Biogas Plant Outlet Chamber Construction

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The Outlet Chamber excavation and manhole is completed concurrently with the digester vessel and the manhole shares a common foundation with the digester vessel.  The manhole of the Outlet Chamber is near the digester wall.  The depth of excavation is less than the digester vessel measured from the top of digester floor by taking the dimension ‘I’ minus the thickness of the digester floor (will depend upon construction material used but generally „I‟ + 13 cm (5”).  The earth behind the manhole and under the outlet floor must be well compacted to prevent cracks in Outlet Chamber walls. The inside dimensions of the outlet can be found on the drawing under A, B and D.  The overflow level is at the top of dimension „D‟ and top of the Outlet Chamber walls is dimension „D‟ + 15 cm.  It is important to use the prescribed dimensions as they determine the useful
capacity of the gasholder.  For the same reason the outlet floor and the top of the walls must be level.  The walls will be vertical and finished with a smooth layer of cement plaster mix: 1 part cement, 3 parts sand.  Outside of the walls must be supported with sufficient compacted earth up to the overflow level to avoid cracks.  The Outlet Chamber walls should slightly higher elevation
than the surrounding ground to reduce chances of surface water entering the outlet during the rainy season. The concrete slabs for the Outlet Chamber should be constructed at the same time of dome casting.  It should be easy to make the additional concrete at this time and the slabs will be well cured before they are placed on the outlet.  The slabs must be 8 cm (3″) thick with proper reinforcement (re-bar) 3 cm (1″) from the bottom side.  The number of slabs should be designed that they can be handled by four men without great difficulty.  Installing re-bar loop handles on the slabs may be useful for the occasional handling of the slabs. When preparing the slab casts use a smoothed, clean sand base and lumber for the frame.  Special care must be taken for the placement of the concrete to prevent small holes that can expose the steel reinforcement to corrosive vapor from the slurry in the Outlet Chamber which will cause corrosion and ultimately lead to the slab collapse.  If holes form in the slab these should be filled with plaster layer.  The Outlet cover slabs are essential to protect people and animals from falling inside and to avoid excessive evaporation of the slurry in dry season.

10.  Construction of Inlet Tank

The Inlet Tank is constructed to mix feedstock and water.  This can be constructed with or without a mixing device.  Installation of a mixing device is preferable because not only it makes plant operation easier but it also improves the quality of mix.  When a mixer is installed, it has to be firmly attached to the structure, easy to operate, effective in the mixing process and the steel
parts in contact with the feedstock should be galvanized.

The top of the structure should not be more than one meter above ground level and both inside and outside of the tank must be covered with a smooth layer of plaster (Mix: 1:3 cement, sand).   The finished bottom of the Inlet Tank must be at least 5 cm above the Outlet Chamber overflow level.  The position of the inlet pipe must be such that a pole or rod can be inserted through it to the digester vessel without obstructions.  This will allow the operator to clear blockages in the inlet pipe.  For the same reason the inlet pipe must be without bends.  Even if a mixing device is not installed, the inlet pit should be round in shape as this is a more economical use of material and easier for hand mixing.

For plants that receive from a toilet, construct a cleanout valve between the water-trap and digester.  The toilet inlet pipe should enter the digester tank no more than 45 degrees from the centerline of the main inlet pipe.  Additionally, the toilet pan level should be at least 25 cm above the outlet overflow level.

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