Organic Waste Recycling

196 Organic waste recycling: technology and management

in which, BODL= 1.42 Px (4.12)

Where: Px= net mass of cells produced

From Monod equation and considering mass balance in a digester without cell
recycle, the term “Px” in kg/day can be expressed as follows:

Px= QYESo × 10-3 (4.13)

1 + kdșc

Where: Y = yield coefficient of anaerobic bacteria (normally = 0.10), and

kd = decay coefficient of anaerobic bacteria (normally = 0.02-0.04

day-1)

șc = mean cell residence time, days

Substituting Equations 4.11, 4.12 and 4.13 into Equation 4.10, the following is
obtained:

Volume of CH 4 produced, m^3 /day

= 0.35 E Q So(10-3) (1- (1.42 Y/ (1+ kdșc )) (4.14)

If the kinetic coefficients Y and kd for cell production and the efficiency of
waste stabilization (E) in a digester are known, then CH 4 production can be
predicted from Equation 4.14. Experimental method to determine values of the
kinetic coefficients for a particular type or mixture of influent feed is outlined in
Metcalf and Eddy Inc. (2003), or these values can be obtained from literature.
It should be noted that Equation 4.14 is applicable to only dispersed-growth
digesters without sludge recycle. Although kinetic models for methane production
and waste stabilization for attached-growth digesters have been developed, their
application to field-scale digesters is not yet widely adopted.
An example showing the application of Equation 4.14 is given in Example 4.1.
Example 4.2 is a simplified method to design a biogas digester using data obtained
from the literature.

Example 4.1

A tapioca industry produces wastewater with the characteristics as shown
below.
Flow rate = 100 m^3 /day
COD = 20,000 mg/L
N = 400 mg/L