1. COD Degradation Principle
COD refers to the total amount of oxidizable organic matter and reducible inorganic substances in water. The core of degradation is to break down organic matter into harmless small molecules, which can be categorized into two types
1. Biochemical degradation: Aerobic microorganisms (such as flocculent bacteria in activated sludge) metabolize organic matter, breaking it down into CO₂ and H₂O while synthesizing their own cells; anaerobic microorganisms decompose macromolecular organic matter into methane, CO₂, and other substances under oxygen-free conditions. This applies to easily degradable COD (e.g., carbohydrates and proteins in domestic wastewater).
2. Physical and Chemical Degradation: For refractory COD (such as aromatic hydrocarbons and heterocyclic compounds in industrial wastewater), advanced oxidation (Fenton, ozone oxidation) is employed to break the chemical bonds of organic matter, or adsorption (activated carbon) is used to directly separate pollutants, thereby reducing the COD value in water bodies.
The composition of Chemical Oxygen Demand (COD) primarily consists of () in water bodies
A. All organic matter
B. Organic matter that can be oxidized by strong oxidizing agents + partially reducible inorganic matter
C. All inorganic substances
D. Degradation of Recalcitrant Organic Matter II. Principles of Ammonia Nitrogen Degradation
Ammonia nitrogen (NH₃-N) degradation primarily involves the transformation of nitrogen elements, with biochemical pathways being the dominant method, while physical-chemical approaches are occasionally employed in specific cases
Biochemical nitrification-denitrification
Nitrification: Under aerobic conditions with suitable pH (7.5-8.5) and temperature (15-30°C), autotrophic nitrifying bacteria (nitrite-oxidizing bacteria + nitrate-oxidizing bacteria) first convert NH₃-N into nitrite nitrogen (NO₂⁻-N), then further transform it into nitrate nitrogen (NO₃⁻-N).
Denitrification reaction: Heterotrophic denitrifying bacteria, under anoxic conditions, use nitrate nitrogen as an electron acceptor, reducing it to N₂, which is released into the atmosphere, thereby completing nitrogen removal.
2. Physicalization Method
◦ Stripping method: Adjust the pH of the wastewater to 10.5-11.5, converting ammonium ions (NH₄⁺) into free ammonia (NH₃), and stripping the ammonia into the atmosphere through aeration.
Breakpoint chlorination method: Adding oxidants such as chlorine to oxidize ammonia nitrogen into N₂, suitable for emergency treatment of low-concentration ammonia nitrogen wastewater.
III. Core Factors Affecting COD Degradation
Water Quality Characteristics: Easily degradable COD (carbohydrates, proteins) is significantly influenced by microbial activity; hard-to-degrade COD (aromatic hydrocarbons, heterocyclic compounds) relies on the oxidation intensity of advanced oxidation processes, which cannot be effectively decomposed by conventional biochemical methods.
2. Microbial Conditions: Aerobic processes require sufficient dissolved oxygen (DO 2-4mg/L) and appropriate sludge concentration (MLSS 2000-4000mg/L); anaerobic processes demand a strictly oxygen-free environment and suitable sludge retention time (SRT). Imbalanced microbial populations will directly reduce degradation efficiency.
3. Environmental Parameters: Water temperature (optimal range: 20-35°C), pH (6.5-8.5). Low temperatures or strong acids/alkalis can inhibit microbial metabolism; toxic substances (heavy metals, phenols) can harm bacterial populations, leading to a sharp decline in COD removal efficiency.
4. Process Operation: The hydraulic retention time (HRT) and reflux ratio in biochemical methods, as well as the dosage of chemical reagents (e.g., the Fe²⁺ to H₂O₂ ratio in Fenton reagent) and reaction time in physicochemical methods, all affect the COD degradation efficiency.
4. Core Factors Influencing Ammonia Nitrogen Degradation
Nitrosation bacteria activity: Nitrosation bacteria are autotrophic, grow slowly, and are sensitive to environmental conditions. They require sufficient dissolved oxygen (DO ≥2mg/L) and a longer sludge retention time (SRT 10-20d). Anoxic conditions or excessively short sludge retention time can lead to stagnation in the nitrosation reaction.
2. Environmental Parameters: Water temperature (15-30°C), below 10°C significantly reduces nitrification rates; pH (7.5-8.5), acidic conditions inhibit nitrifying bacteria activity; toxic substances (such as heavy metals, cyanide) directly kill nitrifying bacteria.
3. Denitrification Conditions: Denitrifying bacteria require an oxygen-deficient environment and sufficient carbon source (C/N ratio ≥ 5:1). Inadequate carbon source prevents complete denitrification, leading to residual nitrate nitrogen from ammonia nitrogen conversion and difficulty in meeting total nitrogen standards.
4. Process parameters: The hydraulic retention time and aeration intensity in the nitrification stage, as well as the pH adjustment accuracy (10.5-11.5) and aeration air volume in the physicochemical method (stripping method), all affect the ammonia nitrogen removal efficiency.
V. Common Influencing Factors
• Influent Load: Excessive fluctuations in COD and ammonia nitrogen concentrations, exceeding the treatment process capacity, will result in超标 effluent water quality.
• Pretreatment Effect: If pre-treatment processes such as grates and grit chambers fail to effectively remove suspended solids and large particulate impurities, they can clog reactors, impair mass transfer efficiency, and indirectly reduce degradation performance.
