JPH0144118B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention is intended to remove heavy metals from wastewater discharged from, for example, civil engineering and construction work, which contains heavy metals such as iron (Fe) and manganese (Mn). This invention relates to a method of oxidation treatment.

[Prior art and its problems]

Groundwater discharged from civil engineering and construction works is
It is often colored by the Fe, Mn, etc. contained in it, and cannot be discharged to the outside as is, so these heavy metals must be removed. By the way, these heavy metals in groundwater are neutral and anaerobic, so they exist in a reduced state.
Iron exists as Fe ²⁺ and manganese exists as Mn ²⁺ .
Therefore, these heavy metals can be removed by oxidizing these metal ions, precipitating them as hydroxides or oxides, and filtering the precipitates.

Conventionally, one such oxidation treatment method involves adding a chlorine-based oxidizing agent such as sodium hypochlorite (NaOCl) to wastewater to oxidize metal ions, increasing the residual chlorine concentration or oxidation-reduction potential (ORP) of wastewater. ) to determine when the oxidation reaction has finished, stop adding the oxidizing agent, and send the treated water to the next process.

However, using the method of determining the end point of the oxidation reaction based on the residual chlorine concentration or ORT, it is difficult to know the exact end point, and therefore, an excessive amount of oxidizing agent is added to compensate for the ambiguity of the end point. The disadvantage was that the loss of oxidizing agent was large. Figures 2 and 3 show ORP, pH, and Fe ²⁺ ions when NaOCl solution was added twice to groundwater from the same sampling site to oxidize Fe ²⁺ ions to Fe ³⁺ ions. It shows changes in concentration and residual chlorine concentration. The drainage shown in Figure 2 is
The one in Figure 3 has a Fe ²⁺ ion concentration of about 34 mg/
The Fe ²⁺ ion concentration is approximately 27 mg/, which is slightly different, but the Fe ²⁺ ion concentration at the end point of the oxidation reaction is
ORP is approximately 550mV in Figure 2 and approximately 550mV in Figure 3.
A large difference appears between 410mV and its absolute value. Therefore, in controlling oxidizing agent addition using ORP absolute value,
If the ORP setting value is large, complete oxidation can be expected, but an excessive amount of oxidizing agent must be added.

On the other hand, as for the residual chlorine concentration, in Figure 2, it is almost 0 before the end of oxidation, and after the end of oxidation, it increases in proportion to the amount of oxidizing agent added. The residual chlorine concentration gradually increases in proportion to the amount added. Therefore, the method of determining the end point based on the residual chlorine concentration has similar problems.

[Means for solving problems]

Therefore, in the present invention, pH is used as an indicator of the oxidation reaction instead of ORP or the amount of residual chlorine, and the increase and decrease in pH as the reaction progresses is measured, and the point at which the sign of the differential value of pH changes is the point at which the reaction ends. By doing so, it was possible to accurately detect the end point of the reaction and thereby accurately control the amount of oxidizing agent injected.

As shown in Figures 2 and 3, the drainage
The inflection point where the change in pH value changes from decreasing to increasing is
It is not affected by the Fe ²⁺ ion concentration and always coincides with the end point of the reaction. This invention was made based on this fact.

This will be explained in detail below.

Figure 1 shows an example of a wastewater treatment device used in the method for treating wastewater containing heavy metals according to the present invention, and reference numeral 1 in the figure indicates an oxidation reaction tank (tank) 1 into which wastewater containing heavy metals flows and undergoes oxidation treatment. be. This oxidation reaction tank 1 is equipped with a pH meter (pH measuring section) 2 that measures the pH of the waste water stored inside, and a stirrer 3 that stirs and mixes the waste water. Further, reference numeral 4 is an oxidizing agent tank for storing an oxidizing agent, and the oxidizing agent is supplied from this tank 4 to the reaction tank 1 by a supply pump 5. The oxidizing agent tank 4 and the supply pump 5 constitute an oxidizing agent supply section. Further, a signal from the pH meter 2 is sent to a control section 6. This control section 6
differentiates the input pH signal, detects the inversion of the sign of this differential value, and controls the operation of the supply pump 5 based on this inversion.

Next, a method for treating wastewater using this device will be explained. First, when a certain amount of wastewater to be treated flows into the oxidation reaction tank 1, the oxidizing agent is supplied from the oxidizing agent tank 4 to the tank 1 by the operation of the supply pump 5.
The oxidizing agent used here is an alkaline chlorine-based oxidizing agent such as sodium hypochlorite, potassium hypochlorite, calcium hypochlorite, or barium hypochlorite. Although the concentration is not particularly limited, Fe ²⁺ and Mn ²⁺ in the target wastewater
It is determined appropriately depending on the concentration, processing time, etc., and usually a 0.1 to 10% aqueous solution is used. The oxidizing agent is normally supplied in a fixed quantity, but it is also possible to perform program control to shorten the processing time.

The added oxidizing agent is stirred and mixed with the wastewater by the stirrer 3, and Fe ²⁺ , Mn ²⁺ , etc. in the wastewater are oxidized. In this oxidation reaction, ferrous Fe ²⁺ →Fe ³⁺ →Fe(OH)↓ ions change to ferric ions, and then to iron hydroxide, as shown in the following equation. As the reaction progresses, the hydroxyl groups in the system are consumed, and the pH gradually decreases as shown in Figures 2 and 3. At this time, the pH differential value in the control unit 6 is negative. Eventually, when the equivalence point is reached, the reaction in the above equation stops, the pH value stops decreasing, and the pH differential value once becomes zero.Then, as the oxidizing agent continues to be added, the pH increases and the differential value becomes positive. . At this point, the control section 6 immediately issues a pump stop signal to stop the operation of the supply pump 5. Next, stirring is continued for some time to promote the formation of iron hydroxide precipitate, and then the treated water is discharged from the reaction tank 1 and sent to the next step. With the above steps, one oxidation treatment is completed.

[Effect]

In this type of treatment method, the end point of the oxidation reaction is detected by a change in the sign of the differential value of the pH value, which is not affected by the concentration of Fe ²⁺ ions in the wastewater. Even with such wastewater, the end point can be accurately determined, eliminating the need to add excessive oxidizing agent.

Note that the timing at which the control unit 6 issues the stop signal for the supply pump 5 may be when the differential value of the pH value is zero or very close to zero, and this prevents excessive addition of oxidizing agent due to inertial operation of the pump 5. It's nice to be able to do it.

Experimental Example 1 A glass electrode of a pH meter was attached to a 200 c.c. glass beaker, and the pH meter was connected to a control device having a differential circuit, a comparison circuit, and a stop signal generation circuit. Further, an automatic brewet was attached to the beaker, and its driving pump was connected so as to be stopped by a stop signal from the control device.

In this beaker, Fe ²⁺ ion concentration is 25mg/,
100 ml of pH 6.4 wastewater was added, and a 0.1% NaOCl aqueous solution was flowed in from an automatic burette at a rate of 0.5 ml/min, followed by stirring with a magnetic stirrer. NaOCl
As the aqueous solution flows in, the pH decreases and reaches a pH of 6.2.
The pH stopped changing for a while and then immediately started rising.
At this point, Viewlet stops working and
The flow of NaOCl aqueous solution was stopped.

After the reaction, the Fe ²⁺ ion concentration in the treated water was measured and found to be 0.08 mg/, and the residual chlorine concentration was 0.05 mg/. From this, almost all of the Fe ²⁺ ions are oxidized, and the amount of residual chlorine is small, so it can be oxidized without adding extra NaOCl solution.
It was found that Fe ²⁺ can be completely oxidized.

Experimental Example 2 Using the apparatus in Experimental Example 1, a similar test was conducted on wastewater with a Fe ²⁺ ion concentration of 40 mg/ and a pH of 6.8, and the pH decreased to 5.8 and then began to rise. At this point, the inflow of the NaOCl aqueous solution was stopped. The Fe ²⁺ ion concentration and residual chlorine concentration in the treated water after the reaction were also very small.

Experimental Example 3 Using the apparatus in Experimental Example 1, Fe ²⁺ ion concentration 35 mg/, Mn ²⁺ ion concentration 4 mg/, pH 6.7
When similar tests were conducted on wastewater from
decreased to 6.0 and started to rise. at this point
The flow of NaOCl aqueous solution was stopped. The Fe ²⁺ ion concentration and residual chlorine concentration in the treated water after the reaction are similarly slight, and the Mn ²⁺ ion concentration is approximately 50% of the initial concentration.
It had declined to .

〔Effect of the invention〕

As explained above, the method for treating wastewater containing heavy metals of the present invention involves measuring the pH when oxidizing metal ions such as Fe ²⁺ and Mn ²⁺ in the wastewater by adding an alkaline chlorine oxidizing agent such as NaOCl. However, the change in the sign of this pH differential value is detected and the addition of the oxidizing agent is stopped based on this.
The end point of the reaction can always be known accurately without being affected by the metal ion concentration in the wastewater or other external factors, and compared to conventional methods that use ORP or residual chlorine concentration as a reaction indicator, the oxidation It is not necessary to add excessive amount of agent, it is economical, and the quality of treated water is stabilized. Additionally, pH meters are inexpensive and easy to calibrate, making them easier to maintain than conventional methods.

[Brief explanation of drawings]

Figure 1 is a schematic configuration diagram showing an example of a wastewater treatment device used in the method for treating wastewater containing heavy metals of the present invention, and Figures 2 and 3 both show wastewater containing Fe ²⁺ ions being oxidized with a NaOCl solution. It is a graph showing changes in ORP, pH, residual chlorine concentration, and Fe ²⁺ concentration during treatment. DESCRIPTION OF SYMBOLS 1... Oxidation reaction tank, 2... pH meter, 4... Oxidizing agent tank, 5... Supply pump, 6... Control part.

Claims (1)

[Claims] 1 When adding an alkaline chlorine oxidizer to wastewater containing heavy metals to oxidize the heavy metals,
A method for treating wastewater containing heavy metals, characterized by measuring pH, detecting a change in the sign of the differential value of this pH value, and stopping the addition of an oxidizing agent.