Technical Field
The present invention relates to a water treatment
process and a water treatment system for reproducing water
to be treated containing Ca ions (Ca2+) , sulfate ions (S04
2~) ,
5 carbonate ions, and silica.
Background Art
It is known that industrial waste water, saline water,
and sewage contain large amounts of ions and silica. In
10 addition, in a cooling tower, heat is exchanged between a
high-temperature exhaust gas discharged from the boiler,
etc., and cooling water. As a result of this heat exchange,
some of the cooling water turns into steam, and,
accordingly, ions and silica in the cooling water are
15 concentrated. Therefore, the cooling water discharged from
the cooling tower (blowdown water) has increased
concentrations of ions and silica.
Water containing a large amount of ions is subjected to
a demineralization treatment and then discharged into the
20 environment. As devices that perform the demineralization
treatment, a reverse osmosis membrane device, a
nanofiltration membrane device, an ion-exchange equipment,
and the like are known.
Among ions contained in the water mentioned above,
25 monovalent cations such as Na+, K+, and NH4
+ and anions such
as Cl~ and N03~ are highly soluble in water. On the other
hand, divalent metal ions such as Ca2+, anions such as S04
2~
and C03
2~, and silica are scale-forming components. Salts and
silica of scale-forming components have low solubility in
30 water, and thus they tend to be deposited as scales. In
particular, the saline water, industrial waste water, and
blowdown water from a cooling tower mentioned above contain
2
large amounts of Ca2+, S04
2~, carbonate ions (C03
2_, HC03~) , and
silica. An example of the property is as follows: pH: 8, Na
ions: 20 mg/L, K ions: 5 mg/L, Ca ions: 50 mg/L, Mg ions: 15
mg/L, HC03 ions: 200 mg/L, Cl ions: 200 mg/L, S04 ions: 120
5 mg/L, P04 ions: 5 mg/L, Si02 ions: 35 mg/L. Among these, the
concentrations of Ca ions, Mg ions, S04 ions, and HC03 ions
are high, and as a result of their reaction, scales (CaS04,
CaC03, etc.) are formed. In addition, depending on the
concentration percentage, silica components present in waste
10 water also serve as scale components adhering to the
instrument, etc. When scales are produced in the device that
performs a demineralization treatment, the treatment
capacity is reduced. Therefore, it is required to perform a
demineralization treatment without allowing for the
15 production of scales.
Here, examples of plants using a water-cooling-type
cooling tower are plants equipped with power generation
facilities (power generation facilities include those for
business purposes for electric power selling and those for
20 industrial purposes for in-house electricity use, and the
power generation is thermal power generation, geothermal
power generation, etc.), plants equipped with power
generation facilities and cooling facilities, etc. In
addition, plants include ordinary chemical plants, steel
25 plants, mining plants, oil field plants, gas field plants,
mechanical plants, etc.
As a process for removing Ca ions, a lime soda process
is known. According to the lime soda process, sodium
carbonate is added to water to be treated, and Ca ions in
30 the water to be treated are deposited/precipitated as
calcium carbonate and thereby removed from the water.
Patent Literature 1 discloses a waste water treatment
3
device including a combination of a chemical softening
device, an ion-exchange equipment, a reverse osmosis
membrane device, and the like using the lime soda process.
5 Citation List
Patent Literature
PTL 1 U.S. Pat. No. 7815804
Summary of Invention
Technical Problem
10 The lime soda process requires the addition of sodium
carbonate for the treatment, and thus the treatment cost is
high. In the lime soda process, when 1 mol of Ca ions are
precipitated as calcium carbonate, 2 mol of Na+ is produced.
Meanwhile, in the case where S04
2~ is contained in water to
15 be treated, it is not removed by the lime soda process. That
is, in the lime soda process, water after the treatment
contains an increased number of moles of ions.
Also in the case where Ca ions are removed using an
ion-exchange equipment, the treatment of 1 mol of Ca ions
20 results in the production of 2 mol of Na+, and water the
after treatment contains an increased number of moles of
ions.
According to the system of Patent Literature 1, water
that has been treated by the lime soda process and in an
25 ion-exchange equipment is further treated in a reverse
osmosis membrane device to remove ion components.
Accordingly, the system of Patent Literature 1 has a problem
in that because of the increased number of moles of ions,
the osmotic pressure in the reverse osmosis membrane device
30 is high, resulting in an increased treatment load. In
addition, with the device of Patent Literature 1, S04
2~ is
not removed but remains in the treated water, and it has
4
been difficult to obtain high water recovery.
In addition, the waste water treatment device of Patent
Literature 1 requires a large amount of chemicals for the
reproduction of the ion-exchange equipment, and thus there
5 has also been the problem of high treatment cost.
An object of the present invention is to provide a
water treatment process and a water treatment system, which
are capable of reproducing water containing salts with high
water recovery.
10
Solution to Problem
A first aspect of the present invention is a water
treatment process including:
a scale inhibitor supplying step of supplying a calcium
15 scale inhibitor which is a scale inhibitor for inhibiting
the deposition of a scale containing calcium and a silica
scale inhibitor which is a scale inhibitor for inhibiting
the deposition of silica to water to be treated containing
Ca ions, SO4 ions, carbonate ions, and silica;
20 a demineralizing step of separating the water to be
treated into concentrated water in which the Ca ions, the S04
ions, the carbonate ions, and the silica are concentrated
and treated water after the scale inhibitor supplying step;
and
25 a crystallizing step of supplying seed crystals of
gypsum to the concentrated water so that gypsum is
crystallized from the concentrated water.
A second aspect of the present invention is a water
treatment system including:
30 a scale inhibitor supplying section that supplies a
calcium scale inhibitor which is a scale inhibitor for
inhibiting the deposition of a scale containing calcium and
5
a silica scale inhibitor which is a scale inhibitor for
inhibiting the deposition of silica to water to be treated
containing Ca ions, S04 ions, carbonate ions, and silica;
a demineralizing section that is installed on a
5 downstream side of the scale inhibitor supplying section and
separates the water to be treated into concentrated water in
which the Ca ions, the S04 ions, and the silica are
concentrated and treated water; and
a crystallizing section including a crystallizing tank
10 that is installed on the downstream side of the
demineralizing section and crystallizes gypsum from the
concentrated water and a seed crystal supplying section that
supplies seed crystals of gypsum to the crystallizing tank.
In the first aspect and the second aspect, owing to the
15 effects of the calcium scale inhibitor and the silica scale
inhibitor supplied, the production of scales in the
demineralizing section and the demineralizing step can be
inhibited. In addition, by adding seed crystals of gypsum to
the first concentrated water in the first crystallizing
20 section and the first crystallizing step, even when a scale
inhibitor is present, gypsum can be crystallized and
separated from the water to be treated. As a result, the
water to be treated containing Ca ions, S04 ions, and silica
can be treated with high water recovery, and the operation
25 cost can be reduced. Further, this is also advantageous in
that high-purity gypsum can be recovered.
In the above aspect, the water treatment process
includes a downstream side demineralizing step of separating
the concentrated water after the crystallizing step on the
30 most downstream of the water to be treated into concentrated
water and treated water, and recovering the separated
treated water.
6
In the above aspect, the water treatment system
includes, on the downstream side of the crystallizing
section on the most downstream of the water to be treated, a
downstream side demineralizing section that separates the
5 concentrated water discharged from the crystallizing section
into concentrated water and treated water.
When the downstream side demineralizing step and the
downstream side demineralizing section are provided, the
water recovery can be further improved. In addition, in the
10 invention, the number of moles of ions in the water to be
treated is significantly reduced. Accordingly, the amount of
salts flowing into the downstream side demineralizing
section can be reduced, and thus the power of the downstream
side demineralizing section can be reduced.
15 In the above aspect, it is preferable that the water
treatment process includes a pH adjusting step of adjusting
the concentrated water to a pH at which a scale inhibition
function of the calcium scale inhibitor is reduced, thereby
promoting the deposition of the gypsum in the crystallizing
20 step.
In the above aspect, it is preferable that the water
treatment system includes a pH adjusting section that is
installed on the downstream side of the demineralizing
section and supplies a pH adjuster to the concentrated water
25 to adjust the pH of the concentrated water to such a value
that a scale inhibition function of the calcium scale
inhibitor is reduced, and the deposition of the gypsum is
promoted.
In this case, it is preferable that after the
30 crystallizing step, the concentrated water after the
adjustment of the pH in the pH adjusting step is adjusted to
a pH at which the calcium scale inhibitor exerts its
7
function.
It is also preferable that the water treatment system
includes, on the downstream side of the crystallizing
section, a pH adjusting section for achieving scale
5 inhibition function that supplies a pH adjuster to the
concentrated water after the adjustment of the pH in the pH
adjusting section to adjust the pH of the concentrated water
to such a value that the calcium scale inhibitor achieves a
function.
10 Alternatively, in the above aspect, the water treatment
process may include a pH adjusting step of adjusting the
concentrated water to a pH at which the silica is soluble in
the crystallizing step. In the above aspect, it is
preferable that the water treatment system includes a pH
15 adjusting section that is installed on the downstream side
of the demineralizing section and supplies a pH adjuster to
the second concentrated water to adjust the pH of the
concentrated water to such a value that the silica is
soluble in the concentrated water in the second
20 crystallizing section.
As a result of this configuration, the formation of
scales containing calcium in the downstream side
demineralizing section can be suppressed.
In the first aspect, it is preferable that the water
25 treatment process includes a upstream side precipitating
step of precipitating at least calcium carbonate from the
water to be treated so that the concentration of the calcium
carbonate in the water to be treated is reduced, before the
scale inhibitor supplying step on the most upstream side of
30 the water to be treated. In this case, it is preferable that
the water treatment process includes a deaerating step of
removing C02 from the water to be treated before the upstream
8
side precipitating step or after the upstream side
precipitating step and before the scale inhibitor supplying
step.
In the second aspect, it is preferable that the water
5 treatment system includes, on the upstream side of the scale
inhibitor supplying section located on the most upstream of
the water to be treated, a upstream side precipitating
section that precipitates at least calcium carbonate from
the water to be treated so that the concentration of the
10 calcium carbonate in the water to be treated is reduced. In
this case, it is preferable that the water treatment system
includes a deaerating section that removes C02 from the water
to be treated on the upstream side of the upstream side
precipitating section or on the downstream side of the
15 upstream side precipitating section and on the upstream side
of the scale inhibitor supplying section.
In this way, by previously removing calcium carbonate
from the water to be treated before flowing into the
demineralizing section, the deposition of calcium carbonate
20 as scales during the water treatment can be inhibited. By
removing calcium carbonate, the purity of gypsum
crystallized in the crystallizing step and the crystallizing
section can be increased.
In the above aspect, it is preferable that the water to
25 be treated contains metal ions, and the water treatment
process includes a precipitating step of precipitating at
least one of calcium carbonate and a metal compound so that
the concentration of at least one of the calcium carbonate
and the metal ions is reduced from the concentrated water,
30 after the crystallizing step. In this case, at least one of
seed crystals of the silica and a precipitant for the silica
is supplied to the concentrated water in the precipitating
9
step.
In the above aspect, it is preferable that the water to
be treated contains metal ions, and the water treatment
system includes, on the downstream side of the crystallizing
5 section, a precipitating section that precipitates at least
one of calcium carbonate and a metal compound so that the
concentration of at least one of the calcium carbonate and
the metal ions in the concentrated water is reduced. In
this case, at least one of seed crystals of.the silica and a
10 precipitant for the silica is supplied to the precipitating
section.
By removing calcium carbonate and a metal compound from
the water to be treated in the precipitating section and the
precipitating step provided after the crystallizing section
15 and the crystallizing step, high water recovery can be
obtained.
The dissolution state of silica changes depending on
the pH of the water to be treated, but silica tends not to
be deposited only by changing the pH. Thus, seed crystals of
20 silica are added in the precipitating section and the
precipitating step to promote the deposition of silica,
whereby the silica removal efficiency can be improved. As a
result, the water recovery can be further improved, and the
operation power can be further lowered.
25 In the water treatment process of the above aspect, it
is preferable that when the water to be treated contains Mg
ions, the amount of the precipitant for silica to be
supplied is adjusted according to the concentration of the
Mg ions.
30 In the above aspect, it is preferable that when the
water to be treated contains Mg ions, the amount of the
precipitant for silica to be supplied is adjusted according
10
to the concentration of the Mg ions in the precipitating
section.
In the above aspect, it is preferable that when the
water to be treated contains Mg ions, the concentrated water
5 in the precipitating step is adjusted to a pH at which a
magnesium compound is deposited so that the concentration of
the Mg ions is reduced, and after the precipitating step,
the concentrated water is adjusted to a pH at which the
magnesium compound is soluble.
10 In the above aspect, it is preferable that when the
water to be treated contains Mg ions, the concentrated water
in the precipitating section is adjusted to a pH at which a
magnesium compound is deposited so that the concentration of
the Mg ions is reduced, and, on the downstream side of the
15 precipitating section, the concentrated water is adjusted to
a pH at which the magnesium compound is soluble.
In the case where Mg ions are contained in the water to
be treated, Mg ions react with silica in the concentrated
water in the precipitating step and the precipitating
20 section, resulting in precipitation. In the present
invention, the amount of precipitant to be supplied is
adjusted according to the balance between Mg ions and silica
in the concentrated water, whereby the precipitant is
efficiently supplied. In the case where the concentration of
25 Mg ions in high relative to silica, the pH of the
concentrated water is adjusted so that a magnesium compound
is deposited in the precipitating step and the precipitating
section. Subsequently, the concentrated water is adjusted to
a pH at which the magnesium compound is soluble, thereby
30 suppressing the formation of scales in the demineralizing
section located on the downstream side of the precipitating
section.
11
In the above aspect, it is preferable that when the
water to be treated contains Mg ions, the water to be
treated in the upstream side precipitating step is adjusted
to a pH at which a magnesium compound is deposited so that
5 the concentration of the Mg ions is reduced, and, after the
upstream side precipitating step, the water to be treated is
adjusted to a pH at which the magnesium compound is soluble.
In the above aspect, it is preferable that when the
water to be treated contains Mg ions, the water to be
10 treated in the upstream side precipitating section is
adjusted to a pH at which a magnesium compound is deposited
so that the concentration of the Mg ions is reduced, and, on
the downstream side of the upstream side precipitating
section, the water to be treated is adjusted to a pH at
15 which the magnesium compound is soluble.
In this way, in the case where Mg ions are contained in
the water to be treated, by efficiently removing Mg ions
before a demineralization treatment, the formation of scales
containing magnesium in the course of water treatment can be
20 inhibited.
In the above aspect, it is preferable that moisture is
evaporated from the concentrated water in the downstream
side demineralizing step, so that a solid in the
concentrated water is recovered.
25 In the above aspect, it is preferable that moisture is
evaporated from the concentrated water in the downstream
side demineralizing step, so that a solid in the
concentrated water is recovered. It is preferable that the
water treatment system includes, on the downstream side of
30 the concentrated water in the downstream side demineralizing
section, an evaporator that evaporates moisture from the
concentrated water to recover a solid in the concentrated
12
water.
According to the water treatment process and the water
treatment system thus configured, when solid matters
produced in the course of water treatment are discharged out
5 of the system as waste, the volume of waste can be reduced.
Advantageous Effects of Invention
According to the water treatment system and the water
treatment process of the present invention, while inhibiting
10 the production of scales such as calcium carbonate and
silica during the treatment, Ca2+ and S04
2~ can be removed as
gypsum from the water to be treated. Accordingly, the water
recovery can be further improved.
Also in the case where magnesium ions are contained in
15 the water to be treated, when the water treatment system or
the water treatment process of the present invention is
used, they can be removed from the water to be treated while
inhibiting the production of scales containing magnesium
during the treatment.
20 Water treated by the present invention has a
significantly reduced number of moles of ions on the
downstream side. Therefore, the power of the demineralizing
section located downstream can be significantly reduced.
Further, the present invention is also advantageous in
25 that high-purity gypsum can be crystallized and recovered.
Brief Description of Drawings
Fig. 1 is a schematic diagram of a water treatment
system according to the first reference embodiment.
30 Fig. 2 shows simulation results for the pH dependency
of the amount of gypsum deposited.
Fig. 3 shows simulation results for the pH dependency
13
of the amount of calcium carbonate deposited.
Fig. 4 is a graph showing the pH dependency of the
amount of silica dissolved.
Fig. 5 shows the results of gypsum deposition
5 experiments performed using simulated water in which gypsum
is supersaturated with changing the pH of the simulated
water.
Fig. 6 shows the results of gypsum deposition
experiments performed using simulated water in which gypsum
10 is supersaturated with changing the concentration of seed
crystals.
Fig. 7 is a microphotograph of gypsum crystallized
under Condition 5.
Fig. 8 is a microphotograph of gypsum crystallized
15 under Condition 3.
Fig. 9 is a schematic diagram of a water treatment
system according to the second reference embodiment.
Fig. 10 is a schematic diagram of a water treatment
system according to the first embodiment.
20 Fig. 11 is a schematic diagram of a water treatment
system according to the third reference embodiment.
Fig. 12 is a schematic diagram of a water treatment
system according to the fourth reference embodiment.
Fig. 13 is a schematic diagram explaining a water
25 treatment system according to the fifth reference
embodiment.
Fig. 14 is a schematic diagram explaining a water
treatment system according to the sixth reference
embodiment.
30 Fig. 15 is a schematic diagram explaining a water
treatment system according to the seventh reference
embodiment.
14
Fig. 16 is a schematic diagram explaining a water
treatment system according to the eighth reference
embodiment.
5 Description of Embodiments
Water that is an object to be treated in the present
invention (water to be treated) contains Ca2+, S04
2~,
carbonate ions, and silica. Specifically, the water to be
treated (raw water) is saline water, sewage, industrial
10 waste water, blowdown water from a cooling tower, or the
like. The water to be treated may also contain metal ions,
such as Mg ions.
First Reference Embodiment
15 Fig. 1 is a schematic diagram of a water treatment
system according to the first reference embodiment of the
present invention. The water treatment system 1 of Fig. 1 is
configured such that two water treatment sections are
connected in the flow direction of the water to be treated.
20 In the water treatment system 1 of this reference
embodiment, depending on the properties of the water to be
treated, the number of water treatment sections may be one,
and it is also possible that three or more water treatment
sections are connected.
25 Each water treatment section includes, from the
upstream side of the water to be treated, a first
demineralizing section 10 (10a, 10b) and a first
crystallizing section 20 (20a, 20b) . The concentration sides
of the first demineralizing sections 10a and 10b are
30 connected to the first crystallizing sections 20a and 20b,
respectively. The water treatment section includes a first
scale inhibitor supplying section 30 (30a, 30b) and a first
15
pH adjusting section 40 (40a, 40b) in the flow path on the
upstream side of each first demineralizing section 10 (10a,
10b) .
The first scale inhibitor supplying section 30 (30a,
5 30b) is made up of a tank 31 (31a, 31b) , a valve VI (Via,
Vlb) , and a control section 32 (32a, 32b) . The control
sections 32a and 32b are connected to the valves Via and
Vlb, respectively. The tanks 31a and 31b have stored therein
a scale inhibitor.
10 The scale inhibitor used in this reference embodiment
serves to inhibit the deposition of scales containing
calcium in the water to be treated. It will be hereinafter
referred to as "calcium scale inhibitor".
The calcium scale inhibitor suppresses the crystal
15 nucleation of gypsum or calcium carbonate in the water to be
treated. At the same time, the calcium scale inhibitor
adheres to the surface of crystal nucleus of gypsum or
calcium carbonate contained in the water to be treated (seed
crystals, small-diameter scales deposited due to the
20 exceeding of the saturation concentration, etc.), and
functions to suppress the crystal growth of gypsum or
calcium carbonate. Alternatively, there is another type of
calcium scale inhibitor, which has the function of
dispersing particles in the water to be treated (inhibiting
25 aggregation), such as deposited crystals.
Examples of calcium scale inhibitors include
phosphonic-acid-based scale inhibitors, polycarboxylic-acidbased
scale inhibitors, and mixtures thereof. A specific
example is FLOCON260 (trade name, manufactured by BWA).
30 In the case where Mg ions are contained in the water to
be treated, a scale inhibitor that inhibits the deposition
of scales containing magnesium (e.g., magnesium hydroxide)
16
in the water to be treated can be used. It will be
hereinafter referred to as "magnesium scale inhibitor".
Examples of magnesium scale inhibitors include
polycarboxylic-acid-based scale inhibitors, etc. A specific
5 example is FLOCON 295N (trade name, manufactured by BWA).
Although Fig. 1 shows only one first scale inhibitor
supplying section 30a/30b in each position, in the case
where two or more kinds of scale inhibitors are loaded, it
is preferable that two or more first scale inhibitor
10 supplying sections are installed. In this case, the scale
inhibitors are sorted according to kind and stored in the
respective tanks.
The first pH adjusting section 40 (40a, 40b) is made up
of a tank 41 (41a, 41b), a valve V2 (V2a, V2b) , a control
15 section 42 (42a, 42b), and a pH meter 43 (43a, 43b). The
tanks 41a and 41b have stored therein an alkali as a pH
adjuster. The alkali is calcium hydroxide or sodium
hydroxide, for example. Calcium hydroxide is particularly
preferable because Ca ions are recovered as gypsum in the
20 below-mentioned crystallizing step, and thus the amount of
ions that reach the demineralizing section on the downstream
side is reduced. The control sections 42a and 42b are
connected to the valves V2a and V2b and the pH meters 43a
and 43b, respectively.
25 In Fig. 1, the first demineralizing sections 10a and
10b are reverse osmosis membrane devices. In addition, the
first demineralizing sections 10a and 10b may also be
electrodialyzers (ED), electro dialysis reversal devices
(EDR), electro de-ionization devices (EDI), ion-exchange
30 equipments (IEx), capacitive de-ionization devices (CDI),
nanofilters (NF), evaporators, etc.
Here, in a nanofilter (NF), an electrodialyzer (ED), an
17
electro dialysis reversal device (EDR), an electro deionization
device (EDI), and a capacitive de-ionization
device (CDI), scale components (divalent ions, Ca2+, Mg2+,
etc.) are selectively removed, while monovalent ions such as
5 Na+ and Cl~ permeate. The use of these demineralizers
suppresses an increase in the ion concentration of ions that
serve as scale components in concentrated water.
Accordingly, the water recovery can be improved, and also
energy saving (e.g., the reduction of pump power, etc.) can
10 be achieved.
In addition, in the case where the water to be treated
is blowdown water from a cooling tower, the reclaimed water
does not have to be pure water, and what is necessary is
that scale components (divalent ions, Ca2+, Mg2+, etc.) are
15 removed. Accordingly, it is advantageous to use a nanofilter
(NF) , etc.
Although only one first demineralizing section lOa/lOb
is shown in Fig. 1, the system may also be configured such
that two or more demineralizers are connected in parallel or
20 in series in the flow direction of the water to be treated.
The first crystallizing section 20 (20a, 20b) is made
up of a first crystallizing tank 21 (21a, 21b) and a first
seed crystal supplying section 22 (22a, 22b). The first seed
crystal supplying sections 22a and 22b are connected to the
25 first crystallizing tanks 21a and 21b, respectively. The
first seed crystal supplying sections 22a and 22b have a
seed crystal tank 23 (23a, 23b) , a valve V3 (V3a, V3b) , and
a control section 24 (24a, 24b) . The control sections 24a
and 24b are connected to the valves V3a and V3b,
30 respectively. The seed crystal tanks 23a and 23b store
gypsum particles as seed crystals.
In the water treatment system 1 of Fig. 1, a first
18
precipitating section 50 (50a, 50b) may be installed on the
downstream side of each of the first crystallizing sections
20a and 20b. The first precipitating sections 50a and 50b
each include a first precipitating tank 51 (51a, 51b) and a
5 first filtration device 52 (52a, 52b).
The water treatment system 1 includes a downstream side
demineralizing section 60 on the downstream side of the
water to be treated of the first crystallizing section 20b
located on the most downstream.
10 In Fig. 1, the downstream side demineralizing section
60 is a reverse osmosis membrane device. The downstream side
demineralizing section 60 may also be an electrodialyzer
(ED) , an electro dialysis reversal device (EDR), an electro
de-ionization device (EDI), an ion-exchange equipment, a
15 capacitive de-ionization device (CDI), a nanofilter (NF), an
evaporator, etc.
In the water treatment system 1, a precipitating tank
71 and a filtration device 72 are installed as a first
upstream side precipitating section 70 on the upstream side
20 of the first scale inhibitor supplying section 30a and the
first pH adjusting section 40a which are located on the most
upstream of the water to be treated. The precipitating tank
71 and the filtration device 72 have the same configuration
as the first precipitating tank 51 and the first filtration
25 device 52 of the first precipitating section 50.
In particular, in the case where Mg ions are contained
in the water to be treated, the first upstream side
precipitating section can be configured such that two or
more precipitating tanks 71 are connected in series in the
30 flow direction of the water to be treated.
In the water treatment system 1 shown in Fig. 1, a
first deaerating section 73 may be provided on the upstream
19
side of the first upstream side precipitating section 70.
Specifically, the first deaerating section 73 is a
deaeration tower equipped with a filler for removing carbon
dioxide or is a separation membrane. On the upstream side of
5 the water to be treated of the first deaerating section 73,
a pH adjusting section for carbonate ions (not shown) that
adjusts the water to be treated to a pH at which carbonate
ions are present in the form of CO2 may be installed.
The first deaerating section 73 may also be installed
10 on the downstream side of the water to be treated of the
first upstream side precipitating section 70 and on the
upstream side of the first scale inhibitor supplying section
30a and the first pH adjusting section 40a.
It is also possible that a deaerating section having
15 the same configuration as the first deaerating section 73 is
installed in the flow path between the first demineralizing
section 10 and the first crystallizing section 20, in the
flow path between the first crystallizing section 20 and the
first precipitating section 50, and on the downstream side
20 of the first precipitating section 50 and in the flow path
between it and the first demineralizing section 10b or the
downstream side demineralizing section 60.
In the case where the concentration of Ca ions in the
water to be treated is high, an ion-exchange equipment (not
25 shown) may be installed on the downstream of the filtration
device 72 and on the upstream of the first scale inhibitor
supplying section 30a and the first pH adjusting section 40a
which are located on the most upstream. The ion-exchange
equipment may be an ion-exchange resin column or an ion-
30 exchange membrane device, for example.
When gypsum in the water to be treated flowing into the
first demineralizing section 10a is already supersaturated,
20
because ions are further concentrated in the first
demineralizing section 10a, the resulting gypsum
concentration is even higher. In this case, the loading of a
large amount of calcium scale inhibitor is required.
5 Further, the concentration of gypsum may become too high for
the calcium scale inhibitor to exert its effect, resulting
in the production of scales in the first demineralizing
section 10a.
Thus, in the case where gypsum in the raw water (water
10 to be treated) is supersaturated, it is possible that an
upstream side crystallizing section (not shown) having the
same configuration as the first crystallizing tanks 21a and
21b are provided on the upstream of the first scale
inhibitor supplying section 30a and the first pH adjusting
15 section 40a on the most upstream, so that the concentration
of gypsum is reduced, and then the water to be treated is
fed to the first demineralizing section 10a.
A process for treating water to be treated using the
water treatment system 1 of the first reference embodiment
20 will be described hereinafter.
First, the deposition behaviors of gypsum, silica, and
calcium carbonate in water will be explained. Fig. 2 shows
simulation results for the pH dependency of the amount of
gypsum deposited. Fig. 3 shows simulation results for the pH
25 dependency of the amount of calcium carbonate deposited. In
the figures, the abscissa is pH, and the ordinate is the
amount of gypsum or calcium carbonate deposited (mol). Using
a simulation software manufactured by OLI, the simulation
was performed under the conditions where 0.1 mol/L of each
30 solid component was mixed with water, and H2S04 and Ca(OH)2
were added as an acid and an alkali, respectively.
Fig. 4 is a graph showing the pH dependency of the
21
amount of silica dissolved (source: Fig. 4 of U.S. Pat. No.
7815804). In the figure, the abscissa is pH, and the
ordinate is the amount of silica dissolved (mg/L).
From Fig. 2, it can be understood that gypsum
5 deposition has no pH dependency, and deposition is possible
over the entire pH range. However, when a calcium scale
inhibitor is added, in a high-pH region, gypsum is present
in the state of being dissolved in water. From Fig. 3,
calcium carbonate is deposited when the pH is more than 5.
10 From Fig. 4, silica tends to dissolve in water when the pH
is 10 or more.

In the case where the water to be treated is industrial
waste water, etc., before the water to be treated flows into
15 the first upstream side precipitating section 70, a step of
removing oils, floating particles, and the like from the
water to be treated and a step of removing organic
substances by a biological treatment or a chemical oxidation
treatment are performed.
20
In the water treatment system 1 of Fig. 1, the water to
be treated before flowing into the first deaerating section
73 is adjusted to a low pH. Carbonic acid in the water to be
treated is in the following equilibrium depending on the pH
25 of the water to be treated.
Chemical Formula 1
C02ZH2C03ZHCOi+H+ZCO^ + 2H+ - • -(1)
In the case where the pH is as low as 6.5 or less, it
30 is mainly present as HC03~ and C02 in the water to be
22
treated.
The water to be treated containing C02 flows into the
first deaerating section 73. C02 is removed from the water
to be treated in the first deaerating section 73. When the
5 water to be treated has been previously adjusted to a pH at
which carbonate ions are present as C02, carbon dioxide can
be efficiently removed.
The water to be treated, whose carbonate ion
concentration has been reduced in the first deaerating step,
10 is fed to the first upstream side precipitating section 70.

In the first upstream side precipitating section 70,
some of Ca ions and carbonate ions are previously removed
from the water to be treated as calcium carbonate.
15 In the case where metal ions other than Ca ions are
contained in the water to be treated, in the first upstream
side precipitating section 70, some of the metal ions are
previously removed from the water to be treated as a metal
compound having low solubility in water. This metal compound
20 is mainly a metal hydroxide, but may also include a
carbonate.
In the precipitating tank 71, Ca(OH)2 and an anionic
polymer (manufactured by Mitsubishi Heavy Industries
Mechatronics Systems, Ltd., trade name: Hishifloc H305) are
25 loaded to the water to be treated, and the pH in the
precipitating tank 71 is controlled to 4 or more and 12 or
less, and preferably 8.5 or more and 12 or less.
As shown in Fig. 3, the solubility of calcium carbonate
is low in this pH range. When calcium carbonate is
30 supersaturated, calcium carbonate is deposited and
precipitated at the bottom of the precipitating tank 71.
The solubility of a metal compound depends on pH. A
23
more acidic pH leads to a higher solubility of metal ions in
water. For many metal compounds, the solubility is low in
the above pH range. In the above pH range, a metal compound
having low solubility in water aggregates in the
5 precipitating tank 71, resulting in precipitation at the
bottom of the precipitating tank 71.
The precipitated calcium carbonate and metal compound
are discharged from the bottom of the precipitating tank 71.
Mg ions form salts that are poorly soluble in water,
10 and thus are components that tend to be deposited as scales.
Mg(OH)2 is deposited at pH 10 or more.
In the case where the water to be treated containing Mg
ions is treated by the water treatment system 1 of this
embodiment, the pH of the water to be treated in the
15 precipitating tank 71 is adjusted to a pH at which a
magnesium compound (mainly magnesium hydroxide) is
deposited. Specifically, the pH of the water to be treated
is adjusted to 10 or more, preferably 10.5 or more, and more
preferably 11 or more. Accordingly, a magnesium compound is
20 deposited from the water to be treated, precipitated at the
bottom of the precipitating tank 71, and removed. As a
result, some of Mg ions in the water to be treated are
removed, resulting in a decrease in the concentration of Mg
ions in the water to be treated.
25 In the above case, it is preferable that the water to
be treated after being discharged from the first upstream
side precipitating section 70 is adjusted to a pH at which
the above magnesium compound is soluble. Specifically, the
pH is adjusted to less than 10. Accordingly, the formation
30 of scales in devices and steps on the downstream side,
particularly the first demineralizing section 10a and the
first demineralizing step, can be inhibited.
24
In the case where two or more stages of precipitating
tanks 71 are provided, Mg ions in the water to be treated
can be reliably removed, and the concentration of Mg ions in
the water to be treated fed to the downstream side can be
5 reduced.
The supernatant in the precipitating tank 71, which is
the water to be treated, is discharged from the
precipitating tank 71. FeCl3 is added to the discharged
water to be treated, and solids in the supernatant, such as
10 calcium carbonate and a metal compound, aggregate with
Fe(OH)3.
The water to be treated is fed to the filtration device
72. The solids aggregated with Fe(OH)3 are removed through
the filtration device 72.
15 In the case where the first deaerating step is
performed after the first upstream side precipitating step,
the pH of the water to be treated is adjusted to a pH at
which carbonate ions can be present as C02, specifically 6.5
or less.
20 Incidentally, depending on the properties of the water
to be treated, the first deaerating step and the first
upstream side precipitating step may be omitted.
In the case where an ion-exchange equipment is
installed, Ca ions in the water to be treated are removed by
25 the ion-exchange equipment. In the case where Mg ions are
contained in the water to be treated, the Mg ions are also
removed by the ion-exchange equipment.
In the case where gypsum in the raw water is
supersaturated, seed crystals of gypsum are loaded to the
30 water to be treated in the upstream side crystallizing
section installed immediately after the filtration device
72, and gypsum is crystallized, thereby reducing the
25
concentration of gypsum in the water to be treated. The
water to be treated having a reduced concentration of gypsum
is fed to the first demineralizing section 10a.

5 The control section 32a of the first scale inhibitor
supplying section 30a opens the valve Via and supplies a
predetermined amount of calcium scale inhibitor to the water
to be treated from the tank 31a. The control section 32a
adjusts the opening of the valve Via so that the
10 concentration of the calcium scale inhibitor is a
predetermined value set according to the properties of the
water to be treated.
In the case where Mg ions are contained in the water to
be treated, a magnesium scale inhibitor is supplied to the
15 water to be treated in the first scale inhibitor supplying
step in the same manner as above. In this case, the calcium
scale inhibitor and the magnesium scale inhibitor are stored
in the tank of each of two or more first scale inhibitor
supplying sections, and each control section adjusts the
20 amounts of calcium scale inhibitor and magnesium scale
inhibitor to be supplied.

The control section 42a of the first pH adjusting
section 40a controls the pH of the water to be treated at
25 the entrance of the first demineralizing section 10a to such
a value that silica is soluble in the water to be treated.
Specifically, the pH of the water to be treated fed to the
first demineralizing section 10a is adjusted to 10 or more,
preferably 10.5 or more, and more preferably 11 or more.
30 The pH meter 43a measures the pH of the water to be
treated at the entrance of the first demineralizing section
10a. The control section 42a adjusts the opening of the
26
valve V2a so that the value measured by the pH meter 43a is
a predetermined pH control value, and allows an alkali to be
loaded to the water to be treated from the tank 41a.

5 In the first demineralizing section 10a, the pHadjusted
water to be treated is treated. In the case where
the first demineralizing section 10a is a reverse osmosis
membrane device, the water that has passed through the
reverse osmotic membrane is recovered as treated water. Ions
10 and scale inhibitors contained in the water to be treated
cannot pass through the reverse osmosis membrane. Therefore,
on the non-permeate side of the reverse osmosis membrane,
there is concentrated water having a high concentration of
ions. Also in the case where other demineralizers, such as a
15 capacitive de-ionization device, are used, for example, the
water to be treated is separated into treated water and
concentrated water having a high concentration of ions
(first concentrated water).
As shown in Fig. 4, as a result of the first
20 demineralizing step, silica is contained in the first
concentrated water in the state of being dissolved in the
water to be treated. Even in the case where gypsum and
calcium carbonate in the first concentrated water are
concentrated to the saturation concentration or higher, the
25 production of scales is suppressed by the calcium scale
inhibitor.
In the case where Mg ions are contained in the water to
be treated, the concentration of Mg ions contained in the
first concentrated water increases as a result of the first
30 demineralizing step. However, the production of scales
containing magnesium is suppressed by the magnesium scale
inhibitor.
27
The first concentrated water is fed toward the first
crystallizing section 20a.

The first concentrated water discharged from the first
5 demineralizing section 10a is stored in the first
crystallizing tank 21a of the first crystallizing section
20a. The control section 24a of the first seed crystal
supplying section 22a opens the valve V3a and adds seed
crystals of gypsum to the first concentrated water in the
10 first crystallizing tank 21a from the tank 23a.
The pH of the first concentrated water from the first
demineralizing section 10a is 10 or more. As mentioned
above, gypsum is in the state of being dissolved in water in
a high-pH region where a calcium scale inhibitor is present.
15 However, when seed crystals are sufficiently present, even
when a scale inhibitor is present, gypsum is crystallized
using the seed crystals as nuclei. In the water treatment
system 1 of Fig. 1, the crystal-grown gypsum having a large
diameter (e.g., having a particle diameter of 10 um or more,
20 more preferably 20 urn or more) is precipitated at the bottom
of the first crystallizing tank 21a. The precipitated gypsum
is discharged from the bottom of the first crystallizing
tank 21a.
Meanwhile, when the pH 10 is or more, silica is present
25 in the state of being dissolved in the first concentrated
water in the first crystallizing tank 21a. Even in the case
where the concentration of silica in the first concentrated
water exceeds the saturation solubility, because seed
crystals of silica are not present, silica is deposited as
30 floating matters in a colloidal form or the like and
unlikely to be precipitated.
With reference to Fig. 3, calcium carbonate tends to be
28
deposited at pH 10 or more. However, because the calcium
scale inhibitor has been added, the deposition of calcium
carbonate is suppressed in the first crystallizing tank 21a.
In addition, in the case where the first upstream side
5 precipitating section or the first deaerating section is
provided, the concentration of calcium carbonate has been
previously reduced. As a result, in the first crystallizing
tank 21a, calcium carbonate is unlikely to be crystallized
using the seed crystals of gypsum as nuclei.
10 Incidentally, although gypsum is crystallized
independent of pH when seed crystals of gypsum are present,
the crystallization rate increases with a decrease in pH.
Fig. 5 shows the result of gypsum deposition
experiments with changing the pH of simulated water in the
15 case where a scale inhibitor (FLOCON260) is added to
simulated water (containing Ca2+, S04
2~, Na+, and CI-) in which
gypsum is supersaturated. The experimental conditions are as
follows:
The degree of gypsum supersaturation in simulated water
20 (25°C): 460%,
The amount of scale inhibitor to be added: 2.1 mg/L,
pH: 6.5 (Condition 1), 5.5 (Condition 2), 4.0
(Condition 3), 3.0 (Condition 4),
The amount of seed crystals to be added: 0 g/L.
25 Two hours and 6 hours immediately after the pH
adjustment, the concentration of Ca in the simulated water
treated under each condition was measured using an atomic
absorption spectrometer (manufactured by Shimadzu
Corporation, AA-7000), and the degree of supersaturation was
30 calculated. The results are shown in Fig. 5. In the figure,
the ordinate is the degree of supersaturation (%).
With reference to Fig. 5, even under conditions where
29
seed crystals are absent, the crystallization rate increases
with a decrease in pH. From this, it can be understood that
in the case where seed crystals are present, gypsum is
crystallized even under Condition 1 (pH 6.5), and the
5 relation of the crystallization rate is such that the
crystallization rate increases with a decrease in pH as
shown in Fig. 5.
In the case where carbonate ions are contained in the
water to be treated, under low-pH conditions, carbonate ions
10 are removed from the water to be treated as CO2 as in
chemical formula (1). In addition, as can be understood from
Fig. 3, in the case where the pH is low, calcium carbonate
is in a dissolved state.
From these results, when the first crystallizing step
15 is performed under low-pH conditions, because of the low
content of calcium carbonate and silica, high-purity gypsum
is crystallized and recovered from the bottom of the first
crystallizing tank 21a. In the case where the first
crystallizing step is performed at low pH, a third pH
20 adjusting section (a pH adjusting section for achieving the
calcium scale inhibition function, not shown) that supplies
an acid as a pH adjuster is installed in the first
crystallizing tank 21a or in the flow path between the first
demineralizing section 10a and the first crystallizing tank
25 21a. The pH adjusting section has the same configuration as
the below-mentioned second pH adjusting section.
Meanwhile, in order to change the pH in the course of
water treatment, it is necessary to supply a large amount of
chemicals (acid or alkali). The use of an acid or an alkali
30 leads to an increase in the amount of ions transferred to
the downstream side of the first crystallizing section 20a,
and this causes an increase in the power of demineralizing
30
sections on the downstream side (in Fig. 1, the first
demineralizing section 10b or the downstream side
demineralizing section 60). In terms of operation cost, it
is more advantageous that the pH is not changed between the
5 first demineralizing step and the first crystallizing step.
The gypsum crystallization rate depends on the loading
of seed crystals. Fig. 6 shows the results of gypsum
deposition experiments with changing the amount of seed
crystals to be added in the case where a calcium scale
10 inhibitor (FLOCON260) is added to simulated water. The
experimental conditions were the same as in Fig. 5 except
that the pH was 4.0, and that gypsum (CaS04-2H20) was added
as seed crystals in the following amounts:
The amount of seed crystals to be added: 0 g/L
15 (Condition 3) , 3 g/L (Condition 5) , 6 g/L (Condition 6) , 3
g/L (Condition 7).
Under Conditions 5 and 6, seed crystals and sulfuric
acid for pH adjustment were added to the simulated water
having added thereto a scale inhibitor. Under Condition 1,
20 seed crystals pre-immersed in the above scale inhibitor were
added to the simulated water having added thereto a scale
inhibitor, and sulfuric acid was added for pH adjustment.
Two hours immediately after the pH adjustment, the
concentration of Ca in the simulated water treated under
25 each condition was measured by the same technique as in Fig.
5. In Fig. 6, the ordinate is the degree of supersaturation
(%) .
From the results of Fig. 6, it can be understood that
although the degree of supersaturation was 215% under
30 Condition 3 where seed crystals are not added, the degree of
supersaturation decreases to 199% (Condition 5) and 176%
(Condition 6) with an increase in the concentration of seed
31
crystals, leading to an increase in the gypsum deposition
rate. Also under high-pH conditions, similarly, the gypsum
deposition rate tends to increase with an increase in the
loading of seed crystals. Condition 5 and Condition 7 are
5 the same test conditions, except for whether the used seed
crystals are not immersed or immersed in a scale inhibitor.
Also under Condition 7 where seed crystals have a scale
inhibitor previously adhering thereto, the degree of
supersaturation is 199%, and it has been confirmed that
10 gypsum is deposited at the same level as under Condition 5.
That is, the results under Condition 5 and 7 show that
independent of the immersion time of seed crystals in a
calcium scale inhibitor, when the pH is reduced to 4.0, the
function of the scale inhibitor is reduced.
15 Figs. 7 and 8 each show a microphotograph of gypsum
resulting from crystallization. Fig. 7 shows results under
Condition 5 (seed crystals added), and Fig. 8 shows results
under Condition 3 (no seed crystals added). Under Condition
5, gypsum having a larger size was deposited than under
20 Condition 3. Generally, the water content decreases with an
increase in the size of deposited gypsum. A low water
content leads to high-purity gypsum. When the average
particle diameter is 10 um or more, preferably 20 urn or
more, the resulting gypsum has a sufficiently reduced water
25 content. The "average particle diameter" in the present
invention is a particle diameter measured by the method
specified in JIS Z 8825 (laser diffractometry).

The supernatant (first concentrated water) in the first
30 crystallizing section 20a is fed to the first precipitating
section 50a. In the first precipitating section 50a, Ca(OH)2
and an anionic polymer (Hishifloc H305) are loaded to the
32
first concentrated water after the crystallizing step, and
the pH in the first precipitating tank 51a is controlled to
4 or more and 12 or less, and preferably 8.5 or more and 12
or less. In the first precipitating tank 51a, calcium
5 carbonate and a metal compound are precipitated and removed
from the first concentrated water. The precipitated calcium
carbonate and metal compound having low solubility in water
are discharged from the bottom of the first precipitating
tank 51a.
10 The water to be treated, which is the supernatant in
the first precipitating tank 51a, is discharged from the
first precipitating tank 51a. FeCl3 is added to the
discharged water to be treated, and solids in the water to
be treated, such as calcium carbonate and a metal compound,
15 aggregate with Fe(OH)3.
The water to be treated is fed to the first filtration
device 52a. The solids aggregated with Fe(OH)3 are removed
through the first filtration device 52a.
Silica in the supernatant in the first crystallizing
20 section 20a may be removed from the first concentrated water
in the first precipitating step, or may also be fed to the
downstream side without being removed.
Whether silica is removed in the first precipitating
step is determined according to the properties of the water
25 to be treated or the first concentrated water.
In the case where silica is not removed, the first
precipitating step is performed without supplying seed
crystals of silica and a precipitant for silica to the first
precipitating tank 51a. In this case, silica is separated
30 from the treated water in demineralizing sections located on
the downstream side (the first demineralizing section 10b
and the downstream side demineralizing section 60).
33
In the case where silica is removed, at least one of
seed crystals of silica and a precipitant for silica is
supplied into the first concentrated water in the first
precipitating section 50a from a supply section (not shown).
5 The seed crystals of silica are a silica gel, for example,
and the precipitant for silica is MgS04 or Na aluminate
(Na[Al(OH)4] ) , for example. In the case where silica is
removed, it is preferable that the first concentrated water
in the first precipitating tank 51a is adjusted to pH 8 or
10 more and 10 or less. In the case where seed crystals of
silica are used, silica is crystallized using the seed
crystals as nuclei. In the case where MgS04 is used as a
precipitant for silica, magnesium silicate is deposited. The
crystallized silica or the crystallized magnesium silicate
15 is precipitated at the bottom of the first precipitating
tank 51a and discharged from the bottom of the first
precipitating tank 51a.
In the case where Mg ions are contained in the water to
be treated, Mg ions react with silica in the first
20 concentrated water in the first precipitating step,
resulting in precipitation. The steps for silica/Mg ion
removal vary depending on the balance between the content of
Mg ions and the content of silica in the first concentrated
water in the first precipitating tank 51a.
25 In the case where the first concentrated water in the
first precipitating step has a lower concentration of Mg
ions relative to the silica content, Mg ions are consumed by
precipitation with silica. In order to remove an excess of
silica that is not consumed by precipitation with Mg ions, a
30 precipitant for silica (MgS04) is supplied. With respect to
the amount of precipitant for silica to be supplied,
according to the content of silica and the content of Mg
34
ions in the first precipitating step, the precipitant is
supplied in such an amount that the excess of silica is
consumed.
In the case where the first concentrated water in the
5 first precipitating step has a higher concentration of Mg
ions relative to the silica content, Mg ions remain as a
result of the precipitation of Mg ions and silica. When the
first concentrated water having a high concentration of
residual Mg ions is discharged from the first precipitating
10 tank 51a, scales containing Mg may be deposited in
demineralizing sections of subsequent stages (the first
demineralizing section 10b in Fig. 1; in the case of the
first precipitating section on the most downstream, the
downstream side demineralizing section 60).
15 Thus, the first concentrated water in the first
precipitating tank 51a is adjusted to such a value that a
magnesium compound (mainly magnesium hydroxide) can be
deposited. Accordingly, a magnesium compound is precipitated
in the first precipitating tank 51a, thereby reducing the
20 concentration of Mg ions in the first concentrated water in
the first precipitating tank 51a. Further, after the first
precipitating step, the first concentrated water discharged
from the first precipitating tank 51a is adjusted to a pH at
which the magnesium compound is soluble, specifically to a
25 pH of less than 10. Accordingly, the deposition of scales
containing Mg in a demineralizing section can be suppressed.
In the case where the treatment is performed in several
stages, the first concentrated water that has passed through
the first filtration device 52a of the first water treatment
30 section of the previous stage flows into the water treatment
section of the subsequent stage as water to be treated. In
the water treatment section of the subsequent stage, the
35
steps from the first scale inhibitor supplying step to the
first precipitating step mentioned above are performed.

The concentrated water (first concentrated water) that
5 has passed through the first precipitating section 50b
located on the most downstream of the water to be treated is
fed to the downstream side demineralizing section 60. The
water that has passed through the downstream side
demineralizing section 60 is recovered as treated water. The
10 concentrated water in the downstream side demineralizing
section 60 is discharged out of the system. The installation
of the downstream side demineralizing section 60 makes it
possible to further recover treated water from water that
has been treated in a water treatment section. Accordingly,
15 the water recovery is improved.
In the water treatment system 1 of this reference
embodiment, ions are concentrated in the first
demineralizing section 10. However, gypsum, calcium
carbonate, silica, etc., have been removed in the first
20 crystallizing section, the first precipitating section, etc.
Accordingly, the water flowing into the downstream side
demineralizing section 60 has a smaller number of moles of
ions than before the treatment. Accordingly, the osmotic
pressure is low in the first demineralizing section 10b or
25 the downstream side demineralizing section 60 located
downstream, and the required power is reduced.
An evaporator (not shown in Fig. 1) may be installed on
the downstream on the concentrated-water side of the
downstream side demineralizing section 60. In the
30 evaporator, water is evaporated from the concentrated water,
and ions contained in the concentrated water are deposited
as a solid and recovered as a solid. Because water is
36
recovered on the upstream side of the evaporator, and the
amount of concentrated water significantly decreases, the
evaporator can be reduced in size, and the energy required
for evaporation can be reduced.
5
Second Reference Embodiment
Fig. 9 is a schematic diagram of a water treatment
system of the second reference embodiment of the present
invention. In Fig. 9, the same configurations as in the
10 first reference embodiment are indicated with the same
reference numerals. In the water treatment system 100 of the
second reference embodiment, a first separating section 180
(180a, 180b) is installed on the downstream side of the
first crystallizing sections 20a and 20b. The water
15 treatment system 100 of Fig. 9 is configured such that two
water treatment sections are connected in the flow direction
of the water to be treated. In the water treatment system
100 of this reference embodiment, depending on the
properties of the water to be treated, the number of water
20 treatment sections may be one, and it is also possible that
three or more water treatment sections are connected.
In Fig. 9, the first separating section 180 (180a,
180b) includes a classifier 181 (181a, 181b) and a
dehydrator 182 (182a, 182b). The classifiers 181a and 181b
25 are liquid cyclones, for example. The dehydrators 182a and
182b are belt filters, for example.
Although the first separating section 180 has only one
classifier installed in Fig. 9, it is also possible that two
or more classifiers are connected in series in the flow
30 direction of the water to be treated.
In the water treatment system 100 of the second
reference embodiment, the water to be treated is treated
37
through the same steps as in the first reference embodiment,
except that the first separating step is performed
immediately after the first crystallizing step.

5 First concentrated water in the first crystallizing
tanks 21a and 21b is transferred to the first separating
sections 180a and 180b. The first concentrated water
transferred here is water containing solid matters deposited
in the first crystallizing tanks 21a and 21b.
10 The first concentrated water discharged from the first
crystallizing tanks 21a and 21b contains gypsum having
various particle diameters, as well as calcium carbonate and
silica deposited due to the exceeding of the saturation
concentration. Because the deposition of calcium carbonate
15 and silica has taken place in the absence of seed crystals,
they have small diameters or are floating matters in a
colloidal form.
When the first concentrated water flows into the
classifiers 181a and 181b, gypsum having a predetermined
20 size, for example, gypsum having an average particle
diameter of 10 jam or more, sediments at the bottom of the
classifiers 181a and 181b, and gypsum having a small
particle diameter, calcium carbonate, and silica remain in
the supernatant. The gypsum sedimented at the bottom of the
25 classifiers 181a and 181b is further dehydrated by the
dehydrators 182a and 182b and recovered. The supernatant
containing gypsum having a small particle diameter, calcium
carbonate, and silica is fed to the first precipitating
sections 50a and 50b.
30 In this reference embodiment, seed crystals are added
to cause crystallization. Therefore, gypsum having an
average particle diameter of 10 um or more is mainly
38
deposited, and the proportion of gypsum having a small
diameter is low. Through the first separating step, gypsum
having a low water content and containing no impurities
(i.e., high-purity gypsum) can be separated and recovered
5 with high recovery.
Some of the gypsum recovered in the first separating
sections 180a and 180b may be circulated through the seed
crystal tanks 23a and 23b as seed crystals.
10 First Embodiment
Fig. 10 is a schematic diagram of a water treatment
system of the first embodiment of the present invention. The
water treatment system 200 of Fig. 10 is configured such
that two water treatment sections are connected in the flow
15 direction of the water to be treated. Depending on the
properties of the water to be treated, the number of water
treatment sections may be one, and it is also possible that
three or more water treatment sections are connected.
In the water treatment system 200 of the first
20 embodiment, each water treatment section includes, from the
upstream side of the water to be treated, a second
demineralizing section 210 (210a, 210b) and a second
crystallizing section 220 (220a, 220b). The concentration
sides of the second demineralizing sections 210a and 210b
25 are connected to the second crystallizing sections 220a and
220b, respectively. The water treatment section includes a
second scale inhibitor supplying section 230 (230a, 230b) in
the flow path on the upstream side of each second
demineralizing section 210 (210a, 210b).
30 The second scale inhibitor supplying sections 230a and
230b are each made up of a tank 231 (231a, 231b), a valve V4
(V4a, V4b), and a control section 232 (232a, 232b). The
39
control sections 232a and 232b are connected to the valves
V4a and V4b, respectively. The tanks 231a and 231b of the
second scale inhibitor supplying sections 230a and 230b have
stored therein a scale inhibitor.
5 Scale inhibitors used in the first embodiment are the
calcium scale inhibitor described in the first reference
embodiment and a scale inhibitor that inhibits the
deposition of silica as scales in the water to be treated
(referred to as "silica scale inhibitor"). Examples of
10 silica scale inhibitors include phosphonic-acid-based scale
inhibitors, polycarboxylic-acid-based scale inhibitors, and
mixtures thereof. A specific example is FLOCON260 (trade
name, manufactured by BWA).
Fig. 10 shows two tanks 231a. For example, a calcium
15 scale inhibitor is stored in one tank 231a, and a silica
scale inhibitor is stored in the other tank 231a.
In Fig. 10, the second demineralizing section 210 is a
reverse osmosis membrane device. In addition, the second
demineralizing section 210 may also be an electrodialyzer
20 (ED), an electro dialysis reversal device (EDR), an electro
de-ionization device (EDI), an ion-exchange equipment, a
capacitive de-ionization device (CDI), a nanofilters (NF) ,
an evaporator, etc.
Although only one second demineralizing section 210 is
25 shown in. Fig. 10, the system may also be configured such
that two or more demineralizers are connected in parallel or
in series in the flow direction of the water to be treated.
The second crystallizing section 220 (220a, 220b) is
made up of a second crystallizing tank 221 (221a, 221b) and
30 a second seed crystal supplying section 222 (222a, 222b).
The second seed crystal supplying section 222 is connected
to the second crystallizing tank 221. The second seed
40
crystal supplying section 222 has a seed crystal tank 223
(223a, 223b), a valve V5 (V5a, V5b), and a control section
224 (224a, 224b). The control section 224 is connected to
the valve V5. The seed crystal tank 223 stores gypsum
5 particles as seed crystals.
In the water treatment system 200 of the first
embodiment, a second pH adjusting section 240 (240a, 240b)
may be installed between the second demineralizing section
210 and the second crystallizing section 220. The second pH
10 adjusting section 240 is made up of a tank 241 (241a, 241b),
a valve V6 (V6a, V6b) , a pH meter 243 (243a, 243b), and a
control section 242 (242a, 242b) . The tank 241 has stored
therein an acid as a pH adjuster. The acid used may be
hydrochloric acid, sulfuric acid, nitric acid, or the like,
15 for example. Sulfuric acid is particularly preferable
because S04
2~ is removed as gypsum in the crystallizing step,
and thus the amount of ions that reach the demineralizing
section on the downstream side can be reduced. The control
section 242 is connected to the valve V6 and the pH meter
20 243. The pH meter 243 may be installed in the flow path
between the second demineralizing section 210 and the second
crystallizing section 220 as shown in Fig. 10, or may also
be installed in the second crystallizing tank 221.
In the water treatment system 200, a precipitating tank
25 271 and a filtration device 272 are installed as a second
upstream side precipitating section 270 on the upstream side
of the second scale inhibitor supplying section 230a located
on the most upstream of the water to be treated. The second
upstream side precipitating section 270 has the same
30 configuration as the first upstream side precipitating
section 70. As in the first reference embodiment, two or
more stages of precipitating tanks 271 may be connected in
41
series in the flow direction of the water to be treated.
In the water treatment system 200, a second deaerating
section 273 may be provided on the upstream side of the
second upstream side precipitating section 270 as shown in
5 Fig. 10. The second deaerating section 273 has the same
configuration as the first deaerating section 73 of the
first reference embodiment.
The second deaerating section 273 may be installed on
the downstream side of the water to be treated of the second
10 upstream side precipitating section 270 and on the upstream
side of the second scale inhibitor supplying section 230a.
It is also possible that a deaerating section having
the same configuration as the second deaerating section 273
is installed in the flow path between the second
15 demineralizing section 210a and the second crystallizing
section 220a, in the flow path between the second
crystallizing section 220 and the second precipitating
section 250, and on the downstream side of the second
precipitating section 250 and in the flow path between it
20 and the second demineralizing section 210b or the downstream
side demineralizing section 60.
As in the first reference embodiment, an ion-exchange
equipment (not shown) may be installed on the downstream of
the filtration device 272 and on the upstream of the second
25 scale inhibitor supplying section 230a located on the most
upstream. In addition, depending on the concentration of
gypsum in the water to be treated, an upstream side
crystallizing section (not shown) having the same
configuration as the second crystallizing section may be
30 installed on the upstream of the second scale inhibitor
supplying section 230a on the most upstream.
In this embodiment, a second separating section 280
42
(280a, 280b) may be installed on the downstream side of the
second crystallizing section 220 as shown in Fig. 10. The
second separating section 280 has the same configuration as
the first separating section 180 and includes a classifier
5 281 (281a, 281b) and a dehydrator 282 (282a, 282b).
In the water treatment system 200 of Fig. 10, a second
precipitating section 250 (250a, 250b) may be installed on
the downstream side of the second crystallizing section 220.
The second precipitating section 250 has the same
10 configuration as the first precipitating section 50 and
includes a second precipitating tank 251 (251a, 251b) and a
second filtration device 252 (252a, 252b).
The water treatment system 200 includes a downstream
side demineralizing section 60 on the downstream side of the
15 water to be treated of the first water treatment section. An
evaporator (not shown in Fig. 10) may be installed on the
downstream on the concentrated-water side of the downstream
side demineralizing section 60.
A process for treating water to be treated using the
20 water treatment system 200 of the first embodiment will be
described hereinafter.

The water to be treated is subjected to the
pretreatment described in the first reference embodiment.
25
In the same manner as in the first deaerating step
described in the first reference embodiment, C02 in the water
to be treated is removed in the second deaerating section
273, whereby the concentration of carbonate ions in the
30 water to be treated is reduced.

In the second upstream side precipitating section 270,
43
some of Ca ions and carbonate ions are previously removed
from the water to be treated as calcium carbonate. In the
case where metal ions other than Ca ions are contained in
the water to be treated, in the second upstream side
5 precipitating section 270, some of a metal compound having
low solubility in water is previously removed from the water
to be treated.
The second upstream side precipitating step is
performed in the same manner as in the first upstream side
10 precipitating step.
In the case where water to be treated containing Mg
ions is treated in the water treatment system 200 of this
embodiment, as in the first reference embodiment, the water
to be treated is adjusted to a pH at which a magnesium
15 compound is deposited in the second upstream side
precipitating section 270, and some of Mg ions in the water
to be treated are removed. Subsequently, it is preferable
that the water to be treated is adjusted to a pH at which
the magnesium compound is soluble on the downstream side of
20 the second upstream side precipitating section 270.
Specifically, the pH is adjusted to less than 10.
Accordingly, the formation of scales in devices and steps on
the downstream side, particularly the second demineralizing
section 210 and the second demineralizing step, can be
25 inhibited.
In the case where the second deaerating step is
performed after the second upstream side precipitating step,
the pH of the water to be treated is adjusted to a pH at
which carbonate ions can be present as CO2, specifically 6.5
30 or less.
Depending on the properties of the water to be treated,
the second deaerating step and the second upstream side
44
precipitating step may be omitted.
In the case where an ion-exchange membrane device is
installed, in the water treatment system 200 of the first
embodiment, Ca ions and Mg ions in the water to be treated
5 are removed by the ion-exchange membrane device.
In the case where an upstream side crystallizing
section is installed, the concentration of gypsum in the
water to be treated is reduced in the upstream side
crystallizing section through the same steps as in the first
10 reference embodiment.

The control section 232a of the second scale inhibitor
supplying section 230a opens the valve V4a and supplies a
predetermined amount of calcium scale inhibitor to the water
15 to be treated from the tank 231a. The control section 232b
of the second scale inhibitor supplying section 230b opens
the valve V4b and supplies a predetermined amount of silica
scale inhibitor to the water to be treated from the tank
231b. The control section 232a and the control section 232b
20 adjust the valve opening of the valve V4a and the valve V4b,
respectively, so that the concentrations of the calcium
scale inhibitor and the silica scale inhibitor are
predetermined values set according to the properties of the
water to be treated.
25 In the water treatment system 200 of the first
embodiment, the pH adjustment of the water to be treated
immediately before flowing into the second demineralizing
section 210 is optionally performed.
For example, in the configuration of Fig. 10, as a
30 result of the addition of FeCl3, the water to be treated is
adjusted to about pH 5 to 6 and then flows into the second
demineralizing section 210a. As shown in Fig. 3, the
45
solubility of calcium carbonate in water is high when the pH
of the water to be treated is 6.5 or less. In addition, as
in formula (1), in the above pH range, carbonic acid is
present mainly in the form of HC03~ and C02 in water. The
5 water to be treated flowing into the second demineralizing
section 210a has a reduced concentration of calcium
carbonate. In such a case, it is not necessary to adjust the
pH immediately before the second demineralizing section
210a.
10 Incidentally, in the case where the pH of the water to
be treated, which is to be treated in the second
demineralizing step, is adjusted, it is possible that a pH
adjusting section having the same configuration as the first
pH adjusting section of the first reference embodiment is
15 installed on the upstream of the second demineralizing
section 210a, and the pH-adjusted water to be treated is fed
to the second demineralizing section 210a.

In the second demineralizing section 210a, the water to
20 be treated containing the scale inhibitors is treated. In
the case where the second demineralizing section 210a is a
reverse osmosis membrane device, the water that has passed
through the reverse osmotic membrane is recovered as treated
water. The water containing ions and the scale inhibitors is
25 discharged from the non-permeate side of the reverse osmosis
membrane as concentrated water (second concentrated water).
As a result of the treatment in the second
demineralizing section 210a, gypsum and silica is
concentrated in the second concentrated water. However, the
30 production of scales is suppressed by the calcium scale
inhibitor and the silica scale inhibitor.
Also in the case where other demineralizers, such as a
46
capacitive de-ionization device, are used, for example, the
water to be treated is separated into treated water and
concentrated water having a high concentration of ions
(second concentrated water). The second concentrated water
5 is fed toward the second crystallizing section 220a.

In this embodiment, the pH of the water to be treated
(second concentrated water) may be adjusted by the second pH
adjusting section 240a between the second demineralizing
10 section 210a and the second crystallizing section 220a.
The second pH adjusting section 240a controls the pH of
the second concentrated water to such a value that the
function of the calcium scale inhibitor is reduced and
gypsum in the second concentrated water can be deposited.
15 The pH meter 243a measures the pH of the second concentrated
water. The control section 242a adjusts the opening of the
valve V6a so that the value measured by the pH meter 243a is
a predetermined pH control value.

20 The second concentrated water is stored in the second
crystallizing tank 221 of the second crystallizing section
220a. The control section 224a of the second seed crystal
supplying section 222a opens the valve V5 and adds seed
crystals of gypsum from the seed crystal tank 223a to the
25 second concentrated water in the second crystallizing tank
221a. Although the second concentrated water contains a
calcium scale inhibitor, when seed crystals are loaded,
gypsum is crystallized, followed by crystal growth.
As shown in Fig. 5, under Condition 1 (pH 6.5), the
30 degree of supersaturation is 460%, and there is no change
from the initial degree of supersaturation even after the
elapse of 6 hours. Under Condition 1, the scale inhibitor
47
exerts its function to suppress the deposition of gypsum.
Meanwhile, under Condition 4 and Condition 2, the degree of
supersaturation decreases.
That is, it has been confirmed that even when seed
5 crystals are not loaded, a decrease in pH leads to a
decrease in the function of the scale inhibitor, whereby
gypsum is crystallized. In addition, according to the
results of Fig. 5, the deposition rate increases with a
decrease in pH.
10 In Fig. 6, as a comparison with Condition 5 (pH 4.0),
under Condition 7 (pH 4.0), seed crystals pre-immersed in
the above calcium scale inhibitor were added to simulated
water having added thereto a calcium scale inhibitor, and
sulfuric acid was added for pH adjustment. Condition 5 and
15 Condition 7 are otherwise the same, and gypsum deposition
experiments were performed under such conditions. Two hours
after the pH adjustment, the concentration of Ca in the
simulated water was measured by the same technique as in
Fig. 3.
20 As a result, as shown in Fig. 6, the degree of
supersaturation was 199% or less both under Condition 6 and
Condition 7. From this, it can be said that independent of
the immersion time of seed crystals in a calcium scale
inhibitor, when the pH is reduced to 4.0, the function of
25 the calcium scale inhibitor is reduced.
In consideration of the effects of the calcium scale
inhibitor, the pH of the second concentrated water is
adjusted in the second pH adjusting step to 6.0 or less,
preferably 5.5 or less, and more preferably 4.0 or less. In
30 particular, when the second concentrated water is adjusted
to pH 4.0 or less, the function of the calcium scale
inhibitor can be significantly reduced. By adjusting the pH
48
of the second concentrated water to such a value that the
scale inhibition function of the calcium scale inhibitor is
reduced, crystallization in the second crystallizing section
220a is promoted. According to the kind of scale inhibitor,
5 the pH range in the second pH adjusting step is suitably
determined.
With reference to Fig. 4, in the case where pH is low,
silica may exceed the saturation solubility. However, in
this embodiment, a silica scale inhibitor is loaded in the
10 second water treatment section. Accordingly, the deposition
of silica is suppressed even at low pH. Even when silica is
deposited in the second crystallizing tank 221a, such silica
is present as small-diameter particles or floating matters
in a colloidal form.
15 In addition, with reference to Fig. 3, calcium
carbonate dissolves in water at pH 6.0 or less.
From the above, high-purity gypsum can be recovered in
the second crystallizing tank 221a of the second water
treatment section.
20 Meanwhile, the second concentrated water in the second
crystallizing step may also be adjusted in the second pH
adjusting step to a pH at which silica is soluble in the
second concentrated water. Accordingly, in the second
crystallizing tank 221a, the deposition of silica from the
25 second concentrated water is suppressed. As a result, in the
case where the second concentrated water discharged from the
second crystallizing tank 221a in the second separating
section 280a is classified, the purity of the recovered
gypsum can be increased.
30
In the case where the second separating section 280a is
installed, the second concentrated water containing solid
49
matters deposited in the second crystallizing tank 221a is
transferred to the second separating section 280a. In the
second concentrated water in the second crystallizing tank
221a, gypsum deposited by crystallization is present. In
5 addition, the second concentrated water may also contain
silica deposited because of an increase in the silica
concentration to be equal to or higher than the
concentration at which the silica scale inhibitor exerts its
function due to the quality change or concentration of the
10 raw water. Silica is present in the second concentrated
water as small-diameter particles or floating matters in a
colloidal form.
Through the same steps as in the second reference
embodiment, the classifier 281a of the second separating
15 section 280a performs separation into gypsum having a
predetermined size (e.g., having an average particle
diameter of 10 um or more) and a supernatant containing
small-diameter precipitates (gypsum, silica). Large-diameter
gypsum .is further dehydrated by the dehydrator 282a and
20 recovered. According to this embodiment, high-purity gypsum
can be recovered. Some of the recovered gypsum may be
circulated through the seed crystal tank 223a as seed
crystals.
In the case where the second separating section 280a is
25 not installed, gypsum precipitated at the bottom of the
second crystallizing tank 221a of the second crystallizing
section 220a is discharged from the second crystallizing
tank 221a. The supernatant in the second crystallizing tank
221a is fed to the second precipitating section 250a.
30
The supernatant (second concentrated water) in the
second crystallizing section 220a or the supernatant (second
50
concentrated water) discharged from the second separating
section 280a is fed to the second precipitating section
250a.
In the second precipitating step, in the same manner as
5 in the first precipitating step described in the first
reference embodiment, calcium carbonate and metal compounds
in the second concentrated water are removed in the second
precipitating tank 251a and the second filtration device
252a.
10 In the second precipitating step, it is also possible
that in the same manner as in the first precipitating step,
at least one of seed crystals of silica and a precipitant
for silica is added to the second precipitating tank 251a to
remove silica from the second concentrated water.
15 In the case where the treatment is performed in several
stages as shown in Fig. 10, the second concentrated water
that has passed through the second filtration device 252a of
the second water treatment section of the previous stage
flows into the water treatment section of the subsequent
20 stage as water to be treated. In the water treatment section
of the subsequent stage, the steps from the second scale
inhibitor supplying step to the second precipitating step
mentioned above are performed.

25 The second concentrated water that has passed through
the second precipitating section 250b located on the most
downstream of the water to be treated is treated in the
downstream side demineralizing section 60. The water that
has passed through the downstream side demineralizing
30 section 60 is recovered as water to be treated. The
concentrated water in the downstream side demineralizing
section 60 is discharged out of the system.
51
Also in this embodiment, an evaporator (not shown) may
be installed on the downstream on the concentrated-water
side of the downstream side demineralizing section 60.
In the first embodiment, in the case where the second
5 concentrated water is adjusted in the second pH adjusting
step to a pH at which the function of the calcium scale
inhibitor is reduced, as a third pH adjusting step (a pH
adjusting step for achieving the calcium scale inhibition
function), the pH of the second concentrated water may be
10 adjusted after the second crystallizing step in order for
the calcium scale inhibitor to exert its function.
Specifically, the pH is preferably adjusted to 4.0 or more,
preferably 5.5 or more, and more preferably 6.0 or more. The
third pH adjusting step is performed after the second
15 crystallizing step and before the second demineralizing
step, or after the second crystallizing step and before the
downstream side demineralizing step.
In the water treatment system 200 of this embodiment,
in order to perform the third pH adjusting step, a third pH
20 adjusting section (not shown in Fig. 10) having the same
configuration as the second pH adjusting section is
installed between the second crystallizing section and the
second demineralizing section immediately thereafter (in
Fig. 10, between the second crystallizing section 220a and
25 the second demineralizing sections 210b, particularly the
second precipitating section 250a and the second
demineralizing section 210b). In addition, a third pH
adjusting section (not shown in Fig. 10) having the same
configuration as the second pH adjusting section is
30 installed between the second precipitating section 250b and
the downstream side demineralizing section 60 on the most
downstream. Accordingly, even in the case where the second
52
concentrated water is treated in the downstream side
demineralizing step, and the concentration of Ca ions is
high on the concentrated-water side, the formation of scales
can be suppressed by the function of the calcium scale
5 inhibitor.
In the water treatment system 200 of this embodiment,
silica is concentrated by the treatment in the second water
treatment section. When the concentration of silica in the
second concentrated water is equal to or higher than the
10 concentration at which the silica scale inhibitor
effectively works, scales of silica may be formed from the
second concentrated water. For example, in the case where
FLOCON260 is used as a silica scale inhibitor, the scaleproduction-
inhibiting effect can be obtained at a silica
15 concentration up to about 200 mg/L. Therefore, the number of
stages of the second water treatment sections is determined
so that silica is concentrated to the concentration at which
the silica scale inhibitor can exert its effect.
By using the water treatment system 200 of the first
20 embodiment, water to be treated containing ions can be
treated with high water recovery.
In particular, in the first embodiment, gypsum is
mainly deposited in the second crystallizing section 220.
Accordingly, the gypsum recovery in the second crystallizing
25 section 220 is high, and the number of moles of ions fed to
the downstream side is further reduced. In addition, the
purity of the gypsum recovered in the second crystallizing
section 220 can be increased.
3 0 Third Reference Embodiment
Fig. 11 is a schematic diagram of a water treatment
system of the third reference embodiment of the present
53
invention. In Fig. 11, the same configurations as in the
first reference embodiment, the second reference embodiment,
and first embodiment are indicated with the same reference
numerals.
5 In the water treatment system 300 of the third
reference embodiment, the water treatment section described
in the first reference embodiment is installed. On the
downstream side of the water to be treated of this water
treatment section, the water treatment section described in
10 the first embodiment is installed.
In the water treatment system 300 of Fig. 11, a first
separating section 180 is installed on the downstream side
of the first crystallizing section 20. In addition, a second
separating section 280, which is the same as the first
15 separating section 180, is installed on the downstream side
of the second crystallizing section 220.
A downstream side demineralizing section 60 is
installed on the downstream side of the water to be treated
of the second crystallizing section 220 located on the most
20 downstream.
The water treatment system 300 of the third reference
embodiment includes the first upstream side precipitating
section 70 described in the first reference embodiment on
the upstream side of the first scale inhibitor supplying
25 section 30 and the first pH adjusting section 40 which are
located on the most upstream of the water to be treated.
Further, the water treatment system 300 of the third
reference embodiment includes a first deaerating section 73,
which is the same as in the first reference embodiment, on
30 the upstream side of the first upstream side precipitating
section 70 as shown in Fig. 11. The first deaerating section
73 may also be installed on the downstream side of the water
54
to be treated of the first upstream side precipitating
section 70 and on the upstream side of the first scale
inhibitor supplying section 30 and the first pH adjusting
section 40.
5 Incidentally, a deaerating section having the same
configuration as the first deaerating section 73 may be
installed in the flow path between the first demineralizing
section 10 and the first crystallizing section 20, in the
flow path between the first crystallizing section 10 and the
10 first precipitating section 50, in the flow path between the
second crystallizing section 220 and the second
precipitating section 250, and in the flow path between the
first precipitating section 50 and the second demineralizing
section 210.
15 Also in the water treatment system 300 of this
reference embodiment, an ion-exchange equipment (not shown)
and an upstream side crystallizing section (not shown) may
be provided on the upstream of the first scale inhibitor
supplying section 30 and the first pH adjusting section 40.
20 In Fig. 11, the water treatment sections from the first
scale inhibitor supplying section 30 to the first
precipitating section 50 and from the second scale inhibitor
supplying section 230 to the second precipitating section
250 are each shown as one stage. However, it is also
25 possible that for each section, two or more stage of water
treatment sections are connected.
In the water treatment system 300 of the third
reference embodiment, first, water to be treated is treated
by the water treatment process described in the first
30 reference embodiment and the second reference embodiment.
First concentrated water after being treated by the process
of the first reference embodiment and the second reference
55
embodiment is treated as water to be treated through the
steps form the second scale inhibitor supplying step to the
second precipitating step described in the first embodiment.
Second concentrated water that has passed through the
5 second precipitating section 250 on the most downstream is
treated in the downstream side demineralizing section 60.
The water that has passed through the downstream side
demineralizing section 60 is recovered as treated water. The
concentrated water in the downstream side demineralizing
10 section 60 is discharged out of the system.
Also in this reference embodiment, an evaporator (not
shown) may be installed on the downstream on the
concentrated-water side of the downstream side
demineralizing section 60.
15 In the third reference embodiment, in the case where
the second concentrated water is adjusted to a pH at which
the function of the calcium scale inhibitor is reduced in
the second pH adjusting step, the third pH adjusting step
described in the first embodiment may be performed.
20
Fourth Reference Embodiment
Fig. 12 is a schematic diagram of a water treatment
system of the fourth reference embodiment of the present
invention. In Fig. 12, the same configurations as in the
25 first reference embodiment, the second reference embodiment,
and first embodiment are indicated with the same reference
numerals.
In the water treatment system 400 of the fourth
reference embodiment, the water treatment section described
30 in the first embodiment is installed. On the downstream side
of the water to be treated of this water treatment section,
the water treatment section described in the first reference
56
embodiment is installed.
In the water treatment system 400 of Fig. 12, a first
separating section 180 and a second separating section 280
are installed.
5 A downstream side demineralizing section 60 is
installed on the downstream side of the water to be treated
of the first crystallizing section 20 located on the most
downstream.
The water treatment system 400 of the fourth reference
10 embodiment includes the second upstream side precipitating
section 270 described in the first embodiment on the
upstream side of the second scale inhibitor supplying
section 230 located on the most upstream of the water to be
treated.
15 Further, the water treatment system 400 of the fourth
reference embodiment has a second deaerating section 273,
which is the same as in the first embodiment, on the
upstream side of the second upstream side precipitating
section 270 as shown in Fig. 12. The second deaerating
20 section 273 may be installed on the downstream side of the
water to be treated of the second upstream side
precipitating section 270 and on the upstream side of the
second scale inhibitor supplying section 230.
Incidentally, a deaerating section having the same
25 configuration as the second deaerating section 273 may be
installed in the flow path between the second demineralizing
section 210 and the second crystallizing section 220, in the
flow path between the first crystallizing section 20 and the
first precipitating section 50, in the flow path between the
30 second crystallizing section 220 and the second
precipitating section 250, and in the flow path between the
second precipitating section 250 and the first
57
demineralizing section 10.
Also in the water treatment system 400 of this
reference embodiment, an ion-exchange equipment (not shown)
and an upstream side crystallizing section (not shown) may
5 be provided on the upstream of the second scale inhibitor
supplying section 230.
In the water treatment system 400 of this reference
embodiment shown in Fig. 12, a first separating section 180
and a second separating section 280 may be installed on the
10 downstream side of the first crystallizing tank 21 and the
second crystallizing tank 221, respectively.
In Fig. 12, the water treatment sections from the
second scale inhibitor supplying section 230 to the second
precipitating section 250 and from the first scale inhibitor
15 supplying section 30 to the first precipitating section 50
are each shown as one stage. However, it is also possible
that for each section, two or more stage of water treatment
sections are connected.
In the water treatment system 400 of the fourth
20 reference embodiment, first, water to be treated is treated
by the water treatment process described in the first
embodiment. Second concentrated water after being treated by
the process of the first embodiment is treated as water to
be treated through the steps form the first scale inhibitor
25 supplying step to the first precipitating step described in
the first reference embodiment and the second reference
embodiment.
First concentrated water that has passed through the
first precipitating section 50 on the most downstream is
30 treated in the downstream side demineralizing section 60.
The water that has passed through the downstream side
demineralizing section 60 is recovered as treated water. The
58
concentrated water in the downstream side demineralizing
section 60 is discharged out of the system.
Also in this reference embodiment, an evaporator (not
shown) may be installed on the downstream on the
5 concentrated-water side of the downstream side
demineralizing section 60.
In the fourth reference embodiment, in the case where
the second concentrated water is adjusted to a pH at which
the function of the calcium scale inhibitor is reduced in
10 the second pH adjusting step, the third pH adjusting step
described in the first embodiment may be performed.
Also by the water treatment system 300 of the third
reference embodiment and the water treatment system 400 of
the fourth reference embodiment, water to be treated
15 containing ions can be treated with high water recovery.
In particular, the fourth reference embodiment is
configured such that gypsum is mainly deposited in the
second crystallizing section 220 on the upstream side of the
water to be treated. Accordingly, the gypsum recovery in the
20 second crystallizing section 220 is high, and the number of
moles of ions fed to the downstream side is further reduced.
Further, the purity of the gypsum recovered in the second
crystallizing section 220 can be increased.
25 Fifth Reference Embodiment
According to the fifth reference embodiment of the
present invention, the amount of seed crystals of gypsum to
be supplied to the first crystallizing tank 21 and the
second crystallizing tank 221 in the first to fourth
30 reference embodiments is controlled. The configuration that
controls the amount of seed crystals to be supplied to the
first crystallizing tank 21 will be described with reference
59
to Fig. 13. The same configuration is also applied to the
second crystallizing tank 221.
In the fifth reference embodiment, a first pH measuring
section 543 that measures the pH of the first concentrated
5 water in the first crystallizing tank 21 is installed. The
first pH measuring section 543 may be installed in the flow
path that connects the first demineralizing section 10 and
the first crystallizing tank 21, or may also be directly
installed in the first crystallizing tank 21. The first pH
10 measuring section 543 is connected to the control section 24
of the seed crystal supplying section 22.
In the fifth reference embodiment, as shown in Fig. 13,
a section 540 for adjusting pH used for controlling the seed
crystal supply is installed. The section 540 for adjusting
15 pH used for controlling the seed crystal supply includes a
tank 541, a control section 542, and a valve V7. The first
pH measuring section 543 is connected to the control section
542 of the section 540 for adjusting pH used for controlling
the seed crystal supply. The section 540 for adjusting pH
20 used for controlling the seed crystal supply controls the pH
of the first concentrated water in the first crystallizing
tank 21 to a predetermined value based on the value measured
by the first pH measuring section 543.
Incidentally, in the case where the amount of seed
25 crystals of gypsum to be supplied to the second
crystallizing tank 221 is controlled, the pH meter 243a
described in the first embodiment corresponds to the second
pH measuring section, and the control section 242 of the
second pH adjusting section corresponds to the control
30 section 542.
Seed crystals stored in the seed crystal tank 23 of the
first seed crystal supplying section 22 may be new
60
chemicals. However, in the case where a first separating
section 180 is installed, the seed crystal tank 23 may also
store gypsum separated by the classifier 181, whose particle
diameter is equal to or greater than a predetermined
5 particle diameter, or gypsum after being dehydrated by the
dehydrator 182.
The control of the amount of seed crystals to be
supplied in the fifth reference embodiment is performed
through the following steps. Hereinafter, the case where the
10 amount of seed crystals to be supplied is constantly
controlled during continuous operation will be explained as
an example.
The first pH measuring section 543 measures the pH of
the first concentrated water in the first crystallizing tank
15 21. The measured pH value is sent to the control section 24
of the seed crystal supplying section 22.
The control section 24 stores the pH range where the
scale inhibition function of a calcium scale inhibitor is
reduced. Specifically, as described for the second
20 crystallizing step, the pH range where the scale inhibition
function of a calcium scale inhibitor is reduced is 6.0 or
less, preferably 5.5 or less, and more preferably 4.0 or
less.
The control section 24 compares the value measured by
25 the first pH measuring section 543 with the above pH range.
In the case where the measured value is within the above pH
range, the control section 24 reduces the opening of the
valve V3 to reduce the amount of seed crystals of gypsum to
be supplied. In the case where the measured value is greater
30 than the above pH range, the control section 24 increases
the opening of the valve V3 to increase the amount of seed
crystals of gypsum to be supplied.
61
Gypsum is deposited when seed crystals are present.
However, in the case where the calcium scale inhibitor
exerts its function, the crystallization rate is low.
Accordingly, the amount of seed crystals is increased to
5 promote crystallization. Meanwhile, in the case where the
function of the calcium scale inhibitor is reduced, a
sufficient crystallization rate can be obtained even when
the amount of seed crystals is small.
In this way, by adjusting the amount of seed crystals
10 to be supplied according to the pH, the amount of seed
crystals used can be reduced.
In this reference embodiment, it is also possible that
the pH is regularly measured during continuous operation,
and seed crystals are supplied intermittently.
15 Alternatively, it is also possible that the time-dependent
variation of pH is obtained at the time of the test run of
the system, for example, and the amount of seed crystals to
be supplied is increased or decreased based on the obtained
time-dependent variation.
20
Sixth Reference Embodiment
The sixth reference embodiment of the present invention
is a water treatment system 600 provided with at least
either a first separating section 180 or a second separating
25 section 280. The water treatment system 600 differs from the
fifth reference embodiment in that gypsum separated in the
separating section is directly supplied to a first
crystallizing tank or a second crystallizing tank as seed
crystals.
30 The configuration that controls the amount of seed
crystals to be supplied to the first crystallizing tank 21
in this reference embodiment will be described with
62
reference to Fig. 14. The same configuration is also applied
to the second crystallizing tank 221.
In Fig. 14, a first circulation line 601, which
performs transfer so that some of the gypsum sedimented at
5 the bottom of the classifier 181 of the first separating
section 180 is supplied directly to the first crystallizing
tank 21, is installed. In addition, a second circulation
line 602, which performs transfer so that some of the gypsum
after being dehydrated by the dehydrator 182 is supplied
10 directly to the first crystallizing tank 21, is installed. A
valve V8 is installed in the first circulation line 601, and
a valve V9 is installed in the second circulation line 602.
Incidentally, this reference embodiment may also be
configured such that either the first circulation line 601
15 or the second circulation line 602 is installed.
The control section 610 is connected to a first pH
measuring section 543, which is the same as in the fifth
reference embodiment, the valve V8, and the valve V9.
The control of the amount of seed crystals to be
20 supplied in the sixth reference embodiment is performed
through the following steps. Hereinafter, the case where the
amount of seed crystals to be supplied is constantly
controlled during continuous operation will be explained as
an example.
25 The first pH measuring section 543 measures the pH of
the first concentrated water in the first crystallizing tank
21. The measured pH value is sent to the control section
610.
The control section 610 stores the pH range where the
30 scale inhibition function of a calcium scale inhibitor is
reduced. Through the same steps as in the fifth reference
embodiment, the control section 610 compares the value
63
measured by the first pH measuring section 543 with the
above pH range to adjust the opening of the valve V8 and the
valve V9.
In the fifth reference embodiment and the sixth
5 reference embodiment, a seed crystal concentration measuring
section (not shown) that measures the concentration of
gypsum seed crystals in the first concentrated water in the
first crystallizing tank 21 may be installed in the first
crystallizing tank 21. The seed crystal concentration
10 measuring section measures the concentration of seed
crystals in the first crystallizing tank 21. The measured
concentration value is sent to the control section 24 or the
control section 610. The control section 24 or the control
section 610 stores the threshold for the concentration of
15 seed crystals, and increases the amount of seed crystals to
be supplied in the case where the concentration of seed
crystals is equal to or less than the threshold.
As a modification of the fifth reference embodiment and
the sixth reference embodiment, a first concentration
20 measuring section (not shown) is installed on the downstream
side of the first crystallizing tank 21 and on the upstream
side of the first precipitating section 50. In the case
where the first separating section 180 is provided, the
first concentration measuring section is preferably
25 installed on the downstream side of the first separating
section 180, but may also be installed on the upstream side
of the first separating section 180. The first concentration
measuring section is connected to the control section 24 or
the control section 610.
30 In the case of the second crystallizing tank 221, a
second concentration measuring section is installed in place
of the first concentration measuring section.
64
The first concentration measuring section measures at
least one of the concentration of Ca ions and the
concentration of sulfate ions in the first concentrated
water discharged from the first crystallizing tank 21. The
5 measured concentration is sent to the control section 24 or
the control section 610.
The concentration of Ca ions and the concentration of
sulfate ions measured by the first concentration measuring
section depend on the crystallization rate in the first
10 crystallizing tank 21. In the case where the residence time
is the same, lower concentrations of Ca ions and sulfate
ions lead to a higher crystallization rate.
The control section 24 and the control section 610
store the threshold for at least one of the concentration of
15 Ca ions and the concentration of sulfate ions.
In the case where at least one of the concentration of
Ca ions and the concentration of sulfate ions measured by
the first concentration measuring section is equal to or
higher than the threshold, the control section 24 increases
20 the opening of the valve V3 to increase the amount of seed
crystals to be supplied. In the case where at least one of
the concentration of Ca ions and the concentration of
sulfate ions measured by the first concentration measuring
section is lower than the threshold, the control section 24
25 reduces the opening of the valve V3 to reduce the amount of
seed crystals to be supplied.
In the case where at least one of the concentration of
Ca ions and the concentration of sulfate ions measured by
the first concentration measuring section is equal to or
30 higher than the threshold, the control section 610 increases
the opening of the valve V8 and the valve V9 to increase the
amount of seed crystals to be supplied. In the case where at
65
least one of the concentration of Ca ions and the
concentration of sulfate ions measured by the first
concentration measuring section is lower than the threshold,
the control section 610 reduces the opening of the valve V8
5 and the valve V9 to reduce the amount of seed crystals to be
supplied.
Also in the case of the second crystallizing tank 221,
the amount of seed crystals to be supplied is controlled
through the same steps as above.
10 In this way, by controlling the amount of seed crystals
to be supplied depending on at least one of the
concentration of Ca ions and the concentration of sulfate
ions after the crystallizing step, the amount of seed
crystals used can be reduced.
15
Seventh Reference Embodiment
Fig. 15 is a partial schematic diagram of a water
treatment system of the seventh reference embodiment of the
present invention. In Fig. 15, the same configurations as in
20 the first reference embodiment, the second reference
embodiment, and first embodiment are indicated with the same
reference numerals.
The water treatment system 700 of Fig. 15 is configured
such that the gypsum separated from the first concentrated
25 water in the first crystallizing section 20 in the water
treatment system of the fourth reference embodiment is
recovered and supplied to the second crystallizing tank 221
of the second crystallizing section 220. Also in the water
treatment system of the third reference embodiment, the same
30 configuration can be employed.
As described in the first reference embodiment, the pH
of concentrated water (first concentrated water) in the
66
first crystallizing tank 21 of the first crystallizing
section 20 is not particularly limited. However, in terms of
operation cost, it is more advantageous to perform the first
crystallizing step without changing the pH from the first
5 demineralizing step. In this case, the first crystallizing
step is performed at a pH at which silica is soluble (10 or
more), but the solubility of calcium carbonate is low in
this pH range.
Meanwhile, as described in the first embodiment, in the
10 second crystallizing section 220 (second crystallizing
step), gypsum is crystallized in a still lower pH range. At
the pH range in the second crystallizing step (6.0 or less,
more preferably 4.0 or less), calcium carbonate is soluble
in water. When gypsum containing calcium carbonate recovered
15 in the first crystallizing section 20 is supplied to the
second crystallizing tank 221 of the second crystallizing
section 220, calcium carbonate, which is an impurity,
dissolves in the second concentrated water, and gypsum is
present as a solid in the second concentrated water. By
20 using the water treatment system 300 of the seventh
reference embodiment, water to be treated can be treated
with high water recovery, and also high-purity gypsum can be
recovered.
2 5 Eighth Reference Embodiment
Fig. 16 is a partial schematic diagram of a water
treatment system of the eighth reference embodiment of the
present invention. In Fig. 16, the same configurations as in
the second reference embodiment are indicated with the same
30 reference numerals.
Incidentally, the eighth reference embodiment will be
described hereinafter using a water treatment process
67
including a first separating step and a water treatment
system including a first separating section. However, the
same configuration is also applicable to the case of a
second separating step and a second separating section.
5 In Fig. 16, the water treatment system 800 includes,
for one first crystallizing section 20, two or more
classifiers (first classifiers) 181 in the flow direction of
the water to be treated. In Fig. 16, two first classifiers
181a and 181b are installed. The size of gypsum to be
10 separated is different between the first classifier 181a
located on the most upstream and the first classifier 181b
located on the downstream side. In this reference
embodiment, the size of gypsum to be separated by the first
classifier 181b is smaller than that of gypsum to be
15 separated by the first classifier 181a. For example, the
first classifier 181a is a classifier that separates
particles having an average particle diameter of 10 urn or
more, and the first classifier 181b is a classifier that
separates particles having an average particle diameter of 5
20 urn or more.
In the case where three or more first classifiers 181
are installed, they are designed such that the size of
gypsum to be separated by each first classifier 181
decreases in the direction from the upstream side toward the
25 downstream side. The number of first classifiers installed
in the flow direction of the water to be treated and the
particle diameter of solid matters that can be separated by
each classifier are suitably determined in consideration of
the water recovery, gypsum recovery, treatment cost, etc.
30 In the water treatment system 800 of the eighth
reference embodiment, the following treatment is performed
in the first separating step.
68
In the first classifier 181a located on the most
upstream, gypsum having an average particle diameter of 10
um or more is classified and sedimented at the bottom of the
first classifier 181a. The sedimented gypsum is discharged
5 from the first classifier 181a and fed to the dehydrator
182. The supernatant in the first classifier 181a is fed to
the first classifier 181b on the downstream side. This
supernatant mainly contains particles having a particle
diameter of less than 10 um (gypsum, calcium carbonate,
10 silica, etc.).
In the first classifier 181b located on the downstream
side, gypsum having an average particle diameter of 5 um or
more is classified and sedimented at the bottom of the first
classifier 181b. The supernatant in the first classifier
15 181b is fed to the first precipitating section 50.
The sedimented gypsum is discharged from the first
classifier 181b. The discharged gypsum is fed to the first
crystallizing tank 21 through a solid matter circulation
line 801 and supplied into the first concentrated water in
20 the first crystallizing tank 21.
The circulated gypsum functions as seed crystals in the
first crystallizing tank 21, and the circulated gypsum is
crystallized, followed by crystal growth. The crystal-grown
circulated gypsum having an average particle diameter of 10
25 um or more is fed from the first crystallizing tank 21 to
the first classifier 181a together with the first
concentrated water, then separated from the first
concentrated water by the first classifier 181a, and
transferred to the dehydrator 182.
30 The supernatant in the first classifier 181b contains
particles having a relatively small diameter of less than 5
um, such as those having a particle diameter of about 2 to 3
69
um. In particular, in the early stage of the operation of
the water treatment system (immediately after start-up,
etc.)/ gypsum is discharged from the first crystallizing
tank 21 before it grows to a sufficient size in the first
5 crystallizing tank 21, and an increased amount of gypsum
flows into the first precipitating tank 51. In such a case,
a large amount of gypsum is contained in the precipitate in
the first precipitating tank 51. Thus, in this reference
embodiment, it is also possible that a circulation line 802
10 that connects the bottom of the first precipitating tank 51
to the first crystallizing tank 21 is provided, and solid
matters containing gypsum precipitated at the bottom of the
first precipitating section 51 are circulated through the
first crystallizing tank 21.
15 According to this reference embodiment, the amount of
gypsum recovered in the first separating section is
increased, and also the water content of the recovered
gypsum can be reduced. The use of the water treatment steps
and the water treatment system of this reference embodiment
20 leads to the reduction of the amount of gypsum particles
having a relatively small diameter flowing out to the
downstream side. Accordingly, the water recovery can be
improved, and also the amount of waste resulting from the
water treatment can be reduced.
25
Reference Signs List
1, 100, 200, 300, 400, 500, 600, 700, 800: Water treatment
system
10: First demineralizing section
30 20: First crystallizing section
21: First crystallizing tank
22: First seed crystal supplying section
7 0
23, 223: Seed crystal tank
24, 32, 42, 224, 232, 242, 542, 610: Control section
30: First scale inhibitor supplying section
31, 41, 231, 241, 541: Tank
5 40: First pH adjusting section
43, 243: pH meter
50: First precipitating section
51: First precipitating tank
52: First filtration device
10 60: Downstream side demineralizing section
70: First upstream side precipitating section
71: Precipitating tank
72: Filtration device
73: First deaerating section
15 180: First separating section
181, 181a, 181b, 281: Classifier
182, 282: Dehydrator
210: Second demineralizing section (demineralizing section)
220: Second crystallizing section (crystallizing section)
20 221: Second crystallizing tank (crystallizing tank)
222: Second seed crystal supplying section (seed crystal
supplying section)
230: Second scale inhibitor supplying section (scale
inhibitor supplying section)
25 240: Second pH adjusting section (pH adjusting section)
250: Second precipitating section (precipitating section)
251: Second precipitating tank (precipitating tank)
252: Second filtration device (filtration device)
280: Second separating section (separating section)
30 540: section for adjusting pH used for controlling the seed
crystal supply
543: First pH measuring section
71
601: First circulation line
602: Second circulation line
801, 802: Solid matter circulation line
72

CLAIMS
1. A water treatment process, comprising:
a scale inhibitor supplying step of supplying a
calcium scale inhibitor which is a scale inhibitor for
5 inhibiting the deposition of a scale containing calcium
and a silica scale inhibitor which is a scale inhibitor
for inhibiting the deposition of silica to water to be
treated containing Ca ions, S04 ions, carbonate ions and
silica;
10 a demineralizing step of separating the water to
be treated into concentrated water in which, the Ca
ions, the S04 ions, the carbonate ions and the silica
are concentrated and treated water after the scale
inhibitor supplying step; and
15 a crystallizing step of supplying seed crystals of
gypsum to the concentrated water so that gypsum is
crystallized from the concentrated water.
2. The water treatment process according to claim 1,
comprising a pH adjusting step of adjusting the
20 concentrated water to a pH at which a scale inhibition
function of the calcium scale inhibitor is reduced,
thereby promoting the deposition of the gypsum in the
crystallizing step.
3. The water treatment process according to claim 1,
25 comprising a downstream side demineralizing step of
separating concentrated water after the crystallizing
step on a most downstream of the water to be treated
into concentrated water and treated water, and
recovering the separated treated water.
30 4. The water treatment process according to claim 2,
73
wherein, after the crystallizing step, the concentrated
water after the adjustment of the pH in the pH
adjusting step is adjusted to a pH at which the calcium
scale inhibitor exhibits its function.
5 5. The water treatment process according to claim 1,
comprising a pH adjusting step of adjusting the
concentrated water to a pH at which the silica is
soluble in the crystallizing step.
6. The water treatment process according to any one of
10 claims 1 to 5, comprising a upstream side precipitating
step of precipitating at least calcium carbonate from
the water to be treated so that the concentration of
the calcium carbonate in the water to be treated is
reduced, before the scale inhibitor supplying step on a
15 most upstream side of the water to be treated.
7. The water treatment process according to claim 6,
comprising a deaerating step of removing CO2 from the
water to be treated before the upstream side
precipitating step or after the upstream side
20 precipitating step and before the scale inhibitor
supplying step.
8. The water treatment process according to any one of
claims 1 to 7,
wherein the water to be treated contains metal
2 5 ions; and
wherein the process comprises a precipitating step
of precipitating at least one of calcium carbonate and
a metal compound so that the concentration of at least
one of the calcium carbonate and the metal ions is
30 reduced from the concentrated water, after the
74
crystallizing step.
The water treatment process according to claim 8,
wherein at least one of seed crystals of the silica and
a precipitant for the silica is supplied to the
concentrated water in the precipitating step.
The water treatment process according to claim 9,
wherein, when the water to be treated contains Mg ions,
the amount of the precipitant for the silica to be
supplied is adjusted according to the concentration of
the Mg ions.
The water treatment process according to claim 8,
wherein, when the water to be treated contains Mg
ions, the concentrated water in the precipitating step
is adjusted to a pH at which a magnesium compound is
deposited so that the concentration of the Mg ions is
reduced, and
wherein, after the precipitating step, the
concentrated water is adjusted to a pH at which the
magnesium compound is soluble.
The water treatment process according to claim 6,
wherein, when the water to be treated contains Mg
ions, the water to be treated in the upstream side
precipitating step is adjusted to a pH at which a
magnesium compound is deposited so that the
concentration of the Mg ions is reduced, and
wherein, after the upstream side precipitating
step, the water to be treated is adjusted to a pH at
which the magnesium compound is soluble.
The water treatment process according to claim 3,
75
wherein moisture is evaporated from the concentrated
water in the downstream side demineralizing step, so
that a solid in the concentrated water is recovered.
A water treatment system, comprising:
a scale inhibitor supplying section that supplies
a calcium scale inhibitor which is a scale inhibitor
for inhibiting the deposition of a scale containing
calcium and a silica scale inhibitor which is a scale
inhibitor for inhibiting the deposition of silica to
water to be treated containing Ca ions, SO4 ions,
carbonate ions and silica;
a demineralizing section that is installed on a
downstream side of the scale inhibitor supplying
section and separates the water to be treated into
concentrated water in which the Ca ions, the SO4 ions,
the carbonate ions and the silica are concentrated and
treated water; and
a crystallizing section including a crystallizing
tank that is installed on a downstream side of the
demineralizing section and crystallizes gypsum from the
concentrated water and a seed crystal supplying section
that supplies seed crystals of gypsum to the
crystallizing tank.
The water treatment system according to claim 14,
comprising a pH adjusting section that is installed on
a downstream side of the demineralizing section and
supplies a pH adjuster to the concentrated water to
adjust the pH of the concentrated water to such a value
that a scale inhibition function of the calcium scale
inhibitor is reduced, and the precipitation of the
gypsum is promoted.
76
16. The water treatment system according to claim 14,
comprising, on a downstream side of the crystallizing
section on a most downstream of the water to be
treated, a downstream side demineralizing section that
5 separates the concentrated water discharged from the
crystallizing section into concentrated water and
treated water.
17. The water treatment system according to claim 15,
comprising, on a downstream side of the crystallizing
10 section, a pH adjusting section for achieving scale
inhibition function that supplies a pH adjuster to the
concentrated water after the adjustment of the pH in
the pH adjusting section to adjust the pH of the
concentrated water to such a value that the calcium
15 scale inhibitor achieves a function.
18. The water treatment system according to claim 14,
comprising a pH adjusting section that is installed on
a downstream side of the demineralizing section and
supplies a pH adjuster to the concentrated water to
20 adjust the pH of the concentrated water to such a value
that the silica is soluble in the concentrated water in
the crystallizing section.
19. The water treatment system according to any one of
claims 14 to 18, comprising, on an upstream side of the
25 scale inhibitor supplying section located on a most
upstream of the water to be treated, a upstream side
precipitating section that precipitates at least
calcium carbonate from the water to be treated so that
the concentration of the calcium carbonate in the water
30 to be treated is reduced.
77
20. The water treatment system according to claim 19,
comprising a deaerating section that removes C02 from
the water to be treated on an upstream side of the
upstream side precipitating section or on a downstream
5 side of the upstream side precipitating section and on
an upstream side of the scale inhibitor supplying
section.
21. The water treatment system according to any one of
claims 14 to 20,
10 wherein the water to be treated contains metal
ions; and
wherein the system comprises, on a downstream side
of the crystallizing section, a precipitating section
that precipitates at least one of calcium carbonate and
15 a metal compound.
22. The water treatment system according to claim 21,
wherein at least one of seed crystals of the silica and
a precipitant for the silica is supplied to the
precipitating section.
20 23. The water treatment system according to claim 21,
wherein, when the water to be treated contains Mg ions,
the amount of the precipitant for the silica to be
supplied is adjusted according to the concentration of
the Mg ions in the precipitating section.
25 24. The water treatment system according to claim 21,
wherein, when the water to be treated contains Mg ions,
the concentrated water in the precipitating section is
adjusted to a pH at which a magnesium compound is
deposited so that the concentration of the Mg ions is
30 reduced, and
78
wherein, on a downstream side of the precipitating
section, the concentrated water is adjusted to a pH at
which the magnesium compound is soluble.
25. The water treatment system according to claim 19,
5 wherein, when the water to be treated contains Mg
ions, the water to be treated in the upstream side
precipitating section is adjusted to a pH at which a
magnesium compound is deposited so that the
concentration of the Mg ions is reduced, and
10 wherein, on a downstream side of the upstream side
precipitating section, the water to be treated is
adjusted to a pH at which the magnesium compound is
soluble.
26. The water treatment system according to claim 16,
15 comprising, on a downstream side of the concentrated
water in the downstream side demineralizing section, an
evaporator that evaporates moisture from the
concentrated water to recover the solids in the
concentrated water.