A modified composite coagulant and its application in treating organic wastewater

Abstract

The present invention relates to the technical field of organic wastewater treatment, and in particular to a modified composite coagulant and its application in treating organic wastewater. The modified composite coagulant includes a modified organic coagulant, an inorganic coagulant and water; wherein the preparation method of the modified organic coagulant includes: mixing chitosan with an acetic acid solution, an initiator and a graft copolymerization monomer, and performing a graft copolymerization reaction under nitrogen protection; the graft copolymerization monomer is a cationic monomer (2-methacryloyloxyethyl) trimethylammonium chloride; the mass ratio of the modified organic coagulant to the inorganic coagulant is 1: (2-5). The present invention provides a modified composite coagulant and its application, which can realize the agglomeration and rapid sedimentation of suspended matter after coagulation, greatly reduce the suspended particles, COD and pollutant content of the supernatant, promote the treatment of high-concentration organic wastewater such as biogas slurry, and has broad application prospects.

Description

Technical Field

[0001] The invention relates to the technical field of organic wastewater treatment, in particular to a modified composite coagulant and application thereof in treating organic wastewater. Background Art

[0002] The production of organic wastewater and waste in my country's livestock and poultry breeding, food processing and other industries is large, and the environmental damage is serious. Anaerobic fermentation is an important technology for realizing the resource utilization of organic wastewater and waste, which can convert organic matter into methane. However, the biogas slurry after anaerobic fermentation still has a high organic matter content and is still a high-concentration organic wastewater. Its treatment and utilization has become an important limiting factor for the rapid development of anaerobic fermentation engineering in my country.

[0003] Whether it is directly discharged organic wastewater or anaerobic fermented biogas, both contain rich nutrients such as nitrogen, phosphorus, and potassium, and are potential liquid organic fertilizers. However, my country's arable land area is limited, especially anaerobic fermentation projects are generally far away from farmland construction, and there is a contradiction between the continuity of wastewater / biogas production and the seasonality of crop fertilizer demand, which makes it difficult to use it in farmland. Therefore, the treatment of organic wastewater / biogas has become the first choice for many anaerobic fermentation projects.

[0004] The organic wastewater treatment process chain is long, mainly including pretreatment, anaerobic fermentation, aerobic fermentation, coagulation and sedimentation, advanced oxidation, disinfection, etc. Among them, coagulation can be used for the treatment of organic wastewater to reduce the back-end treatment pressure, and can also be used to treat the tail water after aerobic fermentation to improve the sludge settling properties and improve the solid-liquid separation efficiency.

[0005] The type of coagulant directly affects the coagulation effect and is an important influencing factor in the coagulation process. Common coagulants include organic coagulants and inorganic coagulants. Due to its biodegradable and environmentally friendly properties, chitosan and other materials are regarded as green organic materials with potential to replace traditional chemical coagulants. Organic coagulants and inorganic coagulants can usually be combined to give full play to their respective advantages and improve the coagulation effect on wastewater. However, there are still many problems that limit the application of natural organic polymer coagulant materials such as chitosan. For example, the poor water solubility and high cost of chitosan limit its direct application in wastewater treatment, and also limit its dissolution and compounding with inorganic coagulants as a new type of coagulant.

[0006] Graft copolymerization and other methods can improve water solubility and adsorption performance by modifying natural organic polymer materials and optimizing their structure, thereby achieving the goal of reducing costs and increasing efficiency. Graft copolymerization is to graft appropriate side chains or functional side groups on the macromolecular chain through chemical bonds to form a new copolymer. In this process, the skeleton of the natural organic material remains unchanged, and suitable grafting monomers can be selected as needed to give it specific chemical activity. But in fact, even the COD removal rate and turbidity removal rate of modified natural polymer organic coagulants still have room for further improvement.

[0007] In view of this, the present invention is proposed. Summary of the invention

[0008] In the existing technology, when optimizing coagulants for treating organic wastewater, researchers often optimize organic coagulants or inorganic coagulants separately. However, in reality, whether it is organic coagulants or inorganic coagulants, their coagulation effects, especially COD removal rate and turbidity removal rate, still have room for further improvement.

[0009] The present invention accidentally thought of dissolving organic coagulants and inorganic coagulants with completely different properties and then compounding them for joint use. As a result, it was found that the effect of the combination of the two was poor, even inferior to the coagulation effect of a single coagulant. Subsequent extensive research found that among conventional modified organic coagulants, only the modified organic coagulant whose graft copolymer monomer is a cationic monomer (2-methacryloyloxyethyl) trimethylammonium chloride has a synergistic effect when compounded with an inorganic coagulant. Specifically, the compounding of the two is conducive to the formation of larger floccules, promoting the aggregation and rapid sedimentation of suspended matter after coagulation, and has a stronger adsorption capacity and a more stable coagulation effect than a single inorganic coagulant, a single modified organic coagulant or a conventional organic-inorganic compound coagulant, and can specifically improve the removal effect of COD, chromaticity and turbidity of organic wastewater.

[0010] In addition, the present invention further discovered during subsequent research that the reason why the conventional organic-inorganic composite coagulants have a poor effect after combination is mainly affected by the mindset of technical personnel in the field when applying the two types of coagulants. Specifically: in the prior art, technical personnel in the field tend to choose a modified organic coagulant that is greater than the inorganic coagulant in order to obtain a better coagulation effect; but the present invention unexpectedly discovered that when the two are compounded, controlling the amount of the modified organic coagulant to be less than the amount of the inorganic coagulant is more conducive to the synergistic effect of the two.

[0011] Based on this, the specific technical solutions of the present invention are as follows:

[0012] In a first aspect, the present invention first provides a modified composite coagulant, comprising a modified organic coagulant, an inorganic coagulant and water;

[0013] Wherein, the preparation method of the modified organic coagulant comprises:

[0014] Chitosan is mixed with acetic acid solution, initiator and graft copolymer monomer, and graft copolymerization reaction is carried out under nitrogen protection; the graft copolymer monomer is cationic monomer (2-methacryloyloxyethyl) trimethylammonium chloride;

[0015] The mass ratio of the modified organic coagulant to the inorganic coagulant is 1:(2-5).

[0016] The present invention finds that when modifying natural polymer materials, (2-methacryloyloxyethyl)trimethylammonium chloride is more conducive to combining with natural polymer materials than other cationic monomers, and the modified organic coagulant obtained after the combination can form a larger flocculent with the inorganic modifier, so that the suspended matter can agglomerate and quickly settle after coagulation, and the COD and pollutant content of the supernatant can be greatly reduced; at the same time, when the dosage relationship of the modified organic coagulant and the inorganic coagulant is controlled within the above range, the coagulation effect can be further improved, the average particle size distribution of the supernatant obtained after being treated with the modified composite coagulant is smaller, and large-particle colloidal substances in the water are effectively removed.

[0017] Further preferably, the mass ratio of the modified organic coagulant to the inorganic coagulant is 1:(2-3).

[0018] Preferably, in the modified composite coagulant, the concentration of the modified organic coagulant is 40-60 g / L, and the concentration of the inorganic coagulant is 80-120 g / L.

[0019] Preferably, the concentration of the acetic acid solution is 0.3-0.5 mol / L.

[0020] Preferably, the initiator includes one or more of potassium persulfate, cerium persulfate and ammonium persulfate.

[0021] More preferably, the amount of the initiator added is 1-3 wt % of the mass of the graft copolymerization monomer, preferably 1 wt %.

[0022] Preferably, the mass ratio of the chitosan to the graft copolymer monomer is 1:(3.75-6.75);

[0023] Preferably, the temperature of the graft copolymerization reaction is 60-80°C, more preferably 70°C.

[0024] Preferably, the graft copolymerization reaction time is 3 to 5 hours, preferably 5 hours.

[0025] Preferably, after the graft copolymerization reaction, the sample is washed several times with propanol or the like, and then dried for 40 to 50 hours to obtain a dry sample, which is then ground and used.

[0026] Preferably, the inorganic coagulant includes one or more of polyferric sulfate, polyaluminium chloride, polyaluminium ferrosilicate and polyferric silicate sulfate; more preferably, the inorganic coagulant is polyferric sulfate.

[0027] The present invention finds that among inorganic coagulants, polyferric sulfate and the modified organic coagulant have better removal effects on COD, chromaticity and turbidity of the organic wastewater, the coagulation performance is less affected by the water temperature, and the residual coagulant has low toxicity.

[0028] As a preferred embodiment of the present invention, the modified composite coagulant comprises a modified organic coagulant, polyferric sulfate and water; wherein the mass ratio of the modified organic coagulant to the polyferric sulfate is 1:(2-3);

[0029] The preparation method of the modified organic coagulant comprises:

[0030] Chitosan, acetic acid solution, initiator and (2-methacryloyloxyethyl) trimethylammonium chloride are mixed, and graft copolymerization reaction is carried out under nitrogen protection; the concentration of the acetic acid solution is 0.3-0.5 mol / L.

[0031] The present invention finds that the compounding of the modified organic coagulant and the inorganic coagulant can exert a better destabilizing agglomeration effect on the colloidal particles in the biogas slurry, further improve the coagulation effect, and reduce the particle size of the particles in the biogas slurry.

[0032] In a second aspect, the present invention provides application of the modified composite coagulant in treating organic wastewater.

[0033] In the present invention, the organic wastewater includes wastewater mainly composed of organic pollutants and suspended particulate matter. These wastewaters are mainly produced in industries such as livestock and poultry breeding and food processing. They have high COD content and complex components, and contain a large amount of carbohydrates, fats, proteins, cellulose, etc. They will emit foul-smelling gases during storage and have high chroma and turbidity.

[0034] Preferably, during the treatment, the pH of the organic wastewater is controlled to be 5-9, such as 5, 6, 7, 8 or 9; more preferably, the pH is 5-7.

[0035] The present invention finds that when the modified composite coagulant is used to treat organic wastewater, controlling the pH of the organic wastewater to be within the above range can improve the coagulation performance and exert a better adsorption and removal effect on organic pollutants and colloidal particles in the organic wastewater.

[0036] In the present invention, those skilled in the art can use sulfuric acid or sodium hydroxide solution to adjust the pH value of the organic wastewater to a desired value, which is not further limited herein.

[0037] Preferably, the dosage of the coagulant mixture when treating organic wastewater is 1.0-2.5 g / L, such as 1.0 g / L, 1.1 g / L, 1.2 g / L, 1.3 g / L, 1.4 g / L, 1.5 g / L, 1.6 g / L, 1.7 g / L, 1.8 g / L, 1.9 g / L, 2.0 g / L, 2.1 g / L, 2.2 g / L, 2.3 g / L, 2.4 g / L or 2.5 g / L.

[0038] The present invention finds that the modified composite coagulant has a preferred method of use when treating the organic wastewater:

[0039] Specifically, the modified composite coagulant is added to the organic wastewater, first stirred quickly and then slowly, and then allowed to settle after stirring. The present invention finds that the above-mentioned method of use is conducive to achieving rapid sedimentation of aggregates, while improving both sedimentation effect and sedimentation efficiency.

[0040] Preferably, the rapid stirring time is 0.5 to 5 minutes, more preferably 3 to 4 minutes; the slow stirring time is 5 to 15 minutes, more preferably 7 to 10 minutes; the static precipitation time is 3 to 10 minutes, more preferably 4 to 8 minutes

[0041] More preferably, the modified composite coagulant is added to the organic wastewater, first stirred at 150 to 500 rpm, then stirred at 40 to 80 rpm, and then allowed to stand after stirring.

[0042] Further preferably, the modified composite coagulant is added to the organic wastewater, first stirred at 300-400 rpm, then stirred at 50-70 rpm, and allowed to stand after stirring.

[0043] Based on the above technical solution, the beneficial effects of the present invention are:

[0044] The present invention provides a modified composite coagulant and its application in treating organic wastewater, which can achieve agglomeration and rapid sedimentation of suspended matter after coagulation, greatly reduce the COD and pollutant content of the supernatant, promote the pretreatment technology of biogas slurry and high-concentration organic wastewater, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0046] Figure 1 This is a schematic diagram of the device for preparing the modified organic coagulant described in Example 1;

[0047] Figure 2 The figures are the physical pictures of the chitosan materials before and after modification in Example 1, wherein A1 is the original chitosan material, A2 is the water-soluble effect of the original chitosan; B1 is the modified chitosan material, and B2 is the water-soluble effect of the modified chitosan;

[0048] Figure 3 The electron microscope scanning images of chitosan before and after modification in Example 1 (before modification (A) and after modification (B));

[0049] Figure 4 The Fourier transform infrared analysis spectra of chitosan before and after modification in Example 1;

[0050] Figure 5 The coagulation treatment effect diagram of organic wastewater represented by biogas slurry by coagulants in Example 1 and Comparative Examples 2 to 5 is shown;

[0051] Figure 6 The graphs are the changes of SS (A), TN (B), turbidity (C), COD (D), TOC (E), and TP (F) of the biogas slurry during the coagulant coagulation process in Example 1 and Comparative Examples 2 to 5;

[0052] Figure 7 The changes in particle size distribution (A) and zeta potential (B) of the biogas slurry before and after coagulant coagulation in Example 1 and Comparative Examples 2-5. DETAILED DESCRIPTION

[0053] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the drawings of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0054] Unless otherwise specified, the various raw materials used in the examples are conventional commercially available raw materials, among which the content of commercially available cationic monomer (2-methacryloyloxyethyl)trimethylammonium chloride (DMC) is 75 wt % (the rest is water), and the technical means used are conventional means well known to those skilled in the art.

[0055] Example 1

[0056] This embodiment first provides a modified composite coagulant CTS-g(DMC)-PFS, which includes water, a modified organic coagulant and polyferric sulfate, the mass ratio of the modified organic coagulant to the polyferric sulfate is 1:2, and the modified composite coagulant contains 60 g / L of modified chitosan solution and 100 g / L of polyferric sulfate solution.

[0057] The preparation method of the modified organic coagulant comprises:

[0058] Weigh 4 g of chitosan and dissolve it in 100 mL of a 0.4 mol / L dilute acetic acid solution. Heat in a 70°C water bath with stirring and introduce a slow flow of nitrogen for 20 min. Control the temperature, add an aqueous solution of potassium persulfate initiator, stir for 20 min, add the cationic monomer (2-methacryloyloxyethyl) trimethylammonium chloride (DMC), react at a constant temperature for 5 h, cool, precipitate with acetone, dry at 60°C to constant weight, and grind to obtain a powder as a modified chitosan coagulant. The mass ratio of chitosan to the cationic monomer (2-methacryloyloxyethyl) trimethylammonium chloride solution (DMC) is 1:9 (i.e., the mass ratio of chitosan to DMC is 1:6.75).

[0059] Figure 1 It is a schematic diagram of the modified organic coagulant preparation device;

[0060] Figure 2 The pictures of the materials before and after chitosan modification are shown in Figure 1. A1 is the original chitosan material, A2 is the water-soluble effect of the original chitosan; B1 is the modified chitosan material, and B2 is the water-soluble effect of the modified chitosan; Figure 2 It can be seen that the modified chitosan material can be directly dissolved in water;

[0061] Figure 3 The electron microscope scanning images of chitosan before and after modification (before modification (A) and after modification (B)); Figure 3 It can be seen that the original crystal structure has been destroyed after modification, forming a more irregular and porous structure.

[0062] Figure 4 The Fourier transform infrared analysis spectra of chitosan before and after modification are shown in Table 1. Figure 4 It can be seen that the functional group structure of chitosan before and after modification is quite different.-1 There is a characteristic peak of quaternary ammonium nitrogen at 1150cm -1 、1730cm -1 The locations are the stretching vibration absorption peaks of the ester group OCC and carbonyl group C=O in the monomer DMC structure, indicating that the monomer has been successfully grafted onto the chitosan chain to synthesize a new material.

[0063] This embodiment further provides the application of the modified composite coagulant in organic wastewater:

[0064] The biogas slurry from an anaerobic fermentation pond in a pig farm in Beijing was used as a typical organic wastewater for research, and the synthesized modified materials were verified and the process was explored. After adjusting the pH of the biogas slurry to 6 with a 10% dilute sulfuric acid solution, a six-joint mixer was used for coagulation experiments, 1.5g of modified composite coagulant / L biogas slurry was added, the fast stirring time was controlled to be 3min, the fast stirring rate was 350rpm, the slow stirring time was 10min, the slow stirring rate was 60rpm, and the solid-liquid separation was performed after standing for 10min, and the supernatant was taken for analysis.

[0065] Example 2

[0066] This embodiment provides a modified composite coagulant CTS-g(DMC)-PFS, which differs from Embodiment 1 only in that the mass ratio of chitosan to the cationic monomer (2-methacryloyloxyethyl)trimethylammonium chloride solution is 1:7 (i.e., the mass ratio of chitosan to DMC is 1:5.25).

[0067] Example 3

[0068] This example uses the same modified composite coagulant CTS-g(DMC)-PFS as in Example 1, and the only difference between this example and Example 1 is that the pH of the biogas slurry is adjusted to 5.

[0069] Example 4

[0070] This example uses the same modified composite coagulant CTS-g(DMC)-PFS as in Example 1, and the only difference between this example and Example 1 is that the pH of the biogas slurry is adjusted to 7.

[0071] Example 5

[0072] This example uses the same modified composite coagulant CTS-g(DMC)-PFS as in Example 1, and the only difference between this example and Example 1 is that the pH of the biogas slurry is 9.

[0073] Example 6

[0074] This embodiment provides a modified composite coagulant, which is different from Embodiment 1 only in that the solute mass ratio of the modified organic coagulant to the polyferric sulfate is 1:3.

[0075] Example 7

[0076] This embodiment provides a modified composite coagulant CTS-g(DMC)-PFS, which is different from Embodiment 1 only in that the mass ratio of the modified organic coagulant to the polyferric sulfate is 1:5.

[0077] Example 8

[0078] This embodiment uses the same modified composite coagulant as that in Embodiment 1, and the only difference between this embodiment and Embodiment 1 is that 1 g of the modified composite coagulant / L of biogas slurry is added.

[0079] Example 9

[0080] This example uses the same modified composite coagulant CTS-g(DMC)-PFS as in Example 1, and the only difference between this example and Example 1 is that 2.5 g of the modified composite coagulant / L of biogas slurry is added.

[0081] Example 10

[0082] This embodiment provides a modified composite coagulant CTS-g(DMC)-PFS, which is different from Embodiment 1 only in that polyferric sulfate is replaced by an equal amount of polyaluminium chloride.

[0083] Embodiment 11

[0084] This embodiment provides a modified composite coagulant CTS-g(DMC)-PFS, which differs from Example 1 only in that: in the coagulation experiment, 1.5 g of the modified composite coagulant / L of biogas slurry was added, the constant stirring rate was controlled to be 100 rpm, the stirring time was 13 min, the mixture was allowed to stand for 10 min for solid-liquid separation, and the supernatant was taken for analysis.

[0085] Comparative Example 1

[0086] This comparative example first provides a modified composite coagulant CTS-g(DMC)-PFS, which is different from Example 1 only in that the mass ratio of the modified organic coagulant to the polyferric sulfate is 2:1.

[0087] Comparative Example 2

[0088] The coagulant in this comparative example is the modified chitosan CTS-g (DMC) prepared in Example 1.

[0089] Comparative Example 3

[0090] The coagulant in this comparative example is the polyferric sulfate (PFS) used in Example 1.

[0091] Comparative Example 4

[0092] The coagulant in this comparative example is chitosan (CTS) used in Example 1.

[0093] Comparative Example 5

[0094] The coagulant in this comparative example is a mixture of chitosan and polyferric sulfate (CTS-PFS) in a mass ratio of 1:2.

[0095] Comparative Example 6

[0096] This comparative example provides a modified composite coagulant CTS-g(DMC)-PFS, which differs from Example 1 only in that (2-methacryloyloxyethyl)trimethylammonium chloride (DMC) is replaced by an equal amount of 3-chloro-2-hydroxypropyltrimethylammonium (CTA).

[0097] Test example

[0098] The present invention tests the treatment effect of the modified composite coagulant on high-suspended solids and high-concentration organic wastewater such as biogas slurry, and the results are shown in Table 1:

[0099] Table 1

[0100]

[0101]

[0102] in, Figure 5 The coagulant treatment effect diagram of biogas slurry in Example 1 and Comparative Examples 2 to 5 is shown;

[0103] Figure 6 The graphs are the changes of SS (A), TN (B), turbidity (C), COD (D), TOC (E), and TP (F) of the biogas slurry during the coagulant coagulation process in Example 1 and Comparative Examples 2 to 5;

[0104] Figure 7 The changes in particle size distribution (A) and zeta potential (B) of the biogas slurry before and after coagulant coagulation in Example 1 and Comparative Examples 2-5.

[0105] like Figure 5 As shown in the figure, the biogas slurry is relatively turbid, and all kinds of coagulants have flocs settling, and the chromaticity and turbidity of the supernatant are reduced, but the new coagulant CTS-g (DMC) -PFS has the best treatment effect. Figure 6It can be seen that after treatment with different coagulants, the contents of SS, total nitrogen (TN), turbidity, COD, total organic carbon (TOC), and TP in the supernatant have decreased significantly. The new coagulant has a better removal effect. The removal rates of SS, COD, and TOC can reach more than 90%, the removal rates of turbidity and TP can reach more than 99%, and the removal rate of TN can reach more than 40%. This shows that the new coagulant has a better removal effect on suspended solids and organic pollutants, which is better than a single coagulant. Therefore, it is proved that the modified CTS-g (DMC) -PFS coagulant saves dosage, reduces the cost of use, and improves its own coagulation performance. Figure 7 It can be seen that the particle size of the particles in the biogas slurry after coagulation treatment showed a significant downward trend, among which the particle size of the CTS-g(DMC)-PFS group was the smallest. At the same time, the zeta potential was measured, and the absolute value of Zeta in each treatment showed a significant downward trend compared with the original solution. The zeta potential of the optimal group (CTS-g(DMC)-PFS) was closest to zero, which showed that the new coagulant improved the electrical neutralization and adsorption bridging effects compared with the coagulant before modification and compounding, and could exert the best instability agglomeration effect on the colloidal particles in the biogas slurry.

[0106] It can be seen that the modified composite coagulant prepared in the above embodiment has the following beneficial effects:

[0107] Compared with traditional coagulants, the present invention utilizes natural polymer materials and iron salts for modification and compounding, reduces dosage, saves dosage costs, and improves pollutant removal capacity, has good decolorization and deodorization effects, and makes the coagulated water clearer. Moreover, this coagulant is easily degradable, green and environmentally friendly, and has no toxic effects on the environment and human health.

[0108] The novel coagulant of the present invention has a better treatment effect on high-suspended solids and high-concentration organic wastewater such as biogas slurry, and has a high removal efficiency for suspended solids and pollutants. When the optimal dosage is reached, the COD and SS removal rates can reach more than 90%, the turbidity and TP removal rates can reach more than 99%, and the TN removal rate can reach more than 40%. The clear liquid after coagulation has a low COD content, which reduces the load of subsequent standard discharge treatment. It can also be used as farmland irrigation water. The generated flocs can be recycled through anaerobic fermentation or aerobic composting to achieve efficient treatment of organic wastewater.

[0109] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. Application of modified composite coagulant in treating organic wastewater, characterized in that: Adding the modified composite coagulant to the organic wastewater, stirring at 300-400 rpm for 0.5-5 min, then stirring at 50-70 rpm for 5-10 min, and letting it stand after stirring; The modified composite coagulant comprises a modified organic coagulant, an inorganic coagulant and water; the inorganic coagulant is polyferric sulfate; the mass ratio of the modified organic coagulant to the inorganic coagulant is 1:(2-3); the concentration of the modified organic coagulant is 40-60 g / L, and the concentration of the inorganic coagulant is 80-120 g / L; Wherein, the preparation method of the modified organic coagulant comprises: Chitosan is mixed with acetic acid solution, initiator and graft copolymer monomer, and graft copolymerization reaction is carried out under nitrogen protection; the graft copolymer monomer is cationic monomer (2-methacryloyloxyethyl) trimethylammonium chloride.

2. The use of the modified composite coagulant according to claim 1 in treating organic wastewater, characterized in that: The concentration of the acetic acid solution is 0.3-0.5 mol / L.

3. The use of the modified composite coagulant according to claim 1 in treating organic wastewater, characterized in that: The mass ratio of the chitosan to the graft copolymer monomer is 1:(3.75-6.75).

4. The use of the modified composite coagulant according to claim 1 in treating organic wastewater, characterized in that: The temperature of the graft copolymerization reaction is 60-80°C.

5. The use of the modified composite coagulant according to claim 1 in treating organic wastewater, characterized in that: During the treatment, the pH of the organic wastewater is controlled to be 5-9.

6. The use of the modified composite coagulant according to claim 1 in treating organic wastewater, characterized in that: The dosage of the modified composite coagulant when treating organic wastewater is 1.0-2.5 g / L.

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