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Response to condom experiment on food waste vs dung for biogas

I got a really interesting response from T.C. Culhane to my earlier blog post on my experiment about biogas in which I tested the theory about kitchen waste vs cow dung for methane biogas.

My condom experiment showed that cow dung was far superior to kitchen waste in gas production

Here’s what Culhane had to say

“Marcel Lenormand called my attention to what you are doing.  It is great to see a small-scale experimental design for biogas which can be easily replicated. With my experience building digestors in Kenya as well as Egypt, Tanzania and the U.S. I thought I might share some thoughts.  First, you will not get any appreciable methane from the food waste container and you are right, it will rot and consume the air (this is the sucking the condom into the cup you observed). This is because food waste contains virtually no methanogens if any at all.  We did the same experiment in Cairo with a 200 liter drum filled with food waste to prove it to ourselves, just in case.  One needs a source of methanogenic bacteria, and normally that is from cow or horse or pig dung (others have used goats and cows and I’ve even started a digestors using my baby’s soiled diaper). However once you have enough bacteria producing gas the kitchen waste is a superior food. In all the cases we’ve observed the dung definitely starts producing gas first because the mix of dung and food can turn too acidic, inhibiting methanogen growth. This is why Dr. Karve always recommends you start your digestor with animal dung first and WAIT until you get your first FLAMMABLE methane before adding food waste.  But from then on you need only add food waste. You just have to make sure you do not add too much.  Karve says (and we’ve confirmed) to start feeding slowly, about 200 grams at first and gradually building up to no more than 2 kilos a day for a 1000 liter system.
It may be that what you are observing in your condoms is not methane at all, but CO2, for this is the first gas to be produced.  Over a period of several weeks the CO2 concentration drops to about 40% and the methane concentration increases to about 60% and that is when it starts burning.  So don’t let the gas production you see at first be the indicator of how well your system is doing. What you are trying to do is grow a stable and large colony of methanogenic bacteria.  Once you have done that, you can start feeding the food waste and you will not have to feed animal wastes again (though you are always welcome to — it is a good thing to add fresh bacteria through new dung introductions but if it gets to be a hassle you don’t have to put more in.) Among the Masai at the Boma we recommend putting any waste cabbage leaves and potato skins and ugale that fell on the floor and mix in any cow dung that is convenient. The addition of the food wastes improves output. Hope that helps!”

Well, Mr. Culhane this really helps a lot and though my dung digetser is working fine now,  I do have to go back to the drawing board and re-do the experiment but will need a new supply of condoms for that!

Read more about the how we installed a flexible biogas system at home on Afrigadget.com


June 21, 2010 - Posted by | Uncategorized

1 Comment »

  1. Thanks T.H. that explains very well what’s going on in Paula’s pots.

    I understand Paula’s (and others) response about feeding food waste to a digester when it’s said “we don’t have any/much food waste”. As T.H. mentions, it’s surprising where such scraps can come from – the floor, washing up water…

    I’ve made a tentative approach to a local supermarket about picking up their spoiled fruit and bread. They were at least initially warm to the suggestion. I’d be very interested to know what grocery stores around Africa would say to that. It would mean an abundant supply of feedstock.

    Comment by Marcel | June 22, 2010 | Reply

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