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Simple designs for DIY home biogas generation? Answered

A friend of mine is working in Sri Lanka at the moment on a biogas project.  Part of the remit is to improve the design of a village-scale rural digester of the "buried concrete tank with a lid" design (as pictured below), by tweaking the operation in terms of feedstock, water:feedstock ratio, temperature and so on.

The other task is "to design an efficient and neat anaerobic digester to market to middle class families."  I say "DIY" because making these of simple and widely-available materials is clearly preferable to involving lots of expensive parts.

Collectively we seem to have a good deal of expertise on similar subjects- biodiesel, algae growing and so on, so I wondered if anyone knows enough about biogas to be able to comment on whether a simple design (possibly something like the Appleseed biodiesel processor) could be practical.  Any lateral thinking on how to overcome the limitations of small batch processing would also be great.



Best Answer 7 years ago

Here is a great, simple, and cheap place to start:
I'm thinking about building one myself.


Answer 7 years ago

Cool- thanks for the link, that blog is right up my street and it might well provide some useful information for the project. Any ideas if you can convert gasoline generators to run on methane? I know it's been done for propane but I don't know how volatility, energy content etc. compare between the two.


Answer 7 years ago

Thanks for the "Best". As for the conversion question, I would suggest making a new "Answers" posting...

Hi I am also building a homemade biogas generator for my industrial project. I have chosen to use a 50L tank to be my digester. But I don't know how much methane will yield out of it. And I also don't know how much manure I should mix with water for a 50 liter biogas generator. Please also advice on the storage and is it necessary to have a fire a arrester in such a system? The system is below, please do advise and share your opinions!


I understand that in china the human effluent from 3 people produces enough gas to light and heat their single room office.

This is what guide we also promote in our biogas company, we make sure that we implement proper guidelines and operations.

Also, answering some of the questions I've seen posted:
1. There is a cheap way to eliminate water and sulfur hydroxide, water can be removed by cooling, such as in an underground pipe, to condense the moisture and H2S can be removed by injecting small quantities of air in the digester to avoid corrosion of engine components (I read this in Bond (2011) "History and future of domestic biogas plants in the developing world").

2. There are already generators that run on biogas, apart from removing the moisture and H2S, biogas must have an appropriate pressure and content on methane. I saw a fabricant called Green Power at http://www.greenpower.cn/, they say biogas must have between 5-8kpa and not less than 55% CH4

3. The combination of manure and crop residues is good to maximize biogas production. The ratio carbon/nitrogen should be approx. 25:1. The optimum quantities of each depend on the feedstock. Food waste alone can be a bit dangerous depending on composition because it degrades very easily and volatile fatty acids can accumulate in the digester, this drops the pH and can cause that your digester doesn't produce any more gas at all. And finally, woody stuff is better for composting than for anaerobic digestion since it contains lignin and this one doesn't degrade easily.

HI everybody! I am doing my MSc thesis in anaerobic digestion at a a small scale and here are some links that I found very interesting. They touch everything from the basics of the anaerobic digestion process to build your own biogas plant. Hope it is useful. Cheers!

http://www.eawag.ch/forschung/sandec/gruppen/swm/projects/anerobic_digestion/index_EN --> Water research institute from Switzerland. Research in small AD in India and Africa

http://www.arti-india.org/--> Floating dome small digesters with food waste in India

http://explow.com/biogas_plant--> Huge collection of information on small biogas plants

http://bio-gas-plant.blogspot.com.au/2012/04/step-by-step-guide-to-constructing.html--> How to build your own floating dome digester

http://bio-gas-plant.blogspot.fr/2012/04/construction-of-biogas-digester_23.html--> Build your own floating dome biogas digester

http://puxinbiogas.en.gongchang.com/product/7329832--> "Sells everything related to AD at a small scale: portable solar digesters, biogas appliances (lamps, rice cookers, stoves...). The company is chinese but has branches in Canada, Kenya, Chile and South Africa. The best websites are those of Chile and South Africa but the Canadian one gives an estimation of the prices and some interesting links to other biogas information websites.

http://www.sswm.info/ --> Website Water management and sanitation Tools for the users to educate themselves and start actions for better water management and sanitation at the local level. It also explain the basics of anaerobic digestion

Your input feed and what you want to use the outputs for (outputs will be both Biogas and undigested solids) will determine what type of digester design you need. A digester or processing unit that will suit Sri Lankan farmers with cow dung and rice husk won't be suitable for middle-class families for example.

Cow dung, for example, can be digested in a simple one-stage digester, similar to the one you have shown. Rice husk can't be easily digested by bacteria, and is more effectively processed by gasification (see www.huskpowersystems.com).

One of the things you haven't mentioned here would be how the undigested outputs are extracted and then used. The solids after digestion are still rich in nutrients and can be used as fertilizers directly applied, or input to a worming-compost setup.

You also mention converting a gasoline generator to run on methane, I presume to produce electricity. Piping methane generated from a biogas process into generators introduces several complications such as scrubbing the biogas for water and sulfur content (both of which will damage and corrode your engine), and pressurizing the gas input. Driving a diesel engine with methane gas is actually not so complicated once you have installed expensive systems to do your cleaning.

One of the problems that I have found with Biogas is that there is water in it. There would have to be some form of water capture so that the gas will actually combust to power a generator. Another issue would be the composition of that Biogas, not all gas has the same chemical amounts. For example, methane can be between 50 and 80 percent of the total volume of gas which creates a somewhat large explosion hazard. The holding tank would have to have some form of AAV to allow expulsion of the gas if pressures exceeded a certain amount dependent on the actual chemical composition of the gas.
Thanks for the diagram. Have you thought of any enzyme additive to speed up the process of decomposition?

Could you define the specific problems that need to be addressed, or is this an "any ideas for improvement, there must be some" question?


More of the second; whoever designed the one my friend is working on now seems to be a little incompetent because of some basic errors (assuming that solid feedstocks in water will flow up pipes against gravity). It sounds like tweaking acidity and temperature will have to take a back seat to actually making the device usable to operate, hence asking for simple effective designs.

You've probably got good advice on this by now. L

A key question is what volume of feedstock do you NEED for a "middle class family" and what IS the feedstock. The REPP organisation offers some very helpful information and discussion groups for some pointers.

That link appears to be a more western-centric site. Bearing in mind these designs are still for rural Sri Lankan families I think the feedstock is "whatever is available" (specifically, mostly cow dung and rice straw) and how much energy/feedstock is needed is "however much we can get".

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