JPH08229571A - Treatment of waste water - Google Patents

Abstract

PURPOSE: To obtain a clean water having a lower content of suspended matter and to facilitate the handling of a dehydrated cake by adding a return sludge added with alkali and a return sludge not added with alkali to a waste water to form a neutralized precipitate. CONSTITUTION: A waste water contg. heavy metal is introduced into a neutralization tank 2 from a pipeline 1, transferred to a flocculation tank 6 from a pipeline 5 and then to a solid-liq. separation tank 9 from a pipeline 8 to separate treated water. The treated water is discharged from an overflow 10 and a pipeline 11, and the sludge deposited on the bottom is discharged from discharge ports 12 and 13. A part of the deposited sludge is returned to a distribution tank 17 from a pump 15 and a pipeline 16, and a part of the return sludge is transferred to an alkali adding tank 20 from a pipeline 18 and converted to an alkaline sludge. The remainder of the return sludge free of alkali is introduced into the neutralization tank 2 from a pipeline 4, and the alkaline sludge is also supplied to the neutralization tank 2 from a pipeline 3. Namely, the waste water is treated with the alkaline sludge and neutralized sludge in the neutralizing tank 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating heavy metal-containing wastewater such as mining and industrial wastewater, and in particular, when removing heavy metals from mining and industrial wastewater, it is possible to suppress the generation of fine particles that cause deterioration of the quality of the treated water. The present invention relates to a wastewater treatment method that makes it possible to obtain treated water that is clearer than that of the method and to obtain a solid content containing heavy metals as a dehydrated cake having excellent handleability.

[0002]

2. Description of the Related Art In various fields of mining industry, various kinds of acidic wastewater containing heavy metals such as acidic mining wastewater, acidic electrolytic wastewater such as plating wastewater, acidic extract and pickling solution are generated. As a method for treating this heavy metal-containing wastewater, an alkali such as lime is added to the wastewater to convert the heavy metal into an insoluble substance such as an oxide, a hydroxide or a salt (herein referred to as a "neutralizing substance"). The method of converting and precipitating and performing solid-liquid separation has been conventionally practiced.

In this case, a method of returning a part of the sludge solid-liquid separated to add alkali without directly adding the alkali to the wastewater and adding the mixture to the wastewater (alkali sludge method)
Is known as an effective treatment method in which the solid content concentration of the generated sludge is high. The outline is shown in FIG. First,
In the first reaction tank, an alkaline neutralizing agent such as lime is mixed with sludge. The sludge used here is a part of the sludge that has been solid-liquid separated by wastewater treatment and is recycled. The sludge containing alkali is introduced into the second reaction tank and mixed with the wastewater. In the second reaction tank, the metal in the wastewater becomes hydroxide due to alkali, or is oxidized by blowing air to form a neutralized precipitate. The suspension containing the neutralization precipitate is led to a thickener and separated into treated water and sludge. A part of the separated sludge is recycled as described above, and the rest is discharged outside the system. (U.S. Patent No. 3,738.932, U.S. Patent No. 4,465,597, Japanese Patent Publication No. 61-156)

[0004]

However, in the conventional alkaline sludge method, since the viscosity of the sludge is too high or the liquid limit value is low, the cake obtained by dehydrating the sludge has a large stickiness and is inferior in handleability. There is a problem that sludge particles become finer and the turbidity of treated water is high, and the sludge is not clarified to the extent that drainage standards are satisfied. In order to eliminate the drawbacks of the conventional alkaline sludge method, the method of JP-A-5-57292 proposes that the amount of sludge to be returned is 15 to 40 times that of the neutralization sludge. Even with this method, the liquidity limit of the dehydrated cake obtained from sludge is not much different from that in the conventional case, and it is still sticky, and the problem that it adheres to the apparatus and the workability is significantly reduced has not been solved. The present invention is to solve the above problems in the conventional treatment method, it is possible to obtain a clearer treated water than the conventional method, it is also an object of the present invention to provide a wastewater treatment method that is easy to treat the dehydrated cake And

[0005]

According to the present invention, in the neutralization step of the alkaline sludge method, the sludge containing no alkali is added together with the sludge containing alkali to the wastewater, preferably in a larger amount than the sludge containing alkali. The unexpected effect of reducing the turbidity of water and improving the handleability of the dehydrated cake was found. The present invention is based on the above findings, and the present invention provides a wastewater treatment method having the following configuration.

(1) When an alkali is added to wastewater containing heavy metals to produce a neutralized precipitate containing heavy metals and solid-liquid separation is performed into sludge containing the neutralized precipitate and treated water. ,
In the treatment method in which the sludge is returned to the alkali addition step, the alkali is mixed with the returned sludge and added to the wastewater, alkali is added to a part of the returned sludge, and the alkali-added return sludge (alkali sludge) and alkali-free A method for treating wastewater, which comprises adding neutralized sludge to the above-mentioned wastewater together with added return sludge (neutralization sludge). (2) The wastewater treatment method according to (1) above, wherein the amount of alkali sludge is 10 to 50% by weight of the returned sludge. (3) The wastewater treatment method according to (1) or (2) above, in which a coagulant is added to the liquid after the neutralized precipitate is formed, and then solid-liquid separation is performed. (4) The liquid limit value of the dehydrated sludge obtained by solid-liquid separation is 60
The wastewater treatment method according to (1) (2) or (3) above, which is at least%. (5) The wastewater treatment method according to any one of (1) to (4) above, wherein the turbidity of the treated water obtained by solid-liquid separation is 10 mg / l or less. (6) Out of the sludge collected by solid-liquid separation, the amount equivalent to the sludge newly generated by the neutralization process is withdrawn to the outside of the system, and the remaining amount is used as return sludge. Any of the above (1) to (5) Wastewater treatment method described in. (7) Until the specified amount of sludge collected by solid-liquid separation is used, the entire amount is used as return sludge. When the amount exceeds the specified amount, the excess amount is extracted to the outside of the system and the remaining amount is used as return sludge. The method for treating wastewater according to any one of (1) to (5) above, which is used.

[0007]

[Detailed Description] The wastewater containing a heavy metal to be treated in the present invention is an acidic wastewater containing a heavy metal ion, a heavy metal chelate complex or the like. Examples thereof include acidic mine drainage, acidic extract, acidic electrolytic waste such as plating waste, and pickling solution. Examples of heavy metals include copper, zinc, nickel, cadmium, manganese, iron, lead and iron. The acidity is not particularly limited.

The method of the present invention will be described below with reference to an apparatus configuration example. FIG. 1 is a schematic diagram showing an example of an apparatus configuration for carrying out the method of the present invention. As shown in the figure, the present treatment apparatus comprises a neutralization tank 2 to which waste water is supplied, a coagulation tank 6 into which neutralized waste water is introduced, a solid-liquid separation tank 9 for solid-liquid separation of waste water, and a separation. Pipeline 16 for returning the returned sludge, distribution tank 17 for distributing the returned sludge, alkali addition tank 20 into which a part of the returned sludge sent from the distribution tank is introduced, return sludge for which no alkali is added sent from the distribution tank To the neutralization tank, and other pipelines. (In the present specification, the returned sludge in which alkali is added may be referred to as alkaline sludge, and the one in which alkali is not added is referred to as neutralized sludge.)

Waste water containing heavy metals is introduced into the neutralization tank 2 through a pipe line 1. Part of the alkali sludge is introduced into the neutralization tank 2 through the pipe line 3. Further, the alkali-free neutralized sludge sent from the distribution tank is introduced into the neutralization tank 2 through the conduit 4. The wastewater, the alkaline sludge and the neutralization sludge are sufficiently stirred in the neutralization tank 2, and the heavy metals in the wastewater are converted into oxides, hydroxides, salts and the like to form neutralization precipitates. In addition, in the neutralization tank, an appropriate aeration means may be provided side by side so as to promote the reaction.

The waste water in the neutralization tank 2 is sent to the coagulation tank 6 through a pipe line 5 after a predetermined residence time. If necessary, a flocculant is added to the flocculation tank 6 through a pipe 7. The type of aggregating agent is not particularly limited, but a polymer aggregating agent such as polyacrylamide can be used. The wastewater that has been subjected to the coagulation treatment in the coagulation tank 6 is sent to a solid-liquid separation tank 9 such as a thickener through a pipe line 8. In the solid-liquid separation tank 9, the sludge containing the neutralized precipitate is deposited on the bottom of the tank and separated from the treated water. The supernatant water (treated water) of the solid-liquid separation tank 9 flows out to the overflow portion 10 and is further discharged to the outside of the system through the pipe line 11. Solid-liquid separation tank 9
The sludge deposited on the bottom of the pipe is connected to the discharge port 12 through the discharge pipe 13
Is extracted through.

A part of the extracted sludge is introduced to the outside of the system and dehydrated to form a dehydrated cake. Further, a part of the sludge is returned to the distribution tank 17 via the pipe 14, the pump 15 and the pipe 16. The sludge may be extracted continuously or intermittently. As a method of continuously extracting sludge, an amount of sludge corresponding to the amount newly generated by the neutralization of wastewater is introduced to the outside of the system, and the remaining amount is returned to the distribution tank 17. As a method of intermittently extracting the sludge, the sludge is accumulated in the solid-liquid separation tank 9 until it reaches a predetermined amount, the sludge is extracted every time the predetermined amount is exceeded, and the excess amount is returned to the distribution tank 17.
Alternatively, until the sludge recovered by solid-liquid separation exceeds a predetermined amount by a certain ratio, the whole amount is returned to the distribution tank 17, and when the predetermined amount is exceeded, the excess amount is extracted from the system and the remaining amount is returned. Good.

The amount of sludge returned is determined by the amount of waste water introduced, the capacity of each part of the system, the amount of alkali added, and the like. Basically, the alkaline sludge added in the neutralization tank 2 contains an amount of alkali sufficient to neutralize the wastewater, and the weight ratio of the alkaline sludge to the alkali-free neutralized sludge will be described later. The amount may be a certain ratio.

The sludge returned to the distribution tank 17 is distributed in a fixed amount ratio, and a part of the sludge is passed through the pipe 18 to the alkali addition tank 2
0, where alkali is added to form alkali sludge. As the alkaline agent, sodium hydroxide or the like is used in addition to slaked lime. These may be added in a suitable state such as a solution or a slurry. The amount of alkali is determined by the heavy metal ion concentration in the wastewater.

On the other hand, the remaining amount of the returned sludge is introduced into the neutralization tank 2 via the pipe line 4 without addition of alkali. The alkaline sludge is supplied to the neutralization tank 2 through the pipe line 3. In this way, alkaline sludge and neutralized sludge without addition of alkali are introduced into the neutralization tank 2 along with the wastewater to be treated, and the wastewater treatment is performed in the state where these coexist. As shown in the examples below, the weight ratio of alkali sludge to neutralized sludge is 1: 9 to 5:
It is appropriate that the range of 5, that is, 10 to 50% by weight of the returned sludge is alkali sludge. The amount of alkaline sludge is 10
If it is less than wt%, the suspension of treated water is lowered, but the amount of alkali is too small and the effect of removing heavy metals is poor. On the other hand, when the amount of alkaline sludge exceeds 50% by weight, the turbidity of the treated water becomes high, and the liquid limit value of the dehydrated sludge cake decreases to the 50% level, and it becomes sticky, which hinders the treatment work. Come here. Incidentally, in order to obtain good handleability, the liquid limit value of the dehydrated cake is appropriately 60% or more, preferably 70% or more.

In the present invention, it is important that the alkaline sludge and the neutralized sludge be present together when the wastewater is neutralized as described above. In the conventional alkaline sludge method, as shown in FIG. 3, only alkaline sludge is used, and neutralized sludge containing no alkali is not used together. Generally, a precipitation nucleus is necessary for the formation of the neutralization precipitate, and when sludge is added, sludge particles serve as the nucleus to form a neutralized precipitate. in this case,
Since alkali sludge has alkali on its particle surface, heavy metal hydroxide or the like is produced by dehydration condensation on the sludge particle surface.

By the way, in the method in which the total amount of the returned sludge is alkali sludge as in the conventional alkaline sludge method, neutralized precipitates preferentially deposit on the surface of most sludge particles, and the particle diameters of both are comparable. Becomes large sludge particles. For this reason, although the solid concentration of the sludge to be deposited is high, the dewatered cake of sludge has a large particle gap and a large amount of free pore water, and therefore the liquid limit value becomes low. For this reason, they tend to adhere to the equipment and the handling becomes poor (see FIG. 2A). In addition, after most of the sludge particle surface is saturated with the neutralized starch, the newly formed neutralized starch is not retained and remains as fine starch particles, so the effect of solid-liquid separation decreases. , The turbidity of treated water becomes high. Furthermore, the sludge particles that have grown excessively are destroyed by stirring in the tank to generate fine particles, which facilitates suspension.

On the other hand, in the method in which the returned sludge is added to the wastewater without any alkali treatment, and the alkali is added to the wastewater separately from the returned sludge, the turbidity of the treated water is reduced, but sludge particles with respect to neutralized sediment are Since the adsorption power is weak and therefore sludge particles are small, the solid concentration of the sludge deposited is low and a relatively long dehydration time is required (see FIG. 2B). In addition, the effect of removing heavy metals is low.

However, in the processing method of the present invention,
Neutralization sludge is used together with alkaline sludge during the neutralization treatment, and since the alkali sludge particle surface has an overwhelmingly high alkali concentration, neutralized precipitates preferentially deposit and become relatively large particles. On the other hand, neutralized sludge, which is smaller than alkaline sludge, exists in large numbers, so even if the particle surface of alkaline sludge is saturated with neutralized sludge, the newly formed neutralized sludge deposits with the neutralized sludge as the nucleus. Therefore, the fine particles in the liquid are effectively removed, and the turbidity of the treated water is significantly reduced. In addition, since relatively large particles with alkaline sludge particles as cores and relatively small particles with neutralized sludge as cores coexist, there are few particle gaps, the solid concentration of deposited sludge is high, and the free gaps of the dehydrated cake are high. Less water, liquid limit value is 6
Since it is as high as 0% or more, preferably 70% or more, a dehydrated cake with less stickiness and good handleability can be obtained (see FIG. 2C).

[0019]

Examples and Comparative Examples Examples 1 to 3 and Comparative Examples 1 to 2 Using the treatment apparatus shown in FIG. 1, mine wastewater having the water quality shown in Table 1 was treated under the treatment conditions shown in Table 1.
The alkali addition tank 20, the neutralization tank 2 and the coagulation tank 6 each have a capacity of 200 liters, and the solid-liquid separation tank has an inner diameter of 2 m and a height of 3 liters.
m. Perform continuous processing for 2 months. Concentration (turbidity) of solids in overflow water discharged from the solid-liquid separation tank when the treatment under each condition reached a steady state, solid concentration of solid sludge separated from sludge, and dehydrated cake obtained from the sludge Was measured for water content and liquid limit. The results are shown in Table 2. The properties of the dehydrated cake are shown in Table 3.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

The dehydrated cake naturally drops from the dehydration chamber to the lower chute, slides down from the chute onto the belt conveyor and is carried to the stockyard. However, in the conventional processing method, the cake does not fall from the discharge port of the dehydration chamber. For some time, it adheres to the filter cloth or remains without sliding off the chute. The peelability of the cake and the slidability from the chute in Table 3 indicate the amount of such a residual portion, respectively. Further, the peeling time and the discharging time represent the time required for each worker to scrape off the cake remaining on the filter cloth or chute, and are numerical values that actually represent the handleability of the cake. The total time required is the sum of the time required for the dehydration treatment of sludge, the removal of the dehydrated cake from the filter cloth, and the scraping from the chute to the belt conveyor.

As shown in Table 2, the turbidity (suspension concentration) of overflow water increases as the proportion of alkaline sludge increases.
In the conventional alkaline sludge method shown in Comparative Example 1, turbidity of overflow water was remarkable. Further, the solid concentration of the sludge discharged from the solid-liquid separation tank increases as the proportion of the alkaline sludge increases, but as shown in Example 4 of the present invention, even if the proportion of the alkaline sludge is about 10%, the solid concentration is low. It is 97 g / l, and the sludge is highly concentrated. On the other hand, the liquid limit value of the dehydrated cake of the discharged sludge is lower as the addition rate of the alkaline sludge is higher, and the liquid sludge is easily liquefied. Therefore, in the conventional method (Comparative Example 1), the dehydrated cake was significantly sticky, and the entire amount of the cake discharged from the dehydration process adhered to the filter cloth, and when this was scraped off on the chute, the entire amount adhered to the chute this time. The work burden of peeling and scraping the cake is significant.

However, in the dehydrated cake obtained by the treatment method of the present invention, the liquid limit values are all about 60% or more, and particularly when the proportion of the neutralized sludge is 20% by weight or more, the value exceeds 70%. The handling property of the dehydrated cake is greatly improved as compared with the conventional treatment method. Also, the turbidity of the treated water after solid-liquid separation is 10 mg / l or less, and clear treated water having a low degree of suspension can be obtained. Furthermore, the fixed concentration of the sludge is 90 g / l or more, preferably 100 g / l or more, and the sludge is highly concentrated.

[0026]

EFFECTS OF THE INVENTION According to the wastewater treatment method of the present invention, clear treated water having a remarkably low degree of suspension can be obtained as compared with the conventional alkaline sludge method. Post-treatment using special chemicals and equipment such as sedimentation pool is not required. Moreover, the solid concentration in the sludge is high, and the effect of removing heavy metals is comparable to that of the conventional alkaline sludge method. Further, the dehydrated cake of sludge is less sticky and easy to handle, so that the working efficiency is greatly improved. Moreover, it is only necessary to divide the returned sludge into alkaline sludge and neutralized sludge and add it.
Since no special equipment is required, it is easy to carry out and the processing cost is low.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a processing process of an example of an apparatus according to a method of the present invention.

FIG. 2 is a schematic diagram showing the state of sludge particles in comparison.

FIG. 3 is a schematic diagram of a processing process of an apparatus example according to a conventional method.

[Explanation of symbols]

2-Neutralization tank, 6-Flocculation tank, 9-Solid-liquid separation tank, 17-Distribution tank, 20-Alkali addition tank

Claims (7)

[Claims] 1. When an alkali is added to wastewater containing heavy metals to produce a neutralized precipitate containing heavy metals and solid-liquid separation is performed into sludge containing the neutralized precipitate and treated water, In the treatment method in which the sludge is returned to the alkali addition step, the alkali is mixed with the returned sludge and added to the wastewater, alkali is added to a part of the returned sludge, and the alkali-added return sludge (alkali sludge) and alkali-free A method for treating wastewater, which comprises adding neutralized sludge to the above-mentioned wastewater together with added return sludge (neutralization sludge). 2. The amount of alkali sludge is 10 to 5 of the amount of returned sludge.
The wastewater treatment method according to claim 1, which is 0% by weight. 3. The wastewater treatment method according to claim 1, wherein a coagulant is added to the liquid after the neutralized precipitate is formed, and then solid-liquid separation is performed. 4. The wastewater treatment method according to claim 1, 2 or 3, wherein the liquid limit value of the dehydrated sludge obtained by solid-liquid separation is 60% or more. 5. The turbidity of the treated water obtained by solid-liquid separation is 10 mg.
The method of treating wastewater according to any one of claims 1 to 4, wherein the amount is not more than 1 / l. 6. The sludge collected by solid-liquid separation, the amount corresponding to the sludge newly generated by the neutralization treatment is withdrawn to the outside of the system, and the remaining amount is used for returning sludge. The described wastewater treatment method. 7. The whole amount of sludge recovered by solid-liquid separation is used for returning sludge until it reaches a predetermined amount, and when the sludge exceeds a predetermined amount, the excess amount is extracted to the outside of the system and the remaining amount is returned. The wastewater treatment method according to any one of claims 1 to 5, which is used for sludge.