- Biogas digesters represent a beautiful concept derived from nature. This technology has been around for many years in developing countries such as India and parts of Africa and South America. Today, developed countries such as the United States, the United Kingdom, Germany, Sweden and Australia are constantly improving this technology for use in their local communities.
|Common Wheelie Bin Full of Food|
The biogas digester can help us become a more environmentally friendly society. Today’s society wastes a great deal of the food it produces. This waste can be attributed to individuals, farms, supermarkets, and businesses. Biogas digesters provide a great way of dealing with potentially harmful organic materials, including food waste, and turning them into valuable new resources. Using food waste, this technology has the potential to provide base-load power to large communities without contributing to climate change.
|Compact Fluorescent Light Globe|
A biogas digester takes in YOUR food scraps, grass clippings, and any other type of organic material and decomposes it through a natural process. This process is known as anaerobic digestion and it is similar to what happens in a cow’s stomach. The outputs of this process are methane and fertiliser. Methane is a gas that can be used to cook food, heat our homes, and produce electricity sustainably. The fertiliser is a nutrient rich liquid that can replace expensive chemical fertilisers and can be used to grow more food. The food scraps can be placed in the biogas digester and continue this sustainable cycle for generations to come.
The biogas digester is a great technology. It helps communities become more efficient and contributes to our fight against today’s environmental issues. If implemented successfully, a biogas digester can power a whole community out of their own organic waste. For more information on food waste, how the biogas digester works, energy production and real life applications of these technologies please click on the icons above.
- Australians throw away up to 20% of the food they purchase.
- Up to 40% of every household rubbish bin is food waste.
- Each year Australia produces enough food waste to fill 450,000 rubbish trucks.
- Each year Australia produces enough food waste to fill the MCG a little over 4 times.
According to foodwise.com.au, Australians are throwing out an average of 4 million tonnes of food every year. Households in Australia are now spending a total of $5.2 billion on food that they do not eat, according to recent research by the Australia Institute. Currently, Australians rank among the people who waste the most food worldwide.
Food waste is created because of regulations, production of too much food, consumer expectations, serving portions and lack of storage. In Australia, about 20% to 40% of fresh fruit and vegetables is thrown out before they reach our shops. Food waste isn’t something that only happens in our homes. This happens in our restaurants, supermarkets, business and farmlands, where product that is not purchased goes to waste. Our wasteful society not only harms our economy, but also our environment.
|Food Waste in the Queen Victoria Market, Melbourne, Australia|
Food waste that isn’t recycled or already being composted will likely end up in one of Australia’s landfills. In these landfills food will break down naturally and will pollute soil and ground water resources. The natural decomposition of food also releases methane. Methane is a greenhouse gas 25 times more harmful than carbon dioxide per unit mass. Methane contributes to climate change by trapping the sun’s heat within our atmosphere. Landfills also create thriving environment for pests, they smell and look bad, and they are a threat to human health.
The Environmental Protection Act (1970) created eleven principles for environmental protection. One of these principles established an order for waste management. Through this, EPA is looking to reduce the amount of waste created in Victoria by making a structure where avoidance is the most preferable practice, followed by reusing, recycling, recovery of energy, treatment, containment and disposal into landfills being the least preferable option. Through the biogas digester we can apply two of these most desirable methods for disposal, which are recovery of energy and recycling food. Placing our food scraps into a biogas digester will not solve our nation wide food waste problem, but it can be a potential start. By recycling our food scraps in biogas digesters, we can reduce our impact on landfills and climate change, help our economy and produce sustainable green energy to cook, and power our homes in a carbon neutral way.
For more information on the topics of food waste, landfills or other related issues please visit the CERES Sustainability Hub or:
Biogas digesters use a relatively simple technology. They take in organic waste, use bacteria to break it down, and produce methane in a process called anaerobic digestion. The decomposed waste can be used as a natural fertiliser and the methane can be burned as a fuel for heating or to produce electricity.
|Rotting Food Waste|
The biogas digester can make use of almost any organic material that can rot. In general, everything from lawn cuttings to leftover sandwiches and vegetables can be processed. While meat and fish can be used, small digesters may have issues with pests and extreme odours. The most noticeable exception is wood because the bacteria have trouble digesting lignin.
The digestion process is done in the absence of air and is comprised of four stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Three types of bacteria sequentially break down the waste into hydrogen and acetic acid (vitamin C). This allows the fourth stage bacteria to feed and produce methane and carbon dioxide as by-products.
For the bacteria, an ideal environment is defined by two factors, the temperature and the acidity [pH]. There are three temperature ranges: low (20-25 °C), medium (35-40 °C), and high (55-60 °C). The CERES digester and most other small digesters run in the medium temperature range, while large digesters often run in the high temperature range. Methane production is reduced when operating in the low temperature range so it is avoided when possible. The pH range is usually from 6.5 to 8. However, in some specialised digesters with multiple tanks, one tank can be at a lower pH to optimise the third stage of the digestion.
Just as food needs to stay in an animal’s stomach for a period of time, the waste needs to stay in the biogas digester for a period of time for the anaerobic digestion to occur. This time span is called the hydraulic retention time and it can range from 15 days for high temperatures to over 100 days for lower temperatures.
Biogas is usually comprised of 60% to 70% methane, the useful flammable gas, the rest being mostly carbon dioxide. Similar to natural gas, biogas can be used in stoves, heaters, or generators to produce electricity.
The liquid, or effluent, that comes out of the digester can be used to replace chemical fertiliser. The key components of this effluent that allow it to be used as a soil replenishing source are Nitrogen, Phosphorous, and Potassium. These are the main three minerals stripped from the ground during farming. In chemical fertiliser, the nutrients are obtained by mining for mineral rich rock or natural gas and then chemically processed. Both the mining and processing are harmful to the environment as well as costly, thus natural fertiliser from the digester has clear advantages.
The CERES Biogas Digester
|CERES Biogas Digester|
The biogas digester at CERES is a 90L plug-flow biogas digester with an estimated maximum daily gas production of 50L. It can store 450L of gas for burning and the burner attached can consume that gas at a rate of 7.2L per minute. When operating in the optimal temperature range of 30-35 °C and the optimal pH range of 6.8 to 8.5, the CERES digester can be fed 3.6L of diluted biomass (slurry) every day. The slurry is half waste – half water by volume and the hydraulic retention time is about 40 days.
Due to the importance of keeping the bacteria within proper temperature and pH ranges, the CERES biogas digester is monitored daily and adjusted when needed. Below you will find typical data for temperature and acidity in March.
For more information on how biogas digesters work please visit the CERES Sustainability Hub
Further information information available at:
How much do YOU waste?
See what you could do with your waste!
Fill in how much organic waste you throw away every week and click ‘Calculate’.
|Gas Stove Burner|
Biogas only contains 60 to 70 percent methane, the rest being mostly carbon dioxide. If incorrect digestion occurs, nitrogen and hydrogen sulphide can also be produced which can affect the burning properties. This composition makes the average energy of a cubic metre of biogas lower than that of methane. In order to use biogas similarly to methane, it can either be used in larger quantities or purified. One kilogram of waste is enough to run a single burner gas stove for one hour.
Biogas can be turned into electricity by running it through a generator. However, 65% of the energy is lost in the conversion process. These generators are usually large power plants that provide for thousands of homes. Some use the excess heat from electricity generation to warm the fermenter and provide an optimal environment for the bacteria. Other generators create steam from the excess heat and distribute it through pipes to heat the local community. The facilities that produce fertiliser pellets even use some excess heat to dry the solid digester output.
- For more information on energy from biogas please visit the
In our Community
|© BIOGEN (UK) Ltd 2013|
Biogas digesters are used on the community level right here in Australia. Eighty thousand tonnes of food waste, enough to fill 9,000 rubbish trucks, are collected from food manufacturers, supermarkets, restaurants, and households in the Sydney suburb of Camellia every year. This food waste is being used sustainably instead of being sent to landfills. The gas produced by the digesters is used to generate green electricity for 3,600 homes. For more information on this facility, visit earthpower.com.au.
Collecting Food Waste
Food waste from households and businesses can be collected by Councils and private companies. Some Councils in the United Kingdom provide a special food waste bin besides rubbish and recycling. The food waste is collected on a weekly basis and taken to a local biogas digester. Also in the U.K, a waste management company works with local businesses to collect their food waste and bring it to biogas digesters on a nearby farm.
|Wastewater Treatment Facility|
Food waste can also be collected with an in-sink food waste disposer. The ground food waste mixes with water and then moves through the sewers to a wastewater treatment plant. In the United States, over 1000 wastewater treatment plants produce biogas. Most of these plants use the biogas for process and building heating but ten per cent of them use the gas to generate electric power. In Australia, electricity is generated with biogas by similar wastewater treatment plants like the Werribee Sewage Plant in Victoria.
|Rural Biogas Digester|
Biogas digesters are also used around the world in homes, on farms, and on industrial sites. Millions of homes in India and China use small biogas digesters. Most of these digesters are located in farming areas and run on human waste, manure, and unused crops. The decomposed waste is used as an agricultural fertiliser. The gas is used for cooking and lighting and is able to provide most of a family’s energy needs. The gas replaces wood as a fuel in these developing countries, reducing the amount of deforestation. Unlike wood, biogas burns cleanly without producing smoke. This makes it more environmentally friendly and less likely to cause health problems for the residents. The use of a biogas digester also reduces health risks by removing human waste from the home as effectively as a sewer system.
Many middle and large-scale biogas digesters that are fed with waste produced on farms. Examples can be found in the United Kingdom, China and Australia. Discarded vegetables, dairy manufacturing waste, manure, and grass silage are a few examples of the types of waste fed into these digesters. The large amount of gas created can be inserted into the national gas grid or used to generate electricity and heat. Megawatts of power can be used locally or sold back to the power company.
|Biogas Digester Facility|
Some of the most advanced biogas digesters in the world are being used in Germany. One biogas plant is capable of producing 20 megawatts of power. This is enough to provide power to 50,000 homes. The plant is mainly fed with farm waste like crop silage. In Germany, there are over 6,000 large agricultural biogas digesters. These digesters contribute to a large portion of the 1,500 million tons of biomass digested each year in the European Union.
For more examples of biogas digesters please visit or:
Science as a Human Endeavour
The Use and Influence of Science
- ACSHE035 – People use science in their daily lives to care for the environment and living things
- identifying the ways humans manage and protect resources, suchas reducing waste and caring for water supplies
Science as a Human Endeavour
- ACSSU219 – Energy from a variety of sources can be used to generate electricity
- considering whether an energy source is sustainable
Nature and Development of Science
- ACSHE099 – Important contributions to the advancement of science have been made by people from a range of cultures
- investigating how people from different cultures have used sustainable sources of energy, for example water and solar power
Use and Influence of Science
- AACSHE100 – Scientific understandings, discoveries and inventions are used to solve problems that directly affect peoples’ lives
- investigating how electrical energy is generated in Australia and around the world
- ACSHE220 – Scientific knowledge is used to inform personal and community decisions
- considering how personal and community choices influence our use of sustainable sources of energy
Science as a Human Endeavour
Nature and Development of Science
- ACSHE120 – Science and technology contribute to finding solutions to a range of contemporary issues; these solutions may impact on other areas of society and involve ethical considerations
- considering how human activity in the community can have positive and negative effects on the sustainability of ecosystems
Science as a Human Endeavour
Use and Influence of Science
- ACSHE135 – Science and technology contribute to finding solutions to a range of contemporary issues; these solutions may impact on other areas of society and involve ethical considerations
- investigating requirements and the design of systems for collecting and recycling household waste
It turns waste into high-quality organic fertiliser
It benefits the environment by capturing methane which otherwise would be produced in landfills and would exit into our atmosphere.
It protects soils and water resources.
It lessens our dependence on landfills for disposing of waste.