Organic Waste Recycling

198 Organic waste recycling: technology and management

0.48 m 1 +0.43 m 2 = 25

0.045 m 1 + 0.009 m 2

m 2 /m 1 = 0.645/0.205 = 3.15 (4.15)

From Table 4.21, choose a methane production rate

= 0.3 m^3 /kg TVS added.

TVS required/day = 10/0.3 ≈ 33.33 kg, or 0.86 m 1 + 0.77 m 2 = 33.33 (4.16)

From Equations 4.15 and 4.16

m 1 = 10.16 kg/day

m 2 = 32 kg/day

Wet weight of nightsoil required = 10.16/0.18 = 56.44 kg/day

Wet weight of rice straw required = 32/0.86 = 37.22 kg/day

Volatile solids added/day

Volume of digester =

Volatile solids loading

The normal range of volatile solids loading

= 1-4 kg VS/(m^3 -day)(Barnett et al. 1978)

Choose a volatile solids loading rate = 2 kg VS/(m^3 -day)
The volume of digester required = 33.33/2 ≈ 17 m^3
To provide additional space for gas storage, the digester volume should be
approximately 22-25 m^3.
The normal hydraulic retention time = 10-60 days (Barnett et
al. 1978 and section 4.3)
Choose a hydraulic retention time = 30 days
Waste volume to be added/day = 17/30 = 0.57 m^3 /day = 570 L/day
Assume bulk densities of 1.1 and 0.1 kg/L for nightsoil and rice straw,
respectively.
Volume of raw wastes to be added each day:
= (56.44/1.1) + (37.22/0.1)
= 423.5 L

Volume of dilution water required to be added to the influent mixture: