JPS61283392A - Treatment of fresh water made by reverse osmosis apparatus - Google Patents

Abstract

PURPOSE:To reduce operation cost, by adjusting not only pH of raw water to be supplied so as to set the hardness of drinking water after passed through a filter to a predetermined value but also the quantity of an alkali agent so as to set pH of drinking water to a predetermined value. CONSTITUTION:Raw sea water receiving the injection of chlorine from a line (a) is sent into a maintenance filter 1 while sulfuric acid is injected in the line (a) from a sulfuric acid tank 2 through a line (b). The pH of raw sea water after the injection of sulfuric acid is detected by a pH detection terminal 5 and the opening degree of a sulfuric acid injection control valve 4 is controlled through a regulator 3 corresponding to the deviation of an objective value and a detection value so as to set the pH value to an objective value. Made-up fresh water is taken out from a line (d) and passed through a filter 8 to be withdrawn through a line (e) but an alkali solution is injected in the line (e) from a line (f) to adjust pH value.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for obtaining drinking water by adding hardness components to fresh water produced by reverse osmosis.

The method is applicable to the treatment of freshwater produced from seawater or brine, but is particularly suitable for the after-treatment of freshwater produced from seawater.

[Conventional technology]

Compared to other seawater desalination technologies such as evaporation, the reverse osmosis method consumes less energy per amount of water produced, and is said to consume less energy than the evaporation method and about 1 to 1 compared to the electrodialysis method. In addition, large-scale equipment has also started operating.

When fresh water produced by seawater desalination using reverse osmosis is used for drinking water, TDS (dissolved solids concentration) is usually 1.
is set to be 500 to 1000 η/l or less. The main component of TDS is sodium chloride, which is also the main component of seawater, and a small amount of various ions dissolved in seawater are also present.
Multivalent ions are almost completely removed, so hardness components such as calcium and magnesium are hardly included.

Furthermore, since original diving is normally operated under acidic conditions, the fresh water produced is also slightly acidic and contains free carbon dioxide gas. Therefore, if left as is, it tends to corrode, so if the above fresh water is directly conveyed using general-purpose water conveyance equipment, the steel pipes used in the water conveyance equipment will corrode and the concrete material will elute, which may impede the functionality of the water conveyance equipment. There is a risk of loss.

On the other hand, when used as drinking water, it not only tastes bad due to the reasons mentioned above, but it has also been pointed out that lack of hardness components can cause heart disease.

Conventionally, for example, as introduced in the document ``Membrane Utilization Technology No. Ndpukku'' (edited by Oya, published in 1983, pages 32-33), carbon dioxide is extracted from fresh water obtained by reverse osmosis using a degassing tower, etc. A method of removing it or a method of readjusting the pH by adding an alkali agent is used.

[Problem that the invention seeks to solve]

If a decarboxylation tower is used to release carbon dioxide contained in fresh water produced by reverse osmosis.

Although pH can be adjusted, hardness components cannot be added. Furthermore, when an alkali such as caustic soda or soda carbonate is used, only the pH can be adjusted in the same way. Quicklime as an alkaline agent.

It is possible to add hardness components by using slaked lime, etc.
Since the pH value and the amount of hardness component added are interrelated, it is difficult to adjust both to predetermined values. Therefore, conventionally, there has been a disadvantage that either the PH value or the hardness component has to be given priority and set to a predetermined value.

The present invention aims to solve the above problems.

[Means for solving problems]

In order to achieve the above object, the present invention makes fresh water produced by a reverse osmosis device pass through a filter consisting of a bed of granules made of at least one of limestone and dolomite, and then an alkaline agent is added thereto to produce drinking water. In the method, the PH value of the raw water supplied to the reverse osmosis device is adjusted so that the hardness of the drinking water after passing through the filter becomes a predetermined value, and the OPH value of the drinking water after passing through the filter becomes the predetermined value. The present invention relates to a freshwater treatment method characterized by adjusting the amount of an alkaline agent added.

[Effect]

The present invention focuses on the fact that the fresh water produced by the reverse osmosis device is slightly acidic and contains free carbon dioxide gas, and the carbon dioxide concentration depends on the OPH value of the raw water supplied to the reverse osmosis device. It was made by

The brine or seawater targeted by the present invention usually contains a carbonic acid component. For example, in seawater there are usually 2 to 3 y+mnt.
/l of carbonic acid component is dissolved. In reverse osmosis equipment, in order to prevent the performance of the reverse osmosis membrane from deteriorating due to the precipitation of carbonic acid components during operation, the raw seawater is often operated on the acidic side with a pH of 5 to 7, as described above.

According to the present invention, when raw water adjusted to be acidic is supplied to a reverse osmosis device, the produced fresh water also contains free carbon dioxide gas, and the free carbon dioxide concentration depends on the pH value of the raw water. This was based on experimental facts.

That is, when the PH value of raw water is lowered, the free carbonate concentration in the fresh water produced increases, and the PH value of the fresh water decreases. Conversely, when the PH value of raw water is increased, the free carbonate concentration in fresh water decreases, and the PH value of fresh water increases.

The fresh water adjusted as described above first passes through a filter consisting of a packed bed of granules made of at least one of limestone and dolomite, and then free carbon dioxide is released according to the following equations (1) and/or (2). The filter agent reacts with the filter agent, and the hardness component dissolves.

CaCO3+ CO2+ H2O4Ca (HCO3)
+1 (1)Ca@Mg(CO3)2 + 2CO
2+ 2H2Og Ca (HCO3)2+Mg (H
At this time, the filter agent is dissolved in an amount commensurate with the free carbon dioxide (CO2) concentration. In other words, if the raw water supplied to the reverse osmosis device in the previous stage is set to a high OPH value,
As a result, the free carbonate concentration in the filtered freshwater decreases.
The hardness of drinking water decreases after passing through the filter. Conversely, if the pH value to be supplied is set low, the hardness of drinking water after passing through the filter will increase.

Also, within the filter, the reactions of equations (1) and (2) proceed from the right side to the left side, and as a result, the PH value of fresh water increases, and in this case, the OPH value of drinking water after passing through the filter is It has been found that, given operating conditions, it depends on the free carbonate concentration of the filter population. In other words, if the free carbon dioxide concentration is high, the pH value after passing through the filter will decrease.

If the free carbonate concentration is low, the pH value will increase.

From the above effects, by adjusting the OPH value of raw water supplied to the reverse osmosis device, it is possible to adjust the hardness of drinking water after passing through the filter.

Furthermore, since the pH value after passing through the above-mentioned filter changes in correlation with the hardness, an alkaline agent is further added to finally set it to a predetermined value. Of course, if the pH value after passing through the filter is a predetermined value, it is not necessary to add an alkaline agent. Caustic soda, soda carbonate, etc. can be used as the alkali agent. If the hardness component reaches a predetermined value, quicklime, slaked lime, etc. can also be used.

In the case of caustic soda and soda carbonate, there is no change in the hardness components (Ca and Mg) due to addition, as shown in the following formula.

CO2+NaOH-+NaHCO3(3)Co2+Na
2CO3+H20→2NaHCO3 (4) On the other hand, when the pH value is adjusted using quicklime, slaked lime, etc., the hardness component is also added at the same time as shown in the quadratic equation.

2CO2+CaO+H20-+Ca (HCO3),
(5)2CO2+Ca (OH)2-+C
a (HCO3)2' (6) Therefore, if the hardness component increases by using quicklime, slaked lime, etc. and the result is a predetermined value, it is possible to use it.

According to the present invention, since the carbonic acid component originally contained in seawater can be effectively used as a hardness component elution source, a carbonic acid source is not particularly required.

It is also advantageous in terms of reducing operating costs.

〔Example〕

FIG. 1 shows the case where the present invention is applied to the treatment of fresh water obtained from seawater.

Water that has undergone chlorine injection or a sand filtration process (not shown) is sent to the safety filter 1 from line a. Sulfuric acid is injected into line a from a sulfuric acid tank 2 through line b. The principle water PH after sulfuric acid injection is detected by the PH detection end 5, and the controller 3 is set so that this PH value becomes the target value.
The opening degree of the sulfuric acid injection control valve 4 is adjusted according to the deviation between the target value and the detected value.

Next, the principle water that has passed through the safety filter 1 is pressurized by a high-pressure pump 6, and then sent to a reverse osmosis device 7, and then taken out from lines C and d as concentrated seawater and manufactured fresh water, respectively. Concentrated seawater C is directly discharged from the system.

In some cases, it may be discharged through a power recovery turbine (not shown).

The produced fresh water passing through line d is then fed to a filter 8 filled with limestone granules, and the fresh water to which hardness components have been added is extracted through line e. An alkaline solution whose injection amount is adjusted by an alkali injection control valve 12 from the alkaline tank 9 is injected into line e through line f, and the drinking water with its pH value adjusted is drawn out of the system through line g. The pH value is detected by the detection end 10, and the opening degree of the alkali injection control valve 12 is adjusted via the controller 11 so that the pH value becomes the target value. The hardness of drinking water is detected by the detection end 13, and the controller 14 is adjusted according to the deviation from the target value so that this hardness component becomes the target value.
The target value of the controller 3 is controlled in a cascade manner.

That is, when the detected drinking water hardness is low, the target pH value is adjusted and lowered by a total of 30 PH, and conversely, when the hardness of the drinking water is high, the target value is increased. Of course, the hardness is detected intermittently, and the controller 3
A method of manually controlling the PH target value is also possible.

Next, an experimental example of the present invention will be shown. In the first experimental example, seawater was treated by the freshwater treatment method shown in FIG. The amount of fresh water produced by the reverse osmosis device was 10 t/-, and the entire amount was passed through a filter 8 to be treated as drinking water. Filter 8 was filled with limestone having a sieve diameter of IWrIn to 4-4. Total hardness of drinking water is 681ng/L ascacO
3 and decided the target value of the Ruyou controller 14. Also, controller 1
The target value of 1 was set to PH=7.5. Twelve samples were taken every 2 hours from line g. The analysis results of drinking water were as follows.

The properties of raw seawater during the above sampling period were as follows.

■ Raw seawater passing through a line (before H2S 04 injection) PH=
8.0 M-alkali = 148 (ppm as (acos
) ■ Raw seawater PH on the upstream side of reverse osmosis device 7 = 6.70 M-alkali = 1107 (pp ascacO3) M
- Arca! J (Measurement method of HCO3 amount specified by JIS.

M-The larger the alkali, the more HCO; therefore, the contained CO
2 amount increases, and the hardness also increases. ) was prepared by adding sodium bicarbonate to seawater. Caustic soda was also used as an alkali agent.

Next, in the second experimental example, the total hardness was 931Ni/1asca.
The target value of the controller 14 was determined to be cO3. or.

The engine was operated with the target value of the controller 11 set to 7.7. Samples were taken periodically from line g.

Other operating conditions and sampling procedures were the same as in Experimental Example 1. The properties of the raw seawater (on the upstream side of the reverse osmosis device 7) are as follows.

PH=6.50 M-alkali=89.8 (ppm ascacO2
) The analysis results of the sampled drinking water were as follows.

As typified by the above two experimental examples, by changing the pH of the raw water supplied to the reverse osmosis device.

The effect of adjusting the total hardness of drinking water to a predetermined value has become clear.

[Effects of the Invention] As described above, according to the two methods of the present invention, the hardness component and PH value of fresh water produced by a reverse osmosis device can be arbitrarily adjusted, and good drinking water with less corrosivity can be obtained. There is. Moreover, the carbon dioxide gas in the raw water originally supplied to the reverse osmosis device can be effectively used, resulting in the effect of reducing operating costs.

[Brief explanation of drawings]

FIG. 1 is a flow chart showing one embodiment of the method of the present invention. 1...Safety filter, 2...Sulfuric acid tank, 3...
・Controller, 4...Sulfuric acid injection control valve, 5.
...PH detection end, 6...high pressure pump, 7...reverse osmosis device, 8...filter, 9...alkali tank. 10...PH detection end, 11...controller, 12...
Alkali injection control valve, 13... sampling cock

Claims (1)

[Claims] In a method for obtaining drinking water by passing fresh water produced by a reverse osmosis device through a filter consisting of a packed bed of granules containing at least one of limestone and dolomite, and adding an alkaline agent to the fresh water after passing through the filter. The pH value of the raw water supplied to the reverse osmosis device is adjusted so that the hardness of the drinking water becomes a predetermined value, and the amount of the added alkali agent is adjusted so that the PH value of the drinking water after passing through the filter becomes a predetermined value. A method for treating freshwater produced by a reverse osmosis device, characterized in that: