
concentration of 2 ppm with respect to the primarily treated
water. An SDI of seawater before passing is 5.2.
25 Fig. 15 shows a measurement result of differential
59
pressures of the coarse-particle separation part and the
filtering part (filter layer). In this figure, the
horizontal axis is an elapsed time (h), and the vertical axis
is the differential pressure (kPa). According to Fig. 15,
during the passing of the water to be treated, a change 5 in
differential pressures of the coarse-particle separation part
and the filtering part was hardly observed.
Fig. 16 shows an SDI measurement result of the filtrate
that has come out from the filtering part. In this figure,
10 the horizontal axis is an elapsed time (h), and the vertical
axis is the SDI (-). According to The Fig. 16, after the
passing of the protrusion forming liquid through the filter
layer, the SDI of the filtrate quickly fell to below 4. It
is presumed that the kaolin is captured to form a protrusion,
15 and the protrusion removes medium sized suspended matters.
Here, it was confirmed that an increase in differential
pressures of the coarse-particle separation part and the
filtering part was small.
As an index that indicates a performance of a filter
20 column, an L/D is used. The L/D is obtained by dividing a
layer thickness L by a particle diameter D. The L/D is a
value proportional to a total area of the filter material per
unit filtration area, and as this value is larger, a surface
area of the filter material per unit filtration area is
25 larger. The L/D of this testing apparatus was 4385. The L
60
was calculated from an input amount of kaolin, and the L/D
calculated by using a particle diameter of 12.5 μm (an
arithmetic average of an average particle diameter) was 0.4.
Thus, it is found that SDI<4 can be satisfied without an
5 increase of the surface area.
(Study 7)
Protrusion forming liquid containing high-molecular
polymer as a protrusion element was fed to primarily treated
water, and a differential pressure of a filtering part and an
10 SDI of filtrate of the filtering part were measured, as with
(Study 6) above. A filtering speed was 10 m/h.
A solid filter material and a filter layer are same as
those in (Study 6) above. As the high-molecular polymer,
there was used Himoloc Q707 (polyamide based, molecular
15 weight (estimate) = 70,000, specific gravity = 1.15) made by
HYMO CORPORATION. The protrusion forming liquid was fed so
as to cause a high-molecular polymer concentration of 0.5 ppm
with respect to the primarily treated water. The water to be
treated is Seawater. An SDI of the seawater before passing
20 was 5.2.
Fig. 17 shows a measurement result of differential
pressures of a coarse-particle separation part and the
filtering part (filter layer). In this figure, the horizontal
axis is an elapsed time (h), and the vertical axis is the
25 differential pressure (kPa) of the filter layer. According
61
to Fig. 17, during the passing of the water to be treated, a
change in differential pressures of the coarse-particle
separation part and the filtering part was hardly observed.
Fig. 16 shows an SDI measurement result of the filtrate
that has come out from the filtering part. According to 5 Fig.
16, although the SDI of seawater was 5.2, the SDI of the
filtrate of the filtering part was decreased to less than 4
after two to three hours of passing of the protrusion forming
liquid. The SDI of the filtrate of the filtering part could
10 be maintained at less than 4, even after the passing of the
protrusion forming liquid was stopped. It was considered
that the high-molecular polymer had utilized suspended
matters in the seawater to form a protrusion on the surface
of the solid filter material, causing a decrease in the SDI.
15 Here, it was confirmed that an increase in differential
pressures of the coarse-particle separation part and the
filtering part was small.
(Study 8)
Protrusion forming liquid containing kaolin and high20
molecular polymer as a protrusion element was fed to
primarily treated water, and a differential pressure of the
filtering part and an SDI of the filtrate of the filtering
part were measured, as with (Study 6) above. A filtering
speed was 10 m/h.
25 A solid filter material and a filter layer are same as
62
those in (Study 6) above. As the kaolin, powder with an
average particle diameter of 10 to 15 μm was used (made by
Takehara Kagaku Kogyo Co., Ltd.). As the high-molecular
polymer, there was used Himoloc Q707 (polyamide based,
molecular weight (estimate) = 70,000, specific 5 gravity =
1.15) made by HYMO CORPORATION. The protrusion forming
liquid was fed so as to cause kaolin of 2 ppm and highmolecular
polymer of 0.5 ppm with respect to the primarily
treated water. The water to be treated is Seawater. An SDI
10 of the seawater before passing was 5.6.
Fig. 18 shows a measurement result of the differential
pressures of the coarse-particle separation part and the
filtering part (filter layer). In this figure, the horizontal
axis is an elapsed time (h), and the vertical axis is the
15 differential pressure (kPa) of the filter layer. According
to Fig. 18, during the passing of the water to be treated, a
change in the differential pressure of the filtering part was
hardly observed at the coarse-particle separation part.
According to Fig. 18, during the passing of the protrusion
20 forming liquid, the differential pressure of the filtering
part was not increased, and even after the passing of the
protrusion forming liquid was stopped, the differential
pressure of the filtering part was not increased.
Fig. 16 shows an SDI measurement result of the filtrate
25 that has come out from the filtering part. According to Fig.
63
16, although the SDI of the seawater before passing was 5.6
or more, the SDI of the filtrate of the filtering part was
decreased to less than 4 after two to three hours of passing
of the protrusion forming liquid. The SDI of the filtrate of
the filtering part could be maintained at less than 4, 5 even
after the passing of the protrusion forming liquid was
stopped. It was presumed that the kaolin and the highmolecular
polymer formed a protrusion on the surface of the
solid filter material, causing a decrease the SDI.
10
Reference Signs List
1, 11, 21 suspended-matter removing apparatus
2 filtering part
2a filter layer
15 2b first opening
2c second opening
3 water-to-be-treated feeding part
3a water-to-be-treated tank
3b first feeding means
20 4 protrusion-element feeding part
4a protrusion element tank
4b second feeding means
5, 15 determination part
6, 16 control part
25 7 first passage
64
8 second passage
9 water-quality inspection means
10 reverse-osmosis-membrane treatment part
12 differential-pressure measurement part
22 coarse-5 particle separation part

CLAIMS:
1. A suspended-matter removing method comprising the steps
of:
by feeding a protrusion element to a filter layer
formed by filling a solid filter material, adding 5 a
protrusion to a surface of the solid filter material;
after feeding of the protrusion element in the step
of adding a protrusion, determining whether or not a
protrusion satisfying a preset standard has been added
10 to the surface of the solid filter material, and when it
is determined that the protrusion has been added,
reducing a feeding amount of the protrusion element as
compared with when adding the protrusion; and
passing water to be treated containing suspended
15 matters through the filter layer having the solid filter
material added with the protrusion in a state in which
the feeding amount of the protrusion element is reduced.
2. The suspended-matter removing method according to claim
1, wherein the feeding of the protrusion element is
20 stopped, in the step of reducing the feeding amount of
the protrusion element.
3. The suspended-matter removing method according to claim
1 or 2, further comprising a step of passing the water
to be treated through the filter layer, in parallel with
66
the step of adding a protrusion.
4. The suspended-matter removing method according to any of
claims 1 to 3, further comprising a step of measuring a
differential pressure between a first side of the filter
5 layer and a second side of the filter layer, wherein
the protrusion element is fed within a range where
the measured differential pressure is less than a
predetermined value, in the step of adding a protrusion.
5. The suspended-matter removing method according to any of
10 claims 1 to 4, further comprising a step of directly or
indirectly measuring an amount of the protrusion element
contained in filtrate that has come out from the filter
layer in the step of adding the protrusion, wherein
it is determined that the protrusion satisfying the
15 preset standard has been added to the surface of the
solid filter material when the measured amount of the
protrusion element becomes equal to or less than a
preset threshold value.
6. The suspended-matter removing method according to any of
20 claims 1 to 3, wherein
a total feeding amount of the protrusion element to
the filter layer in the step of adding a protrusion is
counted, and it is determined that the protrusion
67
satisfying the preset standard has been added to the
surface of the solid filter material when the counted
total feeding amount reaches a preset threshold value.
7. The suspended-matter removing method according to any of
claims 1 to 6, further comprising a step 5 of inspecting
water quality of the filtrate that has come out from the
filter layer in the step of passing the water to be
treated, wherein
when an inspection value of the filtrate exceeds a
10 preset threshold value, it is determined that the
protrusion satisfying the preset standard has not been
added to the surface of the solid filter material, and
the step of adding a protrusion is performed; and when
the inspection value of the filtrate is equal to or less
15 than the preset threshold value, it is determined that
the protrusion satisfying the preset standard has been
added to the surface of the solid filter material, and
the feeding amount of the protrusion element is reduced
as compared with when adding the protrusion.
20 8. The suspended-matter removing method according to any of
claims 1 to 7, wherein
in the step of passing the water to be treated, the
water to be treated is passed through a coarse-particle
separation part to make the water to be treated into
68
water to be primarily treated by mainly separating
suspended matters larger than 10 μm contained in the
water to be treated, and then the water to be primarily
treated is passed through the filter layer to remove
suspended matters having a size of 0.1 μm or more 5 to 10
μm or less.
9. The suspended-matter removing method according to any of
claims 1 to 8, wherein a height of the protrusion is 4
μm or more.
10 10. The suspended-matter removing method according to any of
claims 1 to 9, wherein an average particle diameter of
the solid filter material is 300 μm or more to 2500 μm
or less.
11. The suspended-matter removing method according to any of
15 claims 1 to 10, wherein the protrusion element is made
of kaolin.
12. The suspended-matter removing method according to any of
claims 1 to 10, wherein the protrusion element is made
of iron chloride.
20 13. The suspended-matter removing method according to claim
12, wherein, in the step of reducing the feeding amount
of the protrusion element as compared with when the
protrusion is added, the feeding amount of the
69
protrusion element is reduced such that content of the
protrusion element becomes less than 0.5 ppm as iron in
a solution that passes the filter layer.
14. The suspended-matter removing method according to any of
claims 1 to 13, wherein the protrusion element 5 is made
of high-molecular polymer.
15. A suspended-matter removing apparatus comprising:
a filtering part having a filter layer formed by
filling a solid filter material;
10 a water-to-be-treated feeding part that feeds water
to be treated to a first side of the filtering part to
pass the water to be treated through the filter layer;
a protrusion-element feeding part that feeds a
protrusion element to the first side of the filtering
15 part;
a determination part that, based on a preset
standard, determines whether or not a protrusion has
been added to a surface of the solid filter material;
and
20 a control part that, when the determination part
determines that the protrusion has been added, controls
the protrusion-element feeding part to reduce a feeding
amount of the protrusion element as compared with when
it is determined that the protrusion has not been added.
70
16. The suspended-matter removing apparatus according to
claim 15, wherein, the control part is set to control
the protrusion-element feeding part to stop feeding of
the protrusion element, when the determination part
5 determines that the protrusion has been added.
17. The suspended-matter removing apparatus according to
claim 15 or 16, further comprising a differentialpressure
measurement part that measures a differential
pressure between the first side and a second side of the
10 filtering part, and the control part is set to control
the feeding amount of the protrusion element from the
protrusion-element feeding part such that the
differential pressure measured by the differentialpressure
measurement part becomes less than a
15 predetermined value.