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Waste becomes a resource
Food scraps, slurry, sewage sludge, crop residues and agro-industrial by-products are all digestible.
An illustrated guide to anaerobic digestion
Turning organic waste into energy
Every food scrap, crop residue and barrel of manure holds chemical energy. Microbes working without oxygen can release it as biogas, leaving behind a nutrient-rich fertiliser. Here is how it works.
- 50–70%methane in raw biogas
- 4microbial stages
- 20–40 daystypical retention
- 0 O2oxygen required
Why biogas?
Organic waste left to rot in landfills or open heaps releases methane and CO2 uncontrolled. Digestion captures that gas on purpose.
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Renewable, dispatchable energy
Biogas can be stored and burned when needed for heat and power, unlike wind or solar output.
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Closed nutrient loop
Nitrogen, phosphorus and potassium survive digestion and return to soil as digestate.
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Lower emissions
Capturing methane that would otherwise escape avoids a potent greenhouse gas, and displaces fossil fuels.
The process in four stages
Different communities of microbes hand the material along like a relay. Select a stage to explore it.
- Who does it
- Sweet spot
- Watch out for
What goes in, what comes out
Biogas yield depends on how much biodegradable solid a feedstock holds. Values are typical literature ranges and vary widely between sites.
| Feedstock | Dry matter | Biogas yield (m³ per tonne fresh) | Notes |
|---|---|---|---|
| Cattle slurry | 6–10% | 15–30 | Buffers pH, supplies microbes. Low energy density. |
| Pig slurry | 4–8% | 15–25 | Ammonia can inhibit methanogens at high load. |
| Food waste | 20–30% | 100–150 | Rapidly acidifies. Needs careful feeding. |
| Maize silage | 30–35% | 180–220 | Classic energy crop. Competes with food land. |
| Fats, oils & grease | ~ 90% | 600–1000 | Highest yield, but foams and floats. Co-digest in small shares. |
| Sewage sludge | 3–6% | 10–25 | Established in wastewater plants. |
Biogas
Traces of hydrogen sulphide (H₂S), water vapour and ammonia must be removed before the gas reaches an engine.
Digestate
The leftover slurry keeps nitrogen, phosphorus and potassium in plant-available forms. Separated into liquid and fibre, it replaces part of the mineral fertiliser a farm would buy.
Inside a biogas plant
Hover or tap a numbered point to see what each part does.
Quick energy estimate
An illustrative back-of-envelope calculation. Real projects need lab tests of the actual feedstock.
0m³ biogas / day
0kWh electricity / day
0homes powered*
*Assumes about 10 kWh of electricity per home per day and 9.97 kWh of energy per m³ of methane. Heat recovery from the engine adds a further ~45% of the gas energy as usable heat.
Keeping the microbes happy
Temperature
Mesophilic 35–42 °C is robust and common. Thermophilic 50–58 °C digests faster but is more sensitive.
pH
Methanogens thrive near pH 6.8–7.5. Acid-forming bacteria tolerate lower values, so overfeeding can sour the tank.
Retention time
Material usually stays 20–40 days. Too short and microbes wash out, too long and capacity is wasted.
Organic loading
Feed steadily. Sudden jumps in food waste or fats can outpace methanogens and crash the process.
Mixing
Gentle stirring keeps microbes in contact with food and prevents crusts and sediment.
Inhibitors
Ammonia, hydrogen sulphide, antibiotics, disinfectants and oxygen ingress all suppress methane formation.
You cannot manage what you cannot measure
Digesters and waste bunkers are hostile places: corrosive gas, foam, crusts, condensation. Contact probes foul and fail. Non-contact radar level sensing at 60 GHz sends a frequency-swept (FMCW) beam from above and reads the echo, so the fill level of hoppers, tanks and gas holders can be tracked continuously without touching the material.
Connected through IoT links, level data shows when to collect waste, how evenly a digester is fed and when storage is nearly full. This is the area Eco Energy works in: IoT-enabled level sensing for waste and biogas containers.
Where the biogas goes
Heat & power (CHP)
Burnt in a gas engine to make electricity and recover heat. The most common route.
Boiler heat
Direct combustion for process heat or district heating.
Biomethane
CO₂ and H₂S are stripped out, leaving ~95%+ methane suitable for gas grids.
Vehicle fuel
Compressed or liquefied biomethane powers trucks, buses and tractors.
Questions people ask
Is biogas the same as natural gas?
Both are mostly methane, but natural gas is fossil carbon taken from underground, while biogas recycles carbon recently captured by plants. Upgraded biomethane is interchangeable with natural gas.
Does a biogas plant smell?
A sealed digester is much less odorous than open storage. Smell mostly comes from reception areas and digestate handling, which good design and covered stores control.
What is the difference between composting and digestion?
Composting needs oxygen and releases the energy as heat. Digestion excludes oxygen and captures the energy as combustible gas.
Can any organic waste be digested?
Most can, except material that is woody (lignin resists microbes) or contaminated with plastics, metals or inhibitors. Pre-treatment such as shredding or pasteurising often helps.
Why is the process called anaerobic?
Because the microbes involved work without free oxygen. Oxygen actually poisons the methane-forming archaea.
Glossary
- Anaerobic digestion
- Breakdown of organic matter by microbes in the absence of oxygen.
- Digestate
- Material left after digestion, used as fertiliser.
- CHP
- Combined heat and power: one engine, two useful outputs.
- VFA
- Volatile fatty acids, intermediates such as acetic and butyric acid.
- HRT
- Hydraulic retention time: average days material stays in the digester.
- FMCW radar
- Frequency-modulated continuous-wave radar, which measures distance from the frequency shift of a swept signal.