2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
- 48 -
D. Semi-continuous anaerobic biodegradability with
nitrogen control
Experiments were conducted in three identical semi-
continuous stirred tank 5-L reactors with different
nitrogen concentrations in form of NH
3
Cl. The first
reactor was fed with excess nitrogen according to
suitable basal medium for microorganism involve in
biogas production system. The second and third
reactors were maintained the concentration of NH
4
-N
in excess of 40 and 10 mg L
-1
, respectively. A basal
medium (without yeast extract) and sodium
bicarbonate were supplied for stable digestion. The
working volume was 3 L. The organic loading rate
(OLR) of 3 kgTS m
-3
-day and the feeding
concentration of 3%TS were controlled throughout the
experiment. The temperature was maintained at 37
0
C
by circulating water through a water jacket using a
temperature-controlled water bath.
Daily biogas
production was measured by a liquid displacement
measurement system automatically.
TABLE 1
C
HEMICAL AND ELEMENTAL COMPOSITIONS OF FRESH CASSAVA PULP
(
VALUES ARE THE MEAN
± S.D.
OF
3
DETERMINATIONS
)
Composition
Unit
Concentration
Moisture
% of fresh CP
69.37±0.23
TS
gkg
-1
fresh CP
305±2
VS
gkg
-1
TS
984±1
Ash
gkg
-1
TS
16 ± 1
COD
gkg
-1
VS
1050±5
Starch
gkg
-1
VS
655 ±5
Crude fiber
gkg
-1
VS
320 ± 20
Protein
gkg
-1
VS
10.8 ± 0.3
TKN
gkg
-1
TS
1.73 ±0.01
VFA
gCODkg
-1
VS
2.1 ± 0.3
C
%TS
40.02
H
%TS
7.52
O
%TS
37.51
N
%TS
0.19
S
%TS
N.D.
C/N ratio
-
210
N.D. not detectable
E. Analysis
Total solid (TS), volatile solid (VS), chemical
oxygen demand (COD), pH, and ammonium-nitrogen
(NH4-N) were analyzed according to standard methods
[8]. A spectrophotometer was used to measure
ammonia according to the HACH colorimetric method
10031 (salicylate method) using HACH High Range
Test ′N Tube Nitrogen
-Ammonia AmVer reagent sets
for 0 to 50 mgL
-1
NH
3
-N. The C, H, O, N and S
elemental analysis was performed on ThermoFinnigan
Flash EA 1112 elemental analyzer, following the
manufacturer’s standard procedures. Alkalinity was
measured by titration to pH 4 with 0.1 mol L
-1
H
2
SO
4
.
The biogas production was measured using a liquid
displacement method [8]. The percentages of methane
and carbon dioxide in the biogas were analyzed using
gas chromatography (Shimadzu,Class-GC14B, Japan),
using a Porapak-N column equipped with a thermal
conductivity detector (TCD). The oven, injector, and
detector temperatures were 70, 120 and 120
0
C,
respectively. Helium was used as the carrier gas at a
flow rate of 30 mL min
-1
.
III. R
ESULTS AND
D
ISCUSSION
A. Anaerobic biodegradability batch assays
Fig. 1 illustrates accumulative biogas and methane
production over time. A rapid initial methane
production was observed after day 2 due to hydrolysis
of easily degradable compounds that are present in CP
content (starch 65.5%w/w). The profiles of
accumulated biogas and methane were significantly
different between the reactors with and without an
addition of nitrogen. With the addition of nitrogen,
biogas production were obviously much higher than
that in the no nitrogen addition condition. Table 2
presents the CP digestibility at C/N ratio of 20:1, 25:1,
30:1, and 40:1 to be 48, 45, 40 and 29%, respectively.
The nitrogen addition at C/N ratio of 20:1, 25:1, 30:1
and 40:1 improved CP digestion by 2.8, 2.7, 2.6, and
1.7 times, respectively, comparing to the no nitrogen
addition condition. The maximum specific methane
production rates (Rm) of C/N ratio of 20:1, 25:1, 30:1
and 40:1 observed on day 2 were 15.2, 14.4, 13.6, 14.2
ml g
-1
VS
added
, respectively. The methane yields at the
end of 17 experimental days of C/N ratio 20:1, 25:1,
30:1 and 40:1 were 0.14, 0.16, 0.15 and 0.14 mL CH
4
g
-1
VS ,respectively. The Rm value and the partial
methane yield of the digestion of CP without nitrogen
supply were 6.36 ml g
-1
VS
added
and 0.07 mL CH
4
g
-1
VS, respectively. These results indicated that the
digestibility of CP was affected by a deficiency of
nitrogen in anaerobic digestion [3]. Microorganisms
require nitrogen source to form proteins, cell wall
components, and nucleic acids [9]. Gerhard et al. [10]
reported that increasing of NH
4
+
concentration in the
batch cultures from 40 to 60 mM resulted in methane
production and rapidly growth of
Methanobacterium
thermoautotrophicum
. The batch cultures with NH
4
+
concentration of 60 mM, a specific growth rate of 0.30
hr
-1
, a cell concentration of 4.8 g cell dry weight (DW)
L
-1
, methane production of 0.114 mole L
-1
-hr,
represented a 140% increase over the NH
4
+
concentration of 40 mM. Russell [11] reported that
B.
ruminicola
B
1
4 is unable to regulate glucose transport
and utilization when growth is limited by ammonia.
