Humanure vs. Biogas

Anybody have experience with both? Is there a difference in daily upkeep?

In planning my off grid house, the idea of not needing propane is VERY appealing.  But I want as easy as possible, I would rather haul 20 lb bottles than deal with any more daily upkeep than buckets.  

From what I've read on biogas digesters, they require a minimum temperature of 25 C to work properly. So in any place that gets a proper winter (like Kansas) you either shut down for part of the year, or have a way to keep the reactor vessel heated. I've seen designs that use a part of the produced gas to supply a pilot flame which keeps everything bubbling.

I've looked into them and if you do biogas digesters I would suggest setting it at least halfway into the ground, not unlike a septic tank. It would assist in keeping things warm during the winter. biogas does need to be filtered before use in electric generators and such and I believe it is not compressible

nevermind -- found a link that contradicts me https://www.ijrer.org/ijrer/index.php/ijrer/article/download/3449/pdf

4 bar is not very high, however, equaling about 54 psi, which is roughly a third of a standard propane tank.

https://www.sciencedirect.com/science/article/pii/S0960148104003830

i believe it can be used in place of propane with minimal tinkering required, but since it's methane it has lower energy and is unable to be compressed as much. In practical terms, this means you'd need more storage area or containers, but wouldn't be much other than that.

Humanure could be used to fuel it, but while you do get liquid fertilizer off (not clear on the specifics) it wouldn't be the main goal. I believe the most efficient manures for biogas is human and pig, so in that way they are compatible.

The best way to figure out how a biogas system works, is to look at it like it is a ruminant's stomach, because that is exactly what it is. To get that "stomach" to produce biogas (for a cow or sheep it would be belches or farts) it has to be 103 degrees, just as a cow or sheep is 100 degrees in temperature (whereas humans are 98.6 degrees).

And just like a cow or sheeps stomach, it takes food to get it to cook and produce gas.

Now here is where manures work, but fall short. Because that food has already been gone through a natural biogas unit which is the ruminants four stomachs, that food does not have much biogas left. the cows or sheep have already belched and farted plenty of times before they poo. I visited a 1 megawatt biogas digester at a dairy farm and their 1200 cows could produce 250,000 watts by cow manure. They instead made their farm a municipal waste dump, and so now they can accept wasted food from grocery stores. Doing that (undigested food) they could produce 3/4 more watts, or 1 megawatt instead of only 250,000 watts.

In a home digester a person could do the same. Use animal and human manures for sure, but also put raw food into the system. One of the biggest producers of biogas is corn silage. The leachate that comes off that produces amazing amounts of biogas, but any undigested food is going to produce 3/4 more biogas then digested food.

At the biogas generating plant I was at, to get the biogas digester to heat up, and stay at 103 degrees, their locomotive engine that drove the generator, did not have a radiator. They pumped its coolant water through pex tubing through the digested material to bring the temperature up to 103 degrees. On a home biogas set up, a person could do this same thing. If they used a small propane powered generator that was liquid cooled. The coolant would be diverted to the digester to keep it at 103 degrees so it was always cooking, yet was small enough so it did not consume too much biogas. I happen to have a 3000 watt, gasoline/propane liquid cooled generator. It is not a lot of electricity, but 3000 watts 24/7 using free fuel would do something on my farm.

I did a lot of reading about it and learned a lot. My big concerns were my inability to keep it warm and the need to have a reasonably regular quantity of input material. With dead tree and cheap electricity competing with the biogas side of the equation, I decided that composting was the way to go.

Now if I ever got a decent green house, my opinion might shift! Then again, it might not. With a decent greenhouse, I could process much of the same waste using Black Soldier Flies, and that would feed my chickens and the frass would be good for my growies.

I haven't worked with biogas, but my humanure system requires less than 5 minutes a day to 'charge' the system.  Also, it's not like you can use that time to do much besides read.  When I get 2-3 full buckets, I add them to the pile.  Easy peasy.

This thread is helping to fire me up to get started.

My planned house is earthship/wofati-esque with an attached greenhouse (so biogas could be kept at the optimal temp) and the location cannot have a traditional septic even if I wanted to (rocky and too close to water).  I have used a commercial compost toilet and bucket system in the past (bucket system wins hands down on price and ease of use).  Because of the location, I will probably have to haul buckets a few hundred feet up and over the ridge to get to a good compost site, which makes the bucket system a lot less convenient.  But I would have to deal with the biogas effluent, too, and a bucket of wet sawdust is a lot less messy than a bucket of pure liquid.  I know what liquid manure from a CAFO smells like--if a biogas system smells anything like that, no thank you.  But free cooking gas, hey...

I will have a rocket cookstove and RMH for supplemental heat in the greenhouse, but cooking with wood is complicated--fine if you want to cook a while and need heat anyway but a PITA just for a cup of coffee or it is already 90 degrees outside.

Jay Angler wrote:I did a lot of reading about it and learned a lot. My big concerns were my inability to keep it warm and the need to have a reasonably regular quantity of input material. With dead tree and cheap electricity competing with the biogas side of the equation, I decided that composting was the way to go.

Now if I ever got a decent green house, my opinion might shift! Then again, it might not. With a decent greenhouse, I could process much of the same waste using Black Soldier Flies, and that would feed my chickens and the frass would be good for my growies.

I was like that as well. I learned a lot, and never discounted Biogas out as a method, but it would have been a lot of time and expense to build, and it was not 100% proven. When I was at that biogas dairy farm facility, one issue they had was the quality of the gas. It would cause the engine to shut down. I was never sure how much biogas I would produce, or of what quality. Call me a scared...but what happens if you built everything, and did not work...then what? That is a huge fail.

Progress requires failure.  I don't mind failures that aren't too costly in time or resources, but yeah it would suck to invest all the time and precious greenhouse space for something that doesn't work.

I am also worried about feeding the beast, besides the poo it would take all our veggie scraps and prunings. No chicken or worm food!

Can anyone speak to the efficacy of building a biogas generator such that it is heated like a greenhouse? But a standalone unit, maybe attached to the south side of a home (vs. within a greenhouse)?

R Scott wrote:Anybody have experience with both? Is there a difference in daily upkeep?

In planning my off grid house, the idea of not needing propane is VERY appealing.  But I want as easy as possible, I would rather haul 20 lb bottles than deal with any more daily upkeep than buckets.  

I have no experience with biogas. I don't find making compost to be a burdensome endeavor.

Joe Jenkins

Will Goodfoot wrote:Can anyone speak to the efficacy of building a biogas generator such that it is heated like a greenhouse? But a standalone unit, maybe attached to the south side of a home (vs. within a greenhouse)?

I cannot, but I would think that using compost as a jean Pain Style of heating would be more beneficial than biogas because there is one less element of conversion. In that way when a person was done, not only would their compost have heated their greenhouse, it would be available at some point to be used as a growing medium too.

Those are just loose thoughts though!

At our school we've been trying biogas now and again but only for cow manure. We had a plan to heat it in a simple solar greenhouse. Somehow it hasn't come together yet, but I've been campaigning to start with only cow manure and food waste for the first couple of years.

1) In the low-tech biogas setups I've seen in rural India and Nepal, which always use cow manure, there is a certain amount of manual mixing need.You add (warm) water and stir the manure with a stick to get the right soupy consistency before opening the valve to let it down into the digester tank. With cow manure (especially in the subcontinent where cow dung is not considered shit) this is an acceptable minor daily chore. With human excreta, maybe not.

2) I've heard that human excreta doesn't produce as much biogas as ruminant manure or veggie waste, even for the same volumes or weights of input. I forget the chemistry, but I read a good reason for that. Umm, maybe something about how the biogas is a hydrocarbon so it needs plenty of carbon in the input, which ruminant manure and veggie scraps have? I forget.

3) A friend of mine visited several biogas installations in Nepal, and said the only one that included human excreta was also the only one that smelled bad. Could be coincidence. Or not.

4) The slurry that comes out of biogas has not been hot composted, which would remove pathogens, and it has not been sitting for a long time, which should also reduce pathogens. It has been sitting in anaerobic conditions at the temperature of a mammal's gut, for a few weeks. Human pathogens that like a mammal's gut might come right through. Ruminant manure and veggie waste are  less likely to have human pathogens.  

5) The slurry that comes out of a biogas digester is quite wet, and relatively high in nitrogen compared to what went in. I've heard it is good to have a shallow pond for the slurry to come out into, to dry up to a more manageable consistency, and to air out any harmful gasses. In fact, you can mix in a high carbon material at that point and can hot compost it. If the feedstock is ruminant manure or food waste, this is unobjectionable, but human excreta might make it a less attractive chore.

6) If anything goes wrong in there or gets clogged (and it will, if you are inventing your own details for your installation), you may have to take it apart and get inside, or partially inside. If the input is only ruminant manure and veggie waste, you'll just make jokes about the smell, and go in to fix it. If it's soupy human excreta slurry, you might not be so willing.

I have campaigned among the guys who will actually execute the biogas installation at our school, to do it with only cow manure and maybe food waste, for the first year or two. After seeing if everything works, and tweaking the systems until they are foolproof for a whole year through all seasons, then consider adding human excreta.