JPH0128634B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for treating wastewater containing arsenic (As) and fluorine (F) discharged from semiconductor factories and the like. (Prior Art) The following methods are known as conventional methods for treating As-containing water. () Remove by adsorption with activated carbon, activated alumina, etc. () Remove by ion exchange or collection using ion exchange resin. () Precipitate and remove arsenic sulfide with hydrogen sulfide, sodium sulfide, etc. () Remove by coprecipitation with metal hydroxide. At present, the hydroxide coprecipitation method () is the most reliable and common method. Among the hydroxide coprecipitation methods, iron has the highest removal effect, and the residual arsenic concentration is
It is almost constant depending on the Fe/As ratio. In the case of wastewater from a semiconductor factory containing arsenic and fluorine, one-stage coagulation and sedimentation can reduce the arsenic concentration from 200 mg/ml in the raw water, for example.
To reduce it to 0.05 mg/Fe/As ratio is 25~
A large dose of about 30 doses is required. On the other hand, as a method for removing fluorine from fluorine-containing water, a method is known in which calcium salt is added to precipitate and remove fluorine as calcium fluoride. For trace amounts of residual fluorine, add an aluminum compound such as sulfuric acid to remove the precipitate (Japanese Patent Application Laid-Open No. 1989-1999).
138361). (Problems to be Solved by the Invention) The following problems exist in the treatment of arsenic- and fluorine-containing wastewater according to the prior art. () Environmental standards for arsenic and fluorine are strict. Arsenic is required to be 0.05mg/or less, and fluorine is required to be 15mg/or less. () It takes a long time to measure arsenic, which takes about one day using the JIS method. The polarographic method takes about 50 minutes. Therefore, it is difficult to perform optimal chemical injection in accordance with fluctuations in the raw water, and in order to keep the treated water quality within the above-mentioned standards, excessive chemical injection becomes necessary. () If the quality of the treated water was poor, the treated water was returned to the raw water tank for reprocessing, but the effect of the first stage treatment was that the shortening of reaction time due to returning the treated water led to instability of the treatment. It gets worse. There is also the problem that it takes a long time to replace the water in the treatment system. In view of the above-mentioned problems of the prior art, it is an object of the present invention to provide a more excellent processing method that is improved in the following two points. () Reliable treatment can be performed with a small amount of chemical injection. () When reprocessing treated water that exceeds environmental standard values, ensure that the treatment does not become unstable. (Means for solving the problem) The attached figure shows a flow diagram according to the method of the present invention. With reference to this, the method for treating wastewater containing As and F of the present invention and its characteristics will be explained below. The As concentration of the raw water stored in the raw water tank 2 into which wastewater flows from route 1 is measured, and chloride concentration is added to the first reaction tank 4 into which the raw water is introduced by the raw water pump 3 so that Fe/As = 2 or more. Fe salt such as 2 iron is injected through route 5. In addition, Ca
(OH) ₂ is injected through route 6. The water that has been subjected to the flocculation treatment here flows into the first sedimentation tank 7 and is subjected to the sedimentation treatment.
Most of the As and F in the raw water are removed by this first-stage coagulation-sedimentation treatment. The supernatant separated water from the first precipitation tank 7 is made to flow into the second reaction tank 8 . In the second reaction tank 8, a further trace amount of residual As
In order to remove F and F, Al salt such as sulfuric acid bandate is injected through route 9 so that Al/As becomes 30 or more and Al/F becomes 2 or more, and Ca ( OH) ₂ is injected through route 10.
Here, the water subjected to the second stage flocculation treatment is transferred to the second sedimentation tank 1.
1 and subjected to precipitation treatment. The treated water that has completed the second-stage coagulation-sedimentation treatment is allowed to flow into one of the pair of discharge adjustment tanks 12 and 12', for example 12, which are used alternately, and there the total amount of As is absorbed by a method such as the polarography method. The total As amount is measured by a continuous measuring device, and if it is confirmed that the value is below the standard value after the required measurement time, it is discharged out of the system through route 13 as the final treated water. Discharge adjustment tank 12
During measurement of the amount of As in the water, the second stage coagulation and sedimentation treated water is allowed to flow into another discharge adjustment tank 12'. If the second stage treated water flowing into the discharge control tank 12 has an As concentration that is not allowed to be discharged outside the system,
This is returned to the second reaction tank 8 after the first precipitation tank 7 via a route 14 for reprocessing. The As concentration of the treated water in the second-stage coagulation-sedimentation treatment should be at least much lower than that in the first-stage coagulation-sedimentation treatment, and by returning it to the second reaction tank 8, 4 is to ensure sufficient residence time and therefore reaction time without the influence of the returned water, and
The effectiveness of the stage coagulation and sedimentation treatment can be demonstrated. This is extremely important in ensuring the overall effectiveness of the two-stage process. In addition, when installing a filter, an activated carbon adsorption tower, etc. after the 2nd precipitation tank 11, those washing|cleaning wastewaters are also returned to the 2nd reaction tank 8 and reprocessed. (Action of the invention) The technical reasons for the advantages of the above solution are as follows:
This is because the concentrations of both As and F in the discharge adjustment tank are extremely low, so it is sufficient to return them to the second stage. (Example) In both the following examples () and (), ferric chloride, slaked lime, and a polymer flocculant were added in the first stage to perform coagulation and sedimentation treatment, and in the second stage, sulfuric acid, slaked lime, and a polymer flocculant were added. A flocculant was added to perform flocculation and sedimentation treatment. () Confirmation test for optimum chemical injection amount (i) 1st stage treatment Effect of the amount of Fe salt added (as Fe/As) to raw water with total As of 412 mg/Fe/As As in treated water (mg/) 1.5 0.79 2.0 0.26 (Compatible with the present invention) 5.0 0.06 (Compatible with the present invention) Amount of Ca added to raw water of F52mg/(Ca/F
Ca/F F in treated water (mg/) 3.0 42 5.0 38 10.0 30 20.0 12 (Compatible with the present invention) 35.0 11.6 (Compatible with the present invention) (ii) Second stage treatment Total As0.84mg/, Fe12mg/ Effect of the amount of Al salt added (as Al/As and Al/F) on the first stage treated water

[Table] () As.F-containing wastewater from semiconductor factories (discharge volume 65m ³ /
results of processing on an actual device using the method of the present invention)

[Table] Because the raw water PH is low and the amount of Ca(OH) ₂ required for neutralization is large, F is below the environmental standard of 15mg/in the first stage treatment. If As is reduced to about 1mg/by co-precipitation with Fe in the first stage treatment, it can be easily reduced to below the environmental standard of 0.05mg/in the second stage treatment.
There is no need for return to the reaction tank, and the absolute amount of Al salt to be added is also reduced. (Effects of the Invention) According to the two-stage treatment according to the method of the present invention, the treatment is easy, the treatment results are ensured, and the amount of chemicals added can be reduced. Also, when returning inappropriately treated water, the first
Processing can be stabilized without adversely affecting stage processing.

[Brief explanation of drawings]

The attached figure shows a flow diagram according to the method of the invention. 1, 5, 6, 9, 10, 13, 14...route, 2
...Raw water tank, 3...Raw water pump, 4...First reaction tank, 7
...First precipitation tank, 8...Second reaction tank, 11...Second precipitation tank, 12, 12'...Discharge adjustment tank.

Claims (1)

[Claims] 1. A ferric salt with Fe/As = 2 or more and a calcium salt with Ca/F = 20 or more are added to As.F-containing wastewater to perform coagulation and precipitation in the pH range of 8.0 to 8.5. Al/As = 30 for the first stage treatment process and its supernatant separated water
The second step is to add an aluminum salt with Al/F=2 or more and perform coagulation and precipitation in the pH range of 6.5 to 7.0.
A method for treating arsenic- and fluorine-containing wastewater, the method comprising a step of treating wastewater. 2. The arsenic- and fluorine-containing wastewater according to claim 1, in which the treated water is recycled to the second stage treatment step for retreatment when As or F in the treated water exceeds a permissible value. processing method.