Organic Waste Recycling

Biofuels production 197

Determine the quantity of CH 4 that can be produced from a dispersed-growth
digester treating this tapioca wastewater and the digester size. Assume that at
the șc value of 10 days, there is a waste stabilization efficiency of 65% and the
values of Y and kdwere experimentally found to be experimentally found to be
0.05 and 0.02 day-1, respectively.

Solution

From Equation 4.14,

CH 4 produced = 0.35(0.65) (100) (20,000) (10

-3

) {1- 1.42 (0.05)}

1+0.02(10)

= 423.1 m^3 /day

Note: if COD represents total C in the wastewater, the ratio of COD/N = 50/1
which is more than the optimum value of 25/1. This tapioca wastewater needed to
add with some nitrogenous compounds (such as urea) to adjust the C/N ratio prior
to feeding to the digester.

Volume of this digester is = 100 m^3 /day x 10 days

= 1000 m^3

Construct 2 conventional digesters, each with a working volume of 500 m^3

Example 4.2

A family of five persons needs about 10 m

3

of methane for family uses daily.
Determine the size of biogas digester required and other operating procedures
necessary to maximize the gas production. Raw materials available are nightsoil
and rice straw.

Characteristics of raw materials:

Nightsoil Rice straw

Organic Carbon (C), % total solids 48 43

Kjeldahl Nitrogen (N), % total solids 4.5 0.9

Volatile Solids (TVS), % total solids 86 77

% Moisture 82 14

Let the dry weights of nightsoil and rice straw required to be fed to the
digester be m 1 and m 2 kg/day, respectively. An optimal condition for anaerobic
digestion should have a C/N = 25/1 in the influent feed: