Wastewater Calculations - Mathematics and Statistics for the Environment - page 729

where
Y = Yield coefficient (kg/kg, lb/lb).
K d = Endogenous coefficient (per day).
θ c = Mean cell residence time (days).

Other terms are as defined previously.

■ EXAMPLE 24.113
Problem: Determine the amount of methane generated per kilogram of ultimate BOD stabilized.
Use glucose (C 6 H 12 O 6 ) as BOD and the following information:

Molecular weight of glucose = 180
Molecular weight of methane and carbon dioxide = 48
48/180 = 0.267
Oxidation of methane and carbon dioxide and water = 1.07 kg

Solution: Calculate the rate of the amount of methane generated per kg of BOD converted.

0 267
107

.
.

025
10

.
.

=

Thus, 0.25 kg of methane is produced by each kilogram of BOD stabilized. Now calculate the vol-
ume equivalent of 0.25 kg of methane at the standard conditions (0°C and 1 atm):

Volume = (0.25 × 1000 g)(1 mol/16 g)(22.4 L/mol) = 350 L

24.14 BIOSOLIDS DEWATERING AND DISPOSAL

24.14.1 b iosolids d eWatering
The process of removing enough water from a liquid biosolids to change its consistency to that of
damp solid is called biosolids dewatering . Although the process is also referred to as biosolids dry-
ing , the dry or dewatered biosolids may still contain a significant amount of water, often as much
as 70%. But, at moisture contents of 70% or less, the biosolids no longer behave as a liquid and
can be handled manually or mechanically. Several methods are available to dewater biosolids. The
particular types of dewatering techniques or devices used best describe the actual processes used
to remove water from biosolids and change their form from a liquid to damp solid. The commonly
used techniques and devices include the following:

• Filter presses
• Vacuum iltration
• Sand drying beds

Note: Centrifugation is also used in the dewatering process; however, in this text we concentrate on
those unit processes listed above that are traditionally used for biosolids dewatering.

An ideal dewatering operation would capture all of the biosolids at minimum cost and the resul-
tant dry biosolids solids or cake would be capable of being handled without causing unnecessary
problems. Process reliability, ease of operation, and compatibility with the plant environment would
also be optimized.