Anaerobic digestion is not a single process. It is a cascade of four distinct biological stages, each performed by different groups of microorganisms, each with different environmental requirements, and each dependent on the preceding stage for its feedstock. When all four stages are in balance, the system produces biogas -- a mixture of methane (50-70%), carbon dioxide (30-50%), and trace gases -- at a predictable rate from organic waste. When any stage becomes a bottleneck, the entire system suffers.
This guide explains the science of anaerobic digestion in practical terms. Whether you are operating a covered lagoon, a complete-mix digester, or evaluating whether anaerobic treatment is appropriate for your waste stream, understanding these fundamentals will help you make better decisions about system design, operation, and troubleshooting.
Stage 1: Hydrolysis
Hydrolysis is the first stage of anaerobic digestion, where complex organic polymers are broken down into simpler, soluble molecules. Proteins are broken into amino acids. Carbohydrates (cellulose, starch) are broken into simple sugars. Fats (lipids) are broken into long-chain fatty acids and glycerol. This stage is performed by hydrolytic bacteria that secrete extracellular enzymes (proteases, cellulases, lipases) into the surrounding environment.
Hydrolysis is often the rate-limiting step in anaerobic digestion, particularly for feedstocks with high fiber or cellulose content like crop residues, lignocellulosic biomass, or aged manure. The rate of hydrolysis depends on the particle size and surface area of the feedstock, temperature, pH (optimal range 6.0-7.0 for hydrolytic enzymes), and the inherent biodegradability of the material. Pretreatment methods like mechanical grinding, thermal hydrolysis, or chemical treatment can accelerate this stage.
Stage 2: Acidogenesis
In the acidogenesis stage, acidogenic (fermentative) bacteria convert the soluble products of hydrolysis into volatile fatty acids (VFAs), alcohols, hydrogen, and carbon dioxide. The primary VFAs produced are acetic acid, propionic acid, and butyric acid. This stage proceeds rapidly -- acidogenic bacteria have doubling times of hours rather than days -- and can easily outpace the downstream stages.
When acidogenesis outpaces subsequent stages, VFAs accumulate in the digester, driving pH downward. This is the most common mode of digester upset. If pH drops below 6.0-6.2, the methane-producing archaea in later stages become inhibited, methane production drops, and VFA accumulation accelerates -- creating a feedback loop that can crash the digester if not corrected.
Stage 3: Acetogenesis
Acetogenesis is an intermediary stage where acetogenic bacteria convert the longer-chain VFAs (propionic acid, butyric acid) and alcohols into acetic acid, hydrogen, and CO2. This stage is thermodynamically unfavorable under standard conditions -- it only proceeds when the hydrogen partial pressure is kept very low by the methanogens in Stage 4. This interdependence between acetogens and methanogens is called syntrophic metabolism, and it is one of the most remarkable examples of microbial cooperation in nature.
The practical implication is that acetogenesis and methanogenesis are tightly coupled. If methanogens are inhibited (by toxicity, temperature shock, or pH depression), hydrogen accumulates, acetogenesis stops, and the entire digestion process backs up. This is why maintaining stable conditions for methanogens is the single most important operational priority in any anaerobic digestion system.
Stage 4: Methanogenesis
Methanogenesis is the final stage, where methanogenic archaea convert acetic acid and hydrogen/CO2 into methane. There are two primary pathways: aceticlastic methanogenesis (acetic acid to methane and CO2, performed by Methanosaeta and Methanosarcina species) and hydrogenotrophic methanogenesis (hydrogen plus CO2 to methane and water, performed by Methanobacterium and Methanospirillum species). Approximately 70% of methane in typical digesters comes from the aceticlastic pathway.
- Temperature sensitivity: Methanogens operate in two distinct temperature ranges -- mesophilic (95-105F / 35-40C) and thermophilic (130-140F / 55-60C). Operation between these ranges (105-130F) is unstable because neither mesophilic nor thermophilic populations thrive.
- pH sensitivity: Optimal pH for methanogenesis is 6.8-7.4. Below 6.5, methanogenic activity drops sharply. Below 6.0, it effectively stops.
- Doubling time: Methanogenic archaea have doubling times of 5-15 days, much slower than the acidogenic bacteria in Stage 2. This slow growth rate means that recovery from upset conditions takes weeks, not days.
- Toxicity: Methanogens are sensitive to ammonia (above 3,000-5,000 mg/L as total ammonia nitrogen), heavy metals, sulfide (above 200 mg/L), and certain organic compounds. Feedstock screening for potential inhibitors is important.
Temperature Regimes
The choice between mesophilic and thermophilic operation has significant implications for system design and performance. Mesophilic digestion (95-105F) is more stable, more tolerant of load variations, and less energy-intensive to maintain. Thermophilic digestion (130-140F) achieves faster reaction rates, higher pathogen kill, and better volatile solids reduction, but requires more energy input and is more sensitive to process upsets.
Covered lagoon digesters operate at ambient temperature, which means they function in the psychrophilic (below 68F / 20C) to mesophilic range depending on climate and season. This is the least efficient temperature regime for methanogenesis, which is why covered lagoons require much longer retention times than heated digesters to achieve comparable volatile solids reduction.
Biogas Composition
- Methane (CH4): 50-70% by volume. The energy-carrying component. Methane content varies with feedstock composition and digestion efficiency.
- Carbon dioxide (CO2): 30-50% by volume. Produced in all four stages. CO2 dilutes the energy content of the biogas.
- Hydrogen sulfide (H2S): 100-10,000 ppm. Produced by sulfate-reducing bacteria. Corrosive and toxic. Must be removed before most utilization applications.
- Water vapor: Saturated at digester temperature. Must be removed (condensed) to prevent corrosion and operational problems in gas handling equipment.
- Trace gases: Nitrogen, oxygen (from air intrusion), siloxanes (from landfill gas), and other trace compounds depending on feedstock.
Practical Implications for System Design
Understanding the microbiology of anaerobic digestion leads directly to practical design and operational decisions. Size the system for the slowest stage -- usually hydrolysis or methanogenesis. Maintain temperature stability to protect methanogens. Monitor pH and VFA concentrations as early indicators of process imbalance. Ensure adequate alkalinity (buffering capacity) to prevent pH crashes. Avoid shock loading that overwhelms the slower-growing methanogenic population.
EFI USA designs covered lagoon digester systems with these biological principles at their foundation. Our systems are sized for reliable performance across seasonal temperature variations and normal operational fluctuations. Contact us to discuss your anaerobic digestion project.
