Breaking down wet organic matter like animal dung, leftover food or human waste can be used to produce biogas, a mixture of methane and carbon dioxide, as well as a semi-solid residue. For families in the developing world, using biogas helps ease the drudgery of gathering and using wood for cooking, rids homes of health-damaging smoke, and cuts deforestation and greenhouse gas emissions.
- About 50 million household-size plants are in use, in China, India, Nepal, Vietnam and elsewhere. Over 10,000 larger plants are used for electricity generation.
- Household plants are typically 1 to 12 cubic metres in volume, with the largest plants reaching up to several thousand cubic metres.
- The cost of household plant varies greatly from country to country, but is typically US$550 or more.
How biogas works
A simple biogas plant needs a large container to hold the mixture of decomposing organic matter and water (which is called slurry), and another container to collect the biogas. There must also be ways to add the organic matter (the feedstock), to take the gas to where it will be used, and to remove the residue.
In fixed dome biogas plants (the most common type), the slurry container and gas container are combined, so that the gas collects under a rigid dome over the slurry. As the slurry breaks down, the biogas which is produced pushes some of the slurry into a separate reservoir. When the biogas is taken off, the slurry flows back.
In floating dome plants, the gas container floats in the slurry. The gas container gradually rises up as biogas is produced, and sinks back down as the biogas is used.
A biogas plant needs some methane-producing bacteria to get it started, but once it is producing biogas the bacteria reproduce and keep the process going. Cattle dung contains suitable bacteria, and a small amount of cattle dung is often used as the ‘starter’ for a biogas plant, even when it is not the main feedstock.
How biogas plants are used
Rural families often use animal manure as the feedstock for biogas: the plant needs to be topped up with a mixture of manure and water each day. The manure from two to four cows (or five to ten pigs) can produce enough gas for all household cooking needs, and sometimes for lighting too. Ashden Award-winner BSP-Nepalis a good example of plants that use cattle manure, and Biotech in Kerala, South India, supplies food-waste plants for this urban market. A family or community using just its own food waste can replace between about a quarter and a half of their cooking fuel.
Larger-scale biogas schemes can produce enough gas to run an engine that generates electricity. This is often done in sewage treatment plants in the UK, and there are a growing number of large farm-based plants in Germany and elsewhere. Biotech also supplies plants to manage the waste from vegetable markets, and produce gas for electricity generation. There are also programmes in Europe to clean biogas and feed it into the mains gas supply.
Biogas plants can work for many years, provided that they are constructed well and checked regularly. If the plant is made from masonry, care must be taken to make sure that the structure is water-tight and gas-tight. The slurry needs to be kept at a temperature of about 35°C for the bacteria to work effectively, so away from the tropics some of the gas produced may be needed to heat the plant. In all plants, feedstock must be added regularly so that the bacteria continue to multiply.
What are the benefits of using biogas?
Most household biogas plants replace wood for cooking, and this brings many benefits. Cooking fires and wood stoves produce high levels of indoor air pollution, which kills more than four million people across the globe each year. Biogas burns with a clean flame and cuts pollution to safe levels.
Using biogas also saves valuable time, because collecting manure and feeding it to a biogas plant is much less time consuming than collecting wood and preparing a cooking fire. Studies by BSP-Nepal found that households with biogas plants save three hours per day on average, and women benefit most. Biogas is available whenever it is needed and cooks food quickly, so it is easier to prepare hot food before children go to school.
Biogas plants that use human sewage improve hygiene and sanitation as well as providing cooking fuel. About two thirds of the cattle-manure plants supplied through BSP-Nepal have a household toilet attached as well, as do many of the pig-manure plants supplied through MARD/SNV. In Rwanda, the Kigali Institute of Science of Technology built large biogas plants to manage sewage in prisons, which has improved hygiene and also reduced pollution of nearby land where raw sewage used to be discharged.
The residue from dung-based biogas plants makes a good fertiliser with minimal smell. The fertiliser value can be improved by composting the residue with crop waste, and feeding the compost to earthworms for additional processing (vermicomposting). SKG Sangha in South India provides vermicomposting units with biogas plants.
The cost of biogas plants varies greatly from country to country, because the costs of both materials (brick, concrete and plastic) and labour can be very different. For example, a 10 cubic metre plant sold through the MARD/SNV programme in Vietnam cost about US$550 in 2010, but a similar plant cost more than US$1,500 in Kenya.
Using plastic or steel to pre-fabricate biogas plants usually increases the material cost but can substantially reduce the labour needed for installation.
The economic viability of biogas depends on the cost of the fuel being replaced, and whether there are other financial benefits (for instance, avoided waste disposal costs, or income from selling compost). For example, MARD/SNV plant users can pay back the cost of a plant in about four years through savings in LPG, but even more quickly if the reduced need to buy fertiliser is taken into account.
The potential of biogas plants to reduce greenhouse gases (including methane from uncontrolled dung and sewage management as well as carbon dioxide) means that carbon-offset finance has been a source of funding for some biogas programmes.
According to the Dutch development agency SNV, the ‘biogas giants’ are China and India with , about 43 million and 4.5 million household plants respectively installed at the end of 2011. Biogas use is growing in other parts of Asia, such as Nepal (over 260,000 plants), Vietnam (over 140,000 plants) and Bangladesh (over 50,000). Despite significant work by SNV and others, the use of household biogas is much lower in Africa and Latin America. Around 10,000 larger plants are in use for electricity generation, many in Germany and other parts of Europe.
Biogas plants have huge potential to produce clean fuel from unhygienic, wet organic waste. There are many more rural and peri-urban areas where traditional dung-based plants could be used. There may be even more potential in towns and cities, where waste disposal and sanitation is becoming an increasing challenge as more people move to urban areas. Interest is also growing in the use of larger plants for electricity generation and to supply gas grids