Building a (one-stage) Anaerobic Digester

David of the Kenyan Community Initiative Support (KCIS) recently built a very low tech Anaerobic digester using only a drum, a valve and some pipes:






The balloon shows that gas is being produced. The costs for the drum and professional valves may already be too high for some, and the design isn’t that optimal. They intend to add a storage drum with a water-filled header tank for constant pressure and the loading & desludging processes obviously still require some work.

We still like the approach though, because it does indeed “prove the theory”, as David notes. The theory of building a rather small anaerobic digester that will even work with smaller amounts of organic waste.

Goes to show that producing methane gas from something which would otherwise remain unused (livestock faeces usually kept in such drums for a few weeks without harvesting the methane potential) still is an interesting alternative & well appreciated once costs are covered.

15 thoughts on “Building a (one-stage) Anaerobic Digester”

  1. Hi you guys, excellent work. I just got back from building various biodigester designs in Kenya (with the Masai) and Tanzania (in Kigoma with members of the Jane Goodall Institute and in the village of Kilanzi near Gombe Chimpanzee reserve). Our aim is to provide alternatives to deforestation. We have been using the ARTI India model of using kitchen waste as the feedstock for biodigesters (yielding up to 400 times the amount of gas per kg of feedstock, making small scale digesters feasible). That said, we find that the minimum effective size is 750 to 1000 liters which yields about 2 hours of cooking gas from the previous days organic garbage (about 1 to 2 kg). This is far preferable to using 40 to 70 kg of cow dung. We start our systems with cow dung for the bacteria but then switch to using food waste mixed well with water once first flame is produced. As long as you don’t overfeed (which makes it acid and kills the bacteria) you need only put in animal dung once every six months or year or so if you see gas production slowing down. There is no loading or unloading to worry about — the ARTI design (and our modifications) make it so that every time you add 10 liters of food and water slurry you get 10 liters of “spent fuel” (fertilizer) out, automatically. We build these now on rooftops in Cairo Egypt; they work well in urban areas. We even have one on our porch in Germany. With a 200 liter drum you can get about 30 minutes of cooking gas a day when temps are right (30 degrees C). In Kenya we welded 3 used oil drums together to get over an hour of gas a day. Bravo to you for proving the concept; we welcome you to join us in making it practical for daily use for everyone! Asante sana!

  2. Nice post JKE. I saw a biogas digester (that sounds like the thing Thomas mentioned above) installed in Mbirikani and wanted to do a post on it but it was not yet working. While such a system it may work in an urban environment I think it’s really unrealistic to expect a Masai villager to produce kitchen waste – even if it produces 400 times more gas, any Masai village reveals that all vegetable matter is fed to the cows or goats.What these guys have in enormous excess of, is cow and goat poop! The system I saw was also very expensive using concrete and cement, and costly plastic containers. Dominic Wanjihia is installing flexible bag digesters that are mobile …. he promises to send us material for an afrigadget post soon.

  3. Hi Paula, thx – also saw one of those flexible bag digesters in one of your sisters articles in the organic farming magazine. Makes more sense to me as it provides a better mixture than the drum which would have to be turned upside down from time to time to enable a better gas production and prevent floating sludge. I think the important part will be availability in rural areas and costs, but the example above is just a single attempt with no serious (read: scientific) approach for optimization. Obviously, with an emphasis on “is this possible?” (in Kenya, with given tools & untrained staff).

    @Thomas: vielen Dank! Good blog, bookmarked it and joined your FB group. If you have some pics of your installations in .ke & .tz, we can publish them here on AfriGadget. I like the ARTI design and the reasoning behind it (except of course for the food waste issue, but you are free to tell us how this worked with the Maasai).

    How do you teach the operators to know when the sludge is fully digested? Is there a concept / handling procedure for emptying the reactors? What happens to the digested sludge? Is there any market for the fertilizer?

  4. That is a very ingenious design, although I think you are right that all those valves and fittings would be pretty expensive. It will be neat to see their next version, as the proof of concept is quite impressive.

  5. I like your solution, to store the produced gas in a big plastic bag. This way you can build a digester with a relatively small container. Also the big plastic bag can be produced locally by a small workshop (sowing, welding with an iron).

    Your digester can be adapted to continuous operation, like Thomas’ reactor, by inserting new dung at the bottom and removing spent dung at the top. You could use the same measures like Thomas to maintain the pressure (long pipes IMHO). Also the digester would need to be divided into compartments, because the bacterial communities change during the course of the reaction (Thomas put a plastic table into his giant bucket).

    An interesting design for a continuous reactor is IMHO a horizontal pipe. I’ve seen photos of modern German digesters, and they seem to be horizontal pipes with: diameter 0.5m length 10m. The mixture (mainly maize, it is a total waste in Germany) is pumped in at one end, the spent mixture is taken out at the other end. For extracting the gas you could IMHO simply tilt the pipe slightly to one end, so that the gas accumulates on one end. With this design the bacterial communities can gradually change over the course of the reaction.

    However you have no business case for a continuous digester. You buy two buckets of dung when the gas production ceases. But maybe the Masai have.

    I would like to read more about your digester, and its daily use, because to create a viable product, all the small problems that crop up have to be used. For example what do you do with the spent dung?

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