Smart Solutions

Micro-Biogas Digester

A small-scale physical installation that is fed with local food and garden waste to generate biogas energy for cooking and heating

Implemented inSuderbyn Permaculture Ecovillage

Country : Sweden

What’s the solution?

This small-scale biogas digester can be built and implemented by any rural community.

It mainly consists of a chamber, which is specifically designed to enable it to contain compressed air and which can be fed (through ‘feeding’ pipes) with food and garden waste and water to instigate a natural process of biodigestion generating biogas (which is collected through an ‘outpipe’ with gas cap).

The micro-biodigester has a simple design that can be made locally out of easy-to-make concrete, using concrete moulds. It has no moving parts, making it is relatively unlikely to have technical problems. The digester can run at variable flows and temperatures.

The micro biodigester can be integrated into a full closed-loop system for the sustainable management of waste at a very localised level.

The solution is not only able to enhance hygienic disposal of organic waste products with limited local odour, but it also generates valuable products in terms of fuel and fertiliser (which is also collected through a ‘run-off’ pipe).

What makes it smart?

The solution is smart because it utilises reliable simple technology and minimum investment to transform waste into useful energy and fertiliser resources

The solution demonstrates the following forms of innovation:

Social innovation – it suits neighbourly cooperation to feed and operate the digester and utilise its products, helps people understand the concept of “closing the loop”;

Technological innovation – it makes biogas technology available to everyone at the micro/small scale and with reliable low tech, previously reserved to larger high-tech industrial installations;

Business innovation – it replaces seasonal farm employment with opportunities for year-round production and employment.

How is the solution implemented?

  • Before installing a micro biodigester it is important to identify locally available energy-rich supplies of waste (household, farm, restaurant) and identify local demand for heating and cooking as well as demand for liquid fertiliser.
  • Finding an appropriate site for the digester is also crucial. This can be indoors or outdoors, above or below ground. Crucial aspects to consider are that the site must be able to support the weight of a large tank of water and it should support the minimisation of transport needs in terms of moving inputs and outputs.
  • Once the location is selected, the digester can be constructed using (purchased) concrete moulds and (rented) cement mixers and concrete vibrator, irrigation rings or mortar and brick, to which Feeding and run-off pipes need to be added. A steel or fiberglass gas cap with outpipe for gas should be purchased.
  • Once ready, the biodigester can be filled with water and innoculated with (microbes from) fresh cow manure. After waiting a few weeks it is ready to be fed with organic waste and the digestion process will start naturally, producing gas.
  • The digester temperature should be maintained at temperatures between 20-40 degrees C, which may require specific local solutions.
  • An effective system will then include organised slurry feeding of the digester and storage of both the gas and liquid fertiliser produced.
  • Options to filter the gas can be pursued to improve its quality.

In what local context has it been applied?

Suderbyn Ecovillage established 2008 is an intentional community of 25 persons from 12 countries seeking to develop and test new ways of sustainable living together.

Suderbyn is located in Västerhejde Parish (2300 inhabitants) on Gotland island (60000 inhabitants), 100 kilometers off the Swedish east coast in the middle of the Baltic Sea. Due to its insularity, the island has long worked to become energy self-sufficient with wind, solar and biogas technology and was recently selected by Sweden’s government to be its first fossil-free region by 2035. The island region has a biogas strategy and already produces biogas for biogas cars and buses and will begin to use some biogas for ferries.

Suderbyn Ecovillage wanted to address nutrient leakage of agriculture into the sea by integrating biogas production and food production in a closed loop. It needed a practical solution that it could implement in a small community and on a limited budget.

Who was behind the implementation?

NGO RELEARN - Suderbyn's NGO working with transformative education, research, networking of pro-change initiatives and providing employment in Suderbyn - paid for and runs the digester.

It was also strongly supported by ‘Solar CITIES’ - a U.S.-registered charity focused on delivering biogas solutions within the USA and across the globe.

What was the local journey?

● Prof. T H Culhane of Solar CITIES, donated a set of Chinese-designed family-size biogas digester moulds to build micro-digesters in Europe to the Global Ecovillage Network of Europe.

● Suderbyn Permaculture Ecovillage on the Swedish island of Gotland was chosen as the test site for the first digester in Europe.

● In July 2015 the first micro-digester was built by a group of volunteers. It took several days of planning and 3 days to pour the concrete mixed using two DIY cement mixers to forge the 3 m wide, 3 m tall structure. The amount of concrete needed is difficult to produce with DIY household cement mixers but having concrete delivered by vehicle requires quicker pouring than a volunteer team can manage. We chose a large team of 10-15 volunteers and 2 DIY cement mixers in parallel

● Getting permission from the Municipality to start the digester took 18 months and many documents. We were finally able to satisfy the safe storage requirement by using old CNG car gas tanks. In September 2017 permission was granted by the municipality to start up the digester.

● In June 2018 the digester was filled with water and inoculated with various microbes from cow and other manure.

● August 2018-October 2020 the digester has been fed kitchen waste daily and is constantly producing biogas and liquid fertiliser.

What have been the main outputs & results?

  • Biogas has been produced almost continuously for 2 years despite cold winter conditions.
  • Liquid fertiliser is being produced which is better than animal manure for fertilising gardens and crops.
  • Community members of different gender identities, ages and nationalities have participated in the daily feeding of the digester.

What does it bring the village/community?

  • The micro-biogas digester helps everyone to produce methane and liquid fertilisers for home-use and not just via industrial biogas production of public authorities and larger private businesses.
  • The digester attracts visitors and inspires new thinking about local energy sovereignty.
  • The digester shows that individual citizens and communities can take action to reduce nutrient losses of waste, for climate-neutral energy and prosumer energy citizenship.

What’s needed

Financial resources

Main types of cost:

Construction costs are the main investment component. This includes rebar/wire mesh, cement, sand/gravel, concrete mould oil, rental of cement mixers and concrete vibrator.

Other installation materials/equipment is needed to connect, filter, compress, store, test and use the biogas and grind the food. Safety signs, locks, logbooks and gas alarms are required by law.

Operating costs are mainly electricity to grind food waste and pump water and compress gas.

Financial needs:

• Initial construction and installation costs: 5000 €

• Ongoing/recurring annual costs: 300 €

Funding received:
Private donor5,000 €Digester build, installing initial material/equipment

Human resources

● Digester build requires basic construction knowledge and experience with concrete structures;

● Licensing requires good knowledge of government administration and patience;

● Digester operation requires basic knowledge of microbiology/physics and experience in production processes and operations (which can be built up through training).

Physical resources

● The digester should be kept in an optimal temperature range of 20-40 degrees Celsius - though lower operating temperatures are possible. Heating may be needed depending on geographic location.

● Digesters can be built inside farm buildings or agricultural glasshouses.

● Consider distance to feedstock source and gas consumption point when siting the digester.

What to do…

  • Some amount of training in safe biogas handling and digester construction and operations is recommended.
  • Locate your digester where you produce waste or where you consume gas or would use liquid fertiliser.
  • Identify under-utilised high-quality local digester feedstock available, e g used cooking oil, agricultural waste, restaurant waste.
  • Explain to stakeholders that methane production is a basic natural process happening everywhere all the time including in our own bodies.

and not to do

  • Don’t compress gas if not absolutely necessary. Non-pressurised gas is safer and cheaper to store.
  • Don’t view local micro-production of raw biogas as posing the same danger as industrial production of refined methane.
  • Don’t confuse simple low-tech micro-biogas digesters with complex and high-tech industrial biogas digester.
  • Don’t allow nutrient-rich food and agriculture waste to leak into the environment and cause eutrophication in local wells and waterways.

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