JPS61167495A - Production of potable water - Google Patents

Abstract

PURPOSE:To eliminate entirely the need for an alkaline agent which is hereto fore required or to require just a small amt. thereof even when said agent is necessary by passing a gas contg. no carbon dioxide into fresh water after passage through a filter consisting of limestone, etc., to maintain the pH of said fresh water at a prescribed value. CONSTITUTION:This method produces potable water by blowing the gas contg. the carbon dioxide generated form a desalting device 1 for sea water by an evaporation method to the fresh water formed by said device 1 in a CO2 absorption column 2 to form the carbonic acid-contg. water then passing the water through the filter 3 packed with the granular limestone and/or dolomite. The gas contg. no carbon dioxide is passed from a blower 5 to the fresh water in a water storage tank 4 after the passage through the filter 3 to maintain the pH of the fresh water in the tank 4 at the prescribed value in the above- mentioned method. The need for the alkaline agent which is heretofore required for pH adjustment is eliminated entirely or the amt. thereof to be used is considerably decreased, by which the running cost is considerably reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for obtaining drinking water by cooling a hardness component to fresh water produced in a seawater desalination apparatus using an evaporation method.

[Technical background]

Since the fresh water obtained by the evaporative seawater desalination equipment is distilled water, it contains almost no various ions and dissolved gases, and almost no hardness components such as calcium or magnesium. For this reason, if all of the fresh water mentioned above is used for water transmission using general-purpose water transmission equipment, corrosion of the steel pipes used in the water transmission equipment and elution of the concrete material may occur, which may impair the functionality of the water transmission equipment. There is. On the other hand, when used as drinking water, it has been pointed out that for the reasons mentioned above, it is tasteless and not only tastes bad as drinking water, but can also cause heart disease.

[Conventional technology]

For this reason, for example, the document ``Desalination''
n.

39 (1981) 505-520J, quicklime, slaked lime, limestone, dolomite, etc. have been used as hardness increasing agents, and a method of dissolving these in fresh water has been carried out, but among these, limestone and/or When using dolomite, carbon dioxide gas is blown into a filter filled with granules of dolomite (in drinking water production equipment, it is called a packed bed of limestone and/or dolomite ft filter), and fresh water is introduced. A common method is to increase the hardness of water by eluting calcium or magnesium as bicarbonate.

As a source of carbon dioxide, methods have been proposed, such as recovering carbon dioxide from exhaust gas obtained by separately burning fuel, and transporting carbon dioxide gas in carbon dioxide cylinders. Methods of effective utilization are attracting attention as they are attractive from an economical perspective.

[Problem that the invention seeks to solve]

Seawater desalination equipment using the evaporation method is generally large-scale, and
The amount of fresh water produced per group is 20, OOO~s s, o
o o tons/day, and several to dozens of units are constructed in the same location. Therefore, the amount of fresh water processed by the drinking water conversion equipment, which is ancillary equipment, is also increased, and the consumption of alkaline agents, which are used for the purpose of water after passing through the filter, increases, which poses a problem in terms of operating costs. . As an alkali agent, IJum (hydroxide) or sodium carbonate can be used, but securing the alkali agent becomes a problem when installing a drinking water conversion device in a place where it is difficult to obtain these agents.

[Means for solving problems]

The present invention was discovered as a result of intensive research in order to solve the problems of the above-mentioned conventional methods. This is a method for producing drinking water in which setting the voice to a predetermined value is a t-% sign.

Specifically, fresh water produced in a seawater desalination device using the evaporation method is made into carbonated water by blowing over the carbon dioxide-containing gas generated from the desalination device, and then filtered with limestone and/or dolomite granules. In the method of obtaining drinking water by passing the fresh water through the filter, a complete proposal for a method for producing drinking water that requires completely venting gas that does not contain carbon dioxide to the fresh water after passing through a filter to bring the - of the fresh water to a predetermined value. It is something to do.

[Effect]

The great advantage of the method of the present invention is that the fresh water after passing through the filter
Since a predetermined value is achieved by increasing the output value by fully venting gas that does not contain carbon dioxide gas, there is no need for an alkaline agent, which was conventionally required, or even if it is necessary, only a small amount is required. When carbonated water passes through a filter filled with limestone and/or dolomite, hardness components of calcium and/or magnesium are eluted according to the following Tl1 formula and/or (21 formula).

OaG 01 +OO2+ H2O@ca (HCO
2) 2 (11Ca-Mg(Co)
+2GO+2HO#0a(HGO) +Mg(HGO3
)z(2)is222S
Since the liquid phase reactions shown in equations 2+11 and (21) are equilibrium reactions, unreacted carbon dioxide remains in the water after passing through the filter, and therefore the general Vc-value tends to be low. ...A value as low as 1 is not only unsuitable for drinking ((WHO considers 7.0 to 8.5 to be appropriate), but also the Langel index, which is an index showing the tendency of water to corrode.
iar 5azurationIndex becomes negative,
It becomes impossible to achieve the original purpose of making water into a potable water by improving the water's tendency to corrode. Therefore, the conventional method has been to use alkaline agents such as sodium hydroxide or sodium carbonate to neutralize the carbon dioxide gas remaining in the filter outlet water and adjust the output value, as shown in the following equation.

Co2+ NaOH-j! NaHCC)g,
(31Go2+ Na2Go, + H2O
: 2NaHCO, +41 The inventors of the present invention have conducted extensive studies on a method for reducing the amount of alkaline agents mentioned above, and have found that by aerating gas that does not contain carbon dioxide into the water that has passed through the filter and contains residual carbon dioxide, the residual carbon dioxide can be removed from the water. The present invention was based on the discovery of the fact that the release value increases due to increased release.

As the ventilation gas, a gas containing no carbon dioxide gas such as nitrogen gas is preferable, but it is also possible to use air, which is easily available. However, since air contains about 0.05% carbon dioxide gas, there is a limit to the diffusion of carbon dioxide gas due to vapor-liquid equilibrium, but this can be overcome by adding an alkali agent. Therefore, the present invention also includes adding an altar agent if necessary after gas aeration, depending on the type of gas used and its value in fresh water. As the gas venting device, an aeration tank type device, which has a good track record as a general-purpose facility for water treatment, is sufficient and does not require any special measures. As the aeration device, an aeration grid, a porous aeration pipe, etc. can be used, and if a water storage tank is provided, it is also possible to attach only the aeration pipe to the water storage tank and use it as a gas ventilation device.

...The set value after adjustment is often in the range of 7.5 to 8.5, but if the set value is high, it may be impossible to raise it to the specified μ value with ventilation alone. In the case of kernels, an alkaline agent is further added after aeration to obtain a predetermined value. Of course, in this case as well, the amount of alkaline agent required can be significantly reduced compared to when the μ value is adjusted using an alkaline agent alone. In addition, due to the relationship between gas and liquid, the rate of carbon dioxide dissipation decreases as the liquid level rises, making it necessary to increase the capacity of the gas venting device.To deal with this, it may also be effective to add an alkaline agent. .

[Embodiment]

Next, in order to clarify embodiments of the method of the present invention, a description will be given based on the drawings.

FIG. 1 shows the case where the present invention is applied to the drinking of fresh water obtained from a multi-stage flash evaporation method of seawater.

Nine freshwater produced from the multi-stage flash evaporator 1 of seawater is extracted from line a, bypass line b and line 0.
It is branched by

Next, the fresh water branched by line C is further branched by line d and line 0, and the fresh water passing through line dt is introduced into the CO2 absorption tower 2.

The CO2 absorption tower 2 contains CO extracted from the multi-stage flash evaporator lt, 1 and pressurized by the compressor 7.
2-containing gas is introduced through line ni. The CO2 absorption tower 2 absorbs CO2t and turns it into carbonated water, which is extracted through the line ft-, and after mixing with the fresh water that has gone up through the CO2 absorption tower 2t and the bypassing line e1il, it is transferred to the line gt.
- through which the filter 5VC is introduced. unabsorbed CO2
Gases such as nitrogen and oxygen are discharged to the outside of the system through line Ot.

As the CO2 absorption tower 2, a packed tower or a line mixer is suitable.

Next, the carbonated water sent from line g is filtered to filter 3.
During passing through the filter, the limestone and/or dolomite particles are dissolved, increasing the hardness and total alkalinity, and then extracted from the line h and passing through the air diffuser i
ll'ft: 1# is supplied to the water storage tank 4. Air is introduced into the water storage tank 4 from an air blower 5 through a line mt to diffuse a small amount of CO2 gas remaining in the water and increase the liquid value. Air containing CO2 gas is released from line j to the outside of the yarn. The released gas contains moisture, but since the water temperature is usually below 40°C, the vapor pressure is low and there is little dissipation of produced water.

Water extracted from the water storage tank 4 through line 1 is mixed with fresh water flowing through line bt-a, and after being adjusted to a predetermined hardness, is sent through line jt- to the final conditioning step. In the -adjustment process, an alkali solution is injected from the alkali tank 6 into the line j through the line kt-, and is adjusted to a predetermined value. If the set value is low and the water passing through line j1a has already reached a predetermined value, the final adjustment step described above is not necessary. pHi! 4. The drinking water that has been prepared is taken out from the line 1.

(Example 1) Next, an example will be shown to clarify the effects of the present invention.

Fresh water and carbon dioxide-containing gas produced by a multi-stage flash evaporation device for seawater were separated and treated according to the drinking water production method shown in FIG. The amount of fresh water extracted is 4001/h, of which 200 t/h is branched and sent to the carbon dioxide absorption process via line C, and the rest is sent to the bypass line B.
T-flowed.

As the absorption tower 2, a Raschig ring light filter was used, and for the filter 5, a limestone light filter with a sieve diameter of 111I to 4mm was used. The filling volume was 20I3. 15.3 mN/h of air was fed from an air blower 5 into the water storage tank 4 equipped with an aeration grid for decarbonation.

What is the amount of carbon dioxide-containing gas injected from line n so that the total hardness of the final treated drinking water taken out from line 1 is 60 rng/j based on calcium carbonate? Set. or,
The amount of alkaline agent (caustic soda) injected was adjusted so that the p)I of the drinking water after final treatment was 8.4.

Continuous operation was carried out for two days under the above conditions, and the daily consumption of caustic soda was measured, and the following results were obtained.

Consumption amount of caustic soda = 191 (Example 2) Fresh water was treated by the same method as in Example 1 under the following conditions.

The amount of fresh water taken was 400 l/n, the same as in Example 1, of which 100 t/h was branched off and sent to the carbon dioxide absorption process through line C, and the rest was passed through bypass line bt. The filter was filled with the same limestone T-106 as in Example 1. Other operating conditions were the same as in Example 1, continuous operation was carried out for two days, and the daily consumption of caustic soda was measured, and the following results were obtained.

Consumption amount of caustic soda = 77 g (Comparative Example 1) Continuous operation was carried out for two days under the same conditions as in Example 1 without introducing air into the water storage tank 4, and the daily consumption amount of caustic soda was calculated. When measured, the following results were obtained.

Consumption amount of caustic soda: 265 g (Comparative Example 2) Continuous operation was carried out for two days under the same conditions as in Example 2 without introducing air into the water storage tank 4, and the daily consumption amount of caustic soda was calculated. The measurements yielded the following results:

Consumption amount of caustic soda = 152.9 Comparing the example and the comparative example, the present invention shows that if the fresh water after passing through the filter is decarboxylated by aeration, the amount of alkali agent consumed for final adjustment can be completely reduced. The effects of applying this are clear.

〔effect〕

According to the method of the present invention, the alkaline agent conventionally required for conditioning the fresh water after passing through the filter is not required at all or can be significantly reduced. Since the amount of fresh water that is processed to make it into drinking water is the same as that of an art university, the award for alkaline consumption is also high. Therefore, its reduction is very effective in reducing operating costs.

[Brief explanation of the drawing]

FIG. 1 is a flow sheet for explaining the method for producing drinking water according to the present invention. 1... Multi-stage flash evaporator 2... GO□ absorption tower 3... Filter 4... Water storage tank with aeration device 5... Air blower 6... Alkali agent tank 7... Compressor Sub-agents 1) Meifuku agent Ryo Hagiwara -

Claims (1)

[Claims] Fresh water produced by a seawater desalination device using the evaporation method is made into carbonated water by blowing carbon dioxide-containing gas generated from the desalination device, and then passed through a filter filled with limestone and/or dolomite granules. A method for producing drinking water, which comprises aerating gas not containing carbon dioxide gas into the fresh water after it has passed through a filter to bring the pH of the fresh water to a predetermined value.