Method for recycling sludge

Abstract

The invention discloses a method for recycling sludge. The method comprises the following steps: conveying low-phosphorus sewage in a water distribution tank and calcium hydroxide emulsion in a first lime emulsion tank to a first reaction tank for precipitation; after precipitation, feeding the sewage into a clarification tank, and adding a flocculant for secondary precipitation; sludge is collected and sent to a sludge concentration tank, and supernate and concentrated sludge containing calcium hydroxide are obtained; treating the supernate through a first neutralizing tank, a softening tank, a second neutralizing tank and an intermediate water tank in sequence; and the concentrated sludge is sent to a second lime emulsion tank of the second sewage treatment system as a calcium hydroxide supply source. The method can reduce sludge discharge, relieve filling contradiction, recover effective calcium hydroxide in the sludge, reduce medicament cost, improve system collaboration and efficiency, and realize cost reduction, efficiency improvement and resource circulation of sewage treatment.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sewage treatment, in particular to a sludge recycling method. BACKGROUND

[0002] In the production and operation process of mineral fertilizer, the treatment of phosphorus and fluorine-containing sewage is a key link to ensure that the factory environment meets the standard and the production is stable. With the expansion of production capacity and the gradual expansion of production scale in the mineral fertilizer industry, the problems of cost control pressure, sludge disposal efficiency bottleneck and insufficient resource recycling efficiency in the sewage treatment link have become increasingly prominent, among which the technical shortcomings related to sludge disposal and resource conversion are the most prominent.

[0003] Currently, the existing technical system for treating phosphorus and fluorine-containing sewage in mineral fertilizer production has exposed multiple contradictions in actual application. First, in terms of sludge disposal, the actual sludge discharge under the existing process far exceeds the filling disposal capacity of the factory, resulting in a large amount of sludge that is difficult to be properly treated through a compliant path. In order to temporarily alleviate the disposal pressure, part of the sludge that has not been fully treated is directly introduced into the phosphoric acid extraction tank. Although this reduces the risk of sludge accumulation in the short term, it causes more serious production problems. The impurities and non-target components in the sludge are deeply mixed into the phosphoric acid extraction system, which destroys the process balance stability of phosphoric acid production, directly leading to significant fluctuations in the quality of phosphoric acid products, continuous decline in product purity, and increased energy consumption and cost of subsequent purification process. SUMMARY

[0004] The present application discloses a sludge recycling method for a dust collection system of a diammonium phosphate device to solve the problems of insufficient sludge disposal capacity, resource waste and poor system synergy in the prior art.

[0005] The present application provides a sludge recycling method, which comprises: delivering low-phosphorus sewage in a water distribution tank and calcium hydroxide emulsion stored in a first lime emulsion tank to a first reaction tank of a first sewage treatment system for a precipitation reaction; delivering the sewage after the precipitation reaction to a clarifier, adding a flocculating agent into the clarifier, and performing a secondary precipitation reaction; collecting sludge generated by the secondary precipitation reaction, delivering the sludge to a sludge concentration tank for concentration treatment, and obtaining supernatant and concentrated sludge containing calcium hydroxide; delivering the supernatant to a first neutralization tank, a softening tank, a second neutralization tank and an intermediate water tank of the first sewage treatment system in sequence for water quality treatment; delivering the concentrated sludge containing calcium hydroxide to a second lime emulsion tank of a second sewage treatment system, so that the concentrated sludge serves as a supply source of calcium hydroxide for the second sewage treatment system.

[0006] Optionally, the supernatant is sequentially delivered to a first neutralization tank, a softening tank, a second neutralization tank and an intermediate tank of the first sewage treatment system for water quality treatment, including: Sulfuric acid is added to the first neutralization tank and the second neutralization tank to adjust the pH value of the supernatant to neutral, and soda solution is added to the softening tank; The supernatant treated by the second neutralization tank is delivered to the intermediate tank, and the water quality of the supernatant is detected.

[0007] Optionally, when the water quality of the supernatant meets the standard, the supernatant is discharged; When the water quality of the supernatant does not meet the standard, the supernatant is delivered to the water distribution tank of the first sewage treatment system and re-enters the first reaction tank for precipitation reaction.

[0008] Optionally, the pH value in the first reaction tank is 11-12.

[0009] Optionally, after the concentrated sludge containing calcium hydroxide is delivered to the second lime milk tank of the second sewage treatment system, including: The sewage and the concentrated sludge in the second lime milk tank are delivered to the second reaction tank of the second sewage treatment system for precipitation reaction.

[0010] Optionally, the sewage after precipitation reaction is delivered to the maturation tank, and sodium hypochlorite solution is added to remove ammonia nitrogen in the sewage.

[0011] Optionally, the concentration of the sodium hypochlorite solution is 10%-15%, the addition amount is 5-20 mg per liter of sewage, and the reaction time of the sodium hypochlorite solution and the sewage in the maturation tank is 30-60 minutes.

[0012] Optionally, the temperature of the sewage in the maturation tank is 25-35℃, and the pH value is 10-11.

[0013] Optionally, the sewage treated by the maturation tank is delivered to the mixing tank, and a flocculating agent is added for precipitation reaction.

[0014] Optionally, the supernatant of the sewage after precipitation reaction is recovered, and the sludge after precipitation reaction is delivered to the sludge concentration tank of the second sewage treatment system for concentration treatment.

[0015] From the above technical solutions, the present application has the following advantages: By concentrating the secondary precipitated sludge and directing the concentrated sludge containing calcium hydroxide to the second wastewater treatment system as a calcium hydroxide supply source, the amount of sludge that needs to be discharged or filled can be greatly reduced, the contradiction between sludge discharge amount and filling capacity can be fundamentally alleviated, the problem of not properly treating sludge mixed into the phosphoric acid extraction system to destroy the stability of the production process and cause product quality fluctuations can be avoided, the limitation of independent feeding of the first wastewater treatment system and the second wastewater treatment system can be broken, the recycling of the residual effective calcium hydroxide in the sludge can be realized, and the waste of chemicals and the cost of purchasing chemicals can be reduced. At the same time, the concentration treatment realizes the separate treatment of the supernatant and the concentrated sludge, which not only ensures that the supernatant is stably discharged after being treated by the original process and meets the standard, but also improves the overall synergy and operation efficiency of the wastewater treatment system by supplementing chemicals to the concentrated sludge, reduces the cost of intermediate waste transportation and disposal, and realizes the cost reduction and efficiency increase of the wastewater treatment link and resource recycling. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 A flowchart of an embodiment of the method for recycling sludge provided in the present application; Figure 2 A flowchart of an embodiment of the treatment of wastewater supernatant in the method for recycling sludge provided in the present application; Figure 3 A flowchart of an embodiment of the treatment of sludge in the method for recycling sludge provided in the present application. DETAILED DESCRIPTION

[0017] Please refer to Figure 1 , Figure 1 A flowchart of the process of recycling sludge, the present application provides an embodiment about recycling sludge, which includes: S101, low-phosphorus wastewater in a water distribution tank and calcium hydroxide emulsion stored in a first lime emulsion tank are transported to a first reaction tank of a first wastewater treatment system for precipitation reaction; The low-phosphorus wastewater is collected from a tank outside the plant, and after removing large particles of impurities by pre-filtering, it is transported to the water distribution tank for temporary storage by a lifting pump. The liquid level sensor in the water distribution tank monitors the storage in real time to ensure stable water supply. The first lime emulsion tank is pre-configured with fresh calcium hydroxide emulsion with a concentration of 5% to 10%. When configuring, the emulsion is uniformly ensured by an automatic stirring device, and the concentration is calibrated in real time by a density meter. If the concentration is lower than 5%, solid calcium hydroxide is added; if the concentration is higher than 10%, water is added for dilution. During the transportation stage, a centrifugal sewage pump at the bottom of the water distribution tank sends the wastewater to the first reaction tank; a screw pump at the outlet of the first lime emulsion tank transports the emulsion to the first reaction tank through a stainless steel pipeline. An electric flow regulating valve on the pipeline and a pH sensor in the reaction tank form a closed-loop control to stabilize the pH of the reaction tank at 11-12.

[0018] S102, deliver the sewage after the precipitation reaction to the clarifier, add flocculants to the clarifier, and perform secondary precipitation reaction; The sewage is self-flowing into the clarifier through a pipeline, and a pipeline mixer ensures uniform suspension of particles. A 500 pm aperture filter screen is arranged at the inlet to intercept large particle impurities and avoid clogging the water inlet. An online turbidity instrument on the water inlet pipeline monitors turbidity in real time, and if it exceeds the preset value, it feeds back to the first reaction tank to adjust the calcium hydroxide dosage or reaction time.

[0019] Then flocculant addition and mixing are performed. Polyaluminum chloride is selected as the flocculant, and a 10% concentration solution is prepared by stirring in an automatic dissolving tank at a rate of 40-50 r / min for more than 30 min. A stirring device in the storage tank at 20 r / min prevents the solution from stratifying. Then the flocculant is delivered to the water inlet pipeline mixing section, and the dosage is dynamically adjusted according to turbidity. A static mixer ensures that the flocculant and sewage are fully mixed to form small floc nuclei. Finally, secondary precipitation and sludge discharge control are performed. The clarifier adopts a horizontal flow structure, and the hydraulic retention time is set to 2-4 h to allow the flocs to settle to the bottom sludge. An ultrasonic level meter monitors the supernatant level, and a sawtooth-shaped effluent weir ensures uniform effluent.

[0020] S103, collect the sludge generated by the secondary precipitation reaction, deliver the sludge to the sludge concentration tank for concentration treatment, and obtain the supernatant and concentrated sludge containing calcium hydroxide; The sludge discharged from the clarifier first enters the sludge collection tank, and a stirring device in the tank at 15-20 r / min prevents the sludge from setting. A top odor collection device directs the generated ammonia gas into an activated carbon adsorption tower for treatment to avoid environmental pollution. Then a screw pump delivers the sludge to the sludge concentration tank. The smooth inner wall of the wear-resistant rubber conveying pipe reduces adhesion. The pressure sensor, check valve, and Y-type filter on the pipeline ensure stable delivery.

[0021] Next, the sludge concentration treatment is performed. A circular gravity concentration tank is used, and the sludge scraped by the scraping plate of the periphery transmission type sludge scraper is scraped to the center outlet of the tank. The sludge is left to stand for 8-12 h, with rapid settling in the first 4 h and slow reaching of solid-liquid balance in the last 8 h. Online concentration meters at different depths of the concentration tank monitor the solid content in real time to ensure that the solid content of the bottom sludge after concentration reaches 20%-30%. The supernatant generated by concentration is discharged through the top overflow weir. A 100-150 mm high residue blocking plate in front of the overflow weir intercepts floating residue. The supernatant is delivered to the first neutralization tank through the overflow pipe to realize water resource recovery. The concentrated sludge finally enters the sludge temporary storage bin. The heating device in the bin maintains the temperature at 25-30℃ to prevent low temperature from increasing the sludge viscosity. The liquid level sensor monitors the liquid level, and when it reaches 80%, the subsequent delivery process is started.

[0022] S104, deliver the supernatant to the first neutralization tank, softening tank, second neutralization tank, and intermediate water tank of the first sewage treatment system in sequence for water quality treatment; The supernatant is transported to a first neutralization tank, a metering pump adds 30% to 50% concentration sulfuric acid, and a stirring device in the tank at 20-30 r / min ensures that the acid and base are fully mixed, a pH sensor at the outlet monitors in real time, and the pH is accurately adjusted to 7-8 to avoid excessive sulfuric acid leading to excessively low pH. Then, softening and hardness removal treatment is performed: the neutral supernatant enters a softening tank, a metering pump adds 10% to 20% concentration soda ash, the addition amount is set according to the water quality hardness, a stirring device stirs for 20-30 min to make the soda ash react with the hardness ions to generate calcium carbonate and magnesium carbonate precipitates; the sedimentation area at the bottom of the softening tank is periodically opened to discharge the precipitates to ensure that the water quality hardness after softening is ≤100 mg / L to prevent scaling in subsequent links. Then, a second neutralization tank and an intermediate tank are treated: the softened sewage enters the second neutralization tank, sulfuric acid is added again, and the pH is fine-tuned to 6-9 through a pH sensor; the qualified sewage flows to the intermediate tank, and an online monitor in the tank detects COD, phosphorus, fluorine and other indicators in real time; if the water quality is qualified, it is transported to a monitoring tank by a booster pump for discharge; if it is not qualified, a backflow pump is started to send the sewage back to the distribution tank for sewage treatment again until the water quality is qualified.

[0023] S105, the concentrated sludge containing calcium hydroxide is transported to the second lime milk tank of the second sewage treatment system, so that the concentrated sludge serves as a supply source of calcium hydroxide for the second sewage treatment system.

[0024] The obtained concentrated sludge needs to be transported to the second sewage treatment system for recycling. The sludge is transported to the second lime milk tank of the second sewage treatment system through a special pipeline. A concentration sensor on the pipeline monitors the solid content of the sludge in real time to ensure that it is stable at 20% to 30%. A flow regulating valve is linked with the reaction demand of the second sewage treatment system, and the delivery amount is adjusted according to the pH and phosphorus removal target of each reaction unit to avoid sludge accumulation or insufficient supply in the second lime milk tank.

[0025] Subsequently, the concentrated sludge is recycled. The second lime milk tank uses the concentrated sludge as a supply source of calcium hydroxide to replace fresh calcium hydroxide milk, which is sent to the first-stage reaction tank, the second-stage reaction tank and other units of the second sewage treatment system through a delivery pump. In the first-stage reaction tank, the effective calcium hydroxide in the concentrated sludge adjusts the pH to 9-11, Ca²⁺ reacts with PO4³⁻ in the high-phosphorus sewage to generate calcium phosphate precipitate, and the phosphorus removal rate is ensured to be ≥95%. In the second-stage reaction tank, concentrated sludge is supplemented and added according to the phosphorus concentration in the sewage to further reduce the phosphorus content, and the pH is adjusted by using the calcium hydroxide in the sludge to avoid additional addition of fresh reagent. During the entire utilization process, the pH sensor in each reaction tank is linked with the delivery pump of the second lime milk tank to adjust the supply amount in real time, which not only realizes the recycling of the effective components in the sludge, but also reduces the consumption of fresh calcium hydroxide and the cost of sewage treatment.

[0026] In this embodiment, by concentrating the sludge from secondary sedimentation and then directionally transporting the concentrated sludge containing calcium hydroxide to the second wastewater treatment system as a calcium hydroxide supply source, the amount of sludge requiring external discharge or landfill disposal can be significantly reduced. This fundamentally alleviates the contradiction between sludge discharge volume and landfill capacity, avoids the problem of improperly treated sludge mixing into the phosphoric acid extraction system, which can disrupt the stability of the production process and cause fluctuations in product quality. Furthermore, it breaks the limitation of independent material supply between the first and second wastewater treatment systems, enabling the recovery and reuse of residual effective calcium hydroxide in the sludge, reducing reagent waste and procurement costs. Simultaneously, the separation of the supernatant and concentrated sludge achieved through the concentration treatment ensures that the supernatant meets discharge standards after treatment by the original process. The directional replenishment of reagents through the concentrated sludge improves the overall synergy and operational efficiency of the wastewater treatment system, reduces the cost of intermediate waste transfer and disposal, and achieves cost reduction, efficiency improvement, and resource recycling in the wastewater treatment process.

[0027] Please see Figure 2 , Figure 2 In the process of sludge recycling, the treatment of wastewater supernatant is described in detail below. The supernatant is sequentially transported to the first neutralization tank, softening tank, second neutralization tank, and intermediate water tank of the first wastewater treatment system for water quality treatment, including: S201. Sulfuric acid is added to the first and second neutralization tanks to adjust the pH of the supernatant to neutral, and soda ash solution is added to the softening tank. The supernatant to be treated flows by gravity into the first neutralization tank through pipelines. The tank contains a pre-installed storage tank for industrial sulfuric acid with a concentration of 30%–50%. Sulfuric acid is precisely added via a plunger-type metering pump. Before addition, the theoretical dosage of sulfuric acid is calculated based on the real-time flow rate and initial pH value of the supernatant. A paddle agitator ensures thorough mixing of the sulfuric acid and supernatant, preventing excessively high local acidity. Simultaneously, an online pH sensor is installed at the outlet of the first neutralization tank to monitor the effluent pH value in real time. When pH > 8, the metering pump automatically increases the dosage; when pH < 7, the metering pump automatically decreases the dosage, ultimately stabilizing the supernatant pH to 7-8, eliminating the risk of corrosion from strong alkalinity to the subsequent softening tank equipment.

[0028] The neutral supernatant treated by the first neutralization tank still contains a large amount of calcium and magnesium ions. The supernatant is then transported to a softening tank. A 10%-20% concentration of soda solution storage tank is configured in the tank, and is also added by a metering pump. The addition amount needs to be dynamically adjusted according to the water quality hardness detection results. When the hardness is 200-250 mg / L, the soda addition amount is 50-80 mg / L; when the hardness is 250-300 mg / L, the addition amount is increased to 80-100 mg / L. After adding, the stirring device in the softening tank is started to ensure uniform mixing of soda and supernatant and chemical reaction: CO3²⁺ in soda reacts with Ca²⁺ and Mg²⁺ in water to form calcium carbonate and magnesium carbonate precipitates. The reaction time is controlled for 20-30 min to ensure that the hardness removal rate is ≥60%, which provides protection for avoiding scale blockage of pipelines in subsequent links.

[0029] The supernatant treated by the softening tank has a weak alkaline pH, which rises to 8.5-9.5. It enters the second neutralization tank, which needs to be fine-tuned for pH to ensure that the final pH meets the discharge standard. The addition method is the same as that of the first neutralization tank, but the sulfuric acid addition amount is greatly reduced. The pH of the supernatant is accurately adjusted to 6-9 through real-time monitoring by the pH sensor in the tank to meet the requirements of the comprehensive wastewater discharge standard, while avoiding the decrease of dissolved oxygen in water due to too low pH, which affects the subsequent biological treatment or the stability of the discharged water quality.

[0030] S202, transporting the supernatant treated by the second neutralization tank to an intermediate water tank for water quality detection of the supernatant; The supernatant with fine-tuned pH by the second neutralization tank flows into the intermediate water tank through the pipeline. A filter with a pore size of 500 μm is arranged at the inlet of the pipeline to intercept the calcium carbonate and magnesium carbonate fine precipitates that have not completely settled in the softening tank, preventing impurities from entering the intermediate water tank and affecting the detection accuracy. A low-speed stirrer is installed in the intermediate water tank to prevent local water quality from being uneven due to the settlement of suspended particles in the tank, ensuring that the detection sample is representative. The online monitor detects pH, turbidity, total phosphorus, and fluoride in real time. The detection data are recorded every 5 minutes. If a certain index exceeds the standard range for three times in a row, the system automatically triggers an alarm.

[0031] At the same time, hardness and suspended solids are manually sampled every 2 hours. One sampling point is selected at the middle of the tank and one at the outlet. After mixing, the samples are detected to avoid deviation caused by single-point sampling. All detection data are automatically uploaded to the wastewater treatment central control system of the plant to generate a historical curve, which facilitates tracing the water quality change trend.

[0032] S203, discharging the supernatant when the water quality of the supernatant meets the standard; When the intermediate tank online monitoring system shows that all indicators meet the discharge standard for 10 consecutive minutes, and the manual sampling detection result is verified to be qualified, the central control system automatically generates a standard discharge instruction, opens the electric gate valve at the outlet of the intermediate tank and the discharge pump. Before discharge, check the valve state of the discharge pipeline to ensure that there is no leakage and no blockage, and notify the environmental protection monitoring department to prepare for discharge records.

[0033] The discharge process adopts a staged steady flow method. In the initial stage, the discharge flow is controlled at 50% of the design flow. After observing the data of the online monitoring instrument at the total discharge port and confirming that the indicators have no fluctuations, the flow is increased to 100% to avoid sudden increase of flow causing water quality impact at the total discharge port. During the discharge period, the central control system continuously monitors the intermediate tank liquid level and discharge flow. When the tank liquid level drops to 20%, the discharge pump frequency is automatically reduced to gradually reduce the flow to prevent pump body from idling damage. During the discharge process, the discharge flow and water quality indicator data are recorded every hour. After the discharge is completed, a supernatant discharge report is generated.

[0034] S204, when the water quality of the supernatant does not meet the standard, the supernatant is transported to the distribution tank of the first sewage treatment system to re-enter the first reaction tank for sedimentation reaction.

[0035] When the intermediate tank online monitoring system shows that any indicator exceeds the discharge standard or the manual sampling detection result is unqualified, the central control system immediately triggers the unqualified backflow instruction, closes the discharge gate valve and discharge pump at the outlet of the intermediate tank, and opens the backflow pump and backflow pipeline electric valve on the other side of the tank. Before backflow, the system log is used to locate the unqualified reason, such as insufficient sulfuric acid addition in the first neutralization tank leading to pH exceeding the standard, insufficient soda ash addition in the softening tank leading to hardness exceeding the standard, insufficient sedimentation in the clarifier leading to turbidity exceeding the standard, etc. The unqualified indicators and values are recorded to provide basis for process adjustment.

[0036] The unqualified supernatant is transported to the distribution tank of the first sewage treatment system through the backflow pipeline. A flowmeter and a check valve are installed on the pipeline. The flowmeter monitors the backflow flow in real time, and the check valve prevents the sewage in the distribution tank from flowing back to the intermediate tank. After receiving the backflow supernatant, the distribution tank mixes it with fresh low-phosphorus sewage through the stirring device in the tank. After mixing, the pH and turbidity of the sewage in the distribution tank are detected. If the water quality is still abnormal after mixing, the emergency dosing device in the distribution tank is used to adjust it to the normal range before entering the subsequent process.

[0037] The mixed sewage follows the normal water supply process of the water distribution tank and enters the first reaction tank of step S101 to participate in the sedimentation reaction again. At this time, the process parameters of the first reaction tank need to be adjusted according to the reasons for the previous unqualified results. If the total phosphorus exceeds the standard and backflow occurs, the calcium hydroxide emulsion dosage of the first lime emulsion tank can be appropriately increased, and the reaction time can be extended to 60 minutes. If the turbidity exceeds the standard and backflow occurs, the polyaluminum chloride flocculant dosage of the clarifier can be appropriately increased to ensure that the supernatant after reprocessing can pass the water quality test of step S202. After reprocessing, the water quality test results of the intermediate tank need to be closely tracked until the indicators meet the standards.

[0038] In this embodiment, the first and second neutralization tanks correct the sulfuric acid dosage in real time through online pH sensors. During backflow treatment, the process parameters of the first reaction tank can also be adjusted according to the reasons for unqualified results. The unqualified supernatant is transported to the water distribution tank of the first sewage treatment system through the backflow pipeline, mixed with fresh low-phosphorus sewage, and then reprocessed in the first reaction tank to avoid resource waste and environmental risks caused by direct discharge of unqualified sewage. At the same time, there is no need to build a special unqualified water treatment facility, reducing the equipment investment cost.

[0039] Please refer to Figure 3 , Figure 3 The method for recycling sludge is a treatment method for sludge in wastewater. The sludge in wastewater is described in detail as follows. After the concentrated sludge containing calcium hydroxide is transported to the second lime emulsion tank of the second sewage treatment system, the following steps are included: S301, transporting the wastewater and the concentrated sludge in the second lime emulsion tank to the second reaction tank of the second sewage treatment system for sedimentation reaction.

[0040] The sewage to be treated is delivered to the second reaction tank by a lifting pump, and an electromagnetic flow meter on the delivery pipeline monitors the sewage flow in real time and feeds data back to the central control system. At the same time, the concentrated sludge stored in the second lime emulsion tank is delivered to the second reaction tank by a screw pump, and a concentration sensor on the pipeline monitors the solid content of the sludge in real time. The flow regulating valve dynamically adjusts the sludge delivery amount according to the sewage flow and the target pH value. For high-phosphorus sewage, the sludge addition amount needs to be controlled to stabilize the pH of the reaction tank at 10-11; for medium-phosphorus sewage, the pH can be controlled at 9-10 to meet the phosphorus removal requirement. A paddle stirrer is installed in the second reaction tank to ensure that the sewage and concentrated sludge are fully mixed, avoiding low phosphorus removal rate caused by insufficient local reaction. The reaction time of high-phosphorus sewage is 40-60 min, and that of medium-phosphorus sewage is 20-30 min. The total phosphorus content in the sewage is monitored in real time by an online total phosphorus sensor in the tank. When the total phosphorus concentration is reduced to below 5 mg / L, it is determined that the precipitation reaction is complete, and the subsequent maturation tank treatment is prepared; if the total phosphorus concentration does not meet the standard, the concentrated sludge addition amount or the reaction time is appropriately increased until the treatment requirement is met. In addition, a sludge hopper is provided at the bottom of the reaction tank, and the sludge valve is opened regularly to discharge the calcium phosphate and calcium fluoride precipitate generated in the reaction to the sludge concentration tank of the second sewage treatment system, avoiding the accumulation of precipitate in the tank affecting the reaction efficiency.

[0041] S302, deliver the precipitated and reacted sewage to the maturation tank and add sodium hypochlorite solution to remove ammonia nitrogen in the sewage.

[0042] The sewage precipitated by the second reaction tank is self-flowing into the maturation tank through the pipeline. A filter screen is arranged at the inlet of the pipeline to intercept the fine precipitated particles in the sewage which have not completely settled, preventing them from blocking the subsequent sodium hypochlorite adding pipeline or affecting the ammonia nitrogen removal reaction. The maturation tank is pre-equipped with a 10%-15% concentration sodium hypochlorite solution storage tank. The sodium hypochlorite is accurately added by a plunger type metering pump. The addition amount is dynamically adjusted according to the real-time flow and ammonia nitrogen concentration of the sewage: when the ammonia nitrogen concentration is 15-20 mg / L, the addition amount is 5-10 mg per liter of sewage; when the ammonia nitrogen concentration is 20-30 mg / L, the addition amount is increased to 10-20 mg per liter of sewage, so as to ensure that the molar ratio of sodium hypochlorite to ammonia nitrogen is maintained at 5:1-8:1. The sewage temperature is stably maintained at 25-35℃ by the heating device in the tank. At the same time, the pH value of the sewage in the maturation tank is controlled at 10-11 by using the residual calcium hydroxide in the concentrated sludge delivered by the second reaction tank. If the pH value is lower than 10, a small amount of concentrated sludge or sodium hydroxide solution is added for adjustment. The reaction time is set to 30-60 min. The sodium hypochlorite is fully mixed with the sewage by the stirring device in the maturation tank to avoid local over-concentration of sodium hypochlorite, which leads to waste of reagent. During the reaction process, the ammonia nitrogen concentration in the sewage is monitored in real time by an online ammonia nitrogen sensor. When the ammonia nitrogen concentration is reduced to below 1 mg / L, it is determined that the ammonia nitrogen removal is completed. If the ammonia nitrogen concentration does not meet the standard, the addition amount of sodium hypochlorite is appropriately increased or the reaction time is prolonged until the ammonia nitrogen content meets the requirements of the subsequent mixing pool treatment.

[0043] S303, the sewage treated by the maturation tank is delivered to the mixing pool, and a flocculant is added for precipitation reaction. The sludge generated after the precipitation reaction is delivered to the sludge concentration tank of the second sewage treatment system for concentrated sludge reaction.

[0044] The wastewater after removal of ammonia nitrogen by the maturation tank is transported to the mixing tank, and a polyaluminum chloride or polyacrylamide flocculant storage tank is arranged in the mixing tank. The type of flocculant is selected according to the turbidity of the wastewater. When the turbidity is 30-40 NTU, polyaluminum chloride is selected, and when the turbidity is 40-50 NTU, polyacrylamide is selected. The flocculant is added by a metering pump, and after the addition, a high-speed stirrer in the mixing tank is started to mix the flocculant and the wastewater for 1-2 min to form small floc nuclei. Then, the stirring is switched to low speed for 5-10 min to promote the aggregation of the small floc nuclei to form flocs with larger volume and higher density, which are convenient for subsequent sedimentation and separation. The wastewater treated in the mixing tank is self-flowing into the sedimentation tank. The sedimentation tank adopts a vertical flow structure, the effective water depth is 4-5 m, the hydraulic retention time is 1.5-2.5 h, and the flocs slowly settle to the sludge hopper at the bottom of the tank under the action of gravity. A sawtooth-shaped effluent weir is arranged at the top of the sedimentation tank to ensure uniform outflow of the supernatant. The turbidity of the outflowing supernatant can be reduced to below 10 NTU. The high-phosphorus wastewater treated is transported to the first intermediate tank for use by the phosphoric acid system, and the medium-phosphorus wastewater treated is transported to the outer collection tank and then pumped into the first wastewater treatment system for further treatment. The sludge in the sludge hopper of the sedimentation tank is periodically transported to the sludge thickening tank of the second wastewater treatment system by a sludge pump. The sludge thickening tank adopts gravity thickening, and the sludge is left to stand for 6-8 h. The thickened sludge is scraped to the center outlet of the tank by a peripheral transmission type sludge scraper, and then transported to the sludge dewatering workshop for dewatering treatment, or further increased in phosphorus content according to requirements to meet the productization requirements. The supernatant generated by the thickening tank is returned to the second reaction tank through an overflow pipe to realize the recycling of water resources and residual reagents, and further reduce the treatment cost.

[0045] In the present embodiment, the concentrated sludge containing active calcium hydroxide in the first wastewater treatment system is directionally transported to the second reaction tank to replace the fresh calcium hydroxide emulsion for treating wastewater, which not only recycles the residual active ingredients in the sludge, but also greatly reduces the amount of sludge discharged, thereby alleviating the contradiction between sludge discharge amount and filling capacity. Through the classified recycling of the supernatant of the sedimentation tank and the return of the supernatant of the thickening tank to the second reaction tank, water resources are recycled, combined with reagent addition, 25%-35% of the comprehensive operation cost is reduced, and equipment investment and maintenance cost is reduced.

[0046] It should be noted that the above description of the disclosed embodiments enables a person skilled in the art to implement or use the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the scope of the present application. Therefore, the present application is not limited to the embodiments shown herein, but is intended to conform to the broadest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for recycling sludge, characterized in that, The methods include: The low-phosphorus wastewater in the distribution tank and the calcium hydroxide emulsion stored in the first lime emulsion tank are transported to the first reaction tank of the first wastewater treatment system for precipitation reaction. The wastewater after sedimentation is transported to a clarification tank, where flocculants are added for a secondary sedimentation reaction. Collect the sludge produced by the secondary sedimentation reaction, and transport the sludge to the sludge thickening tank for thickening treatment to obtain supernatant and concentrated sludge containing calcium hydroxide. The supernatant is sequentially transported to the first neutralization tank, softening tank, second neutralization tank and intermediate water tank of the first sewage treatment system for water treatment. Concentrated sludge containing calcium hydroxide is transported to the second lime slurry tank of the second wastewater treatment system, with the concentrated sludge serving as the source of calcium hydroxide for the second wastewater treatment system.

2. The method for sludge recycling according to claim 1, characterized in that, The supernatant is sequentially transported to the first neutralization tank, softening tank, second neutralization tank, and intermediate water tank of the first wastewater treatment system for water treatment, including: Sulfuric acid was added to the first and second neutralization tanks to adjust the pH of the supernatant to neutral, and soda ash solution was added to the softening tank. The supernatant after treatment in the second neutralization tank is transported to the intermediate water tank for water quality testing.

3. The method for sludge recycling according to claim 2, characterized in that, Once the water quality of the supernatant meets the standards, the supernatant is discharged. If the water quality of the supernatant does not meet the standards, the supernatant will be transported to the distribution tank of the first sewage treatment system and re-enter the first reaction tank for sedimentation reaction.

4. The method for sludge recycling according to claim 1, characterized in that, The pH value in the first reaction tank is 11-12.

5. The method for sludge recycling according to claim 1, characterized in that, After the concentrated sludge containing calcium hydroxide is transported to the second lime slurry tank of the second wastewater treatment system, the process includes: Wastewater and concentrated sludge from the second lime slurry tank are transported to the second reaction tank of the second wastewater treatment system for sedimentation reaction.

6. The method for sludge recycling according to claim 5, characterized in that, The wastewater after sedimentation and reaction is transported to a maturation tank, and sodium hypochlorite solution is added to remove ammonia nitrogen from the wastewater.

7. The method for sludge recycling according to claim 6, characterized in that, The concentration of sodium hypochlorite solution is 10% to 15%, the dosage is 5 to 20 mg per liter of wastewater, and the reaction time between sodium hypochlorite solution and wastewater in the maturation tank is 30 to 60 minutes.

8. The method for sludge recycling according to claim 6, characterized in that, The temperature of the wastewater in the maturation tank is 25~35℃, and the pH is 10~11.

9. The method for sludge recycling according to claim 6, characterized in that, Wastewater treated in the maturation tank is transported to the mixing tank, where flocculants are added to induce sedimentation.

10. The method for sludge recycling according to claim 9, characterized in that, The supernatant of the wastewater after sedimentation is recovered, and the sludge after sedimentation is transported to the sludge thickening tank of the second wastewater treatment system for thickening treatment.

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