This is mainly because cooking gas/fuel is very cheap, so biogas is really used in the developing areas of the world, vs the US. In the US, you have digesters on factory farms in the north, where they have cheap sources of bulk feedstock and high fuel demands, making the operation financially feasible.
Daniel Bowman wrote:Hmm.. I get what you are saying. But I still think there is a disproportionate lack of interest in biogas.
You are mistaken, it does not generate enough heat to self maintain. While it will still perform in cooler temps, it will not do so very efficiently. Big digesters in the midwest often freeze, due to poor design.
Daniel Bowman wrote:1) If I am not mistaken, the digestion process generates a lot of its own heat, so the ambient air temp requirements aren't any different than making compost.
Daniel Bowman wrote:2) For a family with a very small yard, the situation for composting toilets would be equally as difficult as biogas. But for anyone with enough land to run chickens and have a decent sized garden, there is plenty of biomass available to harvest methane.
Why? You will basically have to pen up that cow in a small area or follow her around all day to collect the biomass.
Have a cow? Anyone with a couple acres of pasture should really prioritize a biogas setup.
yeah, it is bigger in tropical climates in developing regions. Tropical, because of the temp requirement, and developing, because of the financial side. Most people in those situations are using dung or wood for cooking, so they are moving to gas, and at a relatively low cost of doing so. They are only using biogas for cooking, so their requirements are very low, and they can get by with a dozen pigs or so as their source of biomass.
Daniel Bowman wrote:But no one seems to actually be doing this on an appropriate scale, so maybe I am really missing something. (And most of rural China and India?)
Daniel Bowman wrote:Thanks for the thorough response! Makes my day to see such a good reply. But, even though your overall view is critical, my optimist's spin on it is: Simple designs are entirely possible, even if they are mostly nonexistent, for integrating (warm) flush toilets and (passively) heating a biogas plant in temperate N. hemisphere regions (maybe zone 6 and above).
Daniel Bowman wrote:Also, my experience with pastured cows is that if you pen them up in a barn at night, there is still a fair amount of manure in the morning.
If pigs are in a pasture, they will have several toilet areas, but you can certainly collect it. If you have more than a pig or two, that will be a major chore on any space bigger than an acre. (15 lbs of manure per pig per day)
Daniel Bowman wrote:Also pigs put their manure in a single spot, as do llamas.
I think you'll find that they are actually adding a significant amount of vegetable matter to make it work, and/or their gas needs are very low. Humans just don't produce that much manure (.5 lb a day), so the contribution of humanure to the digester is insignifcant compared to their other feedstock.
In Eugene, OR there is the one digester that I know of in our area. It is an unheated 600 gallon tank which is fueled entirely on kitchen scraps, weeds and humanure, generating the cooking fuel for community meals for everybody. I think they have around 20+ poop-contributing people living there, but no animals.
Well, that depends. I have lived on rain catchment for over 12 years, and water is not cheaper than fuel in my area. In fact, my options for water (other than rain catchment) are expensive. Wells cost $20K+, hauling water would be cheap for the water, expensive for the transportation, etc, etc. Each situation is different. The ROI for rain catchment is quite different in the Chihuahuan Desert (where there isn't a lot of water) compared to Oregon (where there is more).
Water is even cheaper than cooking fuel.
In India, where biogas used a lot, I've been told that the general rule of thumb is that three or four cows produce enough biogas for a family, if the cows are home all day. If it's only nighttime barn collection, more would be needed. I've been told that it takes a lot of humans, maybe a dozen or more, to equal one cow in biogas manure output.
Pathogens are destroyed easily by the heat of aerobic decomposition, but a human gut is anaerobic, and a biodigester is ideally about body temp, so apparently pathogens might be happy in there and make it through to the output.