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Environmental Engineering
Anaerobic Digester Biogas Production Simulator
Design an anaerobic digestion (AD) plant that converts food waste, sewage sludge or animal manure into biogas through methane fermentation. Sliders for feedstock mass, VS, specific yield and reactor volume update biogas output, CHP power, organic loading rate (OLR) and CO2 offset in real time.
Feedstock in → digester (anaerobic fermentation) → gas holder → CHP. Biogas bubbles rise through the slurry and feed the CHP unit. The outline colour reflects OLR (green=safe / orange=high / red=overload).
Biogas production rate and CHP electrical power. LHV(CH₄) = 35.8 MJ/Nm³; η_CHP is the electrical efficiency (~0.35). Methane fermentation proceeds in four stages (hydrolysis → acidogenesis → acetogenesis → methanogenesis); hydrolysis is the rate-limiting step.
Organic loading rate OLR [kg-VS/m³/day] and hydraulic retention time HRT [day]. Stable mesophilic operation typically requires OLR ≤ 4 and HRT ≥ 20 days.
Anaerobic Digestion (Biogas) Process
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"Anaerobic digestion"... you can really get burnable gas out of food scraps? I can't quite picture it.
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You really can. Nature does it all the time — in marshlands, rice paddies and the rumen of a cow. Seal food waste or sewage sludge in an airtight tank at around 35 °C and a community of anaerobic microbes, including methanogenic archaea, breaks the organics down into biogas — typically about 60% methane (CH₄) and 40% carbon dioxide (CO₂). The process happens in four stages: hydrolysis → acidogenesis → acetogenesis → methanogenesis. Hydrolysis is usually the rate-limiting step.
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So you just dump stuff in and warm it. Does that mean the more I feed it, the more gas I get?
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That's the trap. Crank up "Feedstock mass" on the left — biogas does climb, but watch "OLR" rise with it. OLR is the organic loading rate: kilograms of VS fed per cubic metre of reactor per day. Push it past about 4-5 and the methanogens fall behind the acid producers. Volatile fatty acids (VFAs) accumulate, pH drops, the methanogens weaken, and the reactor "sours" and stops producing. Operators call this the souring failure, and it's the single most feared upset in the AD industry.
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Then how do you pick the HRT (retention time)? 25 days sounds long.
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Methanogenic archaea grow slowly — doubling time is measured in days, not minutes like E. coli. If the slurry flushes through too fast you "wash out" the methanogens. Mesophilic AD runs at HRT 20-30 days; thermophilic (55 °C) is two to three times faster, so 12-20 days works. A municipal wastewater plant can easily afford 30 days of volume, but a farm-scale digester often runs short on HRT and the gas yield plateaus.
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What do you do with the biogas afterwards? Just burn it on a gas stove?
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Three main routes. The most common is CHP (combined heat and power): a gas engine generates electricity, and the engine's waste heat keeps the digester at 37 °C. About 35% electrical plus 50% thermal — 85% total energy recovery is achievable. Power goes to the grid, heat goes back into the tank. Route two is biomethane: scrub out CO₂ and H₂S to natural-gas quality and inject into the grid — standard practice in Germany and Denmark. Route three is compressed biogas (Bio-CNG) as vehicle fuel; most municipal buses in Sweden run on it.
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And is the environmental benefit really that big?
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It's a double win, and that's why AD gets such generous EU subsidies. First, replacing fossil natural gas avoids about 2 kg-CO₂ per m³ of methane combusted. Second — and this matters more — if you do nothing, the same organic waste rots anaerobically in landfills or lagoons and releases methane straight to the atmosphere. Methane's global-warming potential is 28-34× CO₂, so capturing it and burning it down to CO₂ is a huge net reduction. The EU's landfill methane directive is largely a push to scale AD plants for this very reason.
Frequently Asked Questions
Take the feedstock mass m [t/day], multiply by the volatile solids fraction VS [%] to get the VS load, then multiply by the specific biogas yield Y_biogas [m³/kg-VS]: V_biogas = m·1000·(VS/100)·Y_biogas. For m=10 t/day, VS=80% and Y=0.6, the VS load is 8,000 kg/day and biogas output is 4,800 m³/day. Typical yields are 0.6-0.8 for food waste, 0.3-0.4 for sewage sludge and 0.2-0.3 for animal manure.
OLR = VS load / digester volume [kg-VS/m³/day] and HRT = digester volume / feed flow [day]. Pushing OLR higher boosts volumetric productivity, but the methanogens cannot keep up with the acid producers; volatile fatty acids accumulate, pH drops and the reactor sours. Mesophilic AD usually runs at OLR 2-4 kg-VS/m³/day with HRT 20-30 days.
The lower heating value of methane is about 35.8 MJ/Nm³, so biogas with 60% CH₄ contains roughly 21 MJ/Nm³. In a CHP unit, typical efficiencies are 30-40% electrical and 45-55% thermal (combined 80-90%). This tool assumes 35% electrical and 50% thermal. For 2,880 m³ CH₄/day that gives about 103,000 MJ/day (28,640 kWh/day), 418 kWe electrical and 597 kWth thermal output.
When biogas-derived methane displaces fossil natural gas, you avoid about 2 kg-CO₂ per m³ of methane (natural gas emission factor ~ 2.0 kg-CO₂/Nm³). On top of that, you also avoid uncontrolled methane emissions from landfill or open storage of the same organic waste (methane GWP is 28-34 times CO₂). This tool reports the substitution effect only.
Real-World Applications
Sewage sludge digestion at wastewater plants: Waste-activated sludge from municipal wastewater treatment is the textbook AD feedstock. Digestion both reduces sludge volume by 30-50% and produces biogas that powers blowers and on-site heating. Most large Japanese and European wastewater treatment plants run mesophilic sludge digesters at VS ≈ 65-75%, Y_biogas ≈ 0.3-0.4 m³/kg-VS and HRT 25-30 days.
Food waste from factories and supermarkets: Food waste is a high-VS (85-90%), high-yield (0.6-0.8 m³/kg-VS) feedstock. Several large food-recycling plants in Japan, the US and the EU now process tens of thousands of tonnes per year and export the CHP electricity to the grid. Feed-in tariffs (Japan's FIT biomass tariff, Germany's EEG) made early projects bankable, and the industry is now shifting toward feed-in-premium and self-consumption models.
Animal manure on farms: Germany, Denmark and the Netherlands have built thousands of on-farm digesters that co-digest manure with silage. Japan is now scaling similar plants in dairy regions like Hokkaido and Kagoshima. Manure has lower VS (10-20%) and modest yield (0.2-0.3), but AD simultaneously solves the manure-disposal problem (odour, nitrate runoff) and generates renewable energy. The liquid digestate is land-applied as fertiliser.
Grid injection and Bio-CNG: If you upgrade biogas to ~95% methane (PSA, membrane separation or water scrubbing) you have "biomethane" suitable for natural-gas pipeline injection. Europe injects over 30 TWh of biomethane annually. In Sweden, Bio-CNG fuels municipal bus fleets and refuse trucks. Japan's gas industry has biomethane in its 2030 methanation roadmap, and the rough sizing this tool provides is a useful first cut for project feasibility studies.
Common Pitfalls & Cautions
The biggest trap is confusing VS (volatile solids) with TS (total solids). The dry residue after evaporation is TS; the part of that TS that burns off in a muffle furnace is VS — only the VS feeds the bugs. The yield Y_biogas in this tool is per kg of VS, and "per kg of TS" can differ by 2-3×. Food waste with TS 20% and VS/TS 90% delivers 1000·0.2·0.9 = 180 kg VS per wet tonne. Always sample your real feedstock and run a VDI 4630 (or equivalent) Biochemical Methane Potential (BMP) test before sizing the plant.
Next, do not assume that raising the temperature simply speeds the process up. Yes, thermophilic AD at 55 °C is 2-3× faster than mesophilic and lets you shrink HRT. But it is fragile — sensitive to ammonia inhibition and to temperature swings of ±1 °C — and the extra heating energy may erode the net energy yield to the point that CHP waste heat alone cannot supply it. Mesophilic operation is far more forgiving and dominates farm-scale and small municipal plants. The choice of temperature must consider reaction rate, C/N ratio, ammonia level, heating-energy balance and the available operator skill.
Finally, do not believe that raw biogas can be used as-is. It carries 100-3,000 ppm of hydrogen sulphide (H₂S) that corrodes cylinders, bearings and exhaust systems in gas engines. Desulphurisation (biological scrubbers or dry sorbents) is mandatory upstream of CHP. The gas is also water-saturated, so without a chiller and demister you will get condensation, plugged piping and freeze damage. Upgrading to biomethane adds CO₂ removal (PSA, membranes or chemical absorption) plus siloxane removal (activated carbon). The cost of these clean-up steps drives a large fraction of project OPEX and must be budgeted from day one.