Biogas self sufficiency

Hi there Permies, my family of 3 is setting up a homestead from scratch in the Northern Free State, South Africa in Jan 2016. We have saved our pennies and are crossing the line!
Originally, I was planning a solar system to power lights, fridge/freezer, washing macine, appliances etc, a solar geyser and a biogas for cooking. Now I am wondering about the feasibility of biogas for everything. So I have a number of questions:
1) How big a unit will I need?
2) As we will keep all our animals contained at night to keep them safe, I will have all their nightsoil and of course our toilet waste at our disposal. That will be 2 cows, 8 sheep, 3 horses and roughly 500 chickens. With that amount of material, will i be able to produce enough gas?
3)What would be the best way to convert the gas to electricity and store it?
4)Would it be better to buy gas appliances where possible to reduce the electricity load?
5)Do I need to change gas injectors on LPG appliances, if so which ones to use and where to get them? I don't think they will be easy to find in SA, but I could get them while still in Europe.
6)Safety features- gas overflow valve, gas detector indoors?
7)Is there a cheap way to build one, preferably avoiding concrete?

Greetings! I have little real life experience with biogas but I have read, watched, listened and observed a ton. Basically 3 cows will give you (very roughy, and I know the veterans and the purists are screaming at their screens now but nonetheless) roughly the equivalent of a 50 lit propane bottle a month in biogas. So with all your animals (again, very rough but good enough for ball parking), excluding the sheep because they have very low volatile solids and their manure is less conducive to turning into gas than the other animals you mentioned, you are looking at having roughly the equivalent of 3-3.5, 50 lit bottles worth of propane in biogas per month.

Now this is not to suggest that you pressurize the gas into bottles (you shouldn't) or that you'll always get the same amount (you won't) or to say that methane has as much energy as propane (it doesn't) but its a nice way I've found of visualizing use. What could you do with 3 to 3.5 50 lit bottles of propane? That is roughly what you'll be able to do with the methane you produce from the animals you listed.

Your best bet with energy, like with everything in nature, is to have diversity. It gets the economic law of 80/20 working for you as opposed to against you. Taking methane, turning methane into electricity via an internal combustion engine, and then trying to store that energy for reuse later, is going to be a very low efficiency use of a gas that is very useful elsewhere and not that easy to get. So yeah, run what you can off your propane directly, this usually means a range or and other heating appliances like a water heater. Maybe a fridge.

Setting up a small solar array for lights and electronics, maybe with a small wind turbine to supplement it. Setting up a gasifier connected to a generator to gig you extra power to run your pumps once a day and to top off your battery bank on cloudy days, will cost not that much and will give you far more energy security than trying to do it all, all one way.

Hi Justus and a big Thank You for your reply! I was beginning to feel rather lonely on this page.....
Your information on gas production quantities and how to use it was most helpful.
Any advice on the size of the unit? I was thinking of one of the 1000L tanks used for storing water/ oil etc
There is a silence in Africa about gas conversion kits, I can't find anyone who sells them. Everyone wants to sell me imported Chinese appliances and generators! Yet I see photos of converted propane ranges being used in Uganda etc. I don't want to pay a premium price for new appliances, I'd rather re-use existing ones.
I would really like to run my fridge/freezer directly off gas as well as my range and have a back up gas hot water system.
I have so many alternative energy ideas I want to use. I want a rocket mass heater for winter and am tinkering with the idea of a rocket mass water heater as a backup for solar & biogas.
I like the idea of biogas for cooking as I want something that will not heat up the house in summer when it's hot enough already.
I love the idea of a gasifer to power a generator to top up my batteries if the weather doesn't play along but don't think I have the technical know how to build one.
I definitely want to spend as little as possible so solar electricity is more for low usage appliances in order to keep the system small.
I have researched biogas builds and get the general idea but there's not much there on what you need to get it to work on standard gas appliances, they talk about scrubbing and removing moisture etc and I have seen the gas supply pipe descending to drain off moisture before entering the house but that's where it ends.
I can't understand how a "third world country" like Uganda can have a standard gas stove running off methane and there's no information on how to do it. I have searched "changing LPG to natural gas" and "LPG to methane" but come up empty. Never mind finding out where to buy stuff and if it's this difficult in europe how much more so in africa?
I know I'm ranting here but I'm looking at the big picture and what we envision to do in South africa-after setting up our homestead and creating an income stream we want to step out into the community and offer alternatives to people who have no options. It would be so nice to have a "school" where we can show communities how to build cheaply and create energy from what is traditionally a logistical nightmare in informal settlements in africa. I am so excited but equally frustrated!
Thanks for listening!

Sarah, glad my comments could be useful. If you use a batch design, you will want to build a digester out of two 1m3 plastic tanks which you mentioned. Connect this to an inner tube reservoir system which you can use to create pressure in your system. Two tanks because when you add material, the digestion process slows down for a few days, then picks up again. I like the batch design because it allows for more fully decomposition of material, but it has an ebb and flow in gas production. My system will use four of these 1m3 plastic containers one of which I will refill every 15 days or so. This will give each container about 60 days to fully decompose the feed stock.

Your question about converting LPG or natural gas appliances to biogas is understandable. One of the reasons why so little information is available on the web is because, frankly, there is very little to convert. Basically your conversion comes down to pressure, flow rate, air mixture and cleanliness of gas. Of these, the quality of the gas is the most important, that's why you find more discussion about it. Well cleaned biogas means you will have fewer problems in using it in your appliances.

When you talk about appliances, there are two kinds. Smart and dumb appliances. In the Dumb category is cooking ranges, lights and engines. They are basically on or off, they are pressure sensitive but only marginally. Smart appliances are anything that either electronically or physically regulate the flow and or pressure of gas automatically. These will be much harder to convert to biogas than the dumb appliances. Fridges, water heaters etc. The main thing is to have a gas reservoir (like an inner tube reservoir) which you can pressurize at a constant level to get constant pressure. Once the problem of gas purity is solved, and the issue of pressure is addressed the next issue is air mixture rate configuration. Most gas stoves or LPG appliances have an air mix adjuster near the area in the appliance where the burning happens. Once you find the air mix valve (sometimes nothing more than a bolt or screw that is tightened or loosened) you simply adjust that till the flame is blue, not yellow. The last issue, flow rate, is the hardest to address. Biogas has less btu's than either LPG or natural gas. So even with the same pressure and the correct air mix and the good clean gas, you still may have trouble with burn rates. In dumb appliances, you just turn up the heat. No problem. In smart appliances, which have complicated adjustments of their own in their small mechanical or electronic brains, this can be much trickier.

LPG has about 2500 btu's worth of potential heat per cubic foot
Natural Gas has about 1000 btu's worth of potential heat per cubic foot.
BioGas has about 660 btus worth of potential heat per cubic foot.

So, for instance, converting a natural gas stove to run on LPG is easy. You get a 2.5 limiter to limit the flow to the device and your good to go. Getting an LPG stove to run on natural gas, however, is harder. You have to INCREASE the flow by 2,5 times. When you start talking about biogas, you start seeing even greater differences in flow rate demands. In dumb appliances this is easy, just turn up the heat, open the gas valve all the way. In smart appliances this usually means either increasing pressure to the system (could be dangerous) or more often drilling the holes in the burners to be larger. Either of these options can be dangerous and takes some trial and error.

I have personally decided to run my range off of BioGas and keep a conversion kit on hand for an engine, but to not bother with any smart appliances. It seems to me that using solar, wind and back up generator connected to a gasifier (or biogas!) can easily produce enough electrical energy for lights, electronics, water pumps, refrigerators and other electric motors. Using solar thermal, rocket mass heater, the waste heat from the backup generator and if need a "dumb" on/off biogas water heater to supply all the domestic hot water and heating needs seems to me you can get around using electricity to heat anything.

Have you read "A Chinese BioGas Manual"? It has all this info and more. Their designs are usually about 10 times bigger than needed due to their cold weather applications, so just keep that in mind. But a lot of good info. Including conversion kits or how to make your own appliances from scratch, like bamboo and clay!! I'm attaching it to this post for you.

If you are willing to pay $25 here is a book that everyone seems to think is the best on the market. I recently started a thread to see if anyone could give me better book recommendations but so far no one is biting. Here is the link to the book sales page.

http://completebiogas.com/workshops.html]

Hope this helps answer your questions. Energy is just another part of the ecosystem we need to live and to thrive in comfort. Nature almost never solves any problem in a single way, but uses interconnected systems to obtain synergy and solve problems with synergy, not just raw energy. Rocket mass heater, Solar thermal, Biogas, solar electric, wind, battery bank and generator with a gasifier (and or biogas) fuel source, and suddenly you have an abundance of energy, locally available and inexpensive to maintain! The beauty of synergy!

PS.

Almost forgot. Journey To Forever has a section on their website about biogas

http://journeytoforever.org/biofuel_biogas.html

One of my favorite collections, There is everything there.

Hi Justus,
In 2 days you have provided me with more resources that I've found in a year! Thank You so much.
I've seen and researched the chinese/nepalese pit but am really trying to stay away from concrete/cement as:
1) raw materials are costly
2)although communities would have enough labour to build these, our family of 3 would take an awful long time!
3) as most settlements we're talking about are squatters i think it would be better to have a moveable system
So your 4 tank idea is very appealing for communities. I like the idea that a tank can be designated for filling and the others sealed off in rotation-less complicated when used by lots of people. For myself i think I'd rather go for a daily fed one in order to deal with the animal waste produced on a daily basis.
But the amount of information in the chinese manual about usage and jets etc is amazing! That website is a gem too-for all aspects of self sufficiency. I liked the articles on the sunken bio digester with pipes coiled around it generating hot water and that converter for cars-oh how I wish I was an engineer!
The book sounds great and I would love it in pdf but the postage to the UK is a killer.
Based on your (very illustrative) description of appliances I can now make an informed decision about gas appliances-thank you!
I am intrigued with the idea of a gasifer-I can't quite grasp the concept-anything mathematical or scientific and my eyes glaze over and all the builds are sooo technical. Is a gasifer like a generator? Will it turn gas into electricity to charge batteries? Do i need a different converter than my solar converter? What's the conversion kit for a generator and is the generator diesel or petrol? Sorry for the bombardment but when I get a willing contributor I tend to hang on!
kind regards,
Sarah

Sarah, Glad I could be of service!

Gasifiers are something I do know a lot about, and am rather passionate about as well. But like all applicable technology, it needs to be fitted appropriately to the situation. The basic prerequisites of gasifier tech are 1). an abundant supply of cheap carbon based fuel. 2). a need for heat. The second is not 100% necessary but if you are going to feel all the benefits of gasification, using the waste heat created by the process (when connected to a generator) is the way to go.

But, what is gasification? Basicaly it is a way of "cold burning" carbon based materials (wood, nut shells, sawdust, etc) to break the carbon bonds and release the hydrogen, nitrogen and methane contained in the materials. "Hot" burning is what you do in a normal fire, combining oxygen and enough heat with materials to create light lots of heat and smoke. "Cold" burning, otherwise known as pyralosis, as I mentioned above, breaks the carbon bonds but does not burn the gasses emitted by limiting the amount of heat and oxygen in the reaction. This process is called, as I mentioned, pyralosis, or gasification! The device used to accommodate this process is called a gasifier. Imagine the gasifier a a kind of stove, that you put carbon based material into, but instead of getting heat and light from it, you get a flow of gasses out of it. This gas is called producer gas or syngas. The gasifier unit also has a filtration system, to filter the ash and tars out of the gas, and a cooling system, so the gas is at an appropriate temperature for use on the exit.

Once you have gas coming out of the outlet valve of your gasifier, there are a number of things you can do with it. One of those things is to use it as your fuel source for an internal combustion engine. During WWII over a million vehicles were powered all over Europe using gasifier units that were built onto the vehicle. A person could load up the gasifier with wood chips, light the hearth, get the gas producing and take off down the road, no petrol needed. This is old tech, but modern advances has made it far more accessible. Personally I do not see the Automotive function as that useful, maybe in extreme crisis or to run farm tractors or trucks, but not for "normal" driving. However there are plenty of people who would argue with me on that one!

The area where gasifiers, in my opinion, have a real place, are in back up generators for off grid applications. The gasifier converts wood, or any other carbon based fuel source, into a vapor gas that can be burned directly in an engine, driving a generator, which in turn charges a battery bank. The "waste" heat from the engine could then be used to heat something, for instance a biogas digester.

So to get electricity from firewood, or other organic material, we use a gasifier to convert the wood to syngas, an engine hooked to a generator to convert the syngas into motion and heat and electricity. The electricity is stored and used by the homestead in lights or pumps or whatever, and the heat is used to heat something.

So that is that! If you want to learn more about gasifiers, here are some links…

http://www.driveonwood.com
http://www.allpowerlabs.com
http://www.woodgasifierplans.com


Ben Petersens Plans are very very thorough and very very easy to follow. Well worth the money.

http://www.woodgasifierplans.com/order/

HI Justus,
You're gonna wish you never bothered responding to my post....

I can understand why gasifers are appealing to people in areas with a reasonable supply of fuel(wood) but I don't think they'll help me much-I find it a bit like the children's song-I know an old lady who swallowed a fly, if you don't know I'll ellaborate- She swallowed a cat to catch the bird, she swallowed the bird to catch the spider, she swallowed the spider to catch the fly....
So I understand the appeal of using a "free" sustainable resource but i think this system is beyond the reach of most average homesteaders who have to weigh up the cost of building a gasifer & buying an engine on top of the cost of a generator and solar system. It sounds a bit pricey for a back up system-for me anyway and definitely for the disadvantaged.
Could you convert a petrol/diesel generator to run on biogas, is there a conversion kit for that? That sounds do-able financially and provides more options for me. I could produce biofuel from waste oils(I know someone in SA who does and runs his car on it) to power the generator when I don't have enough solar or biogas. Being a technoidiot I want a system with as few moving parts as possible-not necessarily as few fuel sources as possible, just the connections between the parts. So I would like to run lights, fridge & wash machine on solar, topping up batteries on wash days and cloud/rainy days with the generator which can run on biogas, biofuel or ethylene. I understand that it would be either biofuel or ethylene and not both. I think....
Do you think that it would be possible to build a communal kitchen/ablution centre where the toilet waste runs straight into the biogas system you described to provide cooking gas and that the gas could also power a generator for a few electric lights in the kitchen and ablutions? We're talking basics here, supplying loo facilities and cooking gas to people who haven't got access to electricity and I hesitate to use gas lamps just because unintentional communities are more informal with no real "command" structure and there are children involved.

My brain is in overdrive and it doesn't help when I battle to grasp the concepts in all the different power systems. I can work out my Ah usage and have studied the solar array system and kinda know all the parts I need but doing this has made me realise how prohibitively expensive solar is-totally beyond the reach of the poor man who probably needs a free source of power the most and because of his poverty is the most inefficient energy user as he is reliant on readily available fossil fuels and not so readily available carbon fuels! It's a case of him spending more in little amounts because he can't afford the one off outlay. What a crazy, unequal world we live in, and I'm on my high horse again aren't I?!

Once again, Thank You for your time and enthusiasm. Sometimes one just needs re-assurance that one's ideas are not totally crazy after all!

Sarah, The expense of Solar is the prime reason I started seriously investigating gasification tech. Again, the tech has to be appropriate for the application, and in your case it may not be. In my case, 7 months out of the year we have very limited sunlight and not very strong light at that. Our farm needs 12 kw per day to operate. Because of where we live, Siberia, I'd have to install 3 times the amount of solar panels to get us through the winter than I would have to install to get us through the summer. I'd need to install about 12 generic 200 watt panels to produce our energy needs 5-6 months out of the year. This is already a big expense. But to get the remaining 6-7 months covered, I'd need an additional 28 panels!!! And that's just crazy! I could save some panels by adding a larger (much larger) battery bank but that only helps marginally. PLus, with ANY solar system you need to have at least 3-4 days worth of battery backup power in case of stormy weather or serious overcast, and that battery bank isn't cheap either. The way most people save money on the battery bank is to get a generator for backup.

So any solar system will consist of PV panels, battery charger, battery bank, inverter and usually a backup generator. So I said to myself, why waste the money on 3-5 days battery bank plus a generator? why spend 3-4 times the money on PV panels to run through the winter?

Why not put up enough solar panels to get me through 4-5 months in the summer, say about 10 panels (instead of 40!!), put in 1 days worth of batteries instead of 3, and get that back up generator I would need anyway, and only add one extra item to the system, the gasifier to supply free fuel to the generator. I buy the plans, pay someone to build it for me, gasifier cost about $2,200 all said and done. What I saved on batteries and panels is way more than this little addition. Maybe in the future I'll add a simple chinese made turbine for wind (you can get pretty good ones not too expensive on Alibaba). And thats how I came to embrace gasifiers.

IT may not work for everyone. It may not work for you. You may have a situation where 10, 200w PV panels will be enough for you 12 months out of the year. But for me, I had to use solar energy in the hard form, that is wood.

Any way, I hope that maybe cleared up my thinking process for you.

Back to biogas.

Using human waste in a biogas reactor, with all due respect to journey to forever, is probably a bad idea. First off, the human body is very very efficient compared to cows or horses or even pigs, and there is not that much potential gas in our poo. Very little potential gas, but a lot of potential pathogens. Biogas reactors use anaerobic digestion, that means without air, which means they don't produce heat, which means the pathogens don't get fried, like they would in a compost pile. So better to compost human waste in a good, very hot compost pile.

Producing electricity out of biogas, on a regular basis, will demand a lot of biogas. There are about 24,370 btus in a cubic meter of bio gas. For comparison, my family uses about 17,500 btus worth of heat energy from gas per day to do all of our cooking right now. Lets saw=y you have a 2kw generator. That generator will use 35,400 btus of energy per hour, or almost 1.5 cubic meters of gas, per hour. If I wanted to run my farm on biogas, i'd need to produce at least 12 kw of energy from my generator (put it in a battery bank and use it when the generator is not running). I'd need 9 cubic meters of gas per day, or about 10 of those 1000 liter tubs to keep the generator running 6 hours a day. Actually more because there are losses involved with converting to the battery bank and converting back to AC but I think you see the point. As a back up fuel, to use occasionally, maybe. But as a full time power solution, I don't think you want that big of a system.

Ofcourse I have no idea what your power needs are. Maybe your power needs are far less than mine. I know I could gt down to about 9kw per day If i tried. But I like having the extra power. Just my thoughts on the Issue!

Hi Justus,
Once again,thanks for replying. I sure did not mean to belittle your power supply choices- I agree that in Siberia you do not have much need for solar panels and a gasifer makes more sense-horses for courses. With a steady supply of wood I'm pretty sure it will work out much cheaper than solar too, lucky you! I was just highlighting why it wouldn't work in either of my scenarios, more's the pity, as I am not looking forward to the installation costs and the fact that I will need to replace batteries and panels at some point so not really self sustainable. I definitely can't afford to go around putting up solar systems in the communities I wish to get involved in! That's why I'm exploring alternatives. But unfortunately South Africa is tree poor for the most part and definitely in areas of squatters as they have already used all the available wood as a fuel source.
Thank you for the clarification on using just human poo, looks like I will have to rethink the whole communal kitchen supply thing.
I noticed in your reply about using a gasifer that you connected your gasifer directly to your generator- does this mean you can run the generator on gas? Is the engine you refer to part of the gasifer or is the generator the engine? Like I said earlier, I'm a technoidiot! Most generators in South Africa are mobile, 2-5kw petrol driven engines which have plug points directly on them to which I would need to attach a battery charger to charge the batteries.

- just a link to a ted talk

https://permies.com/t/51468/biogas/Talk-home-small-scale-Digesters#415146

For the Crafts Big AL

Hello Allen!
Thank you for the link, I am aware of the solarcities site and the 4 "designs" they have-great, do-able stuff! I know I definitely want to "make this at home", it's just getting it to work in my off-grid systems. But the thing this video did highlight was the facebook page. I think I will have to join the facebook page to ask a load of questions about appliances and usage etc. I did send an email direct to solarcities but didn't here back from them-I guess facebook is their preffered means of contact!
I am so inspired by those ladies to go out into my community and spread the word around. There is such a need for cheap, safe, renewable fuel and I can't understand why there isn't more action by governments to make it happen.
Thank you for your interest in my post.
sarah

Hey, if you don't want to totally build it from scratch and via tinkering, HomeBiogas is a good solution. Comes in a do it yourself ikea-like box and manual.
Check it out here: https://www.indiegogo.com/projects/homebiogas-create-your-own-energy#/
Promo price is only for the next few weeks...

I'm about halfway through reading this wildly interesting dialogue. What useful information, thanks for sharing. I'm interested in incorporating biogas into my homestead, but have no idea where to start. We have two cows, two horses, four goats and a bunch of rabbits and poultry. I'd be interested in running one or two stove burners at most. I want to keep it as simple as possible. I heard one design requires three-55 gallon plastic drums, a smaller one set inside the other. Is that doable for what I'd like to achieve? Could you recommend a plan for building one? Much appreciated.

I'm just finishing writing a program for an automated small to medium scale biogas
systems. It is modeled after Comercial biogas systems. The digester is kept warm
with its own gas and is agitated periodically to prevent caking. Gas is passed filtered and dried and compressed for storage. Yes compressed....don't panic. The collection system is always under positive pressure preventing oxygen from entering the collection system. Failure to produce positive pressure shuts the system down. End goal is to be able to use normal natural gas appliances with the standard regulators and controls.
The key to biogas is consistent temperature. Then feedstock.
I'm hoping to have my first trial system up running this (2016) summer. My findings will be documented on my youtube channel (good or bad) user name, dadigitechman.

I am very interested in building a small homescale system or purchasing a ready built system. I am in the USA. Is there any new ideas out here on permies that haven't been covered?
I am in the Midwest so we do have winters to deal with. But we do have barns and greenhouses that we could maintain some constant temperatures.
Also I did wonder about the idea of the system heating itself as in the previous post.
Can anyone recommend a YouTube channel that shows an actual build?
We are a very small farm but propane and electricity are very expensive so replacing both of those is our actual goal.
The farm has two tiny home that will need heat and we have no skills to build our own gasifiers. We have considered hiring a local welder and machine shop. We easily spend over $1000 per year on propane and about $2400 in electricity. Neither will be getting cheaper anytime soon as we are in a small area and our energy is supplied by a very expensive members only co-op.

Hi candy, I see the link above is not working. This:      has a practical how to and links to the biogas facebook page.

Thank you so much Sarah! The hours of sleep and the huge amount of articles i have downloaded and read in addition to this information has given me the knowledge and confidence to get started on this build now.We are gathering the supplies starting today.
I will post pictures if our progress.
I had already planned on building a small cellar under our tiny house. Now it will house our own "baby dragon".
I can't thank you enough.

I am just starting down this path myself.

The solar cities design is good, but more importantly can be adapted to ANY container, including standard septic tanks.  The ibc is the most cost effective choice in many areas, but not always.

Don't forget to feed the system greens! Poo is already partially digested, adding raw green material makes a lot more gas.  Use the effluent to grow more food and fuel.

I look forward to your build biography Candy! I still have not had the opportunity to experiment so I look forward to yours.

Thank you R Scott for the tip on greens, a good way to increase gas yield.

Since starting this thread I have struggled with the problem of running an oven from biogas-apparently you cant due to off-gasses infusing food with toxins. That's what I was told by a biogas appliance seller. However,I was wondering about converting a solid mass stove to run off gas as opposed to wood or coal-put a gas burner in the fuel box. I know it means the gas would be on the whole time and that gas supply may be an issue in a small system, but for those of us who are "timber poor", could it work? It may be a solution to hot water/heating too if you get a range with a water heating facility or just copper pipe wrapped around/behind the mass and thermo siphoning into a storage tank. Any thoughts?

I am surprised about the commments regarding off gases poisoning the food as I am aware that cooking is a major use of such gases in places like Bangladesh .
I am not quite clear what you are intending to do and what type of accommodation you are intending to live in.
I remember Geoff Lawton showing off a system but that it was pretty heavy on concrete .
Also I wonder about putting all your eggs in the same basket .
How about using an older method ? For cooking burning the cow pats there are quite cheap plans for rocket heaters and cookers on the Internet . I am sure that could work . It's relatively risk free and was used in tree poor zones like the Stepps and the Mid West for many years . Since you are keeping chickens as well these will see to your fertilizer needs

It's only cooking in the oven that exposes food apparently. Food cooked in a pot over a flame (hob/stove top) is perfectly safe. I was hoping to avoid mass heating and cooking as it gets very hot here which is why I was interested in biogas for oven cooking.

I think I mentioned diversifying fuel/energy options above- thanks for the reminder about dry cow pats . I suppose I am working on 2 fronts, what I can do for my own homestead and what can be easily and cheaply utilised in poor communities.
The original issue was how far could one go with biogas as an energy source and a few people came up with some great info and useful applications and modifications. My last post referred specifically to OVEN cooking. If biogas is unsuitable for oven cooking, the I have to consider mass again and address the heat problems. I suppose a solution would be an outdoor kitchen for summer use. A communal kitchen shouldn't have too many issues as the space is not living space so discomfort would be limited to a few hours a day. I just thought that a gas fuelled mass oven would be more practical in a communal arrangement as it could end up being more efficient as it's "on" all the time so doesn't cool and need to reheat every time someone wants to use it. There is also no need to arrange a "feeding" schedule (wood, cow pats or charcoal) or the risk of the mass cooling because it wasn't maintained properly. Cast iron ranges are also quite freely available here-old aga s and other ranges.

My question was more a technical one. COULD a communal system feasibly produce enough biogas to run a gas mass oven? Batch fed?

Would a bread or pizza type oven be something to consider?
Using the biogas to heat it up to temperature and let the mass of the oven do the cooking and not the flame of the gas.

I think the off gases (if that is true) would still infiltrate the food, which is why I was thinking of a "mass" style which has a separate fire box and vents out a chimney.

I think it is more unpleasant than dangerous.  Biogas can have sulfur compounds that would make good taste like rotten eggs.  You would probably want to filter them out for your own comfort while cooking, not for safety.

You can build an isolated oven chamber like a wood or kerosene stove, it isn't hard although slightly less efficient.

This thread has been a fantastic read, thanks to everyone who contributed!  

I am a complete newbie to biogas, so please forgive me if I ask really ignorant questions.  A lot of this thread was quite over my head.

In regards to an oven - an oven can be rigged over a stove top burner for small scale baking.  The gas heats the baking space from the outside, so I should think the absorption of fumes would be minimal.  My set up is very simple, a big pot, with something on the bottom to create air space, and a baking dish that sits inside the larger pot.  Then a tight lid.  It works.  I think if you wanted larger scale it could be done, just make sure its heated from the outside.

I need to check out all these awesome links... But here is my question:  what is the risk of explosion with a home made digester?  I am notoriously bad at building things.  I don't want to blow up my house.

Another question... In the video they talk about blending up the waste into a baby food slurry.  Is this really necessary?  What about the manure?  Are you supposed to blend that up too? Does the system ever get to a point where it gunks up with solids?