Introduction
Anaerobic biological treatment is a wastewater treatment process that breaks down organic pollutants in the absence of oxygen. It relies on anaerobic microorganisms to convert complex organic compounds into simpler substances, primarily methane (CH₄) and carbon dioxide (CO₂). This method is widely used for high-strength industrial wastewater and sludge stabilization due to its energy efficiency and low sludge production.
Advantages of Anaerobic Treatment Over Aerobic Treatment
1. Higher Organic Loading Capacity
- The typical sludge loading (F/M) for anaerobic treatment of industrial wastewater is 0.5–1.0 kg BOD₅/(kg MLVSS·d), more than twice that of aerobic processes (0.1–0.5 kg BOD₅/(kg MLVSS·d)).
- Due to the absence of oxygen transfer limitations, the MLVSS (Mixed Liquor Volatile Suspended Solids) in anaerobic systems can reach 5–10 times that of aerobic systems.
- The organic volumetric loading rate for anaerobic treatment is 5–10 kg BOD₅/(m³·d), compared to only 0.5–1.0 kg BOD₅/(m³·d) for aerobic treatment-a 10-fold difference.
2. Lower Sludge Production & Better Sludge Quality
- Anaerobic treatment produces only 5%–20% of the biomass generated in aerobic processes.
- Aerobic methods produce 0.25–0.6 kg of sludge per kg of COD removed, while anaerobic methods yield only 0.02–0.18 kg, with better dewaterability.
- Anaerobic digestion also kills parasite eggs in sludge, improving its hygienic and chemical stability, reducing sludge disposal costs.
3. Lower Nutrient Requirements & Operational Flexibility
- Anaerobic microbes require only 5%–20% of the nutrients (N, P) needed by aerobic processes, making them suitable for nutrient-deficient wastewater.
- Anaerobic microorganisms remain active for months or even years without significant decline and can restart quickly after shutdowns, allowing intermittent operation (ideal for seasonal wastewater).
4. Energy Savings & Methane Production
- Aerobic treatment consumes 0.5–1.0 kWh of electricity per kg of COD removed for aeration, while anaerobic systems eliminate aeration costs.
- Anaerobic digestion produces methane, yielding over 12,000 kJ of energy per kg of COD removed.
- No foam issues (unlike aerobic treatment of surfactant-containing wastewater).
5. Reduced Air Pollution & Broader Degradation Capability
- Aerobic aeration can volatilize organic compounds, causing air pollution, whereas anaerobic systems avoid this issue.
- Anaerobic microbes can degrade certain recalcitrant compounds (e.g., chlorinated hydrocarbons) that aerobic bacteria cannot.
6. Complex Microbial Synergy for Enhanced Degradation
- Anaerobic digestion involves diverse microbial communities working synergistically, enabling the breakdown of hard-to-degrade organics that aerobic treatment cannot fully process.
Disadvantages of Anaerobic Treatment
1. Slow Microbial Growth & Longer Startup Time
- Anaerobic microbes grow slowly, requiring longer startup periods and hydraulic retention times (HRT) than aerobic systems.
2. Effluent Requires Further Treatment
- Anaerobic effluent often does not meet discharge standards and must be polished with aerobic treatment.
3. Alkalinity Supplementation Needed for Low-C/N Wastewater
- Low-concentration or low-C/N wastewater may lack alkalinity, requiring external alkalinity addition.
4. Heating Required for Low-Strength Wastewater
- If methane production is insufficient to maintain optimal temperatures (30–38°C), external heating is necessary.
5. Explosion Risk from Methane
- Biogas (CH₄ + CO₂ + H₂S) is flammable and explosive, requiring explosion-proof reactor designs.
6. Sensitivity to Toxic Compounds
- Chlorinated aliphatics and other toxins inhibit methanogens more severely than aerobic heterotrophs; improper operation can destabilize the system.
7. Strict Temperature Control Needed
- Low temperatures significantly reduce efficiency, and operational management is more complex than in aerobic systems.
8. H₂S Odor & Corrosion Issues
- Sulfate (SO₄²⁻) in wastewater produces H₂S, causing odors and corrosion in pipes, engines, and boilers.
- Sulfate reduction also consumes organic matter, reducing methane yield.
9. No Nitrification
- Anaerobic systems cannot nitrify ammonia; optimal microbial activity requires NH₃-N levels of 40–70 mg/L.