JP7582359B2 - Wastewater treatment method - Google Patents

Description

The present invention relates to a wastewater treatment method that reduces the amount of chemicals used in factory wastewater treatment.

In industry, many chemical products are used, and the wastewater generated is mixed with industrial wastewater and treated.
Methods for treating industrial wastewater include the coagulation and sedimentation process, in which ferric chloride is injected while adjusting the pH with dilute sulfuric acid, hydrated lime, etc., causing impurities in the wastewater to coagulate and settle due to their specific gravity, and the Fenton process, an oxidation process in which hydrogen peroxide and ferric chloride are reacted in an acidic state, causing the Fenton reaction which generates hydroxyl radicals, thereby oxidizing and decomposing organic matter in the wastewater.

For wastewater that is expected to have a high COD (Chemical Oxygen Demand) before treatment, Fenton treatment is performed, followed by coagulation and sedimentation treatment to reduce the COD. On the other hand, for wastewater that is expected to have a low COD, coagulation and sedimentation treatment is used to reduce the COD.

However, in reality, the properties of wastewater are not uniform, so chemicals are generally injected assuming that high-load wastewater is being treated, but when low-load wastewater is being treated, excess chemicals are injected. As a way to eliminate the waste caused by injecting excess chemicals, a method has been proposed in which the optimal amount of chemicals is calculated from data on the wastewater components.

The technology described in Patent Document 1 discloses a wastewater treatment method in which an inorganic flocculant and activated carbon are added to the water to be treated so that they coexist in a flocculation treatment process carried out prior to an oxidation treatment process (Fenton treatment), and an organic flocculant is further added for treatment.
Furthermore, the technology described in Patent Document 2 discloses a water treatment system and a chemical injection control device capable of injecting an appropriate amount of chemical into water to be treated.
Furthermore, the technique described in Patent Document 3 discloses a method for measuring the COD or TOC value of wastewater and controlling the amount of chemicals injected, etc.

JP 2019-214020 A JP 2019-81140 A JP 2010-179272 A

The above-mentioned conventional techniques have the following problems.
In the technology described in Patent Document 1, the persistent COD is efficiently adsorbed and removed by adding activated carbon to perform activated carbon adsorption in the coagulation step prior to the oxidation step, and the oxidation efficiency by the oxidizing agent in the subsequent oxidation step is also improved. However, this is based on the premise that the oxidation step and the coagulation step including activated carbon adsorption are always performed, and the amount of chemicals injected by omitting the process is not significantly reduced.

Furthermore, the method of determining the optimum amount of chemical to be injected by calculation, which is described in Patent Document 2, is effective when changes in the properties of the wastewater to be treated can be predicted to some extent, but when treating industrial wastewater, etc., the properties of the wastewater change significantly, so there is a problem that it is difficult to derive the optimum amount of chemical to be injected by calculation.
The method described in Patent Document 3 makes it possible to control the amount of chemicals to be injected without performing complicated calculations. However, when handling industrial wastewater, the properties of the wastewater may change in a short time, and the change in the wastewater properties may be too large to control the amount of chemicals to be injected.

The present invention was made in consideration of the above circumstances, and aims to propose a wastewater treatment method that reduces the amount of excess chemicals injected even when the wastewater properties change significantly.

The inventors conducted intensive experiments and studies to solve the above problems. As a result, they discovered that a system that can select a wastewater treatment method based on the COD of the wastewater before treatment can select between coagulation sedimentation treatment method alone or a coagulation sedimentation treatment method that also combines oxidation treatment, and that wastewater can be treated with the optimal amount of chemicals injected.

The wastewater treatment method according to the present invention, which advantageously solves the above problems, is configured as follows.
[1] A wastewater treatment method comprising measuring the COD of wastewater before wastewater treatment and selecting either coagulation-sedimentation treatment or one of oxidation treatment and coagulation-sedimentation treatment based on the COD value.
[2] In the above-mentioned [1], the COD is measured in a raw water tank, and a wastewater treatment method is selected based on the amount of change in COD, ΔQ, which is the difference between the maximum and minimum values per hour for the obtained COD value.
[3] The wastewater treatment method according to the above [2], wherein when the amount of change in COD, ΔQ, increases by 1000 mg/L or more, oxidation treatment or coagulation sedimentation treatment is selected as the wastewater treatment.
[4] In any one of the above [1] to [3], the coagulation and sedimentation treatment is a treatment using an iron-based coagulant containing ferric chloride, ferrous sulfate, and polyferric sulfate.
[5] In any one of the above [1] to [3], the wastewater treatment method is a treatment utilizing the Fenton reaction that generates hydroxyl radicals using an aqueous hydrogen peroxide solution and an iron-based catalyst.
[6] The wastewater treatment method according to any one of the above items [1] to [3], wherein the COD of the wastewater is in the range of 200 to 10,000 mg/L.
[7] The wastewater treatment method according to the above [4], wherein the COD of the wastewater is in the range of 200 to 10,000 mg/L.
[8] The wastewater treatment method according to the above [5], wherein the COD of the wastewater is in the range of 200 to 10,000 mg/L.

According to the present invention, since it is now possible to select a treatment method based on the COD value of the wastewater, it is possible to realize a wastewater treatment method that reduces the use of Fenton treatment chemicals that have been used conventionally when the COD of the wastewater is low. The wastewater treatment method according to the present invention can also be applied when treating wastewater whose wastewater properties change significantly. Furthermore, the present invention can also reduce the use of chemicals such as pH adjustment chemicals that accompany the treatment.

FIG. 1 is a schematic diagram of a wastewater treatment flow according to one embodiment of the present invention. 1 is a flow chart of wastewater treatment according to the present invention.

The wastewater treatment method according to one embodiment of the present invention is described in detail below.

<Wastewater treatment equipment>
Figure 1 shows a wastewater treatment facility. A raw water tank 1 stores wastewater from multiple factories. A COD meter 2 is installed to measure the COD of the wastewater in the raw water tank. The measured COD value is transmitted to a control device 3. This control device 3 determines the process of coagulation sedimentation treatment 5, or Fenton treatment 4 and coagulation sedimentation treatment 5, for wastewater treatment based on the COD, and controls the opening and closing of solenoid valves 31 and 32 in the wastewater flow.

The Fenton treatment equipment 4 is shown from the top to the middle of Figure 1. The COD neutralization tank 41 neutralizes the wastewater from the raw water tank. The chemicals used for pH adjustment are preferably dilute sulfuric acid, hydrated lime, and caustic soda. The tank has a buffer pond 42 for stirring the wastewater. Air is preferably used for stirring. The neutralization tank 43 adjusts the pH of the wastewater. Dilute sulfuric acid is preferably used for the adjustment. In the reaction tank 44, ferric chloride and hydrogen peroxide are injected into the wastewater to cause a Fenton reaction and decompose the organic matter. Chemicals that can cause a Fenton reaction, such as ferrous sulfate, may be used instead of ferric chloride. In the decomposition tank 45, the wastewater is stirred and the pH is adjusted to remove hydrogen peroxide remaining in the wastewater. Caustic soda is preferably used for pH adjustment, and air is preferably used for stirring. In the sedimentation tank 46, impurities from the wastewater are settled.

The solenoid valve is not limited to a two-way solenoid valve, but may be, for example, a three-way solenoid valve.

The lower part of Figure 1 shows the coagulation and sedimentation treatment equipment 5. Dilute sulfuric acid and ferric chloride are injected into the decomposition tank 51 to adjust the pH of the wastewater. It is preferable to use dilute sulfuric acid for pH adjustment. The pH of the wastewater is adjusted in the neutralization tank 52. Impurities in the wastewater are separated by settling in the sedimentation tank 53.

<Wastewater treatment method>
A wastewater treatment flow according to an embodiment of the present invention is shown in Fig. 2. First, the COD of the wastewater is measured in the raw water tank 1 (step S1), and this COD value is sent to the control device 3, which compares the measured COD value of the wastewater with a value preset in the control device 3 (step S2). Here, the COD of the wastewater in the raw water tank 1 is measured at a specific time or at specific intervals within the wastewater treatment time.
When the COD of the wastewater is equal to or higher than the set value, the solenoid valve 31 leading from the raw water tank 1 to the Fenton treatment facility 4 is opened, and the solenoid valve 32 leading from the raw water tank 1 to the coagulation sedimentation treatment facility 5 is closed (step S3). At the same time, the control device 3 sends a signal to start the pump that injects the chemicals used in the Fenton treatment facility 4 (step S3). When the COD of the wastewater is lower than the set value, the solenoid valve 31 leading from the raw water tank 1 to the Fenton treatment facility 4 is closed, and the solenoid valve 32 leading from the raw water tank 1 to the coagulation sedimentation treatment facility 5 is opened (step S5). At the same time, the control device 3 sends a signal to stop the pump that injects the chemicals used in the Fenton treatment facility 4 (step S5).

In addition, based on the change amount ΔQ of the COD of the wastewater, the means connected to the wastewater treatment facility can be controlled to select either one of the wastewater treatment methods, i.e., coagulation sedimentation treatment, or oxidation treatment and coagulation sedimentation treatment, and the wastewater treatment conditions can be set for the selected wastewater treatment method to treat the wastewater. Here, the change amount ΔQ of the COD refers to the difference between the maximum and minimum values per hour of the COD of the raw water tank measured within the wastewater treatment time.
For example, the COD of the raw water tank is measured at a specific time or at specific intervals during the wastewater treatment time from before the wastewater treatment, and the difference between the maximum and minimum values per hour in the measured COD is analyzed to output the amount of change ΔQ.

When the change amount ΔQ of the wastewater COD is equal to or greater than the set value, the solenoid valve 31 leading from the raw water tank 1 to the Fenton treatment facility 4 is opened, and the solenoid valve 32 leading from the raw water tank 1 to the coagulation sedimentation treatment facility 5 is closed (step S3). At the same time, the control device 3 sends a signal to start the pump that injects the chemicals used in the Fenton treatment facility 4 (step S3). When the COD of the wastewater is less than the set value, the solenoid valve 31 leading from the raw water tank 1 to the Fenton treatment facility 4 is closed, and the solenoid valve 32 leading from the raw water tank 1 to the coagulation sedimentation treatment facility 5 is opened (step S5). At the same time, the control device 3 sends a signal to stop the pump that injects the chemicals used in the Fenton treatment facility 4 (step S5).
In addition, the selection of the wastewater treatment is preferably made based on the COD change amount ΔQ of 1000 mg/L, and if the amount is 1000 mg/L or more, the oxidation treatment and the coagulation sedimentation treatment are preferably performed.

This wastewater treatment control system can select a wastewater treatment method based on the wastewater's COD value, i.e., when the COD value is equal to or greater than a predetermined set value or change amount, both Fenton treatment (step S4) and coagulation sedimentation treatment (step S6) are performed, and when the COD value is less than the predetermined set value or change amount, only coagulation sedimentation treatment (step S6) is performed to perform wastewater treatment (step S7).

In addition, the cause of the increase in COD may be organic matter such as surfactants or metal ions such as iron. When the COD increases due to metal ions, the increase can be treated only by coagulation and precipitation treatment, and treatment by the Fenton reaction has almost no effect on reducing the COD. When metal ions are predominant among the causes of COD in wastewater, the COD of the wastewater is 200 to 1500 mg/L. Here, the COD in the embodiment of the present invention is a value that takes into account organic matter and metal ion origins, and is in the range of 200 to 10000 mg/L.
Therefore, it is preferable to set the value in advance to 1500 mg/L, but the set value may be changed so as to meet the wastewater standards in actual operation.

After carrying out the above wastewater treatment, the COD of the treated wastewater is measured before discharging it. If the discharge standards are met, the wastewater is discharged. If the standards are not met, the wastewater is treated again with the wastewater treatment method of the present invention.

In the Fenton process 4, the wastewater from the raw water tank 1 is neutralized in a COD neutralization tank 41, and the pH is adjusted to 7-9. The wastewater is then agitated in a buffer pond 42. The pH of the agitated wastewater is adjusted to 2-3 in a neutralization tank 43. After the pH adjustment, the organic matter is decomposed by the Fenton reaction in a reaction tank 44. After this decomposition process, the pH is adjusted to 7-9 in a decomposition tank 45 and agitation is performed to remove any remaining hydrogen peroxide, and impurities are allowed to settle in a settling tank 46.

In the coagulation and sedimentation process 5, dilute sulfuric acid and ferric chloride are injected into the decomposition tank 51 to adjust the pH of the wastewater. Next, slaked lime is added to the neutralization tank 52 to adjust the pH of the wastewater to 7 to 9. A coagulant is injected into the sedimentation tank 53 to form flocs, allowing impurities in the wastewater to settle and be separated.

In addition, wastewater that requires Fenton treatment has always been treated assuming that the COD of the wastewater is high, but in reality, depending on the timing of wastewater collection, the COD may be so low that Fenton treatment is not required. Even in such cases, it is possible to use the wastewater treatment method based on the COD setting value of the present invention to treat the wastewater using only coagulation and sedimentation treatment without Fenton treatment.

The treated water that has undergone coagulation and sedimentation treatment is discharged from the drain after a COD meter has confirmed that the COD concentration is below the standard value. Since this COD meter must be highly accurate, it is preferable to use chemical analysis that complies with JIS K0102-1:2021.

[Example 1]
Wastewater from multiple factories was collected in a raw water tank and the COD was measured. The set value of COD for switching the wastewater treatment method was set to 1500 mg/L, and if the COD was 1500 mg/L or more, both the Fenton treatment and the coagulation precipitation treatment were performed, and if the COD was less than 1500 mg/L, only the coagulation precipitation treatment was performed. Wastewater treatment was performed for three months using the wastewater treatment method according to the COD value of the present invention. After the wastewater treatment was completed, the COD was measured before discharge, and the COD was below the discharge standard in all charges.

[Example 2]
Wastewater from multiple factories was collected in a raw water tank and the COD was constantly measured. Furthermore, the difference between the maximum and minimum COD values was calculated from the COD data converted to one hour from the start of the COD measurement.
The timing for switching the wastewater treatment method was set to 1200 mg/L, where the difference between the maximum and minimum COD values per hour was set. If the difference in COD increased by 1200 mg/L or more, both the Fenton treatment and the coagulation and sedimentation treatment were performed, and if the difference in COD was less than 1200 mg/L, only the coagulation and sedimentation treatment was performed. The treatment time was about 1 to 2 hours. Wastewater treatment was performed for three months using the wastewater treatment method based on the COD change amount of the present invention. After the wastewater treatment was completed, the COD was measured before discharge, and the COD was below the discharge standard in all charges.

In the embodiment of the present invention, it became possible to select a treatment method based on the COD value or change in COD of the wastewater, so when the COD of the wastewater is low, it became unnecessary to use the Fenton treatment chemicals that were conventionally used, and it was possible to reduce the amount of Fenton treatment chemicals used by 25% or more in both treatment methods.

Although the above examples are illustrative of the present invention, the present invention is not limited to these.

According to the present invention, this wastewater treatment control system can be applied even when new wastewater treatment methods are developed, when wastewater treatment methods are improved, or when the options for wastewater treatment methods increase.

W wastewater 1 raw water tank 2 COD measuring device 3 control device 31 solenoid valve connected to Fenton treatment equipment 32 solenoid valve connected to coagulation sedimentation treatment equipment 4 Fenton treatment equipment 41 COD neutralization tank 42 buffer pond 43 neutralization tank 44 reaction tank 45 decomposition tank 46 sedimentation tank 5 coagulation sedimentation treatment equipment 51 decomposition tank 52 neutralization tank 53 sedimentation tank

Claims (7)

A wastewater treatment method characterized in that the COD of wastewater before treatment is measured in a raw water tank , and based on the amount of change in COD, ΔQ, which is the difference between the maximum and minimum values per hour, from the obtained COD value, a coagulation sedimentation treatment or one of the two wastewater treatment methods, an oxidation treatment and a coagulation sedimentation treatment, is selected. 2. The wastewater treatment method according to claim 1 , wherein when the amount of change in COD, ΔQ, increases by 1000 mg/L or more, oxidation treatment and coagulation sedimentation treatment are selected as the wastewater treatment. 3. The wastewater treatment method according to claim 1, wherein the coagulation and sedimentation treatment is a treatment using an iron-based coagulant containing ferric chloride, ferrous sulfate, and polyferric sulfate. 3. The wastewater treatment method according to claim 1 , wherein the oxidation treatment is a treatment utilizing the Fenton reaction, which produces hydroxyl radicals by the action of an aqueous hydrogen peroxide solution and an iron-based catalyst. The wastewater treatment method according to claim 1 or 2 , wherein the COD of the wastewater is in the range of 200 to 10,000 mg/L. The wastewater treatment method according to claim 3 , wherein the COD of the wastewater is in the range of 200 to 10,000 mg/L. The wastewater treatment method according to claim 4 , wherein the COD of the wastewater is in the range of 200 to 10,000 mg/L.

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