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1. Biological Contact Oxidation Process
The biological contact oxidation process is a wastewater biological treatment method derived from the biofilm process. In the biological contact oxidation tank, a certain amount of packing material is installed. Microorganisms grow and attach to the surface of the packing material, forming a biofilm. With sufficient oxygen supply, the microorganisms oxidize and decompose organic matter in wastewater through biological oxidation, thereby achieving purification.
This method combines the characteristics of both the activated sludge process and the biofilm process, incorporating the advantages of both systems. Under biodegradable conditions, it has achieved good economic results when applied to industrial wastewater, aquaculture wastewater, and domestic sewage treatment. Due to its advantages such as high efficiency, energy saving, small footprint, strong resistance to shock loads, and convenient operation and management, this process is widely used in wastewater treatment systems across various industries.
Domestic sewage from residential communities (after septic tank pretreatment) flows by gravity into a fine screen tank, where large settleable solids and suspended materials are removed. The water then enters an equalization tank. From there, the effluent flows into the biological contact oxidation tank. Soft packing materials are installed inside the tank and aerated.
As wastewater passes through the packing layer, biofilms grow on the packing surface. A small blower provides aeration, allowing wastewater to fully contact the biofilm under aerobic conditions. Microorganisms oxidize and decompose the remaining organic matter into carbon dioxide, water, and microbial cells, thereby purifying the wastewater. At the same time, dissolved oxygen levels are controlled to ensure that ammonia nitrogen in the wastewater is converted into nitrate nitrogen through nitrification.
The effluent then enters a sedimentation tank for solid-liquid separation. Afterward, it flows into a filtration tank filled with hard packing materials such as quartz sand. This stage further adsorbs and removes remaining particles, ensuring that the treated water meets discharge standards. Finally, the water enters a disinfection tank where chlorine dioxide is used for disinfection before being discharged.
Underground domestic sewage treatment technology based on the biological contact oxidation process has the advantages of a small footprint and minimal impact on the surrounding landscape. In addition, underground systems generate low noise and minimal odor. The addition of micro-power aeration improves oxygen supply, enhances biological activity, and increases pollutant removal efficiency. The micro-power aeration unit adopts a modular structure, which is suitable for phased construction and expansion of community wastewater treatment facilities.
2. SBR Wastewater Treatment Process
SBR stands for Sequencing Batch Reactor Activated Sludge Process. It is a wastewater treatment technology that operates with intermittent aeration and is also known as the sequencing batch activated sludge process.
Unlike traditional wastewater treatment processes, SBR technology replaces spatial separation with time-based operation. Non-steady biochemical reactions replace steady-state reactions, and static sedimentation replaces conventional dynamic sedimentation. The main characteristic of SBR is its orderly and intermittent operation.
The core component of the SBR system is the SBR reaction tank, which integrates equalization, primary sedimentation, biological degradation, and secondary sedimentation into a single tank without requiring a sludge return system.
Because of these characteristics, the SBR process offers the following advantages:
1. The ideal plug-flow process increases the driving force of biochemical reactions and improves efficiency. Anaerobic and aerobic conditions alternate within the tank, resulting in excellent purification performance.
2. Stable operation. Wastewater settles under ideal static conditions, requiring less time while achieving high efficiency and good effluent quality.
3. Strong resistance to shock loads. The presence of retained treated water provides dilution and buffering effects, helping the system withstand fluctuations in water volume and organic load.
4. The treatment process can be adjusted according to changes in water quality and flow rate, providing flexible operation.
5. Fewer treatment units and simple structures make operation and maintenance easier.
6. The concentration gradient of DO and BOD5 within the reactor helps effectively control sludge bulking.
7. The SBR system is suitable for modular construction, which facilitates the expansion and upgrading of wastewater treatment plants.
8. Nitrogen and phosphorus removal can be achieved by controlling operating conditions to alternate between aerobic, anoxic, and anaerobic states.
3. A/O and A²/O Processes
A/O stands for Anoxic/Oxic. In addition to degrading organic pollutants, it also provides certain nitrogen and phosphorus removal functions. It uses anaerobic hydrolysis technology as a pretreatment stage for the activated sludge process, making it an improved activated sludge method.
The A/O process connects an anoxic stage followed by an aerobic stage. In the anoxic section, the dissolved oxygen (DO) is maintained below 0.2 mg/L, while in the aerobic section the DO is typically maintained between 2–4 mg/L.
In the anoxic stage, heterotrophic bacteria hydrolyze suspended pollutants such as starch, fiber, and carbohydrates, as well as soluble organic matter in wastewater, converting large organic molecules into smaller molecules and organic acids. Insoluble organic matter becomes soluble organic matter, improving the biodegradability of wastewater when it enters the aerobic stage.
During the aerobic process, proteins and fats are ammonified by microorganisms, releasing ammonia (NH₃ and NH₄⁺). Under sufficient oxygen supply, nitrifying bacteria oxidize ammonia nitrogen into nitrate (NO₃⁻). Through controlled recirculation, nitrate returns to the anoxic tank where denitrifying bacteria convert it into nitrogen gas (N₂), completing the ecological cycle of carbon, nitrogen, and oxygen and achieving harmless wastewater treatment.
Based on the above biological denitrification process, the A/O method has the following advantages:
(1) High efficiency. It effectively removes organic matter and ammonia nitrogen. When the total hydraulic retention time exceeds 54 hours, and the effluent undergoes coagulation and sedimentation, COD can be reduced to below 100 mg/L while other indicators also meet discharge standards. The total nitrogen removal rate can exceed 70%.
(2) Simple process, low investment, and low operating costs. Since the organic matter in wastewater acts as the carbon source for denitrification, there is no need to add expensive external carbon sources such as methanol.
(3) High pollutant removal efficiency in the anoxic denitrification stage. Removal rates of COD, BOD₅, and SCN⁻ can reach 67%, 38%, and 59% respectively, while phenol and other organic pollutants can achieve removal rates of 62% and 36%. Therefore, denitrification is an economical and energy-efficient degradation process.
(4) High volumetric loading capacity. Enhanced nitrification and high sludge concentration membrane technology in the denitrification stage significantly increase microbial concentration, resulting in higher volumetric loads compared with similar international processes.
(5) Strong resistance to shock loads. Even when influent quality fluctuates or pollutant concentrations are high, the process can maintain stable operation, making management simple and reliable.
Based on the above analysis of process flows and treatment performance, the biological denitrification process not only removes nitrogen but also degrades phenols, cyanides, COD, and other organic pollutants. Considering the characteristics of wastewater flow and quality, the recommended process is the internal circulation A/O biological denitrification process. This allows the sewage treatment system to achieve nitrogen removal while ensuring that all other discharge indicators meet environmental standards.
