permaculture

["Lawrence F. London, Jr." <venaurafarm@bellsouth.net>]

David wrote:

> With regard to biogas, you should note that without supplementing with 
> food or other material, the wastes from one person will not generate 
> enough biogas to cook their own meals. This is because the feces and 
> urine from one person will, on average, produce one cubic foot of biogas 
> per day, which in turn is enough to run a one burner stove for about 10 
> minutes. (On my stove, it takes 12-15 minutes to heat enough water for 
> three large cups of coffee, and cooking rice takes about 40-50 minutes.)

Well, the composted humanure, finished aerobically 6 months or perhaps
sooner, aerobically, could be used to grow cover crops that could
provide the needed additional C & N to make acceptable quantities of
biogas. Other scenarios might include:
1) Jumpstart the process with immediate addition of vegetable matter to
the humanure, then, after that batch has produced all the biogas that it
will, remove the biomass/humanure, at a measured rate, to a water
impoundment and grow duckweed in the mix, to be regularly harvested to
provide the biomass needed for future biogas batches. Duckweed will do
the job ad can actually grow in slurry containing raw sewage. I have
learned this from a duckweed researcher at NCSU who sells produce at the
same farmers market that I do. The duckweed genome is to be sequenced
and varieties will be genetically engineered for oil content. Duckweed
for biofuel production. Duckweed is amazing, cold hardy too. I have it
growing in the three ponds that collect all runoff from all my farm
fields and gardens; I will harvest the duckweed to use as fertilizer
(if you dry it in an open covered shed it becomes granular, convenient
for distribution over gardens).
2) The duckweed ponds could provide clean irrigation water for food
gardens, orchards and various fruit and nut field crops and coppice
orchards for rocket stove wood fuel.

> That means, in essence, that a community toilet facility which connects 
> to a digester-- again, without supplementation with other wastes-- will 
> most certainly not produce enough biogas to support pressurization of 
> cylinders to distribute to anything except a small fraction of those who 
> "contribute". (And it will take a good deal of the energy from the 
> biogas to run the pump.) Further, it is wasteful to compress raw biogas, 
> since it's about half carbon dioxide, and it is functionally impossible 
> to liquefy it using low-cost pumps.

Got it. The digester designs I have seen on the web include direct
piping to the end user's houses. There is a good case for community
kitchens that use biogas also; efficient use of resources and manpower.

> Again, though, where food or other digestible wastes are added to such a 
> digester, then it could produce more (and perhaps far more) biogas.

It would be interesting to know exactly what types of plant material
work best for this application (I assume duckweed would be ideal, and
there are many varieties to pick from, some for human consumption, some
for ivestock, some for biofuel and maybe some for adding to humanure for
biogas production). Also it would be good to know the proportions of
humanure to biomass for optimal biogas generation.

> You should also know that anaerobic digestion will not destroy some of 
> the more persistent pathogens until the wastes have been allowed to 
> remain anaerobic for 6 or more months. Where fresh material is mixed in, 
> the clock must be restarted. Thus a community wastes digester must deal 
> both with the issue of what to do with the effluent, and how to insure 
> it is safe.

It can be aerobically composted too, if the digester equipment design allows
convenient removal of the slurry; of course it could be pumped. Also
knowing the proportions of water to humanure/biomass in the digester
would be useful.

> Propane, on burning, does /not/ produce fumes that are any nastier or 
> more dangerous than butane or methane. And finally, butane, methane and 

That's good to know.

> propane all require different-sized holes in the burner, so that one 
> stove cannot be designed to efficiently burn any of the three. The issue 
> of designing efficient stoves is indeed very important, but it has as 
> much to do with the design of the pots that go on the stove as it does 
> the stove itself. Quite good biogas stoves have been designed and are 
> being sold, but I am not aware of any that have taken the step of 
> designing pots that contribute to the capture of the heat produced.

Do you have links to sites offering these types of biogas stoves?
Of course, Google will provide, as always.
I would think that some sort of biogas stove and collection of pots
designed to work together for maximum efficiency would be the best route
to follow. I think that the basic rocket stove design could be adapted
to similar stoves using biogas as fuel, with all necessary additional
safety features added.

Thanks for the interesting information you've provided. Great food for
thought.

Cheers,

LL