JP7407750B2 - How to operate desalination equipment - Google Patents

Description

The present invention relates to a method of operating a desalination apparatus, and particularly to a method of operating a desalination apparatus having a first desalination apparatus and a second desalination apparatus.

In desalination equipment such as reverse osmosis membranes, long-term operation can lead to the precipitation of scales such as calcium carbonate, silica, calcium fluoride, etc., as well as membrane clogging due to organic matter, resulting in a decrease in salt removal rate and a decrease in the amount of permeated water. This results in a decrease in the performance of the desalination equipment. In the case of scale blockage, in order to prevent deterioration in the performance of the desalination equipment, a method is adopted in which the ion concentration in the raw water is measured and the concentration of the desalination equipment is operated so as not to exceed the saturation index. Here, the saturation index generally refers to the logarithm of the product of the concentration and ionic strength of each ion species involved in scale generation divided by the solubility product. The desalination equipment is operated in such a range that this saturation index does not exceed zero. Furthermore, in cases where the saturation index is exceeded, scale formation is suppressed, for example, by adding a scale inhibitor, and the desalination equipment is operated.

If the water quality greatly exceeds the saturation index, which cannot be suppressed even by the addition of scale inhibitors, chemical cleaning such as acid cleaning or alkaline cleaning has conventionally been used to remove scale. However, in general cleaning, the process involves stopping the device, adjusting the cleaning liquid before cleaning, and starting water flow after recovering the cleaning liquid, which poses a problem in that cleaning costs increase. Therefore, it has been desired to operate a desalination apparatus that does not require chemical cleaning and whose performance does not deteriorate even after long-term operation.

One of the operating methods for desalination equipment is the flushing method. Here, flushing refers to an operation in which the feed water is discharged from the concentrated water discharge pipe to the outside of the system by opening the on-off valve of the concentrated water discharge pipe while the water supply pump continues to operate. By passing water at a faster flow rate than during normal operation, dirt that clogs the membrane surface can be effectively washed away. Flushing is generally performed at a frequency of 1 to 10 times/day and for 30 to 120 seconds/time. However, the problem is that flushing for several minutes/time is not sufficient to recover a desalination device whose performance has deteriorated, and cleaning must be performed after all. Furthermore, when flushing is performed, since the on-off valve of the concentrated water pipe is opened, permeated water cannot be produced while flushing is performed, and the recovery rate of the desalination apparatus decreases.

Another method is to reverse the flow direction of the water to be treated in the module. According to this method, it becomes possible to easily peel off the suspended solids accumulated in the raw water spacer, and the stability of the desalination apparatus is improved. However, the problem is that the number of valves required to reverse the flow increases significantly, resulting in a significant increase in initial cost. Further, if a failure occurs in a valve, the valve cannot be switched, and the stability of the device is greatly impaired. Furthermore, there is no mention of the peeling effect on scale materials due to flow reversal.

Japanese Patent Application Publication No. 2004-141846 Japanese Patent Application Publication No. 2004-261724

In view of the above-mentioned problems, it is an object of the present invention to provide a method for operating a desalination apparatus that can restore the degraded desalination performance of the desalination apparatus without stopping the operation of the desalination apparatus.

A method of operating a desalination apparatus of the present invention includes a method of operating a desalination apparatus having a first desalination apparatus and a second desalination apparatus, in which water to be treated is supplied to the first desalination apparatus and a first concentration is performed. a normal operation step of separating water and first demineralized water, and supplying the first concentrated water to a second desalination device to separate it into second concentrated water and second demineralized water; Water to be treated is supplied to a desalination device and separated into first concentrated water and first permeated water, and diluted water having a lower concentration than the first concentrated water is passed through the second desalination device. and a recovery operation step of restoring the desalting performance of the second desalting device.

In one aspect of the present invention, a plurality of the second desalination apparatuses are installed in parallel, and while some of the second desalination apparatuses are performing the normal operation process, other second desalination apparatuses are The recovery operation step is performed.

In one aspect of the present invention, in the recovery operation step, dilute water is passed through the second desalination device for 5 to 60 minutes.

In one aspect of the present invention, the water to be treated is used as dilute water.

In one aspect of the present invention, desalinated water from the first desalination device is used as the dilution water.

In one aspect of the invention, a scale inhibitor is added to the dilute water.

In one aspect of the present invention, the desalination device is a reverse osmosis membrane device.

In one aspect of the present invention, the dilute water flow rate is 0.001 to 0.1 m/s.

In one aspect of the present invention, the quality of the first concentrated water is one of the following a to c .
a. Calcium ion concentration 0.1-10mg/L, fluoride ion concentration 3000-8000mg-F/L.
b. Calcium ion concentration 500-1500mg/L, fluoride ion concentration 50-150mg-F/L.
c. Calcium ion concentration 400-1500mg/L, M alkalinity 800-2000mg/L.

In one aspect of the present invention, the first concentrated water is concentrated water obtained by concentrating the water supplied to the first desalination device three times or more.

In the normal operation step of the desalination equipment operating method of the present invention, the water to be treated is passed through the first desalination equipment to be separated into the first desalination water and the first concentrated water, and the first concentrated water is passed through the first desalination equipment. The water is passed through a desalination device and separated into second demineralized water and second concentrated water. If the desalination performance of the second desalination equipment decreases due to the normal operation process, the second desalination equipment can be replaced by passing diluted water through the second desalination equipment while continuing the operation of the first desalination equipment. The desalination performance of the desalination equipment can be restored.

In addition, in an embodiment in which desalinated water from the first desalination device is used as dilute water when performing flux recovery operation of the second desalination device, ancillary equipment such as a tank is not required, and the device configuration is simplified. . In addition, by using the permeated water from the first desalination equipment, which has a low salt concentration, as dilute water, the recovery effect of the second desalination equipment whose performance has deteriorated is greatly improved compared to when the water to be treated is used as diluted water. It becomes possible to do so.

FIG. 2 is a flow diagram illustrating a method of operating the desalination apparatus according to the embodiment. FIG. 3 is a cross-sectional view of a test cell. It is a graph showing the results of Examples and Comparative Examples. FIG. 2 is a flow diagram illustrating a method of operating the desalination apparatus according to the embodiment. FIG. 2 is a flow diagram illustrating a method of operating the desalination apparatus according to the embodiment. FIG. 2 is a flow diagram illustrating a method of operating the desalination apparatus according to the embodiment.

The first embodiment will be described below with reference to FIG. In the embodiment, a reverse osmosis membrane device (RO device) will be described as an example of a desalination device, but the present invention is not limited thereto. The reverse osmosis membrane is preferably a polyamide reverse osmosis membrane, but is not limited thereto. Examples of desalination devices other than reverse osmosis membrane devices include nanofiltration membrane devices, forward osmosis membrane devices, membrane distillation devices, electrodialysis devices, electrodeionization devices, and the like.

FIG. 1 shows the configuration of a desalination apparatus used in a desalination apparatus operating method according to an embodiment. Note that FIG. 11A shows the flow of water during normal operation with a thick solid line, and FIG. 11B shows the flow of water during flux recovery operation with a thick solid line.

[During normal operation]
During normal operation, as shown in FIG. It is extracted as permeate water.

Concentrated water (first concentrated water) from the first RO device 4 is introduced into the relay tank 10 via pipes 7 and 8 and a valve 9. The first concentrated water in the relay tank 10 is supplied from a pipe 13 having a pump 11 and a valve 12 to a second RO device 15 via a pipe 14 .

The permeated water from the second RO device 15 is taken out as second permeated water via a pipe 17 having a valve 16. Concentrated water from the second RO device 15 is taken out as second concentrated water via piping 18, valve 19, and piping 20, 21.

During this normal water flow, the pumps 2 and 11 are operated. Further, valves 5, 9, 12, 16, and 19 are open, and valves 24 and 28, which will be described next, are closed.

In the desalination apparatus shown in FIG. 1, the pipes 7 and 21 are connected by a pipe 23, a valve 24, and a pipe 25 in a bypass manner. Further, the pipes 3 and 14 are connected by a pipe 27, a valve 28, and a pipe 29 in a bypass manner.

PI in the figure represents a pressure sensor, and FI represents a flow rate sensor.

[During flux recovery operation]
During the flux recovery operation to recover the flux of the second RO device 15, the pump 2 is operated and the pump 11 is stopped, as shown in FIG. Further, valves 9 and 12 are closed, and the other valves are opened.

The raw water in the raw water tank 1 is supplied to the first RO device 4 via the pump 2 and piping 3, and the first permeated water is taken out from the piping 6 via the valve 5. The first concentrated water is taken out via the pipes 7 and 23, the valve 24, and the pipes 25 and 21.

Further, during the flux recovery operation of the second RO device 15, a part of the raw water sent from the pump 2 as dilute water is transferred to the second RO device via the pipe 27 branched from the pipe 3, the valve 28, and the pipes 29, 14. 15. Permeated water from the second RO device 15 is taken out via a valve 16 and piping 17. Concentrated water from the second RO device 15 flows out into the pipe 21 via the pipe 18, valve 19, and pipe 20, joins with the first concentrated water from the pipe 25, and is taken out as concentrated water.

In this desalination equipment operating method, when the flux of the second RO equipment 15 decreases due to normal operation, diluted water is added to the second RO equipment 15 while continuing the operation of producing permeated water in the first RO equipment 4. By running the water flow operation, it is possible to recover the flux in the second RO device 15 and prevent a significant decrease in the recovery rate without stopping the desalination device. In particular, by passing dilute water (raw water in the case of Figure 1) for 5 minutes or more (preferably 10 minutes or more but 60 minutes or less, especially 30 minutes or less), scale attached to the membrane surface is dissolved and the membrane is separated. It is possible to greatly recover performance.

Note that although diluted water is normally passed from the water supply side of the desalination apparatus, it may also be passed from the concentrated water side of the desalination apparatus.

The second embodiment will be described below with reference to FIGS. 4, 5, and 6. Note that in the explanation of FIGS. 4 to 6, the permeated water of the reverse osmosis membrane device is referred to as desalinated water. In FIGS. 4 to 6, pipes through which water is flowing are shown by thick solid lines, and pipes through which water is not flowing are shown by thin solid lines.

In the embodiment of FIGS. 4 and 5, two second RO devices 51 and 52 are installed in parallel. In FIG. 4, one second RO device 51 is in normal operation and the other second RO device 52 is in flux recovery operation, and in FIG. 5, one second RO device 51 is in flux recovery operation, and the other second RO device 51 is in flux recovery operation, The device 52 is in normal operation.

[During operation in Figure 4]
As shown in FIG. 4, the raw water in the raw water tank 1 is supplied to the first RO device 4 via the pump 2 and piping 3, and the desalinated water (permeated water) is converted to desalinated water via the piping 31, valve 32, and piping 33. taken out.

The concentrated water (first concentrated water) of the first RO device 4 is supplied to one of the second RO devices 51 via the pipes 34 and 35, the valve 36, and the pipe 37.

Desalinated water (permeated water) from the second RO device 51 joins the pipe 33 via pipes 61 and 62, and is taken out as desalted water. Concentrated water from the second RO device 51 is taken out as concentrated water via piping 63 , valve 64 , piping 65 , valve 66 , and piping 67 .

A pipe 38 branched from the pipe 34 is connected to the water supply port of the other second desalination device 52 via a valve 39 and a pipe 40. In FIG. 4, the valve 39 is closed.

In the desalination apparatus shown in FIG. 4, the pipes 31 and 37 are connected by a pipe 41 and a valve 42. Further, the pipes 31 and 40 are connected by a pipe 43 and a valve 44. In FIG. 4, the valve 42 is closed and the valve 44 is open. Therefore, a part of the first demineralized water in the pipe 31 is supplied to the water supply port of the second demineralizer 52 via the pipes 43 and 40, and the second demineralizer 52 is operated for flux recovery.

During the flux recovery operation of the other second RO device 52, the desalted water of the other second RO device 52 joins the pipe 33 via the pipes 73 and 62, and is taken out as desalted water. Concentrated water from the second RO device 52 is returned to the raw water tank 1 via pipes 74 and 77, a valve 78, and pipes 79 and 71.

[During operation in Figure 5]
Contrary to FIG. 4, in FIG. 5, one of the second desalination devices 51 is in flux recovery operation, and the other second desalination device 52 is in normal operation. In this case as well, the raw water in the raw water tank 1 is supplied to the first RO device 4 via the pump 2 and piping 3, and desalinated water (permeated water) is taken out as desalinated water via piping 31, valve 32, and piping 33. It will be done.

In FIG. 5, the valve 36 is closed and the valve 39 is open. Therefore, the concentrated water (first concentrated water) of the first RO device 4 is supplied to the other second RO device 52 via the pipes 34 and 38, the valve 39, and the pipe 40.

Desalinated water (permeated water) from the second RO device 52 joins the pipe 33 via pipes 73 and 62, and is taken out as desalted water. The concentrated water from the second RO device 52 flows into the pipe 65 via the pipe 74 and the valve 76, and is taken out as concentrated water.

Further, in FIG. 5, the valve 42 is open and the valve 44 is closed. Therefore, a part of the first desalinated water in the pipe 31 is supplied to the water supply port of one of the second desalination devices 51 via the pipes 42 and 37, and the second desalination device 51 is operated for flux recovery. The demineralized water from the second RO device 51 joins the pipe 33 via the pipes 61 and 62, and is taken out as demineralized water. Concentrated water from the second RO device 51 is returned to the raw water tank 1 via pipes 63 and 68, a valve 69, and pipes 70 and 71.

In this way, in the operating method of the desalination apparatus shown in FIGS. 4 and 5, one of the two second desalination apparatuses 51 and 52 installed in parallel is operated normally, while the other one passes the first desalination water. By performing flux recovery operation using water, it is possible to prevent a significant drop in the recovery rate without stopping the desalination equipment.
In FIGS. 4 and 5, one first RO device 4 is installed and a total of two second RO devices 51 and 52 are installed, but more than two RO devices may be installed.
FIG. 6 shows an example, in which four first RO devices 4A to 4D are installed in parallel, and four second RO devices 51 to 54 are installed in parallel.
Water to be treated is distributed and supplied to the first RO devices 4A to 4D through the pipe 3 and a pipe 81 branched from the pipe, and desalinated water from each of the first RO devices 4A to 4D is passed through pipes 82, 31, and 33 as desalinated water. taken out.
Concentrated water from the first RO devices 4A to 4D can be switched and supplied from the confluence pipe 83 to the second RO devices 51, 52 and the second RO devices 53, 54 via branch pipes 88 to 91 each having valves 84 to 87. ing. Further, each of the branch pipes 88 to 91 is connected to the demineralized water pipe 31 via pipes 91, 93, 95, and 97 having valves 92, 94, 96, and 98, respectively.
In FIG. 6, the second RO devices 51 and 52 are in normal operation, and the second RO devices 53 and 54 are in flux recovery operation. That is, valves 84 and 85 are open, valves 86 and 87 are closed, valves 92 and 94 are closed, and valves 96 and 98 are open.
Desalinated water from each of the second RO devices 51 to 54 joins the pipe 33 from pipes 101, 103, 105, and 107, and is taken out as desalted water.
Concentrated water from the second RO devices 51 to 54 is taken out as concentrated water via pipes 102, 104, 106, and 108 and a confluence pipe 109.
By opening and closing the valves 84 to 87 and valves 92, 94, 96, and 98 in the opposite manner to the above, the second RO devices 51 and 52 perform flux recovery operation, and the second RO devices 53 and 54 perform normal operation. be exposed.
In FIG. 6, four first RO devices and four second RO devices are shown, but the number may be two, three, five or more.
In FIGS. 4 to 6, the number of second RO devices that are normally operated and those that are operated for flux recovery are the same, but they may be different. Also in FIG. 6, the concentrated water of the second RO device during the flux recovery operation may be returned to the raw water tank 1.

[Water flow rate of dilute water]
The dilute water flow rate can be determined as appropriate depending on the blockage state of the desalination equipment, but for example, in the case of a reverse osmosis membrane, it is 0.001 to 1 m/s, particularly 0.02 to 0.2 m/s. is preferred. Specifically, it is preferably 300 to 2000 L/Hr per 4-inch module, and 1.8 to 10 m ³ /Hr per 8-inch module. Further, the pressure on the concentrated water discharge side of the desalting device is preferably 0.1 to 2 MPa.

[Timing for switching from normal operation to recovery operation]
In the present invention, it is possible to switch to recovery operation after normal operation for a predetermined period of time, but it is preferable to switch to recovery operation at the timing when scale is generated in the second desalination device. To illustrate the illustrated case in which a reverse osmosis membrane is used as a desalination device, switching is performed when the permeation flux of the second RO device decreases by a set ratio from the initial stage of operation, for example, by 5%. Note that this 5% is just an example, and any value selected from 1 to 20%, particularly 1 to 10%, may be used. In particular, it is preferable to measure changes in the permeation flux of the terminal RO, which is most concentrated, in the second RO device.

Since the actual permeation flux is affected by the operating pressure, water temperature, and salt concentration in the water supply, it is desirable to specify the corrected permeation flux as data indicating the performance of the RO device.

Here, the corrected permeation flux is generally calculated by the method described in the standardization method of reverse osmosis membrane element and module permeation water amount performance data as shown in JIS K 3805:1990.

That is, the permeated water amount performance data is corrected using the following equation (1) to calculate the corrected permeated flux F _ps .

Here, Q _pa : Permeated water amount under actual operating conditions (m ³ /d)
_Pfa : Operating pressure under actual operating conditions (kPa)
ΔP _fba : Module differential pressure under actual operating conditions (kPa)
P _pa : Pressure on the permeated water side under actual operating conditions (kPa)
Π _fba : Osmotic pressure of the average solute concentration on the supply side and concentration side under actual operating conditions (kPa)
TCF _a : Temperature conversion coefficient under actual operating conditions
P _fs : Operating pressure under standard operating conditions (kPa)
ΔP _fbs : Module differential pressure under standard operating conditions (kPa)
P _ps : Pressure on the permeated water side under standard operating conditions (kPa)
Π _fbs : Osmotic pressure of the average solute concentration on the supply side and concentration side under standard operating conditions (kPa)
TCF _s : Temperature conversion coefficient under standard operating conditions

In addition to the amount of decrease from the initial corrected permeate flux, there are other causes such as an excess of the differential pressure in the second desalination device, a decrease in the amount of water processed by the second desalination device, and a small RO installed on the concentrated water side of the second desalination device. The normal operation may be switched to the recovery operation depending on a decrease in the amount of permeated water, a sensor using ultrasonic waves is set in the pipe, and whether or not supersaturated precipitates are detected in the pipe.

In the above embodiment, raw water is used as the dilute water, but permeated water from the first RO device or the second RO device may be used as the dilute water. Alternatively, a mixture of permeated water from the first RO device or the second RO device and raw water may be used as the dilute water. Furthermore, a mixture of the permeate water from the first RO device or the second RO device and the first concentrated water may be used as the dilute water.

[Water quality of first concentrated water]
The present invention can be suitably used when calcium fluoride scale or calcium carbonate scale is generated in the second RO device. Specifically, the first concentrated water supplied to the second RO device can be suitably used in the following cases a to e. Note that a and b are cases in which calcium fluoride scale is generated, and c is a case in which calcium carbonate scale is generated.
a. Calcium ion concentration 0.1-10mg/L, fluoride ion concentration 3000-8000mg-F/L.
b. Calcium ion concentration 500-1500mg/L, fluoride ion concentration 50-150mg-F/L.
c. Calcium ion concentration 400-1500mg/L, M alkalinity 800-2000mg/L.

[Scale inhibitor added to dilute water]
It is preferable to add a scale inhibitor to the dilute water. By adding a scale inhibitor, it is possible to improve the scale dissolving power and prevent the dissolved scale from re-deposition.

The scale inhibitor can be selected as appropriate depending on the type of desalination equipment used and the raw water.If a reverse osmosis membrane device is used as the desalination equipment and the water to be treated generates calcium carbonate scale, 2- Phosphonic acids such as phosphonobutane-1,2,4-tricarboxylic acid, copolymer polymers of acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid, polyacrylic acid, etc. can be used, and calcium fluoride scale is generated. In the case of such water to be treated, phosphonic acids such as 2-phosphonobutane-1,2,4-tricarboxylic acid, polyacrylic acid, etc. can be used. Further, the amount of these scale inhibitors added is about 10 to 1000 mg/L.

Note that a pH adjuster may be added to the dilute water to improve the scale dissolving power and prevent the dissolved scale from re-deposition.

[Direction of dilute water flow]
When raw water is passed through as diluted water as shown in Figure 1, it is preferable to pass the water from the water supply side of the second RO device. When using one with good quality, water may be passed through from the concentrated water outlet side of the second RO device. Further, while diluted water is being passed through, the recovery rate may be maintained within a range of less than the saturated solubility of concentrated water in the desalination apparatus, and water may be passed while producing treated water.

In the above embodiment, the RO devices are installed in two stages, but they may be installed in three or more stages. In addition, when it is installed in three or more stages, it is preferable that the desalination device for passing diluted water be the one in the last stage.

[Scale dissolution test]
<Test purpose>
A calcium carbonate fine particle dispersion liquid and a calcium fluoride fine particle dispersion liquid are prepared, diluted water is added to each liquid, and the solubility characteristics of the fine particles are measured.

<Preparation of calcium carbonate fine particle dispersion>
Pour 500 mL of ultrapure water into a 500 mL conical beaker to prepare an aqueous solution containing 340 mg/L of calcium chloride, 10 mg/L of scale inhibitor, and 1300 mg/L of sodium bicarbonate, and then add an aqueous solution of sodium hydroxide or The pH was adjusted to 8.5 with an aqueous hydrochloric acid solution to prepare a starting solution. The starting solution was stirred using a stirrer under room temperature conditions of 25° C. until scale was generated, and left for a predetermined period of time. By varying the standing time, scale solutions with different particle sizes were prepared.

<Preparation of calcium fluoride fine particle dispersion>
Pour 500 mL of ultrapure water into a 500 mL conical beaker to prepare an aqueous solution containing 100 mg/L of sodium fluoride, 5 mg/L of scale inhibitor, and 400 mg/L of calcium chloride, and add aqueous sodium hydroxide solution or The pH was adjusted to 5.5 with an aqueous hydrochloric acid solution to prepare a starting solution. The starting solution was stirred using a stirrer under room temperature conditions of 25° C. until scale was generated, and left for a predetermined period of time. By varying the standing time, scale solutions with different particle sizes were prepared.

For each liquid, the particle size of the generated scale was measured using a particle size distribution meter (Shimadzu SALD-7500nano).

<Mixing with dilute water>
In a 500 mL conical beaker, the fine particle dispersion prepared as above and diluted water (RO permeated water from the RO membrane for wastewater recovery at Kurita Industrial Development Center (Nogi-machi, Shimotsuga-gun, Tochigi Prefecture)) are shown in Table 1. After 5 minutes had elapsed, the mixture was visually observed and measured for particle size distribution to determine the presence or absence of dissolution of fine particles. The results are shown in Table 1.

<Consideration>
No. In No. 1 and No. 2 (CaCO ₃ scale particle diameter 5 μm), the scale fine particles were dissolved by adding dilute water, but in No. 4, 5 (mixing ratio 50/50 or 60/40) and No. In samples 6 to 9 (CaCO ₃ scale particle size 20 μm), the scale fine particles were not completely dissolved and remained.

No. In No. 3 (CaF ₂ scale particle size 0.1 μm, mixing ratio 10/90), the fine particles were dissolved by adding dilute water, but in No. When the mixing ratio was 10 to 12 (mixing ratio 20/80 to 60/40), the scale particles were not completely dissolved and remained.

[Examples 1 to 6]
The flat membrane RO device shown in Figure 2, which is equipped with a rectangular RO membrane with a membrane surface size of 95 mm x 146 mm, was equipped with simulated raw water and simulated diluted water (in Example 4, a scale preventive agent was added) prepared as follows. Added simulated dilute water) was passed in the following three steps in order.

<Water flow order>
First simulated raw water passing step: Simulated raw water was passed through with an initial permeation flux of 0.45 m/D and a water flow rate of 0.1 m/s, and operation was performed with a constant amount of permeated water. (Therefore, the corrected transmission flux gradually decreases over time.)
Dilute water flow process: After the corrected permeation flux after water flow has decreased by 20 to 25% compared to the initial corrected permeation flux, dilute water is passed at a flow rate of 0.1 m/s for the time shown in Table 2. , to pass water.
Second simulated raw water passage step: The simulated raw water is passed at the same pressure and permeate amount as in the first simulated raw water passage process.

<Simulated raw water>
Calcium chloride dihydrate and sodium hydrogen carbonate were dissolved in pure water to give a Ca concentration of 600 mg/L and a Na concentration of 600 mg/L.

M alkalinity: 850mg/L as _CaCO3
pH: 8.4-8.5
Water temperature: 30℃

<Simulated dilute water>
The simulated raw water with a Ca and Na concentration of 1/10 was used. pH: 7.2-7.3, water temperature 30°C.

<Simulated diluted water with anti-scaling agent added>
The above simulated dilute water was prepared by adding 10 mg/L of 2-phosphonobutane-1,2,4-tricarboxylic acid as a scale inhibitor.

[Comparative example 1]
Simulated raw water was passed in the same manner as in Example 1 except that dilute water was not passed.

[Results and discussion]
The ratio F/F ₀ between the flux F and the initial flux F ₀ immediately before the end of the first simulated raw water passage process is shown in Table 2 as "Flux ratio before dilute water passage".

The ratio F'/F ₀ of the flux F' immediately after the start of the second simulated raw water passage process and the initial flux F ₀ is shown in Table 2 as "Flux ratio after dilute water passage".

The value of [Flux ratio after passing dilute water]/[Flux ratio before passing dilute water] is shown in Table 2 as a "recovery ratio".

As shown in Table 2, the flux is recovered (increased) by performing the dilute water flow step. In particular, as in Examples 1 to 4, the flux is sufficiently recovered by carrying out the dilute water flow step for 10 minutes or more, particularly 30 minutes or more. Further, as in Example 4, by adding a scale inhibitor to the diluted water, the flux is more fully recovered.

[Example 7]
<Simulated raw water>
As the simulated raw water, calcium chloride dihydrate, magnesium chloride, aluminum chloride, and sodium fluoride were dissolved in pure water so that the concentrations of Ca, Mg, Al, and F were as follows.

Ca: 0.5mg/L
Mg: 4mg/L
Al: 0.25mg/L
F: 4000mg/L
(Water temperature 22-23℃, pH: 5.5)

<Dilute water>
As the diluted water, the permeated water obtained when the simulated raw water was passed through the RO apparatus shown in FIG. 2 was used.

<Water flow order>
The water flow order was as follows. The water flow time in each step was as shown in FIG. 4.
First simulated raw water passing step: Simulated raw water is passed through at a constant amount of permeated water (0.45 m/D).
Diluted water passing step: Diluted water is passed for 45 minutes.
Second simulated raw water passage process: Simulated raw water is passed through at the same water supply pressure as in the first simulated raw water passage process.

<Results and discussion>
The results are shown in Table 3 and FIG. 3(a). As shown in Table 3 and FIG. 3(a), according to Example 7, the flux was sufficiently recovered.

[Comparative example 2]
Simulated raw water and diluted water were prepared in the same manner as the simulated raw water preparation method of Example 7 so that each concentration of Ca, Mg, Al, and F was as follows.

The water flow order was the same as in Example 7. The water flow time was as shown in FIG. 3(b).

<Simulated raw water>
Ca: 0.4mg/L
Mg: 2mg/L
Al: 0.25mg/L
F: 4000mg/L
(Water temperature 22-23℃, pH: 5.5)

<Simulated dilute water>
Ca: 0.1mg/L
Mg: 0.8mg/L
Al: 0.8mg/L
F: 500mg/L

<Results and discussion>
The results are shown in Table 3 and FIG. 3(b). As shown in Table 3 and FIG. 3(b), in Comparative Example 2, the salt concentration of the diluted water is high, so the flux does not recover even if diluted water is passed.

[Examples 8 to 10, Comparative Examples 3 to 6]
The simulated raw water and simulated diluted water (simulated diluted water with scale inhibitor added in Examples 9 and 10 and Comparative Examples 3 to 6) prepared as shown below were added to the flat membrane type RO device in Figure 2 in the following three steps. Water was passed in sequence.

<Water flow order>
First simulated raw water passing step: Simulated raw water was passed through with an initial permeation flux of 0.45 m/D and a water flow rate of 0.1 m/s, and operation was performed with a constant amount of permeated water. (Therefore, the corrected transmission flux gradually decreases over time.)
Dilute water flow process: After the corrected permeation flux after water flow has decreased by 20 to 25% compared to the initial corrected permeation flux, dilute water is passed at a flow rate of 0.1 m/s for the time shown in Table 2. , to pass water.
Second simulated raw water passage step: The simulated raw water is passed at the same pressure and permeate amount as in the first simulated raw water passage process.

<Simulated raw water>
Calcium chloride dihydrate and sodium fluoride were dissolved in pure water to give a Ca concentration of 650 mg/L and an F concentration of 70 mg/L.
pH: 5.5
Water temperature: 22-23℃

<Simulated dilute water>
In Example 8, RO treated water from the RO equipment in the wastewater treatment equipment of Kurita Industries, Ltd. Development Center was used as the simulated dilution water. (pH: 5.5)

<Simulated diluted water with anti-scaling agent added>
In Example 9, 10 mg/L of 2-phosphonobutane-1,2,4-tricarboxylic acid was added to the simulated dilute water of Example 8 as a scale inhibitor.
In Example 10, calcium chloride dihydrate and sodium fluoride were further added to the simulated dilute water of Example 9 so as to have the concentrations shown in Table 3.
In Comparative Example 3, calcium chloride dihydrate and sodium fluoride were further added to the simulated diluted water of Example 9 so as to have the concentrations shown in Table 3.
In Comparative Example 4, the simulated dilute water of Comparative Example 3 was adjusted to pH 3.5 with hydrochloric acid.
In Comparative Examples 5 and 6, calcium chloride dihydrate and sodium fluoride were further added to the simulated dilute water of Example 9 to have the concentrations shown in Table 3, and the pH was adjusted to 3 with hydrochloric acid. .

[Results and discussion]
The ratio F/F ₀ between the flux F and the initial flux F ₀ immediately before the end of the first simulated raw water passage process is shown in Table 4 as "Flux ratio before dilute water passage".

The ratio F'/F ₀ of the flux F' immediately after the start of the second simulated raw water passage process and the initial flux F ₀ is shown in Table 4 as "Flux ratio after dilute water passage".

The value of [Flux ratio after passing dilute water]/[Flux ratio before passing dilute water] is shown in Table 4 as a "recovery ratio".

As shown in Table 4, the flux is recovered (increased) by performing the dilute water flow step. In particular, as in Examples 8 and 9, the flux is sufficiently recovered by lowering the salt concentration of the dilute water.

1 Raw water tank 4, 4A to 4D 1st RO device 10 Relay tank 15, 51 to 54 2nd RO device

Claims (9)

In a method for operating a desalination device having a first desalination device and a second desalination device,
The water to be treated is supplied to a first desalination device and separated into first concentrated water and first desalinated water, and the first concentrated water is supplied to a second desalination device to produce second concentrated water. and a normal operation step of separating into second demineralized water and second demineralized water;
The water to be treated is supplied to the first desalination device and separated into the first concentrated water and the first desalinated water, and the second desalination device is supplied with water having a lower concentration than the first concentrated water and having a saturated solubility. A recovery operation step of passing diluted water for 10 to 60 minutes to dissolve scale attached to the second desalting device and restoring the desalting performance of the second desalting device. How to operate salt equipment. A claim in which a plurality of the second desalination apparatuses are installed in parallel, and while some of the second desalination apparatuses are performing the normal operation process, other second desalination apparatuses are performing the recovery operation process. How to operate the desalination equipment in Section 1. The method of operating a desalination apparatus according to claim 1 or 2, wherein the water to be treated is used as dilute water. 3. The method for operating a desalination apparatus according to claim 1, wherein the desalination water of the first desalination apparatus is used as the dilution water. The method for operating a desalination apparatus according to any one of claims 1 to 4, wherein a scale inhibitor is added to the diluted water. The method of operating a desalination apparatus according to claim 1, wherein the first desalination apparatus and the second desalination apparatus are reverse osmosis membrane apparatuses. The method for operating a desalination apparatus according to claim 6, wherein the dilute water flow rate is 0.001 to 1 m/s. The method for operating a desalination apparatus according to any one of claims 1 to 7, wherein the quality of the first concentrated water is one of the following a to c.
a. Calcium ion concentration 0.1-10mg/L, fluoride ion concentration 3000-8000mg-F/L.
b. Calcium ion concentration 500-1500mg/L, fluoride ion concentration 50-150mg-F/L.
c. Calcium ion concentration 400-1500mg/L, M alkalinity 800-2000mg/L. 9. The method of operating a desalination apparatus according to claim 1, wherein the first concentrated water is concentrated water that is three times or more concentrated the water supplied to the first desalination apparatus.

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