2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
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respectively. These results were not significantly
different in any the nitrogen concentration and were
similar to batch assays. The TVA concentration in R1,
R2 and R3 were 1728±780.6, 1217±436.7 and
1288±201.1 mg L
-1
as CaCO
3
. The TVA concentration
showed similar trend with the VS reduction. The
highest efficiency of VS reduction and TVA
concentration were obtained from R1. While the biogas
and methane yield of R1 shows the similar results to
R2 and R3. The nitrogen utilization rates at R1, R2 and
R3 were 16.99, 11.96 and 11.41 mg gTS
-1
respectively.
However, the deterioration of the nitrogen adding for
CP digestion was resulting in rapid VFA accumulation
and insufficient buffering capacity. It can be seen, the
VFA:Alkalinity ratio was 0.47, 0.39 and 0.39 in R1, R2
and R3 respectively. The highest sodium bicarbonate
adding was R1. The sodium bicarbonate was added of
2.57±0.49, 2.04±0.67 and 2.10±0.57 in R1, R2 and R3
respectively.
TABLE 3
PERFORMANCE DATA OF SEMI
-CSTR
TREATING
CP
WITH
MAINTAINED NITROGEN CONCENTRATION
Nitrogen
excess* (R1)
Nitrogen 40
mgL
-1
(R2)
Nitrogen 10
mgL
-1
(R3)
Nitrogen
concentration in
reactor (mgL
-1
)
467.33±31.65
41.09±5.90
14.52±4.49
VS reduction (%)
64.37±0.40
59.24±0.50
57.88±2.61
Specific biogas
yield
(mLg
-1
TS added)
517.15±155.24 536.76±50.65 546.93±53.66
Specific CH
4
yield
(mLg
-1
TS added)
281.95±99.01 294.25±103.19 288.88±35.76
CH
4
content (%)
54.52±1.28
53.82±1.74
53.80±1.57
TVA
(mg L
-1
as CaCO
3
) 1728±780.6
1217±436.7
1288±201.1
Alk (mg L
-1
as
CaCO
3
)
3672±471.3
3080±375.2
3143±357.4
TVA:Alkalinity
ratio
0.47
0.39
0.40
Na
2
HCO
3
adding
(g d
-1
)
2.57±0.49
2.04±0.67
2.10±0.57
Nitrogen
utilization rate
(mg gTS
-1
)
16.99
11.96
11.41
*according to basal medium of Li and Noike, 1992 [14]
The results reveal that the level of nitrogen adding
can enhance CP digestion especially
easily degradable
compounds that are present in CP content. However,
the reactor which was controlled concentration of
nitrogen of 10 mg L
-1
achieved the same biogas and
methane production efficiencies as the reactors
operated with excess nitrogen adding and nitrogen
control of 40 mg L
-1
. According to Speece [7]
suggested that an excess of 40
–
70 mg NH
4
–
N L
-1
must
be maintained in the reactor to prevent a reduction of
microbial activity and the acetate utilization rate was
only 54% of maximum potential at 12 mgL
-1
of NH4
–
N
in the reactor. Nevertheless, the nitrogen utilization rate
depend on microbial activity, the acetate utilization rate
and typical of feedstock. Ammary [15] suggested that
the optimum C/N ratio (20:1 to 25:1) was assumed that
the efficiency of removal is 100%, the fact that
different substrate have different biomass yields.
Therefore, the requirement of nutrient should be taken
into account both the microbial yield and the efficiency
of COD removal.
IV. C
ONCLUSIONS
This study determined nitrogen adding strategy in
order to enhance cassava pulp digestion. The C/N
ratios of 20:1, 25:1, 30:1 and 40:1
improved CP
digestion by 2.82 2.65 2.35 and 1.7 times, respectively,
comparing to no nitrogen addition. The C/N ratio of
40:1 caused a nitrogen limiting growth of acidogenic
microbial. The semi-continuous stirred tank reactors of
CP with nitrogen concentration of 10 mg L
-1
provided
sufficient nitrogen for the anaerobic digestion of
cassava pulps.
A
CKNOWLEDGMENT
Authors gratefully thank to King
Mongkut’s university
of Technology Thonburi for the financial support. The
authors also wish to thank Nunsurakit’s Tapioca Flour
Ltd., Part and K P Farm for sup- porting raw materials
used in this study. Appreciation is as well extended to
Dr. Warinthorn Songkasiri for her helpful English
grammar correction.
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