The digester foundation is placed using cobbles and gravel as aggregate then filled with concrete.The foundation should be 15 cm thick and allowed.t the center of the pit, a straight rod or pipe (the 0.5″ GI gas pipe) must be placed in an exactvertical position. The vertical pipe will be used during the construction as a field-expedientguide to ensure symmetry of the biogas plant. At ground level, a rigid pole, pipe or cord is placed horizontally across the diameter of the pit. The vertical pipe is secured to the horizontalpipe, pole or cord. After securing, the vertical pipe should be checked to ensure it is still in the plumb/vertical position. A string or wire is attached to the vertical pipe.
The length of this wire can be found on Figure 2.1, dimension „F‟. Add one cm length to this length to allow space for plastering. Every brick or stone that is laid in the round-wall will be exactly F+ 1 cm away from the vertical pipe. After the Foundation has cured for at least two days, the round wall is constructed. The first two
rows of bricks must be positioned side by side so that 23 cm (9″) wide base is made. It is essential that first row be placed on a firm, untouched and level foundation. Subsequent rows of bricks are positioned on their lengths so that the wall thickness is maintained at 23 cm (9″) wide. It is not necessary to build in support columns or pillars in the wall however, the backfilling
between wall and pit-side must be compacted with great care. Backfilling should be done no sooner than 12 hours following brick course placement to allow mortar to cure. Earth should be well compacted by adding water and gentle ramming along the circumference of the digester. Poor compaction will lead to cracks in round-wall and dome. The cement mortar used can be 1 part cement to 4 parts sand (1:4) up to 1 part cement-6 parts sand (1:6) depending on the quality of the sand.
The height of the round-wall is detailed on the drawing in Figure 2.1 under dimension ‘H’ as measured from the finished floor. The feedstock inlet pipe (and toilet pipe, if installed) must be placed in position when the round-wall is 30-36 cm high. To reduce the risk of blockages, the inlet pipe(s) must be placed as vertical as practically possible. Exactly to the opposite of the main feedstock inlet pipe, a 60 cm wide opening must be left in the round-wall that serves as a manhole. The digested slurry will flow to the outlet tank through this opening. Additional inlet pipes should be placed as close as possible to the main feedstock inlet
pipe with a maximum distance of 45 degrees from the inlet-center-manhole line. When the round-wall has reached the correct height, the inside must be plastered with a smooth layer of cement mortar with mix of 1:3 cement-sand.