A biodigester is a container in which methane,
or biogas, is captured in the processing of
organic material by anaerobic bacteria. In the
model shown above, the organic wastes are
stored in the plastic bag, and the methane is
captured. A pipe leading from the middle of
the plastic cover takes the methane to the
kitchen for cooking.
Carbon Dioxide (CO2) emissions are being addressed all over the world through alternate forms of energy, carbon credits, and simple lifestyle adjustments. Yet there is a fight that needs to be fought against a more powerful greenhouse gas; that of methane. Methane, because of its molecular structure is twenty times a more dangerous greenhouse gas than CO21. Due to the release of methane from natural waste, the agricultural industry is one of the leading causes for the increase in global temperatures.

One of the ways to fight this is through the introduction of biodigesters on farms whether large or small. Small scale farms can capture methane to use in their homes, but those of a larger scale can implement biodigestors to sell methane, or even turn it in to electricity. Biodigesters are a key factor in the sustainability efforts of farmers, and have many positive ramifications if installed.

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Definition

What the heck is a biodigester? A biodigester is a container in which methane, or biogas, is captured in the processing of organic material by anaerobic bacteria. There are several varieties and most plans are simple, cheap, and easy to build2.

Our Experience

As part of our service-learning experience, we worked with a
farmer in Agua Buena to install a biodigester at his home.
While working on coffee farms in Agua Buena, we found ourselves working on a biodigester, something that we had previously become acquainted with at Earth University but only to a certain point. Now we had the chance to help build one, or at least watch one being built. There was a hired worker who specialized in biodigesters who was the brains and head of the construction, and due to the necessity of precision whilst making it, we could not help much in the actual building of the biodigester itself. However there were still tasks to be done, for example moving a pile of cement that was to be mixed down the construction cite, and digging a trench behind the biodigester so there is room to walk around it.

Materials

One of the important aspects of biodigesters is that they are so simple to make, in terms of materials and costs. In theory, all you need is some PVC piping, a large plastic bag and some pigs, but to maximize efficiency there are other necessary materials. When dealing with the cost of one, they can range from $200 to $1000, but $300 should be suitable to fund a perfectly good biodigester. The actual material that is used can range from human and animal feces, or even plant matter. The problem with human waste is that a lot is needed to produce a significant amount of methane, and if you are thinking of modifying your septic tank to capture the gas, chemicals flushed down the toilet can kill the anaerobic bacteria that are used to break the waste down. The situation with plants is that they are low-energy and often very slow to break down. The tank would serve best if built of cement, with plastic used for the pipes. Any metals used can potentially harm the content of the organic material.3

Types of Biodigesters

In Costa Rica, two types of biodigesters are most commonly built. There is the "Media-bolsa," in which a plastic sheet stretched over a tank captures the biogas, and there is the "Salchicha," where a large, sausage like plastic bag has organic matter introduced to one end and is eventually flushed out the other.

Used Waste

After the manure or other organic material is processed, what is left can be used as a very effective fertilizer that easily benefits the farmer. One of the reasons it is effective is while in the biodigester it becomes sterilized, which eliminates any disease-causing bacteria such as E-Coli. This is one of the reasons the biodigester is such a useful sustainable farming tool.4

Economic, Social, and Environmental Advantages

Having methane available can change the lifestyle of a family. The biogas can be used for heating, cooking, and even lighting, thus eliminating the costs for one or maybe each of the utilities. Not only does the biodigester give the family more disposable income, but it also makes safer living conditions.5 Before families had methane available to them, they would use propane, kerosene, or firewood. Using the latter leads to deforestation and a possible loss of biodiversity, while the first two can become very costly. Also there are health risks with using firewood because it is often lit with materials that would release carcinogens into the indoor kitchens.6 The methane is also not compressed, like the propane tank would be, eliminating another risk.

How it works

The Following is an explanation of how the "Media-bolsa" model of a biodigester functions, slightly varying from the "Salchicha" model

Before the organic material enters the biodigester, it needs to be mixed with water. When dealing with pig manure the amount of water added equals the amount of manure entering the system. In Costa Rica cows are grass fed so twice as much water as cow manure is needed to break down the fiber.7 The organic mix sits in the bottom of the tank, while the anaerobic bacterium breaks it down and reproduces creating more bacteria. This process creates biogas which escapes the surface of the material and is trapped by the plastic cover that balloons up when filled with methane. There are various factors that effects the amount of time organic material sits in the tank; the type of organic material used, temperature, and unbalanced acidity and alkalinity levels.8 The anaerobic bacterium does best in warm temperatures so in cold weather the digester should be heated (in Costa Rica temperature is not a problem). Twenty degrees Celsius is the temperature in which methane cannot be produced. A pipe leading from the middle of the plastic cover takes the methane to the kitchen for cooking. The waste exits through a pipe that is level with the mixture of manure and water so any material introduced to the tank pushes old material out.9

This small generator was being used by a farmer in Agua
Buena to generate electricity from his biodigester. These
simple-to-build generators can provide families with a
back-up energy source, and can also help diminish energy
costs.
The methane can also be bottled and sold, or hooked up to a generator to create electricity. At Earth University we saw a large generator but not enough biogas was being produced to run it. In Agua Buena we saw a small generator that was very simple to build. Methane is lit under a tank filled with water which evaporates in to another tank above it. The steam is pressurized and when released shoots out a pipe and spins a small turbine creating electricity.


Visit this page for a diagram of the biodigester and precise construction instructions.









1 The EPA's Climate Change Division and Office of Atmospheric Programs. (2007). Methane. Retrieved September 4, 2007 from http://www.epa.gov/methane/
2, 3 Thomas Carmona. (2007). Biogas and Biodigester FAQs. In Rural Costa Rica. Retrieved August 1, 2007 from http://www.ruralcostarica.com/biogas-faqs.html
4 The Appropriate Infrastructure Development Group. (2007). Biodigesters. Retrieved August 7, 2007 from http://www.aidg.org/biodigesters.htm
5, 6 Fundacion para el Desarollo de las Comunidades del Sur (FUDECOSUR) and Fundacion Union y Desarollo de las Comunidades Campesinas (FUNDECOCA). (2007). Biodigesters in Rural Communities Costa Rica. Retrieved August 9, 2007 from http://sgp.undp.org/download/SGP_CostaRica1.pdf
7, 8, 9 Thomas Carmona. (2007). Biodigester Design & Construction. In Rural Costa Rica. Retrieved August 13, 2007 from http://www.ruralcostarica.com/biodigester.html