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~) , and
5 carbonate ions.
Background Art
It is known that industrial waste water, saline water,
and sewage contain large amounts of ions. In addition, in a
cooling tower, heat is exchanged between a high-temperature
10 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 in the cooling
water are concentrated. Therefore, the cooling water
discharged from the cooling tower (blowdown water) has
15 increased concentrations of ions.
Water containing a large amount of ions is subjected to a
demineralization treatment and then discharged into the
environment. As devices that perform the demineralization
treatment, a reverse osmosis membrane device, a nanofiltration
20 membrane device, an ion-exchange equipment, and the like are
known.
Among ions contained in the water mentioned above,
monovalent cations such as Na + , K+, and NH4+ and anions such
as CI- and N03- are highly soluble in water. On the other
25 hand, divalent metal ions such as Ca2+ and anions such as
S042- and C032- are scale-forming components. Salts of scaleforming
components have low solubility in water, and thus they
tend to be deposited as scales. In particular, the saline
water, industrial waste water, and blowdown water from a
30 cooling tower mentioned above contain large amounts of Ca2+,
S042-, and carbonate ions (C032-, HC03-) . 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,
2
CI ions: 200 mg/L, S04 ions: 120 mg/L, P04 ions: 5 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. When scales are produced in
5 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
production of scales.
Here, examples of plants using a water-cooling-type
10 cooling tower are plants equipped with power generation
facilities (power generation facilities include those for
business purposes for electric power selling and those for
industrial purposes for in-house electricity use, and the
power generation is thermal power generation, geothermal power
15 generation, etc.), plants equipped with power generation
facilities and cooling facilities, etc. In addition, plants
include ordinary chemical plants, steel plants, mining plants,
oil field plants, gas field plants, mechanical plants, etc.
As a process for removing Ca ions, a lime soda process is
20 known. According to the lime soda process, sodium carbonate is
added to water to be treated, and Ca ions in 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
25 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.
Citation List
30 Patent Literature
PTL 1 U.S. Pat. No. 7815804
3
Summary of Invention
Technical Problem
The lime soda process requires the addition of sodium
carbonate for the treatment, and thus the treatment cost is
5 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 be
treated, it is not removed by the lime soda process. That is,
in the lime soda process, water after the treatment contains
10 an increased number of moles of ions.
Also in the case where Ca ions are removed using an ionexchange
equipment, the treatment of 1 mol of Ca ions results
in the production of 2 mol of Na+, and water the after
treatment contains an increased number of moles of ions.
15 According to the system of Patent Literature 1, water
that has been treated by the lime soda process and in an ionexchange
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
20 the increased number of moles of ions, the osmotic pressure in
the reverse osmosis membrane device 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 been difficult to obtain high water
25 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 has
also been the problem of high treatment cost.
30 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.
4
Solution to Problem
A first aspect of the present invention is a water
treatment process including: a demineralizing step of
5 separating water to be treated containing Ca ions, S04 ions,
and carbonate ions into concentrated water in which the Ca
ions, the S04 ions, and the carbonate ions are concentrated and
treated water; a crystallizing step of supplying seed crystals
of gypsum to the concentrated water so that gypsum is
10 crystallized from the concentrated water; and a separating
step of separating the gypsum from the concentrated water
after the crystallizing step.
A second aspect of the present invention is a water
treatment system including: a demineralizing section that
15 separates water to be treated containing Ca ions, S04 ions, and
carbonate ions into concentrated water in which the Ca ions
and the S04 ions are concentrated and treated water; a
crystallizing section including a crystallizing tank that is
installed on the downstream side of the demineralizing section
20 and crystallizes gypsum from the concentrated water and a seed
crystal supplying section that supplies seed crystals of
gypsum to the crystallizing tank; and on the downstream side
of the crystallizing section, a separating section that
separates the gypsum from the concentrated water.
25 According to the first aspect and the second aspect, seed
crystals of gypsum are added to the concentrated water in the
crystallizing section and the crystallizing step, whereby
gypsum can be crystallized and separated from the water to be
treated. As a result, the water to be treated containing Ca
30 ions and S04 ions can be treated with high water recovery, and
the operation cost can be reduced. Further, this is also
advantageous in that high-purity gypsum can be recovered.
A third aspect of the present invention is a water
treatment process including: a first scale inhibitor supplying
5
step of supplying a calcium scale inhibitor which is a scale
inhibitor for inhibiting the deposition of a scale containing
calcium to water to be treated containing Ca ions, S04 ions,
carbonate ions, and silica; a first pH adjusting step of
5 adjusting the water to be treated to a pH at which the silica
is soluble in the water to be treated; a first demineralizing
step of separating the water to be treated into first
concentrated water in which the Ca ions, the S04 ions, the
carbonate ions, and the silica are concentrated and treated
10 water after the first scale inhibitor supplying step and the
first pH adjusting step; a first crystallizing step of
supplying seed crystals of gypsum to the first concentrated
water so that gypsum is crystallized from the first
concentrated water; and, a first separating step of separating
15 the gypsum from the first concentrated water after the first
crystallizing step.
A fourth aspect of the present invention is a water
treatment system including: a first scale inhibitor supplying
section that supplies a calcium scale inhibitor which is a
20 scale inhibitor for inhibiting the deposition of a scale
containing calcium to water to be treated containing Ca ions,
S04 ions, carbonate ions and silica; a first pH adjusting
section that supplies a pH adjuster to the water to be treated
to adjust the pH of the water to be treated to such a value
25 that the silica is soluble in the water to be treated; a first
demineralizing section that is positioned on a downstream side
of the first scale inhibitor supplying section and the first
pH adjusting section and separates the water to be treated
into first concentrated water in which the Ca ions, the S04
30 ions, the carbonate ions and the silica are concentrated and
treated water; a first crystallizing section including a first
crystallizing tank that is positioned on a downstream side of
the first demineralizing section and crystallizes gypsum from
the first concentrated water and a first seed crystal
6
supplying section that supplies seed crystals of gypsum to the
first crystallizing tank; and on a downstream side of the
first crystallizing section, a first separating section that
separates the gypsum from the first concentrated water.
5 According to the third aspect and the fourth aspect, a
calcium scale inhibitor is added, and also the water to be
treated is adjusted to a pH at which silica is soluble,
followed by a water treatment. Accordingly, the production of
scales in the first demineralizing section and the first
10 demineralizing step can be inhibited. In addition, by adding
seed crystals of gypsum to the first concentrated water in the
first crystallizing 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
15 result, while inhibiting the production of scales, the water
to be treated containing Ca ions, S04 ions, carbonate ions, and
silica can be treated with high water recovery. In addition,
the amount of chemicals required for the treatment and the
power required for the operation can be reduced, and also
20 maintenance is facilitated. Accordingly, the operation cost
can be reduced. In addition, high-purity gypsum can be
recovered in the course of water treatment.
A fifth aspect of the present invention is a water
treatment process including: a second scale inhibitor
25 supplying step of supplying 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, S04 ions, carbonate
30 ions, and silica; a second demineralizing step of separating
the water to be treated into second concentrated water in
which the Ca ions, the S04 ions, the carbonate ions, and the
silica are concentrated and treated water after the second
scale inhibitor supplying step; a second crystallizing step of
7
supplying seed crystals of gypsum to the second concentrated
water so that gypsum is crystallized from the second
concentrated water; and a second separating step of separating
the gypsum from the second concentrated water after the second
5 crystallizing step.
A sixth aspect of the present invention is a water
treatment system including: a second scale inhibitor supplying
section that supplies a calcium scale inhibitor which is a
scale inhibitor for inhibiting the deposition of a scale
10 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 second demineralizing section that is
positioned on a downstream side of the second scale inhibitor
15 supplying section and separates the water to be treated into
second concentrated water in which the Ca ions, the SO4 ions
and the silica are concentrated and treated water; a second
crystallizing section including a second crystallizing tank
that is positioned on a downstream side of the second
20 demineralizing section and crystallizes gypsum from the second
concentrated water and a second seed crystal supplying section
that supplies seed crystals of gypsum to the second
crystallizing tank; and on the downstream side of the second
crystallizing section, a second separating section that
25 separates the gypsum from the second concentrated water.
In the fifth aspect and the sixth aspect, owing to the
effects of the calcium scale inhibitor and the silica scale
inhibitor, the production of scales in the second
demineralizing section and the second demineralizing step can
30 be inhibited. In addition, by adding seed crystals of gypsum
to the first concentrated water in the first crystallizing
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
8
treated containing Ca ions, S04 ions, and silica can be treated
with high water recovery, and the operation cost can be
reduced. Further, this is also advantageous in that highpurity
gypsum can be recovered.
5 In the present invention, the water treatment processes
of the third aspect and the fifth aspect and the water
treatment systems of the fourth aspect and the sixth aspect
can be combined in the flow direction of the water to be
treated to perform a water treatment.
10 In the above aspect, it is preferable that the water
treatment process includes a second separating step that
separates the gypsum from the second concentrated water after
the second crystallizing step.
In the above aspect, it is preferable that the water
15 treatment system includes a second separating section that
separates the gypsum from the second concentrated water on the
downstream side of the second crystallizing section.
According to the above aspect, high-purity gypsum can be
recovered in the course of water treatment.
20 In addition, in the above aspect, by separating and
recovering the gypsum having a predetermined size, the water
content of gypsum can be reduced. As a result, high-purity
gypsum can be recovered.
In the above aspect, it is preferable that the gypsum
25 separated in the first separating step is used as seed
crystals of the gypsum. It is preferable that the gypsum
separated in the second separating step is used as seed
crystals of the gypsum.
In the above aspect, it is preferable that the gypsum
30 separated in the first separating section is used as seed
crystals of the gypsum. It is preferable that the gypsum
separated in the second separating section is used as seed
crystals of the gypsum.
According to the above aspect, seed crystals of gypsum
9
are efficiently supplied, and thus the amount of seed crystals
of gypsum to be used can be reduced.
In the above aspect, it is preferable that the first
separating step is carried out a plurality of times in the
5 flowing direction of the water to be treated per the first
crystallizing step, the size of the gypsum to be separated in
the first separating step carried out on the downstream side
of the water to be treated is smaller than that in the first
separating step carried out on the most upstream of the water
10 to be treated, and the gypsum separated in the first
separating step carried out on the downstream side of the
water to be treated is supplied into the first concentrated
water in the first crystallizing step. It is preferable that
the second separating step is carried out a plurality of times
15 in the flowing direction of the water to be treated per the
second crystallizing step, the size of the gypsum to be
separated in the second separating step carried out on the
. downstream side of the water to be treated is smaller than
that in the second separating step carried out on the most
20 upstream of the water to be treated, and the gypsum separated
in the second separating step carried out on the downstream
side of the water to be treated is supplied into the second
concentrated water in the second crystallizing step.
In the above aspect, it is preferable that wherein the
25 first separating section includes a plurality of classifiers
in a flowing direction of the water to be treated per the
first crystallizing section, wherein the size of the gypsum to
be separated in the first classifier located on a downstream
side of the water to be treated is smaller than in the first
30 classifier located on a most upstream of the water to be
treated, and the gypsum separated in the first classifier
located on the downstream side of the water to be treated is
supplied to the first concentrated water in the first
crystallizing section.
10
It is preferable that the second separating section
includes a plurality of second classifiers in the flowing
direction of the water to be treated per the second
crystallizing section, the size of the gypsum to be separated
5 in the second classifier located on the downstream side of the
water to be treated is smaller than in the second classifier
located on the most upstream of the water to be treated, and
the gypsum separated in the second classifier located on the
downstream side of the water to be treated is supplied to the
10 second concentrated water in the second crystallizing section.
According to the above aspect, gypsum having a relatively
small size is further crystallized as seed crystals.
Therefore, the recovery of high-purity gypsum can be improved,
and the purity of gypsum can also be increased. In addition,
15 the amount of gypsum flowing out to the downstream side is
reduced, and thus the amount of precipitate recovered as waste
in the precipitating section, for example, can be reduced.
In the above aspect, it is preferable that a third pH
adjusting step of adjusting the second concentrated water to a
20 pH at which calcium carbonate is soluble, wherein in the
second crystallizing step, the gypsum separated in the first
separating step is supplied into the second concentrated water
after the adjustment of the pH in the third pH adjusting step.
In the above aspect, it is preferable that wherein the
25 second pH adjusting section adjusts the second concentrated
water to a pH at which calcium carbonate is soluble, and
supplies the gypsum separated in the first separating section
to the second crystallizing section.
According to the above aspect, high-purity gypsum can be
30 recovered in the course of water treatment.
{Advantageous Effects of Invention}
According to the water treatment system and the water
treatment process of the present invention, while inhibiting
the production of scales such as calcium carbonate during the
11
treatment, Ca2+ and S042- can be removed as gypsum from the
water to be treated. Accordingly, the water recovery can be
further improved.
Water treated by the present invention has a
5 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
that high-purity gypsum can be crystallized and recovered.
10 Brief Description of Drawings
Fig. 1 is a schematic diagram of a water treatment system
according to the first reference embodiment.
Fig. 2 shows simulation results for the pH dependency of
the amount of gypsum deposited.
15 Fig. 3 shows simulation results for the pH dependency 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 experiments
20 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 is
supersaturated with changing the concentration of seed
25 crystals.
Fig. 7 is a microphotograph of gypsum crystallized under
Condition 5.
Fig. 8 is a microphotograph of gypsum crystallized under
Condition 3.
30 Fig. 9 is a schematic diagram of a water treatment system
according to a first embodiment.
Fig. 10 is a schematic diagram of a water treatment
system according to a first example of a second embodiment.
Fig. 11 is a schematic diagram of a water treatment
12
system according to a second example of the second embodiment.
Fig. 12 is a schematic diagram of a water treatment
system according to a third embodiment.
Fig. 13 is a schematic diagram explaining a water
5 treatment system according to a forth embodiment.
Fig. 14 is a schematic diagram explaining a water
treatment system according to a second reference embodiment.
Fig. 15 is a schematic diagram explaining a water
treatment system according to a fifth embodiment.
10 Fig. 16 is a schematic diagram explaining a water
treatment system according to a sixth embodiment.
Fig. 17 is a schematic diagram explaining a water
treatment system according to a seventh embodiment.
Description of Embodiments
15 Water that is an object to be treated in the present
invention (water to be treated) contains Ca2 + , S042-,
carbonate ions, and silica. Specifically, the water to be
treated (raw water) is saline water, sewage, industrial waste
water, blowdown water from a cooling tower, or the like. The
20 water to be treated may also contain metal ions, such as Mg
ions.
First Reference Embodiment
Fig. 1 is a schematic diagram of a water treatment system
according to the first reference embodiment of the present
25 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. In
the water treatment system 1 of this reference embodiment,
depending on the properties of the water to be treated, the
30 number of water treatment sections may be one, and it is also
possible that three or more water treatment sections are
connected.
Each water treatment section includes, from the upstream
13
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 connected to the first
5 crystallizing sections 20a and 20b, respectively. The water
treatment section includes a first scale inhibitor supplying
section 30 (30a, 30b) and a first pH adjusting section 40
(40a, 40b) in the flow path on the upstream side of each first
demineralizing section 10 (10a, 10b).
10 The first scale inhibitor supplying section 30 (30a, 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.
15 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
20 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 exceeding of the
25 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 aggregation), such as deposited crystals.
30 Examples of calcium scale inhibitors include phosphonicacid-
based scale inhibitors, polycarboxylic-acid-based scale
inhibitors, and mixtures thereof. A specific example is
FLOCON260 (trade name, manufactured by BWA).
In the case where Mg ions are contained in the water to
14
be treated, a scale inhibitor that inhibits the deposition of
scales containing magnesium (e.g., magnesium hydroxide) in the
water to be treated can be used. It will be hereinafter
referred to as "magnesium scale inhibitor".
5 Examples of magnesium scale inhibitors include
polycarboxylic-acid-based scale inhibitors, etc. A specific
example is FLOCON 295N (trade name, manufactured by BWA).
In the case where silica is contained in the water to be
treated, a scale inhibitor that inhibits the deposition of
10 silica as scales in the water to be treated can be used. It
will be hereinafter referred to as a "silica scale inhibitor".
Examples of silica scale inhibitors include phosphonicacid-
based scale inhibitors, polycarboxylic-acid-based scale
inhibitors, and mixtures thereof. A specific example is
15 FLOCON260 (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 supplying
20 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
section 42 (42a, 42b), and a pH meter 43 (43a, 43b). The tanks
25 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 below-mentioned
crystallizing step, and thus the amount of ions that reach the
30 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.
In Fig. 1, the first demineralizing sections 10a and 10b
are reverse osmosis membrane devices. In addition, the first
15
demineralizing sections 10a and 10b may also be
electrodialyzers (ED), electro dialysis reversal devices
(EDR), electro de-ionization devices (EDI), ion-exchange
equipments (IEx), capacitive de-ionization devices (CDI),
5 nanofilters (NF), evaporators, etc.
Here, in a nanofilter (NF), an electrodialyzer (ED), an
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
10 selectively removed, while monovalent ions such as Na+ and Clpermeate.
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
15 reduction of pump power, etc.) can 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 removed.
20 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 in
series in the flow direction of the water to be treated.
25 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
first crystallizing tanks 21a and 21b, respectively. The first
30 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, respectively. The
seed crystal tanks 23a and 23b store gypsum particles as seed
16
crystals.
In the water treatment system 1 of Fig. 1, a first
precipitating section 50 (50a, 50b) may be installed on the
downstream side of each of the first crystallizing sections
5 20a and 20b. The first precipitating sections 50a and 50b each
include a first precipitating tank 51 (51a, 51b) and a 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
10 to be treated of the first crystallizing section 20b located
on the most downstream.
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) ,
15 an electro dialysis reversal device (EDR), an electro deionization
device (EDI), an ion-exchange equipment, a
capacitive de-ionization device (CDI), a nanofilter (NF), an
evaporator, etc.
In the water treatment system 1, a precipitating tank 71
20 and a filtration device 72 are installed as a first upstream
side precipitating section 70 on the upstream side 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
25 filtration device 72 have the same configuration as the first
precipitating tank 51 and the first filtration 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
30 section can be configured such that two or more precipitating
tanks 71 are connected in series in the 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 side of
17
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 the water to be
5 treated of the first deaerating section 73, a pH adjusting
section (not shown) that adjusts the water to be treated to a
pH at which carbonate ions are present in the form of C02 may
be installed.
The first deaerating section 73 may also be installed on
10 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 the
15 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 of the
20 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,
because ions are further concentrated in the first
demineralizing section 10a, the resulting gypsum concentration
18
is even higher. In this case, the loading of a large amount of
calcium scale inhibitor is required. Further, the
concentration of gypsum may become too high for the calcium
scale inhibitor to exert its effect, resulting in the
5 production of scales in the first demineralizing section 10a.
Thus, in the case where gypsum in the raw water (water 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
10 provided on the upstream of the first scale inhibitor
supplying section 30a and the first pH adjusting 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.
15 A process for treating water to be treated using the
water treatment system 1 of the first reference embodiment
will be described hereinafter.
First, the deposition behaviors of gypsum, silica, and
calcium carbonate in water will be explained. Fig. 2 shows
20 simulation results for the pH dependency of the amount of
gypsum deposited. Fig. 3 shows simulation results for the pH
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
25 simulation software manufactured by OLI, the simulation was
performed under the conditions where 0.1 mol/L of each 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 amount
30 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 deposition
has no pH dependency, and deposition is possible over the
19
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. From Fig. 4, silica
5 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
the first upstream side precipitating section 70, a step of
10 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.

15 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 of
the water to be treated.
20 {Chemical Formula 1}
C02ZH2C03ZHCOi+H+ZCO^+2H+ - • -(1)
In the case where the pH is as low as 6.5 or less, it is
mainly present as HC03- and C02 in the water to be treated.
The water to be treated containing C02 flows into the
25 first deaerating section 73. C02 is removed from the water to
be treated in the first deaerating section 73. When the 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.
30 The water to be treated, whose carbonate ion
20
concentration has been reduced in the first deaerating step,
is fed to the first upstream side precipitating section 70.

In the first upstream side precipitating section 70, some
5 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 first upstream
side precipitating section 70, some of the metal ions are
10 previously removed from the water to be treated as a metal
compound having low solubility in water. This metal compound
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
15 Mechatronics Systems, Ltd., trade name: Hishifloc H305) are
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
20 is low in this pH range. When calcium carbonate is
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 more
acidic pH leads to a higher solubility of metal ions in water.
25 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 precipitating tank 71,
resulting in precipitation at the bottom of the precipitating
tank 71.
30 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, and
thus are components that tend to be deposited as scales.
21
Mg(0H)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
reference embodiment, the pH of the water to be treated in the
5 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 deposited from the
10 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.
15 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 of scales
20 in devices and steps on the downstream side, particularly the
first demineralizing section 10a and the first demineralizing
step, can be inhibited.
In the case where two or more stages of precipitating
tanks 71 are provided, Mg ions in the water to be treated can
25 be reliably removed, and the concentration of Mg ions in the
water to be treated fed to the downstream side can be reduced.
The supernatant in the precipitating tank 71, which is
the water to be treated, is discharged from the precipitating
tank 71. FeC13 is added to the discharged water to be treated,
30 and solids in the supernatant, such as 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.
22
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.
5 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 the ion-
10 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
15 water to be treated in the upstream side crystallizing section
installed immediately after the filtration device 72, and
gypsum is crystallized, thereby reducing the 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
20 demineralizing section 10a.

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
25 to be treated from the tank 31a. The control section 32a
adjusts the opening of the valve Via so that the 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
30 be treated, a magnesium scale inhibitor is supplied to the
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
23
in the tank of each of two or more first scale inhibitor
supplying sections, and each control section adjusts the
amounts of calcium scale inhibitor and magnesium scale
inhibitor to be supplied.
5
The control section 42a of the first pH adjusting section
40a controls the pH of the water to be treated at the entrance
of the first demineralizing section 10a to such a value that
silica is soluble in the water to be treated. Specifically,
10 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.
The pH meter 43a measures the pH of the water to be
treated at the entrance of the first demineralizing section
15 10a. The control section 42a adjusts the opening of the 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.

20 In the first demineralizing section 10a, the pH-adjusted
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 and scale
25 inhibitors contained in the water to be treated cannot pass
through the reverse osmosis membrane. Therefore, on the nonpermeate
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 capacitive
30 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).
24
As shown in Fig. 4, as a result of the first
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
5 carbonate in the first concentrated water are concentrated to
the saturation concentration or higher, the 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
10 first concentrated water increases as a result of the first
demineralizing step. However, the production of scales
containing magnesium is suppressed by the magnesium scale
inhibitor.
The first concentrated water is fed toward the first
15 crystallizing section 20a.

The first concentrated water discharged from the first
demineralizing section 10a is stored in the first
crystallizing tank 21a of the first crystallizing section 20a.
20 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 first
crystallizing tank 21a from the tank 23a.
The pH of the first concentrated water from the first
25 demineralizing section 10a is 10 or more. As mentioned above,
gypsum is in the state of being dissolved in water in a highpH
region where a calcium scale inhibitor is present. However,
when seed crystals are sufficiently present, even when a scale
inhibitor is present, gypsum is crystallized using the seed
30 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 urn or more, more preferably 20 urn or
more) is precipitated at the bottom of the first crystallizing
25
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
in the state of being dissolved in the first concentrated
5 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 floating
matters in a colloidal form or the like and unlikely to be
10 precipitated.
With reference to Fig. 3, calcium carbonate tends to be
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
15 addition, in the case where the first upstream side
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
20 using the seed crystals of gypsum as nuclei.
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
25 with changing the pH of simulated water in the case where a
scale inhibitor (FLOCON260) is added to simulated water
(containing Ca2+, S042-, Na+, and C1-) in which gypsum is
supersaturated. The experimental conditions are as follows:
The degree of gypsum supersaturation in simulated water
30 (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.
26
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,
5 AA-7000), and the degree of supersaturation was 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
seed crystals are absent, the crystallization rate increases
10 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 relation
of the crystallization rate is such that the crystallization
rate increases with a decrease in pH as shown in Fig. 5.
15 In the case where carbonate ions are contained in the
water to be treated, under low-pH conditions, carbonate ions
are removed from the water to be treated as C02 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
20 dissolved state.
From these results, when the first crystallizing step 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
25 crystallizing tank 21a. In the case where the first
crystallizing step is performed at low pH, a third pH
adjusting section (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
30 and the first crystallizing tank 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
27
chemicals (acid or alkali) . The use of an acid or an alkali
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
5 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 first
demineralizing step and the first crystallizing step.
10 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 inhibitor (FLOCON260)
is added to simulated water. The experimental conditions were
15 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 (Condition
3), 3 g/L (Condition 5), 6 g/L (Condition 6), 3 g/L (Condition
20 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 7, seed
crystals pre-immersed in the above scale inhibitor were added
25 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 each
condition was measured by the same technique as in Fig. 5. In
30 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
Condition 3 where seed crystals are not added, the degree of
supersaturation decreases to 199% (Condition 5) and 176%
28
(Condition 6) with an increase in the concentration of seed
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
5 loading of seed crystals. Condition 5 and Condition 7 are 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
10 supersaturation is 199%, and it has been confirmed that 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
15 scale inhibitor is reduced.
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,
20 gypsum having a larger size was deposited than under 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
jam or more, preferably 20 um or more, the resulting gypsum has
25 a sufficiently reduced water content. The "average particle
diameter" in the present invention is a particle diameter
measured by the method specified in JIS Z 8825 (laser
diffractometry).

30 The supernatant (first concentrated water) in the first
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
29
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 carbonate
5 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. FeC13 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, aggregate with
15 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 to be
25 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 from
30 the treated water in demineralizing sections located on the
downstream side (the first demineralizing section 10b and the
downstream side demineralizing section 60).
In the case where silica is removed, at least one of seed
crystals of silica and a precipitant for silica is supplied
30
into the first concentrated water in the first precipitating
section 50a from a supply section (not shown) . 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]),
5 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 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
10 the case where MgS04 is used as a precipitant for silica,
magnesium silicate is deposited. The crystallized silica or
the crystallized magnesium silicate is precipitated at the
bottom of the first precipitating tank 51a and discharged from
the bottom of the first precipitating tank 51a.
15 In the case where Mg ions are contained in the water to
be treated, Mg ions react with silica in the first
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
20 the content of silica in the first concentrated water in the
first precipitating tank 51a.
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
25 precipitation with silica. In order to remove an excess of
silica that is not consumed by precipitation with Mg ions, a
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 ions in the
30 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
first precipitating step has a higher concentration of Mg ions
relative to the silica content, Mg ions remain as a result of
31
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 tank 51a,
scales containing Mg may be deposited in demineralizing
5 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).
Thus, the first concentrated water in the first
10 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
concentration of Mg ions in the first concentrated water in
15 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 pH
of less than 10. Accordingly, the deposition of scales
20 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
section of the previous stage flows into the water treatment
25 section of the subsequent stage as water to be treated. In the
water treatment section of the subsequent stage, the steps
from the first scale inhibitor supplying step to the first
precipitating step mentioned above are performed.

30 The concentrated water (first concentrated water) that
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
32
has passed through the downstream side demineralizing section
60 is recovered as treated water. The concentrated water in
the downstream side demineralizing section 60 is discharged
out of the system. The installation of the downstream side
5 demineralizing section 60 makes it possible to further recover
treated water from water that has been treated in a watertreatment
section. Accordingly, the water recovery is
improved.
In the water treatment system 1 of this reference
10 embodiment, ions are concentrated in the first demineralizing
section 10. However, gypsum, calcium carbonate, silica, etc.,
have been removed in the first crystallizing section, the
first precipitating section, etc. Accordingly, the water
flowing into the downstream side demineralizing section 60 has
15 a smaller number of moles of ions than before the treatment.
Accordingly, the osmotic pressure is low in the first
demineralizing section 10b or the downstream side
demineralizing section 60 located downstream, and the required
power is reduced.
20 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 evaporator,
water is evaporated from the concentrated water, and ions
contained in the concentrated water are deposited as a solid
25 and recovered as a solid. Because water is 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.
30 First Embodiment
Fig. 9 is a schematic diagram of a water treatment system
of the first embodiment of the present invention. In Fig. 9,
the same configurations as in the first reference embodiment
33
are indicated with the same reference numerals. In the water
treatment system 100 of the first embodiment, a first
separating section 180 (180a, 180b) is installed on the
downstream side of the first crystallizing sections 20a and
5 20b. The water 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 embodiment, depending on the
properties of the water to be treated, the number of water
10 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 are liquid
15 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
20 direction of the water to be treated.
In the water treatment system 100 of the first
embodiment, the water to be treated is treated through the
same steps as in the first reference embodiment, except that
the first separating step is performed immediately after the
25 first crystallizing step.

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
30 is water containing solid matters deposited in the first
crystallizing tanks 21a and 21b.
The first concentrated water discharged from the first
crystallizing tanks 21a and 21b contains gypsum having various
34
particle diameters, as well as calcium carbonate and silica
deposited due to the exceeding of the saturation
concentration. Because the deposition of calcium carbonate and
silica has taken place in the absence of seed crystals, they
5 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 size,
for example, gypsum having an average particle diameter of 10
10 urn 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 classifiers 181a and 181b is
further dehydrated by the dehydrators 182a and 182b and
15 recovered. The supernatant containing gypsum having a small
particle diameter, calcium carbonate, and silica is fed to the
first precipitating sections 50a and 50b.
In this embodiment, seed crystals are added to cause
crystallization. Therefore, gypsum having an average particle
20 diameter of 10 um or more is mainly 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 with high recovery.
25 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.
[First Example of Second Embodiment]
Water that is an object to be treated in the present
30 invention (water to be treated) contains Ca2+, S04
2~, and
carbonate ions. Specifically, the water to be treated (raw
water) is saline water, sewage, industrial waste water,
blowdown water from a cooling tower, or the like. The water to
35
be treated may also contain metal ions, such as Mg ions.
Fig. 10 is a schematic diagram of a water treatment
system of the first example of the second embodiment of the
present invention. The water treatment system 201 of Fig. 10
5 is configured such that two water treatment sections are
connected in the flow 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
10 connected.
In the water treatment system 201 of the first example of
the second 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
15 crystallizing section 220 (220a, 220b). The concentration
sides of the second demineralizing sections 210a and 210b 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
20 path on the upstream side of each second demineralizing
section 210 (210a, 210b).
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
25 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 store a scale
inhibitor.
The scale inhibitor used in the first example of this
30 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
nucleation of gypsum or calcium carbonate in the water to be
36
treated. At the same time, it 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 exceeding of the saturation
5 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 aggregation), such as deposited crystals.
10 Examples of calcium scale inhibitors include phosphonicacid-
based scale inhibitors, polycarboxylic-acid-based scale
inhibitors, and mixtures thereof. A specific example is
FLOCON260 (trade name, manufactured by BWA).
Fig. 10 shows tanks 231a and 231b. The tanks 231a and
15 231b store a calcium scale inhibitor.
In Fig. 10, the second demineralizing sections 210a and
210b are reverse osmosis membrane devices. In addition, the
second demineralizing sections 210a and 210b may also be
electrodialyzers (ED), electro dialysis reversal devices
20 (EDR), electro de-ionization devices (EDI), ion exchangers
(IEx), capacitive de-ionization devices (CDI), nanofilters
(NF), evaporators, etc.
Here, in a nanofilter (NF), an electrodialyzer (ED), an
electro dialysis reversal device (EDR), an electro de-
25 ionization device (EDI), and a capacitive de-ionization device
(CDI), scale components (divalent ions, Ca2+, Mg2+, etc.) are
selectively removed, while monovalent ions such as Na+ and Cl~
permeate. The use of these demineralizers suppresses an
increase in the ion concentration of ions that serve as scale
30 components in concentrated water. Accordingly, the water
recovery can be improved, and also energy saving (e.g., the
reduction of pump power, etc.) can be achieved.
In addition, in the case where the water to be treated is
blowdown water from a cooling tower, the reclaimed water does
37
not have to be pure water, and what is necessary is that scale
components (divalent ions, Ca2+, Mg2+, etc.) are removed.
Accordingly, it is advantageous to use a nanofilter (NF), etc.
Although only one second demineralizing section 210a/210b
5 is 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 a
10 second seed crystal supplying section 222 (222a, 222b). The
second seed crystal supplying sections 222a and 222b are
connected to the second crystallizing tanks 221a and 221b,
respectively. The second seed crystal supplying sections 222a
and 222b have a seed crystal tank 223 (223a, 223b), a valve V5
15 (V5a, V5b), and a control section 224 (224a, 224b). The
control sections 224a and 224b are connected to the valves V5a
and V5b, respectively. The seed crystal tanks 223a and 223b
store gypsum particles as seed crystals.
In the water treatment system 201 of Fig. 10, a second
20 precipitating section 250 (250a, 250b) may be installed on the
downstream side of the second crystallizing sections 220a and
220b. The second precipitating sections 250a and 250b each
include a second precipitating tank 251 (251a, 251b) and a
second filtration device 252 (252a, 252b).
25 The water treatment system 201 includes a downstream side
demineralizing section 60 on the downstream side of the water
to be treated of the second crystallizing section 220b located
on the most downstream.
In Fig. 10, the downstream side demineralizing section 60
30 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 deionization
device (EDI), an ion exchanger, a capacitive deionization
device (CDI), a nanofilter (NF), an evaporator,
38
etc.
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. In the evaporator,
5 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 recovered on the
upstream side of the evaporator, and the amount of
concentrated water significantly decreases, the evaporator can
10 be reduced in size, and the energy required for evaporation
can be reduced.
In the water treatment system 201 of the first example of
the second embodiment, a second pH adjusting section 240
(240a, 240b) may be installed between the second
15 demineralizing section 210 and the second crystallizing
section 220. The second pH 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
20 may be hydrochloric acid, sulfuric acid, nitric acid, or the
like, 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
25 section 242 is connected to the valve V6 and the pH meter 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.
30 In the water treatment system 201, a precipitating tank
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
39
upstream side precipitating section 270 has the same
configuration as the first precipitating tank 251 and the
first filtration device 252 of the first precipitating section
250.
5 In particular, in the case where Mg ions are contained in
the water to be treated, the second upstream side
precipitating section 270 can be configured such that two or
more precipitating tanks 271 are connected in series in the
flow direction of the water to be treated.
10 In the water treatment system 201, a second deaerating
section 273 may be provided on the upstream side of the second
upstream side precipitating section 270 as shown in Fig. 10.
Specifically, the second deaerating section 273 is a
deaeration tower equipped with a filler for removing carbon
15 dioxide or is a separation membrane. On the upstream side of
the water to be treated of the second deaerating section 273,
a pH adjusting section (not shown) that adjusts the water to
be treated to a pH at which carbonate ions are present in the
form of C02 may be installed.
20 The second deaerating 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 230a.
It is also possible that a deaerating section having the
25 same configuration as the second deaerating section 273 is
installed in the flow path between the second demineralizing
section 210 and the second crystallizing section 220, in the
flow path between the second crystallizing section 220 and the
second precipitating section 250, and on the downstream side
30 of the second precipitating section 250 and in the flow path
between it and the second demineralizing section 210b 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 exchanger (not shown) may
40
be installed on the downstream of the filtration device 272
and on the upstream of the second scale inhibitor supplying
section 230a located on the most upstream. The ion exchanger
may be an ion-exchange resin column or an ion-exchange
5 membrane device, for example.
When gypsum in the water to be treated flowing into the
second demineralizing section 210a is already supersaturated,
because ions are further concentrated in the second
demineralizing section 210a, the resulting gypsum
10 concentration is even higher. In this case, the loading of a
large amount of calcium scale inhibitor is required. 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 second demineralizing section
15 210a.
Thus, in the case where gypsum in the raw water (water to
be treated) is supersaturated, it is possible that an upstream
side crystallizing section (not shown) having the same
configuration as the second crystallizing sections 221a and
20 221b is provided on the upstream of the second scale inhibitor
supplying section 230a on the most upstream, so that the
concentration of gypsum is reduced, and then the water to be
treated is fed to the second demineralizing section 210a.
In the first example of this embodiment, a second
25 separating section 280 (280a, 280b) is 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 281 (281a, 281b) and a dehydrator 282
30 (282a, 282b). The classifiers 281a and 281b are liquid
cyclones, for example. The dehydrators 282a and 282b are belt
filters, for example.
Although the second separating section 280 has only one
classifier installed in Fig. 10, it is also possible that two
41
or more classifiers are connected in series in the flow
direction of the water to be treated.
A process for treating water to be treated using the
water treatment system 201 of the first example of the second
5 embodiment will be described hereinafter.
First, the deposition behaviors of gypsum 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
10 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 solid
15 component was mixed with water, and H2SO4 and Ca(OH)2 were
added as an acid and an alkali, respectively.
From Fig. 2, it can be understood that gypsum deposition
has no pH dependency, and deposition is possible over the
entire pH range. However, when a calcium scale inhibitor is
20 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.

In the case where the water to be treated is industrial
25 waste water, etc., before the water to be treated flows into
the second upstream side precipitating section 270, 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
30 performed.
Oecond Deaerating Step>
In the water treatment system 201 of Fig. 10, the water
42
to be treated before flowing into the second deaerating
section 273 is adjusted to a low pH. Carbonic acid in the
water to be treated is in the following equilibrium depending
on the pH of the water to be treated.
5 [Chemical Formula 2]
C02ZH2C03tHCO^+H+ZCO^ + 2H+ • • • (1)
In the case where the pH is as low as 6.5 or less, it is
mainly present as HCC>3~ and C02 in the water to be treated.
The water to be treated containing CO2 flows into the
10 second deaerating section 273. C02 is removed from the water
to be treated in the second deaerating section 273. When the
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.
15 The water to be treated, whose carbonate ion
concentration has been reduced in the second deaerating step,
is fed to the second upstream side precipitating section 270.

In the second upstream side precipitating section 270, Ca
20 ions and carbonate ions are previously roughly 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 precipitating
section 270, the metal ions are previously roughly removed
25 from the water to be treated as a metal compound having low
solubility in water. This metal compound is mainly a metal
hydroxide, but may also include a carbonate.
In the precipitating tank 271, Ca(OH)2 and an anionic
polymer (manufactured by Mitsubishi Heavy Industries
30 Mechatronics Systems, Ltd., trade name: Hishifloc H305) are
43
loaded to the water to be treated, and the pH in the
precipitating tank 271 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
5 is low in this pH range. When calcium carbonate is
supersaturated, calcium carbonate is deposited and
precipitated at the bottom of the precipitating tank 271.
The solubility of a metal compound depends on pH. A more
acidic pH leads to a higher solubility of metal ions in water.
10 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 precipitating tank 271,
resulting in precipitation at the bottom of the precipitating
tank 271.
15 The precipitated calcium carbonate and metal compound are
discharged from the bottom of the precipitating tank 271.
Mg ions form salts that are poorly soluble in water, and
thus are components that tend to be deposited as scales.
Mg(OH)2 is deposited at pH 10 or more.
20 In the case where the water to be treated containing Mg
ions is treated by the water treatment system 201 of the first
example of this embodiment, the pH of the water to be treated
is adjusted to a pH at which a magnesium compound (mainly
magnesium hydroxide) is deposited in the second upstream side
25 precipitating section 270. 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 deposited from the water to be treated,
precipitated at the bottom of the precipitating tank 271, and
30 removed. As a result, Mg ions in the water to be treated are
roughly removed, resulting in a decrease in the concentration
of Mg ions in the water to be treated.
In the above case, it is preferable that the water to be
treated after being discharged from the second upstream side
44
precipitating section 270 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 of scales
in devices and steps on the downstream side, particularly the
5 second demineralizing section 210 and the second
demineralizing step, can be inhibited.
In the case where two or more stages of precipitating
tanks 271 are provided, Mg ions in the water to be treated can
be reliably removed, and the concentration of Mg ions in the
10 water to be treated fed to the downstream side can be reduced.
The supernatant in the precipitating tank 271, which is
the water to be treated, is discharged from the precipitating
tank 271. FeCl3 is added to the discharged water to be
treated, and solids in the supernatant, such as calcium
15 carbonate and a metal compound, aggregate with Fe(OH)3.
The water to be treated is fed to the filtration device
272. The solids aggregated with Fe(OH)3 are removed through
the filtration device 272.
In the case where the second deaerating step is performed
20 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 or less.
Depending on the properties of the water to be treated,
the second deaerating step and the second upstream side
25 precipitating step may be omitted.
In the water treatment system 201 of the first example of
the second embodiment, in the case where an ion-exchange
membrane device is installed, Ca ions in the water to be
treated are removed by the ion exchanger. In the case where Mg
30 ions are contained in the water to be treated, the Mg ions are
also removed by the ion exchanger.
In the case where gypsum in the raw water is
supersaturated, seed crystals of gypsum are loaded to the
water to be treated in the upstream side crystallizing section
45
installed immediately after the filtration device 272, and
gypsum is crystallized, thereby reducing the concentration of
gypsum in the water to be treated. The water to be treated
having a reduced concentration of gypsum is fed to the second
5 demineralizing section 210a.

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
10 to be treated from the tank 231a. The control section 232a
adjusts the opening of the valve V4a so that the 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
15 be treated, a magnesium scale inhibitor is supplied to the
water to be treated in the second 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 second scale inhibitor
20 supplying sections, and each control section adjusts the
amounts of calcium scale inhibitor and magnesium scale
inhibitor to be supplied.
In the water treatment system 201 of the first example of
the second embodiment, the pH adjustment of the water to be
25 treated immediately before flowing into the second
demineralizing section 210 is optionally performed.
For example, in the configuration of Fig. 10, as a result
of the addition of FeCl3, the water to be treated is adjusted
to about pH 5 or 6 and then flows into the second
30 demineralizing section 210a. As shown in Fig. 3, the
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 water to be
46
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.
5 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 second pH adjusting
section is installed on the upstream of the second
10 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
be treated containing the scale inhibitors is treated. In the
15 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. Ions
and scale inhibitors contained in the water to be treated
cannot pass through the reverse osmosis membrane. Therefore,
20 on the non-permeate side of the reverse osmosis membrane,
there is concentrated water having a high concentration of
ions (second concentrated water).
Even in the case where gypsum and calcium carbonate in
the second concentrated water are concentrated to the
25 saturation concentration or higher as a result of the
treatment in the second demineralizing section 210a, the
production of scales is suppressed by the calcium scale
inhibitor.
In the case where Mg ions are contained in the water to
30 be treated, the concentration of Mg ions contained in the
second concentrated water increases as a result of the second
demineralizing step. However, the production of scales
containing magnesium is suppressed by the magnesium scale
47
inhibitor.
Also in the case where other demineralizers, such as a
capacitive de-ionization device, are used, for example, the
water to be treated is separated into treated water and
5 concentrated water having a high concentration of ions (second
concentrated water). The second concentrated water is fed
toward the second crystallizing section 220a.

In the first example of this embodiment, the pH of the
10 water to be treated (second concentrated water) may be
adjusted by the second pH adjusting section 240a between the
second demineralizing section 210a and the second
crystallizing section 220a.
The second pH adjusting section 240a controls the pH of
15 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. The pH
meter 243a measures the pH of the second concentrated water.
The control section 242a adjusts the opening of the valve V6a
20 so that the value measured by the pH meter 243a is a
predetermined pH control value.

The second concentrated water discharged from the second
demineralizing section 210a is stored in the second
25 crystallizing tank 221a of the second crystallizing section
220a. The control section 224a of the second seed crystal
supplying section 222a opens the valve V5a and adds seed
crystals of gypsum from the seed crystal tank 223a to the
second concentrated water in the second crystallizing tank
30 221a.
The pH of the second concentrated water from the second
demineralizing section 210a is 10 or more. As mentioned above,
gypsum is in the state of being dissolved in water in a high-
48
pH region where a calcium scale inhibitor is present. 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 201 of Fig.
5 10, the crystal-grown gypsum having a large diameter (e.g.,
having a particle diameter of 10 urn or more, more preferably
20 |am or more) is precipitated at the bottom of the second
crystallizing tank 221a. The precipitated gypsum is discharged
from the bottom of the second crystallizing tank 221a.
10 With reference to Fig. 3, calcium carbonate tends to be
deposited at pH 10 or more. However, because the calcium scale
inhibitor has been added, the deposition of calcium carbonate
is suppressed in the second crystallizing tank 221a. In
addition, in the case where the second upstream side
15 precipitating section or the second deaerating section is
provided, the concentration of calcium carbonate has been
previously reduced. As a result, in the second crystallizing
tank 221a, calcium carbonate is unlikely to be crystallized
using the seed crystals of gypsum as nuclei.
20 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 case where a
25 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
(25°C) : 460%,
30 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.
49
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,
5 AA-7000) , and the degree of supersaturation was 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
seed crystals are absent, the crystallization rate increases
10 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 relation
of the crystallization rate is such that the crystallization
rate increases with a decrease in pH as shown in Fig. 5.
15 In the case where carbonate ions are contained in the
water to be treated, under low-pH conditions, carbonate ions
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
20 dissolved state.
From these results, when the second crystallizing step is
performed under low-pH conditions, because of the low content
of calcium carbonate, high-purity gypsum is crystallized and
recovered from the bottom of the second crystallizing tank
25 221a. In the case where the second crystallizing step is
performed at low pH, a third pH adjusting section (not shown)
that supplies an acid as a pH adjuster is installed in the
second crystallizing tank 221a or in the flow path between the
second demineralizing section 210a and the second
30 crystallizing tank 221a. The pH adjusting section has the same
configuration as the 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

CLAIMS
1. A water treatment process comprising:
a demineralizing step of separating water to be
5 treated containing Ca ions, S04 ions, and carbonate ions
into concentrated water in which the Ca ions, the S04 ions,
and the carbonate ions are concentrated and treated water;
a crystallizing step of supplying seed crystals of
gypsum to the concentrated water so that gypsum is
10 crystallized from the concentrated water; and
a separating step of separating the gypsum from the
concentrated water after the crystallizing step.
2. The water treatment process according to claim 1, wherein
15 among the gypsum deposited in the crystallizing step,
gypsum having a predetermined size is recovered in the
separating step.
3. The water treatment process according to claim 1 or 2,
20 wherein the gypsum separated in the separating step is used
as seed crystals of the gypsum.
4. The water treatment process according to any one of claims
1 to 3,
25 wherein the separating step is carried out a
plurality of times in a flowing direction of the water to
be treated per the crystallizing step,
wherein the size of the gypsum to be separated in
the separating step carried out on a downstream side of
30 the water to be treated is smaller than that in the
separating step carried out on a most upstream of the
water to be treated, and
the gypsum separated in the separating step carried
out on the downstream side of the water to be treated is
89
supplied into the concentrated water in the crystallizing
step.
5. A water treatment system comprising:
5 a demineralizing section that separates water to be
treated containing Ca ions, S04 ions and carbonate ions
into concentrated water in which the Ca ions and the S04
ions are concentrated and treated water;
a crystallizing section which is positioned on a
10 downstream side of the demineralizing section and which
includes a crystallizing tank that crystallizes gypsum
from the concentrated water and a seed crystal supplying
section that supplies seed crystals of gypsum to the
crystallizing tank; and
15 a separating section that is positioned on a
downstream side of the crystallizing section and separates
the gypsum from the concentrated water.
6. The water treatment system according to claim 5, wherein
20 the separating section separates the gypsum having a
predetermined size, followed by recovery.
7. The water treatment system according to claim 5 or 6,
wherein the gypsum separated in the separating section is
25 used as seed crystals of the gypsum.
8. The water treatment system according to any one of claims 5
to 7, wherein the separating section includes a plurality
of classifiers in a flowing direction of the water to be
30 treated per the crystallizing section,
wherein the size of the gypsum to be separated in
the classifier located on a downstream side of the water
to be treated is smaller than in the classifier located on
a most upstream of the water to be treated, and
90
the gypsum separated in the classifier located on
the downstream side of the water to be treated is supplied
to the concentrated water in the crystallizing section.
A water treatment process comprising:
a first scale inhibitor supplying step of supplying
a calcium scale inhibitor which is a scale inhibitor for
inhibiting the deposition of a scale containing calcium to
water to be treated containing Ca ions, S04 ions, carbonate
ions and silica;
a first pH adjusting step of adjusting the water to
be treated to a pH at which the silica is soluble in the
water to be treated;
a first demineralizing step of separating the water
to be treated into first concentrated water in which the
Ca ions, the S04 ions, the carbonate ions and the silica
are concentrated and treated water after the first scale
inhibitor supplying step and the first pH adjusting step;
a first crystallizing step of supplying seed
crystals of gypsum to the first concentrated water so that
gypsum is crystallized from the first concentrated water;
and
a first separating step of separating the gypsum
from the first concentrated water after the first
crystallizing step.
The water treatment process according to claim 9,
comprising, after the first crystallizing step:
a second scale inhibitor supplying step of supplying
the calcium scale inhibitor and a silica scale inhibitor
which is a scale inhibitor for inhibiting the deposition
of the silica to the water to be treated;
a second demineralizing step of separating the water
to be treated into second concentrated water in which the
91
Ca ions, the S04 ions, the carbonate ions and the silica
are concentrated and treated water after the second scale
inhibitor supplying step; and
a second crystallizing step of supplying seed
5 crystals of gypsum to the second concentrated water so
that gypsum is crystallized from the second concentrated
water.
11. The water treatment process according to claim 10,
10 comprising a second separating step of separating the
gypsum from the second concentrated water after the second
crystallizing step.
The water treatment process according to claim 9, wherein
among the gypsum deposited in the first crystallizing step,
gypsum having a predetermined size is recovered in the
first separating step.
The water treatment process according to claim 11, wherein
among the gypsum deposited in the second crystallizing
step, gypsum having a predetermined size is recovered in
the second separating step.
14. The water treatment process according to claim 9, wherein
25 the gypsum separated in the first separating step is used
as seed crystals of the gypsum.
15. The water treatment process according to claim 11, wherein
the gypsum separated in the second separating step is used
30 as seed crystals of the gypsum.
16. The water treatment process according to claim 9 or 12,
wherein the first separating step is carried out a
plurality of times in a flowing direction of the water to
92
12.
15
13.
20
be treated per the first crystallizing step,
wherein the size of the gypsum to be separated in
the first separating step carried out on a downstream side
of the water to be treated is smaller than that in the
first separating step carried out on a most upstream of
the water to be treated, and
the gypsum separated in the first separating step
carried out on the downstream side of the water to be
treated is supplied into the first concentrated water in
the first crystallizing step.
The water treatment process according to claim 11 or 13,
wherein the second separating step is carried out a
plurality of times in a flowing direction of the water to
be treated per the second crystallizing step,
wherein the size of the gypsum to be separated in
the second separating step carried out on a downstream
side of the water to be treated is smaller than that in
the second separating step carried out on a most upstream
of the water to be treated, and
the gypsum separated in the second separating step
carried out on the downstream side of the water to be
treated is supplied into the second concentrated water in
the second crystallizing step.
The water treatment process according to claim 10,
comprising a third pH adjusting step of adjusting the
second concentrated water to a pH at which calcium
carbonate is soluble,
wherein in the second crystallizing step, the gypsum
separated in the first separating step is supplied into
the second concentrated water after the adjustment of the
pH in the third pH adjusting step.
93
A water treatment process, comprising:
a second scale inhibitor supplying step of supplying
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, S04 ions, carbonate ions and
silica;
a second demineralizing step of separating the water
to be treated into second concentrated water in which the
Ca ions, the SO4 ions, the carbonate ions and the silica
are concentrated and treated water after the second scale
inhibitor supplying step;
a second crystallizing step of supplying seed
crystals of gypsum to the second concentrated water so
that gypsum is crystallized from the second concentrated
water; and
a second separating step of separating the gypsum
from the second concentrated water after the second
crystallizing step.
The water treatment process according to claim 32,
comprising, after the second crystallizing step:
a first scale inhibitor supplying step of supplying
the calcium scale inhibitor to the water to be treated;
a first pH adjusting step of adjusting the water to
be treated to a pH at which the silica is soluble in the
water to be treated;
a first demineralizing step of separating the water
to be treated into first concentrated water in which the
Ca ions, the S04 ions, the carbonate ions and the silica
are concentrated and treated water after the first scale
inhibitor supplying step and the first pH adjusting step;
and
94
a first crystallizing step of supplying seed
crystals of gypsum to the first concentrated water so that
gypsum is crystallized from the first concentrated water.
5 21. The water treatment process according to claim 20,
comprising a first separating step of separating the gypsum
from the first concentrated water after the first
crystallizing step.
10 22. The water treatment process according to claim 19,
comprising a second separating step of separating the
gypsum from the second concentrated water after the second
crystallizing step.
15 23. The water treatment process according to claim 21, wherein
among the gypsum deposited in the first crystallizing step,
gypsum having a predetermined size is recovered in the
first separating step.
20 24. The water treatment process according to claim 19, wherein
the gypsum separated in the second separating step is used
as seed crystals of the gypsum.
25. The water treatment process according to claim 21, wherein
25 the gypsum separated in the first separating step is used
as seed crystals of the gypsum.
26. The water treatment process according to claim 19 or 22,
wherein the second separating step is carried out a
30 plurality of times in a flowing direction of the water to
be treated per the second crystallizing step,
wherein the size of the gypsum to be separated in
the second separating step carried out on a downstream
side of the water to be treated is smaller than that in
95
the second separating step carried out on a most upstream
of the water to be treated, and
the gypsum separated in the second separating step
carried out on the downstream side of the water to be
treated is supplied into the second concentrated water in
the second crystallizing step.
The water treatment process according to claim 21 or 23,
wherein the first separating step is carried out a
plurality of times in a flowing direction of the water to
be treated per the first crystallizing step,
wherein the size of the gypsum to be separated in
the first separating step carried out on a downstream side
of the water to be treated is smaller than that in the
first separating step carried out on a most upstream of
the water to be treated, and
the gypsum separated in the first separating step
carried out on the downstream side of the water to be
treated is supplied into the first concentrated water in
the first crystallizing step.
The water treatment process according to claim 21,
comprising a third pH adjusting step of adjusting the
second concentrated water to a pH at which calcium
carbonate is soluble,
wherein in the second crystallizing step, the gypsum
separated in the first separating step is supplied into
the second concentrated water after the adjustment of the
pH in the third pH adjusting step.
A water treatment system comprising:
a first scale inhibitor supplying section that
supplies a calcium scale inhibitor which is a scale
inhibitor for inhibiting the deposition of a scale
96
containing calcium to water to be treated containing Ca
ions, S04 ions, carbonate ions and silica;
a first pH adjusting section that supplies a pH
adjuster to the water to be treated to adjust the pH of
the water to be treated to such a value that the silica is
soluble in the water to be treated;
a first demineralizing section that is positioned on
a downstream side of the first scale inhibitor supplying
section and the first pH adjusting section and separates
the water to be treated into first concentrated water in
which the Ca ions, the SO4 ions, the carbonate ions and the
silica are concentrated and treated water;
a first crystallizing section which is positioned on
a downstream side of the first demineralizing section and
which includes a first crystallizing tank and crystallizes
gypsum from the first concentrated water and a first seed
crystal supplying section that supplies seed crystals of
gypsum to the first crystallizing tank; and.
a first separating section that is positioned on a
downstream side of the first crystallizing section and
separates the gypsum from the first concentrated water.
The water treatment system according to claim 29,
comprising, on a downstream side of the water to be treated
in the first crystallizing section:
a second scale inhibitor supplying section that
supplies the calcium scale inhibitor and a silica scale
inhibitor which is a scale inhibitor for inhibiting the
deposition of silica to the water to be treated;
a second demineralizing section that is positioned
on a downstream side of the second scale inhibitor
supplying section and separates the water to be treated
into second concentrated water in which the Ca ions, the
SO4 ions, the carbonate ions and the silica are
97
concentrated and treated water; and
a second crystallizing section which is positioned
on a downstream side of the second demineralizing section
and which includes a second crystallizing tank and
5 crystallizes gypsum from the second concentrated water and
a second seed crystal supplying section that supplies seed
crystals of gypsum to the second crystallizing tank.
The water treatment system according to claim 30,
comprising, a second separating section that is positioned
on a downstream side of the second crystallizing section
and separates the gypsum from the second concentrated
water.
15 32. The water treatment system according to claim 29, wherein
the first separating section separates the gypsum having a
predetermined size, followed by recovery.
33. The water treatment system according to claim 31, wherein
20 the second separating section separates the gypsum having a
predetermined size, followed by recovery.
34. The water treatment system according to claim 29, wherein
the gypsum separated in the first separating section is
25 used as seed crystals of the gypsum.
35. The water treatment system according to claim 31, wherein
the gypsum separated in the second separating section is
used as seed crystals of the gypsum.
30
36. The water treatment system according to claim 29 or 32,
wherein the first separating section includes a plurality
of classifiers in a flowing direction of the water to be
treated per the first crystallizing section,
98
31.
10
wherein the size of the gypsum to be separated in
the first classifier located on a downstream side of the
water to be treated is smaller than in the first
classifier located on a most upstream of the water to be
treated, and
the gypsum separated in the first classifier located
on the downstream side of the water to be treated is
supplied to the first concentrated water in the first
crystallizing section.
The water treatment system according to claim 31 or 33,
wherein the second separating section includes a plurality
of classifiers in a flowing direction of the water to be
treated per the second crystallizing section,
wherein the size of the gypsum to be separated in
the second classifier located on a downstream side of the
water to be treated is smaller than in the second
classifier located on a most upstream of the water to be
treated, and
the gypsum separated in the second classifier
located on the downstream side of the water to be treated
is supplied to the second concentrated water in the second
crystallizing section.
The water treatment system according to claim 30, wherein
the second pH adjusting section adjusts the second
concentrated water to a pH at which calcium carbonate is
soluble, and supplies the gypsum separated in the first
separating section to the second crystallizing section.
A water treatment system, comprising:
a second scale inhibitor supplying section that
supplies a calcium scale inhibitor which is a scale
inhibitor for inhibiting the deposition of a scale
99
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;
a second demineralizing section that is positioned
on a downstream side of the second scale inhibitor
supplying section and separates the water to be treated
into second concentrated water in which the Ca ions, the
SO4 ions, the carbonate ions and the silica are
concentrated and treated water;
a second crystallizing section which is positioned
on a downstream side of the second demineralizing section
and which includes a second crystallizing tank and
crystallizes gypsum from the second concentrated water and
a second seed crystal supplying section that supplies seed
crystals of gypsum to the second crystallizing tank; and
a second separating section that is positioned on a
downstream side of the second crystallizing section and
separates the gypsum from the second concentrated water.
The water treatment system according to claim 39,
comprising, on a downstream side of the water to be treated
in the second crystallizing section:
a first scale inhibitor supplying section that
supplies the calcium scale inhibitor to the water to be
treated;
a first pH adjusting section that supplies a pH
adjuster to the water to be treated to adjust the pH of
the water to be treated to such a value that the silica is
soluble in the water to be treated;
a first demineralizing section that is positioned on
a downstream side of the first scale inhibitor supplying
section and the first pH adjusting section and separates
the water to be treated into first concentrated water in
100
which the Ca ions, the S04 ions, the carbonate ions and the
silica are concentrated and treated water; and
a first crystallizing section which is positioned on
a downstream side of the first demineralizing section and
5 which includes a first crystallizing tank and crystallizes
gypsum from the first concentrated water and a first seed
crystal supplying section that supplies seed crystals of
gypsum to the first crystallizing tank.
10 41. The water treatment system according to claim 40,
comprising, a first separating section that is positioned
on a downstream side of the first crystallizing section and
separates the gypsum from the first concentrated water.
15 42. The water treatment system according to claim 39, wherein
the second separating section separates the gypsum having a
predetermined size, followed by recovery.
43. The water treatment system according to claim 41, wherein
20 the first separating section separates the gypsum having a
predetermined size, followed by recovery.
44. The water treatment system according to claim 39, wherein
the gypsum separated in the second separating section is
25 used as seed crystals of the gypsum.
45. The water treatment system according to claim 41, wherein
the gypsum separated in the first separating section is
used as seed crystals of the gypsum.
30
46. The water treatment system according to claim 39 or 42,
wherein the second separating section includes a plurality
of classifiers in a flowing direction of the water to be
treated per the second crystallizing section,
101
wherein the size of the gypsum to be separated in
the second classifier located on a downstream side of the
water to be treated is smaller than in the second
classifier located on a most upstream of the water to be
treated, and
the gypsum separated in the second classifier
located on the downstream side of the water to be treated
is supplied to the second concentrated water in the second
crystallizing section.
The water treatment system according to claim 41 or 43,
wherein the first separating section includes a plurality
of classifiers in a flowing direction of the water to be
treated per the first crystallizing section,
wherein the size of the gypsum to be separated in
the first classifier located on a downstream side of the
water to be treated is smaller than in the first
classifier located on a most upstream of the water to be
treated, and
the gypsum separated in the first classifier located
on the downstream side of the water to be treated is
supplied to the first concentrated water in the first
crystallizing section.
The water treatment system according to claim 41, wherein
the second pH adjusting section adjusts the second
concentrated water to a pH at which calcium carbonate is
soluble, and supplies the gypsum separated in the first
separating section to the second crystallizing section.