A biogas slurry coagulation method based on the removal of suspended matter by agglomeration and floating

Abstract

The present invention relates to the technical field of sewage treatment, and specifically to a biogas slurry coagulation method based on the agglomeration and floating removal of suspended matter, comprising adding a composite coagulant to the biogas slurry, stirring and coagulating, generating floating flocs, and then solid-liquid separation; the composite coagulant comprises chitosan or its derivatives and an inorganic coagulant. The present invention uses a composite of chitosan or its derivatives and an inorganic coagulant for biogas slurry coagulation, which can break the biogas slurry stability, and promote the biogas slurry to generate carbon dioxide bubbles in the process of generating flocs. The bubbles and flocs adhere to each other to enhance the stability of the flocs, and at the same time, the flocs are floated, which is conducive to timely slag and mud removal. The present invention not only saves the cost of aeration flotation and reduces energy consumption, but also realizes good spontaneous solid-liquid separation, has good decolorization and deodorization effects on biogas slurry, and synergistically retains most of the nutrients while efficiently removing suspended matter, and has good application prospects.

Description

Technical Field

[0001] The invention relates to the technical field of sewage treatment, and in particular to a biogas slurry coagulation method based on the removal of suspended matter by agglomeration and floating. Background Art

[0002] my country has a large amount of organic waste, with an annual total of more than 6 billion tons. Anaerobic fermentation technology plays an important role in the treatment of organic waste. At present, my country's anaerobic fermentation projects are mainly based on wet fermentation with a solid content of less than 10%. While treating organic waste, a large amount of biogas slurry is produced. my country's annual biogas slurry production is about 800-100 million tons.

[0003] Biogas slurry is rich in suspended solids (SS), organic matter, nitrogen (N), phosphorus (P), potassium (K) and other macroelements and plant growth hormones such as gibberellins, and has the potential to be used as fertilizer. At present, the treatment methods for biogas slurry at home and abroad mainly include direct return to the field, membrane concentration fertilizer production and biochemical treatment to meet the discharge standards. Direct return to the field is the main way to utilize biogas slurry in my country. However, high concentrations of suspended solids in biogas slurry will block pipelines, reduce soil permeability, cause soil compaction and reduce fertility; in addition, there are contradictions between biogas projects and farmland for consumption, and the continuous production of biogas slurry and the seasonal fertilizer requirements of crops in China. Biogas slurry is difficult to be consumed and utilized in time during the process of returning to the field. Biogas slurry membrane concentration technology can achieve the concentration of nutrients and volume reduction of biogas slurry, and realize the utilization of water and fertilizer. However, high content of suspended solids and colloidal particles will cause serious membrane pollution, which restricts the promotion and application of membrane concentration technology. In addition, the treatment load of biogas slurry is high, and biochemical treatment has high technical requirements, high energy consumption and waste of resources. Due to the high load of biogas slurry treatment and the emission of malodorous gases, there is an urgent need to seek green and efficient biogas slurry pretreatment methods to ensure the sustainable application of the above-mentioned treatment and resource utilization technologies.

[0004] Coagulation can not only effectively reduce the sensory indicators of biogas slurry such as turbidity and color, but also remove SS and a variety of toxic and harmful pollutants. After the addition of coagulant, the colloids are destabilized and coagulate with each other, gradually agglomerating to form flocs. At present, most of the flocs produced after coagulation settle at the bottom and gradually accumulate to form sludge. The bottom sludge is difficult to remove and excessive sludge accumulation will affect the effluent water quality; in addition, the solid-liquid separation process is prone to floc disturbance, causing the flocs to disperse and pollutants to be released, causing secondary pollution. Compared with sedimentation-type flocs, floating flocs are easy to discharge and reduce floc disturbance during the removal process, thereby achieving a good solid-liquid separation effect and improving resource utilization, but it is usually necessary to add additional slow-release flotation agents and aeration devices to achieve floc floating.

[0005] The type of coagulant is a key factor affecting the coagulation effect. Common coagulants include organic coagulants, inorganic coagulants and composite coagulants, which remove pollutants through compression of the double layer, electrical neutralization, adsorption bridging and net capture and sweeping. Inorganic coagulants mainly exert their coagulation properties through electrical neutralization and net capture and sweeping, while organic coagulants mainly rely on their long chain structure and the groups on them to adsorb and bridge and aggregate particulate matter. The combination of the two can enhance pollution removal by utilizing the principle of synergistic enhancement. At present, the main goal of the biogas slurry coagulation process is to remove pollutants such as suspended particulate matter, and less attention is paid to the retention of nutrients such as N, P and K in the biogas slurry.

[0006] At present, the coagulation treatment technology for biogas slurry mainly has the following problems:

[0007] 1) The properties of biogas slurry are complex, the treatment load is high and it emits foul-smelling gases. The existing coagulants have poor decolorization and deodorization effects. The amount of coagulant used is large and the amount of coagulant residue in the biogas slurry is large, which is not conducive to subsequent treatment and utilization.

[0008] 2) The existing coagulants mainly aim to destabilize the colloids in the biogas slurry and aggregate SS, without considering the retention of nutrients such as N, P, and K in the biogas slurry, which is not conducive to the fertilizer utilization of the biogas slurry.

[0009] 3) The existing biogas slurry coagulation mainly achieves solid-liquid separation through the sedimentation of flocs. The flocs gradually accumulate to form bottom sludge, which is difficult to remove. Excessive sludge accumulation will further affect the effluent quality and is not conducive to subsequent treatment.

[0010] 4) The solid-liquid separation process of existing biogas slurry coagulation is prone to cause sediment disturbance, destroy the stability of flocs, cause floc dispersion, release pollutants and form secondary pollution, which is not conducive to the treatment and utilization of biogas slurry.

[0011] In summary, the current biogas slurry coagulation mainly has problems such as difficulty in slag removal, poor solid-liquid separation and unclear nutrient retention. There is a lack of flocculent floating coagulants and their applications that are suitable for biogas slurry and can take into account both nutrient retention and suspended matter removal. Summary of the invention

[0012] The purpose of the present invention is to provide a biogas slurry coagulation method based on the removal of suspended matter by agglomeration and flotation, which takes into account the characteristics of nutrient retention and suspended matter agglomeration and flotation, can achieve good spontaneous solid-liquid separation, and promote the full resource utilization of biogas slurry, and has broad application prospects.

[0013] To achieve the above object, the present invention provides a biogas slurry coagulation method, comprising: adding a composite coagulant to the biogas slurry, carrying out a coagulation process to form floating flocs, and then spontaneously performing solid-liquid separation;

[0014] The composite coagulant comprises chitosan or its derivatives and an inorganic coagulant.

[0015] The present invention has found that by using chitosan or its derivatives (chitosan quaternary ammonium salt, etc.) and inorganic coagulants, the hydrolysis product of the coagulant affects the balance of carbonate and bicarbonate in the biogas slurry to produce carbon dioxide bubbles to promote the floating of flocs, and the bubbles and flocs adhere to each other to enhance the stability of the flocs, thereby achieving the functions of coagulation and flotation at the same time, which is beneficial to the removal of slag and mud. This method not only saves the cost of aeration and flotation, reduces energy consumption, but also achieves good solid-liquid separation, and can also take into account nutrient retention and suspended matter removal. It is a new floc floating biogas slurry coagulation technology with good application prospects.

[0016] Furthermore, the biogas slurry includes carbonate and / or bicarbonate, which are anions originally contained in the biogas slurry, and usually there is no need to specifically regulate their content. That is, the present invention performs coagulation treatment on the biogas slurry stock solution to promote the balanced movement of carbonate and / or bicarbonate in the biogas slurry to release carbon dioxide, and utilizes the hydrophobic groups on the chitosan to promote the mutual adhesion of bubbles and flocs, thereby forming floating flocs.

[0017] In some embodiments, the total concentration of carbonate and bicarbonate in the biogas slurry is 500-8000 mg / L, and common concentrations are 1500 mg / L, 2000 mg / L, 2500 mg / L, 3000 mg / L, 3500 mg / L, 4000 mg / L, 4500 mg / L, etc.; wherein the bicarbonate concentration is 300-5000 mg / L, for example, 400 mg / L, 500 mg / L, 1000 mg / L, 2000 mg / L, 3000 mg / L, 4000 mg / L, etc.; the carbonate concentration is 200-3000 mg / L, for example, 200 mg / L, 500 mg / L, 800 mg / L, 1000 mg / L, 2000 mg / L, 3000 mg / L, etc.

[0018] Furthermore, the inorganic coagulant includes one or more of polyaluminium chloride, polyferric sulfate and ferric chloride, preferably polyferric sulfate. The present invention adheres to the principle of green environmental protection, selects commonly used chitosan (CTS)-polyferric sulfate (PFS) and the like as representative organic coagulants and inorganic coagulants, and realizes a floating coagulant and its application technology for efficient removal of suspended solids in biogas slurry and coordinated nutrient retention.

[0019] Furthermore, the preparation method of the composite coagulant includes: dissolving the chitosan or its derivative in an acetic acid solution to obtain a chitosan acetic acid solution; and then mixing and stirring with an inorganic coagulant solution to obtain the composite coagulant.

[0020] Furthermore, the concentration of chitosan or its derivatives in the chitosan acetic acid solution is 30-50 g / L, the concentration of acetic acid is 0.3-0.5 mol / L; and the concentration of the inorganic coagulant solution is 80-120 g / L.

[0021] Furthermore, the mass ratio of the chitosan acetic acid solution to the inorganic coagulant solution is 1:(2-8), preferably 1:6; the total dosage of the composite coagulant is 6-18 g / L, preferably 6-9 g / L.

[0022] Since the coagulant of the present invention is mainly composed of natural organic polymers CTS and PFS, the components remaining in the clear liquid are mainly CTS and iron ions, which are green and non-toxic. The clear liquid after coagulation can be returned to the field for use or compounded into liquid fertilizer through membrane concentration, and the flocs can be used for anaerobic fermentation or aerobic composting, thereby realizing efficient treatment and resource utilization of biogas slurry.

[0023] Furthermore, 30-90 mg / L of cationic polyacrylamide is added to the biogas slurry, preferably 60-80 mg / L.

[0024] Furthermore, the original pH value of the biogas slurry is usually 7.5-9.5; before coagulation, the pH value of the biogas slurry is adjusted to 6.5-11, preferably to 6.5-9.5, and more preferably to 6.5-7.5; by sodium hydroxide and / or dilute sulfuric acid. pH is a key factor affecting coagulation performance, and a suitable pH value helps to remove suspended matter and promote the floating of aggregates. In the experiment of the present invention, the composite coagulant itself contains acetic acid and is acidic after hydrolysis. The addition of coagulant will acidify the biogas slurry, and adjusting the pH of the biogas slurry to near neutral to weak alkalinity is conducive to the hydrolysis of the coagulant to produce positively charged hydrolysis products, and exert a strong adsorption bridging and electrical neutralization effect. If the pH is adjusted to acidic, it is not conducive to floc formation, and the biogas slurry after coagulation is highly acidic, which is not conducive to subsequent treatment and utilization.

[0025] Furthermore, the stirring and coagulation includes: stirring at a rotation speed of 150-450 r / min for 1-5 min, and then stirring at a rotation speed of 40-80 r / min for 8-12 min.

[0026] Preferably, the stirring is performed at a speed of 300-400 r / min for 2-4 min, and then at a speed of 50-70 r / min for 9-11 min. The stirring speed has a certain influence on the removal of suspended matter. Too slow a speed is not conducive to floc agglomeration, while too fast a stirring speed is easy to break the floc.

[0027] Furthermore, the biogas slurry coagulation method comprises the following steps:

[0028] S1. Adjust the pH value of the biogas slurry to 6.5-11;

[0029] S2, adding 6-18 g / L of the composite coagulant and 30-90 mg / L of cationic polyacrylamide to the biogas slurry; stirring at a speed of 150-450 r / min for 1-5 min, and then stirring at a speed of 40-80 r / min for 8-12 min, until the flocs float;

[0030] S3. After the biogas slurry is left to stand for 20-40 minutes, solid-liquid separation is performed to remove the floating flocs.

[0031] Furthermore, the removal rate of SS in the biogas slurry is ≥60%, preferably ≥90%, and the retention rate of N+P2O5+K2O is ≥65%, preferably ≥80%. The invention effectively achieves a significant reduction in biogas slurry color and odor; while efficiently removing suspended matter, it also synergistically retains the nutrients in the biogas slurry, optimally removing 90% of SS while retaining more than 80% of N+P2O5+K2O, which is beneficial to the subsequent resource utilization of biogas slurry.

[0032] Furthermore, the clear liquid obtained after solid-liquid separation is returned to the field for utilization or used to prepare liquid fertilizer, and the flocs are used for anaerobic fermentation or aerobic composting.

[0033] In summary, the present invention belongs to a new type of coagulation technology that combines suspended particle aggregation with flotation function. The floating flocs have strong stability, are conducive to slag removal and mud discharge, avoid secondary pollution during solid-liquid separation, and achieve good spontaneous solid-liquid separation. It is also green and environmentally friendly. The coagulated effluent can be used for compounding liquid fertilizers or returning to the fields; the surface agglomerated sludge can be used for anaerobic fermentation or aerobic composting, realizing the full and efficient resource utilization of biogas slurry.

[0034] The beneficial effects of the present invention are as follows:

[0035] 1. The biogas slurry coagulation method provided by the present invention adopts chitosan or its derivatives and inorganic coagulants to compound, and in the process of destabilizing and agglomerating suspended matter in the biogas slurry, it also promotes the generation of carbon dioxide bubbles, thereby floating flocs, and the mutual adhesion of bubbles and flocs can also enhance the stability of flocs, thus facilitating the removal of suspended matter and the removal of slag and mud. Compared with the prior art, there is no need to add an additional aeration device, thereby saving the cost of aeration and flotation.

[0036] 2. Compared with traditional coagulants, the present invention has better decolorization and deodorization effects on biogas slurry. Under the optimal composite coagulant ratio and process parameters, the chromaticity of biogas slurry after coagulation by composite coagulant is greatly reduced, and the effluent is relatively clear and almost odorless.

[0037] 3. Compared with traditional coagulants, the present invention can effectively remove suspended solids while retaining nutrients in biogas slurry. Under the optimal coagulation process parameters, the biogas slurry SS removal rate is as high as 90%, and the N+P2O5+K2O retention rate is above 80%.

[0038] 4. Compared with traditional coagulants, the present invention has developed a coagulant for biogas slurry that is both environmentally friendly and resource-based. Since the coagulant of the present invention is mainly composed of natural organic polymers and inorganic coagulants, the components remaining in the clear liquid are mainly CTS and iron ions, which are green, non-toxic and synergistic. The clear liquid after coagulation can be returned to the field for utilization or compounded into liquid fertilizer through membrane concentration, and the flocs can be used for anaerobic fermentation or aerobic composting, thereby achieving efficient treatment and resource utilization of biogas slurry. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. 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.

[0040] Figure 1 It is the entity diagram after coagulation treatment with CTS-PFS composite coagulant under the optimal process parameters (A is the original biogas slurry, B and C are the entity diagrams after coagulation treatment with CTS-PFS);

[0041] Figure 2 The effects of CTS-PFS composite coagulant ratio, total dosage, and coagulant dosage on the removal rate of SS in biogas slurry;

[0042] Figure 3 The influence of CTS-PFS composite coagulant ratio, total dosage, and coagulant dosage on the retention rate of N+P2O5+K2O in biogas slurry;

[0043] Figure 4 The effect of CTS-PFS composite coagulant on the removal rate of SS in biogas slurry under different process parameters;

[0044] Figure 5 The effect of CTS-PFS composite coagulant on the retention rate of N+P2O5+K2O in biogas slurry under different process parameters;

[0045] Figure 6 This is the flow chart of biogas slurry coagulation;

[0046] Figure 7 This is an electron microscope scan of the floc surface produced by CTS-PFS coagulation;

[0047] Figure 8 The density change (A) and viscosity change (B) of the biogas slurry system before and after coagulation with different coagulants;

[0048] Fig. 9 The changes of anion content in biogas slurry before and after coagulation with different coagulants. DETAILED DESCRIPTION

[0049] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention is described clearly and completely below. 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.

[0050] Experimental Example 1: Study on the influence of the combination ratio of CTS and PFS, the total dosage of CTS-PFS, and the dosage of CPAM

[0051] a. Preparation of coagulant: 4 g chitosan (CTS) was dissolved in 100 mL 0.4 mol / L acetic acid solution to obtain 40 g / L CTS acetic acid solution, and 100 g / L polyferric sulfate (PFS) stock solution and 5 g / L cationic polyacrylamide (CPAM) solution were prepared. CTS acetic acid solution and PFS solution were mixed in the proportion shown in Table 2, and stirred thoroughly to obtain CTS-PFS composite coagulant;

[0052] b. Stirring procedure: Do not adjust the pH of the biogas slurry, that is, pH = 9. After adding CTS-PFS coagulant, turn on the fast stirring function of the six-in-one stirrer, then add CPAM and stir slowly. The fast stirring speed (hereinafter referred to as fast stirring speed) is 250r / min, and the fast stirring time (hereinafter referred to as fast stirring time) is 3min; the slow stirring speed is 60r / min, and the slow stirring time is 10min;

[0053] c. Standing: After the stirring process is completed, let it stand for 30 minutes, and then perform solid-liquid separation after the standing period.

[0054] The specific implementation plan is as follows:

[0055] Using orthogonal test, the effects of three compounding conditions, namely, the ratio of CTS:PFS, the dosage of coagulant and the dosage of coagulant aid on CTS-PFS coagulation were comprehensively investigated, with SS removal rate and total nutrient (N+P2O5+K2O) retention rate as evaluation indicators. Four levels were set (Table 1), and L 16 4 3 The orthogonal table was used to form 16 groups of biogas slurry coagulation test schemes (Table 2), so as to determine the efficient coagulation compound scheme of biogas slurry.

[0056] Table 1 Different factors and corresponding level designs of orthogonal experiment

[0057]

[0058] Table 2 Orthogonal test design table

[0059]

[0060]

[0061] from Figure 2 and Figure 3 From the orthogonal test results, it can be seen that the comprehensive effect is optimal when the combination ratio of CTS and PFS is 1:6, the total dosage of CTS-PFS is 6g / L, and the dosage of CPAM is 70mg / L.

[0062] Experimental Example 2: Study on the influence of pH value, fast stirring speed and fast stirring time

[0063] A biogas slurry coagulation method based on the removal of suspended matter by agglomeration and floating, such as Figure 6 As shown, the details are as follows:

[0064] a. Preparation of coagulant: Take 4g chitosan (CTS) and dissolve it in 100mL 0.4mol / L acetic acid solution to obtain 40g / L CTS acetic acid solution, prepare 100g / L polyferric sulfate (PFS) stock solution and 5g / L cationic polyacrylamide (CPAM) solution. Mix CTS acetic acid solution and PFS solution at a mass ratio of 1:6, and stir thoroughly to obtain CTS-PFS composite coagulant;

[0065] b. Adjustment of biogas slurry pH: Use 10% sodium hydroxide and dilute sulfuric acid solution to adjust the biogas slurry pH to the required pH of the test;

[0066] c. Stirring procedure: Add 6g / L CTS-PFS coagulant to start the fast stirring function of the six-joint stirrer, then add 70mg / L CPAM and stir slowly. The fast stirring speed and fast stirring time are set according to the test plan, the slow stirring speed is 60r / min, and the slow stirring time is 10min;

[0067] d. Standing: After the stirring process is completed, stand for 30 minutes, and then perform solid-liquid separation.

[0068] The specific implementation plan is as follows:

[0069] Using orthogonal test, the effects of three key process parameters, pH, fast stirring speed and fast stirring time, on CTS-PFS coagulation were comprehensively investigated, with SS removal rate and N+P2O5+K2O retention rate as evaluation indicators. Three levels were set (Table 3), and L93 was used. 3 The orthogonal table was used to form 9 groups of biogas slurry coagulation test schemes (Table 4), thereby determining the process parameters for efficient biogas slurry coagulation.

[0070] Table 3 Different factors and corresponding level designs of orthogonal experiment

[0071]

[0072] Table 4 Orthogonal test design table

[0073]

[0074] Figure 4 and Figure 5 The orthogonal test results of the above three factors are comprehensively analyzed to determine the effect of CTS-PFS composite coagulant on the SS removal rate of biogas slurry ( Figure 4 ) and N+P2O5+K2O retention rate ( Figure 5 ), the optimal process parameters of CTS-PFS high-efficiency coagulation are determined to be: A1, B3, C2, that is, pH 7, fast stirring speed 350r / min, and fast stirring time 3min. The present invention uses biogas slurry as the stock liquid for biogas slurry coagulation treatment, and adopts a six-connected electric stirrer for coagulation treatment under normal temperature conditions. By screening the pH, fast stirring speed and fast stirring time of biogas slurry, more efficient coagulation process parameters are selected to further promote the development of biogas slurry coagulation pretreatment technology.

[0075] like Figure 1 As shown, it can be seen that the biogas slurry is relatively turbid. After the CTS-PFS coagulation treatment, the suspended matter aggregates to form flocs that float up, and at the same time, a relatively clear lower layer of liquid is obtained. It can be seen that the present invention can produce floating flocs during the coagulation process without the need for an additional aeration device, which not only saves the cost of aeration flotation and reduces energy consumption, but also achieves good solid-liquid separation. Since the N+P2O5+K2O retention rate of the clear liquid after coagulation is relatively high, the clear liquid after coagulation can be returned to the field for utilization or compounded into liquid fertilizer through membrane concentration, and the flocs can be used for anaerobic fermentation or aerobic composting, thereby realizing efficient resource utilization of biogas slurry, and has a good comprehensive application prospect.

[0076] from Figure 7 It can be seen that the flocs formed by CTS-PFS have a rough surface, a large floc size, a loose and porous structure, and bubbles exist on the surface and inside the floc layer.

[0077] In order to further reveal the mechanism of CTS-PFS coagulant producing floating flocs, three groups of coagulation treatments were carried out using CTS, PFS and CTS-PFS composite coagulants respectively. The formation mechanism of floating flocs was analyzed by combining the density and viscosity of the biogas slurry coagulation system (e.g. Figure 8 There was no significant difference in the density of the clear liquid after coagulation in each treatment group, which was similar to that of water ( Figure 8 -A), but the floc density varies greatly ( Figure 8-B). The density of the PFS group flocs is greater than that of water, and the flocs are in a sinking state. The density of the CTS and CTS-PFS groups flocs is lower than that of water, which is consistent with the observation that both groups of flocs are floating. The CTS-PFS group has the lowest floc density, which may be due to the formation of a "bubble-floc" composite structure with an overall density lower than that of water, which floats upward with the buoyancy generated by the bubbles. As an organic polymer coagulant, CTS has viscosity and surface hydrophobic groups that further increase the adhesion between bubbles and flocs, promoting the floating of flocs.

[0078] By measuring the types and contents of anions in the clear liquid during the three treatments, the gas components produced during the coagulation process were analyzed ( Fig. 9 ). By comparing the anion content in the unadjusted pH biogas slurry and the pH=7 biogas slurry, the H + The introduction of the effect on the type and content of anions in the biogas slurry. By comparing several anions, it was found that after coagulation, HCO3 - 、CO3 2- and PO4 3- This may be due to the fact that the organic coagulant hydrolyzes to produce H + Promote CO3 2- To HCO3 - Conversion, thus promoting HCO3 - and H + Combined to produce CO2, the specific chemical equilibrium reaction is as follows:

[0079]

[0080]

[0081]

[0082] Among them, HCO3 in the PFS group - and CO3 2- The contents are close to zero, iron ions and CO3 2- The reaction will speed up CO3 2- consumption and generation of CO2, but the buoyancy of the bubbles is not enough to offset the gravity of the flocs produced by PFS, so the flocs settle.

[0083] In summary, the present invention adheres to the principle of green environmental protection, selects commonly used chitosan (CTS)-polyferric sulfate (PFS) as representative organic coagulants and inorganic coagulants, proposes a CTS-PFS composite coagulant compounding scheme, and determines a set of biogas slurry coagulation process parameters, thereby realizing a floating coagulant and application technology for efficient removal of suspended matter in biogas slurry and coordinated nutrient retention.

[0084] 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. A biogas slurry coagulation method, characterized in that: include: Add composite coagulant to the biogas slurry, stir and coagulate to produce floating flocs, and then separate the solid and liquid; The biogas slurry coagulation method comprises the following steps: S1. Adjusting the pH value of the biogas slurry to 6.5-11; the biogas slurry comprises carbonate and / or bicarbonate; and the total concentration of carbonate and bicarbonate in the biogas slurry stock solution is 500-8000 mg / L; S2, adding 6-18 g / L of the composite coagulant to the biogas slurry, stirring at a speed of 150-450 r / min for 1-5 min; then adding 30-90 mg / L of cationic polyacrylamide, stirring at a speed of 40-80 r / min for 8-12 min, and the flocs float; S3, after the biogas slurry is left to stand for 20-40 minutes, spontaneous solid-liquid separation is performed to remove the floating flocs; The composite coagulant comprises chitosan or its derivatives and an inorganic coagulant, which promotes the generation of carbon dioxide bubbles in the process of destabilizing and agglomerating suspended matter in the biogas slurry, so that flocs float, and the bubbles and flocs adhere to each other to enhance the stability of the flocs.

2. The biogas slurry coagulation method according to claim 1, characterized in that: The inorganic coagulant includes one or more of polyaluminium chloride, polyferric sulfate and ferric chloride.

3. The biogas slurry coagulation method according to claim 1, characterized in that: The preparation method of the composite coagulant comprises: dissolving the chitosan or its derivative in an acetic acid solution to obtain a chitosan acetic acid solution; and then mixing and stirring with an inorganic coagulant solution to obtain the composite coagulant.

4. The biogas slurry coagulation method according to claim 3, characterized in that: The concentration of chitosan or its derivatives in the chitosan acetic acid solution is 30-50 g / L, the concentration of acetic acid is 0.3-0.5 mol / L; the concentration of the inorganic coagulant solution is 80-120 g / L; And / or, the mass ratio of the chitosan acetic acid solution to the inorganic coagulant solution is 1:(2-8).

5. The biogas slurry coagulation method according to claim 4, characterized in that: The total dosage of the composite coagulant is 6-9 g / L.

6. The biogas slurry coagulation method according to any one of claims 1 to 5, characterized in that: 60-80 mg / L of cationic polyacrylamide is added to the biogas slurry.

7. The biogas slurry coagulation method according to any one of claims 1 to 5, characterized in that: The pH value of the biogas slurry is adjusted by sodium hydroxide and / or dilute sulfuric acid.

8. The biogas slurry coagulation method according to claim 7, characterized in that: The pH value of the biogas slurry is 6.5-7.

5.

9. The biogas slurry coagulation method according to any one of claims 1 to 5, characterized in that: The stirring and coagulation comprises: stirring at a rotation speed of 300-400 r / min for 2-4 min, and then stirring at a rotation speed of 50-70 r / min for 9-11 min.

10. The biogas slurry coagulation method according to any one of claims 1 to 5, characterized in that: The removal rate of suspended matter in the biogas slurry is ≥60%, and the retention rate of N+P2O5+K2O is ≥65%.

11. The biogas slurry coagulation method according to claim 10, characterized in that: The removal rate of suspended matter in the biogas slurry is ≥90%.

12. The biogas slurry coagulation method according to claim 10, characterized in that: The retention rate of N+P2O5+K2O in the biogas slurry is ≥80%.

13. The biogas slurry coagulation method according to any one of claims 1 to 5, characterized in that: The clear liquid obtained after solid-liquid separation is returned to the field for integrated water and fertilizer utilization to reduce pipe blockage; or liquid fertilizer is compounded to avoid interference from impurities; the flocs are used for anaerobic fermentation or aerobic composting, thereby realizing the full resource utilization of the biogas slurry.

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