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Biogas in Rural Costa Rica with the Santa Fe Women's Group |
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Biogas and Biodigester FAQ's
What is biogas?
Biogas is a gas produced by anaerobic digestion (in the absence of oxygen) of organic material,
largely comprised of methane (about two-thirds). Biogas is often called "marsh gas"
or "swamp gas" because it is produced by the same anaerobic processes that occur during the underwater
decomposition of organic material in wetlands.
What are some online biogas resources?
Aside from our main biogas page, we have compiled a number of different valuable sources of biogas information:- Learn about biodigester construction - See the biodigester design that the women's group used for its biogas project that was featured in the Summer 2007 issue of the Permaculture Activist.
- Biogas and Biodigester FAQ's - Learn the basics about biogas and get some of your specific biogas questions answered
- Video on how to make an anaerobic methane digester - Transcription of the English translation with link to the video
- Video on biogas in Pune, India - See how biogas works in this Indian city where they use leftover food, instead of manure, for their biogas energy needs
- Video on biogas digesters in Tamil Nadu, India - See how this rural Indian state utilize its abundant raw materials to create energy independence through biogas.
- Video of the UC Davis Biogas Plant - Interesting large-scale biogas plant that processes solid organic wastes with minimal preparation
- Read our collection of online biogas articles - Learn about biogas in other parts of the world from a newly-updated compilation of online biogas articles
- Learn how to make your own homemade biodigester - This book offers an easy-to-use, step-by-step illustrated manual on how to make your own homemade biodigester to create biogas in your very own backyard.
- Cow Power - This is a short video that speaks to the importance of biogas production.
- What is Anaerobic Digestion? - A video introduction to the biogas production through anaerobic digestion.
What is a biodigester?
A biodigester is a tank that processes the organic material that produces biogas. A biodigester can come in different shapes and sizes, depending on the needs of the people using it and the local possibilities in building materials. Come see the biodigester design that the Santa Fe Women's Group used.What material can I use to feed a biodigester?
In theory, any organic material can be decomposed anaerobically to produce biogas, but some materials work better than others. In general, materials need to be rich in energy and easily digestible. Manure works very well, coming from cows, pigs, or horses. Biodigesters can be fashioned from septic tanks, but the waste production is often not enough to produce enough biogas, and cleaning agents (bleach etc.) kill the anaerobic bacteria necessary for digestion. Plant material can be used, but acidic matter should be avoided, for they disturb the anaerobic processes. Plant matter is also often low-energy and slow to digest, creating a number of difficulties for digesters relying solely on such material.What is the production capacity of a biodigester?
This is probably THE MOST frequently asked question, and is the most difficult to answer for the following reasons:- Biogas production is best measured by scientific processes not common in rural Costa Rica and with tools not available in rural Costa Rica (sorry)
- Biogas production varies with the type of material you use to feed the biodigester
- Biogas production varies with the temperature of the mixture inside the tank
- Biogas production varies with the acidity or alkalinity of the mixture inside the tank
- Other factors, such as defects in construction, can make true measurements of biogas production very difficult
Are there different styles of biodigesters?
Yes. There are many different styles of biodigesters. The Santa Fe Women's Group used a style that in Costa Rica is known as "media bolsa," consisting of a large underground tank that is covered by a large, inflatable plastic sheet. Another style used in Costa Rica is called "salchicha" ('sausage' in English), consisting of a plastic bag spread out in a ditch with tubes attached at each end to put in and take out the organic material. I am very interested in this style and will make a page describing the instructions if I get the chance to build this type of biodigester.How does climate affect biogas production?
Tropical climes generally have no problems with temperature because the anaerobic bacteria thrive in higher temperatures. If you live in a more temperate climate, you may need to heat the tank during colder months. If temperatures within the tank reach temperatures below 20°C, the biogas production slows down. Under freezing conditions, you will not have digestion—only a big poopsicle.What is the most cost-efficient way to create a biodigester?
This will depend largely on the supply of certain materials that you have in your area. If you have access to affordable cement, I would recommend that you make a tank with cement. However, if you have very clay-like soil, you can maintain a biodigester tank in pure dirt with minimal draining. In April of 2006, this biodigester that the Santa Fe Women's Group built costed 161.000 colones, around $310 US at the time.My biogas production has dropped quite a bit. How do I figure out what's wrong with my biodigester?
There are a number of things that can affect the biogas production in a biodigester.Biogas leaks
If there is very little biogas, there may be a leak somewhere. If you are using water to seal the tank, make sure that water levels are sufficient enough to make a strong seal. In a biodigester like the ones the Santa Fe Women's Group made, if there are no problems with the water levels, you should check the plastic to see if there is a hole. With two or three people, you can remove the PVC frame from the plastic and hold the plastic up to sunlight to spot any holes that may have formed. If you don't find any holes, you can then check the PVC tubing that carries the biogas to the kitchen. To do this you can fill the tubing completely with water to see where, if anywhere, water is leaking through.Temperature problems
As stated before, if temperatures reach below 20°C, you will experience a drastic decrease in biogas production. If this is the case, look to adapt a heating system to your biodigester.Problems with the biodigester's pH
The pH in the biodigester tank should be as close to neutral (7) as possible. Since the anaerobic processes in a biodigester produce acids, the most common pH problem is one of acidity. If you do a simple litmus test on the biodigester's contents and the result is below 7, you should add a small amount of lime to normalize the tank's pH. Since excessive amounts of lime will not be soluble in the mixture and may harm the bacteria, you should never exceed a lime concentration of 500mg for every liter of mixture in the biodigester tank.Other problems
There are a number of other problems that can arise during the life of a biodigester. To investigate problems, it is best to think back to the basics of what makes a biodigester work (organic material, strong water seal, warmth) and eliminate anything else that could possibly harm its functioning. For example be careful not to introduce unnecessary chemicals into the tank, and try not to use livestock that has recently been given antibiotics or other medications, for these chemicals present in the manure may cause damage to the bacteria in the biodigester tank. Also, make sure to use non-corrosive materials for the structure of the tank. Cement and plastic cause no harm to the mixture in the tank, but metals should be avoided for use in the tank, or any of the tubing through which the biogas travels.Why does the gas not escape from the biodigester tank?
In the biodigesters that the Santa Fe Women's Group made, there are small openings for the entrance and exit of waste material through which small amounts of biogas can escape. However, a vast majority of the biogas escapes through the surface of the mixture inside the confines of the plastic that covers the tank. Any biogas that escapes is negligible.Why put steel wool in the biodigester tubing?
Steel wool (about three pieces) is placed in the tubing that carries the biogas to the kitchen in order to filter out impurities in the biogas. Biogas often contains elements that can stain the bottoms of pots and pans. Steel wool is not necessary, but is recommended.How do you remove the digested material from the biodigester?
If the mixture in the tank is at the same level of the exit tube, anything introduced through the entrance tube will force the same volume out the exit tube on the other end. Ureka!What can I use the digested waste for?
The digested material that comes out the exit tube of the biodigester is a liquid material that accumulates near the surface of the tank and is eventually forced out as more undigested material enters the tank. This liquid can be used as a convenient growth stimulant for nearby plants. In rural Costa Rica people often plant banana trees or vegetable gardens around their biodigesters, taking advantage of this great organic material.Do you have any more information on biogas and biodigesters?
Copyright 2007 Rural Costa Rica, a Costa Rica Travel Affiliate and member of the Green Energy Network