Predict daily biogas, methane and electricity output from anaerobic digestion of manure, food waste, sewage sludge or energy crops. Vary substrate, reactor type, HRT, temperature and C/N ratio to find a stable, cost-effective design window.
Parameters
Substrate
Sets B_max, hydrolysis constant k and total solids (TS)
Feed rate
t/day
Volatile solids VS
%
Gasifiable organic fraction of total solids
Digester type
Mixing / flow configuration (yield model is shared)
Volume V
m³
Temperature T
°C
Mesophilic 35-40 C / thermophilic 50-55 C bands
HRT
day
Hydraulic retention time; too short and methanogens wash out
Animated CSTR with mixer, biogas dome, feed inlet, digestate outlet and CHP unit. The four AD steps (hydrolysis, acidogenesis, acetogenesis, methanogenesis) light up in sequence. Colour reflects OLR and temperature health.
B_max is the substrate-specific maximum yield (m³/kg VS) and k is the hydrolysis constant (1/d). For a continuous reactor the completion ratio at hydraulic retention time HRT is used: $B_{actual} = B_{max}\cdot k\,HRT/(1+k\,HRT)$.
OLR is the organic loading rate (kg VS/m³/d) and φ_CH4 is the methane fraction (0.50-0.65). Stable operation needs OLR ≤ 5 and HRT ≥ 15 d as rough rules of thumb.
LHV_CH4 ≈ 35.8 MJ/m³, gas-engine efficiency η_elec ≈ 0.35. Multiply GJ by 277.8 to obtain kWh.
Anaerobic Digestion Biogas Yield — HRT, VS Loading and C/N
🙋
"Anaerobic digestion" - is that the tank that turns food waste and manure into burnable gas? I never really understood how it works.
🎓
Yes. You seal a tank from air and let four bacterial guilds run a relay race: hydrolysis breaks long molecules into sugars and amino acids; acidogenesis and acetogenesis turn those into short organic acids; finally methanogens convert acetate and hydrogen into CH4. The "biogas" coming out is 55-65% methane, 30-50% CO2 plus a little H2S and ammonia. Try switching the substrate from cattle manure to food waste in the panel - B_max jumps from 0.20 to 0.50 because food has lots of sugars and fats.
🙋
Then food waste is by far the best substrate! Why not run the plant on food only?
🎓
Here is the trap. Food-only digesters acidify too fast - volatile fatty acids (VFAs) accumulate, pH falls below 6, methanogens stop and the reactor dies within a week. Watch OLR while you push feed rate up at constant 3000 m³ volume. The rule is OLR > 5 kg VS/m³/d = red, around 3-4 = yellow. So in practice food waste is co-digested with manure or sludge to bring the C/N ratio into the 20-30 sweet spot and add buffering nitrogen.
🙋
Wouldn't a shorter HRT let me process more in the same tank?
🎓
Tempting, but it backfires. Slide HRT from 25 down to 10 days and the first-order factor drops from 0.77 to 0.57, taking specific yield with it. Below about 7 days the methanogens (doubling time 5-15 days) physically wash out and gas production crashes. UASB sidesteps this by granulating the biomass so it settles and stays in the reactor even at HRT < 1 day. Japan's Kisarazu food-waste plant (6,000 m³, 1,000 t/d) is a well-known example.
🙋
How do I pick the temperature? Why are there two peaks in the curve - 38 C and 55 C?
🎓
Mesophilic (35-40 C) is the workhorse - stable, easy to operate, about 80% of installed AD capacity. Thermophilic (50-55 C) doubles the reaction rate, lets you shrink HRT to about 15 d and kills Salmonella and weed seeds. The catch is sensitivity: a 2 C swing can sour the reactor, plus the heating load is much larger. Denmark's centralised manure AD network runs almost entirely mesophilic; Germany's ~8,000 plants (Bauer Group and others) mix both.
🙋
What do you actually do with the biogas? Just sell electricity?
🎓
Three options. (1) Combined heat and power - gas engine giving roughly 35% electricity and 50% recoverable heat. (2) Direct firing for hot water or steam in boilers. (3) Upgrading - PSA or water scrubbing strips CO2 down to pipeline-grade biomethane (CH4 > 97%) for grid injection or CNG vehicles. EPC players like Veolia, Suez and Hitachi Zosen package the whole chain, and many projects stack Verra VCS-J carbon credits on top to make the economics work.
Frequently Asked Questions
For design we take a substrate-specific maximum specific yield B_max (m^3/kg VS) and multiply it by a first-order completion factor k*HRT/(1+k*HRT), a temperature factor and a C/N factor to get the actual specific yield B_actual. Daily biogas production is B_actual times the daily VS load (kg/day). This tool uses B_max = 0.20 for cattle manure, 0.30 for pig manure, 0.50 for food waste, 0.28 for sewage sludge and 0.45 for energy crops, with the temperature factor peaking at 35-40 C, C/N held at 1.0 for 20-30 and a linear penalty outside that band.
Typical mesophilic CSTRs run at 20-30 days HRT and 2-5 kg VS/m^3/d OLR. A short HRT washes methanogens out and yield drops fast; an excessively long HRT wastes reactor volume. OLR above 5 kg VS/m^3/d allows volatile fatty acids (VFAs) to accumulate, dropping pH and souring the reactor. The tool flags OLR > 5 as NG and warns when HRT < 15 d or when the temperature factor falls below 0.5.
Thermophilic operation around 55 C roughly doubles reaction rate, allows HRT to drop to about 15 d and kills more pathogens and weed seeds. The downside is high temperature sensitivity - a 2 C swing can sour the reactor - and significantly higher heating duty. Mesophilic (35-40 C) is far more stable and is the workhorse for farm-scale plants. The temperature factor in this tool captures both peaks so you can see the trade-off directly.
Raw biogas contains 55-65% methane, 30-50% CO2 and several hundred to several thousand ppm H2S. Gas engines (LHV ~ 21-23 MJ/Nm^3) accept it directly but need H2S removal (iron sponge, activated carbon) and moisture removal, otherwise cylinder corrosion and spark-plug fouling cut engine life below 5,000 hours. With further upgrading the gas can reach pipeline-quality biomethane (CH4 > 97%) for grid injection or CNG vehicle fuel.
Real-world applications
On-farm manure digesters: Germany operates roughly 8,000 farm-scale AD plants (Bauer Group and many others) that mix livestock manure with energy-crop silage (maize), generate electricity in gas engines and recover waste heat for barn or dryer heating. Denmark coordinates around 100 centralised manure digesters across all dairy farms, with biomethane injected into the national gas grid.
Municipal food-waste AD: In Japan, the Kisarazu Biomass Utilisation Centre (6,000 m³, ~1,000 t/d of food waste) is a flagship plant. Heavy front-end shredding and contaminant removal are needed, and OLR is typically held at about 3 kg VS/m^3/d to stay on the safe side. The biogas usually fuels boilers in neighbouring incinerators or wastewater plants.
Sewage-sludge digestion: Waste activated sludge has VS up to 70-80%, and AD halves its volume while producing enough biogas to cover 30-50% of the wastewater plant's electricity. Veolia, Suez, METAWATER and other EPCs deliver these systems worldwide, making sludge AD the most mature large-scale AD application.
Carbon credit projects: AD avoids fugitive methane (GWP 28) from manure lagoons and food waste. Methodologies under Verra VCS-J (waste management) and Japan's J-Credit scheme allow the avoided emissions to be issued as t-CO2 credits, stacked on top of energy revenue. Any AD feasibility study should back-calculate CO2 savings from the gas volumes this tool predicts.
Common misconceptions and pitfalls
The biggest misconception is that "more feed = proportionally more gas". Once OLR climbs above ~5 kg VS/m^3/d the VFAs produced by hydrolysis outpace the methanogens, pH crashes, methanogens stop, more VFAs accumulate and within about a week the reactor is dead. Recovery means re-seeding and several weeks of re-acclimation - easily millions of yen in lost output. That is exactly why the tool turns the verdict red as you push feed rate up at fixed volume.
Next, treating B_max as a fixed design number. Real B_max swings by 30-50% with pre-treatment (shredding, thermal, enzymes), co-digestion mix and inhibitors (NH3, H2S, Na, antibiotics). Chicken manure and food waste are nitrogen-rich; once free ammonia exceeds about 100 mg/L methanogens are inhibited and B_actual can halve. The design number is an upper bound - the real value is tuned during 3-6 months of commissioning.
Finally, assuming the economics are "electricity = biogas x LHV x efficiency". Real budgets must include engine maintenance (typically ~JPY 1,500/kW/year extra to counter H2S damage), digestate dewatering and land application, feedstock transport (food waste is usually only economic within a 20 km radius) and operating consumables (pH buffer, trace metals). Ignoring these overstates project returns by 20-30%. With FIT tariffs declining in Japan, viable AD projects now layer heat sales and carbon credits on top of the electrical output this tool estimates.
How to Use
Enter feedstock feed rate in tonnes/day (typical range: 0.5–50 t/day for farm digesters, 5–200 t/day for municipal plants)
Specify digester volume in m³ and operating temperature in °C (mesophilic: 35–38°C; thermophilic: 50–55°C)
Simulator calculates VS load, organic loading rate (OLR), specific yield, daily biogas volume, methane percentage, and net electricity output
Worked Example
Dairy farm digester processing 5 t/day cattle manure (21% VS content) in a 75 m³ reactor at 37°C mesophilic conditions: VS load = 1,050 kg/day; OLR = 14 kg VS/m³/d (stable operation <2.5 kgVS/m³/d); specific yield = 0.22 m³ biogas/kg VS degraded; daily biogas = 220 m³; methane content = 58%; methane yield = 128 m³/day; electricity output = 165 kWh/day (assuming 0.75 kWh/m³ biogas via combined heat-power engine at 35% electrical efficiency).
Practical Notes
OLR stability: rates exceeding 3 kg VS/m³/d risk acidification and methane collapse; reduce feed rate or increase reactor volume if acetate accumulates
Temperature sensitivity: 5°C drop from 37°C mesophilic reduces biogas yield ~12–15%; maintain insulation and heating for northern climates
Substrate mix: co-digesting manure with food waste (5–15% blend) boosts yield from 0.18 to 0.24 m³/kg VS without toxicity; avoid fats above 5% (grease coating)
Retention time: systems with 20–30 day hydraulic residence time achieve 75–85% VS destruction; shorter times (<15 days) compromise conversion efficiency