What’s the Big Deal with “Biogas” and Anaerobic Digestion?
Anaerobic digestion is a method of producing and collecting methane gas produced by methanogenic anaerobes by providing an environment free of oxygen and abundant in organic matter in which they can flourish. This form of gas production-despite its recent popularity-is a form of renewable energy that has been around for a bit longer than many of us might expect. Below is a rather succinct excerpt from an article posted on pigproducer.net showing a snapshot of Biogas around the world in history:
SHORT HISTORY OF BIOGAS
- b.c. - Marco Polo mentions the use of covered sewage tanks in ancient Chinese
-In 1776 - Alessandro Volta concluded that there was a direct correlation between
the amount of decaying organic matter and the amount of flammable gas produced.
-In the 1930s - the development of microbiology as a science led to research by
Buswell and others in the 1930s to identify anaerobic bacteria and the conditions that
promote methane production.
-In 1937 – municipal park cars of several German cities (e.g. Muenchen) ran on biogas
from sewage treatment,
-In 1972 – due to the oil-crisis, construction of biogas plants became interesting
-Today (for instance); in Germany in 1992 there were as few as 100 biogas plants,
but in 2005 their number due to favored legislation has increased to 4.000 capable of
cumulative power production of nearly 1.000 MW
The beauty of a biogas digester is that it is incredibly flexible in its scale and design. There are biogas digesters that power industry as well as the subsistence farmer. Another attractive feature of biogas digestion is that it allows you to harvest a valuable energy source like manure without degrading its effectiveness as a fertilizer. This fertilizer is of course the effluent discharge from the biogas digester. What is notable about this effluent is that it has a greatly diminished population of dangerous pathogens that are found in untreated manure and spoiled food waste that fuel the system. For these reasons and many more, biogas is a great alternative and supplement to conventional natural gas, and can be readily added to existing natural gas infrastructures providing that a refinement of the raw gas is met (which I will not go into here).
My First Experience
Though I was actually introduced to Biogas in the 1985 Post Apocalyptic Sci-Fi flick Mad Max Beyond Thunder dome, (http://www.youtube.com/watch?v=Hgq4w4dqKsU) in which the city called “Barter Town” is run on the excrement of pigs, my first real hands on experience was at a small farm in Heredia Costa Rica.
When I attended the TAM (tropical agriculture and missions) course last year we were taken to the farm of Don Rafael. As you can see from the pictures below the reactor used by Don Rafael's family is not very large, yet it is capable of supplying enough biogas to provide cooking fuel for his family. His bioreactor has a concrete base and a flexible bladder. The swell of gas contained in this bladder is primarily methane. It is fueled mainly by fruit and vegetable, many of which can be found beneath the many fruit producing plants on his property. If I remember correctly he used to have some live stock that contributed fuel, but had since sold them and the reactor is still producing effective volumes of gas.
Not too long after seeing the Biogas reactor at Don Rafael's farm we saw another at the dairy farm across the street from CATIE and yet another at Earth university. The latter (seen below at EARTH) provided cooking gas for the cafeteria and is powered by the student body. What you are seeing below is a storage bladder suspended above the actual biogas-reactor.
As you can see in the following pictures, the biogas reactor built on the ADE farm is quite large (yet not as big as EARTH’s). I have heard some estimates that it could produce enough gas to power five households. The bag is made of polyethylene and is around ten meters long and has an internal lumen big enough to park a car in.
Here in Vara Blanca there are two agricultural products that dominate: strawberries and milk. For the biogas reactor, rotten strawberries and cow pies should work wonderfully as reactor feed, providing the pH is regulated due to the fruits acidity. To that end the supplementation of fruit and vegetable waste to that of cow manure provides a great primary material for a high yield of CH4 (methane). A study in India has shown that such mixtures of primary material can produce a CH4 yield that is in the high 70 - 80% range (Narayani et al. 2012).
In spite of the plentiful cow pies in Vara Blanca, the ADE farm does not yet have many of its own (nor does it have many spare strawberries), so at present we must go and collect five gallons of this green gold everyday from our next door neighbor Don Carlos who so kindly lets us pick as many pies as we please.
April 17th we primed the digester with the first batch of cow manure. We have yet to close the outlet pipe that will carry biogas once the reactor is active, so we are still dealing with an aerobic system. This means that starting the clock as far as the anaerobic process will have to wait until we get all the fittings and valves set. Once we have closed the system we should have to wait around two weeks for the population of bacteria to take off and the processing of organic material into methane water and carbon dioxide.