JP2008264626A - Method for treating soil-washed waste water - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for simply and easily treating waste water produced by washing heavy metal-containing soil. <P>SOLUTION: The method for treating soil-washed waste water is characterized by subjecting the waste water produced by washing heavy metal-containing soil to alkali coagulation treatment at pH 8.5 to 10. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for treating wastewater from which heavy metal-containing soil has been washed.

In order to purify heavy metal-containing soil, a method of eluting and removing heavy metals from soil using a cleaning agent such as an aqueous solution or water containing various chemicals has been studied.
When the soil pH is adjusted to 3 or less with a cleaning agent, the soil cleaning effect can be enhanced (Patent Document 1). At this time, heavy metals mainly composed of cadmium are eluted from the soil to be purified, but aluminum is eluted from the soil mineral, and in particular, a 2: 1 type intermediate clay containing polymerized hydroxyaluminum ions between clay layers. In the case of soil rich in minerals, a large amount of aluminum is mixed in the washing wastewater.

  Wastewater containing heavy metals or the like needs to be purified to a level equal to or lower than the wastewater standard value, and a coagulation sedimentation method is used for the treatment of such wastewater. In the coagulation precipitation method, a hydroxide precipitate of heavy metal is generated by alkali treatment and is removed by coagulation precipitation. However, since aluminum is an amphoteric metal, it cannot be removed by re-dissolution with alkali. The supernatant water containing water-treated aluminum that has been subjected to alkali treatment is neutralized in the next step and discharged after passing through the final filter, but in this neutralization treatment step, aluminum precipitates as a hydroxide and becomes cloudy, The final filter was blocked in a short period of time, and stable operation was not possible.

In this case, as Step 1, precipitation is performed by generating heavy metal hydroxide with a high alkalinity of pH 10.5 to 11, and as Step 2, the supernatant water is adjusted to pH 5 to 6 where aluminum solubility is minimized. Thus, a two-stage reactive agglomeration process is required in which a hydroxide is generated and a precipitation process is performed.
Such a two-stage reaction precipitation process requires a large wastewater treatment apparatus, and the soil cleaning method that performs wastewater treatment in situ has problems such as an increase in installation area and cost.

On the other hand, as described above, colloidal aluminum hydroxide is mixed in the soil washing wastewater, and the dewaterability by the filter press is extremely lowered. Therefore, improvement of dewatering efficiency has been an important problem.
Furthermore, the soil washing wastewater generated a maximum of 3000 ppm of hydroxide precipitation due to the alkali treatment, and in order to perform a large amount of wastewater treatment, a huge sedimentation tank was required, making it difficult to perform the wastewater treatment in situ.
Japanese Patent Application Laid-Open No. 2005-163981

  Accordingly, an object of the present invention is to provide a method for simply treating wastewater from which heavy metal-containing soil has been washed.

  As a result of various studies in view of such circumstances, the present inventors have found that wastewater washed from heavy metal-containing soil can be easily treated by alkali coagulation treatment at pH 8.5 to 10 and completed the present invention. .

  That is, this invention provides the processing method of the soil washing wastewater characterized by carrying out the alkali coagulation process of the wastewater which wash | cleaned the heavy metal containing soil at pH 8.5-10.

  ADVANTAGE OF THE INVENTION According to this invention, the waste water which wash | cleaned the heavy metal containing soil can be processed simply and efficiently. In particular, it is suitable for wastewater treatment when soil with high aluminum concentration such as allophane soil is washed.

  The present invention treats wastewater when heavy metal-containing soil is extracted and washed with heavy metals in the soil using cleaning agents such as aqueous solutions and water containing various chemicals. Examples of heavy metal-contaminated soil to be purified include urban areas, mountain forests, factory sites, agricultural land, swamps, and soils that contain heavy metal elements such as lead, cadmium, and arsenic, compounds, or ions. .

  Extracting and washing heavy metals in soil is not only mixing the soil and aqueous solution of the drug directly, but also adding the chemical and water separately to the soil and washing them, mixing the chemical into the soil containing water. Cleaning methods are also included.

  The washing liquid from which the heavy metal has been extracted is separated into solid and liquid by natural sedimentation or active dehydration, separated and removed from the soil, and used as waste water.

Thus, the heavy metal containing waste liquid discharged | emitted after washing | cleaning soil performs an alkali aggregation process first. The alkali aggregation treatment is performed at pH 8.5 to 10, preferably pH 9 to 9.8, particularly preferably pH 9 to 9.5. If the pH is less than 8.5, the heavy metals in the wastewater cannot be reduced below the environmental standard. If the pH exceeds 10, the aluminum is eluted in a large amount.
The alkali aggregation treatment can be performed, for example, by adding an alkali such as slaked lime, quick lime, calcium carbonate, acetylene residue, sodium carbonate, sodium hydroxide, magnesium hydroxide, magnesium oxide or the like.

  In the present invention, it is preferable to further neutralize the waste liquid. The neutralization treatment is preferably performed at pH 5.8 to 7.0, particularly pH 6.0 to 6.5. The neutralization treatment can be performed, for example, by adding an acid such as hydrochloric acid or sulfuric acid.

In the present invention, wastewater treatment includes alkali coagulation treatment step, chelate reaction treatment step, polymer coagulation treatment step, precipitation separation treatment step, concentrated sludge dehydration treatment step, supernatant water pH neutralization treatment step, and final drainage filtration step. Is preferred.
These wastewater treatments are preferably carried out in situ at the contaminated site.

Chelate reaction treatment can be performed by adding liquid heavy metal scavengers such as pyrrolidine, imine, carbamic acid, etc. and stirring vigorously. Cadmium that was not precipitated as hydroxide by aeration treatment and neutralization treatment The heavy metal ion containing is used as a floc to form an insoluble precipitate.
In addition to supplementing heavy metal ions containing cadmium as a floc, the re-elution of heavy metals can be suppressed in the disposal of dewatered cakes collected by sedimentation separation processing and concentrated sludge dewatering processing in the latter stage. Processing is possible.

  Polymer agglomeration treatment includes sodium alginate, sodium CMC, sodium polyacrylate, polyacrylamide partially hydrolyzed salt, maleic acid copolymer, water-soluble aniline resin, polythiourea, polyethylenimine, quaternary ammonium salt, polyvinylpyridine , Polyacrylamide, polyoxyethylene, causticized starch and the like can be added, and gently agitated to grow and coarsen flocs and improve sedimentation.

  The precipitation separation process can use any sedimentation method such as a cylindrical sedimentation tank, a cross-flow sedimentation tank, a center-driven circular sedimentation tank, or a slurry circulation sedimentation tank, but a large number of inclined plates are inserted into the sedimentation tank, It is preferable to use an inclined plate type high-speed sedimentation tank that increases the effective separation area and promotes sedimentation of sludge-like flocs, because high processing efficiency with respect to the installation area of the apparatus can be obtained.

Concentrated sludge dewatering can be used in any dewatering method such as vacuum dewatering, centrifugal dewatering, filter press, belt press, screw press, multiple disk dehydrator, multiple plate outer cylinder screw press and the like. The sludge-like floc accumulated in the lower layer in the sedimentation process is dehydrated, and heavy metals containing cadmium are recovered as a dehydrated cake.
In this concentrated sludge dewatering treatment, it is preferable to add a dewatering aid 1 to 5 times, particularly 2 to 3 times, the solid content of the concentrated sludge. The dehydration aid is not particularly limited, but a pearlite dehydration aid is preferred.

  The pH neutralization treatment of the supernatant water can be performed by adding an acid such as hydrochloric acid or sulfuric acid, and is preferably performed at pH 5.8 to 7, particularly pH 6 to 6.5. If the pH is less than 5.8, the pH of the wastewater is below the environmental standard. If the pH exceeds 7, the aluminum ions remaining in the supernatant water become hydroxides and the treated wastewater becomes cloudy, and in the final wastewater filtration step performed later, The machine will be blocked early.

Final drainage filtration can be performed with any type of filter such as sand filter, diatomaceous earth filter, cartridge filter, membrane filter, micro strainer, screen, centrifuge, activated carbon adsorber, etc. Collect and remove the solid suspended matter.
After performing the above treatment, the waste water can be discharged.

  EXAMPLES Next, although an Example is given and this invention is demonstrated further in detail, this invention is not restrict | limited to these at all.

Example 1
The wastewater which wash | cleaned the air-dried soil extract | collected in Toyama Prefecture with the 30 mM ferric chloride solution 2 mass times with respect to the soil was used. The Al concentration in the waste water was 120 mg / L, and the Cd concentration was 0.1 mg / L. Sodium hydroxide was added thereto, and alkali aggregation treatment was performed at pH 8.0 to 11.0. After stirring for 15 minutes, the mixture was allowed to stand, and the Al concentration and Cd concentration in the supernatant water were measured by ICP-AES. The results are shown in Table 1.

  As a result of Table 1, when the treatment pH was less than 8.5, the Cd concentration did not satisfy the environmental standard (water quality 0.01 mg / L), and when the pH exceeded 10.0, a large amount of aluminum was eluted.

Example 2
The supernatant wastewater treated in Example 1 was neutralized with hydrochloric acid so as to have a pH of 5.8 to 7.0. Thereafter, the treatment liquid was suction filtered with a filter paper and then dried with a 105 ° C. dryer to determine the dry matter amount of the precipitate. The results are shown in Table 2.

  From the results in Table 2, at pH 7.0 or less, Al exists in a water-soluble state, and can be drained as a water-soluble state at pH 6.0, which is particularly close to the wastewater standard pH 5.8, without generating hydroxide. The water could be discharged.

Example 3
55 L of simulated washing wastewater containing 2000 mg / L of iron hydroxide and 433 mg / L of aluminum hydroxide and a total of 2433 mg / L of precipitate was prepared. To this, no dehydration aid was added, or dehydration aid (Mitsui Metal Mining Co., Ltd., RocaHelp) twice the mass of the precipitate was added. The filter was pressed under the conditions of a filtration area of 0.24 m ² , a filtration chamber capacity of 3.3 L, and a water supply pressure of 0.5 Mpa, and the dehydration time and dehydrated cake water content were determined. The results are shown in Table 3.

  From the results shown in Table 3, by adding twice the amount of dehydration aid, the dehydration time was significantly shortened and the moisture content of the dehydrated cake was also greatly reduced.

Claims (5)

  A method for treating soil washing wastewater, comprising subjecting wastewater washed with heavy metal-containing soil to an alkali agglomeration treatment at pH 8.5 to 10.   Furthermore, the processing method of the soil washing wastewater of Claim 1 which performs a neutralization process by pH 5.8-7.   Furthermore, the processing method of the soil washing | cleaning wastewater of Claim 1 or 2 which adds 1-5 mass times of solid content in concentration sludge, and performs dewatering processing of concentration sludge.   The alkali coagulation treatment, the chelate reaction treatment step, the polymer coagulation treatment step, the precipitation separation treatment step, the concentrated sludge dehydration treatment step, the supernatant water pH neutralization treatment step, and the final wastewater filtration step. The processing method of the soil washing wastewater as described.   The method for treating soil washing wastewater according to any one of claims 1 to 4, wherein the wastewater treatment is carried out in situ at the contaminated site.