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re: creating biogas from familly waste
18 apr 2004
melodie de l'epine   wrote:

>i have found a lot of information about communities and farmers 
>(especially in vietnam and china) creating biogas (for cooking/heating) 
>from their toilet , kitchen and farm wastes.

i saw slides of an interesting south american "manure sausage" at the
sustainable resources conf in boulder, co last october. a polyethylene
film circular greenhouse ventilation air duct was laid on the ground and
partially filled with water. each end was raised, and fermentables dumped
into one end and scooped out of the other, after they traveled the length
of the duct in the water. the duct might have been 30 feet (9 m) long and
30 inches (75 cm) in diameter, about 4 feet (1,2m) wide when laid flat.

the gas was trapped in the upper part of the duct by two horizontal boards
about a foot above the ground and perpendicular to the duct a few feet from
each end. this requires a lot less labor and material than an underground
masonry chinese pit digester :-) the duct costs about 25 cents per foot
(1 euro per meter) in rolls. it's treated for uv sun protection, and it
should last at least 4 years unless punctured with pitchforks. in a cold
northern climate, it might run east-west to collect sun, with a reflective
wall on the north side. it might be covered with shadecloth in summertime
to lower the temperature and make the poly film last longer. 

>is it practicable/possible to create biogas when you only have 1 family's 
>wastes (from dry toilets and kitchen scraps)?

the phoenixville, pa sewage treatment plant has two 70' diameter x 28' high
anaerobic digesters with floating covers that move up and down 7', which
are kept at 100 f by boilers burning their own methane. metcalf and eddy's
wastewater engineering book says secondary sewage treatment plants produce
about one cubic foot of biogas per capita per day. not much...

the nraes-20 on-farm biogas production manual says one "sow unit" (an
average animal weight of 1300 pounds of pigs and sow producing an average
of 16 pigs per year) makes 1.75 ft^3 of manure per day, yielding 56 ft^3
of biogas (55-70% methane, plus co2) over a retention time of about 15 days.
manure from one cow produces about the same.

the chinese biogas manual (trans. michael crook, int'l tech pubs, 1979,
isbn 0 903031 65 5) says a metric tonne of dried "general stable manure"
makes 270 m^3 of biogas, dried pig manure makes 561, horse manure, 250,
fresh grass, 630, straw, 342, leaves, 250, and sludge, 640. they also give
c:n ratios to help in making an optimum 25:1 mix. "fresh human manure" is
about 3:1, horse and cow manure are about 25:1, leaves are 40:1; dry straw
is about 90:1. know anyone with a horse and straw bedding?

they say a family of 5 in the countryside needs about 1 m^3/day of biogas
for cooking and lighting, and each m^3 of pit digester produces about 0.2
m^3/day in summer and 0.1 in winter (each 10 c increase in temperature
doubles the gas production rate.) making 18 ft^3 (0,5 m^3) of biogas per day
might require a 3 m^3 digester fed with one person's manure and something
like an average of 5 pounds (2 kg) of dry leaves per day. using 10 pounds
of leaves or 5 pounds of straw might make 1 m^3/day with a better c:n ratio.  

so it looks like this could work on a small scale, with a source of leaves
or straw or grass clippings. a meter of fully-inflated 30 inch round poly
duct has a volume of 0.456 m^3...

bonne chance,

nick




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