CN112079494B - Method for treating emulsion wastewater - Google Patents

Abstract

The invention belongs to the technical field of wastewater treatment, and provides a method for treating emulsion wastewater. Performing coagulative precipitation with potassium ferrate as an auxiliary, treating the emulsion wastewater by combining an electrochemical oxidation method, controlling the reaction temperature to be 35-40 ℃ by using PAC as a coagulant and potassium ferrate as a coagulant aid, performing coagulative precipitation pretreatment on the emulsion wastewater for 10min, and standing for 30-40 min; after the emulsion wastewater coagulating sedimentation pretreatment is finished, a BDD electrode electrochemical oxidation process is connected to carry out advanced treatment on the effluent of the coagulating sedimentation process. The potassium ferrate is used as the coagulant aid, so that the removal effect of the coagulant on organic pollutants can be improved, and the product of the potassium ferrate after hydrolysis cannot cause harm to human bodies and secondary pollution. The biodegradability of the effluent is improved. The electrochemical method can efficiently degrade pollutants in the wastewater, does not generate harmful substances, and does not cause secondary pollution. Compared to membrane processes, it is easy to frame and does not require cleaning.

Description

A kind of treatment method of emulsion waste water

technical field

The invention belongs to the technical field of wastewater treatment, and in particular relates to a treatment method for emulsion wastewater.

Background technique

At present, the key to the treatment of emulsion wastewater is effective demulsification, that is, oil-water separation. The existing process mainly relies on coagulation and precipitation to add flocculants to enhance the effects of electric neutralization, adsorption and bridging in the solution, so as to destabilize and coalesce the colloidal particles in the solution to form large-particle flocs, which are then added by adding flocculants. Coagulation aids are used to further enhance the size and density of flocculation; biological methods or membrane filtration methods are used after the coagulation and sedimentation process to perform advanced treatment on the effluent of the coagulation process section. So that the effluent reaches the standard and then discharges.

In order to solve the serious problem of ultrafiltration pollution and blockage in the emulsion wastewater treatment system of steel mills, Wang Ming et al. determined the process parameters through the experimental results: after adjusting the temperature of the emulsion in the tank and adding acid to break the demulsification, add liquid alkali to control the pH value between 7 and 9. , choose PAFC as flocculant, cationic PAM as coagulant aid. The field trial operation shows that: by adjusting the dosing concentrations of PAFC and PAM, the COD _Cr removal rate of the air flotation water of the emulsion system reaches 93.7%, and the air flotation water can be directly discharged into the dilute alkali oil wastewater system for secondary treatment. It solves the problem that the emulsion system cannot operate normally due to ultrafiltration fouling.

In the treatment of wastewater in a certain processing industry, the wastewater containing concentrated oil and emulsion discharged from the pickling production line is passed through the distribution tank into the wastewater conditioning tank containing concentrated oil and emulsion. The waste water in the conditioning tank is heated and skimming the oil slick, and then pumped to the paper belt filter for filtration to remove the coarse scum in the waste water. The effluent from the paper belt filter enters the ultrafiltration circulation tank, and the effluent from the circulation tank enters the ultrafiltration system, where the interception effect of the ultrafiltration membrane is used to separate oil and water, and to remove part of COD _Cr and other impurities. The COD _Cr of the concentrated oil wastewater is still high after ultrafiltration treatment. In order to ensure that the effluent COD _Cr meets the standard, the ultrafiltration effluent needs to be biochemically treated. The pH of the ultrafiltration effluent is adjusted to neutral in the pH adjustment tank, and then pumped into the cooling tower to cool down. The cooled wastewater enters the facultative oxygen tank for hydrolysis to degrade the macromolecular organic matter, thereby improving the biodegradability of the wastewater, which is beneficial to the normal operation of the subsequent membrane bioreactor. The effluent of the facultative aerobic tank enters the aeration tank of the membrane bioreactor, and further degrades COD _Cr and BOD in the wastewater through the aerobic activity of aerobic microorganisms. The retention effect of the membrane makes the microorganisms in the wastewater be retained, maintains a higher sludge concentration in the aeration tank, and improves the treatment efficiency of the biochemical system. The biochemical effluent of the membrane bioreactor meets the discharge requirements and is directly discharged to the final discharge pool.

At present, the main problems existing in the existing emulsion wastewater treatment technology are as follows:

Using flocculant and coagulant aid polyacrylamide (PAM) in the coagulation and sedimentation section to treat emulsified wastewater, although effective oil-water separation can occur, the biodegradability of the effluent is still low; and improper use of PAM is not conducive to flocs. aggregation. At the same time, excessive use of PAM will produce certain toxicity and affect the human body.

Subsequent advanced treatment is generally connected to biological treatment or ultrafiltration membrane treatment. Biochemical treatment mainly uses microorganisms to degrade pollutants in wastewater. Mainly divided into biological filter and activated sludge method. A biofilter is a bioreactor filled with an inert material that degrades pollutants mainly by relying on the biological community on the material. The activated sludge method relies on the metabolism of various microorganisms in the sludge to decompose macromolecular pollutants into small molecular organic or inorganic substances through the process of eutrophication or anaerobic reaction. The main problems of advanced biological treatment of emulsified wastewater are: long residence time and large area; improper anti-seepage treatment will pollute groundwater, and at the same time, it is easy to emit foul odor in summer, breed mosquitoes and flies, and cause pollution.

Ultrafiltration treatment of emulsion wastewater mainly utilizes the significant difference in molecular size of oil and water, and adopts cross-flow filtration to filter oil and water. The water molecules are smaller than the pores and pass through the ultrafiltration membrane, and the oil molecules are larger than the pores and cannot pass through the ultrafiltration membrane, thereby realizing the separation of oil and water. Ultrafiltration method uses many organic membranes, but the cost of organic membranes is too high, and they are not resistant to high temperature, low mechanical strength, and easy to hydrolyze. At the same time, the ultrafiltration membrane has problems such as the rapid decrease of the membrane flux and the easy fouling of the membrane. Regular cleaning is required, and additional economic costs are incurred.

SUMMARY OF THE INVENTION

In order to solve the technical problems existing in the current emulsion wastewater treatment process, the present invention provides a treatment method for the emulsion wastewater.

The present invention is realized by the following technical solutions: a method for treating emulsion waste water, adopts potassium ferrate to assist coagulation and precipitation, and combines electrochemical oxidation method for joint treatment of emulsion waste water, and the specific steps are as follows:

(1) Coagulation and sedimentation process: adjust the pH value of the emulsion wastewater to 9, use polyaluminum chloride PAC as a coagulant, potassium ferrate as a coagulant aid, and the dosage of coagulant is 50-100mg/L. The dosage of coagulant is 50-75mg/L; stir to make the agent evenly dispersed in the solution, control the reaction temperature to 35-40℃, carry out the coagulation pretreatment of the emulsion wastewater for 10min, and then let it stand for 30-40min;

(2) Electrochemical oxidation process: After the coagulation and sedimentation pretreatment of the emulsion wastewater is completed, the BDD electrode electrochemical oxidation process is connected to perform advanced treatment on the effluent of the coagulation and sedimentation process. The specific method is: Discharge into a new reservoir, with the BDD electrode as the anode and the Ti electrode as the cathode, adjust the initial pH between 7 and 9, control the current density within the range of 50-100 mA/ ^cm2 , and control the reaction temperature at 30-40 °C , electrolyze the effluent after coagulation and precipitation pretreatment, and the electrolysis time is 4-5h.

The coagulant is any one of polyaluminum chloride PAC, polyferric sulfate PFS, aluminum sulfate or ferric sulfate. The preferred coagulant is polyaluminum chloride PAC.

The dosage of PAC was 50 mg/L, the dosage of potassium ferrate was 50 mg/L, the reaction temperature was 40° C., stirred for 10 min, and allowed to settle for 30 min.

Step (2) In the electrochemical oxidation process, the initial pH is 9, the current density is 75 mA/cm ² , the reaction temperature is 40° C., and the electrolysis reaction is performed for 5 hours.

The use of potassium ferrate as a coagulant in the present invention can not only improve the effect of removing organic pollutants, but at the same time, the product of potassium ferrate after hydrolysis will not cause harm to the human body, and will not cause secondary pollution. Improve the biodegradability of the effluent. The electrochemical method acts as an advanced oxidation method, which can efficiently degrade the pollutants in the wastewater without producing harmful substances and causing secondary pollution. Compared with membrane treatment, it is easy to frame and does not require cleaning.

The present invention adopts potassium ferrate to assist PAC coagulation and precipitation, followed by BDD electrode electrochemical oxidation technology to treat the emulsified liquid waste water, and the existing process usually adopts organic flocculant PAM as a coagulant to assist inorganic flocculants to jointly pretreat and emulsify However, the main problems are that the composition of the emulsion is relatively complex, the flocculant cannot effectively break the emulsion wastewater, the separation of water and oil is not obvious, and the removal effect of organic pollutants is poor. Adding PAM to increase the floc volume also does not achieve the desired effect.

In the present invention, potassium ferrate is used as the coagulant aid, and the removal effect of the solution on organic pollutants can be greatly improved by virtue of its strong oxidizing property. At the same time, the positively and highly charged hydrolysate produced by ferrate in the reduction process can improve the treatment effect of PAC on emulsified wastewater in the coagulation and sedimentation process; and potassium ferrate can effectively change the floc structure, enhance the floc density and speed up the floc of sedimentation, shortening the time required for the process. Under neutral and weak alkaline conditions, good results can be achieved, and the removal efficiency of COD and turbidity can reach more than 90%, which improves the biodegradability for subsequent processing.

As a green water purifier, potassium ferrate will not cause secondary pollution and can effectively reduce the harm of excess aluminum. The wastewater treated by the coagulation and precipitation process section is then connected to the BDD electrode as an electrochemical oxidation process. This process uses the strong oxidant OH produced by the electrolysis of the anode surface to react with the organic substances in the wastewater to quickly and efficiently convert the organic substances. It becomes CO ₂ and H ₂ O or small molecular inorganics, which are effectively removed.

The process is fast and efficient and does not require the addition of additional oxidants to degrade pollutants. In addition, the electrochemical oxidation of BDD electrode does not produce pollutants and will not cause secondary pollution; and the boron-doped diamond film is an inert material, which has strong acid and corrosion resistance, and has a wide potential window. Has a long service life. In contrast, membrane filtration or ultrafiltration can easily cause problems such as membrane blockage or membrane fouling after being used for many times, and the filtration membrane must be cleaned in time. In addition, the price of membranes is relatively expensive, and the required area is large. Improper handling will cause pollution and economic losses; while the use of biological treatment requires a larger site, which increases operating costs, and is likely to cause secondary pollution, resulting in Larger amounts of sludge require further treatment, resulting in additional cost requirements.

To sum up, the method of the present invention can not only improve the removal efficiency of pollutants in the existing process, but also effectively improve the biodegradability; it can also effectively avoid the secondary pollution, high cost and large area of the existing process. question. It is a cost-effective and feasible process.

The present invention adopts potassium ferrate-assisted PAC coagulation precipitation + BDD electrode electrochemical oxidation two-stage process to treat emulsion wastewater, and adopts the optimal experimental conditions of the two-stage combined process determined by single-factor experiments, that is, the initial pH of the flocculation and precipitation process section is 9 , The dosage of PAC is 50mg/L, the dosage of potassium ferrate is 50mg/L, the reaction temperature is 40°C, stirring for 10min, standing and settling for 30min; the initial pH of the electrolysis process section is 9, the current density is 75mA/cm2, The reaction temperature was 40°C, and the electrolysis reaction was carried out for 5h. After successive coagulation precipitation + electrochemical oxidation combined process treatment, the effluent was colorless and odorless, COD _Cr was 23mg/L, c(NH ₃ -N) was 0.35mg/L, TOC was 11.75mg/L, and turbidity was 2.1 NTU. The main indicators all meet the first-class standard of "Integrated Wastewater Discharge Standard" (GB8978-1996), and at the same time meet the requirements of "Water Quality Standard for Sewage Discharged into Urban Sewers" (GB/T 31962-2015).

Description of drawings

Figure 1 shows the effect of potassium ferrate dosage on the removal of emulsion wastewater when the dosage of PAC is 50 mg/L;

Figure 2 shows the effect of potassium ferrate dosage on the removal of emulsion wastewater when the dosage of PFS is 50 mg/L;

Figure 3 shows the effect of potassium ferrate dosage on the removal of emulsion wastewater when the dosage of aluminum sulfate is 100 mg/L;

Figure 4 shows the effect of potassium ferrate dosage on the removal of emulsion wastewater when the dosage of ferric sulfate is 100 mg/L;

Figure 5 shows the effect of initial pH on the removal of pollutants by coagulation and sedimentation;

Figure 6 shows the effect of reaction temperature on the removal of pollutants by coagulation and sedimentation;

Figure 7 shows the effect of settling time on the removal of pollutants by coagulation and sedimentation;

Figure 8 shows the effect of PAC dosage on the removal of pollutants under the conditions that the pH of the solution is 9, the dosage of potassium ferrate is 50 mg/L, and the reaction temperature is 30 °C;

Figure 9 shows the effect of electrolyte concentration on the degradation of pollutants. In the figure: (1) is the removal effect of COD _Cr ; (2) is the removal effect of NH ₃ -N; (3) is the removal effect of TOC;

Figure 10 shows the effect of current density on the degradation of pollutants; in the figure: (1) is the removal effect of current density on COD _Cr ; (2) is the removal effect of current density on NH ₃ -N;

Figure 11 shows the effect of electrolysis pH on the degradation of pollutants; in the figure: (1) the removal effect of different electrolysis pH on COD _Cr ; (2) the removal effect of different electrolysis pH on NH ₃ -N;

Figure 12 shows the effect of electrolysis temperature on the degradation of pollutants; in the figure: (1) the removal effect of different electrolysis temperatures on COD _Cr ; (2) the removal effect of different electrolysis temperatures on NH ₃ -N;

FIG. 13 is a graph showing the change of TOC removal rate with time.

Detailed ways

The specific embodiments of the present invention will be further described below.

A method for treating emulsion waste water, which adopts potassium ferrate to assist coagulation and precipitation, and combines electrochemical oxidation method to jointly treat emulsion waste water. The specific steps are as follows:

(1) Coagulation and sedimentation process: adjust the pH value of the emulsion wastewater to 9, use polyaluminum chloride PAC as a coagulant, potassium ferrate as a coagulant aid, and the dosage of coagulant is 50-100mg/L. The dosage of coagulant is 50-75mg/L; stir to make the agent evenly dispersed in the solution, control the reaction temperature to 35-40℃, carry out the coagulation pretreatment of the emulsion wastewater for 10min, and then let it stand for 30-40min;

Analysis of reaction mechanism: PAC, as a polymer inorganic flocculant, is hydrolyzed to produce a large number of positively charged hydrolysis products, and the generated Al(OH) ₃ colloid has a strong adsorption effect. Therefore, adding PAC promotes the neutralization of electricity in the solution. It strengthens the destabilization and coalescence of colloidal particles in the solution; potassium ferrate is a water treatment agent that integrates the functions of oxidation, flocculation, coagulation and disinfection.

In the present invention, adjusting the pH to 9 helps potassium ferrate to exert its strong oxidizing property and oxidize macromolecular organic matter into small molecular organic matter, thereby promoting the coagulation and removal of organic pollutants by PAC. At the same time, in the process of reducing the hexavalent iron ions of potassium ferrate to ferric ions, a positive high-valence intermediate product with a positive charge and a network structure is generated, which strengthens the net capture effect of PAC. Moreover, potassium ferrate can effectively change the floc structure and promote the aggregation of flocs, thereby accelerating the sedimentation, which is conducive to improving the efficiency of the industry and thus saving costs. And the generated Fe(OH) ₃ colloid can co-precipitate with the Al(OH) ₃ colloid, which can further treat the pollutants in the solution and improve the biodegradability of the wastewater. At the same time, the effluent temperature of this industry is about 30 °C, and the temperature is appropriately increased to 35 to 40 °C, which helps to accelerate the Brownian motion between flocs in the solution, thereby accelerating the aggregation of flocs.

(2) Electrochemical oxidation process: After the coagulation and sedimentation pretreatment of the emulsion wastewater is completed, the BDD electrode electrochemical oxidation process is connected to carry out advanced treatment of the effluent of the coagulation and sedimentation process. The specific method is: discharge the effluent of the coagulation and sedimentation process into The new reservoir, with BDD electrode as anode and Ti electrode as cathode, adjusts the initial pH between 7 and 9, the current density is controlled within the range of 50 to 100 mA/cm ² , and the reaction temperature is controlled between 30 and 40 °C. , electrolyze the effluent from the previous process. The electrolysis time is between 4 and 5 hours.

The surface of BDD electrolysis produces a large amount of strong oxidizing OH, which can effectively degrade the organic pollutants in the effluent, and can directly degrade the organic pollutants into carbon dioxide and water on the anode surface. At the same time, increasing the reaction temperature can accelerate the generation rate of OH, thereby effectively increasing the indirect oxidation rate of the BDD electrode. At the same time, the Brownian motion of the solute in the solution is strengthened, the diffusion rate of pollutants in the solution and the chemical reaction rate on the electrode surface are improved, which effectively promotes the electrolysis reaction. Therefore, the removal of pollutants in wastewater is greatly improved, the biodegradability of effluent is improved, and the guarantee for subsequent advanced treatment is provided.

Experimental Example 1: Comparative Experiment of Potassium Ferrate Compound Coagulant

A. Potassium ferrate compound PAC: adjust the initial pH=7 with 1mol/L sulfuric acid and sodium hydroxide solution, set the temperature to 30℃, and add PAC to the wastewater when the dosage is 50mg/L Coagulation aid potassium ferrate (1% solution), so that the content of potassium ferrate in the water sample is 0, 20, 30, 50, 100 mg/L, respectively.

The effect of potassium ferrate dosage on the removal of pollutants is shown in Figure 1. It can be found from Figure 1 that with the increase of potassium ferrate dosage from 0 to 50 mg/L, the CODCr and turbidity removal rates of the solution increased rapidly from 48.6% and 62.1% to 84.8% and 92.07%, respectively. Table 1 shows the effect on the removal efficiency of emulsion wastewater when only adding PAC, and Table 2 shows the effect on the removal efficiency of emulsion wastewater when only adding potassium ferrate. Combining with Tables 1 and 2, it can be seen that when only PAC (50mg/L) or potassium ferrate (50mg/L) was added, the removal rates of CODCr were 48.6% and 14.25%, respectively, and the turbidity removal rates were 48.6% and 14.25% respectively. 62.1%, 20.56%. When the two are combined, the CODCr removal rate reaches 84.8%, and the turbidity removal rate is 92.07%, forming an obvious synergistic effect.

Table 1 Effects of only adding PAC on the removal efficiency of emulsion wastewater

Table 2 The effect of only adding potassium ferrate on the removal efficiency of emulsion wastewater

B. Potassium ferrate compound PFS: adjust the initial pH=7, set the reaction temperature to 30℃, and add the coagulant aid potassium ferrate (1 % solution), so that the content of potassium ferrate in the water sample was 0, 20, 30, 50, and 100 mg/L, respectively. Table 3 shows the effect of PFS dosage on the removal effect of emulsion wastewater only by adding PFS. It can be seen from Table 3 that when the dosage of PFS is 50 mg/L and potassium ferrate is not added, the removal rates of CODCr and turbidity in the emulsion wastewater are 46.75% and 57.96%, respectively.

Figure 2 shows the effect of potassium ferrate dosage on the removal effect of emulsion wastewater when the dosage of PFS is 50mg/L. It can be seen from Figure 2 that the removal rate of CODCr and turbidity of wastewater increases with high iron Potassium acid increased rapidly from 0 to 50 mg/L from 48.6% and 62.1% to 81.25% and 90.2%, respectively. Combining with Table 2, Table 3 and Figure 2, it can be seen that when only adding PFS (50mg/L) or potassium ferrate (50mg/L), the removal rate of CODCr is lower, 46.75% and 14.25% respectively, while When the two chemicals were added to the emulsified waste liquid, the removal rate of CODCr reached 81.25%, and the obvious change of turbidity can be seen intuitively, which shows that the addition of potassium ferrate strengthens the coagulation and precipitation effect of PFS. .

Table 3 Effect of PFS dosage on the removal of emulsion wastewater

C, potassium ferrate compound Al ₂ (SO ₄ ) ₃ : Table 4 is to add aluminum sulfate separately, the result of the influence of aluminum sulfate dosage on the removal effect of emulsion waste water, as can be seen from Table 4, when adding aluminum sulfate ( 100 mg/L), the removal rates of CODCr and turbidity in the emulsion wastewater were 31.1% and 40.36%, respectively.

Figure 3 shows the effect of potassium ferrate dosage on the removal effect of emulsion wastewater when the dosage of aluminum sulfate is 100mg/L; it is also found from Figure 3 that as the dosage of potassium ferrate increases from 0 to 50mg /L, the removal rates of CODCr and turbidity of the solution increased rapidly from 31.1% and 40.36% to 75.68% and 84.88%, respectively. Combining with Table 2 and Table 4, it can be seen that when aluminum sulfate (100mg/L) or potassium ferrate (50mg/L) is added alone, the removal rate of CODCr is relatively low, 31.1% and 14.25% respectively, but the removal rate of CODCr is relatively low. When ferric sulfate and potassium ferrate are compounded into the emulsion wastewater, the removal rate of CODCr can reach 75.68%, indicating that potassium ferrate can effectively improve the removal effect of aluminum sulfate on the emulsion wastewater.

Table 4 Addition of aluminum sulfate alone, the effect of aluminum sulfate dosage on the removal effect of emulsion wastewater

D, potassium ferrate compound Fe When the potassium dosage was 0 mg/L, the removal rates of CODCr and turbidity in the emulsion wastewater were 27.63% and 37.76%, respectively.

Figure 4 shows the effect of potassium ferrate dosage on the removal effect of emulsion wastewater when the dosage of ferric sulfate is 100 mg/L; it can be found from Figure 4 that the removal rate of CODCr and turbidity of wastewater increases with the increase of ferric acid. The potassium dosage increased continuously (from 0 to 50 mg/L) from 27.63% and 37.76% to 71.75 and 79.5%, respectively. Analysis of Table 2 and Table 5 shows that when ferric sulfate (100mg/L) or potassium ferrate (50mg/L) is added alone, the removal rates of CODCr and turbidity are 27.63%, 14.25% and 37.76%, 20.56% respectively. , the removal effect is poor, but when the waste emulsion is treated with the two chemicals at the same time, the removal rates of CODCr and turbidity can reach 71.75% and 79.5%.

Table 5 Adds ferric sulfate alone, and the effect of ferric sulfate dosage on the removal effect of emulsion wastewater

Potassium ferrate combined with different coagulants (PAC, PFS, aluminum sulfate and ferric sulfate) can effectively improve the removal effect of CODcr and turbidity in wastewater when removing CODcr and turbidity in emulsion wastewater at the same time. Comparing the experimental results, it can be seen that the optimal order of the effect of potassium ferrate and four flocculants in the treatment of emulsion wastewater is: potassium ferrate + PAC > potassium ferrate + PFS > potassium ferrate + aluminum sulfate > potassium ferrate + ferric sulfate.

The removal rate of potassium ferrate compounded with polyaluminum chloride PAC reached the maximum, so PAC was selected as the coagulant in subsequent experiments.

Existing studies have also demonstrated that potassium ferrate as a coagulant can improve the removal of organic pollutants. Potassium ferrate is a strong oxidant with strong oxidizing ability, and its oxidizing ability is stronger than potassium permanganate, O ₃ and other substances. At the same time, K ₂ FeO ₄ has the advantages of high efficiency, non-toxicity and harmlessness. It is a new type of high-efficiency water purifier that integrates oxidation, adsorption, coagulation, flocculation and other functions and can remove organic matter and heavy metal ions. ^[1-3]

Potassium ferrate can effectively change the structure of flocs, enhance the density of flocs, speed up the settling of flocs, and shorten the time required for the process. And under the conditions of neutrality and weak alkalinity, good results can be achieved, and the removal efficiency of COD and turbidity can reach more than 90%, which improves the biodegradability for subsequent processing.

Experimental Example 2: Single factor experiment in coagulation and sedimentation stage: 100 mL of emulsion wastewater was added to the beaker, and the pH was adjusted with 1 mol/L H ₂ SO ₄ and NaOH solution. The specified amount of coagulant PAC (10% solution) and coagulant potassium ferrate (1% solution) were added successively, and magnetic stirring was performed at a fixed speed at the set temperature. The reaction was stirred for 10 minutes, and after standing for 30 minutes, the supernatant was taken to measure the COD _Cr and turbidity values. The investigated factors include: initial pH (3, 5, 7, 9, 11), dosage of PAC (0, 10, 20, 30, 50, 100 mg/L), dosage of potassium ferrate (0, 20, 30, 50, 100 mg/L), reaction temperature (20, 30, 40, 50, 60 °C), sedimentation time.

The effect of initial pH on the coagulation and sedimentation removal effect of pollutants is shown in Figure 5. From the experimental results in Figure 5, it is determined that in the subsequent experiments, the initial pH value is selected as 9.

1. The effect of reaction temperature on the removal of pollutants: Under the conditions that the pH of the solution is 9, and the dosage of PAC and potassium ferrate is 50 mg/L, the reaction temperatures are 20, 30, 40, 50, and 60 °C, respectively. The removal effect of pollutant coagulation and sedimentation was shown in Figure 6. It can be seen from the figure that when the reaction temperature increased from 20 °C to 40 °C, the removal rates of COD _Cr and turbidity in the solution increased from 73.9% and 89.9% to 86.3% and 96.6%, respectively. Generally speaking, with the increase of water temperature, the Brownian motion in the solution is strengthened, which greatly improves the contact probability between flocs; however, if the water temperature is too high, the hydrolysis reaction speed of PAC and potassium ferrate will be accelerated, and the flocculation will be loose and not easy to settle. Coagulation and precipitation have adverse effects ^[4,5] . When the water temperature is low, the hydrolysis of the coagulant is slow, the viscosity of the water is high, and the Brownian motion is weak, which is not conducive to the anisotropic flocculation of the destabilized colloidal particles. As shown in the figure, when the reaction temperature continued to rise, the COD _Cr and turbidity removal rates of the solution increased slowly or even decreased. According to the experimental results, 40°C was selected as the optimum reaction temperature. Due to the characteristics of the processing technology, the actual temperature of the waste emulsion is roughly between 20 and 30 °C. Considering the influence of reaction temperature on coagulation and the application of practical engineering, it is recommended to treat the generated wastewater as soon as possible, thereby reducing the dosage of medicines, reducing the energy consumption of heating and saving industrial costs.

2. The effect of sedimentation time on the removal of pollutants: Under the conditions that the initial pH of the solution is 9, the dosage of PAC and potassium ferrate is 50 mg/L, and the reaction temperature is 40 °C, the solution is stirred at a constant speed for 10 minutes and then left to settle. The influence of the settling time on the flocculation treatment effect of the composite agent was investigated as shown in Figure 7. It can be seen from Figure 7 that in the early stage of settlement (between 0-30 min), with the increase of settling time, the removal rates of COD _Cr and turbidity increased rapidly; if the settling time continued to extend, the removal rates of COD _Cr and turbidity increased gradually slowed down.

This indicates that the settling time directly affects the flocculation effect of the flocs in adsorbing pollutants in wastewater. As time goes on, it can be observed that after potassium ferrate assists PAC to treat wastewater, the fine flocs formed in the solution aggregate and squeeze each other to form a cluster structure. Under the action of gravity, the water in the lower layer is continuously squeezed out, and obvious stratification occurs. As the settling time progressed, the water layer became thicker and clearer, and the flocs became denser. After 30 min, the settlement basically became stable, and no obvious changes were observed.

When the settling time is less than 30min, the COD _Cr and turbidity removal rates are low due to the short settling time, the insufficient adsorption of the flocs to the organic pollutants in the wastewater, and the incomplete settling of the destabilized flocs. When the sedimentation time exceeded 30 min, the removal rate of COD _Cr and turbidity did not change significantly or even decreased to a certain extent, which may be caused by the re-dissolution of some organic matter during the long sedimentation process. Comprehensive consideration, in the process of potassium ferrate-assisted PAC treatment of emulsion wastewater, the optimal settling time was determined to be 30min.

3. The influence of the dosage of PAC on the removal of pollutants: under the conditions that the pH of the solution is 9, the dosage of potassium ferrate is 50 mg/L, and the reaction temperature is 30 °C, 0, 10, 20, 30, 50, and 100 mg/L PAC, and the effect of PAC dosage on the removal of pollutants was investigated. The results are shown in Figure 8. It can be seen from the figure that when the same amount of coagulant is added, the dosage of PAC has a significant impact on the flocculation and sedimentation effect; with the continuous increase of the dosage, the COD _Cr and turbidity removal rates of the wastewater both increase rapidly. .

When the dosage of potassium ferrate was 50 mg/L, with the increase of the dosage of PAC from 10 to 50 mg/L, the removal rate of COD _Cr in the solution increased from 14.25% to 84.8%, and the turbidity removal rate increased from 20.56%. to 92.07%. Increasing the dosage of PAC resulted in more high-valent polynuclear complexes produced by hydrolysis in the solution, which promoted the compression of the electric double layer and the effect of adsorption charge neutralization. When the dosage of PAC reached 100 mg/L, the removal rates of COD and turbidity decreased to 82.01% and 83.54%, respectively. Since the excess flocs surround the colloidal particles, the collision probability of the colloidal particles will be reduced, and the adsorption and bridging effect will be affected, so that the flocculation and precipitation cannot achieve the desired effect.

4. The influence of the dosage of coagulant aid on the removal of pollutants: under the conditions that the pH of the solution is 9, the dosage of PAC is 50 mg/L, and the reaction temperature is 30 °C, the dosage of coagulant aid is 0, and the reaction temperature is 30 °C. The effects of 20, 30, 50, and 100 mg/L on the pollutant removal effect are shown in Figure 1. As the dosage of potassium ferrate increased from 0 to 50 mg/L, the removal rates of solution and turbidity increased rapidly from 48.6% and 62.1% to 84.8% and 92.07%, respectively. Combining with Tables 1 and 2, it can be seen that COD _Cr , when only adding PAC (50mg/L) or potassium ferrate (50mg/L), the removal rate of COD _Cr is lower, respectively 48.6% and 14.25%, while the two The removal rate of COD _Cr reached 84.8% when the compound was added, forming an obvious synergistic effect.

On the one hand, the strong oxidizing property of potassium ferrate destroys the organic protective layer on the surface of the colloid in solution, and destabilizes the wastewater through electrical neutralization. On the other hand, the produced Fe(OH) ₃ flocs can adsorb fine colloidal particles and co-precipitate with Al(OH) ₃ colloids, which can remove pollutants from wastewater more effectively. At the same time, adding potassium ferrate can change the floc structure, make the floc more compact and easy to settle. When potassium ferrate is added to a certain amount, the stability of potassium ferrate becomes poor, the decomposition is accelerated, and the excess Fe(OH) ₃ recombines with the difficult-to-settlement substances in the wastewater to form suspended solids, which removes COD _Cr and turbidity. rate decreased slightly. After comprehensively considering the removal effect of pollutants and the cost of medicines, the dosage of PAC and potassium ferrate was determined to be 50 mg/L.

Single factor experiments were used to determine the optimal experimental conditions of the two-stage combined process, that is, the initial pH of the flocculation and precipitation process was 9, the dosage of PAC was 50 mg/L, the dosage of potassium ferrate was 50 mg/L, and the reaction temperature was 40 °C. Stir for 10min, stand and settle for 30min; the initial pH of the electrolysis process section is 9, the current density is 75mA/cm ² , the reaction temperature is 40°C, and the electrolysis reaction is 5h. After successive coagulation precipitation + electrochemical oxidation combined process treatment, the effluent was colorless and odorless, COD _Cr was 23mg/L, c(NH ₃ -N) was 0.35mg/L, TOC was 11.75mg/L, and turbidity was 2.1 NTU. The main indicators all meet the first-class standard of "Integrated Wastewater Discharge Standard" (GB8978-1996), and at the same time meet the requirements of "Water Quality Standard for Sewage Discharged into Urban Sewers" (GB/T 31962-2015).

By investigating the removal effect of flocculant dosage, potassium ferrate dosage, reaction temperature, initial pH of flocculation, and sedimentation time on the removal of emulsified wastewater by each compound combination, the compound of potassium ferrate and polyaluminum chloride was finally selected. The combination is the optimal combination of the emulsion wastewater treatment. According to the single factor experiment, the optimal conditions for each process parameter are: the dosage of PAC is 50 mg/L, the dosage of potassium ferrate is 50 mg/L, the initial pH is 9, the reaction temperature is 40 °C, and the settling time is 30 min. . At this time, the effluent quality of the coagulation and sedimentation process section is shown in Table 6. The removal rates of CODcr and turbidity were 89.5% and 92.5%, respectively.

Table 6: Water quality of effluent from coagulation and sedimentation process

Experimental Example 3: Electrochemical oxidation stage experiment: 500 mL of wastewater treated with optimal conditions in the coagulation and precipitation process was injected into a beaker with an effective volume of 500 mL, and 500 mg/L Na ₂ SO ₄ electrolyte was added to improve the conductivity of the solution. The BDD anode and the Ti cathode were placed vertically in a beaker, the distance between the plates was 10 mm, the immersion area was both 10 cm ² , and a set current density was applied to conduct galvanostatic electrochemical degradation. During the reaction, the beaker was placed on a constant temperature magnetic stirrer, so that the concentration of the reaction solution was uniform and the temperature was constant; the concentration values of CODCr, ammonia nitrogen (NH3-N) and total organic carbon (TOC) were determined by sampling regularly. The investigated factors include: current density (20, 30, 50, 75, 100 mA/cm2), initial pH of the electrolysis process section (3, 5, 7, 9, 11) and electrolysis reaction temperature (20, 30, 40, 50, 60°C).

1. The effect of electrolyte concentration on electrochemical oxidation: In order to study the effect of supporting electrolyte concentration on wastewater degradation efficiency, Na2SO4 was selected as the supporting electrolyte, and the supporting electrolyte concentrations were set to 300 mg/L, 500 mg/L and 1000 mg/L, respectively. Take 500 mL of effluent from the coagulation and sedimentation process section in a beaker, set the current density to 50 mA/cm ² , and keep the distance between the plates at 10 mm for the experiment. The degradation effect of the supporting electrolyte concentration on the effluent of the BDD electrode electrolytic oxidation coagulation precipitation process section is shown in Figure 9.

It can be seen from Figure 9 (1), (2), (3) that when degraded under three different supporting electrolyte concentrations, the removal rates of CODCr, ammonia nitrogen and TOC of the solution increased rapidly as the electrolysis time increased from 0 to 180 min. high. At the same time, it can be seen that under the same electrolysis time, as the concentration of the supporting electrolyte increases from 300 mg/L to 1000 mg/L, the degradation degree of each pollutant in the wastewater also increases gradually. The analytical reason may be that the addition of the auxiliary electrolyte increases the ion concentration in the solution, thereby enhancing the mass transfer ability of the ions in the solution. At the same time, the conductivity of the solution also increases continuously, which is helpful for electron transfer and electrolysis. After electrolysis for 180 min, the removal rates of CODCr, ammonia nitrogen and TOC at electrolyte concentrations of 300 mg/L, 500 mg/L and 1000 mg/L reached 58.6%, 65.28%, 66.32%, 74.58%, 88.46% and 90.89%, respectively. and 58.45%, 65.64%, 70.16%. Continue to prolong the electrolysis time, the removal rate keeps rising slowly.

It can be seen from the figure that when the supporting electrolyte concentrations are 500mg/L and 1000mg/L, respectively, the CODCr, ammonia nitrogen and TOC removal effects of the solution are not significantly different. It can be seen that when the supporting electrolyte concentration reaches a certain level, its influence on the electrolysis process will gradually weaken or even disappear. Therefore, 500mg/L Na2SO4 was selected as the optimum supporting electrolyte concentration under the condition of ensuring the efficient removal effect and reducing the cost of chemicals.

2. Influence of current density on pollutant removal effect: Current density is an important factor affecting the electrochemical oxidation process. Take the wastewater with COD _Cr =456mg/L, c (NH ₃ -N)=26mg/L after the best coagulation and precipitation treatment, when the current density is 20, 30, 50, 75, 100mA/cm ² , the investigation The effect of current density on the degradation effect of wastewater treated in the electrolysis process section is shown in Figures 10(1) and (2).

The mechanism of BDD electrode oxidizing organic pollutants mainly relies on oxidants such as OH and hypochlorous acid generated on the anode surface to indirectly oxidize and degrade organic pollutants. The reactions involved are shown in formulas (1)-(4).

BDD+H ₂ O→BDD(·OH)+H ⁺ +e ^- (1)

BDD(·OH)+R→BDD+CO ₂ +H ₂ O (2)

2Cl ^- →Cl ₂ +2e ^- (3)

Cl ₂ +H ₂ O→HClO + Cl ⁻ +H ⁺ (4).

It can be seen from Figure 10 that under the same electrolysis time, when the current density increases from 20 mA/cm ² to 75 mA/cm ² , the removal rates of COD _Cr and ammonia nitrogen gradually increase. This is because increasing the current density is conducive to electron transfer and generates high concentration of OH, thereby accelerating the oxidative degradation of the anode surface; in addition, due to the presence of Cl ^- in the wastewater treated by the flocculation and precipitation process, with the increase of the current density, it may be Promote the formation of HClO oxidant. When the current density increased to 100mA/cm ² , the removal rates of COD _Cr and ammonia nitrogen showed a downward trend. Because the excessively high current density aggravates the side reaction of oxygen evolution in the water splitting of the anode, a large number of micro-bubbles adhere to the electrode surface and prevent organic pollutants from getting close to the electrode surface to be oxidized. Therefore, 75 mA/cm ² was chosen as the optimum anode current density.

3. The effect of electrolysis pH on the removal of pollutants: In order to investigate the effect of different initial pH on the removal of pollutants in the electrolysis process, 1mol/LH ₂ SO ₄ and NaOH solutions were used to adjust the electrolysis pH to 3, 5, 7, 9, 11 conduct experiments separately. The effect of electrolysis pH on the pollutant removal effect is shown in Figure 11(1), (2). It can be seen from the figure that the degradation effect is the best under weak alkaline conditions; the next is under neutral and weak acid conditions; and the degradation effect is the worst under strong acid and strong alkaline conditions. Since the chlorine evolution ability of the BDD electrode is weak and the generated HClO is less, the main reaction mechanism of the electrolytic oxidation is the indirect oxidation of OH. The electron abstraction of hydroxide at the anode is one of the main ways to generate ·OH. As the electrolysis pH increases, the hydroxide concentration increases, and more OH is generated on the surface of BDD electrolysis. However, when the hydroxide in the solution is excessive, the production of OH cannot be further promoted, but it will reduce the oxygen evolution potential of the anode and aggravate the side reaction of oxygen evolution, which is not conducive to the removal of organic pollutants by the BDD electrode. For ammonia nitrogen, under neutral to weak alkaline conditions, NH ₃ -N can achieve a high removal rate (>80%) at 3h. Under neutral and alkaline conditions, NH ₃ -N mainly exists in the form of free ammonia, and its anodized products are mainly nitrogen and water. Therefore, the optimum electrolytic pH for electrochemical degradation of organics is 9.0.

4. The influence of electrolysis temperature on wastewater treatment: The initial temperature of electrolysis reaction is set to 20℃, 30℃, 40℃, 50℃, 60℃ respectively, and the degradation effect of BDD electrode electrolytic oxidation treatment of wastewater at different electrolysis temperatures is investigated, see Figure 12(1), (2). It can be seen from the figure that when the initial temperature increased from 20 °C to 40 °C, the removal rates of COD _Cr and ammonia nitrogen in the solution increased continuously. Due to the increase of temperature, the generation rate of OH is accelerated, thereby effectively improving the indirect oxidation rate of BDD electrodes. At the same time, the Brownian motion of the solute in the solution is strengthened, the diffusion rate of pollutants in the solution and the chemical reaction rate on the electrode surface are improved, which effectively promotes the electrolysis reaction. When the electrolysis time is the same, the electrolysis efficiency of BDD electrode at the initial temperature of 50 °C and 60 °C is significantly lower than that of 40 °C. The main reason is that the high temperature aggravates the deactivation of free radicals, thereby reducing the reaction rate and hindering the degradation of organic matter. Considering the removal effect and the energy consumption required for heating, 40 °C was selected as the initial temperature. Since heat is generated spontaneously during the electrolysis process, it is suggested that the initial temperature of the influent water can be appropriately lowered in engineering applications, and the initial temperature of the electrolysis can be appropriately lowered to reduce energy consumption and cost.

5. TOC removal rate: According to the above experiments, the optimal experimental conditions for the electrolysis process section are as follows: the current density is 75mA/cm ² , the initial pH value of the solution is 9, and the solution temperature is 40°C. The TOC concentration analysis found that after 5h electrolysis treatment, the TOC removal rate was as high as 97.8%, indicating that the BDD electrode treatment almost mineralized all the organic matter in the wastewater. The TOC versus time curve is shown in Figure 13.

6. Coagulation-electrochemical combined treatment effect: the best experimental conditions of the two-stage combined process determined by single-factor experiments, that is, the initial pH of the flocculation and precipitation process section is 9, the dosage of PAC is 50mg/L, and the dosage of potassium ferrate is The dosage was 50 mg/L, the reaction temperature was 40 °C, the stirring was performed for 10 min, and the settling was carried out for 30 min; the initial pH of the electrolysis process section was 9, the current density was 75 mA/cm ² , the reaction temperature was 40 °C, and the electrolysis reaction was performed for 5 h. After successive coagulation precipitation + electrochemical oxidation combined process treatment, the effluent is colorless and odorless, COD _Cr is 23mg/L, c (NH ₃ -N) is 0.35mg/L, TOC is 11.75mg/L, and turbidity is 2.1 NTU. The main indicators all meet the first-class standard of "Integrated Wastewater Discharge Standard" (GB8978-1996), and at the same time meet the requirements of "Water Quality Standard for Sewage Discharged into Urban Sewers" (GB/T 31962-2015).

references:

1. Liu Wei. A new type of water treatment agent ferrate. (China Construction Industry Press, 2007).

2. Ma Ronghua, Liu. Preparation of potassium ferrate and its application in water treatment. Water Treatment Technology, 297-298.

3. Guo Shicheng & Lin Qiuhua. Potassium ferrate and drinking water treatment. Chemistry Education (2005).

4. Lizama Allende, K., McCarthy, D. T. & Fletcher, T. D. The influence of media type on removal of arsenic, iron and boron from acidicwastewater in horizontal flow wetland microcosms planted with Phragmitesaustralis. Chemical Engineering Journal 246, 217-228, doi:https ://doi.org/10.1016/j.cej.2014.02.035 (2014).

5. Song Hua & Wang Yuanyuan. Stability of potassium ferrate in neutral and acidic media. Chemical Bulletin 71, 696-700 (2008).

Claims (5)

1. a treatment method of emulsion waste water, it is characterized in that: adopt potassium ferrate auxiliary coagulation precipitation, in conjunction with electrochemical oxidation method combined treatment emulsion waste water, concrete steps are as follows: (1) Coagulation and sedimentation process: adjust the pH value of the emulsion wastewater to 9, add 50-75mg/L potassium ferrate as a coagulant, add 50-100mg/L coagulant; stir to make the agent evenly dispersed in the In the solution, control the reaction temperature to be 35-40°C, carry out coagulation and precipitation pretreatment of emulsion wastewater for 10min, and then stand for 30-40min; (2) Electrochemical oxidation process: After the coagulation and sedimentation pretreatment of the emulsion wastewater is completed, the BDD electrode electrochemical oxidation process is connected to perform advanced treatment on the effluent of the coagulation and sedimentation process. The specific method is: Discharge into a new reservoir, with the BDD electrode as the anode and the Ti electrode as the cathode, adjust the initial pH between 7 and 9, control the current density within the range of 50-100 mA/ ^cm2 , and control the reaction temperature at 30-40 °C , electrolyze the effluent after coagulation and precipitation pretreatment, and the electrolysis time is 4-5h. 2. The treatment method of a kind of emulsion waste water according to claim 1, is characterized in that: described coagulant is any one in polyaluminum chloride PAC, polyferric sulfate PFS, aluminum sulfate or iron sulfate. 3 . The method for treating emulsion wastewater according to claim 2 , wherein in the coagulation and sedimentation process in step (1): the coagulant is polyaluminum chloride PAC. 4 . 4. A method for treating emulsion wastewater according to claim 3, characterized in that: in the coagulation and precipitation process in step (1): the dosage of the coagulant PAC is 50 mg/L, potassium ferrate The dosage was 50 mg/L, the reaction temperature was 40° C., the mixture was stirred for 10 minutes, and allowed to settle for 30 minutes. 5 . The method for treating emulsion wastewater according to claim 1 , wherein in the electrochemical oxidation process in step (2), the initial pH is 9, the current density is 75 mA/cm ² , the reaction temperature is 40° C., and the electrolysis The reaction was carried out for 5 hours.

Images