Carbon source recovery method for sludge reduction and sewage treatment

Abstract

This invention discloses a method for sludge reduction and carbon source recovery for wastewater treatment, comprising the following steps: adding a calcium-based conditioner to the remaining sludge, stirring and conditioning, and then performing a primary hot water hydrolysis reaction at a temperature of 100-150°C; dewatering the sludge after the primary hot water hydrolysis reaction to obtain a primary sludge dissolving liquid and primary dewatered sludge; adding sodium hydroxide to the primary dewatered sludge, stirring and conditioning, and then performing a secondary hot water hydrolysis reaction at a temperature of 130-170°C; dewatering the sludge after the secondary hot water hydrolysis reaction to obtain sludge with low water content and a secondary sludge dissolving liquid that can be used as a carbon source for wastewater treatment. This invention, through secondary alkaline hot water hydrolysis treatment with a calcium-based conditioner and sodium hydroxide, can obtain sludge with a water content of less than 40% and a secondary sludge dissolving liquid with high COD and low total nitrogen, making the secondary sludge dissolving liquid more suitable as a carbon source for wastewater treatment.

Description

Technical Field

[0001] This invention relates to the field of sludge resource utilization and disposal, and in particular to a method for sludge reduction and carbon source recovery for wastewater treatment. Background Technology

[0002] Sludge, a byproduct of wastewater treatment, accumulates 30%–50% of the organic matter and other pollutants in the wastewater, possessing both "pollution" and "resource" attributes. If sludge is not effectively treated, it will cause serious environmental pollution and resource waste. According to data from the China Environmental Statistics Yearbook, sludge production exceeded 70 million tons in 2022. Sludge hydrolysis involves treating excess sludge under specific temperature and pressure conditions, breaking down sludge flocs and cell structures, and dissolving the organic matter within the sludge. To improve the efficiency of hydrolysis, alkali is added to the sludge. The sludge dissolution liquid after hydrolysis or alkaline hydrolysis contains a high concentration of organic matter, which can serve as a carbon source for wastewater treatment systems. Furthermore, the treated sludge can be dewatered mechanically to reduce its moisture content to less than 40%, achieving good sludge reduction and resource recovery effects.

[0003] Currently, the COD of sludge disintegration liquid obtained through high-temperature hot water hydrolysis and alkaline hot water hydrolysis can reach over 20,000, but the Kjeldahl nitrogen (ammonia nitrogen and organic nitrogen) concentration is also typically as high as several thousand. When carbon sources are used in wastewater treatment, they are mainly intended to provide carbon for the denitrification process to remove nitrogen from wastewater. However, when sludge disintegration liquid is used as a carbon source, it introduces a high concentration of nitrogen, increasing the denitrification load on the wastewater treatment system and leading to an increase in the total nitrogen concentration in the effluent, which contradicts the original purpose of adding a carbon source. Summary of the Invention

[0004] This invention aims to overcome the problem that existing technologies introduce high concentrations of nitrogen when using sludge dissolving liquid as a carbon source, increasing the denitrification load on wastewater treatment systems. It provides a method for sludge reduction and carbon source recovery in wastewater treatment. Through secondary alkaline hydrolysis treatment with calcium-based conditioning agents and sodium hydroxide, sludge with a water content of less than 40% and a secondary sludge dissolving liquid containing high COD and low total nitrogen can be obtained, making the secondary sludge dissolving liquid more suitable as a carbon source for wastewater treatment.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A method for sludge reduction and carbon source recovery in wastewater treatment includes the following steps: (1) First alkaline hot water hydrolysis: Add calcium-based conditioner to the remaining sludge, stir and condition it, and then carry out a first hot water hydrolysis reaction at a reaction temperature of 100-150℃. (2) Primary dewatering: The sludge after the primary hot hydrolysis reaction is dewatered to obtain primary sludge decomposition liquid and primary dewatered sludge; (3) Secondary alkaline hot water hydrolysis: Add sodium hydroxide to the primary dewatered sludge, stir and adjust, and then carry out secondary hot water hydrolysis reaction at a reaction temperature of 130-170℃. (4) Secondary dewatering: The sludge after the secondary hot hydrolysis reaction is dewatered to obtain sludge with low water content and secondary sludge dewatering liquid that can be used as a carbon source for sewage treatment.

[0006] This invention first adds a calcium-based conditioner to the remaining sludge for a single alkaline hot hydrolysis. During this single hydrolysis, the sludge flocs and cell structure are destroyed, causing the organic matter in the sludge to dissolve. Simultaneously, with the help of calcium ions, the dissolved organic matter in the sludge can undergo flocculation and be transformed back into sludge flocs. Furthermore, the calcium... 2+ The flocculation of Ca is selective. 2+ The sludge exhibits extremely high binding capacity to EPS (extracellular polymeric substances) in sludge, with strong binding to polysaccharide hydroxyl groups and weaker binding to protein amide groups. Therefore, the primary sludge dissolution solution contains a high concentration of nitrogen while leaching relatively low COD. Then, during the secondary alkaline hydrolysis, NaOH is added. Its alkaline hydrolysis enhances the sludge dissolution capacity and avoids calcium ion re-flocculation, allowing for the re-dissolution of organic matter from the sludge. However, because the sludge dissolution solution after the primary hydrolysis removes a large amount of nitrogen, the secondary dissolution solution obtained by this invention is characterized by high COD and low total nitrogen, making it more suitable as a carbon source for wastewater treatment.

[0007] In this invention, different reaction temperatures are used in the primary and secondary alkaline hydrolysis processes. During the primary alkaline hydrolysis, although calcium ions have a re-flocculation effect, organic matter will still dissolve. If the reaction temperature is too high, the organic matter concentration in the primary sludge dissolution liquid will be high, which is not conducive to the subsequent carbon source recovery of the secondary sludge dissolution liquid. In the secondary alkaline hydrolysis, the organic matter in the sludge is released as much as possible, so the temperature is slightly higher than that of the primary alkaline hydrolysis.

[0008] Preferably, the calcium-based conditioner mentioned in step (1) is calcium hydroxide and / or calcium oxide, and the amount of calcium-based conditioner added is 10% to 30% of the dry weight of the sludge. If the amount of calcium-based conditioner added is too low, the effect on sludge disintegration will be poor; while if the amount of calcium-based conditioner added is too high, the amount of dewatered sludge produced will increase, which is not conducive to sludge reduction.

[0009] Preferably, the stirring and conditioning time in step (1) is 10 to 20 minutes.

[0010] Preferably, the time for the first hot hydrolysis reaction in step (1) is 30 to 60 minutes.

[0011] Preferably, in step (1), water is added first to adjust the moisture content of the remaining sludge to 95±3wt%, and then a calcium-based conditioner is added. Adjusting the moisture content of the sludge to this range can maintain its fluidity and facilitate subsequent operations.

[0012] Preferably, the amount of sodium hydroxide added in step (3) is 5% to 20% of the dry weight of the sludge. If the amount of sodium hydroxide added is too low, the sludge disintegration effect will be poor, which is not conducive to the recovery of carbon source; while if the amount of sodium hydroxide added is too high, the pH of the secondary sludge disintegration liquid will be too high, which is not conducive to the subsequent utilization of carbon source.

[0013] Preferably, the stirring and conditioning time in step (3) is 10 to 20 minutes.

[0014] Preferably, the time for the secondary hot hydrolysis reaction in step (3) is 30 to 60 minutes.

[0015] Preferably, in step (3), water is first added to the primary dewatered sludge to adjust its moisture content to 95±3wt%, and then sodium hydroxide is added.

[0016] Therefore, the present invention has the following beneficial effects: First, a calcium-based conditioner is added for a first alkaline hot hydrolysis. By utilizing the re-flocculation effect of calcium ions, nitrogen elements in the sludge can be removed through the first sludge dissolving liquid during the first alkaline hot hydrolysis process. Then, sodium hydroxide is added for a second alkaline hot hydrolysis to obtain a second sludge dissolving liquid with high COD and low total nitrogen characteristics, which is more suitable as a carbon source for wastewater treatment. Detailed Implementation

[0017] The present invention will be further described below with reference to specific embodiments.

[0018] In this invention, unless otherwise specified, all equipment and raw materials are commercially available or commonly used in the industry. The methods described in the following embodiments are conventional methods in the art, unless otherwise specified. The residual sludge used in the embodiments of this invention originated from a municipal wastewater treatment plant in Hangzhou.

[0019] General Implementation Examples: A method for sludge reduction and carbon source recovery in wastewater treatment includes the following steps: (1) First alkaline hot water hydrolysis: Add water to adjust the moisture content of the remaining sludge to 95±3wt%, add calcium-based conditioner, which is calcium hydroxide and / or calcium oxide, and the amount of calcium-based conditioner added is 10% to 30% of the dry weight of the sludge; after stirring and conditioning for 10 to 20 minutes, carry out a first hot water hydrolysis reaction at a reaction temperature of 100 to 150℃ and a reaction time of 30 to 60 minutes; (2) Primary dehydration: Dehydrate the sludge after the primary hot hydrolysis reaction to obtain primary sludge dissolving liquid and primary dehydrated sludge; (3) Secondary alkaline hot hydrolysis: Add water to the primary dehydrated sludge to adjust its moisture content to 95±3wt%, and then add sodium hydroxide. The amount of sodium hydroxide added is 5% to 20% of the dry weight of the sludge; after stirring and conditioning for 10 to 20 minutes, carry out the secondary hot hydrolysis reaction at a reaction temperature of 130 to 170℃ and a reaction time of 30 to 60 minutes; (4) Secondary dewatering: The sludge after the secondary hot hydrolysis reaction is dewatered to obtain sludge with low water content and secondary sludge dewatering liquid that can be used as a carbon source for sewage treatment.

[0020] Example 1: A method for sludge reduction and carbon source recovery in wastewater treatment includes the following steps: (1) Primary alkaline hot water hydrolysis: Transfer the remaining sludge to a sludge conditioning tank, add water to adjust the moisture content of the remaining sludge to 95 wt%, add calcium hydroxide, the amount of calcium hydroxide added is 10% of the dry weight of the sludge; stir and condition for 15 min, and then send the conditioned sludge to the primary hot water hydrolysis reactor through a pipeline pump, heat and stir at 120℃ for 40 min to carry out the primary hot water hydrolysis reaction; (2) Primary dehydration: The sludge after the primary hot hydrolysis reaction is dehydrated to obtain primary sludge dissolving liquid and primary dehydrated sludge; (3) Secondary alkaline hot hydrolysis: The primary dehydrated sludge is transferred to the secondary conditioning tank by a screw pump, water is added to the primary dehydrated sludge to adjust its moisture content to 95wt%, and then sodium hydroxide is added. The amount of sodium hydroxide added is 10% of the dry weight of the sludge; the mixture is stirred and conditioned for 15min, and then the conditioned sludge is sent to the secondary hot hydrolysis reactor by a pipeline pump. The mixture is heated and stirred at 150℃ for 40min to carry out the secondary hot hydrolysis reaction; (4) Secondary dewatering: The sludge after the secondary hot hydrolysis reaction is mechanically dewatered to obtain sludge with a water content of <40wt% and secondary sludge dewatering liquid that can be used as a carbon source for sewage treatment.

[0021] Example 2: A method for sludge reduction and carbon source recovery in wastewater treatment includes the following steps: (1) Primary alkaline hot water hydrolysis: Transfer the remaining sludge to a sludge conditioning tank, add water to adjust the moisture content of the remaining sludge to 92 wt%, add calcium hydroxide, the amount of calcium hydroxide added is 15% of the dry weight of the sludge; stir and condition for 20 min, and then send the conditioned sludge to a primary hot water hydrolysis reactor through a pipeline pump, heat and stir at 100℃ for 60 min to carry out the primary hot water hydrolysis reaction; (2) Primary dehydration: The sludge after the primary hot hydrolysis reaction is dehydrated to obtain primary sludge dissolving liquid and primary dehydrated sludge; (3) Secondary alkaline hot hydrolysis: The primary dehydrated sludge is transferred to the secondary conditioning tank by a screw pump, water is added to the primary dehydrated sludge to adjust its moisture content to 93wt%, and then sodium hydroxide is added. The amount of sodium hydroxide added is 5% of the dry weight of the sludge; the mixture is stirred and conditioned for 10 minutes, and then the conditioned sludge is sent to the secondary hot hydrolysis reactor by a pipeline pump. The mixture is heated and stirred at 170℃ for 30 minutes to carry out the secondary hot hydrolysis reaction; (4) Secondary dewatering: The sludge after the secondary hot hydrolysis reaction is mechanically dewatered to obtain sludge with a water content of <40wt% and secondary sludge dewatering liquid that can be used as a carbon source for sewage treatment.

[0022] Example 3: A method for sludge reduction and carbon source recovery in wastewater treatment includes the following steps: (1) Primary alkaline hot water hydrolysis: Transfer the remaining sludge to a sludge conditioning tank, add water to adjust the moisture content of the remaining sludge to 96 wt%, add calcium hydroxide, the amount of calcium hydroxide added is 15% of the dry weight of the sludge; stir and condition for 10 min, and then send the conditioned sludge to a primary hot water hydrolysis reactor through a pipeline pump, heat and stir at 150℃ for 30 min to carry out the primary hot water hydrolysis reaction; (2) Primary dehydration: The sludge after the primary hot hydrolysis reaction is dehydrated to obtain primary sludge dissolving liquid and primary dehydrated sludge; (3) Secondary alkaline hot hydrolysis: The primary dehydrated sludge is transferred to the secondary conditioning tank by a screw pump, water is added to the primary dehydrated sludge to adjust its moisture content to 92wt%, and then sodium hydroxide is added. The amount of sodium hydroxide added is 10% of the dry weight of the sludge; the mixture is stirred and conditioned for 20 minutes, and then the conditioned sludge is sent to the secondary hot hydrolysis reactor by a pipeline pump. The mixture is heated and stirred at 130℃ for 60 minutes to carry out the secondary hot hydrolysis reaction; (4) Secondary dewatering: The sludge after the secondary hot hydrolysis reaction is mechanically dewatered to obtain sludge with a water content of <40wt% and secondary sludge dewatering liquid that can be used as a carbon source for sewage treatment.

[0023] Example 4: A method for sludge reduction and carbon source recovery in wastewater treatment includes the following steps: (1) Primary alkaline hot water hydrolysis: Transfer the remaining sludge to a sludge conditioning tank, add water to adjust the moisture content of the remaining sludge to 95 wt%, add calcium oxide, the amount of calcium oxide added is 20% of the dry weight of the sludge; stir and condition for 15 min, and then send the conditioned sludge to the primary hot water hydrolysis reactor through a pipeline pump, heat and stir at 110℃ for 45 min to carry out the primary hot water hydrolysis reaction; (2) Primary dehydration: The sludge after the primary hot hydrolysis reaction is dehydrated to obtain primary sludge dissolving liquid and primary dehydrated sludge; (3) Secondary alkaline hot hydrolysis: The primary dehydrated sludge is transferred to the secondary conditioning tank by a screw pump, water is added to the primary dehydrated sludge to adjust its moisture content to 95wt%, and then sodium hydroxide is added. The amount of sodium hydroxide added is 20% of the dry weight of the sludge; the mixture is stirred and conditioned for 15min, and then the conditioned sludge is sent to the secondary hot hydrolysis reactor by a pipeline pump. The mixture is heated and stirred at 140℃ for 45min to carry out the secondary hot hydrolysis reaction; (4) Secondary dewatering: The sludge after the secondary hot hydrolysis reaction is mechanically dewatered to obtain sludge with a water content of <40wt% and secondary sludge dewatering liquid that can be used as a carbon source for sewage treatment.

[0024] Example 5: A method for sludge reduction and carbon source recovery in wastewater treatment includes the following steps: (1) Primary alkaline hot water hydrolysis: Transfer the remaining sludge to a sludge conditioning tank, add water to adjust the moisture content of the remaining sludge to 95 wt%, add calcium oxide, the amount of calcium oxide added is 30% of the dry weight of the sludge; stir and condition for 15 min, and then send the conditioned sludge to the primary hot water hydrolysis reactor through a pipeline pump, heat and stir at 130℃ for 35 min to carry out the primary hot water hydrolysis reaction; (2) Primary dehydration: The sludge after the primary hot hydrolysis reaction is dehydrated to obtain primary sludge dissolving liquid and primary dehydrated sludge; (3) Secondary alkaline hot hydrolysis: The primary dehydrated sludge is transferred to the secondary conditioning tank by a screw pump, water is added to the primary dehydrated sludge to adjust its moisture content to 95wt%, and then sodium hydroxide is added. The amount of sodium hydroxide added is 15% of the dry weight of the sludge; the mixture is stirred and conditioned for 15min, and then the conditioned sludge is sent to the secondary hot hydrolysis reactor by a pipeline pump. The mixture is heated and stirred at 160℃ for 35min to carry out the secondary hot hydrolysis reaction; (4) Secondary dewatering: The sludge after the secondary hot hydrolysis reaction is mechanically dewatered to obtain sludge with a water content of <40wt% and secondary sludge dewatering liquid that can be used as a carbon source for sewage treatment.

[0025] Comparative Example 1 (without calcium supplements): A method for sludge reduction and carbon source recovery in wastewater treatment includes the following steps: (1) Alkaline hot water hydrolysis: Transfer the remaining sludge to a sludge conditioning tank, add water to adjust the moisture content of the remaining sludge to 95 wt%, add sodium hydroxide, the amount of sodium hydroxide added is 10% of the dry weight of the sludge; stir and condition for 15 min, and then send the conditioned sludge to a primary hot water hydrolysis reactor through a pipeline pump, heat and stir at 150℃ for 40 min to carry out the hot water hydrolysis reaction; (2) Dewatering: The sludge after the hot hydrolysis reaction is dewatered to obtain sludge with a water content of <40wt% and sludge dissolving liquid that can be used as a carbon source for sewage treatment.

[0026] Comparative Example 2 (without sodium hydroxide): A method for sludge reduction and carbon source recovery in wastewater treatment includes the following steps: (1) Alkaline hot water hydrolysis: Transfer the remaining sludge to a sludge conditioning tank, add water to adjust the moisture content of the remaining sludge to 95 wt%, add calcium hydroxide, the amount of calcium hydroxide added is 10% of the dry weight of the sludge; stir and condition for 15 min, and then send the conditioned sludge to a primary hot water hydrolysis reactor through a pipeline pump, heat and stir at 120℃ for 40 min to carry out the hot water hydrolysis reaction; (2) Dewatering: The sludge after the hot hydrolysis reaction is dewatered to obtain sludge with a water content of <40wt% and sludge dissolving liquid that can be used as a carbon source for sewage treatment.

[0027] Comparative Example 3 (calcium hydroxide and sodium hydroxide were added simultaneously): A method for sludge reduction and carbon source recovery in wastewater treatment includes the following steps: (1) Alkaline hot water hydrolysis: Transfer the remaining sludge to a sludge conditioning tank, add water to adjust the moisture content of the remaining sludge to 95 wt%, add calcium hydroxide and sodium hydroxide, the amount of calcium hydroxide added is 10% of the dry weight of the sludge, and the amount of sodium hydroxide added is 10% of the dry weight of the sludge; stir and condition for 15 min, and then send the conditioned sludge to the hot water hydrolysis reactor through a pipeline pump. First, heat and stir at 120℃ for 40 min, and then heat and stir at 150℃ for 40 min to carry out the hot water hydrolysis reaction; (2) Dewatering: Mechanically dewater the sludge after the hot hydrolysis reaction to obtain sludge with a water content of <40wt% and sludge dewatering liquid.

[0028] Comparative Example 4 (changing the order of addition of calcium hydroxide and sodium hydroxide): A method for sludge reduction and carbon source recovery in wastewater treatment includes the following steps: (1) Primary alkaline hot water hydrolysis: Transfer the remaining sludge to a sludge conditioning tank, add water to adjust the moisture content of the remaining sludge to 95 wt%, add sodium hydroxide, the amount of sodium hydroxide added is 10% of the dry weight of the sludge; stir and condition for 15 min, and then send the conditioned sludge to the primary hot water hydrolysis reactor through a pipeline pump, heat and stir at 120℃ for 40 min to carry out the primary hot water hydrolysis reaction; (2) Primary dehydration: The sludge after the primary hot hydrolysis reaction is dehydrated to obtain primary sludge dissolving liquid and primary dehydrated sludge; (3) Secondary alkaline hot hydrolysis: The primary dehydrated sludge is transferred to the secondary conditioning tank by a screw pump, water is added to the primary dehydrated sludge to adjust its moisture content to 95wt%, and then calcium hydroxide is added. The amount of calcium hydroxide added is 10% of the dry weight of the sludge; the mixture is stirred and conditioned for 15min, and then the conditioned sludge is sent to the secondary hot hydrolysis reactor by a pipeline pump. The mixture is heated and stirred at 150℃ for 40min to carry out the secondary hot hydrolysis reaction; (4) Secondary dewatering: The sludge after the secondary hot hydrolysis reaction is mechanically dewatered to obtain sludge with a water content of <40wt% and secondary sludge dewatering liquid that can be used as a carbon source for sewage treatment.

[0029] Comparative Example 5 (the temperature of the primary hydrolysis reaction was too low): A method for sludge reduction and carbon source recovery in wastewater treatment includes the following steps: (1) Primary alkaline hot water hydrolysis: Transfer the remaining sludge to a sludge conditioning tank, add water to adjust the moisture content of the remaining sludge to 95 wt%, add calcium hydroxide, the amount of calcium hydroxide added is 10% of the dry weight of the sludge; stir and condition for 15 min, and then send the conditioned sludge to the primary hot water hydrolysis reactor through a pipeline pump, heat and stir at 80℃ for 40 min to carry out the primary hot water hydrolysis reaction; (2) Primary dehydration: The sludge after the primary hot hydrolysis reaction is dehydrated to obtain primary sludge dissolving liquid and primary dehydrated sludge; (3) Secondary alkaline hot hydrolysis: The primary dehydrated sludge is transferred to the secondary conditioning tank by a screw pump, water is added to the primary dehydrated sludge to adjust its moisture content to 95wt%, and then sodium hydroxide is added. The amount of sodium hydroxide added is 10% of the dry weight of the sludge; the mixture is stirred and conditioned for 15min, and then the conditioned sludge is sent to the secondary hot hydrolysis reactor by a pipeline pump. The mixture is heated and stirred at 150℃ for 40min to carry out the secondary hot hydrolysis reaction; (4) Secondary dewatering: The sludge after the secondary hot hydrolysis reaction is mechanically dewatered to obtain sludge with a water content of <40wt% and secondary sludge dewatering liquid that can be used as a carbon source for sewage treatment.

[0030] Comparative Example 6 (the temperature of the primary hydrolysis reaction was too high): A method for sludge reduction and carbon source recovery in wastewater treatment includes the following steps: (1) Primary alkaline hot water hydrolysis: Transfer the remaining sludge to a sludge conditioning tank, add water to adjust the moisture content of the remaining sludge to 95 wt%, add calcium hydroxide, the amount of calcium hydroxide added is 10% of the dry weight of the sludge; stir and condition for 15 min, and then send the conditioned sludge to a primary hot water hydrolysis reactor through a pipeline pump, heat and stir at 170℃ for 40 min to carry out the primary hot water hydrolysis reaction; (2) Primary dehydration: The sludge after the primary hot hydrolysis reaction is dehydrated to obtain primary sludge dissolving liquid and primary dehydrated sludge; (3) Secondary alkaline hot hydrolysis: The primary dehydrated sludge is transferred to the secondary conditioning tank by a screw pump, water is added to the primary dehydrated sludge to adjust its moisture content to 95wt%, and then sodium hydroxide is added. The amount of sodium hydroxide added is 10% of the dry weight of the sludge; the mixture is stirred and conditioned for 15min, and then the conditioned sludge is sent to the secondary hot hydrolysis reactor by a pipeline pump. The mixture is heated and stirred at 150℃ for 40min to carry out the secondary hot hydrolysis reaction; (4) Secondary dewatering: The sludge after the secondary hot hydrolysis reaction is mechanically dewatered to obtain sludge with a water content of <40wt% and secondary sludge dewatering liquid that can be used as a carbon source for sewage treatment.

[0031] Comparative Example 7 (Secondary hydrolysis reaction temperature too low): A method for sludge reduction and carbon source recovery in wastewater treatment includes the following steps: (1) Primary alkaline hot water hydrolysis: Transfer the remaining sludge to a sludge conditioning tank, add water to adjust the moisture content of the remaining sludge to 95 wt%, add calcium hydroxide, the amount of calcium hydroxide added is 10% of the dry weight of the sludge; stir and condition for 15 min, and then send the conditioned sludge to the primary hot water hydrolysis reactor through a pipeline pump, heat and stir at 120℃ for 40 min to carry out the primary hot water hydrolysis reaction; (2) Primary dehydration: The sludge after the primary hot hydrolysis reaction is dehydrated to obtain primary sludge dissolving liquid and primary dehydrated sludge; (3) Secondary alkaline hot hydrolysis: The primary dehydrated sludge is transferred to the secondary conditioning tank by a screw pump, water is added to the primary dehydrated sludge to adjust its moisture content to 95wt%, and then sodium hydroxide is added. The amount of sodium hydroxide added is 10% of the dry weight of the sludge; the mixture is stirred and conditioned for 15min, and then the conditioned sludge is sent to the secondary hot hydrolysis reactor by a pipeline pump. The mixture is heated and stirred at 120℃ for 40min to carry out the secondary hot hydrolysis reaction; (4) Secondary dewatering: The sludge after the secondary hot hydrolysis reaction is mechanically dewatered to obtain sludge with a water content of <40wt% and secondary sludge dewatering liquid that can be used as a carbon source for sewage treatment.

[0032] Comparative Example 8 (Secondary hydrolysis reaction temperature too high): A method for sludge reduction and carbon source recovery in wastewater treatment includes the following steps: (1) Primary alkaline hot water hydrolysis: Transfer the remaining sludge to a sludge conditioning tank, add water to adjust the moisture content of the remaining sludge to 95 wt%, add calcium hydroxide, the amount of calcium hydroxide added is 10% of the dry weight of the sludge; stir and condition for 15 min, and then send the conditioned sludge to the primary hot water hydrolysis reactor through a pipeline pump, heat and stir at 120℃ for 40 min to carry out the primary hot water hydrolysis reaction; (2) Primary dehydration: The sludge after the primary hot hydrolysis reaction is dehydrated to obtain primary sludge dissolving liquid and primary dehydrated sludge; (3) Secondary alkaline hot hydrolysis: The primary dehydrated sludge is transferred to the secondary conditioning tank by a screw pump, water is added to the primary dehydrated sludge to adjust its moisture content to 95wt%, and then sodium hydroxide is added. The amount of sodium hydroxide added is 10% of the dry weight of the sludge; the mixture is stirred and conditioned for 15min, and then the conditioned sludge is sent to the secondary hot hydrolysis reactor by a pipeline pump. The mixture is heated and stirred at 180℃ for 40min to carry out the secondary hot hydrolysis reaction; (4) Secondary dewatering: The sludge after the secondary hot hydrolysis reaction is mechanically dewatered to obtain sludge with a water content of <40wt% and secondary sludge dewatering liquid that can be used as a carbon source for sewage treatment.

[0033] Comparative Example 9 (Excessive Addition of Calcium-Based Conditioner): A method for sludge reduction and carbon source recovery in wastewater treatment includes the following steps: (1) Primary alkaline hot water hydrolysis: Transfer the remaining sludge to a sludge conditioning tank, add water to adjust the moisture content of the remaining sludge to 95 wt%, add calcium hydroxide, the amount of calcium hydroxide added is 40% of the dry weight of the sludge; stir and condition for 15 min, and then send the conditioned sludge to the primary hot water hydrolysis reactor through a pipeline pump, heat and stir at 120℃ for 40 min to carry out the primary hot water hydrolysis reaction; (2) Primary dehydration: The sludge after the primary hot hydrolysis reaction is dehydrated to obtain primary sludge dissolving liquid and primary dehydrated sludge; (3) Secondary alkaline hot hydrolysis: The primary dehydrated sludge is transferred to the secondary conditioning tank by a screw pump, water is added to the primary dehydrated sludge to adjust its moisture content to 95wt%, and then sodium hydroxide is added. The amount of sodium hydroxide added is 10% of the dry weight of the sludge; the mixture is stirred and conditioned for 15min, and then the conditioned sludge is sent to the secondary hot hydrolysis reactor by a pipeline pump. The mixture is heated and stirred at 150℃ for 40min to carry out the secondary hot hydrolysis reaction; (4) Secondary dewatering: The sludge after the secondary hot hydrolysis reaction is mechanically dewatered to obtain sludge with a water content of <40wt% and secondary sludge dewatering liquid that can be used as a carbon source for sewage treatment.

[0034] Comparative Example 10 (too much sodium hydroxide added): A method for sludge reduction and carbon source recovery in wastewater treatment includes the following steps: (1) Primary alkaline hot water hydrolysis: Transfer the remaining sludge to a sludge conditioning tank, add water to adjust the moisture content of the remaining sludge to 95 wt%, add calcium hydroxide, the amount of calcium hydroxide added is 10% of the dry weight of the sludge; stir and condition for 15 min, and then send the conditioned sludge to the primary hot water hydrolysis reactor through a pipeline pump, heat and stir at 120℃ for 40 min to carry out the primary hot water hydrolysis reaction; (2) Primary dehydration: The sludge after the primary hot hydrolysis reaction is dehydrated to obtain primary sludge dissolving liquid and primary dehydrated sludge; (3) Secondary alkaline hot hydrolysis: The primary dehydrated sludge is transferred to the secondary conditioning tank by a screw pump, water is added to the primary dehydrated sludge to adjust its moisture content to 95wt%, and then sodium hydroxide is added. The amount of sodium hydroxide added is 30% of the dry weight of the sludge; the mixture is stirred and conditioned for 15min, and then the conditioned sludge is sent to the secondary hot hydrolysis reactor by a pipeline pump. The mixture is heated and stirred at 150℃ for 40min to carry out the secondary hot hydrolysis reaction; (4) Secondary dewatering: The sludge after the secondary hot hydrolysis reaction is mechanically dewatered to obtain sludge with a water content of <40wt% and secondary sludge dewatering liquid that can be used as a carbon source for sewage treatment.

[0035] The COD and total nitrogen in the secondary sludge dissolving liquid or sludge dissolving liquid used as carbon sources in the above embodiments and comparative examples were tested, and the results are shown in Table 1.

[0036] Table 1: Performance test results of sludge dissolving fluid.

[0037] As can be seen from the above results, after treating the remaining sludge using the method of the present invention in Examples 1 to 5, the sludge moisture content can be reduced to below 40%, and a secondary sludge dissolving liquid with high COD and low total nitrogen can be obtained, which is suitable for use as a carbon source for sewage treatment, thereby realizing sludge reduction and carbon source recovery.

[0038] In Comparative Example 1, only sodium hydroxide was added for a single alkaline thermal treatment of the sludge. Due to the lack of calcium ion re-flocculation, the COD and total nitrogen of the sludge disintegration liquid were very high, but the carbon-to-nitrogen ratio was very low, making it unsuitable as a carbon source. In Comparative Example 2, only calcium hydroxide was added for a single alkaline thermal treatment of the sludge. Due to the selective re-flocculation effect of calcium ions, the COD of the sludge disintegration liquid was low, but the total nitrogen was very high, and the carbon-to-nitrogen ratio was very low, again making it unsuitable as a carbon source. In Comparative Example 3, calcium hydroxide and sodium hydroxide were added simultaneously for a single alkaline thermal treatment. Because the disintegration effect of sodium hydroxide was stronger than the re-flocculation effect of calcium ions, the COD and total nitrogen concentration of the sludge disintegration liquid were very high, and the carbon-to-nitrogen ratio was also low. In Comparative Example 4, the order of addition of calcium hydroxide and sodium hydroxide was changed. Sodium hydroxide was added first for a single alkaline thermal treatment, and then calcium hydroxide was added for a second alkaline thermal treatment. Because the first hot water hydrolysis disintegration effect was better, a large amount of organic matter was released, resulting in lower COD and total nitrogen in the second disintegration liquid, and a very small carbon-to-nitrogen ratio, which did not meet the requirements for carbon source use.

[0039] In Comparative Example 5, the temperature during the primary hot water hydrolysis reaction was too low. Due to the limited catalytic effect of calcium ions and their re-flocculation effect, the COD and total nitrogen of the secondary sludge decomposition liquid were very high, while the carbon-to-nitrogen ratio was low. In Comparative Example 6, the temperature during the primary hot water hydrolysis reaction was too high. Because the primary hot water hydrolysis released a large amount of organic matter, the COD and total nitrogen concentrations of the secondary sludge decomposition liquid were low, and the carbon-to-nitrogen ratio was low. In Comparative Example 7, the temperature during the secondary hot water hydrolysis reaction was too low. Due to the poor decomposition effect of the secondary hot water hydrolysis, the COD and total nitrogen concentrations of the secondary sludge decomposition liquid were very low, and the carbon-to-nitrogen ratio was low. In Comparative Example 8, the temperature during the secondary hot water hydrolysis reaction was too high. Because the decomposition effect of the secondary hot water hydrolysis was very significant, the COD and total nitrogen concentrations of the sludge decomposition liquid released by the secondary hot water hydrolysis were very high, and the carbon-to-nitrogen ratio was very low.

[0040] In Comparative Example 9, excessive calcium hydroxide was added during the primary hot water hydrolysis reaction, leading to a significant increase in the amount of sludge after hydrolysis due to the excess calcium ions. In Comparative Example 10, excessive sodium hydroxide was added during the secondary hot water hydrolysis reaction. Since its catalytic hydrolysis effect has an upper limit, the COD and total nitrogen concentrations of the secondary sludge hydrolysis solution did not increase significantly, and the carbon-to-nitrogen ratio was low. However, the cost of conditioning agents would increase significantly, and the pH of the secondary sludge hydrolysis solution would be high, which is not conducive to the subsequent use of carbon sources.

Claims

1. A method for sludge reduction and carbon source recovery in wastewater treatment, characterized in that, Includes the following steps: (1) First alkaline hot water hydrolysis: Add calcium-based conditioner to the remaining sludge, stir and condition it, and then carry out a first hot water hydrolysis reaction at a reaction temperature of 100~150℃; (2) Primary dewatering: The sludge after the primary hot hydrolysis reaction is dewatered to obtain primary sludge dissolving liquid and primary dewatered sludge; (3) Secondary alkaline hot water hydrolysis: Sodium hydroxide is added to the primary dewatered sludge, and after stirring and conditioning, a secondary hot water hydrolysis reaction is carried out at a reaction temperature of 130~170℃; (4) Secondary dewatering: The sludge after the secondary hot hydrolysis reaction is dewatered to obtain sludge with low water content and secondary sludge dewatering liquid that can be used as a carbon source for sewage treatment.

2. The method for sludge reduction and carbon source recovery for wastewater treatment according to claim 1, characterized in that, The calcium-based conditioner mentioned in step (1) is calcium hydroxide and / or calcium oxide, and the amount of calcium-based conditioner added is 10% to 30% of the dry weight of sludge.

3. The method for sludge reduction and carbon source recovery for wastewater treatment according to claim 1 or 2, characterized in that, The stirring and conditioning time in step (1) is 10~20 minutes.

4. The method for sludge reduction and carbon source recovery for wastewater treatment according to claim 1 or 2, characterized in that, The time for one hot hydrolysis reaction in step (1) is 30~60 min.

5. The method for sludge reduction and carbon source recovery for wastewater treatment according to claim 1 or 2, characterized in that, In step (1), water is added first to adjust the moisture content of the remaining sludge to 95±3wt%, and then calcium-based conditioner is added.

6. The method for sludge reduction and carbon source recovery for wastewater treatment according to claim 1, characterized in that, In step (3), the amount of sodium hydroxide added is 5% to 20% of the dry weight of the sludge.

7. The method for sludge reduction and carbon source recovery for wastewater treatment according to claim 1 or 6, characterized in that, The stirring and conditioning time in step (3) is 10~20 minutes.

8. The method for sludge reduction and carbon source recovery for wastewater treatment according to claim 1 or 6, characterized in that, The time for the secondary hot hydrolysis reaction in step (3) is 30~60 min.

9. The method for sludge reduction and carbon source recovery for wastewater treatment according to claim 1 or 6, characterized in that, In step (3), water is first added to the primary dewatered sludge to adjust its moisture content to 95±3wt%, and then sodium hydroxide is added.

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