Biodigestion - A Bahamas First!

On the 13^th of April, 2011, The Cape Eleuthera Island School created the first recorded experiment of Biodigestion in The Commonwealth of the Bahamas.  Biodigestion is a way for the energy within animal waste to be harnessed and made into usable forms.  This concept has been around for thousands of years, but has recently gained popularity as the population exploded and significant issues with untreated human waste became apparent.  Currently, The Island School has a “blackwater” filtration system on campus known as the “Poo-Poo Garden” from which plants can draw nutrients.  This system does not remove all of the pathogens nor can it deal with the waste from nearly 100 people.

The campus has been actively searching for a solution to this problem and arrived at biodigestion due in part to the favorable conditions that The Bahamas provides, but more so the availability of glycerol.  Glycerol is a by-product of the bio-diesel production and was considered a waste product because of its inability to be used in large quantities.

After reading numerous articles on construction methods and types of digesters, our 6 students built scale models of the system in order to run tests on how different amount of glycerol affect the production of methane.  Methane, as known as natural gas, is a gas that when unused does about 20 times more damage to the atmosphere than the equivalent amount of carbon dioxide.  Therefore, it is important to harness this energy source and greenhouse gas before it enters the atmosphere.

Our primary input is pig waste, weighing on kilogram, mixed with water to create a viscous slurry.  When inoculated, the slurry will begin a stage called hydrolysis.  Moving through the stages of breakdown, called acetogenesis and acidogenesis, the slurry begins methanogensis.  At this point, methane begins to produce, changing the pressure within the first container.  A tube connects the first container to the second, and similarly the second to the third.  The second container has water in it, which becomes displaced into the third as gas pressure increases.

Once the process is completed, the methane can be captured from the second container and collected or burned outright.  As mentioned earlier, at upwards of 20 times more dangerous than carbon dioxide, the free release of methane into the atmosphere poses a great danger to the Earth.  When burned, methane changes into on CO₂ molecule and a water molecule in the form of steam.  Also, trace compounds have been found but none as harmful as pure methane. 

As you might imagine, the amount of gas that can be harnessed from one kilogram of waste can be considerable.  When mixed with glycerol, an agent which increases the rate of production, this amount is increased over a shorter period of time.  Two of our trial run digesters failed after the pressure inside the containers caused the caulking to fail.  After 5 days, we still had enough gas in our remaining two digesters from the 8 pigs on campus to get a sustained burn of 20 seconds. 

Currently, we are running a second trial using what we learned in the first trial to figure out the best ratio of glycerol to waste to maximize our output.  In a large scale production, this technique has the ability to power all of the cooking needs for a village of 50-75 people, roughly the size of our on-campus community.  As a benefit to the community, we reduce the amount of glycerol that goes unused, reduce septic waste, and get a nice pathogen-free fertilizer which helps plants grow faster.