Method for application of concentrated residual sludge in denitrification treatment

Abstract

The application discloses a method for applying concentrated residual sludge in denitrification treatment. The method comprises the following steps: concentrating and dewatering residual sludge generated by a biochemical treatment system of waste water and waste to a water content of not less than 75%, and directly adding the residual sludge into waste water containing nitrate nitrogen, waste acid liquid or NO2 absorption liquid; under the stress of nitrate nitrogen, a denitrification bacterial group in the residual sludge secretes extracellular polymers as a carbon source, and reduces the nitrate nitrogen into nitrogen; after the reaction, sludge and water are separated, and the residual sludge is returned to be recycled. The application is suitable for municipal sewage plants, industrial continuous flow waste water, high-concentration waste liquid and waste nitric acid treatment, and can realize cross-plant cooperation and nitrogen oxide waste gas purification. The application can replace purchased carbon sources and commercial bacterial agents with the residual sludge, and simultaneously realizes sludge reduction and stabilization.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment and exhaust gas purification technology, specifically relating to a method for denitrification treatment using concentrated residual sludge as a carbon source and microbial strains, applicable to the treatment of wastewater containing nitrate nitrogen, waste acid liquid, and exhaust gas containing nitrogen oxides. Background Technology

[0002] Urban sewage and industrial wastewater (from fertilizer, electronics, photovoltaic, electroplating, etc.) often contain high concentrations of nitrate nitrogen (NO3⁻-N). Biological denitrification is the most economical and effective means of removing nitrate nitrogen. Its principle is that under anaerobic conditions, denitrifying bacteria use organic carbon sources to reduce nitrate nitrogen to nitrogen gas.

[0003] However, existing denitrification technologies generally suffer from the following drawbacks:

[0004] 1. External carbon sources are required, which is costly: Most industrial wastewater has an extremely low carbon-to-nitrogen ratio, requiring the continuous addition of purchased carbon sources such as sodium acetate, glucose, and methanol, which is costly and some carbon sources pose safety risks.

[0005] 2. Denitrifying bacteria need to be purchased separately or acclimatized over a long period of time: Industrial wastewater treatment systems often lack activated sludge and need to purchase dewatered sludge for acclimatization or purchase commercial bacterial agents, resulting in a long start-up period.

[0006] 3. Traditional processes are complex and require a large footprint: they require independent anoxic tanks and mixed liquor reflux systems, making them difficult to modify.

[0007] 4. High pressure to dispose of excess sludge: The excess sludge from wastewater treatment plants has a high water content and large volume, and the disposal cost is increasing year by year.

[0008] The applicant has previously filed several patents related to "waste-to-waste" treatment. Building on this, they further discovered that concentrating and dewatering excess sludge to a moisture content of no less than 75% allows it to simultaneously serve as a slow-release carbon source and denitrifying bacteria, enabling efficient denitrification when directly added to a nitrate-nitrogen-containing medium. Simultaneously, the high-concentration nitrate environment triggers an emergency response in the denitrifying bacteria, causing them to actively secrete extracellular polymeric substances (EPS) as a carbon source, achieving self-supply and self-consumption, thus reducing sludge volume. Summary of the Invention

[0009] Technical solution

[0010] S1. Preparation of concentrated excess sludge:

[0011] The excess sludge generated by the wastewater biological treatment system is collected and then concentrated by gravity, mechanical dewatering, or centrifugal dewatering to obtain concentrated excess sludge with a moisture content of not less than 75% (typical range 75%-90%). This sludge is in the form of a slurry or paste, which is easy to pump, transport, and add. It is enriched with a large number of facultative denitrifying bacteria and contains abundant extracellular polymers and biodegradable organic matter.

[0012] S2. Add sludge:

[0013] The concentrated residual sludge can be directly added to wastewater containing nitrate nitrogen, waste acid, or aqueous solution after NO2 absorption, and dispersed by hydraulic or mechanical action. There is no need to specifically limit the dissolved oxygen at the addition point.

[0014] S3. Denitrification reaction:

[0015] After addition, the denitrifying bacteria in the concentrated waste sludge are in a high-concentration nitrate nitrogen environment. This environment triggers the microbial emergency response mechanism: the denitrifying bacteria actively release bioavailable organic carbon sources by secreting extracellular polymeric substances (EPS) and accelerating their own metabolism. These carbon sources are then used as electron donors by the denitrifying bacteria, using nitrate nitrogen in the wastewater as electron acceptors, reducing it to nitrogen gas. The slurry / paste-like structure of the concentrated sludge keeps the bacteria in a high-density aggregated state, with EPS locally enriched and slowly released within the aggregates, ensuring a continuous supply of carbon sources.

[0016] This process achieves a dual purpose: ① Nitrate nitrogen in wastewater is removed (reduced to N2); ② Organic matter in sludge (including EPS and the biomass of the microbial community itself) is oxidized and decomposed, and the sludge is gradually rendered inorganic, stabilized and reduced in volume.

[0017] In particular, the reaction may produce vigorous bubbles (N2), which is a direct indication of active denitrification. The reaction time is related to sludge activity (EPS secretion capacity, microbial density) and nitrate nitrogen concentration, and the actual treatment effect shall prevail.

[0018] S4. Sludge-water separation and sludge recycling:

[0019] The mixture after the reaction is separated into mud and water, which can be done by sedimentation, filtration or centrifugation.

[0020] The separated sludge is returned to step S2 for continued use, achieving sludge recycling. During the recycling process, the sludge continuously consumes its own organic matter, gradually achieving inorganicization, stabilization, and volume reduction.

[0021] The separated effluent meets discharge standards or enters subsequent treatment units.

[0022] The term "concentrated residual sludge" refers to sludge that is concentrated and dehydrated during storage and transportation to reduce its volume, making it convenient for transport and storage; and that is easily dissolved and evenly dispersed in water for rapid reaction during use, rather than becoming a drier sludge cake, thus eliminating the need for mechanical crushing.

[0023] Beneficial effects

[0024] 1. Completely overturn the "external carbon source" model: replace purchased carbon sources with the organic matter of the remaining sludge itself, reducing carbon source costs by 80%-100%.

[0025] 2. Resource utilization, productization, and commercialization of excess sludge: Transform excess sludge into concentrated products, increase solids content, reduce volume, and make it available for internal use, storage and transportation in drums, or sales to generate revenue.

[0026] 3. The process is extremely flexible and no new tanks are required: Denitrification can be completed in existing facilities such as pipelines, equalization tanks, and sedimentation tanks, reducing investment by 50%-70%.

[0027] 4. "One bucket of sludge and one bucket of liquid" mixed mode: suitable for on-site treatment of high-concentration nitrate waste liquid in the plant, with simple equipment and flexible operation.

[0028] 5. Utilizing a microbial emergency response mechanism to achieve sludge self-consumption and carbon self-supply: A high-concentration nitrate nitrogen environment triggers an emergency response in the denitrifying bacteria, which actively secrete extracellular polymeric substances (EPS) as a carbon source, achieving self-sufficiency in carbon. Simultaneously, during denitrification, the bacteria consume their own biomass, resulting in the inorganicization, stabilization, and volume reduction of the sludge. This mechanism differs from the traditional denitrification model of "external carbon source input + sludge proliferation."

[0029] 6. Wide range of applications: It can be used for direct denitrification or denitrification after nitrification; it can be used for continuous treatment or batch treatment; it is applicable to various scenarios such as municipal, industrial, cross-plant collaboration, and waste gas denitrification.

[0030] 7. Complementarity with the application submitted on the same day: This method complements the "Method for Denitrification and Digestion of Excess Sludge from Wastewater Treatment Plants" submitted on the same day. This method uses concentrated excess sludge as a product to treat external wastewater / waste gas containing nitrate nitrogen; the latter introduces wastewater containing nitrate nitrogen (including NO2 absorbent generated from the catalytic oxidation of waste ammonia) into a sludge tank to digest the excess sludge. Together, they form a two-way closed loop of "sludge leaving the plant for industrial denitrification" and "nitrate-containing wastewater entering the plant to digest sludge," achieving material synergy between wastewater treatment plants and industrial users. Detailed Implementation

[0031] Example 1 (Urban Water Purification Plant)

[0032] A municipal wastewater treatment plant, with a daily treatment capacity of 100,000 tons, uses the AAO process. During operation, it faces a carbon source shortage. To ensure total nitrogen meets standards, sodium acetate is continuously added as a carbon source, consuming approximately 3 tons daily, resulting in high carbon source costs. Excess sludge needs to be conditioned and filtered to a certain moisture content before being transported for further treatment.

[0033] Using the method of this invention:

[0034] The original mixed liquor recirculation system (nitrified liquor from the aerobic tank was recirculated to the anoxic tank) was eliminated. The aged, concentrated excess sludge from the bottom of the sludge thickening tank was directly pumped into the downstream section of the aerobic tank using a screw pump. After dissolving, it was thoroughly mixed with the treated water. The hydraulic retention time was 0.5-1 hour, and low-intensity aeration and stirring were maintained in the tank (to keep the sludge suspended and prevent sedimentation). After the sludge entered this section, the denitrifying bacteria used the organic matter carried by the sludge as a carbon source, rapidly consuming dissolved oxygen in the water to create a localized anoxic microenvironment, and then began denitrification. After the reaction, the mixed liquor entered the sedimentation tank, and part of the separated sludge was returned to the front end of the system, while the rest entered the sludge thickening tank. This sludge had already consumed its own organic matter during the denitrification process, achieving inorganication, stabilization, and volume reduction, significantly reducing the burden on subsequent dewatering and disposal.

[0035] Execution result:

[0036] • Discontinuing the use of sodium acetate completely eliminates carbon source costs;

[0037] • Eliminating the mixing liquid reflux reduces the reflux pump power consumption by approximately 30%;

[0038] • The total nitrogen in the effluent is consistently below 10 mg / L, meeting the Class A standard;

[0039] • The aged sludge in the thickening tank is disposed of in situ, reducing the amount of excess sludge by about 20% and improving dewatering performance;

[0040] • The increased inorganic sludge content reduces the cost of subsequent anaerobic digestion or incineration.

[0041] Example 2 (Continuous Treatment of Nitrate-Containing Wastewater from the Photovoltaic Industry)

[0042] A photovoltaic company's wastewater treatment system, after defluorination and nitrification, produces effluent with NO⁻-N concentrations of 120-150 mg / L, COD of approximately 50 mg / L, and an extremely low C / N ratio (<0.5), failing to meet denitrification requirements. The original process continuously added sodium acetate as a purchased carbon source at a dosage of approximately 300 mg / L, costing about 4.5 yuan per ton of water. Simultaneously, the system's own residual sludge, after plate and frame filtration, had a moisture content of approximately 80% and was in a pasty state, originally intended for off-site solid waste disposal.

[0043] Modification using the method of this invention:

[0044] Excess sludge discharged from the company's UASB anaerobic reactor (pretreatment unit) was filtered through a plate and frame filter press until the moisture content was approximately 80%. An inlet was installed on the effluent pipe of the nitrification tank, and sludge was continuously injected via a screw pump. The dosage was controlled to increase the MLSS of the mixed wastewater by approximately 2000 mg / L. After the sludge and wastewater were mixed at a pipe bend (where strong turbulence facilitates dispersion), they entered an equalization tank (retention time approximately 4 hours) for in-situ denitrification. Operational observation revealed numerous fine nitrogen bubbles escaping from the surface of the equalization tank, indicating active denitrification. The resulting mixture entered a sedimentation tank, and the separated sludge was returned to the injection point for recycling, with only a small amount of fresh concentrated sludge added weekly (approximately 5%-10% of the initial dosage).

[0045] Execution result:

[0046] • Completely discontinuing the use of sodium acetate reduces carbon source costs to 0 yuan / ton of water;

[0047] • The NO3⁻-N concentration at the outlet of the equalization tank is reduced to below 15 mg / L, with a removal rate >88%;

[0048] • The COD of the effluent is consistently below 60 mg / L, with no carbon source penetration issues;

[0049] • The amount of excess sludge transported off-site decreased by approximately 15%;

[0050] • The sedimentation tank exhibits good sludge settling performance, with SVI < 80 mL / g.

[0051] Example 3 (Treatment of high-concentration nitrate wastewater using the "one bucket of sludge, one bucket of liquid" model)

[0052] A plating plant generates a batch of aging bath solution containing nitrate nitrogen each month, with a NO3⁻-N concentration of approximately 80 g / L (i.e., 80,000 mg / L), and a production volume of about 5 m³ / month. The plant's biological treatment system has limited capacity and cannot directly handle such a high concentration of nitrate wastewater. The original treatment method involved outsourcing the transportation and disposal, incurring monthly disposal costs of approximately 6,000 yuan, which was costly and cumbersome to manage.

[0053] The method of this invention is used for on-site processing:

[0054] A previously unused 10 m³ tank within the wastewater treatment plant was repurposed as a denitrification reaction tank. One ton of concentrated excess sludge (approximately 78% moisture content, transported in drums) was purchased from a nearby wastewater treatment plant and poured into the tank along with 5 m³ of aging tank liquid. A 750W submersible pump was installed at the bottom of the tank for intermittent circulation and agitation of the liquid within the tank (running for 15 minutes per hour, then stopping for 45 minutes) to prevent sludge sedimentation and promote mixing. Once the submersible pump was started for circulation and agitation, the denitrification reaction began, and dense nitrogen bubbles were visible being generated within the tank.

[0055] During operation, excess sludge generated at the wastewater treatment plant (originally intended for solid waste disposal) is continuously transferred to the denitrification tank. The high concentration of nitrate nitrogen oxidizes and stabilizes the excess sludge. The reaction cycle is approximately 5-7 days, with the endpoint determined by the near cessation of bubble generation and a reduction in nitrate nitrogen concentration to the target value. After the reaction, bag filtration separates the effluent from the sludge. The NO3⁻-N concentration in the filtered effluent has decreased to below 35 mg / L, allowing for direct discharge into the plant's integrated wastewater treatment station for further processing. The sludge retained by filtration is an inorganic, stabilized solid residue; it is small in volume, odorless, and easily dewatered, and can be directly stored as general solid waste or disposed of externally.

[0056] Execution result:

[0057] • The NO3⁻-N in the effluent is reduced to below 35 mg / L, with a removal rate >99.9%;

[0058] • The monthly outsourcing cost of 6,000 yuan has been completely eliminated;

[0059] • The cost of purchasing sludge (1 ton of concentrated sludge) is about 500 yuan, and the operating electricity cost is extremely low (750W submersible pump operates intermittently). The overall treatment cost is reduced by more than 90% compared with outsourcing.

[0060] • It simultaneously absorbed the excess sludge generated by the wastewater treatment plant itself, achieving in-situ stabilization and volume reduction of the sludge;

[0061] • No purchased carbon sources or commercial denitrifying bacteria agents were used throughout the entire process;

[0062] • The equipment investment is extremely low (using only an idle water tank and a small submersible pump), and the operation is simple, making it especially suitable for the "on-site treatment" of high-concentration nitrate waste liquid in small and medium-sized industrial enterprises.

[0063] This embodiment fully demonstrates the core concept of the present invention of "using waste to treat waste and making the most of mud", as well as the flexibility and economy of the "one bucket of mud and one bucket of liquid" model.

[0064] Example 4 (Direct Denitrification Treatment of Waste Nitric Acid)

[0065] A company generates waste nitric acid (concentration approximately 3%-10%), which is directly added to concentrated residual sludge (80% moisture content) at a ratio of sludge to waste liquid approximately 1:20. The reaction is stirred for 48 hours. Results: NO⁻-N removal rate >95%; pH naturally rises from 3-4 to 7-8 during the reaction without the need for external alkali; sludge is rendered inorganic, and dewatering performance is improved.

[0066] Example 5 (Cross-plant co-processing - using municipal sludge for denitrification of industrial wastewater)

[0067] A municipal wastewater treatment plant has a daily treatment capacity of 200,000 tons and produces approximately 50 tons of residual sludge (after dewatering) per day annually, with a moisture content of 75%-80%. The plant's sludge disposal cost is approximately 300 yuan per ton, and the annual expenditure on transporting it to landfill or incinerating it exceeds 5 million yuan.

[0068] A photovoltaic company (or an electronics / electroplating / chemical company) has a wastewater treatment system with an influent total nitrogen (mainly nitrate nitrogen) concentration of approximately 150-300 mg / L and an extremely low C / N ratio. It has been using sodium acetate as a carbon source for a long time, with a daily carbon source cost of approximately 8,000 yuan.

[0069] The cross-plant collaborative processing mode of this invention is adopted:

[0070] The municipal wastewater treatment plant transports the dewatered, concentrated residual sludge (75%-80% moisture content) in drums, tankers, or tank trucks to photovoltaic (PV) plants via specialized vehicles (transport distance approximately 50 km, freight cost approximately 100 RMB / ton). Upon receiving the sludge, the PV plant adds it directly to a denitrification tank or equalization tank at a sludge:high-nitrate wastewater mass ratio of approximately 1:20, dispersing it through mechanical stirring or hydraulic mixing. The mixed liquor remains for 24-48 hours for denitrification. During the reaction, numerous nitrogen bubbles are visible as nitrate nitrogen is reduced to nitrogen gas, while organic matter in the sludge is oxidized and decomposed.

[0071] After the reaction is complete, the mixture enters a sedimentation tank or a plate and frame filter press for separation. The NO3⁻-N concentration in the separated supernatant is reduced to ≤15 mg / L, meeting the discharge standards. The separated sludge has been inorganicized and stabilized, with its volume reduced by approximately 20%-30% compared to when it was added. Its dewatering performance is significantly improved (filtration speed increases by approximately 30%), and it can be disposed of on-site as general solid waste, or returned to the municipal wastewater treatment plant for further concentration and dewatering before incineration / landfill.

[0072] Operational Results and Economic Analysis:

[0073] Operational Results and Economic Analysis:

[0074] project Original Schema (Discrete) Collaborative mode of the present invention change Wastewater treatment plant sludge disposal costs 300 yuan / ton Revenue from sludge removal (or free disposal) Costs reduced to zero or turned into revenue Carbon source cost for photovoltaic companies 8,000 yuan / day 0 yuan / day -100% Denitrification effect of photovoltaic enterprises External carbon source meets standards Equal to standard Consistent Final sludge volume 50 tons / day (before disposal) Approximately 35-40 tons / day (after inorganic processing) -20%~-30% Inter-factory transportation costs none Approximately 100 yuan / ton New additions, but offset by carbon source savings. Comprehensive environmental benefits Carbon source consumption + sludge treatment Carbon source reduction + sludge reduction Double benefits

[0075] The core significance of this embodiment:

[0076] • The sludge from urban wastewater treatment plants is no longer "waste," but a "denitrification aid product" with carbon source and microbial value;

[0077] • Industrial users: Replacing purchased carbon sources and commercial microbial agents with sludge significantly reduces costs;

[0078] • A win-win situation: wastewater treatment plants save on treatment costs, industrial users save on carbon source costs, and the environment achieves double reduction.

[0079] This model is particularly suitable for establishing a long-term, stable material closed loop between urban wastewater treatment plants and surrounding high-nitrate nitrogen industrial wastewater treatment facilities.

[0080] Industrial applicability

[0081] This invention converts a large amount of excess sludge generated by a wastewater biological treatment system into a concentrated excess sludge product, which is then used for the denitrification treatment of wastewater containing nitrate nitrogen. The method is as follows:

[0082] • Suitable for industries such as urban sewage treatment plants, fertilizer, photovoltaic, electronics, electroplating, chemicals, metal surface treatment, slaughtering, and food processing;

[0083] • It can continuously process large volumes of water, and also process high-concentration waste liquid in a "one bucket of mud, one bucket of liquid" manner.

[0084] • It can be denitrified independently or in series with existing nitrification processes;

[0085] • Sludge can be recycled, requiring only a small amount of replenishment;

[0086] • Simultaneously achieve wastewater denitrification and sludge reduction and stabilization.

[0087] This invention fully utilizes the residual sludge generated by the wastewater biochemical treatment system itself as an integrated "carbon source + microbial strain" addition material, completely overturning the traditional technical route of "denitrification must add an external carbon source, commercial microbial strains or long-term sludge domestication", and has significant economic and environmental benefits.

[0088] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for applying concentrated excess sludge in denitrification treatment, characterized in that, Includes the following steps: The residual sludge generated by the wastewater biological treatment system is concentrated and dewatered to a moisture content of not less than 75% to obtain concentrated residual sludge. The concentrated residual sludge is directly added to wastewater containing nitrate nitrogen. The denitrifying bacteria in the sludge use the organic matter carried by the sludge as a carbon source to reduce the nitrate nitrogen in the wastewater to nitrogen gas. The mixture after the reaction is subjected to mud-water separation, and the separated sludge is returned for continued use.

2. The method according to claim 1, characterized in that: The nitrate-containing wastewater is the effluent from the aerobic tank of the municipal wastewater treatment plant. The dosing point is located at the downstream section of the aerobic tank or the inlet of the sedimentation tank. The hydraulic retention time is 0.5-2 hours, and there is no need to set up a separate anoxic tank and mixed liquor recirculation system.

3. The method according to claim 1, characterized in that: The nitrate-containing wastewater is industrial wastewater (such as photovoltaic, electronics, fertilizer, electroplating and other industries) that has undergone nitrification treatment or directly contains nitrate nitrogen. The addition point is located at the effluent pipe of the nitrification tank, the equalization tank or the inlet of the denitrification tank, and the sludge is recycled.

4. The method according to claim 1, characterized in that: The nitrate-containing wastewater is a high-concentration nitrate-containing waste liquid with a nitrate nitrogen concentration of not less than 1 g / L. It adopts a batch treatment mode: the concentrated residual sludge and the nitrate-containing waste liquid are mixed in a container in proportion. After the reaction is completed, the water is filtered or centrifuged to separate the water. The separated solids are inorganically stabilized and then disposed of.

5. The method according to claim 4, characterized in that: The high-concentration nitrate-containing waste liquid is a nitrate-containing nitrogen waste liquid generated in scenarios such as electroplating, chemical industry, metal surface treatment or waste nitric acid recovery, and is intermittently mixed by stirring or circulation devices during the reaction process.

6. The method according to claim 1, characterized in that: In the method described, the concentrated residual sludge simultaneously provides the carbon source and denitrifying bacteria required for denitrification, without the need for additional externally purchased carbon source or additional commercial denitrifying bacteria agent.

7. The method according to claim 1, characterized in that: The concentrated residual sludge is stored and transported in drums, bags, cans or tank trucks and used as a denitrification treatment product.

8. The method according to claim 1, characterized in that: The concentrated residual sludge is obtained from the municipal wastewater treatment plant and transported to the site where nitrate-nitrogen-containing wastewater is generated via drums, bags, cans, or tank trucks. After being mixed with high-nitrate-nitrogen wastewater for denitrification, the separated sludge is inorganicated, stabilized, and reduced in volume, and can be directly disposed of or returned to the municipal wastewater treatment plant for further treatment.

9. The method according to claim 1, characterized in that: High-concentration nitrate or nitrate-containing wastewater is directly discharged into the biochemical waste sludge thickening tank for denitrification. The sludge after the reaction is dewatered by filter press, and the filter effluent is returned to the equalization tank for treatment.

10. The method according to claim 1, characterized in that: The nitrate-containing wastewater is a nitric acid or nitrite solution formed after absorbing nitrogen oxide-containing waste gas; the concentrated residual sludge is directly added to this solution to carry out denitrification reaction, thereby simultaneously achieving waste gas denitrification and sludge reduction.