Low-concentration municipal sewage deep denitrification system and method based on MBBR coupled pyrite autotrophic denitrification

Abstract

The invention discloses a low-concentration municipal sewage treatment system and method based on MBBR coupled pyrite autotrophic denitrification, and belongs to the technical field of sewage treatment. The system sequentially comprises a front nitrification MBBR unit, a ferrous sulfide autotrophic-MBBR coupling denitrification unit and a micro-aerobic post-treatment unit along the water flow direction. The core of the device is that the coupled denitrification unit is filled with a sulfur-iron composite functional filler, the filler takes a porous MBBR suspended filler as a carrier, and composite particles of elemental sulfur and zero-valent iron are loaded inside and on the surface of the carrier in an embedding and fixing manner. The treatment method comprises the following steps: nitrifying sewage in the front unit, and controlling dissolved oxygen in effluent; nitrification effluent enters a coupling denitrification unit, and nitrogen removal is performed by using autotrophic denitrifying bacteria attached to filler under an anoxic condition; and the effluent is finally purified by the post-treatment unit. Through deep coupling of the carrier and the ferrous sulfide matrix, efficient retention of a biological membrane and synergistic slow release of an electron donor are achieved, and the device has the remarkable advantages of being fast in starting, high in denitrification load, stable in operation, free of additional organic carbon sources and capable of effectively controlling sulfate generation and pH fluctuation and is particularly suitable for deep denitrification treatment of municipal sewage with the low carbon-nitrogen ratio.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, specifically to a process system and method for deep denitrification of municipal wastewater with low C / N ratios (carbon-nitrogen ratios). In particular, it is an innovative coupling of moving bed biofilm reactor (MBBR) technology with sulfur-iron composite matrix autotrophic denitrification technology to achieve a wastewater treatment system and method with high efficiency, stability, and low operating costs for deep denitrification. Background Technology

[0002] Currently, urban municipal wastewater in vast areas of my country is generally characterized by "low carbon and high nitrogen," with the ratio of chemical oxygen demand (COD) to total nitrogen (TN) (C / N) often below 5:1, or even lower. Traditional heterotrophic denitrification biological nitrogen removal processes require the addition of large amounts of exogenous organic carbon sources (such as methanol and sodium acetate) due to severe carbon source shortages, resulting in high operating costs and risks of secondary pollution from excessive or residual carbon sources, as well as increased sludge production.

[0003] Sulfur autotrophic denitrification technology uses elemental sulfur (S) 0 Using carbon dioxide or carbonate as the carbon source and employing electron donors, this method eliminates the need for organic carbon sources, offering a new approach to solving the aforementioned problems. The reaction process can be simplified as follows:

[0004] 55S 0 + 50NO3⁻ + 38H2O + 20CO2 + 4NH4⁺ → 4C5H7O2N (bacterial cells) + 25N2 +55SO4²⁻ + 64H⁺ However, this technology has two significant drawbacks: 1) The reaction produces a large amount of sulfate (SO4²⁻), which may lead to excessive sulfate concentration in the effluent; 2) The reaction process produces acid (releasing H⁺), which causes the pH value of the system to drop, thereby inhibiting microbial activity and even causing the reaction to stop.

[0005] In recent years, research has proposed to incorporate zero-valent iron (Fe) 0 Iron compounds are introduced into the sulfur autotrophic denitrification system. Iron is introduced through a corrosion process (Fe... 0 → Fe²⁺ + 2e⁻) can provide additional electrons to participate in denitrification, while consuming H⁺ (Fe) in the system. 0+ 2H⁺→ Fe²⁺ + H₂), theoretically, can neutralize acidity and reduce the net formation of sulfate. However, directly adding sulfur powder and iron filings (or iron compounds) to a reactor (such as a fixed bed or upflow reactor) presents the following insurmountable engineering bottlenecks: 1) The biofilm attached to the surface of sulfur-iron particles is easily detached under long-term operation or water flow scouring, making it difficult to retain biomass, resulting in slow system start-up, low and unstable denitrification load; 2) Iron filings are prone to caking and passivation in water, and their electron supply capacity decays rapidly, making it difficult to maintain and stably exert the synergistic effect with sulfur; 3) The sulfur-iron mixed bed is prone to clogging, resulting in large head loss and difficult maintenance.

[0006] Moving bed biofilm reactors (MBBRs) are renowned for their high biomass retention capacity, strong resistance to shock loads, and ease of operation and maintenance. Existing technologies have attempted to combine functional materials with MBBR carriers; for example, patent (CN117819698A) discloses a "slow-release iron-sulfur-based modified biological carrier packing," which mixes iron-sulfur-based powder with a biological carrier (including MBBR packing) using a binder. However, these technologies primarily focus on the "slow-release" function of the packing. Their simple mixing or surface loading methods, in the dynamic fluidized, long-term operation of MBBR environments, still struggle to address issues such as biofilm detachment due to friction and collision, loss of functional materials, and iron filings passivation. More importantly, existing technologies lack a design that considers this modified packing within a complete, parameter-optimized process system, failing to address the challenge of matching the iron-sulfur electron donor release rate with the microbial utilization rate in low-concentration wastewater scenarios, as well as the fine-tuning of system pH and sulfate byproducts.

[0007] Therefore, how to deeply and systematically combine ferrous sulfate materials with MBBR technology to build a ferrous sulfate autotrophic denitrification system that can start up quickly, operate under high load, operate stably, and effectively control byproducts has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0008] (a) The technical problem to be solved by the present invention

[0009] This invention aims to overcome the shortcomings of existing sulfur autotrophic denitrification technologies and their simple sulfur-iron mixture or loading processes, as well as the problems of easy biofilm detachment, easy material passivation, and short-lived synergistic effects of functional packing materials in engineering applications. This invention provides a low-concentration municipal wastewater treatment system and method based on deep coupling of MBBR and sulfur-iron composite matrix, characterized by rapid start-up, high nitrogen removal load, long-term stable operation, no need for external organic carbon sources, and simultaneous and effective control of sulfate generation and system pH.

[0010] (II) Technical Solution of the Invention

[0011] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0012] A deep denitrification system for low-concentration municipal wastewater based on MBBR coupled with sulfur-iron autotrophic denitrification includes the following components connected sequentially along the water flow direction: A pre-nitrification MBBR unit: used to efficiently and stably oxidize ammonia nitrogen in wastewater to nitrate nitrogen. This unit employs the MBBR process, with suspended biological packing added to the tank, and an aerobic environment maintained through aeration. A sulfur-iron autotrophic MBBR coupled denitrification unit: the core unit of this invention. A specially formulated sulfur-iron composite functional packing is added to the tank. This packing uses porous MBBR suspended packing (e.g., hydrophilic modified polyethylene or polypropylene packing, specific surface area ≥ 500 m² / m³) as a carrier, and through hot-melt embedding or high-strength adhesive fixing processes, composite particles composed of elemental sulfur (e.g., sulfur powder) and zero-valent iron (e.g., reduced iron powder) in a specific ratio are firmly fixed to the pores inside the carrier and its outer surface. This unit is equipped with a low-speed stirring device (such as a submersible propeller) to maintain the packing material in a uniform fluidized state (avoiding sedimentation and caking) while ensuring a strictly anoxic environment (dissolved oxygen DO < 0.5 mg / L). The micro-aerobic post-treatment unit is typically a contact tank or filter with micro-aeration. It consumes residual dissolved oxygen from the preceding unit, further converts any remaining nitrite, and utilizes the flocculation effect of ferrous / ferric ions generated during denitrification to enhance phosphorus removal, ensuring that the final effluent meets all standards.

[0013] Furthermore, in the sulfur-iron composite functional packing, the mass ratio of elemental sulfur to zero-valent iron is (4:1)-(6:1). This ratio range has been experimentally verified to optimally balance electron supply, acidity neutralization, and sulfate control. Furthermore, the filling volume ratio of the sulfur-iron composite functional packing in the coupled denitrification unit is 40%-60%. This filling rate ensures sufficient biofilm attachment surface area and electron donor reserves, while also ensuring good fluidization. Furthermore, the effluent dissolved oxygen concentration of the pre-nitrification MBBR unit is controlled at 0.5-2.0 mg / L. This control point is crucial, ensuring complete nitrification while preventing excessive dissolved oxygen from entering subsequent anoxic units and disrupting the denitrification environment. No additional deoxygenation facilities are required, resulting in energy savings and high efficiency. Furthermore, the hydraulic retention time (HRT) of the sulfur-iron autotrophic-MBBR coupled denitrification unit is designed to be 1.5-2.5 hours. For influent with low nitrate concentrations (typically <30 mg / L), this HRT range allows for reactor miniaturization while ensuring high removal rates.

[0014] A wastewater treatment method using the above system includes the following steps:

[0015] S1: The pre-treated (grit screen, grit removal, primary sedimentation) low C / N ratio municipal wastewater is pumped into the pre-nitrification MBBR unit, and the dissolved oxygen is controlled at 2-4 mg / L. The nitrifying bacteria attached to the suspended packing oxidize ammonia nitrogen into nitrate, and the dissolved oxygen in the effluent is precisely controlled within the range of 0.5-2.0 mg / L.

[0016] S2: The nitrified effluent from S1 is introduced into the sulfur-iron autotrophic-MBBR coupled denitrification unit. Under anoxic conditions (DO <0.5 mg / L), the autotrophic denitrifying bacteria (mainly sulfur-oxidizing denitrifying bacteria) pre-enriched and firmly attached to the sulfur-iron composite functional packing material gradually reduce nitrate nitrogen to nitrogen gas using sulfur and iron slowly released from the packing material as combined electron donors. Simultaneously, iron corrosion consumes H⁺, effectively buffering the system pH. S3: The denitrified effluent from S2 is introduced into the micro-aerobic post-treatment unit. After further purification through micro-aeration (DO approximately 0.5-1.0 mg / L), the water is discharged or reused.

[0017] Furthermore, in step S2, by adjusting the reflux ratio from the nitrification unit to the denitrification unit or by directly adjusting the influent flow rate, the surface load of nitrate nitrogen entering the coupled denitrification unit is controlled within the optimized range of 0.05-0.15 kg N / (m³·d) to achieve the best treatment efficiency and material utilization.

[0018] Furthermore, the rapid start-up method of the system involves inoculating the sulfur-iron autotrophic-MBBR coupled denitrification unit with sludge taken from a mature sulfur autotrophic denitrification system, using low-concentration nitrate as influent in the initial stage of start-up, and gradually increasing the load. This allows the system to reach the design denitrification performance within 10-15 days, and the start-up speed is significantly faster than that of the traditional sulfur-iron mixed bed.

[0019] (III) Beneficial Effects of the Invention

[0020] Compared with the prior art, the present invention has the following significant advantages and unexpected technical effects:

[0021] 1. Deep Coupling and Synergistic Effect: This invention does not simply mix sulfur-iron materials with the MBBR carrier, but achieves deep coupling through "internal embedding and fixation within the porous carrier." The MBBR carrier provides a stable and large attachment surface for microorganisms, while its porous structure provides physical protection for the sulfur-iron particles, greatly reducing the loss of functional materials and biofilm detachment caused by water flow shear and filler collision. Sulfur and iron are tightly bound in an optimized ratio, generating a continuous synergistic electron supply and pH buffering effect under the action of microorganisms, realizing a highly efficient cycle of "carrier retaining biofilm - biofilm utilizing matrix - matrix synergistic reaction."

[0022] 2. Rapid start-up and stable operation: Due to the superior biofilm attachment environment and readily available electron donors, the system's biofilm formation start-up speed is more than 50% faster than that of traditional sulfur-iron fixed-bed reactors. The carrier's isolation effect on iron filings effectively delays their passivation and caking, enabling the system to maintain high and stable nitrogen removal performance (>85% nitrate removal rate) for several months of operation, and exhibiting strong resistance to water quality fluctuations.

[0023] 3. Highly efficient byproduct control: By optimizing the sulfur-iron mass ratio (4:1-6:1) and operating parameters, the intervention of iron in the sulfate formation pathway and its consumption of H⁺ are maximized. Examples show that, under the same denitrification efficiency, the sulfate concentration in the effluent of the system of this invention can be reduced by 30%-50% compared to traditional sulfur autotrophic reactors, and the system pH can be automatically maintained within the optimal range of 6.5-7.5 for microorganisms, without the need for external alkalinity.

[0024] 4. System Integration and Energy Saving Advantages: Utilizing a pre-emergence MBBR nitrification system with controlled effluent DO, seamless and efficient integration of nitrification and denitrification biological processes is achieved. This avoids additional deoxygenation steps, simplifies the process, and reduces energy consumption. The entire system is compact, suitable for upgrading existing wastewater treatment plants or constructing new compact wastewater treatment facilities.

[0025] 5. Low operating costs: Relying entirely on the sulfur-iron composite packing as the electron donor, no external carbon source is required, resulting in extremely low reagent costs. The system's sludge production rate is far lower than that of heterotrophic denitrification systems, reducing sludge treatment and disposal costs. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the process flow of the wastewater treatment system described in this invention.

[0027] In the diagram: 1-Pre-nitrification MBBR unit; 2-Sulfur-iron autotrophic-MBBR coupled denitrification unit; 3-Micro aerobic post-treatment unit; 4-Microporous aeration disc; 5-Suspended biological packing; 6-Sulfur-iron composite functional packing; 7-Low-speed flow promoter; 8-Micro aeration head. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention.

[0029] Example 1: System construction and packing material preparation as follows Figure 1 As shown, an experimental system with a processing capacity of 1 m³ / d was constructed.

[0030] 1. Pre-nitrification MBBR unit (1): Effective volume 0.5 m³, with polyethylene suspended biological packing material (5) of specific surface area 650 m² / m³ added, and a filling rate of 40%. Microporous aeration discs (4) are used for oxygen supply, and the DO controller maintains dissolved oxygen at 3.0±0.5 mg / L. Through online monitoring, the DO of the effluent from this unit is controlled at 1.0±0.3 mg / L.

[0031] 2. Sulfur-iron autotrophic-MBBR coupled denitrification unit (2): effective volume 0.8 m³, which is the core of this invention.

[0032] Preparation of sulfur-iron composite functional filler (6): Take 200-mesh sulfur powder and reduced zero-valent iron powder (300-mesh) and mix them evenly at a mass ratio of 5:1. Mix the mixed powder with molten low-density polyethylene (as a binder) at a mass ratio of 3:2 in a heated stirrer, and then pour it into a porous polypropylene MBBR carrier (diameter 25 mm, specific surface area 550 m² / m³) in a pre-made mold. After cooling, sulfur-iron composite particles are formed and embedded in the pores and surface of the carrier to form functional fillers. The mass ratio of sulfur-iron composite in each filler is about 25%.

[0033] Unit assembly: The prepared sulfur-iron composite functional packing is put into the unit with a filling rate of 50% (by volume). A low-speed flow promoter (7) is installed at the bottom of the pool, and the speed is adjusted to about 30 rpm to ensure that the packing is uniformly fluidized and there are no obvious vortices on the water surface (to maintain anoxic conditions).

[0034] 3. Micro-aerobic post-treatment unit (3): effective volume 0.2 m³, using micro-aeration head (8), controlling DO at 0.5-0.8 mg / L.

[0035] Example 2: System Startup and Operation Method

[0036] 1. Influent water quality: Simulates low-concentration municipal sewage, with the following main indicators: COD=50-80 mg / L, NH4⁺-N=25-35 mg / L, TN=28-38 mg / L, pH=7.2-7.8, C / N≈2.

[0037] 2. Start-up phase: Simulated wastewater is introduced into the pre-nitrification unit and aerated normally. After the ammonia nitrogen removal rate stabilizes at >95%, subsequent startup begins. Approximately 10 L of sludge taken from a mature sulfur autotrophic denitrification filter column is added to the coupled denitrification unit (2) as inoculum. The initial influent is diluted nitrified effluent (NO3⁻-N≈10 mg / L), and the HRT is approximately 4 hours. The effluent nitrate is monitored every 2 days. When the removal rate stabilizes at >80%, the influent nitrate concentration is gradually increased and the HRT is shortened.

[0038] 3. Stable Operation: After approximately 12 days of startup, the system enters a stable operation period. The nitrate load entering unit (2) is controlled at approximately 0.08 kg N / (m³·d), and the HRT is set to 2.0 hours. The system performance is monitored after 30 days of continuous operation.

[0039] Example 3: Treatment Effect and Comparative Example

[0040] Comparative Example A: Following the approach in Comparative Document 1, a simple mixed packing was prepared: sulfur powder and iron powder in the same proportion were mixed with cellulose-based binder and simply adhered to the surface of the same MBBR carrier, and then filled into a reactor of the same volume to form a "surface-loaded sulfur-iron MBBR reactor".

[0041] Comparative Example B: Construction of a conventional sulfur autotrophic MBBR reactor: The carrier is the same MBBR packing material, but without loading any sulfur iron material. It is operated by adding sulfur powder (suspended) and a small amount of sodium bicarbonate (to provide carbon source and alkalinity) to the feed water.

[0042] Under the same influent water quality (effluent after nitrification, NO3⁻-N≈25 mg / L) and similar HRT (2 hours), the operating performance of the system of the present invention (example), Comparative Example A, and Comparative Example B (average value of one week during the stabilization period) was compared, and the results are shown in the table below:

[0043] project This invention (exemplary) Comparative Example A (Surface Load) Comparative Example B (Traditional Sulfur Autotrophic) <![CDATA[NO3⁻-N removal rate]]> 92.5% 78.3% 85.6% <![CDATA[SO4²⁻ concentration in the effluent]]> 162 mg / L 205 mg / L 288 mg / L System pH value 7.1 ± 0.2 6.5 ± 0.4 6.0 ± 0.5 (alkali needs to be added) Startup to stabilization time 12 days >25 days Approximately 20 days Packing condition after 30 days of operation The packing material is intact, the biofilm is thick, and there is no caking. The surface coating is partially worn, iron powder is visibly lost, and the biofilm in some areas is thinned. Sulfur powder deposition requires regular sludge removal, and the biofilm is prone to fluctuations.

[0044] Results analysis:

[0045] 1. Denitrification efficiency: The system of this invention has the highest denitrification efficiency, thanks to the dual protection of the biofilm and functional materials provided by the embedded structure, which offers the most stable reaction microenvironment. Comparative Example A suffers from decreased performance and instability due to the easy loss of surface-loaded materials.

[0046] 2. Byproduct control: This invention produces the lowest sulfate concentration and most stable pH in the effluent, fully demonstrating the enhancing effect of optimized sulfur-iron ratio and encapsulation structure on the synergistic effect of iron. Comparative Example B requires external alkalinity and has a high sulfate yield.

[0047] 3. Start-up and Stability: This invention starts up the fastest and maintains the best packing condition after long-term operation, verifying its advantages in engineering applications. Comparative Example A showed significant physical wear and tear on the packing.

[0048] In summary, this invention, through the innovative deep coupling design of MBBR carrier and sulfur-iron composite matrix, and the coordinated parameter control of the entire process system, successfully solves key technical problems such as carbon source shortage, by-product control, and system stability in deep denitrification of low-concentration municipal wastewater, achieving a comprehensive effect that is significantly superior to existing technologies.

[0049] Funding Statement

[0050] This invention was completed with the support of the National Natural Science Foundation of China (Project No.: 52370099).

Claims

1. A deep denitrification system and method for low-concentration municipal wastewater based on MBBR coupled with autotrophic denitrification of sulfur and iron, characterized in that, The system comprises, sequentially connected along the water flow direction, a pre-nitrification MBBR unit (1), a sulfur-iron autotrophic-MBBR coupled denitrification unit (2), and a micro-aerobic post-treatment unit (3); wherein, the sulfur-iron autotrophic-MBBR coupled denitrification unit (2) is filled with sulfur-iron composite functional packing material, which is based on porous MBBR suspended packing material, with sulfur-iron composite particles embedded and fixed in its internal channels and on its surface by hot melting or adhesive; the pre-nitrification MBBR unit (1) is used to efficiently convert influent ammonia nitrogen into nitrate, and its effluent dissolved oxygen concentration is controlled at 0.5-2.0 mg / L; The sulfur-iron autotrophic-MBBR coupled denitrification unit (2) is equipped with a low-speed stirring device to maintain the fluidization state of the sulfur-iron composite functional packing and ensure that the unit is in an oxygen-deficient environment; the filling rate of the sulfur-iron composite functional packing is 40%-60%; the micro-aerobic post-treatment unit (3) is used to consume residual dissolved oxygen, remove trace amounts of nitrite and remove phosphorus by flocculation with iron ions.

2. The system according to claim 1, characterized in that, In the sulfur-iron composite particles, the mass ratio of elemental sulfur to zero-valent iron is (4:1)-(6:1).

3. The system according to claim 1, characterized in that, The porous MBBR suspension packing is made of hydrophilic modified polyethylene or polypropylene, with a specific surface area of ​​not less than 500 m² / m³.

4. The system according to claim 1, characterized in that, The sulfur-iron composite particles have a particle size of 0.1-0.5 mm, and the mass percentage of sulfur-iron composite particles in the sulfur-iron composite functional filler is 15%-35%.

5. The system according to claim 1, characterized in that, The hydraulic retention time (HRT) of the sulfur-iron autotrophic-MBBR coupled denitrification unit (2) is 1.5-2.5 hours.

6. A wastewater treatment method using the system described in any one of claims 1-5, characterized in that, Includes the following steps: S1: Low C / N ratio municipal wastewater is introduced into the pre-nitrification MBBR unit (1), and the dissolved oxygen is controlled at 2-4 mg / L to oxidize ammonia nitrogen into nitrate. At the same time, the dissolved oxygen in the effluent is controlled at 0.5-2.0 mg / L. S2: The nitrified effluent from S1 is introduced into the sulfur-iron autotrophic-MBBR coupled denitrification unit (2). Under anoxic conditions, the autotrophic denitrifying bacteria attached to the sulfur-iron composite functional packing material are used to carry out denitrification with sulfur and iron as combined electron donors. The stirring speed of this unit is controlled to make the packing material fluidized uniformly and without obvious turbulence. S3: The denitrified effluent from S2 is introduced into the micro-aerobic post-treatment unit (3) to achieve final water purification through micro-aeration.

7. The method according to claim 6, characterized in that, In step S2, the nitrate load entering the sulfur-iron autotrophic-MBBR coupled denitrification unit (2) is controlled within the range of 0.05-0.15 kg N / (m³·d) by adjusting the reflux ratio or the influent flow rate.

8. The method according to claim 6, characterized in that, When the system is started, sulfur autotrophic denitrification sludge is inoculated into the sulfur-iron autotrophic-MBBR coupled denitrification unit (2). During the start-up period, the nitrate influent concentration gradient is controlled to rise, and the designed denitrification load is reached within 10-15 days.

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