Geoscience Reference
In-Depth Information
■
EXAMPLE 24.112
Problem:
Using the digester data provided below, determine the percent moisture reduction and
percent volatile matter reduction for the digester.
Raw biosolids
% Solids
9%
% Moisture
91% (100% - 9%)
Digested biosolids
% Solids
15%
% Moisture
85% (100% - 15%)
Solution:
(0.91
−
0.85)
×
100
%Moisturereduction=
= 44%
0.91
− 0.91
(
×
0.85)
24.13.4.7 Gas Production
When measuring the performance of a digester, gas production is one of the most important param-
eters. Typically, gas production ranges from 800 to 1125 L of digester gas per kilogram volatile
solids destroyed. Gas produced from a properly operated digester contains approximately 68%
methane and 32% carbon dioxide. If carbon dioxide exceeds 35%, the digestion system is operating
incorrectly. The quantity of methane gas produced can be calculated in SI and British units, respec-
tively, by the following equations(McCarty, 1964):
(
)
V
=
350
Q SS
−
(
1000 g/kg
)
−
142
.
P
(24.14 4)
o
x
[
(
)
]
V
=
562
.
Q SS
−
8 34
.
−
142
.
P
(24.145)
o
x
where
V
= Volume of methane produced at standard conditions (0°C, 32°F and 1 atm), L/d or ft
3
/d).
350 = Theoretical conversion factor for the amount of methane produced per kg of ultimate
BOD oxidized (L/kg).
5.62 = Theoretical conversion factor for the amount of methane produced per lb of ultimate
BOD oxidized (ft
3
/ lb).
Q
= Flow rate (m
3
/day, MGD).
S
o
= Influent ultimate BOD (mg/L).
S
= Effluent ultimate BOD (mg/L).
8.34 = Conversion factor (lb/MG/mg/L).
P
x
= Net mass of cell tissue produced (kg/day, lb/day).
For a complete-mix, high-rate, two-stage anaerobic digester (without recycle), the mass of biological
solids synthesized daily (
P
x
) can be estimated by the following equations (in SI and British system
units, respectively):
[
]
(
)
YQSS
K
−
o
dc
P
=
(24.146)
x
1
+
θ
[
]
(
)
YQSS
K
−
834
.
o
P
=
(24.147)
x
1
+
θ
dc
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