Anaerobic Digestion: What It Is, Process, Outputs, Uses

Anaerobic digestion is a natural, oxygen‑free process where microbes break down organic materials—such as manure, food waste and wastewater solids—inside sealed tanks to produce two outputs: biogas and digestate. The biogas is mainly methane and carbon dioxide and can fuel heat and power or be purified into renewable natural gas (biomethane). The digestate is a nutrient‑rich solid/liquid material that can return to land as fertiliser or be further processed. In short, it turns waste into useful energy and plant nutrients.

This article explains what anaerobic digestion is and how it works, what goes into a digester, and what comes out. You’ll find a clear overview of core technologies, operating conditions, biogas and digestate quality and uses, upgrading to biomethane and CO2 capture, where AD is deployed, benefits and risks, compliance essentials, project economics, plus a quick comparison with aerobic digestion. It’s a practical guide for evaluating, operating or specifying AD systems.

How anaerobic digestion works

At its core, anaerobic digestion is staged microbiology in a sealed, oxygen‑free reactor. Prepared organic feedstocks are pumped in, mixed and held at controlled temperature so specialised microbes convert volatile solids to gas. Biogas rises to the headspace and is drawn off; partially stabilised slurry (digestate) flows out for separation. Co‑digestion—blending manures with food wastes or FOG—often boosts gas yield and process stability. Depending on temperature and design, hydraulic retention typically spans days to weeks.

1. Hydrolysis: Enzymes break complex polymers (carbohydrates, proteins, lipids) into soluble monomers.

2. Acidogenesis: Fermenters turn monomers into volatile fatty acids, plus CO2, H2 and ammonia.

3. Acetogenesis: VFAs are converted to acetate, additional CO2 and H2—key precursors for methanogens.

4. Methanogenesis: Archaea produce methane and carbon dioxide, forming the bulk of usable biogas.

In practice, raw biogas (methane, CO2, traces of H2S and water vapour) is collected for heat/power or upgrading, while digestate is dewatered into liquid and solid streams for agronomic use. This is the operational heartbeat behind what is anaerobic digestion.

Inputs and feedstocks: what digests well (and what doesn’t)

Anaerobic digestion thrives on biodegradable, consistent organics. Gas yield and stability depend on volatile solids content, moisture and steady supply. Co‑digestion—blending manures with food wastes or fats, oils and greases (FOG)—commonly boosts methane output and smooths operation. If you’re evaluating what is anaerobic digestion in practice, start with the recipe: fresher materials and predictable blends reduce process risk and downtime.

• Dairy/cattle manure: Proven backbone for farm‑based systems with reliable performance.

• Food wastes and by‑products: Break down rapidly and raise biogas production when co‑digested.

• FOG (fats, oils, greases): High‑energy co‑substrate when dosed and controlled.

• High‑strength industrial/municipal solids: Established AD feedstocks in wastewater applications.

• Energy crops and yard waste (dry systems): Effective where economics support them and in high‑solids designs.

• Poultry or swine manure alone: Higher nitrogen; blend with other materials to avoid inhibition.

• Highly diluted manure/washwater: Low solids reduce efficiency; bypass or pre‑manage where possible.

• Sand, grit and inorganics: Settle, displace volume and force costly clean‑outs.

• Cleaning agents and chemicals: Can inhibit microbes; keep them out of the digester.

• Sudden recipe changes or inconsistent loads: Introduce new feedstocks gradually; use reputable suppliers and screening.

The optimal feedstock mix directly informs the digester technology you should select next.

Digester technologies you should know

Technology choice should follow your recipe, solids content and operating envelope. In anaerobic digestion, the higher the solids, the harder it is to mix—so you either pick a reactor that tolerates thick slurries or add water and heat to aid mixing. If you’re evaluating what is anaerobic digestion for your site, start by matching feedstocks and stability needs to one of these proven designs.

• Complete mix (CSTR): Enclosed, heated tanks with mechanical, hydraulic or gas mixing; most common for liquid manures and co‑digestion, offering robust, stable operation.

• Plug flow: Long, narrow heated channels with minimal mixing; well suited to scraped dairy manure with higher solids, often built partially below grade.

• Mixed plug flow: Hybrid adding mixing to handle thicker slurries without heavy dilution; popular on dairies seeking higher throughput.

• Dry/garage batch: High‑solids piles in enclosed cells with percolate recirculation; ideal for yard waste, energy crops and source‑separated organics.

• Covered lagoon: Lined ponds with gas‑tight covers; lowest capex, limited heating/mixing, with biogas output that tracks seasonal temperature.

Your reactor choice drives biogas yield, digestate handling and upgrading options—and ultimately the project economics.

Operating conditions and control: temperature, mixing and stability

Stable anaerobic digestion depends on keeping microbes in their comfort zone: consistent temperature, appropriate mixing, steady loading and an oxygen‑free environment. Three temperature regimes are common. Mesophilic (about 35–40°C) is the most widely used and comparatively easy to run, with retention times of weeks. Thermophilic (about 50–55°C) accelerates breakdown and allows shorter retention (around 3–5 days), but it is more sensitive to temperature swings and nitrogen/ammonia, and demands more insulation and heat. Psychrophilic (roughly 15–25°C) is very stable and simple, but requires much longer retention to achieve equivalent gas and pathogen reduction.

Mixing should be “just right” to keep solids suspended, avoid dead zones, scum and foam, and maintain contact between microbes and substrate—hence the popularity of stirred tanks (CSTRs). Covered lagoons typically lack heat and mixing, so gas output tracks seasons and can dip sharply in winter. Avoid abrupt recipe changes; introduce new co‑substrates gradually to protect methanogens.

• Temperature control: Hold tight bands within the chosen regime; insulate, heat and monitor continuously.

• Mixing regime: Size mixers for viscosity; prevent stratification without over‑shearing.

• Loading discipline: Keep daily feed steady; ramp changes, especially with high‑energy FOG or food wastes.

• Solids and grit management: Screen and remove sand/grit to preserve active volume.

• Process monitoring: Track pH/alkalinity, hydraulic retention time, biogas rate and composition (CH4, CO2, H2S), and watch for foaming—early signals of instability.

Getting these levers right is the practical answer to what is anaerobic digestion at reliable, commercial scale.

Biogas and digestate: outputs, quality and uses

Understanding what is anaerobic digestion means recognising its two value streams: biogas and digestate. Biogas bubbles off the reactor and typically contains 50–75% methane, plus CO2, H2S and water vapour; its quality reflects feedstocks, temperature and retention time, and improves with basic clean‑up. Digestate leaves as a pumpable slurry that is commonly separated into liquid and solid fractions, with reduced odour and pathogens and nutrients retained for reuse.

• Biogas—what’s in it: Predominantly methane and carbon dioxide with trace H2S and moisture that must be managed.

• Biogas—how it’s used: Burn for heat, generate electricity or CHP; or condition it for downstream upgrading.

• Biogas—conditioning essentials: Dehumidify and remove H2S to protect engines, boilers and downstream equipment.

• Digestate—how it’s handled: Separate into liquid and solids; store to apply when crops can use nutrients.

• Digestate—agronomic uses: Liquids and solids as fertiliser/soil amendment with more plant‑available nutrients than raw manure.

• Digestate—material uses: Solids for animal bedding, composting, or as a base for bio‑based products.

A clear plan for gas clean‑up and nutrient use is central to reliable, bankable AD operations.

From biogas to biomethane (RNG) and CO2 capture

Once you understand what anaerobic digestion produces, the next step is upgrading biogas into biomethane (renewable natural gas, RNG). Upgrading strips out water, hydrogen sulphide and carbon dioxide to raise methane content and meet grid or vehicle‑fuel specifications. Well‑designed trains also recover a pure CO2 stream, creating new value and improving project carbon intensity.

• Pre‑treat and compress: Condition raw gas; boost pressure for efficient clean‑up and stable flow.

• H2S removal: Use catalytic or adsorptive media to protect equipment and meet emissions limits.

• Dehydration: Dry to a low dew point to prevent corrosion and freezing.

• CO2 removal: Apply membranes, PSA or amine solvents to achieve high‑methane RNG (typically 96–99%+ CH4).

• Polishing and QA: Remove siloxanes/trace oxygen (to very low ppm where required), odourise and continuously monitor quality.

• Delivery: Inject RNG into the natural gas grid, or compress/liquefy for vehicle fuel.

• CO2 capture: Liquefy high‑purity CO2 for sale or utilisation, enhancing credits and overall project returns.

Modern, integrated packages can automate these steps end‑to‑end, delivering very high biomethane recovery and deep CO2e reductions.

Where anaerobic digestion is used: farm, municipal and industrial

Understanding where anaerobic digestion fits helps translate what is anaerobic digestion into real projects. You’ll most often see it on farms handling manure and co‑digestion streams, at municipal wastewater plants stabilising biosolids, and at industrial sites treating high‑strength organics. Siting close to feedstocks cuts haulage, while CHP, heat recovery or RNG injection monetise the gas; digestate nutrients return to land or enter managed outlets.

• Farm-based: Dairy and cattle manure systems, often with co‑digested food wastes/FOG, produce steady biogas for CHP or RNG and yield low‑odour digestate for fields.

• Municipal: Wastewater treatment works digest sewage biosolids to cut volume and odour, generate on‑site heat/power, and increasingly upgrade to RNG.

• Industrial (food/beverage): On‑site digesters treat high‑strength wastewater and residuals, trim energy bills with CHP, and control compliance risks.

• Centralised/urban hubs: Non‑farm facilities receive food‑processing waste and source‑separated organics; heat can serve neighbours, with digestate nutrients exported to agriculture.

Benefits and environmental impact

Understanding what is anaerobic digestion shows it is both a waste‑treatment and energy‑generation solution with clear environmental gains. By capturing methane from manures, biosolids and food wastes in sealed reactors, AD avoids uncontrolled emissions, produces useful energy, and returns stabilised nutrients to land. Upgrading biogas to biomethane (RNG) displaces fossil natural gas, while digestate delivers more plant‑available nutrients than raw manure with far less odour.

• Greenhouse gas mitigation: Captures and utilises methane; reduces on‑farm emissions versus conventional storage and handling.

• Renewable energy, 24/7: Biogas powers heat, electricity or RNG for grids and vehicles.

• Nutrient recycling: Digestate liquids/solids replace synthetic fertiliser with improved predictability.

• Odour and pathogen reduction: Noticeably lower odour; effective pathogen reduction, especially at higher temperatures.

• Waste diversion: Co‑digestion of food by‑products and FOG keeps organics out of disposal pathways.

• Operational improvements: Easier pumping and spreading; fewer viable weed seeds reported.

• Local value: New revenue streams and rural economic opportunities alongside environmental compliance.

Risks, safety and compliance essentials

Anaerobic digestion is very safe when well designed and operated, but the hazards are real: combustible methane, toxic hydrogen sulphide, oxygen‑deficient spaces, pressure systems, and nutrient‑laden liquids. Understanding what is anaerobic digestion means planning for engineering controls, trained operators and clear procedures, and meeting local codes and environmental approvals (for example in Ontario, TSSA/ESA oversight and specific approvals may apply).

• Gas hazards: Detect CH4/H2S, ventilate, use rated equipment and flame arrestors.

• Pressure safety: Specify relief devices, emergency flare, rupture protection and proof‑tested controls.

• Confined spaces: Permit entry only; monitor atmosphere and have rescue plans.

• Electrical/mechanical: Apply hazardous area classification, guarding and lockout/tagout.

• Spill and odour control: Secondary containment, stormwater management and odour abatement.

• Process upsets: Prevent foaming/acidification with steady feed, monitoring and alarms.

• Training and SOPs: Competency, drills and incident reporting baked into daily operations.

• Compliance: Secure required environmental, energy and building permits before commissioning.

Economics and revenue streams to plan for

Understanding what is anaerobic digestion from a financial angle means stacking multiple income lines while controlling interconnection, operations and nutrient management costs. Most projects are underpinned by energy offtake (power or RNG), steady feedstock contracts, and a clear digestate plan. RNG can command a premium over fossil gas but requires confirmed buyers; carbon markets and incentives improve returns. On farms, energy sales typically dominate, while tipping fees help but are often secondary. Capturing and using surplus heat further strengthens the case.

• Energy sales: Power via CHP with PPAs or net metering; or upgrade to RNG for grid injection or vehicle fuel.

• Tipping fees: Paid receipts for off‑farm organics; size varies with quality, alternatives and haul distance; often modest versus energy.

• CO2 and credits: Monetise captured high‑purity CO2 where markets exist; pursue applicable carbon/renewable credits.

• Digestate value: Offset fertiliser purchases; niche sales of solids or bedding reuse, net of processing costs.

• Heat utilisation: Use cogenerated heat for digesters, buildings or greenhouses to reduce fuel spend.

• Capex: Digester, pre‑treatment, gas clean‑up/upgrading, storage, interconnection and permitting.

• OpEx: Heat/mixing power, H2S and dehydration media, routine maintenance, monitoring and insurance.

• Grid/gas access: Interconnection timelines, fees and metering requirements can drive schedule and cost.

• Feedstock logistics: Transport, reception, screening and consistent recipes to avoid upsets.

• Grit/solids management: Sand/inorganics reduce active volume and can require periodic clean‑outs.

Anaerobic vs aerobic digestion: key differences and when to use each

Both approaches stabilise organics, but they do so in opposite atmospheres and deliver different value. Understanding what is anaerobic digestion clarifies the choice: AD runs without oxygen to produce energy‑rich biogas and nutrient‑dense digestate; aerobic systems use oxygen to convert organics to CO2, water, heat and a compost‑like material.

• Oxygen and energy use: AD is sealed and energy‑recovering; aerobic needs continuous aeration (energy‑consuming).

• Outputs: AD yields biogas (typically 50–75% CH4) and digestate; aerobic yields heat and stabilised solids without usable gas.

• Best fit for AD: High‑strength wet wastes (manure, food residues), odour reduction, year‑round energy/RNG, nutrient recycling.

• Best fit for aerobic: Structured, drier organics (yard waste), when energy recovery isn’t a priority and space is available.

• Complexity and safety: AD adds gas handling, H2S/CH4 safety and compliance; aerobic is simpler but still needs odour and leachate control.

Optimisation tips and common pitfalls

For reliable, high‑yield operation, optimise the basics: feedstock quality, temperature and mixing, gas clean‑up and nutrient logistics. Many issues are avoidable with steady routines and simple controls—the practical side of what is anaerobic digestion in daily operation.

• Keep recipes consistent: Introduce new co‑substrates gradually.

• Use fresh, warm manure: Avoid highly diluted washwaters.

• Blend high‑nitrogen manures: Poultry/swine benefit from co‑digestion.

• Stabilise temperature and mixing: Hold tight bands; prevent stratification.

• Remove grit/inorganics: Screen sand; plan periodic clean‑outs.

• Protect equipment: Strip H2S and moisture before engines/upgrading.

• Plan nutrient use: Separate/store digestate; apply at crop uptake.

• Know your design limits: Covered lagoons dip in winter—plan capacity/outputs.

• Standardise control: SOPs plus basic monitoring (loading, pH, gas composition).

Planning a project: quick checklist

Turning the idea of anaerobic digestion into a bankable project means de‑risking the basics—inputs, permits, technology, offtakes, operations and safety—before you spend heavily. Use this concise checklist to structure diligence, defend your numbers and keep stakeholders aligned from feasibility through commissioning.

• Feedstock study: quantity, seasonality, contaminants, volatile solids.

• Site and permits: local approvals (e.g., REA/ECA/NMS where applicable).

• Match technology: digester type and temperature to the recipe.

• Energy offtake: PPA/RNG buyer and grid/gas interconnection plan.

• Gas treatment: clean‑up/upgrading spec; H2S/H2O control; CO2 capture.

• Digestate plan: separation, storage, agronomy timing and odour.

• Safety/codes: hazardous areas, gas detection, flare and reliefs.

• O&M readiness: monitoring, SOPs, operator training, spares/media.

• Financial model: capex/opex, incentives, tipping fees, sensitivities.

Key takeaways

Anaerobic digestion converts manures, food wastes and biosolids into two assets—biogas for energy and digestate for nutrients—by running a staged microbial process in sealed, oxygen‑free reactors. Success hinges on matching feedstocks to the right digester, holding stable temperature and mixing, cleaning/upgrading gas to specification, and planning nutrient reuse and compliance from day one. RNG and optional CO2 capture can transform the business case, especially where offtakes, credits and heat recovery are secured.

• Design to the recipe: Steady feeds beat occasional peak yields.

• Run tight temperatures: Avoid shock loads; manage grit and inorganics.

• Treat the gas: Dry it, remove H2S, then upgrade to RNG.

• Monetise more: Use heat, secure credits/tipping fees, consider CO2 sales.

• Keep it safe: Gas detection, reliefs, permits and trained operators.
  
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