Environmental Engineering Reference
In-Depth Information
Solution:
(0.74
−
0.55)
×
100
%VMreduction
=
]
= 57
%
[
0.74
−
(
0.74
×
0.5
5)
Percent Moisture Reduction in Digested Sludge
(
)
×
%Moisture%Moisture
−
100
in
ou
t
%Moisturereduction
=
[
]
(
)
%Moisture
−
Moisture
×
%Moisture
in
in
ou
t
■
Example 5.6
Problem:
Using the digester data provide below, determine the percent moisture
reduction and percent volatile matter reduction for the digester.
Raw sludge percent solids = 6%
Digested sludge percent solids = 14%
Solution:
Note:
Percent moisture = 100% - Percent solids.
(0.94
−
0.86)
×
100
%Moisturereduction
=
]
=
61
%
[
0.94
−
(
0.
94
×
0.86
)
Anaerobic Digestion of Agricultural Wastes
Animal waste accounts for 10% of methane emissions in the United States. Ruminant
animals, particularly cows and sheep, contain bacteria in their gastrointestinal sys-
tems that help to break down plant material. Some of these microorganisms use
the acetate from the plant material to produce methane, and, because these bacteria
live in the stomachs and intestines of ruminants, whenever the animal burps or def-
ecates it emits methane as well (Spellman and Whiting, 2007). When not correctly
managed, farm waste slurries can also seriously pollute local watercourses. Small
anaerobic digesters have been installed on farms to treat excess animal slurries that
cannot be placed on the land. The biogas formed is normally used for heat but can
also be used to fuel engines and other onsite energy needs such as electricity and
heating. Onsite biogas production and management also reduce offensive odors from
overloaded or improperly managed manure storage facilities. These odors impair
air quality and may be a nuisance to nearby communities. Anaerobic digestion of
animal waste reduces these offensive odors because the volatile organic acids, the
odor-causing compounds, are consumed by biogas-producing bacteria. In addition to
biogas, another important byproduct of anaerobic digestion is ammonium, which is
the major constituent of commercial fertilizer, which is readily available and utilized
by crops. The bottom line on the production of biogas on the farm: Biogas recovery
can improve profitability while improving environmental quality.
