Reinforced concrete is obtained by adequately mixing specific proportions of aggregates (gravels and sand), cement, and water (Bartali, 1999). The water:cement ratio is 0.53 L kg-1 and the cement: sand: gravel mass ratio is 1:2.2:3.7 for floors, driveways, structural beams, and columns (Lindley & Whitaker, 1996). Cylindrical cast-in-place concrete tanks are commonly used in biogas plants for storing liquid manure during long periods. A serviceable tank should be watertight to prevent groundwater pollution and corrosion of the reinforcing rods. Therefore, these tanks should be designed to withstand different design loads, i. e. the loads of the soil outside the digester which is buried underground level and loads of the liquid stored inside the digester. Liquid manure is often stored in large cylindrical concrete tanks, which are partially underground. The dimensions of these tanks vary from 18 to 33 m in diameter with heights from 2.4 to 4.9 m and a uniform wall thickness varying from 150 to 200 mm (Ghafoori & Flynn, 2007; Godbout et al., 2003).

The internal volume of the tank can be calculated by multiplying the volume of substrates that should be stored in the tank by 1.10 in order to consider 10% as headspace. The cement mass (kg), gravels volume (m3), and sand volume (m3) required to build the tank can be calculated by multiplying the concrete volume of the tank by the constants C, G, and S, respectively, where C represents the mass of cement required to make 1 m3 of concrete (325 kg m-3), G is the volume of gravel required for 1 m3 concrete (0.8 m3 of gravel per m-3 of concrete), and S is the volume of sand required for 1 m3 concrete (0.4 m3 of sand per m-3 of concrete). The type of iron rods should be selected. The different types (N0D m-1, where N is the number of iron rods per meter length, and D is the diameter of the iron rod) are 606 m-1 (0.666 kg m-1) and 608 m-1 (0.888 kg m-1). In the case of constructing a tank without a concrete top, both types can be used. On the other side, in the case of building a tank with a concrete top, the type 608 m-1 must be used with two iron grids (Samer, 2008, 2010, 2011; Samer et al., 2008). The thickness of digester wall should be 35 cm and is built using reinforced concrete to bear the loads of the materials stored in the digester. Tables 1 through 3 show the typical digester specifications for a commercial biogas plant, the required quantities of construction materials to build the digester, and the quantities of the substrates.

1Consider a duration of 3 days for mixing and pumping, daily manure deposition of 6 m3 day-1, 1.8 m3 cow-1 month-1, and 100 cows

2Consider a storage duration of 7 days and liquid organic matter deposition of 3 m3 day-1 3Consider 40 days of storage duration and liquid substrate deposition of 2 m3 day-1 4Consider digester load of 4 kg m3 day-1 and density of 1.2 kg m-3

5Total quantity of substrates (10750 m3) that should be stored in a digester having a capacity of 11820 m3

Table 3. Quantities of the substrates