Contact Rancho Mastatal or Siempre Verde, Mastatal, Costa Rica for more. They have several other basic humanure systems and appear to have built a more durable biogas system since I was there.
Story from mother earth news about low-tech chinese digesters:
The chinese have built over 7 million low-tech methane digesters that produce safe(?) effluent for fertilizer.
These people have some interesting things to say about digesters, including the data about the values gained from vegetable waste vs manure as feedstock which was new to me ....
Well, it does make sense to me. By the time biomass has passed through my system and become manure, a great deal of gas has already been produced... ops:
A non-manure based methane digester avoids some sanitation issues that could be a factor with widespread adoption. And efficiency is great. But the fact remains that poop has to be disposed of somehow, and it might as well produce energy before it turns into fertilizer.
If the digester is more efficient, but you have to expend extra energy to obtain feedstock, what's the use? Also, kitchen scraps might be diverted from other potential uses, the primary one being chook food...which leads to more manure and more kitchen scraps. If the basic idea is to cycle energy through as many times as possible before it becomes so dispersed as to be useless, then it makes more sense to first use it as chook or soldier fly food, turn it into manure and then make biogas before using the effluent as fertilizer. If we look just at the primary yield of biogas, it may make more sense to use kitchen scraps, but when start matching inputs and outputs in a robust permie system, we end up with a wide range of possible yields.
Of course, these folks are aiming at urban users, and in a great many situations, a kitchen-scraps-only system may be the most appropriate technology.
I wonder what is the range of feedstock this digester can handle? Just how starchy does it need to be? Will it take tender garden weeds? Dried leaves? Woody yard trimmings? I have a hard time imagining producing enough kitchen scraps to cover the pinky-toe of my carbon footprint, but if a wider range of biomass will work well, then I want one.
Otherwise you are looking for local restaurants and stores to top up your feedstock supplies, maybe..around here asking neighbors for their vegetable kitchen scraps would lead them to consider calling the men in the little white coats..
Apparently, pine resin stops the composting process after only two months, and it's worth putting at least three feet of compostables around the tank in any direction.
pH is important, and there are a few other important lessons in it all.
An important point from this same author, is that neighbors are happy to see you clear invasive brush. Happy enough not to call the nice young men in their clean white coats. It is significant work to cut and chip, but on a per-ton basis, perhaps less so than tracking down so many kitchen scraps. It also seems to be potentially a net win energy-wise, despite the internal combustion to (typically) power the cutting and chipping and hauling.
Another important point is that any wood over 1" diameter should probably be chipped separately. Potentially, this could work with a greywater system and a barrel breeder to feed poultry.
Problems : (1) : Well the truck inner tube inflates nicely and the great thing is that it provides gas pressure which is useful when burning (gets to about three atmosphere in pressure). However the pressure inside the inner tube meant that the whole system became pressurized from the digester itself through all the tubes and the hydrogen sulfide scrubber and the carbon dioxide scrubber. This increased leaks issues as the pressure tests the system really well. But it is doable. It is important to flush the first lot of gas build up as the first lot can contain residual air and air and methane mixed inside an inner tube that you connect to an ignition source is always a bad idea.
(2) : The amount of iron oxide (what is essentially just rust) required is small. The amount of hydrogen sulfide produced per liter of gas is only milligrams so 1 kilogram of iron oxide is able to process a massive amount of gas. But the amount of pickling salt required to process the carbon dioxide was a lot due to the large quantities of carbon dioxide in the gas (If I calculated it correctly I was using about 750 grams per 1,000 liters of gas). I did collect the gas at first without the carbon dioxide scrubbers but the collection volume was greater and I am looking at using it for inside cooking gas at a later date so I didn`t want to introduce all that carbon dioxide into the house.
Another process of removing the carbon dioxide and the hydrogen sulfide is to absorb them in water. Hydrogen sulfide and carbon dioxide both adsorb into water quite well but methane doesn`t. However I read that for best results it is best to pressurize the processing system to 10 atmospheres and that level of mechanization for a system introduces too many issues. So I preferred the chemical way of scrubbing the gasses. If you are only ever going to cook outside on it then I guess the carbon monoxide scrubbing isn`t required. Although as a personal preference I would still do the hydrogen sulfide scrubbing as hydrogen sulfide is poisonous.
I was getting around 70 liters of gas (after scrubbing most of the carbon dioxide out of it, so I guess that is mostly methane with some water residue) for every 1 kilogram (2.2 pounds) of grass clippings I put into it. The water from the digester made great fertilizer after I diluted it down for the kitchen garden. I put grass clippings and things like newspapers etc in the digester. I don`t keep my own animals and personally if I did would probably prefer to compost it as the gas digester doesn`t kill all the pathogens in the manure however proper composting should kill most things due to the heat. For the same reason I definitely would never methane digest human manure, but of course in certain countries doing so is much more preferable than having it deposited where it can get into the water system.
I operated it in a Japanese Spring and Summer and early Autumn (the temperature where I live in summer often stays around 30 degree`s Celsius even at night in summer). So heating it wasn`t an issue. But I`m making plans for a bigger system at a later date and would prefer to use a solar water heater system for it in the cooler months as opposed to burning some of the methane to heat it. Burning the methane involves a flame and mixing a flame and a methane gas producing unit together … I`m sure it could be made safe but personally not my cup of tea.
One thing to note is that methane is about 50% lighter than air so at least if there are leaks the gas mostly dissipates upwards and away which is great (unlike say propane which accumulates in the lowest place). A lot of the collection systems seem to be quite high such as up by the barn ceiling so that would really help with gas dispersal in the case of leaks. I wouldn`t try anything with the gas unless you have the space where it can be away from the house.
I got the idea for using inner tubes as a collection source from this site;
When I first started playing around I used plastic softdrink bottles with different mixtures in them with aquarium tubing going into a large party balloon. A T-junction in the aquarium tubing between the bottle top and the balloon meant that I could draw of the gas as it built up and test it.