Bio-digester

Well, I thought I was going on another survey trek this week but the cards were not in my favor. Not the end of the world though because I have plenty of stuff to catch up on here in Brikama. The last few weeks have been full. First, I was on another survey trek to the Tanji Wetland Preserve, and then it was off to our Close of Service Conference for the group of volunteers I arrived with, and finally conducting training for the latest group of environment volunteers on beekeeping, bio-char, and bio-digesters.

I know I told a few of you about the project I have been working on with an agriculture student at the college building a bio-digester. I thought I would write something up on it quickly so you could actually see what we’ve been up to.

First off, what is a bio-digester and is it supposed to be hyphenated? In our case a bio-digester is a device or structure that ferments biological waist in an anaerobic environment to produce, among other things, methane gas. That gas can be used for a whole range of things including cooking, lighting, running equipment, etc. For our purposes we’re only concerned with cooking. As for the question of hyphenation, I could use an assist from the English major crowd out there.

I heard about the concept of bio-digesters from reading on the Internet and talking with some other people who had seen them implemented in other places. I thought it would be an interesting thing to try out here in The Gambia since there is a major problem with deforestation due to people harvesting wood for cooking and there is no shortage of organic waste, namely animal dung. So, I started looking around for someone to work with and in the process talked to the head of the agriculture department at The Gambia College to see if he knew any students who might be interested in working on such a project as part of their final dissertation. A few weeks later a student came into my office and said he wanted to work on the project. He has proved to be a very motivated and determined individual and the rest is history.

I won’t bore you with all the technical details but we did some more research to figure out what type of digester we wanted to make and settled on a design from ECHO, a organization based in Florida. We wanted to build a semi-portable unit that we could use as a demonstration to get people interested in the idea and see where it would go from there. All the materials we bought locally and for less than $75 which we could significantly reduce the second time around knowing what we know now. We strung the whole construction process out over several weeks because we were working around two schedules but you could build it in a day if you had to.

So how does it work? I’ll let the pictures do most of the talking.


Here is Malik, my counterpart during the construction phase.


Here is the whole digester (yes, I know its not the glorious), the left barrel is the digester, its full of a 50/50 mix of pounded cow dung and water, as the bacteria break down the dung the methane rises inside the tank and is forced out the clear hose. The drum on the right is full of plain water. The gas that is forced through the clear hose bubbles up through that water and is captured in an inverted trash can (you can’t see the trash can in this picture). The gas is kept there until it is ready for use.


Here is what the trash can looks like when it’s almost full of gas and floating on the water in the second drum.



Another picture of it all put together. You can see the trash can has a little bit of gas in it. You can also see our sweet prototype, shower head flame thrower attached to the orange gas output hose.


When we want to “charge” the digester by adding new waste we pound up some of the dung to make it easier to digest


Then we mix it with water


Then pour it into the input pipe on the first drum.


In order to add more waste we need to take some of the digested material out using this ladle thingy. This “manure tea” is great fertilizer for the garden.


This is another burner we made from a milk tin and a coffee tin. It worked pretty well but we have since made a better one from a single smaller coffee tin.



One of our first tests when we got something to burn. Fire! Sweet!


It’s been a great project. The Gambian TV station GRTS and a local newspaper have done stories on the project and Malik has made some great contacts with other organizations in the country that are interested in bio-gas. Most recently The Department of Energy invited him to help in construction of a digester they want to build in the near future. We did a demo for all the incoming PCVs during their recent training and it seemed to generate quite a bit of enthusiasm. Hopefully people will take the idea and try building some of the larger family size units in their villages.