Introduction
With the rapid development of industrialization, wastewater treatment technologies are also constantly evolving. Among them, aerobic, anoxic, and anaerobic are three common biological treatment processes. They can effectively remove pollutants such as organic matter, nitrogen, and phosphorus in wastewater, and each has different characteristics. Today, in this article, we will explore the roles, processes, and how to select the appropriate one for your water treatment application of these three processes together with you.
What are Aerobic, Anoxic, Anaerobic Processes, and Their Roles?
Aerobic, anoxic, and anaerobic are three different biological treatment processes. The main differences among them lie in the presence or absence of oxygen and the oxygen concentration in the treatment tank. Here are their definitions and comparisons:
|
Process Type |
Oxygen Concentration |
Main Microorganisms |
Typical Reactions |
|
Aerobic |
High concentration of oxygen |
Aerobic bacteria |
Oxidation of organic matter into CO₂ and H₂O |
|
Anoxic |
Low concentration of oxygen |
Facultative anaerobic bacteria |
Reduction of nitrate into nitrogen gas |
|
Anaerobic |
No oxygen |
Anaerobic bacteria |
Decomposition of organic matter into methane and CO₂ |
Aerobic Process
The aerobic process refers to the oxidation and decomposition of organic matter in wastewater into carbon dioxide and water by aerobic microorganisms under the condition of sufficient oxygen. Common aerobic treatment processes include the activated sludge process, biological filters, and oxidation ditches.
(1) Advantages:
• High treatment efficiency, suitable for large-scale sewage treatment plants operating continuously.
• Good sludge settling performance, facilitating subsequent solid-liquid separation.
• Applicable to domestic sewage, industrial wastewater, and low-concentration organic wastewater.
(2) Disadvantages:
• Continuous oxygen supply is required through aeration equipment, resulting in high costs.
• Limited treatment effect on high-concentration organic wastewater.
• A large amount of excess sludge is generated, requiring additional sludge treatment facilities.
Anoxic Process
It refers to the process of biochemical reactions carried out by facultative anaerobic microorganisms in an environment with a low oxygen concentration. Anoxic treatment is mainly used for the denitrification process, that is, reducing nitrate into nitrogen gas to remove nitrogen pollutants in wastewater. Anoxic tanks and denitrification filters are common examples.
(1) Advantages:
• High-efficiency nitrogen removal to control eutrophication of water bodies.
• No oxygen supply is required, resulting in low energy consumption.
• Suitable for wastewater containing a high concentration of nitrate.
(2) Disadvantages:
• High requirements for the control of dissolved oxygen (DO).
• The activity of microorganisms in the anoxic process is greatly affected by temperature.
• In the denitrification process, it needs to be combined with the previous aerobic process to generate nitrate and cannot be used alone.
Anaerobic Process
In water treatment, generally, under the condition of no oxygen, anaerobic microorganisms are used to decompose organic matter into methane and carbon dioxide. This process is most suitable for the treatment of high-concentration organic wastewater, such as food processing wastewater and sludge digestion. Its representatives include anaerobic digesters, upflow anaerobic sludge blanket (UASB), and anaerobic filters.
(1) Advantages:
• The biogas produced can be recycled as an energy source.
• No oxygen supply is required, suitable for treating high-concentration organic wastewater.
• Less sludge is produced, which can save treatment costs.
(2) Disadvantages:
• The growth of its microbial community is slow, usually taking several weeks or even months.
• Anaerobic microorganisms are sensitive to heavy metals and sulfides in wastewater and are easily inhibited.
• The effluent quality is poor, and subsequent aerobic processes are required for further purification.
Their Main Roles in Water Treatment
Aerobic Process
(1) Removal of Organic Matter: Aerobic microorganisms convert the biodegradable organic matter (BOD) in wastewater into carbon dioxide (CO₂) and water (H₂O) through oxidation and decomposition.
(2) Removal of Ammonia Nitrogen (Nitrification): Nitrifying bacteria in the aerobic process convert ammonia nitrogen (NH₄⁺) in wastewater into nitrate (NO₃⁻). This is divided into two steps:
• Ammonia-oxidizing bacteria convert ammonia nitrogen into nitrite (NO₂⁻).
• Nitrite-oxidizing bacteria further convert nitrite into nitrate (NO₃⁻).
(3) Removal of Part of Phosphorus: Under aerobic conditions, some phosphorus-accumulating bacteria can absorb phosphates in wastewater and store them in cells, thus reducing the phosphorus content in wastewater. However, this needs to be combined with the anaerobic process to achieve.
Anoxic Process
(1) Removal of Nitrate (Denitrification): The core of the anoxic process is denitrification, that is, reducing nitrate (NO₃⁻) into nitrogen gas (N₂).
(2) Utilization of Supplementary Carbon Source: In the denitrification process, it can use the organic matter in wastewater as a carbon source, reducing the operation cost.
(3) Synergistic Nitrogen Removal: When combined with the aerobic process, this process can form the A/O (anoxic/aerobic) or A²/O (anaerobic/anoxic/aerobic) process.
Anaerobic Process
(1) Degradation of High-Concentration Organic Matter: Anaerobic microorganisms decompose the complex organic matter in wastewater into simple organic matter and finally convert it into methane (CH₄) and carbon dioxide (CO₂).
(2) Production of Biogas: The anaerobic process produces biogas during the degradation of organic matter. Its main components are methane and carbon dioxide.
(3) Removal of Part of Suspended Solids: It removes part of the suspended solids (SS) in wastewater through the adsorption and precipitation of microorganisms, thus improving the effluent quality.
How to Select These Processes for Your Application
Selecting the appropriate wastewater treatment process needs to consider various aspects, including the wastewater treatment objectives, operation costs, and site conditions. Here are some suggestions for selecting the process:
1. Wastewater Treatment Objectives: Each treatment process has a different role. If your water treatment objective is to remove organic matter, you can choose the aerobic treatment process. If it is for nitrogen removal, you can choose the anoxic treatment process. And the anaerobic treatment is generally used for high-concentration organic wastewater.
2. Operation Costs: The operation cost of the aerobic treatment process is relatively high, mainly because sufficient oxygen needs to be provided. While the operation cost of the anaerobic treatment process is relatively low. Since it does not require any equipment to supply oxygen and can produce biogas as an energy source.
3. Site Conditions: The aerobic treatment process requires more space and equipment costs. Because it will generate a lot of sludge during the water treatment process, and additional equipment is needed for cleaning and transportation. The same is true for the anaerobic process, which will produce biogas.
4. Combined Processes: Sometimes, you can also combine these three processes for use. According to our tests, they can effectively improve the water quality and reduce the production of traditional activated sludge.
Conclusion
From the above content, we have learned about the roles and characteristics of the aerobic, anoxic, and anaerobic treatments in water treatment, as well as how to select the appropriate one for the application. As a professional water treatment supplier, AquaSust has rich experience in project cooperation. If you want to customize, design, install, or renovate a water treatment project, please contact us to upgrade your treatment efficiency.