Anaerobic Digestion Process Steps: The 4 Stages And More

Anaerobic digestion is the natural, oxygen‑free breakdown of organic matter—such as food scraps, manure, and wastewater sludges—by communities of microbes. It turns waste into two useful products: biogas (mainly methane and carbon dioxide) and a nutrient‑rich digestate for soil. Crucially, it isn’t a single reaction but a relay of four linked stages—hydrolysis, acidogenesis, acetogenesis, and methanogenesis—each handled by different microorganisms that pass their intermediates to the next group.

This article gives you a clear, step‑by‑step map of the process. You’ll see how feedstock is prepared and pre‑treated, how each biochemical stage works and what it needs to stay stable, and how reactor choices (batch vs continuous, mesophilic vs thermophilic, wet vs dry) affect performance. We’ll outline control ranges for pH, temperature, loading and retention time, common pitfalls and how to spot trouble early, and what to do with digestate. We’ll finish at the outlet: upgrading raw biogas to biomethane, drying, and options for CO₂ capture—with a simple flow to tie it all together. Let’s begin with the high‑level process from feedstock to biogas and digestate.

Process overview: from feedstock to biogas and digestate

Organic feedstocks arrive, are screened and conditioned into a pumpable slurry, then flow into an airtight digester. Inside, the anaerobic digestion process steps run in sequence—hydrolysis, acidogenesis, acetogenesis, and methanogenesis—transforming complex organics into gas. The plant produces two saleable outputs: biogas, typically 50–80% methane with CO₂ and trace gases, captured for heat, power, or upgrading; and digestate, a nutrient‑rich liquid/solid stream that is dewatered and routed to agronomic use or further treatment.

At a glance: Feedstock → pre‑treatment → digester (hydrolysis → acidogenesis → acetogenesis → methanogenesis) → biogas handling → digestate handling.

Upstream feedstock preparation and pre-treatment

Upstream preparation sets digester stability and yield. The goal is a clean, consistent, pumpable (or stackable) substrate at the right solids, C:N and temperature so the anaerobic digestion process steps run smoothly. A typical line removes contaminants, size‑reduces, homogenises, adjusts solids, equalises, pre‑heats, and blends co‑substrates to 20:1–30:1 C:N.

• Contaminant removal: remove metals/plastics/grit to protect equipment.

• Solids regime: 4–8% TS for wet; 15–40% for dry systems.

• Pre‑treatment: mechanical, thermal, or biological to speed hydrolysis; gains up to 360% reported for some feeds, with biological best for protein‑rich.

Hydrolysis: breaking down complex organics

Among the anaerobic digestion process steps, hydrolysis is the opening gate: hydrolytic bacteria secrete extracellular enzymes that split insoluble polymers into soluble molecules the rest of the community can use. Glycosidases, peptidases, and esterases cleave carbohydrates, proteins, and lipids into sugars, amino acids, and long‑chain fatty acids. It is often the rate‑limiting step, especially with fibrous or fat‑rich feeds. Performance improves with slightly acidic pH (~5–6), stable temperature under the chosen regime (mesophilic 35–37°C or thermophilic 50–57°C), good mixing, and effective mechanical/thermal/biological pre‑treatment to speed surface attack and shorten the hand‑off to acidogenesis.

Acidogenesis: fermenting to volatile fatty acids and alcohols

Among the anaerobic digestion process steps, acidogenesis takes the soluble sugars and amino acids from hydrolysis and ferments them into intermediates—volatile fatty acids (VFAs), alcohols, CO2 and H2—ready for the next stage. Acidogenic bacteria commonly produce acetic, propionic and butyric acids, with acetate often dominating the VFA pool (roughly 40–88%). Pathways hinge on pH: around 4–4.5 favours acetate–ethanol products, while pH above 5.0 drives butyric‑type fermentation that yields more acetate, butyrate and hydrogen. Good mixing and buffering curb VFA build‑up, hold bulk pH near neutral, and protect downstream methanogens.

Acetogenesis: converting intermediates to acetate and H2/CO2

Acetogenesis is the third hand‑off in the anaerobic digestion process steps, where specialist syntrophic bacteria convert the higher VFAs and alcohols from acidogenesis into acetate, CO2 and H2. Key players include Syntrophobacter wolinii and Syntrophomonas wolfei. These reactions only proceed when hydrogen stays at very low partial pressure; methanogenic archaea continuously consume H2 and CO2, keeping conditions favourable for acetogens. In practice, keeping conditions stable and avoiding sudden loading shocks helps prevent H2 accumulation and ensures a strong acetate feed into methanogenesis.

Methanogenesis: making methane and biogas quality

Among the anaerobic digestion process steps, methanogenesis is the finale: methanogenic archaea convert intermediates into energy‑rich gas via two main routes—acetoclastic (from acetate) and hydrogenotrophic (from H2 + CO2). Around two‑thirds of methane typically derives from acetate. These microbes are strictly anaerobic and perform best at near‑neutral pH (about 6.5–8) under the chosen mesophilic or thermophilic regime. By scavenging hydrogen, they keep the low H2 partial pressure that lets acetogens run. The resulting biogas is usually 50–80% CH4 with CO2 and traces of H2S and moisture; stable pH, temperature and loading minimise VFA build‑up and protect gas quality.

Reactor configurations and operating modes

Reactor choice and operating mode set stability, throughput, and OpEx. Match configuration to feedstock solids, scale, and how tightly you need to steer the anaerobic digestion process steps. Below are the proven options.

• Batch vs continuous: Batch is simple; continuous delivers higher throughput and steadier biology.

• Single‑stage CSTR vs two‑stage: CSTR co‑locates all steps; two‑stage separates acids from methane for tighter control and shorter HRT.

• Wet vs dry: Wet (<15% TS) is pumpable; dry (15–40% TS) tolerates contamination and needs less pre‑processing.

• Mesophilic vs thermophilic: Mesophilic 30–38°C is robust; thermophilic 50–57°C is faster with better pathogen kill but higher heat demand and sensitivity.

Key operating parameters and control ranges

High yield depends on holding the biology inside safe bands. Keep the bulk environment steady so each of the anaerobic digestion process steps hands off cleanly—hydrolysers tolerate mild acidity, methanogens need neutrality and zero oxygen. Use these proven control ranges to minimise shocks and protect gas quality.

• Temperature: Mesophilic 30–38°C (optimum 35–37°C); thermophilic 50–57°C; daily drift ≤0.6°C.

• pH: Bulk 6.5–8; hydrolysis favours 5–6; acidogenesis pathways differ at ~4–4.5 vs >5.

• Solids regime: Wet 4–8% TS; dry 15–40% TS.

• C:N ratio: Target 20:1–30:1.

• Residence time: Single‑stage mesophilic 15–40 days; thermophilic ~14 days; shorter in two‑stage setups.

• Atmosphere and mixing: Strictly oxygen‑free; uniform mixing; avoid sudden load shocks.

Performance metrics, yields, and gas composition

Performance across the anaerobic digestion process steps hinges on methane per unit of organics and gas quality. Benchmark specific methane yield (SMY = CH4 volume / gVS added) with BMPs: cooked food waste ≈ 328 ml CH4/gVS; textile waste ≈ 174 ml CH4/gVS. In operation, track gas composition—methane 50–80% with CO2 and traces of H2S, oxygen ~0—and VS destruction (e.g., 45.6% to 33.8% as moisture fell 97%→89%).

Monitoring, inhibition, and troubleshooting

Daily control should mirror the anaerobic digestion process steps: track gas flow and composition (CH4/CO2), digester pH, temperature drift (keep ≤0.6°C/day), solids regime, and any sign of VFA build‑up. Rising VFAs with falling pH, a drop in methane percentage, or a spike in hydrogen are early markers that syntrophic hand‑offs between acetogenesis and methanogenesis are slipping.

• VFA surge/low pH: Cut loading, dilute, improve buffering, stabilise near‑neutral pH.

• Hydrogen spike: Smooth feed pulses, check mixing, give methanogens time to recover.

• Temperature shocks: Return to setpoint gradually; avoid step changes between regimes.

• Oxygen ingress: Find and seal leaks; maintain strictly oxygen‑free conditions.

• Slow hydrolysis: Enhance pre‑treatment/mechanical size reduction; blend to 20:1–30:1 C:N.

Digestate handling, dewatering, and end uses

Digestate is the second output of the anaerobic digestion process steps. On discharge, it is commonly split into a fibre and a liquor to lower haulage and match crop demand. Dewatering with a screw press or centrifuge yields a stackable solid and a pumpable fertiliser. Basic conditioning—pH adjustment, dilution, odour management—and covered storage safeguard nutrients and enable compliant field use.

• Solid fraction: soil amendment or compost blend.

• Liquid fraction: fertiliser via storage and injection.

• Where required: further treatment or nutrient recovery.

From biogas to biomethane: upgrading, drying, and CO2 capture

After the anaerobic digestion process steps yield raw biogas (about 50–80% CH4 with CO2, H2S, moisture and trace O2), upgrading turns it into saleable biomethane. A compact, automated train dries, removes sulphur and oxygen, separates CO2, and conditions gas for injection or sale.

• Desulphurisation: proprietary catalyst converts H2S to a non‑hazardous by‑product.

• Oxygen management: controlled O2 for H2S oxidation; catalytic removal to <10 ppm O2.

• CO2 and H2O: medium‑pressure drying, then CO2 separation; optional liquid CO2 capture (12–66 bar, >99 mole% purity).

Safety, odour, and compliance basics

AD plants are process‑safety assets. Keep systems strictly oxygen‑free; use fixed gas detection for CH4 and toxic H2S; ensure ventilation and pressure‑rated vessels/pipework built to API/ASME/CSA/EPA standards. Limit odour with enclosed reception, tight digester seals, desulphurisation, and covered digestate storage. Hold active permits, log emissions and incidents, and document stable anaerobic digestion process steps for inspections and land‑application compliance.

Key takeaways

From tipping floor to polished gas, anaerobic digestion is a disciplined relay between four specialist crews. Keep contaminants out, feed a steady, well‑balanced substrate, and hold temperature and pH in range so each stage passes a cleaner baton to the next—ending in predictable biogas and agronomic digestate. Upgrading then converts raw gas to biomethane with optional CO2 capture.

• Four stages: hydrolysis → acidogenesis → acetogenesis → methanogenesis.

• Control bands: meso 35–37°C; thermo 50–57°C; bulk pH 6.5–8.

• Feed basics: C:N 20:1–30:1; wet 4–8% TS; dry 15–40% TS.

• Watchpoints: VFAs, H2, CH4%, temperature drift ≤0.6°C/day.

• Value chain: raw gas 50–80% CH4; upgrading to biomethane with CO2 capture.

If you’re designing or retrofitting AD‑to‑biomethane plants, the biggest wins come from stable biology and smart, integrated upgrading. See how the compact BioTreater system streamlines desulphurisation, drying, oxygen control, and high‑recovery CO2 separation at guaranteed performance via 99pt5.