Biogas systems use bacteria to break down wet organic matter like animal dung, human sewage or food waste. This produces biogas, which is a mixture of methane and carbon dioxide, and also a semi-solid residue. The biogas is used as a fuel for cooking, lighting or generating electricity. Using biogas can save the labour of gathering and using wood for cooking, minimise harmful smoke in homes, and cut deforestation and greenhouse gas emissions. Biogas plants can also improve sanitation, and the residue is useful as a fertiliser.
Individual biogas systems are already benefitting several million households in Nepal, India, China and elsewhere. Larger systems are also used, for instance to process farm waste in Germany, and at sewage treatment works in the UK.
Read on for more information about biogas, or go to our database for films and case studies of Ashden Award winners who use biogas.
How biogas works
A simple biogas plant has a container to hold the decomposing organic matter and water (slurry), and another to collect the biogas. There must also be systems to feed in 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. Once the plant 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 dung as the feedstock for a biogas plant. The dung from two to four cows (or five to ten pigs) can produce enough gas for all cooking, and sometimes lighting too. The family needs to feed the plant once each day with a mixture of dung and water. Ashden Award winner BSP-Nepal coordinates a programme which has sold over 170,000 fixed-dome plants throughout rural Nepal.
Food waste can also be used as the feedstock. Food waste breaks down and produces gas more quickly than dung, so the slurry does not need to be held for as long; these plants are therefore smaller and more suitable for urban homes. A family or community using just their own food waste can replace between 25% and 50% of their cooking fuel.
Larger-scale biogas schemes can produce sufficient gas to generate electricity. This is frequently done in sewage treatment plants in the UK, and there are a number of large farm-based plants in Germany and elsewhere. BIOTECH in Kerala, South India, supplies plants to manage the waste from vegetable markets, and produce gas for electricity generation.
Biogas plants can work well 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 35oC for the bacteria to work effectively, and feedstock must be added regularly so that they continue to multiply.
What are the benefits of using biogas?
The immediate benefit of biogas is replacing other fuels for cooking. In rural areas, biogas usually replaces fuelwood, which is often in short supply. Studies by BSP-Nepal show that households with biogas plants save three hours per day on average, because collecting dung and feeding it to a biogas plant takes much less time than collecting fuelwood and preparing a cooking fire. 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 reduces indoor air pollution because it burns with a clean flame. This means that women do not have to breathe wood smoke, which is a major cause of respiratory and eye disease and responsible for an estimated 1.6 million deaths each year.
If the fuelwood source is unsustainable (i.e. not re-growing fast enough to keep up with use) then a biogas plant reduces deforestation and cuts CO2 emissions. CO2 is also saved when biogas replaces kerosene or LPG for cooking.
Sanitation and hygiene can be improved when human sewage is used as a biogas feedstock. The Shaanxi Mothers in China provide household plants based on pig dung, but with a latrine attached as well. In Rwanda, the Kigali Institute of Science of Technology has built large biogas plants to manage sewage in prisons, which has improved prison hygiene and 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 cheapest, fixed-dome biogas plants are made mainly of masonry, either brick or concrete. A 6 m3 plant using cattle dung, to provide gas for a single family costs about US$500.
Steel and plastic are used in some floating-dome plants. These can be pre-fabricated and thus installed very quickly. BIOTECH sells prefabricated plants for suburban homes with the gas container made of fibreglass-reinforced-plastic and the slurry container from ferrocement, and a 1m3 plant costs about US$220.
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, BIOTECH plant users can pay back the cost of a plant in about three years through savings in LPG.
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 is becoming a significant source of funding.
The number of domestic biogas plants currently in use is difficult to estimate. Nepal, with about 170,000 biogas plants in 2008 has the largest per capita use. India had an estimated 2 million plants in use in 2000. Around 8 million plants were installed in China in the early 1980s, but quality was poor and many fell into disuse. The programme in China is now continuing with a better quality design but at a slower rate. Biogas programmes are growing in many other parts of the world, such as Vietnam, Brazil and Tanzania.
Biogas plants have huge potential to produce a clean fuel from unhygienic, wet organic waste. There are many more rural 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.