Method for carrying out anaerobic digestion to recover phosphorus by adding sludge-based biochar

Abstract

The invention discloses a method for recovering phosphorus through anaerobic digestion by adding sludge-based biochar, which comprises the following steps of: firing dewatered sludge serving as a raw material under an anaerobic condition to obtain carbonized carbon; or firing under an aerobic condition to obtain pyrolytic carbon, adding any sludge-based biochar obtained by firing into the hydrothermal dewatered sludge, and carrying out anaerobic digestion on the hydrothermal dewatered sludge; the hydrothermal dewatered sludge is obtained by performing hydrothermal pretreatment on dewatered sludge; and carrying out phosphorus recovery on the sludge-based biochar subjected to anaerobic digestion by utilizing a magnetic separation method. Compared with the traditional method for recovering the phosphate fertilizer from the sludge incineration ash, the recovery method disclosed by the invention is green and pollution-free, belongs to a sludge treatment mode of treating waste with waste, realizes effective combination of a heat treatment mode and a biological treatment mode, and has a good practical application value. Moreover, the prepared sludge-based biochar has the property of magnetism, so that the recovery rate of the subsequent blue iron ore is increased, and the separation difficulty and economic investment are reduced.

Description

Technical Field

[0001] This invention belongs to the field of sludge recycling technology, and relates to the recovery of lapis lazuli from waste activated sludge, and particularly to a method for recovering phosphorus by adding sludge-based biochar for anaerobic digestion. Background Technology

[0002] To prevent eutrophication of rivers and lakes, many wastewater treatment plants are equipped with phosphorus recovery equipment, as phosphorus accumulates in the sludge after treatment. Among these methods, phosphorus recovery in the form of lapis lazuli is receiving increasing attention.

[0003] Because waste activated sludge is widely accepted as a secondary feedstock for phosphorus, various methods have been explored in recent years to improve the efficiency of phosphorus recovery from sludge. These methods include: 1. Biomass fermentation, which promotes the decomposition of organic matter to release phosphorus. 2. Membrane separation technology, characterized by selective filtration. 3. Bioextraction, which utilizes microorganisms to accumulate and subsequently release phosphorus for recovery. For example, CN111792636A discloses a method for recovering lapis lazuli from sludge incineration ash, which ultimately obtains high-purity lapis lazuli product through acid leaching of sludge incineration ash, adsorption of leaching filtrate and adsorption of filter residue; CN112279478A discloses a method for recovering phosphorus from residual sludge in the form of lapis lazuli, which involves adding ferric chloride to concentrated secondary sedimentation tank sludge samples for anaerobic digestion, and precipitating and recovering lapis lazuli crystals in the fermentation supernatant.

[0004] However, the aforementioned treatment methods are largely affected by the complex nature of the sludge itself and the stringent recovery conditions for lapis lazuli. Due to the complexity of the sludge's composition, the phosphorus release rate is low, making it difficult to efficiently recover lapis lazuli. Besides being rich in iron and phosphorus, lapis lazuli formation also requires a reducing environment and a neutral pH. Currently, the hydroxyapatite and struvite crystallization method is relatively mature for phosphorus recovery, but it is not the optimal method due to its high cost, low product value, complex operating conditions, and low recovery efficiency. Summary of the Invention

[0005] In order to overcome the shortcomings of the prior art, the present invention aims to provide a method for anaerobic digestion and phosphorus recovery by adding sludge-based biochar, which has the characteristics of low cost, simple preparation and good phosphorus adsorption effect.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for anaerobic digestion and phosphorus recovery by adding sludge-based biochar includes the following steps:

[0008] Step 1: Prepare sludge-based biochar using dewatered sludge as raw material;

[0009] Step 2: Add the sludge-based biochar to the hydrothermal dewatered sludge to carry out anaerobic digestion of the hydrothermal dewatered sludge; the hydrothermal dewatered sludge is obtained by hydrothermal pretreatment of dewatered sludge;

[0010] Step 3: Use magnetic separation to recover phosphorus from anaerobic digested sludge-based biochar.

[0011] In one embodiment, the dewatered sludge described in this invention is sludge whose moisture content has been significantly reduced after dewatering treatment. It is a semi-solid waste generated during wastewater treatment, typically originating from sedimentation tanks, biological treatment units, or sludge thickening stages in wastewater treatment plants.

[0012] In one embodiment, step 1 involves preparing the sludge-based biochar in one of the following ways:

[0013] Method 1: Dewatered sludge is burned under anaerobic conditions to obtain carbonized char.

[0014] Method 2: Burn the dewatered sludge under aerobic conditions to obtain pyrolytic char.

[0015] In one embodiment, the firing conditions for Method 1 are: temperature 400-600℃, time 1-2h; and the firing conditions for Method 2 are: 800-1000℃, 1-2h.

[0016] In one embodiment, the dewatered sludge used in step 2 is the same as the dewatered sludge used in step 1.

[0017] In one embodiment, the hydrothermal pretreatment is performed under the following conditions: an oxygen-rich atmosphere, hydrothermal treatment at 160-180°C for 20-40 minutes.

[0018] In one embodiment, the sludge treated by the hydrothermal process is in a liquid state, and the sludge-based biochar is added to the hydrothermally dewatered sludge at a rate of 10 g / L.

[0019] In one embodiment, the hydrothermal dewatered sludge undergoes anaerobic digestion, with a weight ratio of hydrothermal dewatered sludge to dewatered sludge inoculum of 1:1-1.5. For example, the main sludge parameters are: TS = 11.37% VS = 6.68%, and the inoculum is an inoculum cultured in a laboratory anaerobic digester using dewatered sludge as raw material, with main parameters: TS = 6.05% VS = 3.34%.

[0020] In one embodiment, the hydrothermal dewatered sludge undergoes anaerobic digestion with a fermentation cycle of 15-20 days and a temperature of 35±0.5℃.

[0021] In one embodiment, step 3 involves freeze-drying the anaerobic digested biogas residue and recovering the sludge-based biochar using magnetic separation, wherein the lapis lazuli produced during anaerobic digestion is enriched on the sludge-based biochar.

[0022] Compared to traditional methods of recovering phosphate fertilizer from sludge incineration ash, this invention offers a green and pollution-free recovery method, representing a "waste-to-waste" sludge treatment approach. It effectively combines thermal and biological treatment methods, demonstrating significant practical application value. Furthermore, the sludge-based biochar prepared in this invention possesses magnetic properties, improving the subsequent recovery rate of lapis lazuli and reducing the difficulty and economic investment in sorting. For solid waste such as dewatered sludge from wastewater treatment plants, this method uses thermal treatment to prepare different types of biochar for conditioning and dewatering, improving sludge properties, reducing the land area required for dewatered sludge, and minimizing environmental harm. Overall, this method provides an economical and environmentally friendly way to recover lapis lazuli from dewatered sludge. Attached Figure Description

[0023] Figure 1 This is the main flowchart of the present invention.

[0024] Figure 2 This is a schematic diagram of the preparation process of the two types of sludge-based biochar of the present invention.

[0025] Figure 3 These are schematic diagrams of two sludge-based biochar products prepared by this invention, wherein the left diagram shows pyrolytic biochar and the right diagram shows carbonized biochar.

[0026] Figure 4 The images show SEM characterizations of different sludge-based biochars before and after anaerobic digestion, where (a) is before anaerobic digestion of hydrothermal biochars and (b) is after anaerobic digestion of hydrothermal biochars.

[0027] Figure 5 The images show SEM characterizations of different sludge-based biochars before and after anaerobic digestion, where (a) is the biochar before anaerobic digestion and (b) is the biochar after anaerobic digestion.

[0028] Figure 6 yes Figure 5 A magnified view of the dashed box portion in (b).

[0029] Figure 7 The images show SEM characterizations of different sludge-based biochars before and after anaerobic digestion, where (a) is before anaerobic digestion of the biochar and (b) is after anaerobic digestion of the biochar.

[0030] Figure 8 yes Figure 7 A magnified view of the dashed box portion in (b).

[0031] Figure 9 The images show the XRD characterization of biochar recovered from R (addition of pyrolytic carbon) treatment, where red represents the standard card for Vivianite Fe3(PO4)2·8H2O and blue represents the standard card for SiO2.

[0032] Figure 10 The images show the XRD characterization of biochar recovered from T (addition of carbon carbide) treatment, where red represents the standard card for Vivianite Fe3(PO4)2·8H2O; and blue represents the standard card for SiO2.

[0033] Figure 11 The images show the initial state of a seed germination experiment. The left image shows an ultrapure water culture medium, while the right image shows a solution with 1% T liquid phase blue iron ore added.

[0034] Figure 12 This is a diagram showing the state of seed germination after 48 hours. From left to right, the culture medium is ultrapure water, 1% R solid phase blue iron ore is added, and 1% T solid phase blue iron ore is added.

[0035] Figure 13 This is a schematic diagram of seed germination rate obtained from a seed germination experiment. The horizontal axis represents the treatment method, and the vertical axis represents the seed germination rate. Detailed Implementation

[0036] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0037] Example 1:

[0038] This embodiment describes a method for anaerobic digestion and phosphorus recovery by adding sludge-based biochar. The sludge used in the experiment was dewatered sludge from Huayu Water Quality Company in Yangling District, Xianyang City, Shaanxi Province. The aim was to effectively recover phosphorus (blue iron ore) from the dewatered sludge and alleviate the current problem of phosphorus resource scarcity. Figure 1 As shown, the main steps are as follows:

[0039] Step 1: Using dewatered sludge as raw material, sludge-based biochar was prepared using two different methods, named pyrolysis biochar and carbonized biochar, respectively. The specific preparation methods are as follows: Figure 2 Finished product such as Figure 3 As shown.

[0040] The present invention prepares pyrolytic char and carbonized char using the following methods.

[0041] Method 1: Dewatered sludge is burned under anaerobic conditions to obtain carbonized char; the burning conditions are: temperature 400-600℃, time 1-2h.

[0042] Method 2: The dewatered sludge is burned under aerobic conditions to obtain pyrolytic char; the burning conditions are: 800-1000℃, 1-2h.

[0043] It is expressed as follows:

[0044]

[0045] In this embodiment, the dehydrated sludge can be dried and pulverized first, and then calcined. The drying temperature is selected as 105℃. In calcination method one, the temperature is selected as 500℃, and calcination is carried out in a muffle furnace for 2 hours under oxygen-free conditions. In calcination method two, the temperature is selected as 900℃, and calcination is carried out in a muffle furnace for 2 hours under oxygen-exposed conditions. After calcination, the sludge is washed with anhydrous ethanol, filtered, dried again, and passed through a 100-mesh sieve for later use.

[0046] Step 2: Prepare hydrothermal dewatered sludge by hydrothermal pretreatment of the dewatered sludge.

[0047] In this embodiment, 30g of dehydrated sludge was added to a hydrothermal reactor and subjected to hydrothermal treatment at 170℃ for 30 minutes under aerobic conditions to obtain hydrothermally dehydrated sludge. After cooling, the sludge was removed and stored in a refrigerator at 4℃ for later use. It is easy to understand that the conditions for the hydrothermal reaction do not need to be overly stringent. In the experiment, effective hydrothermal pretreatment could be achieved within a temperature range of 160-180℃ and a time range of 20-40 minutes.

[0048] The hydrothermal pretreatment process in this step is mainly to promote the release of phosphorus during the anaerobic digestion of sludge, so as to open up the complex structure of the sludge and facilitate the subsequent anaerobic process. The solid-liquid ratio in the hydrothermal process can be designed to be 1:10 (w / w). Obviously, preferably, the dewatered sludge used in this step is the same as the dewatered sludge used in step 1.

[0049] Step 3: The prepared pyrolytic carbon and carbonized carbon were mixed with hydrothermal dewatered sludge to conduct an anaerobic digestion test of the hydrothermal dewatered sludge.

[0050] In this embodiment, the working volume was 800 mL. By weight, the ratio of hydrothermal dewatered sludge (TS = 11.37% VS = 6.68%) to inoculum (TS = 6.05% VS = 3.34%) (the inoculum was taken from the inoculum cultured from dewatered sludge in the laboratory anaerobic digester) was 1:1-1.1. 8 g of biochar was added to each bottle (at a rate of 10 g / L). The fermentation cycle was 15-20 days. 20 days was selected in this embodiment. The experiment was conducted at 35±0.5℃ and 120 rpm in a shaker. Six replicates were set up for each treatment, and samples were taken on days 0, 3, 5, 10, 15, and 20.

[0051] Step 4: The anaerobic digestion product sludge from day 20 is subjected to vacuum freeze-drying. Magnetic biochar in the sludge is separated using a magnet, washed with oxygen-free water, and dried in a vacuum freeze dryer for 24–36 hours. Finally, the separated magnetic biochar is characterized by SEM and XRD. The lapis lazuli produced during anaerobic digestion is enriched on the sludge-based biochar, which is convenient for further utilization.

[0052] refer to Figure 4 As shown, the surface of the hydrothermal carbon after anaerobic digestion exhibits a layered morphological structure, which matches the morphology of vivianite.

[0053] refer to Figure 5 , Figure 6 As shown, the pyrolytic carbon recovered by magnetic separation after anaerobic digestion has a wrinkled structure compared with that before anaerobic digestion, which is consistent with the morphology of vivianite.

[0054] refer to Figure 7 , Figure 8 As shown, the carbon carbide recovered by magnetic separation after anaerobic digestion has a wrinkled structure compared with that before anaerobic digestion, which matches the morphology of vivianite.

[0055] Step 5: The separated magnetic biochar was used to conduct germination tests on Chinese cabbage seeds at a concentration of 1%. Two sheets of filter paper were placed in three sterile petri dishes, and 10 ml of sterile water and a 1% solution of two phosphate fertilizers were added to each dish. The dishes were placed in an incubator in the dark for 25% for 48 hours. The germination rate of seeds in each treatment group was calculated to verify the fertilizer effect.

[0056] refer to Figure 9 As shown, XRD characterization of the carbon carbide after anaerobic digestion revealed that the surface of the recovered pyrolytic carbon was loaded with lapis lazuli crystals, proving that pyrolytic carbon loaded with lapis lazuli was recovered. This further demonstrates that the addition of carbon carbide during anaerobic digestion produced lapis lazuli.

[0057] refer to Figure 10 As shown, XRD characterization of the carbon carbide after anaerobic digestion revealed that the surface of the recovered carbon carbide was loaded with lapis lazuli crystals, proving that carbon carbide loaded with lapis lazuli was recovered. This further demonstrates that the addition of carbon carbide during anaerobic digestion produced lapis lazuli.

[0058] refer to Figure 11 , Figure 12 As shown, verifying fertilizer efficacy through seed germination rate is a commonly used bioassay method to assess the promoting effect of fertilizers on plant growth and their potential toxicity. By observing the germination of seeds in a fertilizer-containing medium, the efficacy and safety of the fertilizer can be quickly and intuitively determined. Figure 11 , Figure 12This study demonstrates the seed germination test and germination rate of lavite recovered from biogas slurry in treatment group T using ferric hydroxide. The results showed that the germination rate of group T reached 84.55%, while that of the control group (CK) was 64%. Compared with the CK group which had ultrapure water added, the 1% solid-phase recovered lavite from group T resulted in the best germination effect for Chinese cabbage seeds, further illustrating the good fertilizer application value of lavite recovered from liquid phase in group T.

[0059] Figure 13 This study demonstrates seed germination experiments and germination rates of lapis lazuli recovered using magnetic separation in the R and T treatment groups. Results showed that the seed germination rate increased from 64% in the control group to 91% in the R group and 84% in the T group. This indicates that the presence of phosphate fertilizer recovered by magnetic separation in the R and T groups using sludge-based biochar, after dilution, improved seed germination rates. Furthermore, the significant impact of phosphate fertilizer on seed germination rate and early germination rate is likely due to the physical adsorption of lapis lazuli during the growth stages. Phosphorus and iron are essential nutrients for plant growth, and lapis lazuli can serve as a slow-release fertilizer, providing long-term nutrients to plants while also improving soil structure and promoting plant growth.

[0060] Through the above steps, this invention reduces the volume of dewatered sludge through calcination and hydrothermal pretreatment, thereby reducing its environmental pollution. By adding sludge-based biochar for anaerobic digestion, the anaerobic digestion process is not only improved, but also the release and recovery of phosphorus is promoted. At the same time, the sludge-based biochar added in this invention can effectively adhere to the phosphate fertilizer (blue iron ore) generated during anaerobic digestion, obtaining high-quality blue iron ore products, alleviating the shortage of phosphorus resources. Moreover, the process is simple and easy to operate.

Claims

1. A method for anaerobic digestion and phosphorus recovery by adding sludge-based biochar, characterized in that, Includes the following steps: Step 1: Prepare sludge-based biochar using dewatered sludge as raw material; Step 2: Add the sludge-based biochar to the hydrothermal dewatered sludge to carry out anaerobic digestion of the hydrothermal dewatered sludge; the hydrothermal dewatered sludge is obtained by hydrothermal pretreatment of dewatered sludge; Step 3: Use magnetic separation to recover phosphorus from anaerobic digested sludge-based biochar.

2. The method for anaerobic digestion and phosphorus recovery by adding sludge-based biochar according to claim 1, characterized in that, The dewatered sludge is a semi-solid waste generated during the wastewater treatment process.

3. The method for anaerobic digestion and phosphorus recovery by adding sludge-based biochar according to claim 1, characterized in that, Step 1 involves preparing the sludge-based biochar in one of the following ways: Method 1: Dewatered sludge is burned under anaerobic conditions to obtain carbonized char. Method 2: Dehydrated sludge is burned under aerobic conditions to obtain pyrolytic char.

4. The method for anaerobic digestion and phosphorus recovery by adding sludge-based biochar according to claim 3, characterized in that, Method 1 has the following firing conditions: temperature 400-600℃, time 1-2h; Method 2 has the following firing conditions: 800-1000℃, time 1-2h.

5. The method for anaerobic digestion and phosphorus recovery by adding sludge-based biochar according to claim 1, characterized in that, The dewatered sludge used in step 2 is the same as the dewatered sludge used in step 1.

6. The method for anaerobic digestion and phosphorus recovery by adding sludge-based biochar according to claim 1 or 5, characterized in that, The hydrothermal pretreatment conditions are: oxygen-rich atmosphere, hydrothermal treatment at 160-180℃ for 20-40 minutes.

7. The method for anaerobic digestion and phosphorus recovery by adding sludge-based biochar according to claim 1, characterized in that, The sludge-based biochar is added to the hydrothermal dewatered sludge at a rate of 10 g / L.

8. The method for anaerobic digestion and phosphorus recovery by adding sludge-based biochar according to claim 1, characterized in that, The anaerobic digestion of hydrothermal dewatered sludge involves a weight ratio of hydrothermal dewatered sludge to dewatered sludge inoculum of 1:1-1.

5.

9. The method for anaerobic digestion and phosphorus recovery by adding sludge-based biochar according to claim 1, characterized in that, The hydrothermal dewatering sludge undergoes anaerobic digestion with a fermentation cycle of 15-20 days and a temperature of 35±0.5℃.

10. The method for anaerobic digestion and phosphorus recovery by adding sludge-based biochar according to claim 1, characterized in that, In step 3, the anaerobic digested biogas residue is freeze-dried and the sludge-based biochar is recovered by magnetic separation. The lapis lazuli produced during the anaerobic digestion process is enriched on the sludge-based biochar.

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