When the round wall of the digester is complete, the dome is then constructed. Before filling the pit with earth to make the mould for dome, backside of the round wall should be filled with proper compacted back-fill. If this is not done, the pressure of earth for the mould can lead to cracks in the round-wall. The dome is constructed using a mould or cast technique. This can be accomplished by constructing a timber frame then placing the arth-cast atop for proper arch design. Once the dome is cast, the timber frame and earth will be removed through the outlet chamber manhole. Mark on the vertical center pipe the distance „J‟ from the finished floor noted on drawing in
Figure 2.1. The vertical pipe should remain in place as the mould is constructed.
A dome arch-guide is made for ensuring the mould is the correct shape. The guide can be manufactured from an iron rod with a loop at one end to ride on the vertical pole of the biogas plant and the other end bent to seat flush to the top of the digester wall. The arch shaped should be an 80 to 90-degree arc from the Mark „J‟ on the vertical pipe and interior edge of the completed digester wall. The mould is shaped rotating the guide around the central vertical pipe. The top of the round-wall must be clean when the guide is in use.
It is important that the earth of the mould is well compacted. If the earth is further compressed after casting the dome, by its own weight and that of the concrete, it can lead to cracks in the dome. When the earth mould has the exact shape of the guide, a thin layer of fine sand is spread on the mould-top by gently patting it on the surface. The sand layer will prevent the earth from adhering to the cast. The earth used for the mould needs to be damp to prevent dry earth from soaking up water from freshly casted concrete.
Before start of the cast work, sufficient labor and construction materials like sand, gravel, cement
and water must be staged on the site and ready for use. The casting must be done as quickly as
possible and without interruptions as this will negatively affect the quality of the cast. A
constant, adequate supply of concrete (mix: 1 cement, 3 sand, 3 gravel – 1:3:3) must be made for
the mason. No concrete older than 30 minutes should be used.
Special care should be taken to maintain the thickness of the dome while casting, i.e. the thickness near the outer edge should be greater than the thickness at the center. For the 4, 6, 8 and 10 m3 plant, the thickness at the edge should be 25 cm (10”) where as the thickness in the center should be 7 cm (2 ¾”). Similarly, for 15 and 20 m3 plants, the thickness at the edge should be 25 cm (10”) and the thickness in the center should be 8 and 9 cm (3 and 3 ½”) respectively. An alternative to concrete cast construction technique is the use of baked clay brick in corbel-arch construction. The compression of the brick and mortar in a spherical shape will support the dome. The clay brick dome will have a near-uniform thickness compared to the cast-concrete dome that thins towards the center. A continuous application of mortar along the sand mould is necessary as the bricks are placed. The brick dome should be placed continuously and use a mortar mix of 1:4 cement to sand. Once the bricks for the dome have all been placed, the exterior is covered with 1:3 cement to sand plaster. During the casting, the concrete has to be protected against strong sunlight by covering it with wetted burlap, jute bags or straw mats. This protection has to be left in place for at least one week. The day after the casting, the turret must be made. The turret is made with brick, 36 cm square and 50 cm tall. The turret is plastered with 1:3 concrete. Any delays during dome construction can lead to leakage between main gas pipe and dome.
Following completion of the dome, the structure must be sprinkled with water 3 to 4 times a day during the curing period (up to one week). After the dome has cured for approximately one week, the timbers and earth of the mould can be removed through the manhole. When all earth is removed, the inside of the dome has to be thoroughly cleaned with a stiff brush and clean water.
On the clean surface of the dome interior, the following plaster coats must be applied to make the dome gas-tight from first to last coats:
a) Cement and water wash (1:1)
b) 10 mm layer: 1 part cement, 2 parts sand (1:2)
c) 5 mm layer: 1:1
d) Cement/acrylic emulsion paint coating: 1.5 parts paint, 20 parts cement
e) Cement/acrylic emulsion paint coating: 1 part paint, 2 parts cement
Allow at least one day between plaster coats. When a layer of plaster is applied, the work must be executed with great care and without interruptions. The performance of the biogas plant is dependent on the gas tightness of the dome. For proper insulation during the cold season and as counter weight against the gas pressure inside, a minimum top cover of 40 cm (16”) compacted earth is required on the dome. If the top cover will be prone to erosion due to wind and rain proper protection with gravel, circular wall, or straw matting should be applied.