Method for controlling generation amount of reclaimed water disinfection by-product haloacetonitrile by pre-oxidation enhanced coagulation process

Abstract

The invention relates to the field of sewage recycling, and discloses a method for controlling the generation amount of a reclaimed water disinfection by-product haloacetonitrile. The method comprises the following steps: in a reclaimed water advanced treatment process, before secondary effluent of a municipal sewage treatment plant is disinfected; and the potassium permanganate pre-oxidation enhanced coagulation advanced treatment process is adopted to reduce the yield of haloacetonitrile in the subsequent chlorination disinfection process. According to the method, the feeding amount of the oxidant potassium permanganate, the feeding amount of the coagulant aluminum polychlorid and the initial pH value are optimized, theoretical guidance is provided for improving the haloacetonitrile generation control efficiency, technical support is provided for controlling the haloacetonitrile content in the reclaimed water in actual engineering, and the process is easy to operate, low in cost, good in control effect, free of secondary pollution and suitable for industrial production. And the method has high practicability and wide applicability.

Description

Technical Field

[0001] The present invention belongs to the technical field of sewage recycling and reuse, and particularly relates to a method for controlling the generation amount of haloacetonitrile, a disinfection by-product of reclaimed water, by pre-oxidation with potassium permanganate to enhance coagulation process. Background Art

[0002] Municipal domestic sewage is an important source for preparing reclaimed water. Compared with drinking water, reclaimed water has a more complex composition and relatively poor stability. Its health and ecological risks in the effluent, especially pathogenic microorganisms, nutrients (such as nitrogen and phosphorus), etc., are important factors restricting the development of reclaimed water reuse. To ensure the safety of the effluent, reclaimed water plants will kill harmful microorganisms in the water through disinfection treatment in the final treatment stage. The precursors of disinfection by-products (DBPs) in secondary effluent of sewage are large in quantity and more complex, especially the concentration level of nitrogen-containing organic compounds is much higher than that of drinking water source water, resulting in a greatly increased formation potential of nitrogen-containing disinfection by-products (N-DBPs) haloacetonitrile after sewage reclamation treatment. It is necessary to effectively control it.

[0003] Haloacetonitrile (HANs) is the N-DBPs with the largest concentration, and its average content is only second to trihalomethanes (THMs) and haloacetic acids (HAAs). However, the cytotoxicity of HANs is 150 times that of THMs and 100 times that of HAAs; the genotoxicity is 13 times that of THMs and 4 times that of HAAs. Due to its high cytotoxicity, high genotoxicity, as well as teratogenic, carcinogenic and mutagenic properties, it is necessary to control and remove its generation amount during the sewage reclamation treatment process. Among them, source control, that is, water source protection and removal of the precursors of DBPs in raw water, is the most effective technology for controlling the generation of DBPs at present.

[0004] The organic matters that are not completely removed and newly generated during the secondary sewage treatment process need to be removed through subsequent advanced treatment. Commonly used advanced treatment methods include coagulation sedimentation filtration, disinfection, and when necessary, processes such as activated carbon adsorption, membrane filtration, oxidation and natural treatment can be adopted. The coagulation process can effectively remove macromolecular organic matters in water, and can strengthen coagulation by adjusting the pH value, adding coagulant aids, etc. The US Environmental Protection Agency (USEPA) believes that enhanced coagulation is an effective method for removing the precursors of DBPs in the water treatment process. However, since coagulation can only selectively remove macromolecular, negatively charged hydrophobic organic matters, and has a very poor removal effect on small molecular hydrophilic organic matters, which are important precursors of HANs, it is necessary to combine it with other processes to achieve the purpose of effectively controlling the generation of HANs, such as pre-oxidation to enhance coagulation to change the properties of DOM in water and then better combine with the coagulant and be removed.

[0005] Potassium permanganate (KMnO4) is a commonly used oxidant in sewage treatment. It has a relatively strong degradation rate for organic matter in water under near-neutral conditions, and its oxidation ability is even stronger under acidic conditions. The standard redox potential is 1.69V. Potassium permanganate has stable properties and low price, and it is an environmentally friendly green oxidant. Its final oxidation product, manganese dioxide (MnO2), not only has certain adsorption, oxidation, and coagulation-aiding effects on pollutants, but also is insoluble in water, easy to separate, and rarely produces toxic by-products to cause secondary pollution to water bodies. KMnO4 is often used as a pre-oxidant to control the formation of DBPs after disinfection, and it can destroy the single formation pathway of HANs, showing good effects on controlling the formation of HANs.

[0006] Process control and end control will involve the transformation of water plant equipment. The transformation process is difficult and increases the cost of disinfection treatment, making it difficult to promote in practical applications. Several other commonly used oxidants in water plants, such as chlorine, chlorine dioxide, and ozone, may produce secondary pollutants such as halogenated organic compounds after oxidation. They are easily affected by the environment and water quality during storage, transportation, and use, have poor stability, are not easy to operate and manage, and have relatively high use costs. Summary of the Invention

[0007] In view of the deficiencies of the prior art, the present invention provides a method for pre-oxidation enhanced coagulation to control the generation amount of haloacetonitrile, a disinfection by-product of reclaimed water, which is characterized by including the following steps:

[0008] Before reclaimed water disinfection during the reclaimed water treatment process, first perform a pre-oxidation - coagulation sedimentation filtration process.

[0009] (1) Pre-oxidation stage:

[0010] Add C1 - C2 mg / L of potassium permanganate to the secondary effluent water sample of the sewage treatment plant, ensure complete oxidation with an oxidation time of t minutes, and adjust the pH to pH1 - pH2 to preferentially oxidize the active groups in the nitrogen-containing precursors.

[0011] (2) Enhanced coagulation stage:

[0012] Add C3 - C4 mg / L of polyaluminum chloride (PAC) to the water sample after pre-oxidation in step (1), and conduct a coagulation experiment through a six-blade stirrer. The coagulation procedure is as follows: v1 r / min, rapid stirring for t1 minutes; add the coagulant PAC, medium-speed stirring at v2 r / min for t2 minutes; v3 r / min, slow stirring for t3 minutes; static sedimentation for t4 minutes. After coagulation, take the supernatant of the water sample, filter it, and use it for the subsequent chlorination experiment.

[0013] (3) Chlorine disinfection stage:

[0014] Using NaClO as the chlorine disinfectant, the chlorination reaction is carried out in the dark in a sealed container with a volume of V mL. Take V1 mL of the water sample and put it into a brown culture bottle, then add V2 mL of phosphate buffer solution. Next, place the culture bottle in a constant temperature incubator for stirring cultivation. The pH is adjusted using a pH = 7.0 phosphate buffer solution, and the initial value is controlled at pH 3; the temperature of the constant temperature incubator is controlled at T °C; the cultivation time is t5 h; the remaining chlorine content after cultivation is maintained at c1 mg / L. When sampling, an appropriate amount of ascorbic acid is added to terminate the reaction. In this experiment, m1 g of ascorbic acid is added to every 100 mL of the sample to remove the residual disinfectant and terminate the chlorination reaction. To avoid experimental errors, three parallel experiments are set up for each test, and the average value of each group is used as the experimental result.

[0015] (4) Sample collection and analysis:

[0016] The production amounts of 4 kinds of HANs in the disinfected water sample are determined by purge and trap-gas chromatography-mass spectrometry. Take the water sample to be tested, after filtering through a 0.22 μm filter membrane, add V3 mL of the sample and 1,2-dibromopropane with a mass concentration of c2 μg / L as the internal standard into the purge bottle, and place it on the sample tray for sample measurement.

[0017] First, the sample is automatically purged according to the set parameters, and then the target products obtained by purge and trap of the sample are analyzed and determined using a gas chromatography-mass spectrometer.

[0018] Furthermore, in step (1), C1 is 0.5 mg / L, C2 is 10 mg / L, the oxidation time t is 60 min, pH1 = 3, and pH2 = 11.

[0019] Furthermore, in step (2), C3 is 2 mg / L, C4 is 30 mg / L, v1 is 300 r / min, t1 is 1 min; v2 is 150 r / min, t2 is 6 min; v3 is 50 r / min, t3 = 10 min; t4 is 30 min.

[0020] Furthermore, in step (3), V = 500 mL, V1 = 250 mL, V2 = 25 mL; pH3 = 7.0 ± 0.2; the temperature T = 20 °C; the cultivation time is t5 = 24 h; the remaining chlorine content c1 = 1.0 ± 0.4 mg / L; the mass of ascorbic acid m1 = 0.2 g.

[0021] Furthermore, in step (4), V3 = 20 mL, c2 = 5 μg / L.

[0022] Furthermore, the 4 kinds of haloacetonitriles (HANs) in step (4) are respectively: trichloroacetonitrile (TCAN), dichloroacetonitrile (DCAN), bromochloroacetonitrile (BCAN), and dibromoacetonitrile (DBAN).

[0023] The method provided by the present invention has at least the following beneficial effects:

[0024] (1) By combining pre-oxidation and coagulation processes, the present invention avoids the disadvantage that separate coagulation can only selectively remove macromolecular, negatively charged hydrophobic organic substances, but has a very poor removal effect on small molecular hydrophilic organic substances, and can synergistically remove different characteristic precursors to effectively control the generation of HANs.

[0025] (2) The oxidant KMnO4 used in the present invention belongs to an environmentally friendly green oxidant, and no toxic secondary pollutants such as bromate or halogenated organic substances will be generated after disinfection, avoiding harm to the water quality safety of reclaimed water.

[0026] (3) The process of the present invention will preferentially react with the electron-rich group organic substances in the secondary effluent of sewage, selectively destroy the unsaturated bonds of the organic substances, and destroy the single generation pathway of HANs, and has a good control effect on the generation of HANs, the disinfection by-products of reclaimed water. Description of the Drawings

[0027] Figure 1 Shows the influence of different dosages of oxidant KMnO4 on the removal efficiency of precursors in secondary effluent (the influence of KMnO4 concentration on water quality parameters (a) DOC, UV254, SUVA254; (b) DON, DON / DOC).

[0028] Figure 2 Shows the influence of different dosages of oxidant KMnO4 on the efficiency of controlling the generation amount of HANs (the influence of KMnO4 concentration on the generation amount of HANs).

[0029] Figure 3 Shows the influence of different dosages of coagulant PAC on the removal efficiency of precursors in secondary effluent (the influence of PAC concentration on water quality parameters (a) DOC, UV254, SUVA254; (b) DON, DON / DOC).

[0030] Figure 4 Shows the influence of different dosages of coagulant PAC on the efficiency of controlling the generation amount of HANs (the influence of PAC concentration on the generation amount of HANs).

[0031] Figure 5 Shows the influence of different pH values on the efficiency of the pre-oxidation - coagulation process in removing precursors in secondary effluent (the influence of pH value on water quality parameters (a) DOC, UV254, SUVA254; (b) DON, DON / DOC).

[0032] Figure 6 Shows the influence of different pH values on the efficiency of the pre-oxidation - coagulation process in controlling the generation amount of HANs (the influence of pH value on the generation amount of HANs).

[0033] Figure 7 The effectiveness of controlling the formation of HANs in the advanced treatment of sewage regeneration by the KMnO4 pre-oxidation - coagulation process under the optimal process conditions (the effectiveness of the pre-oxidation enhanced coagulation process in controlling the formation of HANs). Specific embodiments

[0034] The preferred embodiments of the present invention are described below. It should be understood that the embodiments are for better explaining the present invention and are not used to limit the present invention.

[0035] In the present invention, the CAS number of trichloroacetonitrile is 545 - 06 - 2, the CAS number of dichloroacetonitrile is 3018 - 12 - 0, the CAS number of bromochloroacetonitrile is 83463 - 62 - 1, the CAS number of dibromoacetonitrile is 3252 - 43 - 5, and the CAS number of 1,2 - dibromopropane is 78 - 75 - 1. Specific Example 1

[0037] This example is used to optimize the dosage of the oxidant in the sewage regeneration treatment by the KMnO4 pre-oxidation - coagulation process:

[0038] (1) Water sample collection:

[0039] The raw water used in this example is taken from the secondary effluent of a municipal sewage treatment plant. A glass bottle with a polypropylene (PP) lid is used for on-site water sampling and preservation. The retrieved water sample is immediately filtered through a 0.45 μm microfiltration membrane and then refrigerated at 4°C for later use. The conventional water quality indicators are measured within 24 hours, which are: DOC: 16.52 mg / L, UV 254 : 0.195 cm -1 , SUVA: 1.18 L / (m·mg), DON: 3.07 mg / L.

[0040] (2) Pre-oxidation stage:

[0041] 0.5, 1.0, 2.0, 5.0, and 10.0 mg / L of KMnO4 are respectively added to the secondary effluent water sample of the sewage treatment plant, the oxidation time is 60 minutes, and the pH is adjusted to 7.0.

[0042] (3) Enhanced coagulation stage:

[0043] 10 mg / L of polyaluminum chloride (PAC) is added to the pre-oxidized water sample, and a coagulation experiment is carried out through a six-blade stirrer. The coagulation procedure is as follows: 300 r / min, rapid stirring for 1 minute; add the coagulant PAC, medium-speed stirring at 150 r / min for 6 minutes; 50 r / min, slow stirring for 10 minutes; static precipitation for 30 minutes. After coagulation, the supernatant of the water sample is taken, filtered, and used for the subsequent chlorination experiment.

[0044] (4) Chlorine disinfection stage:

[0045] Put 250 mL of the water sample into a brown culture bottle, add 25 mL of a pH = 7.0 phosphate buffer solution, then place the culture bottle in a constant temperature incubator for stirring culture, and adjust the initial pH to 7.0; the temperature of the constant temperature incubator is controlled at 20 °C; the culture time is 24 h; the residual chlorine content after culture is maintained at 1.0 ± 0.4 mg / L. When sampling, add an appropriate amount of ascorbic acid to terminate the reaction. In this experiment, 0.2 g of ascorbic acid is added to every 100 mL of the sample to remove the residual disinfectant and terminate the chlorination reaction. To avoid experimental errors, three parallel experiments are set up for each test, and the average value of each group is used as the experimental result.

[0046] The effects of different dosages of the oxidant KMnO4 on the removal of precursors in the secondary effluent and the control of the formation amount of HANs are as Figure 1 and Figure 2 shown. As the concentration of KMnO4 increases, DOC and DON show a downward trend, and the total amount of HANs formed also gradually decreases. When the concentration of KMnO4 is 10 mg / L, the removal rate of the total amount of HANs formed is 45.34%. Specific Example 2

[0048] This example is used to optimize the dosage of the coagulant in the sewage reclamation treatment by the KMnO4 pre-oxidation - coagulation process:

[0049] (1) Water sample collection:

[0050] The raw water used in this example is taken from the secondary effluent of a municipal sewage treatment plant. A glass bottle with a polypropylene (PP) lid is used for on-site water sampling and preservation. The retrieved water sample is immediately filtered through a 0.45 μm microfiltration membrane, and then refrigerated at 4 °C for later use. The conventional water quality indicators are measured within 24 h, which are: DOC: 16.52 mg / L, UV 254 : 0.195 cm -1 , SUVA: 1.18 L / (m·mg), DON: 3.07 mg / L.

[0051] (2) Pre-oxidation stage:

[0052] Add 2.0 mg / L of KMnO4 to the secondary effluent water sample of the sewage treatment plant, with an oxidation time of 60 min, and adjust the pH to 7.0.

[0053] (3) Enhanced coagulation stage:

[0054] 2, 5, 10, 20, and 30 mg / L of polyaluminum chloride (PAC) were respectively added to the pre-oxidized water samples, and a coagulation experiment was carried out using a six-joint stirrer. The procedure was as follows: 300 r / min, rapid stirring for 1 min; adding the coagulant PAC, medium-speed stirring at 150 r / min for 6 min; 50 r / min, slow stirring for 10 min; static sedimentation for 30 min. After coagulation, the supernatant of the water sample was taken, filtered, and used for the subsequent chlorination experiment.

[0055] (4) Chlorination disinfection stage:

[0056] Take 250 mL of water sample and put it into a brown culture bottle, then add 25 mL of pH = 7.0 phosphate buffer solution. Then place the culture bottle in a constant temperature incubator for stirring and cultivation, and adjust the initial pH = 7.0; the temperature of the constant temperature incubator is controlled at 20 °C; the cultivation time is 24 h; the residual chlorine content after cultivation is maintained at 1.0 ± 0.4 mg / L. When sampling, an appropriate amount of ascorbic acid was added to terminate the reaction. In this experiment, 0.2 g of ascorbic acid was added to every 100 mL of the sample to remove the residual disinfectant and terminate the chlorination reaction. To avoid experimental errors, three parallel experiments were set for each test, and the average value of each group was used as the experimental result.

[0057] The effects of different dosages of the coagulant PAC on the removal of precursors in the secondary effluent and the efficacy of controlling the formation amount of HANs are as Figure 3 and Figure 4 shown. As the PAC concentration increases, DOC and UV 254 gradually decrease and finally tend to be stable, and the total amount of HANs formed first decreases and then increases. When the PAC concentration is 20 mg / L, the control effect on the total amount of HANs formed is the best, and the removal rate of the total amount of HANs formed is 34.79%. Specific Example 3

[0059] This example is used to optimize the pH value in the sewage regeneration treatment of the KMnO4 pre-oxidation-coagulation process:

[0060] (1) Water sample collection:

[0061] The raw water used in this example was taken from the secondary effluent of a municipal sewage treatment plant. A glass bottle with a polypropylene (PP) lid was used for on-site water sampling and preservation. The retrieved water sample was immediately filtered through a 0.45 μm microfiltration membrane and then refrigerated at 4 °C for later use. The conventional water quality indicators were measured within 24 h, which were: DOC: 16.52 mg / L, UV 254 : 0.195 cm -1 , SUVA: 1.18 L / (m·mg), DON: 3.07 mg / L.

[0062] (2) Pre-oxidation stage:

[0063] Add 2.0 mg / L of KMnO4 to the secondary effluent water sample of the sewage treatment plant. The oxidation time is 60 min, and the initial pH values are adjusted to pH = 3, pH = 5, pH = 7, pH = 9, and pH = 11 respectively.

[0064] (3) Enhanced coagulation stage:

[0065] Add 10 mg / L of polyaluminum chloride (PAC) to the pre-oxidized water sample, and conduct a coagulation experiment through a six-joint stirrer. The coagulation procedure is as follows: 300 r / min, rapid stirring for 1 min; add the coagulant PAC, medium-speed stirring at 150 r / min for 6 min; 50 r / min, slow stirring for 10 min; static precipitation for 30 min. After coagulation, take the supernatant of the water sample, filter it, and use it for the subsequent chlorination experiment.

[0066] (4) Chlorine disinfection stage:

[0067] Put 250 mL of water sample into a brown culture bottle, add 25 mL of pH = 7.0 phosphate buffer solution, and then put the culture bottle into a constant temperature incubator for stirring and culturing; the temperature of the constant temperature incubator is controlled at 20 °C; the culture time is 24 h; the remaining chlorine content after culturing is maintained at 1.0 ± 0.4 mg / L. Add an appropriate amount of ascorbic acid to terminate the reaction during sampling. In this experiment, 0.2 g of ascorbic acid is added to every 100 mL of sample to remove the residual disinfectant and terminate the chlorination reaction. To avoid experimental errors, three parallel experiments are set for each test, and the average value of each group is used as the experimental result.

[0068] The effects of different pH values on the removal of precursors in the secondary effluent and the efficiency of controlling the formation amount of HANs are as Figure 5 and Figure 6 shown. As the pH increases, DOC and UV 254 first decrease and then increase, and the total amount of HANs generated gradually decreases. When pH = 11, the control effect on the total amount of HANs generated is the best, and the removal rate of the total amount of HANs generated is 46.64%. Specific Example 4

[0070] This example is used to determine the efficiency of controlling the formation amount of HANs in the advanced treatment of sewage regeneration by the KMnO4 pre-oxidation - coagulation process under the optimal process conditions:

[0071] (1) Water sample collection:

[0072] The raw water used in this example is taken from the secondary effluent of a municipal sewage treatment plant. A glass bottle with a polypropylene (PP) lid is used for on-site water intake and preservation. The retrieved water sample is immediately filtered through a 0.45 μm microfiltration membrane, and then refrigerated at 4 °C for later use. The conventional water quality indicators are measured within 24 h, which are: DOC: 16.52 mg / L, UV 254 : 0.195 cm-1 , SUVA: 1.18 L / (m·mg), DON: 3.07 mg / L.

[0073] (2) Pre-oxidation stage:

[0074] To ensure process economy and prevent a large water color after high-concentration KMnO4 oxidation treatment, 2.0 mg / L of KMnO4 was added to the water sample, with an oxidation time of 60 min. According to the urban wastewater reuse water quality standard, the pH was adjusted to 8.0.

[0075] (3) Enhanced coagulation stage:

[0076] 20 mg / L of polyaluminum chloride (PAC) was added to the pre-oxidized water sample, and a coagulation experiment was carried out using a six-joint stirrer. The coagulation procedure was as follows: rapid stirring at 300 r / min for 1 min; adding the coagulant PAC and medium-speed stirring at 150 r / min for 6 min; slow stirring at 50 r / min for 10 min; static precipitation for 30 min. After coagulation, the supernatant of the water sample was taken, filtered, and used for the subsequent chlorination experiment.

[0077] (4) Chlorine disinfection stage:

[0078] 250 mL of the water sample was placed in a brown culture bottle, 25 mL of pH = 8.0 phosphate buffer solution was added, and then the culture bottle was placed in a constant temperature incubator for stirring and culturing; the temperature of the constant temperature incubator was controlled at 20 °C; the culture time was 24 h; the residual chlorine content after culturing was maintained at 1.0 ± 0.4 mg / L. When sampling, an appropriate amount of ascorbic acid was added to terminate the reaction. In this experiment, 0.2 g of ascorbic acid was added to every 100 mL of the sample to remove the residual disinfectant and terminate the chlorination reaction. To avoid experimental errors, three parallel experiments were set up for each test, and the average value of each group was used as the experimental result.

[0079] The water quality indicators of the water samples before and after the advanced treatment process of pre-oxidation - coagulation - sedimentation - filtration - disinfection were measured. The results are shown in Table 1. The water quality indicators of the reclaimed water after advanced treatment all meet the standard limits of "Urban Miscellaneous Water Quality for Urban Wastewater Reuse" (GB / 18920 - 2020).

[0080] Table 1 Changes in the water quality indicators of the secondary effluent after advanced wastewater treatment Continued Table 1

[0080] The efficacy of controlling the formation of HANs by KMnO4 pre-oxidation - coagulation process in advanced wastewater reclamation treatment under the optimal process conditions is as Figure 7As shown in the figure. Under the condition of pH = 8, the dosage of the oxidant KMnO4 is 2 mg / L. After 60 minutes of oxidation, 20 mg / L of PAC is added for coagulation. The removal rate of the total output of HANs after treatment is 33.36%.

[0081] As mentioned above, the above is only a specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for controlling the generation amount of haloacetonitrile, a disinfection by-product of reclaimed water, by pre-oxidation enhanced coagulation process, which is characterized in that: When the secondary effluent of the sewage treatment plant is regenerated, a pre-oxidation - coagulation sedimentation - filtration process is carried out before chlorination disinfection.

2. The method according to claim 1, characterized in that: The oxidant is potassium permanganate (KMnO4); the coagulant is polyaluminum chloride (PAC); the chlorine disinfectant is sodium hypochlorite (NaClO).

3. The method according to claim 2, wherein: The dosage range of the oxidant potassium permanganate is 0.5 - 10 mg / L.

4. The method according to claim 2, wherein: The dosage range of the coagulant polyaluminum chloride is 2 - 30 mg / L.

5. The method according to claim 2, wherein: The chlorine dosage is 5 mg / L.

6. The method according to any one of claims 1 to 5, characterized in that: The disinfection by-products of the reclaimed water are dichloroacetonitrile, trichloroacetonitrile, dibromoacetonitrile and bromochloroacetonitrile.

7. A method for controlling the generation amount of haloacetonitrile, a disinfection by-product of reclaimed water, in a potassium permanganate pre-oxidation enhanced coagulation process according to any one of claims 1 to 6, characterized in that: Compared with direct chlorination disinfection of reclaimed water, when the dosage of the oxidant KMnO4 is 2 mg / L, the dosage of the coagulant PAC is 20 mg / L, and the initial pH value is 8, the total amount of the four kinds of haloacetonitriles generated is reduced by 33.36%.

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