Operation principle and application of AAO wastewater treatment technology

Date public: 05-06-2024||View: 2052

AAO Wastewater Treatment Technology (Anaerobic-Anoxic-Oxic) plays an important role in treating polluted wastewater. With this capability, AAO technology not only helps clean water sources but also contributes to environmental protection and human health. Let's explore in detail the operating principles, as well as the advantages and disadvantages of AAO technology provided by Môi trường Đại Nam.

1. Concept of AAO Wastewater Treatment Technology

AAO technology stands for Anaerobic – Anoxic – Oxic. This is a biological method that relies on the growth and development of anaerobic, anoxic, and aerobic microorganisms to decompose pollutants in wastewater. Through the action of these microorganisms, wastewater is effectively treated and meets the discharge standards before being released into the environment.

2. The Operating Process of AAO Wastewater Treatment Technology

Through the degradation of pollutants in wastewater by anaerobic, anoxic, and aerobic microorganisms, wastewater is thoroughly treated.

Anaerobic Process: Removal of hydrocarbons, precipitation of heavy metals, precipitation of phosphorus, and removal of active chlorine.
Anoxic Process: Reduction of nitrate to nitrogen gas (N2), reduction of BOD and COD levels in wastewater.
Aerobic Process: Conversion of NH4 to NO3, removal of BOD, COD, and sulfides.
Sterilization: Using microfiltration or chemicals—mainly calcium hypochlorite (Ca(OCl)2 —to eliminate pathogenic microorganisms.

2.1 Anaerobic Biological Process (Anaerobic)

In anaerobic tanks, the process of decomposing dissolved organic matter and colloidal substances in wastewater occurs with the participation of anaerobic microorganisms. During growth and development, these microorganisms absorb dissolved organic matter in wastewater, decompose it, and convert it into gaseous compounds. Gas bubbles produced adhere to sludge particles, which float to the surface, causing local circulation in the suspended sludge layer.
The decomposition of organic matter by anaerobic microorganisms can be simplified by the following chemical equations:

Organic matter + Anaerobic bacteria → CO2 + H2S + CH4 + other substances + energy
Organic matter + Anaerobic bacteria + energy → C5H7O2N (new bacterial cells)

The gas mixture produced is commonly known as biogas. The anaerobic decomposition process is divided into three main stages: the breakdown of high-molecular-weight organic substances (hydrolysis), acid formation, and methane formation, which occur simultaneously during the anaerobic decomposition of organic matter.

Hydrolysis: Under the action of enzymes secreted by bacteria, complex substances and insoluble materials (polysaccharides, proteins, lipids) are converted into simpler compounds or soluble substances (sugars, amino acids, fatty acids). This process occurs slowly, and the rate of hydrolysis depends on pH, particle size, and the biodegradability of the substrate. Lipid hydrolysis is particularly slow.
Acidogenesis: In this stage, fermentative bacteria convert soluble substances into simple compounds such as volatile fatty acids, alcohols, lactic acid, methanol, CO2, H2, NH3, H2S, and new biomass.
Methanogenesis: Acetic acid, H2, CO2, formic acid, and methanol are converted into methane, CO2, and new biomass.

2.2 Anoxic Biological Process (Anoxic)

The anoxic tank is crucial in the biological treatment of nitrogen in wastewater. The most common method for nitrogen removal in wastewater today is nitrification and denitrification. The second step after nitrification is the reduction of nitrate-nitrogen to nitrogen gas, nitrous oxide (N2O), or NO, which is carried out in an anoxic environment and requires an electron donor, either organic or inorganic.

2.3 Aerobic Biological Process (Oxic)

Aerobic decomposition is based on the activity of aerobic microorganisms, which utilize dissolved oxygen in water to break down organic matter (pollutants to be treated). Microorganisms such as *Pseudomonas denitrificans*, *Bacillus licheniformis*, etc., reduce nitrate to N2, which is released into the atmosphere. The general conditions for nitrifying bacteria are pH = 5.5–9, with the optimal being 7.5. When pH < 7, bacterial growth slows, dissolved oxygen needs to be at 0.5 mg/L, and the temperature should be between 5–40°C.
This process is vigorous if influenced by factors such as aeration, increased microbial activity by adding activated sludge, adjusting nutrient levels, and inhibiting toxic substances that affect microbial activity. Additionally, the suitable temperature for treatment is 20–40°C, with the optimal range being 25–30°C.
The breakdown of organic matter in wastewater involves three stages:

Stage 1: Oxidation of organic matter: CxHyOz + O2 → CO2 + H2O + ∆H
Stage 2: Synthesis and cell construction: CxHyOz + O2 → Microbial cells + CO2 + H2O + C5H7NO2 – ∆H
Stage 3: Oxidation of cellular material: C5H7NO2 + 5O2 → 5CO2 + 2H2O + NH3 ± ∆H’

3. Applications of AAO Wastewater Treatment Technology

Currently, AAO technology is applied to wastewater with high organic content, such as domestic wastewater, hospital wastewater, and wastewater from various production and processing industries.
To enhance wastewater treatment efficiency, AAO technology can be combined with MBBR and MBR technologies.

4. Advantages and Disadvantages of AAO Technology

Advantages:

Cost-effective due to low construction, machinery, and operation costs.
Reduced sludge generation compared to other aerobic biological treatment technologies.
Depending on the design objectives, the treated water quality can meet standards A or B. The technology achieves high BOD removal efficiency (90–95%).
Easy to operate, with the ability to add new equipment without dismantling existing connections or devices.
Can effectively treat various types of wastewater and pollutants.

Disadvantages:

Factors such as microbial treatment efficiency, activated sludge settling capacity, temperature, pH, MLSS concentration, and input load will affect the output water quality.
Requires a large area for construction.
AAO technology requires maintaining a sludge concentration of 3–5 g/L. If the sludge concentration is too high, it will be difficult to settle and may be washed out. If the sludge concentration is too low, its treatment capacity will be low, leading to sludge overloading, death, and washout.
Post-treatment water sterilization is mandatory.

If your company has any further questions related to wastewater treatment, please contact Đại Nam Environmental Solutions Co., Ltd. for advice on building, installing, and operating the AAO wastewater treatment system. Additionally, Đại Nam uses many other modern technologies. Contact the hotline at 0909 378 796 to find the most suitable technology for your company.