How To Build a 100 Cubic Foot a Day Methane Gas Plant

Enlarge Image Image Gallery These instructions are for an underground, single-stage, double-chamber plant designed to digest 100 pounds of manure every 24 hours   ILLUSTRATION: KIM F. ZARNEY

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. This plan comes from "Gobar Gas: Experiments in Renewable Energy Sources."  

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 inch stone aggregate.

The digester will be built on this base from 1:2:4 concrete using 1/2 inch aggregate. The floor and walls will be 3 inches thick, giving an inside diameter of 5 feet and 6 inches. The walls will be 16 feet high and reinforced with eight 3/8 inch machine steel vertical rods and 15 horizontal rings of the same material.

Inlet and outlet pipes of 4 inch galvanized iron should be positioned before pouring the walls so that the pipes are positioned 1 1/2 feet 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 inch 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 inch 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 feet 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.