Method for recycling aerobic granular sludge
By using eggshell powder as a calcium source during sludge pyrolysis, the phosphorus form is regulated to be converted into apatite phosphorus, which solves the problems of low phosphorus conversion rate and high cost in traditional sludge pyrolysis technology, and achieves efficient phosphorus utilization and improved biochar stability.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-02-05
- Publication Date
- 2026-03-17
AI Technical Summary
Existing sludge pyrolysis technology has problems such as high cost, difficulty in reproducing, and poor performance at low temperatures when converting phosphorus in sludge into apatite-like phosphorus. In particular, when using calcium oxide as a calcium source, calcium carbonate is easily generated, which affects the effect.
Eggshell powder is used as a calcium source and mixed with aerobic granular sludge at a certain calcium-to-phosphorus molar ratio. The mixture is then pyrolyzed under an inert atmosphere at a controlled temperature of 300-900℃ to directionally regulate the conversion of phosphorus into apatite phosphorus or slow-release calcium phosphate.
It increases the content and proportion of apatite phosphorus in sludge char, promotes the effective utilization of phosphorus, reduces sludge melting and agglomeration, produces more stable biochar-based products, and enhances the slow-release performance of phosphorus.
Smart Images
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Figure SMS_1
Abstract
Description
BACKGROUND
[0001] Phosphorus is a key nutrient element for life and agriculture, but the global high-quality phosphorus resources are facing the risk of exhaustion and are listed as strategic resources. In the process of purifying water quality in modern sewage treatment plants, about 90% of phosphorus in sewage is transferred to sludge, and a large amount of phosphorus is lost or stored with sludge being landfilled or improperly disposed, and even may cause secondary pollution. Aerobic granular sludge (AGS) as a leading sewage treatment technology is concerned for its compact structure, excellent settling performance and strong simultaneous nitrogen and phosphorus removal capacity. Due to its special "aerobic-anoxic-anaerobic" layered structure, AGS can remove phosphorus in multiple ways, on the one hand, it can enrich phosphorus accumulating organisms to achieve biological phosphorus removal, on the other hand, it can chemically remove phosphorus in the form of mineral precipitation in a complex microenvironment. This makes the phosphorus content of AGS usually significantly higher than that of traditional activated sludge, becoming a very potential "urban phosphorus mine". It is reported that the phosphorus content in aerobic granular sludge can reach more than 15%, and the content of polyphosphate in phosphorus can also be high, up to 20%-50%. Polyphosphoric acid is a long-chain structure connected by the polymerization of multiple phosphoric acid molecules through specific chemical bonds, and its salt is the "energy conversion station" of phosphorus accumulating organisms: phosphorus accumulating organisms decompose organic matter and synthesize polyphosphate during the aerobic period.
[0002] Sludge pyrolysis technology can effectively reduce the volume of sludge and degrade or eliminate toxic and harmful substances therein, but the phosphorus in the sludge may be converted into non-apatite inorganic phosphorus (such as phosphate combined with iron and aluminum) with poor solubility and difficult for plants to use during the pyrolysis process. In order to better convert the sludge phosphorus into apatite state phosphorus (phosphorus combined with calcium and magnesium ions), some people add calcium oxide, calcium hydroxide and calcium salt in the pyrolysis system, but there are defects such as high cost and difficulty in reproduction, for example, calcium oxide is easily converted into calcium carbonate and calcium hydroxide and other substances at room temperature, which affects the actual use process and effect.
[0003] The specific conditions of sludge and calcium additives can also affect the conversion rate of apatite inorganic phosphorus. Although some people mention adding calcium carbonate and biological shell substances, the essence is still to use calcium oxide, and the calcium carbonate decomposes into calcium oxide at high temperature, and the existing technology tends to believe that the low-temperature undecomposed calcium carbonate affects the conversion of phosphorus. SUMMARY
[0004] In order to solve the resourceization of aerobic granular sludge, in order to overcome the bias in traditional sludge pyrolysis technology, the present application improves the traditional pyrolysis technology, and investigates its performance as a fertilizer.
[0005] In the first aspect, this application provides a method for treating aerobic granular sludge of AGS, including mixing eggshell powder with AGS sludge at a Ca / P ratio of 1-2:1 (calcium:phosphorus), and pyrolyzing the resulting mixture under an inert atmosphere to obtain sludge char, with pyrolysis temperatures of 300-900℃, 300-700℃, and 300-500℃.
[0006] This invention introduces eggshell waste as a calcium source to directionally regulate phosphorus form during the pyrolysis of aerobic granular sludge, stably converting it into apatite phosphorus or slow-release calcium phosphate. Pyrolysis temperatures ranging from 300-900℃ all increase the content of apatite phosphorus in the sludge char and the proportion of apatite phosphorus in the total phosphorus of the sludge char. The applicant also found that higher pyrolysis temperatures result in better conversion of non-apatite phosphorus to apatite phosphorus, and a higher content and proportion of apatite phosphorus in the resulting sludge char.
[0007] Accordingly, in a second aspect, this application provides methods for AGS resource utilization, such as methods for increasing the content of apatite phosphorus in sludge char and methods for increasing the proportion of apatite phosphorus in total phosphorus of sludge char.
[0008] In the above aspects, aerobic granular sludge is usually neutral or slightly alkaline in pH. The AGS sludge used may contain polyphosphate, metal-phosphate precipitates, or sludge from the end of the aerobic phase in the operation cycle of a sequencing batch reactor. The polyphosphate content (wt%) in the total phosphorus of the sludge may be 20%-50%, such as 26.4%.
[0009] Optionally, the metal-bound inorganic phosphorus crystals of the aerobic granular sludge used are mainly apatite-state phosphorus, and may contain almost no AlPO4 or FePO4. If AlPO4 is not present (the inorganic crystals in the sludge can be seen in the attached figure).
[0010] Optionally, the content of apatite phosphorus in the total phosphorus of AGS is 10%-30% by wt%, such as 26.76%.
[0011] Optionally, the volatile matter ratio (VSS / SS) of the aerobic granular sludge used is 0.7-0.82, or 0.81; the phosphorus content of the AGS dry sludge can be 6-15%, such as 8.44%.
[0012] Optionally, the aerobic granular sludge used was found by SMT method to contain 26.76% apatite-form inorganic phosphorus, 54.42% non-apatite-form inorganic phosphorus, and 18.82% organic phosphorus.
[0013] Optionally, the main elemental composition of the dry basis of the aerobic granular sludge used is as follows: carbon 45-62%; nitrogen 4-8%; oxygen 26-32%; phosphorus 1.5-8.4%; calcium 0.43-10.46%; iron 0.14-0.75%; and potassium 1.28-3.39%.
[0014] Optionally, a pretreatment is performed before mixing, which includes drying, pulverizing and sieving in sequence.
[0015] Optionally, the sludge used can be ground and passed through a 100-200 mesh sieve.
[0016] Optionally, the eggshells used should be sieve size that passes through a 100-200 mesh sieve.
[0017] Optionally, the calcium carbonate content in the eggshells used can be 83-95%.
[0018] Optionally, the proportion of eggshells used as a byproduct of the sludge dry weight is 5%, 10%, 15%, or 20%.
[0019] Optionally, the pyrolysis process temperature can be 300-900℃.
[0020] Optionally, the pyrolysis process can take 1-3 hours, such as 2 hours.
[0021] Optionally, the heating rate of the pyrolysis process is 5-20℃ / min, or 5-10℃ / min.
[0022] Optionally, the pyrolysis process can use an inert gas to create an oxygen-free atmosphere, and the flow rate can be 100-1200 mL / min.
[0023] The specific operation example is as follows: Aerobic granular sludge from a sequencing batch reactor that had been running for about 200 days was selected, with a sludge concentration of 8.42 g / L and an MLVSS / MLSS ratio of 0.81.
[0024] The sludge contains 44% carbon, 30.7% oxygen, 8.4% phosphorus, 9.7% calcium, 0.28% magnesium and 0.15% iron, etc.; its calcium-to-phosphorus molar ratio is 0.866, and polyphosphate accounts for 26.4% of the total phosphorus.
[0025] Sludge is collected at the aerobic end of the sequencing batch reactor, washed, dried at 105°C, ground, and passed through a 150-mesh sieve.
[0026] Eggshells with a calcium carbonate content of approximately 90% were washed, dried at 105°C, ground, and passed through a 150-mesh sieve.
[0027] Eggshell powder is thoroughly mixed with dried aerobic granular sludge at a mass ratio of 5%-20% to achieve Ca / P ratios of 29:28, 32:25, 33:23, and 5:3.
[0028] The mixture was pyrolyzed in a tube furnace at a temperature of 300-900℃, a heating rate of 5℃ / min, and a pyrolysis time of 2h. After pyrolysis, the mixture was allowed to cool naturally to obtain the pyrolysis product. N2 was introduced during the pyrolysis process at a flow rate of 100mL / min.
[0029] The speciation and content of phosphorus in pyrolysis products were determined using surface metallography (SMT).
[0030] The slow-release performance of phosphorus pyrolysis products was analyzed by using 2% citric acid to simulate soil pH.
[0031] Thirdly, this application provides sludge char prepared by the above method and phosphate fertilizer containing the sludge char. Specifically, as the Ca:P ratio increases from 29:28, 32:25, and 33:23 to 5:3 (eggshell addition ratio increases from 0% to 20%), the proportion of apatite phosphorus continuously increases at 300 °C and 500 °C without a significant plateau, and the conversion effect of non-apatite phosphorus at 300 °C is slightly better than at 500 °C. At 700 °C, the apatite phosphorus content is highest when 15% eggshell is added (69.21 mgP / g), accounting for 0.94% of the total phosphorus. At 900 °C, the apatite phosphorus content reaches its highest level (77.69 mgP / g) when 10% eggshell is added, also accounting for the largest proportion of total phosphorus (0.99%). Peat containing 20% eggshells at 300 ℃ and 900 ℃ can promote the growth of plant morphology (plant height, root length) and biomass.
[0032] The beneficial effects of this invention are mainly reflected in: The addition of eggshell powder not only serves as a calcium source but also reduces the melting and clumping of sludge, resulting in biochar-based products with more developed pore structures and more stable properties.
[0033] The addition of eggshells can promote the transformation of phosphorus forms in aerobic granular sludge at low temperatures. By adding eggshell powder, it was found that at 300℃, the proportion of apatite phosphorus in the total phosphorus increased significantly from 27.19% to 61.93%; at 500℃, the proportion of apatite phosphorus increased from 36.19% to 65.46%, and hydroxyapatite was formed.
[0034] This invention cleverly utilizes the potential advantages of low-temperature pyrolysis in retaining nitrogen and organic matter, and overcomes the disadvantage of low phosphorus availability by adding eggshell powder, thus achieving synergistic nutrient retention. Attached Figure Description
[0035] Figure 1 XRD analysis of the dried sludge used in the embodiments of the present invention. Detailed Implementation
[0036] Unless otherwise specified, the detection methods used in the following embodiments are all conventional methods in the art. For example, the SMT method developed under the framework of the European Committee for Standardization and Testing is used to detect the phosphorus speciation in prepared sludge or biochar: total phosphorus is measured by extracting the supernatant with 3.5 mol / L HCl after ignition; inorganic phosphorus is measured by extracting the supernatant with 1.0 mol / L HCl, and organic phosphorus is measured by extracting the residue with 1.0 mol / L HCl again after ignition; non-apatite phosphorus is measured by extracting with 1.0 mol / L NaOH and reacting the supernatant with 3.5 mol / L HCl, and apatite phosphorus is measured by reacting the residue with the supernatant of 1.0 mol / L HCl.
[0037] For example, 2% citric acid was used to simulate soil pH to analyze the slow-release performance of phosphorus, a pyrolysis product.
[0038] All reagents and materials used were commercially available or could be further prepared. For example, the eggshells used came from a school cafeteria, and the dried eggshells contained approximately 90% calcium carbonate. Similarly, the AGS used were derived from the aerobic end of the sequencing batch reactor's operating cycle (approximately 200 days of operation; the phosphorus concentration in the influent to the sequencing batch reactor was 20 mg / L, and polyphosphate-accumulating bacteria participated in AGS formation), with a sludge concentration of 8.42 g / L and an MLVSS / MLSS ratio of 0.81. The XRD results of the dried sludge are shown below. Figure 1 As shown, the SMT method determined that the phosphorus content was 26.76% apatite-based inorganic phosphorus, 54.42% non-apatite-based inorganic phosphorus, and 18.82% organic phosphorus. The main indicators are as follows: Example 1
[0039] The granular sludge from the aerobic end is dried in an oven, ground, and then passed through a 150-mesh sieve. After cleaning and drying, the eggshells are ground and passed through a 150-mesh sieve. The sieved eggshells and aerobic granular sludge were thoroughly mixed at a mass ratio of 1:19 to achieve a Ca / P ratio of 29:28. The mixture was pyrolyzed in a tube furnace at a heating rate of 5℃ / min, and then cooled after pyrolysis at 300℃ for 2 hours. Nitrogen gas was introduced at a rate of 100mL / min throughout the process to ensure an oxygen-free environment.
[0040] Analysis of the pyrolysis products showed that, at 300℃, with 5% eggshell powder added, the yield of the pyrolysis products was 72.93%, the total phosphorus content was 76.37 mg / g, the apatite phosphorus content was 25.86 mg / g, and the apatite phosphorus accounted for 33.86% of the total phosphorus; the proportion of total nutrients (nitrogen, phosphorus, and potassium) was approximately 12.05%. The cumulative phosphorus release from the pyrolysis products on day 21 was measured to be 16.37 mg / g. Example 2
[0041] The granular sludge from the aerobic end is dried in an oven, ground, and then passed through a 150-mesh sieve. After cleaning and drying, the eggshells are ground and passed through a 150-mesh sieve. The sieved eggshells and aerobic granular sludge were thoroughly mixed at a mass ratio of 1:9 to achieve a Ca / P ratio of 32:25. The mixture was pyrolyzed in a tube furnace at a heating rate of 5℃ / min, and then cooled after pyrolysis at 300℃ for 2 hours. Nitrogen gas was introduced at a rate of 100mL / min throughout the process to ensure an oxygen-free environment.
[0042] Analysis of the pyrolysis products showed that at 300℃, with 10% eggshell powder added, the yield of the pyrolysis products was 73.10%, the total phosphorus content was 68.56 mg / g, the apatite phosphorus content was 28.65 mg / g, and the apatite phosphorus accounted for 43.27% of the total phosphorus; the proportion of total nutrients (nitrogen, phosphorus, and potassium) was approximately 11.27%. The cumulative phosphorus release from the pyrolysis products on day 21 was measured to be 14.30 mg / g. Example 3
[0043] The granular sludge from the aerobic end is dried in an oven, ground, and then passed through a 150-mesh sieve. After cleaning and drying, the eggshells are ground and passed through a 150-mesh sieve. The sieved eggshells and aerobic granular sludge were thoroughly mixed at a mass ratio of 3:17 to make the Ca / P ratio 33:23. The mixture was pyrolyzed in a tube furnace at a heating rate of 5℃ / min, and then cooled after pyrolysis at 300℃ for 2 hours. Nitrogen gas was introduced at a rate of 100mL / min throughout the process to ensure an oxygen-free environment.
[0044] Analysis of the pyrolysis products showed that, at 300℃, with 15% eggshell powder added, the yield of the pyrolysis products was 74.94%, the total phosphorus content was 64.87 mg / g, the apatite phosphorus content was 27.98 mg / g, and the apatite phosphorus accounted for 43.33% of the total phosphorus; the proportion of total nutrients (nitrogen, phosphorus, and potassium) was approximately 10.90%. The cumulative phosphorus release from the pyrolysis products on day 21 was measured to be 8.43 mg / g. Example 4
[0045] The granular sludge from the aerobic end is dried in an oven, ground, and then passed through a 150-mesh sieve. After cleaning and drying, the eggshells are ground and passed through a 150-mesh sieve. The sieved eggshells and aerobic granular sludge were thoroughly mixed at a mass ratio of 1:4 to achieve a Ca / P ratio of 5:3. The mixture was pyrolyzed in a tube furnace at a heating rate of 5℃ / min, and then cooled after pyrolysis at 300℃ for 2 hours. Nitrogen gas was introduced at a rate of 100mL / min throughout the process to ensure an oxygen-free environment.
[0046] Analysis of the pyrolysis products showed that at 300℃, with 20% eggshell powder added, the yield of the pyrolysis products was 75.78%, the total phosphorus content was 59.43 mg / g, the apatite phosphorus content was 36.21 mg / g, and the apatite phosphorus accounted for 61.93% of the total phosphorus; the proportion of total nutrients (nitrogen, phosphorus, and potassium) was approximately 10.35%. The cumulative phosphorus release from the pyrolysis products on day 21 was measured to be 5.79 mg / g.
[0047] Example 5 The granular sludge from the aerobic end is dried in an oven, ground, and then passed through a 150-mesh sieve. After cleaning and drying, the eggshells are ground and passed through a 150-mesh sieve. The sieved eggshells and aerobic granular sludge were thoroughly mixed at a mass ratio of 1:19 to achieve a Ca / P ratio of 29:28. The mixture was pyrolyzed in a tube furnace at a heating rate of 5℃ / min, and then cooled after pyrolysis at 500℃ for 2 hours. Nitrogen gas was introduced at a rate of 100mL / min throughout the process to ensure an oxygen-free environment.
[0048] Analysis of the pyrolysis products showed that, at 500℃, the yield of pyrolysis products with 5% eggshell powder was 62.60%, the total phosphorus content was 78.31 mg / g, the apatite phosphorus content was 32.96 mg / g, and the apatite phosphorus accounted for 42.09% of the total phosphorus; the proportion of total nutrients (nitrogen, phosphorus, and potassium) was approximately 9.76%. The cumulative phosphorus release from the pyrolysis products on day 21 was measured to be 11.72 mg / g. Example 6
[0049] The granular sludge from the aerobic end is dried in an oven, ground, and then passed through a 150-mesh sieve. After cleaning and drying, the eggshells are ground and passed through a 150-mesh sieve. The sieved eggshells and aerobic granular sludge were thoroughly mixed at a mass ratio of 1:9 to achieve a Ca / P ratio of 32:25. The mixture was pyrolyzed in a tube furnace at a heating rate of 5℃ / min, and then cooled after pyrolysis at 500℃ for 2 hours. Nitrogen gas was introduced at a rate of 100mL / min throughout the process to ensure an oxygen-free environment.
[0050] Analysis of the pyrolysis products showed that at 500℃, with 10% eggshell powder added, the yield of the pyrolysis products was 63.26%, the total phosphorus content was 72.68 mg / g, the apatite phosphorus content was 37.78 mg / g, and the apatite phosphorus accounted for 51.98% of the total phosphorus; the proportion of total nutrients (nitrogen, phosphorus, and potassium) was approximately 9.21%. The cumulative phosphorus release from the pyrolysis products on day 21 was measured to be 8.46 mg / g. Example 7
[0051] The granular sludge from the aerobic end is dried in an oven, ground, and then passed through a 150-mesh sieve. After cleaning and drying, the eggshells are ground and passed through a 150-mesh sieve. The sieved eggshells and aerobic granular sludge were thoroughly mixed at a mass ratio of 3:17 to make the Ca / P ratio 33:23. The mixture was pyrolyzed in a tube furnace at a heating rate of 5℃ / min, and then cooled after pyrolysis at 500℃ for 2 hours. Nitrogen gas was introduced at a rate of 100mL / min throughout the process to ensure an oxygen-free environment.
[0052] Analysis of the pyrolysis products showed that at 500℃, with 15% eggshell powder added, the yield of the pyrolysis products was 66.58%, the total phosphorus content was 66.55 mg / g, the apatite phosphorus content was 43.32 mg / g, and the apatite phosphorus accounted for 65.09% of the total phosphorus. Hydroxyapatite formation was also detected. The proportion of total nutrients (nitrogen, phosphorus, and potassium) was approximately 8.59%. The cumulative phosphorus release from the pyrolysis products on day 21 was measured to be 6.95 mg / g. Example 8
[0053] The granular sludge from the aerobic end is dried in an oven, ground, and then passed through a 150-mesh sieve. After cleaning and drying, the eggshells are ground and passed through a 150-mesh sieve. The sieved eggshells and aerobic granular sludge were thoroughly mixed at a mass ratio of 1:4 to achieve a Ca / P ratio of 5:3. The mixture was pyrolyzed in a tube furnace at a heating rate of 5℃ / min, and then cooled after pyrolysis at 500℃ for 2 hours. Nitrogen gas was introduced at a rate of 100mL / min throughout the process to ensure an oxygen-free environment.
[0054] Analysis of the pyrolysis products showed that at 500℃, with 20% eggshell powder added, the yield of the pyrolysis products was 66.69%, the total phosphorus content was 61.98 mg / g, the apatite phosphorus content was 40.57 mg / g, and the apatite phosphorus accounted for 65.46% of the total phosphorus. Hydroxyapatite formation was also detected. The proportion of total nutrients (nitrogen, phosphorus, and potassium) was approximately 8.13%. The cumulative phosphorus release from the pyrolysis products on day 21 was measured to be 4.37 mg / g.
[0055] Comparative Example 1 Select the aerobic end, take granular sludge, dry it in an oven, grind it, and then pass it through a 150-mesh sieve; Aerobic granular sludge powder is pyrolyzed in a tubular furnace at a heating rate of 5℃ / min, and then cooled after pyrolysis at 300℃ for 2 hours. Nitrogen gas is introduced at a rate of 100mL / min throughout the process to ensure an anaerobic environment.
[0056] Analysis of the pyrolysis products showed that, without the addition of eggshell powder, the yield of the pyrolysis products of aerobic granular sludge at 300℃ was 72.35%, the total phosphorus content was 81.29 mg / g, the apatite phosphorus content was 22.11 mg / g, and the apatite phosphorus accounted for 27.19% of the total phosphorus; the proportion of total nutrients (nitrogen, phosphorus, and potassium) was approximately 12.54%. The cumulative phosphorus release from the pyrolysis products was measured to be 19.43 mg / g on day 21. Comparative Example 2
[0057] Select the aerobic end, take granular sludge, dry it in an oven, grind it, and then pass it through a 150-mesh sieve; Aerobic granular sludge powder is pyrolyzed in a tubular furnace at a heating rate of 5℃ / min, and then cooled after pyrolysis at 500℃ for 2 hours. Nitrogen gas is introduced at a rate of 100mL / min throughout the process to ensure an anaerobic environment.
[0058] Analysis of the pyrolysis products showed that, without the addition of eggshell powder, the yield of the pyrolysis products of aerobic granular sludge at 500℃ was 62.05%, the total phosphorus content was 82.75 mg / g, the apatite phosphorus content was 29.95 mg / g, and the apatite phosphorus accounted for 36.19% of the total phosphorus; the proportion of total nutrients (nitrogen, phosphorus, and potassium) was approximately 11.21%. The cumulative phosphorus release from the pyrolysis products was measured to be 16.74 mg / g on day 21.
Claims
1. A method for treating aerobic granular sludge (AGS) includes mixing eggshell powder with AGS sludge at a Ca / P molar ratio of 1-2:1, and then pyrolyzing the resulting mixture under an inert atmosphere to obtain sludge char at a pyrolysis temperature of 300-900℃, 300-700℃, or 300-500℃.
2. The method as described in claim 1, characterized in that: The AGS sludge used contains polyphosphate. If it is the sludge at the end of the aerobic phase in the operation cycle of the sequencing batch reactor, the polyphosphate content in the total phosphorus of the sludge can be 20-50 wt%, such as 26.4 wt%.
3. The method as described in any of the prior claims, characterized in that, The content of apatite phosphorus in the total phosphorus of AGS sludge is 10%-30%, such as 26.76 wt%.
4. The method as described in claim 1, characterized in that, The phosphorus content of AGS sludge is 6-10% of the dry weight of the sludge, such as 8.44%.
5. The method as described in claim 1, characterized in that: The volatile matter content (VSS / SS) of the AGS sludge used is 0.7-0.82; it can be 0.
81.
6. The method as described in claim 1, characterized in that: The Ca / P ratio is 29:28, 32:25, 33:23 or 5:
3.
7. The method as described in claim 1, characterized in that: The pyrolysis heating rate is 5-20℃ / min, and the pyrolysis time can be 1-3h, such as 2h.
8. The method as described in claim 1, characterized in that: Before mixing, a pretreatment process was performed, which consisted of drying, pulverizing, and sieving in sequence.
9. Sludge char prepared by the method of any of the prior claims.
10. A phosphate fertilizer containing the sludge carbon as described in the preceding claim.