Biogas: digestion and plant design
How anaerobic digestion turns manure into biogas: the four stages, operating conditions, digester types, gas yields, digestate and a sizing sum.
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Cattle dung, pig and poultry manure, crop residues and food waste can be turned into a clean cooking and engine fuel and a nutrient-rich fertiliser, all in a sealed tank and without any added energy. Anaerobic digestion by mixed communities of microbes produces biogas, mostly methane and carbon dioxide. Tens of millions of small household digesters operate in China, India, Nepal, Viet Nam and other countries, and large farm and industrial plants in Europe and elsewhere generate electricity and biomethane.
Biogas production, plant types and design calculations are standard topics in renewable energy, agricultural engineering and integrated farming courses.
The four stages of anaerobic digestion
- Hydrolysis: bacteria break down complex polymers (cellulose, starch, proteins, fats) into sugars, amino acids and fatty acids. This is often the slowest step for fibrous materials.
- Acidogenesis: fermentative bacteria convert these into volatile fatty acids, alcohols, hydrogen and carbon dioxide.
- Acetogenesis: acetogens convert volatile fatty acids and alcohols into acetate, hydrogen and carbon dioxide.
- Methanogenesis: methanogenic archaea produce methane, about two-thirds from acetate and one-third from hydrogen and carbon dioxide.
Methanogens grow slowly and are sensitive, so the process must be kept stable: if acids accumulate faster than methanogens use them, the pH drops and gas production fails ("souring").
Biogas composition and uses
- Methane (CH4): typically 50 to 70 per cent.
- Carbon dioxide (CO2): 30 to 45 per cent.
- Small amounts of hydrogen sulphide, water vapour, nitrogen and hydrogen.
Biogas has a calorific value of roughly 20 to 25 MJ per m3, depending on methane content. It is used for cooking and lighting, in engines and combined heat and power units for electricity and heat, and after upgrading (removing CO2 and H2S) as biomethane in gas grids or vehicles.
Operating conditions
- Temperature: most plants run in the mesophilic range (about 30 to 40 °C); thermophilic plants (about 50 to 55 °C) are faster but less stable. Small unheated plants slow sharply in cold weather.
- pH: about 6.8 to 7.5.
- Carbon to nitrogen ratio: about 20 to 30 : 1. Dung is near this range; straw has too much carbon and poultry manure too much nitrogen, so mixing feedstocks (co-digestion) helps.
- Solids content: about 8 to 10 per cent total solids for wet slurry digesters, usually achieved by mixing dung and water about 1 : 1.
- Hydraulic retention time (HRT): the average time slurry stays in the digester, typically 30 to 60 days for small unheated plants, less for heated plants.
- Toxic substances: antibiotics, disinfectants, pesticides and excess ammonia can inhibit microbes.
Digester types
- Fixed-dome (Chinese type): an underground masonry digester with a fixed gas dome; no moving parts and long life, but gas pressure varies and the dome must be perfectly gas-tight.
- Floating-drum (Indian or KVIC type): a steel drum floats on the slurry and rises as gas collects, giving constant pressure; the drum can corrode.
- Balloon or tubular digesters: plastic bags, cheap and quick to install but less durable.
- Plug-flow digesters for thicker manure, and large continuously stirred tank reactors (CSTR) with heating and mixing on commercial farms and industrial plants.
Digestate
The digested slurry, or digestate, keeps most of the nitrogen, phosphorus and potassium of the feedstock, with more of the nitrogen in readily available ammonium form, fewer pathogens and weed seeds, and less odour than raw manure. It is a valuable fertiliser if stored and applied carefully to limit ammonia losses.
Benefits and cautions
Benefits: clean cooking fuel replacing firewood and dung cakes, reducing indoor smoke and women's labour; renewable energy; fertiliser; reduced methane emissions from manure storage; sanitation when toilets are connected.
Cautions: plants fail without enough feedstock, water, maintenance and gas-tight construction; methane leaks reduce climate benefits; hydrogen sulphide is toxic and corrosive; gas is flammable and must be handled safely.
Sizing a household plant
A household keeps 4 cows, each producing about 10 kg of fresh dung a day.
- Dung = 4 × 10 = 40 kg per day.
- Gas yield, about 0.035 to 0.040 m3 per kg of fresh cattle dung: 40 × 0.035 to 0.040 ≈ 1.4 to 1.6 m3 of biogas a day.
- Cooking needs about 0.25 m3 per person per day, so a family of five needs about 1.25 m3: the plant meets it.
Digester volume. Dung mixed 1 : 1 with water gives 80 litres of slurry a day. With a retention time of 40 days: digester volume = 0.08 m3 × 40 = 3.2 m3, plus space for gas storage; a plant of about 4 m3 total would be chosen from standard sizes.
Energy. At 22 MJ per m3, 1.5 m3 supplies about 33 MJ a day, roughly equal to the useful heat from 6 to 8 kg of firewood burnt in a traditional stove.
Key points
- Anaerobic digestion: hydrolysis, acidogenesis, acetogenesis and methanogenesis.
- Biogas is about 50–70 per cent methane, 30–45 per cent CO2, with about 20–25 MJ per m3.
- Conditions: mesophilic 30–40 °C, pH 6.8–7.5, C:N 20–30, about 8–10 per cent solids, HRT 30–60 days for small plants.
- Digesters: fixed-dome, floating-drum, balloon or tubular, plug-flow and CSTR.
- Digestate is a good fertiliser with more available nitrogen.
- Cattle dung gives about 0.035–0.040 m3 of gas per kg; digester volume = daily slurry × HRT.
What the exams ask
- School-leaving and entrance papers: what biogas is, its uses and the materials used.
- Degree and research-fellowship entrances: stages of digestion, factors affecting gas production, plant types and their comparison, and digestate value.
- Agricultural engineering and renewable energy examinations: plant sizing, gas yield and energy calculations, upgrading and safety.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why does my biogas plant produce less gas in winter?
Methanogens work slowly at low temperatures. Insulating the plant, building it partly underground, using warm water for mixing, or covering it with a greenhouse can help.
Can crop residues be used?
Yes, especially mixed with manure, though fibrous straw digests slowly and may need chopping or pre-treatment. Food waste and some energy crops are also used in larger plants.
Is biogas slurry safe for vegetables?
Digestion reduces many pathogens, but not all. Well-digested slurry, stored or composted and applied before planting or to the soil rather than the edible parts, is a safe practice.
Sources
- Fulford, D. Running a Biogas Programme: A Handbook. Intermediate Technology Publications, 1988.
- Deublein, D. and Steinhauser, A. Biogas from Waste and Renewable Resources, 2nd edition. Wiley-VCH, 2011.
- Weiland, P. "Biogas production: current state and perspectives." Applied Microbiology and Biotechnology 85, 849–860 (2010).
- IEA Bioenergy. Task 37: Energy from Biogas, technical reports. IEA, various years.
- Bond, T. and Templeton, M. R. "History and future of domestic biogas plants in the developing world." Energy for Sustainable Development 15, 347–354 (2011).
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