Large-scale discharge of nitrogen and phosphorus can cause eutrophication of water bodies.

Large-scale discharge of nitrogen and phosphorus can cause eutrophication of water bodies. Therefore, China uses ammonia nitrogen and total phosphorus as important assessment indicators for evaluating the treatment effect of sewage treatment plants. At present, sewage treatment mainly relies on biological denitrification, which converts nitrogen in sewage into harmless nitrogen gas through aerobic nitrification and anoxic denitrification.

Total nitrogen refers to the nitrogen content in soluble and suspended particles, including inorganic nitrogen such as NO3-, NO2-, and NH4+, and organic nitrogen such as amino acids, proteins, and organic amines. Biological denitrification first involves converting organic nitrogen into ammonia nitrogen through ammonification in an anaerobic environment. This process is easily carried out and can be completed in most treatment facilities. Then, in an aerobic environment, ammonia nitrogen is converted into nitrate nitrogen through nitrification. Finally, in an anoxic environment, nitrate nitrogen is converted into ammonia gas through denitrification, which escapes from the water.

The main denitrification processes include activated sludge processes (A2O, oxidation ditch, SBR, etc.) and biofilm processes (biological filters, biological contact oxidation tanks, biological rotating discs, etc.), which have good removal effects on nitrogen in wastewater, but have certain limitations and complexities in terms of process and operation.

The A2O process, or anaerobic-anoxic-aerobic activated sludge process, involves wastewater flowing through three distinct functional zones—anaerobic, anoxic, and aerobic—where different microbial communities remove organic matter, nitrogen (N), and phosphorus (P). The A2O process is the simplest simultaneous phosphorus and nitrogen removal process, characterized by a short total hydraulic retention time. Under alternating anaerobic, anoxic, and aerobic conditions, it inhibits the growth of filamentous bacteria, overcomes sludge bulking, and typically achieves a sludge viscosity (SVI) of less than 100. This facilitates the separation of treated wastewater from sludge. The anaerobic and anoxic sections require only gentle agitation during operation, resulting in low operating costs.

Advantages: This process is the simplest simultaneous nitrogen and phosphorus removal, with a small total hydraulic retention time and a small total floor space; under anaerobic-aerobic alternating operation conditions, filamentous bacteria cannot proliferate in large quantities, and there is no sludge bulking; the sludge has a high phosphorus concentration and high fertilizer effect; no chemicals are required during operation, only gentle stirring is needed, and the operating cost is low.

Disadvantages: Phosphorus removal efficiency is difficult to improve further, sludge growth has a certain limit and is not easy to improve; nitrogen removal efficiency is also difficult to improve further, the internal circulation volume should not be too high, otherwise it will increase operating costs; a certain concentration of dissolved oxygen should be maintained in the sedimentation tank, the residence time should be reduced, and the dissolved concentration should not be too high to prevent the circulating mixed liquor from interfering with the reactor.

Oxidation ditches, also known as continuous circulation reactors, are a modification and development of the conventional activated sludge process and a special form of extended aeration.

Its main functions are to supply oxygen; ensure that the activated sludge is in a suspended state, allowing for thorough mixing and contact between wastewater, air, and sludge; and drive water to circulate along the length of the tank at a certain flow rate (not less than 0.25 m/s), which is crucial for maintaining the purification function of the oxidation ditch. Oxidation ditches offer advantages such as good effluent quality, strong resistance to shock loads, high phosphorus and nitrogen removal efficiency, easy sludge stabilization, low energy consumption, and ease of automated control.

However, in actual operation, a series of problems still exist, such as sludge bulking, foaming, sludge floating, uneven flow rate, and sludge deposition.

The intermittent activated sludge process, or SBR process for short, has an operating cycle that can be divided into five stages: influent, reaction, sedimentation, effluent, and idle. This integrated process is characterized by its simplicity. Since there is only one reaction tank, there is no need for a secondary sedimentation tank, return sludge and related equipment. Generally, an equalization tank is not required, and in most cases, a primary sedimentation tank can be omitted.

Features: In most cases, there is no need to set up an equalization tank; the SVI value is low, it is easy to settle, and sludge bulking generally does not occur; phosphorus and nitrogen removal reactions are carried out by adjusting the operation mode; the degree of automation is high; when done properly, the treatment effect is better than continuous treatment; the unit investment is relatively small; the footprint is large, but the water volume treated is small.

Problems: Both A2O and oxidation ditch processes require large tank areas, resulting in high infrastructure costs; sludge return and sedimentation processes are complex and energy-intensive, making them difficult for ordinary small wastewater treatment plants to undertake and unsuitable for wastewater treatment plant retrofitting. The SBR process requires high-precision decanters to ensure effluent quality, and a subsequent equalization tank is needed to regulate the effluent flow rate, placing high demands on automation.

Biological filters require a large area, and the fixed carriers in biological contact oxidation tanks are difficult to construct and maintain; both are also prone to clogging, posing significant challenges to the long-term stable operation of wastewater treatment plants. Biological rotating discs, on the other hand, handle smaller volumes of wastewater and are more suitable for wastewater treatment plants with smaller treatment capacities.

The MBBR process is developed based on biological filters and biological fluidized bed processes. By simultaneously leveraging the advantages of biofilm and activated sludge processes, it overcomes the problems of packing blockage and high energy consumption of backwashing that are often encountered in biofilm processes, as well as the problems of sludge loss in activated sludge processes, making its biological treatment effect more effective.

MBBR carriers are made of polymer materials that incorporate various trace elements that promote rapid microbial attachment and growth. They are modified and constructed using special processes, resulting in carriers with advantages such as large specific surface area, good hydrophilicity, high biological activity, rapid biofilm formation, good treatment effect, and long service life.

Microorganisms can attach extensively to the MBBR carrier, resulting in a significant increase in biomass in the biological treatment system while maintaining a constant sludge concentration. This leads to a corresponding improvement in the system's treatment capacity and efficiency, and enhances its resistance to shock loads from varying water qualities. When the biofilm attached to the MBBR carrier reaches a certain thickness, it creates a dissolved oxygen gradient, resulting in anoxic zones within the carrier in the aerobic tank. This allows denitrifying bacteria to perform denitrification within the carrier, i.e., simultaneous nitrification and denitrification. This effectively conserves carbon sources, enabling good nitrogen removal capacity even at lower carbon-to-nitrogen ratios.

MBBR carriers all have a density of less than 1, and after biofilm formation, their density is similar to that of water, allowing them to remain suspended in water. In actual operation, aeration combined with stirring is used to fluidize the carriers in the water, forming a gas-liquid-solid three-phase fluidization. This enhances the contact between the gas, liquid, and carrier phases, significantly improving oxygen utilization efficiency and effectively reducing aeration volume and energy consumption.

The MBBR process only requires the addition of a specific additive and the installation of a carrier screen on top of the existing biological treatment process. It achieves enhanced nitrogen removal capacity without extensive infrastructure construction, significantly reducing investment costs. It shows promising development prospects in the upgrading and retrofitting of wastewater treatment plants.

Traditional denitrification processes oxidize NH4+ to NO2-, and then to NO3-. The active agents are nitrite-oxidizing bacteria and nitrifying bacteria, collectively known as nitrifying bacteria. The following conclusions can be drawn: nitrite oxidation produces more energy than nitrification, hence its faster reaction rate; nitrite oxidation generates a large amount of H+, lowering the system pH, while nitrification has no effect on the system pH; the aerobic ratio between nitrite oxidation and nitrification is 3:1; nitrite-oxidizing bacteria and nitrifying bacteria have largely similar physiological characteristics, but nitrite-oxidizing bacteria have shorter lifespans and faster growth, thus they are better able to adapt to shock loads and adverse environmental conditions.

When nitrifying bacteria are inhibited, NO2- will accumulate. Clearly, in the traditional nitrification-denitrification nitrogen removal process, under the action of denitrifying bacteria, denitrification can begin from either nitrate or nitrite. However, the repeated conversion from NO2- to NO3-, and then from NO3- back to NO2-, consumes more dissolved oxygen and organic carbon sources. If, in actual processes, this conversion process is controlled so that all or most of NH4+ is converted to NO2- instead of NO3-, and denitrification occurs directly from NO2-, this process is called short-cut nitrification-denitrification. Through the unremitting efforts of environmental workers, short-cut nitrification-denitrification has been achieved in many reactors.

Compared with traditional denitrification processes, short-cut nitrification-denitrification exhibits the following advantages.

1. Energy saving: During the nitrification stage, oxygen supply is reduced by nearly 25%, thus reducing energy consumption;
2. Saves external carbon source: The denitrification process from NO2- to N2 reduces the organic carbon source by 40% compared to the process from NO3- to N2;
3. It can shorten the hydraulic retention time: In a high ammonia environment, the nitrification rate of NH4+ and the denitrification rate of NO2- are faster than the oxidation rate of NO2- and the denitrification rate of NO3-. Therefore, the hydraulic retention time can be shortened and the reactor volume can be reduced accordingly.
4. Reduced sludge production: The apparent yield coefficient of nitrite-oxidizing bacteria is 0.04~0.13gVSS/gN, the apparent yield coefficient of nitrifying bacteria is 0.02~0.07gVSS/gN, and the apparent yield coefficients of NO2-denitrifying bacteria and NO3-denitrifying bacteria are 0.345gVSS/gN and 0.765gVSS/gN, respectively. Therefore, sludge production can be reduced by 24~33% during short-cut nitrification and denitrification, and by 50% during denitrification.

Problems: Short-cut nitrification-denitrification (SCD) processes are currently in the research stage, with limited practical engineering applications. Due to the difficulty in controlling factors such as temperature and pH during the SCD stage, more sophisticated online detection and fuzzy control technologies are needed to achieve stable SCD processes and expand their application.

Anaerobic ammonia oxidation is a biological reaction process in which anaerobic ammonia-oxidizing bacteria use nitrite as an electron acceptor to oxidize ammonia nitrogen into nitrogen gas under anaerobic conditions. This reaction usually has relatively harsh requirements on external conditions (pH, temperature, dissolved oxygen, etc.), but because it does not require the participation of oxygen and organic matter, its research and process development are of sustainable development significance.

Anaerobic ammonia nitrogen treatment typically involves a pre-treatment short-cut nitrification process to convert a portion of the ammonia nitrogen in wastewater into nitrite. There are already successful examples of its application in treating wastewater from coking plants and landfill leachate.

Anaerobic ammonium oxidation is a microbial reaction that produces nitrogen gas. It offers several advantages: because ammonia directly acts as an electron donor in denitrification, exogenous organic matter is eliminated, saving operating costs and preventing secondary pollution; oxygen is effectively utilized, reducing oxygen supply energy consumption; and because some ammonia participates directly in anaerobic ammonium oxidation without undergoing nitrification, acid production is reduced and alkali production is zero, thus decreasing the amount of chemical reagents needed for neutralization, lowering operating costs, and mitigating secondary pollution.

This process removes suspended solids (SS), chemical oxygen demand (COD), and BOD, and performs nitrification, denitrification, phosphorus removal, and AOX (a harmful substance). It is characterized by integrating biological oxidation and suspended solids interception, saving the need for a subsequent sedimentation tank (secondary sedimentation tank). It has a large volumetric load and hydraulic load, a short hydraulic retention time, requires less infrastructure investment, produces good effluent quality, has low operating energy consumption, and saves operating costs.

BAF is a third-generation biofilm reactor that not only has the advantages of biofilm technology, but also plays an effective role in spatial filtration by using special filter media and proper gas distribution design.

Technological characteristics:

1. The air-water flow is horizontal and upward, which ensures excellent uniformity of air and water, prevents the formation of air bubbles and blockage in the filter media layer, and results in high oxygen utilization and low energy consumption.
2. In contrast to downflow filtration, upflow filtration maintains positive pressure conditions across the entire height of the filter bed, which can better avoid the formation of channeling or short-circuiting, thereby avoiding air traps that could affect the filtration process by forming channeling.
3. Upflow creates favorable semi-column push conditions for the process, ensuring the long-term stability and effectiveness of the BAF process even with high filtration rates and loads.
4. The use of horizontal upward airflow allows for better utilization of the filtration space. Air can carry solid matter deep into the filter bed, resulting in a high load and uniform solid matter in the filter tank, thereby extending the backwashing cycle and reducing cleaning time and the amount of air and water used during cleaning.
5. The cutting effect of the filter media on air bubbles prolongs the residence time of air bubbles in the filter bed, thereby improving oxygen utilization.
6. Due to the excellent sludge interception capacity of the filter, there is no need to install a secondary sedimentation tank after the BAF.