U.S. patent application number 14/175309 was filed with the patent office on 2014-06-05 for multi-stage seawater desalination apparatus and operation control method of multi-stage seawater desalination apparatus.
This patent application is currently assigned to MITSUBISHI HEAVY INDUSTRIES, LTD.. The applicant listed for this patent is MITSUBISHI HEAVY INDUSTRIES, LTD.. Invention is credited to Takayoshi Hori, Yoshiaki Ito, Hideo Iwahashi, Katsunori Matsui, Kazuhisa Takeuchi, Kenji Tanaka.
Application Number | 20140151278 14/175309 |
Document ID | / |
Family ID | 42225546 |
Filed Date | 2014-06-05 |
United States Patent
Application |
20140151278 |
Kind Code |
A1 |
Takeuchi; Kazuhisa ; et
al. |
June 5, 2014 |
MULTI-STAGE SEAWATER DESALINATION APPARATUS AND OPERATION CONTROL
METHOD OF MULTI-STAGE SEAWATER DESALINATION APPARATUS
Abstract
A high-pressure pump P.sub.1 that increases the pressure of raw
water, a high-pressure reverse osmosis device including a
high-pressure reverse osmosis membrane for concentrating a salt
content in high-pressure feed water, a first drain valve mounted on
a permeated water line for supplying the permeated water downstream
and temporarily draining permeated water of an initial start-up
from a discharge line, a low-pressure pump that is mounted on a
permeated water line provided downstream of the first drain valve
and reduces the pressure of the permeated water, a low-pressure
reverse osmosis device including a low-pressure reverse osmosis
membrane for concentrating a salt content in low-pressure feed
water, and a second drain valve mounted on a discharge line at the
concentrated water side of the low-pressure reverse osmosis device
that temporarily discharges the low-pressure feed water of the
initial start-up supplied to the low-pressure reverse osmosis
device.
Inventors: |
Takeuchi; Kazuhisa;
(Nagasaki-ken, JP) ; Ito; Yoshiaki; (Nagasaki-ken,
JP) ; Tanaka; Kenji; (Nagasaki-ken, JP) ;
Iwahashi; Hideo; (Nagasaki-ken, JP) ; Matsui;
Katsunori; (Nagasaki-ken, JP) ; Hori; Takayoshi;
(Nagasaki-ken, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MITSUBISHI HEAVY INDUSTRIES, LTD. |
Tokyo |
|
JP |
|
|
Assignee: |
MITSUBISHI HEAVY INDUSTRIES,
LTD.
Tokyo
JP
|
Family ID: |
42225546 |
Appl. No.: |
14/175309 |
Filed: |
February 7, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13131769 |
May 27, 2011 |
8685249 |
|
|
PCT/JP2009/064058 |
Aug 7, 2009 |
|
|
|
14175309 |
|
|
|
|
Current U.S.
Class: |
210/195.2 ;
210/258 |
Current CPC
Class: |
B01D 61/12 20130101;
C02F 1/441 20130101; C02F 2209/03 20130101; B01D 61/025 20130101;
B01D 2311/14 20130101; B01D 61/022 20130101; C02F 2103/08 20130101;
C02F 2301/08 20130101; B01D 2317/025 20130101; Y02A 20/131
20180101 |
Class at
Publication: |
210/195.2 ;
210/258 |
International
Class: |
C02F 1/44 20060101
C02F001/44 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 27, 2008 |
JP |
2008-303218 |
Claims
1. A multi-stage seawater desalination apparatus comprising: a
high-pressure pump that is mounted on a raw water line through
which raw water is supplied, and increases pressure of the raw
water to a predetermined high pressure; a high-pressure reverse
osmosis device that includes a high-pressure reverse osmosis
membrane for concentrating a salt content in high-pressure feed
water whose pressure is increased by the high-pressure pump; a
first drain valve that is mounted on a permeated water line through
which permeated water that has passed through the high-pressure
reverse osmosis device is supplied downstream, and temporarily
drains the permeated water of an initial start-up; a low-pressure
pump that is mounted on the permeated water line provided
downstream of the first drain valve, and reduces pressure of the
permeated water to a predetermined low pressure; a low-pressure
reverse osmosis device that includes a low-pressure reverse osmosis
membrane for concentrating a salt content in low-pressure feed
water whose pressure is reduced by the low-pressure pump; and a
second drain valve that is mounted on a discharge line on a
concentrated water side of the low-pressure reverse osmosis device,
and temporarily discharges the low-pressure feed water of the
initial start-up supplied to the low-pressure reverse osmosis
device.
2. The multi-stage seawater desalination apparatus according to
claim 1, further comprising a plurality of the low-pressure reverse
osmosis devices, wherein the second drain valve is mounted on the
discharge line at the concentrated water side of each of the
low-pressure reverse osmosis devices, and temporarily discharges
the low-pressure feed water of the initial start-up supplied to the
low-pressure reverse osmosis devices.
3. The multi-stage seawater desalination apparatus according to
claim 1, further comprising a returned concentrated water line that
returns the concentrated water from the low-pressure reverse
osmosis device upstream.
4. The multi-stage seawater desalination apparatus according to
claim 1, wherein the high-pressure reverse osmosis device is a
membrane with chlorine-resistance.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a divisional of U.S. application Ser. No. 13/131,769
filed May 27, 2011, which is a 371 of PCT/JP2009/064058 filed Aug.
7, 2009, the entire contents of which is incorporated herein by
reference.
TECHNICAL FIELD
[0002] The present invention relates to a multi-stage seawater
desalination apparatus that can obtain high-quality product water
from seawater using a reverse osmosis method, and an operation
control method of the multi-stage seawater desalination
apparatus.
BACKGROUND ART
[0003] Conventional methods for obtaining fresh water from seawater
include an evaporation method in which seawater is evaporated, and
a reverse osmosis method in which fresh water is obtained by
filtering seawater through a filtration membrane called a reverse
osmosis membrane (RO membrane) by applying pressure to the
seawater, concentrating the salt in the seawater, and discarding
thereof.
[0004] The latter reverse osmosis method is energy efficient
compared to the evaporation method. However, in the reverse osmosis
method, a careful pretreatment (treatment with an "ultrafilter
membrane (UF membrane)" or a "microfilter membrane (MF membrane)"
for removing suspended matters in seawater, or raw water) is
required to prevent the RO membrane from being clogged by microbes
and deposits in seawater, and the maintenance and the like is
expensive.
[0005] It is also difficult to obtain water quality as good as that
produced by the evaporation method. To obtain highly purified water
quality, a plurality of stages of reverse osmosis devices needs to
be combined.
[0006] FIG. 10 is a schematic of a multi-stage seawater
desalination apparatus of a conventional reverse osmosis
method.
[0007] As shown in FIG. 10, a conventional multi-stage seawater
desalination apparatus 100 includes a high-pressure pump P.sub.1
that increases the pressure of raw water (seawater) 101 from which
suspended matters are removed by pretreatment to a predetermined
high pressure, a high-pressure reverse osmosis device 103 that
includes a high-pressure reverse osmosis membrane 103a for
concentrating a salt content in high-pressure feed water 102 whose
pressure is increased by the high-pressure pump P.sub.1, an
intermediate tank 110 that temporarily stores therein permeated
water 104 that has passed through the high-pressure reverse osmosis
device 103, a low-pressure pump P.sub.2 that reduces the pressure
of the permeated water 104 supplied from the intermediate tank 110
to a predetermined low pressure, and a low-pressure reverse osmosis
device 106 that has a low-pressure reverse osmosis membrane 106a
for concentrating a salt content in low-pressure feed water 105
whose pressure is reduced by the low-pressure pump P.sub.2, and
obtains product water (fresh water) 107. Because the pressure in
the intermediate tank 110 is returned to normal, the pressure is
released. While the system is stopped, pH of the permeated water
104 is adjusted, by adding a pH adjusting agent 111, thereby
preventing microbial contamination (see Non-Patent Document 1
"Fukuoka District Waterworks Agency: Mechanism of Seawater
Desalination").
[0008] FIG. 10 is a schematic of concentrated water 103b from the
high-pressure reverse osmosis device 103, and concentrated water
106b from the low-pressure reverse osmosis device 106. [0009]
[Non-Patent Document 1] "Fukuoka District Waterworks Agency
Mechanism of Seawater Desalination"
URL:http://www.f-suiki.or.jp/seawater/facilities/mechanism.php
DISCLOSURE OF INVENTION
Problem to be Solved by the Invention
[0010] Conventionally, to combine the plurality of stages of
reverse osmosis devices, as shown in FIG. 10, the operation is
normally performed by placing the intermediate tank 110 between the
devices. However, because the intermediate tank 110 is likely a
cause of microbial contamination, the pH adjusting agent 111 needs
to be added constantly.
[0011] At the start-up, the permeated water 104 to which the pH
adjusting agent 111 is added needs to be discarded. This is a
problem, because the conversion rate of the product water 107 is
reduced as much as the amount being discarded.
[0012] Another problem is that the product water 107 cannot be
supplied at the right timing of the system start-up, corresponding
to the discard.
[0013] Accordingly, elimination of the intermediate tank 110 may be
considered. When the plurality of stages is combined, the salt
concentration of the concentrated water 106b at the subsequent
stage (low-pressure reverse osmosis device 106) is lower than the
salt concentration of the high-pressure feed water 102 at the
preceding stage. Consequently, when a circulation operation in
which the concentrated water 106b at the subsequent stage is
returned to the side of the feed water at the preceding stage is
performed, the salt concentration at the side of the high-pressure
feed water 102 becomes low, thereby reducing the osmotic pressure,
and reducing the operating pressure. As a result, when the reverse
osmosis devices are directly connected without interposing the
intermediate tank 110 therebetween, the pressure fluctuations at
the start-up is large, thereby preventing a stable start-up
operation.
[0014] In a combined system in which the high-pressure RO membrane
103a with chlorine-resistance is provided at the preceding stage
and an RO membrane without chlorine-resistance is provided at the
subsequent stage, it is difficult to carry out chlorination at the
preceding stage, when the concentrated water 106b at the subsequent
stage is returned to the side of the high-pressure feed water 102
at the preceding stage.
[0015] In isolated islands and desert regions where seawater
desalination apparatuses are to be installed, a desalination
technology that can stably supply a large amount of product water
(2000 t/day to 5000 t/day) is required.
[0016] The water used in chemical plants and the like built in the
desert regions may be required to have higher quality and less salt
content (such as pure water whose salt concentration is equal to or
less than 5 parts per million) than that of drinking water (salt
concentration is equal to or less than 250 parts per million).
[0017] Accordingly, in the reverse osmosis method including a
plurality of stages of RO membranes, a multi-stage seawater
desalination apparatus that can obtain water quality as good as
that produced by the evaporation method, that can prevent microbial
contamination by eliminating the intermediate tank, and that
enables serial start-up and operation has been much demanded.
[0018] The present invention is made in view of the above
circumstances and intended to provide a multi-stage seawater
desalination apparatus in series that can obtain high-quality
product water from seawater using a reverse osmosis method and an
operation control method of the multi-stage seawater desalination
apparatus.
Means for Solving Problem
[0019] According to an aspect of the present invention, a
multi-stage seawater desalination apparatus includes: a
high-pressure pump that is mounted on a raw water line through
which raw water is supplied, and increases pressure of the raw
water to a predetermined high pressure; a high-pressure reverse
osmosis device that includes a high-pressure reverse osmosis
membrane for concentrating a salt content in high-pressure feed
water whose pressure is increased by the high-pressure pump; a
first drain valve that is mounted on a permeated water line through
which permeated water that has passed through the high-pressure
reverse osmosis device is supplied downstream, and temporarily
drains the permeated water of an initial start-up; a low-pressure
pump that is mounted on the permeated water line provided
downstream of the first drain valve, and reduces pressure of the
permeated water to a predetermined low pressure; a low-pressure
reverse osmosis device that includes a low-pressure reverse osmosis
membrane for concentrating a salt content in low-pressure feed
water whose pressure is reduced by the low-pressure pump; and a
second drain valve that is mounted on a discharge line on a
concentrated water side of the low-pressure reverse osmosis device,
and temporarily discharges the low-pressure feed water of the
initial start-up supplied to the low-pressure reverse osmosis
device.
[0020] Advantageously, the multi-stage seawater desalination
apparatus further includes a plurality of the low-pressure reverse
osmosis devices. The second drain valve is mounted on the discharge
line at the concentrated water side of each of the low-pressure
reverse osmosis devices, and temporarily discharges the
low-pressure feed water of the initial start-up supplied to the
low-pressure reverse osmosis devices.
[0021] Advantageously, the multi-stage seawater desalination
apparatus further includes a returned concentrated water line that
returns the concentrated water from the low-pressure reverse
osmosis device upstream.
[0022] Advantageously, in the multi-stage seawater desalination
apparatus, the high-pressure reverse osmosis device is a membrane
with chlorine-resistance.
[0023] According to another aspect of the present invention, an
operation control method of a multi-stage seawater desalination
apparatus that uses the multi-stage seawater desalination apparatus
described above, at an initial start-up of the multi-stage seawater
desalination apparatus, includes: a step of supplying raw water to
the high-pressure reverse osmosis device; a step of temporarily
draining permeated water that has passed through the high-pressure
reverse osmosis membrane of the high-pressure reverse osmosis
device through the first drain valve; a step of supplying a whole
amount of the permeated water to a side of the low-pressure reverse
osmosis device, while closing the first drain valve, when a
pressure of the permeated water has reached a pressure around an
inlet pressure of the low-pressure pump; a step of temporarily
draining low-pressure feed water of the initial start-up supplied
to the low-pressure reverse osmosis device through the second drain
valve; and a step of producing product water by making the
low-pressure feed water pass through the low-pressure reverse
osmosis membrane while closing the second drain valve, and
switching operation to a rated seawater desalination operation.
[0024] According to still another aspect of the present invention,
an operation control method of a multi-stage seawater desalination
apparatus that uses the multi-stage seawater desalination apparatus
described above, during a sterilization operation of the
high-pressure reverse osmosis device of the multi-stage seawater
desalination apparatus, includes: a step of supplying raw water to
which chlorine (Cl) is added is supplied to the high-pressure
reverse osmosis device without feeding a reducing agent from a
first reducing agent supplying unit; a step of temporarily draining
low-pressure feed water including a reducing agent through the
second drain valve, after the reducing agent is added to permeated
water with chlorine that has passed through the high-pressure
reverse osmosis membrane of the high-pressure reverse osmosis
device from a second reducing agent supplying unit; a step of
switching operation to a normal operation, by adding a reducing
agent from the first reducing agent supplying unit to the raw
water, and stopping addition of the reducing agent from the second
reducing agent supplying unit, after a predetermined sterilization
operation is finished; and a step of producing product water, by
switching operation to a rated seawater desalination operation,
while closing the second drain valve.
[0025] Advantageously, in the operation control method of a
multi-stage seawater desalination apparatus, concentrated water at
a low pressure side is returned upstream through a returned
concentrated water line, after the sterilization operation is
finished.
[0026] According to still another aspect of the present invention,
an operation control method of a multi-stage seawater desalination
apparatus that uses the multi-stage seawater desalination apparatus
described above, during a sterilization operation of the
high-pressure reverse osmosis device of the multi-stage seawater
desalination apparatus, includes: a step of supplying raw water to
which chlorine (Cl) is added to the high-pressure reverse osmosis
device without feeding a reducing agent from a first reducing agent
supplying unit; a step of neutralizing permeated water with
chlorine that has passed through the high-pressure reverse osmosis
membrane of the high-pressure reverse osmosis device, by adding a
reducing agent less than a neutralization equivalent from a second
reducing agent supplying unit, and then supplying rest of the
reducing agent for neutralizing to low-pressure feed water, while
measuring an oxidation-reduction potential thereof with an
oxidation-reduction potential meter; a step of producing product
water by making the low-pressure feed water pass through the
low-pressure reverse osmosis membrane; and a step of producing
product water by switching operation to a rated seawater
desalination operation, after the sterilization operation is
finished.
[0027] Advantageously, in the operation control method of a
multi-stage seawater desalination apparatus, concentrated water at
a low pressure side is returned upstream through a returned
concentrated water line during the sterilization operation and the
rated operation.
Effect of the Invention
[0028] With the present invention, in a reverse osmosis method
having a plurality of stages of RO membranes, it is possible to
obtain water quality as good as that is produced by the evaporation
method, prevent microbial contamination occurred when the system is
stopped by eliminating the intermediate tank, and perform serial
start-up and operation.
BRIEF DESCRIPTION OF DRAWINGS
[0029] FIG. 1 is a schematic of a multi-stage seawater desalination
apparatus according to a first embodiment.
[0030] FIG. 2 is a process chart of the multi-stage seawater
desalination apparatus according to the first embodiment.
[0031] FIG. 3A is an operational schematic of the multi-stage
seawater desalination apparatus according to the first embodiment
at Step 2.
[0032] FIG. 3B is an operational schematic of the multi-stage
seawater desalination apparatus according to the first embodiment
at Steps 3 and 4.
[0033] FIG. 3C is an operational schematic of the multi-stage
seawater desalination apparatus according to the first embodiment
at Step 5.
[0034] FIG. 4 is a schematic of a multi-stage seawater desalination
apparatus according to a second embodiment.
[0035] FIG. 5 is a schematic of a multi-stage seawater desalination
apparatus according to a third embodiment.
[0036] FIG. 6A is an operational schematic of the multi-stage
seawater desalination apparatus according to the third embodiment
at Steps 1 and 2.
[0037] FIG. 6B is an operational schematic of the multi-stage
seawater desalination apparatus according to the third embodiment
at Step 3.
[0038] FIG. 7 is a schematic of a multi-stage seawater desalination
apparatus according to a fourth embodiment.
[0039] FIG. 8 is an illustration of variation of
oxidation-reduction potential (ORP) when a reducing agent (SBS) is
added.
[0040] FIG. 9 is a schematic of a multi-stage seawater desalination
apparatus according to a fifth embodiment.
[0041] FIG. 10 is a schematic of a multi-stage seawater
desalination apparatus according to a conventional example.
BEST MODE(S) FOR CARRYING OUT THE INVENTION
[0042] Exemplary embodiments of the present invention will be
described in detail with reference to the accompanying drawings.
However, the present invention is not limited by the embodiments.
Constituting elements in the embodiments include elements that can
be easily conceived by those skilled in the art, or elements being
substantially the same as those elements.
First Embodiment
[0043] A multi-stage seawater desalination apparatus according to a
first embodiment of the present invention is described with
reference to the accompanying drawings.
[0044] FIG. 1 is a schematic of the multi-stage seawater
desalination apparatus according to the first embodiment.
[0045] As shown in FIG. 1, this multi-stage seawater desalination
apparatus 10A according to the present embodiment includes a
high-pressure pump P.sub.1 that increases the pressure of raw water
11 to a predetermined high pressure and is mounted on a raw water
line L.sub.1 through which raw water (such as seawater) 11 is
supplied, a high-pressure reverse osmosis device 13 that includes a
high-pressure reverse osmosis membrane (high-pressure RO membrane)
13a for concentrating a salt content in high-pressure feed water 12
whose pressure is increased by the high-pressure pump P.sub.1, a
first drain valve 21 that temporarily drains permeated water 14 of
the initial start-up through a discharge line L.sub.6 and is
mounted on a permeated water line L.sub.2 through which the
permeated water 14 that has passed through the high-pressure
reverse osmosis device 13 is supplied downstream, a low-pressure
pump P.sub.2 that reduces the pressure of the permeated water 14 to
a predetermined low pressure and is mounted on a permeated water
line L.sub.3 provided downstream of the first drain valve 21, a
low-pressure reverse osmosis device 16 that has a low-pressure
reverse osmosis membrane (low-pressure RO membrane) 16a for
concentrating a salt content in low-pressure feed water 15 whose
pressure is reduced by the low-pressure pump P.sub.2, and a second
drain valve 22 that temporarily discharges the low-pressure feed
water 15 of the initial start-up supplied to the low-pressure
reverse osmosis device 16 and is mounted on a discharge line L5 at
the concentrated water side of the low-pressure reverse osmosis
device 16.
[0046] To filter the raw water 11, the pretreatment may be carried
out by using an ultrafilter membrane (UF membrane), a microfilter
membrane (MF membrane), or the like, that removes suspended matters
in seawater, similar to the conventional manner.
[0047] In FIG. 1, the reference numeral 13b denotes concentrated
water from the high-pressure reverse osmosis device 13, the
reference numeral 16b denotes concentrated water from the
low-pressure reverse osmosis device 16, and the reference numeral
18 denotes a switch valve that switches the flow path of product
water 17a having a value equal to or less than a defined value and
is mounted on a product water line L.sub.7.
[0048] In the present embodiment, a three-way valve is used for the
first and the second drain valves 21 and 22. However, the present
invention is not limited thereto.
[0049] To start the system by using the multi-stage seawater
desalination apparatus 10A, the system is started by the following
procedure.
[0050] FIG. 2 is the process chart.
[0051] <Step 1> At Step 1, at the initial start-up of the
system, the raw water 11 is supplied to the high-pressure reverse
osmosis device 13 through the high-pressure pump P.sub.1 (S-1). At
this time, the concentrated water 13b is discharged outside through
a concentrated water line L.sub.4.
[0052] <Step 2> At Step 2, the permeated water 14 that has
passed through the high-pressure reverse osmosis membrane 13a of
the high-pressure reverse osmosis device 13 is temporarily drained
from the discharge line L.sub.6 through the first drain valve 21
(S-2: see FIG. 3A).
[0053] <Step 3> At Step 3, when the pressure of the permeated
water 14 that passed through the first drain valve 21 has reached
the pressure around the inlet pressure of the low-pressure pump
P.sub.2, the whole amount of the permeated water 14 having a
predetermined pressure is supplied to the side of the low-pressure
reverse osmosis device 16 through the low-pressure pump P.sub.2,
while closing the first drain valve 21 (S-3: see FIG. 3B).
[0054] <Step 4> At Step 4, the low-pressure feed water 15 of
the initial start-up supplied to the low-pressure reverse osmosis
device 16 is temporarily drained through the second drain valve 22
(S-4: see FIG. 3B).
[0055] <Step 5> At Step 5, when the pressure of the
low-pressure feed water 15 has reached a predetermined permeate
pressure of the low-pressure RO membrane 16a of the low-pressure
reverse osmosis device 16, the low-pressure feed water 15 is passed
through the low-pressure reverse osmosis membrane 16a, while
closing the second drain valve 22. By switching operation to the
rated seawater desalination operation, product water 17 is produced
(S-5: see FIG. 3C).
[0056] In this manner, when the plurality of stages (two-stage in
the present embodiment) of RO device is connected directly, the
permeated water 14 is drained through the first drain valve 21, and
waits until the pressure of the permeated water 14 reaches a
predetermined pressure at the inlet pressure of the low-pressure
pump P.sub.2. When it is confirmed that the pressure of the
permeated water 14 has reached the pressure around the inlet
pressure of the low-pressure pump P.sub.2, the whole amount of the
low-pressure feed water 15 is supplied to the side of the
low-pressure reverse osmosis device 16, while closing the first
drain valve 21.
[0057] The low-pressure feed water 15 of the initial start-up
supplied to the low-pressure reverse osmosis device 16 is then
temporarily drained through the second drain valve 22, and waits
until the pressure of the low-pressure feed water 15 reaches the
permeate pressure of the low-pressure RO membrane 16a of the
low-pressure reverse osmosis device 16.
[0058] When it is confirmed that the pressure of the low-pressure
feed water 15 has reached near the permeate pressure of the
low-pressure RO membrane 16a of the low-pressure reverse osmosis
device 16, the low-pressure feed water 15 is passed through the
low-pressure reverse osmosis membrane 16a, while closing the second
drain valve 22. By switching operation to the rated seawater
desalination operation, the product water 17 is produced.
[0059] Accordingly, even if the conventional intermediate tank is
not installed, it is possible to prevent the pressure fluctuations
at the initial start-up, and constantly perform start-up operation
in a stable manner.
Second Embodiment
[0060] FIG. 4 is a schematic of a multi-stage seawater desalination
apparatus according to a second embodiment. The same reference
numerals are given to the same elements as those in the multi-stage
seawater desalination apparatus in FIG. 1, and their repeated
descriptions will be omitted.
[0061] As shown in FIG. 4, this multi-stage seawater desalination
apparatus 10B according to the present embodiment returns the
concentrated water 16b concentrated by the low-pressure reverse
osmosis device 16 in the multi-stage seawater desalination
apparatus 10A according to the first embodiment to the raw water
line L.sub.1 through a returned concentrated water line
L.sub.8.
[0062] Accordingly, product water can be obtained again by
reconcentrating those discarded as the concentrated water, thereby
reducing the amount of discarded concentrated water and increasing
the amount of product water.
[0063] To start the system by using the multi-stage seawater
desalination apparatus 10B, the system is started by the following
procedure.
[0064] <Step 1> At Step 1, at the initial start-up of the
system, the raw water 11 is supplied to the high-pressure reverse
osmosis device 13 through the high-pressure pump P.sub.1. At this
time, the concentrated water 13b is discharged outside through the
concentrated water line L.sub.4. Because the pressure of the
concentrated water 13b is high, the energy is collected by driving
a turbine T.
[0065] <Step 2> At Step 2, the permeated water 14 that has
passed through the high-pressure reverse osmosis membrane 13a of
the high-pressure reverse osmosis device 13 is temporarily drained
from the discharge line L.sub.6 through the first drain valve
21.
[0066] <Step 3> At Step 3, when the pressure of the permeated
water 14 has reached the pressure around the inlet pressure of the
low-pressure pump P.sub.2, the whole amount of the permeated water
14 is supplied to the side of the low-pressure reverse osmosis
device 16 through the low-pressure pump P.sub.2, while closing the
first drain valve 21.
[0067] <Step 4> At Step 4, the low-pressure feed water 15 of
the initial start-up supplied to the low-pressure reverse osmosis
device 16 is temporarily drained through the second drain valve
22.
[0068] <Step 5> At Step 5, when the pressure of the
low-pressure feed water 15 has reached the permeate pressure of the
low-pressure RO membrane 16a of the low-pressure reverse osmosis
device 16, the low-pressure feed water 15 is passed through the
low-pressure reverse osmosis membrane 16a, while closing the second
drain valve 22. By switching operation to the rated seawater
desalination operation, the product water 17 is produced
[0069] <Step 6> At Step 6, after a predetermined rated
concentration operation is carried out in the low-pressure reverse
osmosis device 16, the concentrated water 16b is returned to the
raw water line L.sub.1 through the returned concentrated water line
L.sub.8, as returned concentrated water 26, while adjusting a
flowmeter FC and a switching valve 25.
[0070] Accordingly, because the concentrated water is not simply
returned as the returned concentrated water 26, the concentrated
water 16b can be returned to the side of the raw water 11 from the
low-pressure reverse osmosis device 16 at the subsequent stage.
Consequently, it is possible to prevent the reduction of the
operating pressure, due to the reduced osmotic pressure.
[0071] With the first and the second embodiments, it is possible to
provide water quality as good as that is produced by the
evaporation method, and because the intermediate tank is
eliminated, it is possible to significantly reduce microbial
contamination.
Third Embodiment
[0072] FIG. 5 is a schematic of a multi-stage seawater desalination
apparatus according to a third embodiment. The same reference
numerals are given to the same elements as those in the multi-stage
seawater desalination apparatuses in FIGS. 1 and 4, and their
repeated descriptions will be omitted.
[0073] As shown in FIG. 5, this multi-stage seawater desalination
apparatus 10C according to the present embodiment disposes the
high-pressure RO membrane 13a with chlorine-resistance in the
high-pressure reverse osmosis device 13, and disposes the
low-pressure RO membrane 16a without chlorine-resistance in the
low-pressure reverse osmosis device 16, in the multi-stage seawater
desalination apparatus 10B according to the second embodiment.
After a predetermined period of operation, addition of a reducing
agent 32 that neutralizes the chlorine in the raw water 11 is
stopped, and sterilization is carried out for a predetermined
period of time, by using the chlorine in the raw water.
Accordingly, high-quality product water can be obtained in a stable
manner.
[0074] In the present embodiment, a cellulose acetate membrane is
used as the high-pressure RO membrane 13a with chlorine-resistance.
A polyamide membrane is used as the low-pressure RO membrane 16a
without chlorine-resistance.
[0075] As shown in FIG. 5, during a normal operation, the reducing
agent 32 is supplied to the raw water line L.sub.1 from a first
reducing agent supplying unit 31-1, to neutralize chlorine.
[0076] In this case, the reducing agent 32 is not supplied to the
permeated water line L.sub.2 from a second reducing agent supplying
unit 31-2.
[0077] SBS may be used as the reducing agent 32. However, the
present invention is not limited thereto.
[0078] At the initial operation, the operation is started as the
first embodiment, and the rated operation is being continued.
[0079] A first sterilization operation mode to carry out
sterilization treatment will now be explained.
[0080] FIGS. 6A and 6B are the sterilization operation mode (first
mode).
[0081] As shown in FIG. 6A, at Step 1, raw water 11.sub.Cl to which
chlorine (Cl) is added, is supplied to the high-pressure reverse
osmosis device 13, without feeding the reducing agent 32 from the
first reducing agent supplying unit 31-1 (Step 1).
[0082] At Step 2, permeated water 14.sub.Cl with chlorine that has
passed through the high-pressure reverse osmosis membrane 13a of
the high-pressure reverse osmosis device 13 is neutralized by
adding the reducing agent (SBS) 32 from the second reducing agent
supplying unit 31-2. The permeated water 14 is then passed through
the low-pressure RO membrane 16a of the low-pressure reverse
osmosis device 16, and low-pressure feed water 15.sub.SBS with the
reducing agent (SBS) 32 is temporarily drained through the second
drain valve 22 (Step 2). At Step 2, the low-pressure feed water
15.sub.SBS includes the reducing agent. Accordingly, the
low-pressure feed water 15.sub.SBS is stopped from being returned
to the side of the raw water 11 through the returned concentrated
water line L.sub.8, by switching the switching valve 25. This is to
prevent the reducing agent from being supplied to the raw water,
and interfering with the sterilization operation.
[0083] As shown in FIG. 6B, at Step 3, after a predetermined
sterilization is being operated, the reducing agent 32 is added to
the raw water 11.sub.Cl from the first reducing agent supplying
unit 31-1, and the addition of the reducing agent 32 from the
second reducing agent supplying unit 31-2 is stopped, thereby
switching operation to a normal operation (Step 3).
[0084] At Step 4, while closing the second drain valve 22, the
product water 17 is produced, by switching operation to the rated
seawater desalination operation (Step 4).
[0085] The low-pressure reverse osmosis device 16 is interposed
between oxidation-reduction potential (ORP) meters 41-1 and 41-2,
and the oxidation-reduction potential of the low-pressure feed
water 15 is measured thereby.
[0086] The period of time for the sterilization treatment may be
modified suitably according to the equipment and the operation
load. As an example, after six hours to ten hours of operation,
chlorination sterilization is carried out for about an hour.
[0087] After the sterilization is complete, the concentrated water
at the low-pressure side is returned upstream through the returned
concentrated water line L.sub.8 as the returned concentrated water
26.
[0088] In this manner, by chlorination sterilizing every time the
apparatus is operated for a predetermined period of time, it is
possible to sterilize the membranes, and to constantly produce
clean product water equal to or higher than a certain level of
quality.
Fourth Embodiment
[0089] FIG. 7 is a schematic of a multi-stage seawater desalination
apparatus according to a fourth embodiment. The same reference
numerals are given to the same elements as those in the multi-stage
seawater desalination apparatus in FIG. 5, and their repeated
descriptions will be omitted.
[0090] The present embodiment is a second sterilization operation
mode to carry out sterilization treatment.
[0091] FIG. 7 is the sterilization operation mode (second mode). As
shown in FIG. 7, this multi-stage seawater desalination apparatus
10D according to the present embodiment operates the
oxidation-reduction potential (ORP) meter in the multi-stage
seawater desalination apparatus 10C according to the third
embodiment, and produces product water also during the
sterilization treatment.
[0092] As shown in FIG. 7, the following steps are executed during
the sterilization operation of the high-pressure reverse osmosis
device of the multi-stage seawater desalination apparatus.
[0093] At Step 1, the raw water 11.sub.Cl to which chlorine (Cl) is
added is supplied to the high-pressure reverse osmosis device 13,
without feeding the reducing agent 32 from the first reducing agent
supplying unit 31-1 (Step 1).
[0094] At Step 2, the reducing agent (SBS) 32 less than the
neutralization equivalent is added to the permeated water 14.sub.Cl
with chlorine that has passed through the high-pressure reverse
osmosis membrane 13a of the high-pressure reverse osmosis device
13, from the second reducing agent supplying unit 31-2. The rest of
reducing agent 32 for neutralizing is then supplied to the
low-pressure feed water 15 from a third reducing agent supplying
unit 31-3, while measuring the oxidation-reduction potential with
the oxidation-reduction potential (ORP) meter 41-1 (Step 2).
[0095] The reasons for taking such measures are as follows:
[0096] To remove chlorine in the permeated water 14.sub.Cl of the
high-pressure reverse osmosis device 13, the reducing agent 32 is
supplied in excess by the first reducing agent supplying unit 31-1,
from the safety point of view.
[0097] The ORP meter 41-1 indicates a large value, when chlorine is
present. However, it is difficult to determine the supplied amount
of reducing agent, after the chlorine is removed.
[0098] FIG. 8 illustrates the variation of ORP when a reducing
agent (SBS) is added to the water with a chlorine concentration of
0.3 parts per million at the exit.
[0099] As shown in FIG. 8, the ORP value is high when a large
amount of chlorine is present. The ORP value is low, if chlorine is
not present (minus).
[0100] Accordingly, when the chlorine has been consumed, the
measurement of the concentration of reducing agent is difficult,
even if the reducing agent is supplied in excess. The is because,
when the product water 17 is produced while a large amount of
reducing agent 32 is supplied, the produced product water 17
includes a large amount of reducing agent, compared to the product
water produced during the normal operation. To prevent this from
happening, in the third embodiment, the low-pressure feed water
15.sub.SBS with reducing agent is drained through the second drain
valve 22 (Step 2 of the third embodiment: see FIG. 6A).
[0101] Alternatively, in the present embodiment, the supply of the
reducing agent is monitored with the oxidation-reduction potential
meter 41-1, and the value is controlled within a high sensitivity
area (at this time, 200 millivolts to 400 millivolts). A certain
amount (at this time, approximately 1 part per million) of the
reducing agent 32 is then supplied by the third reducing agent
supplying unit 31-3 including a metering pump. Accordingly, the
chlorine can be completely removed, and the reducing agent can be
prevented from being oversupplied.
[0102] In this manner, the supply of the reducing agent can be
suitably managed, and the product water with an extremely low
amount of reducing agent can also be produced during the
sterilization operation.
Fifth Embodiment
[0103] FIG. 9 is a schematic of a multi-stage seawater desalination
apparatus according to a fifth embodiment. The same reference
numerals are given to the same elements as those in the multi-stage
seawater desalination apparatus in FIG. 1, and their repeated
descriptions will be omitted. As shown in FIG. 9, this multi-stage
seawater desalination apparatus 10E according to the present
embodiment includes two stages of the low-pressure reverse osmosis
device 16 (low-pressure reverse osmosis device 16-1 and
low-pressure reverse osmosis device 16-2) in the multi-stage
seawater desalination apparatus 10A according to the first
embodiment.
[0104] The concentrated water concentrated by the first
low-pressure reverse osmosis device 16-1 is returned to the raw
water line L.sub.1 through the returned line L.sub.8 as returned
concentrated water 26-1, and the concentrated water concentrated by
the second low-pressure reverse osmosis device 16-2 is returned to
the permeated water line L.sub.3 through a returned concentrated
water line L.sub.12 as returned concentrated water 26-2.
[0105] In FIGS. 9, 15-1 and 15-2 denote low-pressure feed water,
21-1 and 21-2 denote first drain valves, 22-1 and 22-2 denote
second drain valves, 25-1 and 25-2 denote switching valves, L.sub.9
denotes a low-pressure water transmission line, L.sub.10 denotes a
discharge line, and L.sub.11 denotes a discharge line.
[0106] At the initial start-up of the system, the first
low-pressure reverse osmosis device 16-1 is operated, and when a
predetermined pressure is reached, the low-pressure feed water 15-2
is supplied to the second low-pressure reverse osmosis device 16-2,
while draining the low-pressure feed water 15-1 through the first
drain valve 21-1. When a predetermined pressure is reached at the
first drain valve 21-2, the product water 17 is produced by the
second low-pressure reverse osmosis device 16-2.
[0107] In the present embodiment, if the flow rate of the raw water
11 is "100", the raw water 11 with the flow rate of "104" is
supplied to the high-pressure reverse osmosis device 13, because
the returned concentrated water 26-1 is returned.
[0108] The permeated water 14 with the flow rate of "44" passes
through the high-pressure RO membrane 13a of the high-pressure
reverse osmosis device 13, and the concentrated water 13b with the
flow rate of "60" is supplied to the side of the turbine T as
high-pressure discharge water, where the energy is collected.
[0109] Permeated water 19 with the flow rate of "40" passes through
the low-pressure RO membrane 16a of the first low-pressure reverse
osmosis device 16-1, and the concentrated water with the flow rate
of "4" is returned to the raw water 11 as the returned concentrated
water 26-1 as discharge water, and is reused.
[0110] The product water 17 with the flow rate of "36" passes
through the low-pressure RO membrane 16a of the second low-pressure
reverse osmosis device 16-2, and the concentrated water with the
flow rate of "4" is returned to the permeated water 14 as the
returned concentrated water 26-2 as discharge water, and is
reused.
[0111] While the sterilization treatment is carried out, the
high-quality product water is constantly manufactured by executing
either the third or the fourth embodiment.
[0112] In this manner, with the fifth embodiment, the low-pressure
reverse osmosis device has a plurality of stages. Accordingly, when
the concentration of salt in the raw water 11 is 220,000 parts per
million, the high-pressure reverse osmosis device 13 can reduce the
concentration of salt to 500 parts per million, the first
low-pressure reverse osmosis device 16-1 can reduce the
concentration of salt to approximately 100 parts per million, and
the second low-pressure reverse osmosis device 16-2 can reduce the
concentration of salt to equal to or less than 5 parts per million.
Accordingly, pure water equal to or more than drinking water
(approximately 250 parts per million) can be manufactured in a
stable manner.
[0113] Because a part of the concentrated water is reused, it is
also possible to reduce the discharge of concentrated water and
increase the amount of product water being produced.
INDUSTRIAL APPLICABILITY
[0114] In this manner, with the operation control method of the
multi-stage seawater desalination apparatus according to the
present invention, it is possible to provide water quality as good
as that is produced by the evaporation method. Because the
intermediate tank is eliminated, it is possible to significantly
reduce microbial contamination. Accordingly, it can be suitably
used for seawater desalination equipment.
EXPLANATIONS OF LETTERS OR NUMERALS
[0115] 10A, 10B, 10C, 10D, and 10E multi-stage seawater
desalination apparatus [0116] 11 raw water (such as seawater)
[0117] 22 high-pressure feed water [0118] 13a high-pressure reverse
osmosis membrane [0119] 13 high-pressure reverse osmosis device
[0120] 14 permeated water [0121] 15 low-pressure feed water [0122]
16a low-pressure reverse osmosis membrane [0123] 16 low-pressure
reverse osmosis device [0124] 17 product water [0125] 21 first
drain valve [0126] 22 second drain valve
* * * * *
References