How to Generate Power from Garbage
(Page 7 of 12)
May/June 1970
By the Mother Earth News editors
A suitable baffle should be provided between the digestion tank and the latrine pit to prevent the escape of valuable gas. Care should be taken during the operation of the plant to see that this seal is maintained. It will also be necessary to maintain in the latrine an adequate level of water in order to ensure operation in a clean and sanitary manner.
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In the design shown, the digesters are placed partly below ground level so that the floors of the latrines are not too high above ground. Steps could be made, however, if you desire to have more of the tank above ground level.
The digester's gas piping is very simple. The only requirement is that the opening of the pipe collector should be sufficiently high (12-18CM) above water level to prevent it from becoming clogged by floating scum. In some tanks a ball-shaped dome is built as part of the roof for easier collection of gas. Outside the digester, a petcock followed by a shut-off valve should be installed, the petcock for occasional testing of gas-burning quality and the valve for shutting off and isolating the digester compartment when so desired.
The digester should be provided with a short discharge pipe, leading outside to a spigot or valve, through which the tank's liquid can be drained into a lined pit. As this liquid is rich in nutrients and is seeded with micro-organisms which are adapted to the environment, it is essential that it be returned to the land and not wasted. Some of the liquid should be used again with the next batch of manure loaded into the digester, the excess is what is put onto the land as organic fertilizer. The capacity of the pit need only be 1/2-1 m 3 for each 10 m 3 of digester space.
GAS-HOLDER
The gas-holder consists essentially of a reinforced-concrete or masonry tank, filled with water and equipped with a floating cover. The cover moves up and down through the water according to the quantity of gas the holder receives from the digester(s) (Figure 8). The water caught in the space between the cover and the tank walls maintains a permanent seal against any escape of gas. The gas-holder may be any shape, but circular is usually the most satisfactory.
Under the pressure of gas arriving from the digester, the cover moves upwards, the water under it is pressed down, and, at the same time, the water level in the space between the cover and the tank's walls moves upward, the difference in water levels corresponding to the actual pressure of gas stored under the cover. In designing floating covers for digesters, allowance should be made in the freeboard on the tank to account for the difference in water elevation inside and outside the cover (Figure 9).
The tank should be sunk in the ground for structural reasons, and to help prevent freezing of the water in cold climates. It is good practice to build the tank walls slightly higher than the bell-shaped cover. The cover is usually made of sheet iron, 2-3 mm thick, and should be strengthened and framed with angle iron or cross-braces. Otherwise the thin iron sheets may warp and bind against the wall surfaces of the tank.
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