Anaerobic digestion is the microbial conversion of organic matter to biogas — chiefly methane and carbon dioxide — in the absence of oxygen. It proceeds through four coupled biochemical stages (hydrolysis, acidogenesis, acetogenesis and methanogenesis) and, at 35 °C, yields about 0.35 m³ of methane per kilogram of COD destroyed.
What are the four stages of anaerobic digestion?
Anaerobic digestion is not a single reaction but a sequential microbial food chain in which the products of one guild become the substrate of the next. Four stages act in series:
- Hydrolysis. Extracellular enzymes cleave particulate biopolymers — carbohydrates, proteins and lipids — into soluble monomers (sugars, amino acids, long-chain fatty acids). For particulate-rich feeds such as sludge this step is rate-limiting.
- Acidogenesis (fermentation). Fermentative bacteria convert the monomers to short-chain volatile fatty acids (VFAs) — acetate, propionate, butyrate — plus hydrogen, CO2 and alcohols. This is the fastest step, which is why VFAs accumulate when a digester is overloaded.
- Acetogenesis. Obligate proton-reducing (syntrophic) acetogens oxidise propionate and butyrate to acetate, H2 and CO2.
- Methanogenesis. Archaea produce methane by two routes: acetoclastic (CH3COO- → CH4 + CO2) and hydrogenotrophic (4H2 + CO2 → CH4 + 2H2O). Roughly two-thirds of methane in a conventional digester comes from the acetate route.
These stages are the core of biological anaerobic wastewater treatment, and the same sequence governs both sludge stabilisation and high-rate industrial reactors.
Why does syntrophy control digester stability?
The acetogenic oxidation of propionate is thermodynamically unfavourable under standard conditions (ΔG°′ > 0). It only proceeds when the dissolved hydrogen partial pressure is kept extremely low — typically 10-4 to 10-6 atm — by hydrogenotrophic methanogens that continuously consume H2. This obligate mutualism is called interspecies hydrogen transfer.
ΔG°′ = +76 kJ/mol (endergonic in isolation). Coupled to hydrogenotrophic methanogenesis (ΔG°′ = −131 kJ/mol per mol CH4) the overall reaction becomes exergonic. Design implication: keep H2 partial pressure low by never letting methanogens wash out or be inhibited.
Because acetogens and methanogens are slow-growing and tightly coupled, any disturbance that inhibits methanogenesis (a temperature shock, an ammonia or sulphide spike, or hydraulic overload) causes H2 and then VFAs to accumulate, the pH to fall, and the digester to 'sour'. Stability is therefore a property of the whole consortium, not of any single organism.
How much methane can a feedstock yield? The Buswell equation
For a substrate of known elemental formula CnHaOb, the stoichiometric (theoretical maximum) methane yield under complete conversion is given by the Buswell equation:
where n, a, b are the moles of carbon, hydrogen and oxygen per mole of substrate. The equation assumes all carbon is converted and neglects the fraction diverted to cell synthesis, so it is an upper bound.
For glucose (C6H12O6: n=6, a=12, b=6) the coefficients give 3 CH4 + 3 CO2 per mole — a 50% methane biogas. Lipids, being highly reduced (large a, small b), yield the most methane per gram (≈1.0 m³/kg VS) while carbohydrates yield least (≈0.4 m³/kg VS); proteins sit between the two but release ammonia on degradation.
How is methane yield linked to COD? (0.35 m³/kg)
The most robust design basis is the electron balance expressed as chemical oxygen demand (COD). Because methane exerts a COD of 4 g O2 per gram CH4 (i.e. 64 g COD per mole of CH4, which occupies 22.4 L at STP), every unit of COD removed as methane maps to a fixed gas volume:
Derivation: 22.4 L CH4 / 64 g COD = 0.35 L/g = 0.35 m³/kg. At 35 °C the ideal-gas correction raises this to about 0.40 m³/kg. Some COD is diverted to biomass, so realised yields are typically 85–95% of theoretical.
This COD closure is invaluable in practice: it lets you predict gas production, check a mass balance and detect measurement errors, because the COD removed from the liquid must reappear either as methane or as new sludge. It is the same accounting used when designing energy recovery from sludge digestion and biogas capture.
Worked example: biogas from a distillery effluent
Consider a high-strength distillery wastewater treated in a UASB reactor:
- Flow: Q = 500 m³/day
- Influent COD: 15,000 mg/L = 15 kg/m³
- COD removal efficiency: 90%
- Operating temperature: 35 °C (mesophilic)
Step 1 — COD load applied: 500 m³/day × 15 kg/m³ = 7,500 kg COD/day.
Step 2 — COD removed: 0.90 × 7,500 = 6,750 kg COD/day.
Step 3 — assume 5% of removed COD is diverted to cell synthesis, leaving 95% converted to methane: 0.95 × 6,750 = 6,413 kg COD/day as CH4.
Step 4 — methane volume at 35 °C (0.40 m³/kg COD): 6,413 × 0.40 = 2,565 m³ CH4/day.
Step 5 — biogas volume, assuming methane is 65% of biogas: 2,565 / 0.65 = 3,946 m³ biogas/day.
Step 6 — recoverable energy: with a methane lower heating value of 35.8 MJ/m³, the methane carries 2,565 × 35.8 ≈ 91,800 MJ/day ≈ 25,500 kWh/day of primary energy. At 38% electrical conversion in a CHP engine that is roughly 400 kW of continuous electrical output — enough to make the plant a net energy exporter.
What loading rates and retention times govern design?
Three linked parameters size a digester. The organic loading rate (OLR) is the mass of COD or volatile solids applied per unit reactor volume per day (kg COD/m³·d). Hydraulic retention time (HRT) is the reactor volume divided by flow. Solids retention time (SRT) is the mean residence time of biomass.
where Q = flow (m³/d), S0 = influent COD (kg/m³) and V = reactor volume (m³). Conventional mixed digesters run at OLR 1–4 kg COD/m³·d; high-rate granular systems reach 10–30. Because methanogens grow slowly (μmax ≈ 0.1–0.4 d-1), SRT must exceed a critical washout value — commonly ≥12–15 days at 35 °C.
In a completely mixed reactor without biomass retention, HRT equals SRT, forcing large tanks. The central innovation of high-rate reactors is to decouple SRT from HRT: by retaining biomass as granules or a fixed film, they hold a long SRT in a small volume, allowing far higher OLR and much shorter HRT. Reaction rate follows Monod kinetics, r = μmax·S/(Ks + S), so above the half-saturation constant Ks the rate is substrate-independent and biomass concentration governs throughput.
How do you keep a digester stable? pH, VFA and inhibition
The methanogenic archaea have a narrow optimum near pH 6.8–7.4. Stability rests on the buffering provided by the bicarbonate/CO2 system, quantified by the VFA-to-alkalinity ratio:
- < 0.3 — stable, well-buffered operation.
- 0.3–0.5 — caution; VFAs are beginning to outrun methanogenesis.
- > 0.5 — impending souring; reduce loading and, if needed, dose alkalinity.
The principal inhibitors are free ammonia (NH3, the un-ionised form, problematic above roughly 700–1,100 mg/L as N and rising sharply with pH and temperature) from protein-rich feeds, and hydrogen sulphide (H2S) from sulphate reduction, where sulphate-reducing bacteria also outcompete methanogens for hydrogen and acetate. Both are pH- and temperature-dependent equilibria, so an ammonia or sulphide problem often worsens precisely when the operator raises temperature to lift the rate.
Conversely, methanogens have specific micronutrient demands: trace metals such as nickel, cobalt, iron and molybdenum are cofactors in the key enzymes (nickel sits at the active site of the methyl-coenzyme M reductase that forms the final C–H bond of methane). Deficiency limits the rate as surely as toxicity does, which is why many high-rate industrial digesters are dosed with a trace-element supplement. Sodium, potassium and calcium show the classic dual response — stimulatory at low concentration, inhibitory in excess — so a stable digester is one held within a window on every ion, not merely below a single toxicity threshold.
Mesophilic vs thermophilic, and which reactor to choose
Digestion runs in two temperature regimes: mesophilic (30–38 °C) and thermophilic (49–57 °C). Thermophilic operation roughly doubles reaction rates (consistent with an Arrhenius Q10 ≈ 2), allowing shorter SRT and giving superior pathogen kill, but it is more sensitive to inhibition, demands more heating energy and produces a higher fraction of free ammonia. Pre-treatment such as thermal hydrolysis of sludge accelerates the rate-limiting hydrolysis step and lets a downstream digester run at much higher solids and OLR.
Reactor selection follows the feedstock. Particulate, high-solids feeds (municipal sludge, manure) suit completely mixed tanks; soluble, high-strength industrial effluents suit granular high-rate reactors. The table compares the main configurations.
| Reactor type | Typical OLR (kg COD/m³·d) | HRT | Best-suited feed |
|---|---|---|---|
| CSTR (completely mixed) | 1–4 | 15–30 days | Sludge, manure, high-solids slurries |
| UASB (upflow, granular sludge) | 10–20 | 4–12 hours | Soluble industrial effluent (brewery, distillery, food) |
| EGSB (expanded granular bed) | 15–30 | 2–6 hours | Dilute or low-temperature soluble effluent; high upflow velocity |
| Anaerobic filter / fixed film | 5–15 | 0.5–4 days | Low-solids effluents where granulation is poor |
UASB and EGSB reactors depend on the formation of dense, fast-settling microbial granules that give exceptional biomass retention. For guidance on matching these technologies to a specific stream, see our overview of industrial wastewater treatment.
Frequently asked questions
What is anaerobic digestion in simple terms?
Anaerobic digestion is the breakdown of organic matter by microorganisms in the absence of oxygen. It converts biodegradable material into biogas — a mixture of about 55–70% methane and 30–45% carbon dioxide — plus a stabilised residual digestate. It is used both to stabilise sewage sludge and to treat high-strength industrial effluents while recovering energy.
How much biogas does anaerobic digestion produce?
Yield depends on feedstock, but a reliable rule is 0.35 m³ of methane per kilogram of COD removed at STP, rising to about 0.40 m³/kg at 35 °C. On a volatile-solids basis, carbohydrates yield roughly 0.4 m³/kg, proteins around 0.5, and lipids up to 1.0 m³/kg because they are the most chemically reduced.
What is the difference between mesophilic and thermophilic digestion?
Mesophilic digestion operates at 30–38 °C and thermophilic at 49–57 °C. Thermophilic roughly doubles reaction rates, allowing shorter retention times and better pathogen destruction, but it needs more heating energy and is more prone to ammonia inhibition and process upset. Mesophilic operation is more robust and is the more common choice.
Why do anaerobic digesters fail or "go sour"?
Souring happens when volatile fatty acids are produced faster than the slow-growing methanogens can consume them, so acids accumulate and the pH falls. It is usually triggered by organic overloading, a temperature shock, or inhibition by free ammonia or hydrogen sulphide. Monitoring the VFA-to-alkalinity ratio — keeping it below about 0.3 — gives early warning.
What is the Buswell equation used for?
The Buswell equation predicts the theoretical maximum methane and carbon dioxide yield from a substrate of known elemental formula CnHaOb. It provides an upper bound on biogas production for feedstock screening and mass-balance checks, though real yields fall below it because some carbon is diverted to microbial cell synthesis and not all material is biodegradable.
What is the role of the VFA-to-alkalinity ratio?
It is the key early-warning indicator of digester stability. It compares the concentration of volatile fatty acids to the bicarbonate buffering capacity. A ratio below 0.3 signals stable, well-buffered operation; above 0.5 warns of impending acidification, prompting the operator to reduce the organic loading rate or dose alkalinity before the pH collapses.