How To Build a 100 cu. ft./day Methane Gas Plant
(These instructions are for an underground,
single-.stage, double-chamber plant designed to digest 100
pounds of manure every 24 hours — five cows'
worth — but may be scaled upward to
construct a plant capable of producing 500 feet of gas a
day).
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Dig a hole 13 feet deep and 12 feet in diameter, cutting
away trenches for the inlet and outlet pipes to angle down
through.
In the center of the hole, pour a slab of concrete six
inches thick and six feet in diameter. The composition of
the concrete should be 1 part cement, 4 parts sand and 8
parts of 1" stone aggregate.
The digester will be built on this base from 1:2:4 concrete
using 1/2" aggregate. The floor and walls will be 3" thick,
giving an inside diameter of 5'6". The walls will be 16'
high and reinforced with eight 3/8" machine steel vertical
rods and 15 horizontal rings of the same material.
Inlet and outlet pipes of 4" galvanized iron should be
positioned before pouring the walls so that the pipes are
positioned 1-1/2' above the digester floor and in from the
walls. This is so that when the dividing wall is built
across the center of the digester, each pipe will be
centered in its chamber. The concrete must be tightly
packed around the pipes to prevent leakage.
Another wall of brick or concrete will be built three feet
outside the digester wall and to the same height (i.e. four
feet above ground level). This space will be filled with an
insulating material: straw, sawdust, shavings, etc.
Provide some means of descending into this space —
perhaps rungs of machine steel rod extending from the
digester wall to the brick retaining wall — in case
it should ever become necessary to empty the insulation.
Seal the top of this area to prevent water from getting in,
and leave bare earth in the bottom for drainage.
Bisecting the digester will be a wall of 4" reinforced
concrete eight feet high, at the top of which an iron
support structure with a guide pipe for the gas collector
will be placed. This structure is made of angle iron and
the guide pipe is eight feet of 3" galvanized iron pipe.
The structure will be set in the digester walls and solidly
fixed atop the chamber-dividing wall. The pipe must be in
the exact center of the digester, allowing the gas
collector to descend into the slurry when empty and rise to
ground level when full. This requires 4' of vertical
travel, thus the top eight feet of the digester are left
for the gas collector while the bottom eight feet contain
the dividing wall.
