2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
- 49 -
TABLE 2
M
ETHANE YIELD AND BIODEGRADABILITY OF
CP
AT THE DIFFERENT
C/N
RATIO
C/N ratio
Without
nitrogen adding
20:1 25:1 30:1 40:1
CP digestion (%)
48
45
40
29
17
maximum specific
methane production rate ;
Rm (ml d
-1
)
15.23 14.39 13.62 14.22
6.36
Partial methane yield
(17 day)
(mL CH
4
g
-1
VS added)
0.14 0.16 0.15 0.14
0.07
Partial biogas yield (17
day)
(mL biogas g
-1
VS
added)
0.32 0.34 0.31 0.30
0.17
Partial VFA production
(hours 48
th
)
(mg L
-1
as acetic acid)
2,028 2,101 1,907 817
663
Fig. 1. Biogas and methane accumulation through time of different
C/N ratios. (
20:1,
25:1,
S
30:1,
¼
40:1 and
§
without nitrogen
adding).
The study monitored VFA productivity over time.
The average VFA concentrations produced during the
first 48 hours at the C/N ratio of 20:1, 25:1 and 30:1
were 2,028, 2,101, and 1,907 mg L
-1
as acetic acid,
respectively. Fig. 2 presents the specific VFA and
ethanol concentrations. The most dominant VFA
species were acetic acid and butyric acid. Noike et al.
[12] found similar results that the major VFA species
produced in anaerobic digestion of starch feed were
acetic acid and butyric acid. Butyric acid was the most
abundant fermentation product in low SRT. At a C/N
ratio of 40:1 and with not nitrogen addition, the VFA
productivity was 817 and 663 mg L
-1
as acetic acid,
respectively which lower than the experiments with the
C/N ratio of 20:1, 25:1 and 30:1. The results revealed
that the nitrogen concentration at a C/N ratio of 40
affected the VFA production. However, the Rm value
and the partial methane yield were similar to the other
C/N ratios (that of 20:1, 25:1 and 30:1). The lower
acetic acid and propionic acid concentrations at a C/N
ratio of 40:1 caused by nitrogen limiting growth of the
acidogenic microbia. However, at a C/N ratio of 40:1
wasn’t effect to methane production because this ratio
was sufficient for methanogenic bacteria. Lettinga et al.
[13] reported that nitrogen requirements are based on
the empirical chemical composition of the microbial
cell. Fermentative bacteria utilizing carbohydrates had
a relative high yield coefficient (0.15 gVSS/gCOD).
The nitrogen requirement may be about 2-3 times of
the fermentative bacteria utilizing volatile acids (with a
low yield coefficient of 0.05 gVSS/gCOD). The value
of COD:N:P (C:N:P) ratio of high yield coefficient and
low yield coefficient should be 1000:5:1 (330:5:1) and
350:5:1 (130:5:1), respectively.
Fig. 2. The concentration of specific VFA and ethanol of various C/N
ratio at hours 48
th
.
B. Semi-continuous anaerobic biodegradability with
maintained nitrogen concentration
Three identical semi-continuous stirred tank reactors
were operated at organic loading rate of 3 kgTS m
-3
-
day and 10 day of HRT. They were maintained in
different nitrogen concentrations. Reactor 1 (R1) was
fed with excess nitrogen. Reactor 2 (R2) was
maintained at 40 mg L
-1
according to Speece et al. [7].
Reactor 3 (R3) was maintained of 10 mg L
-1
following
the results of the batch assays (data not shown). Table
3 provides the performance indices of each reactor. The
nitrogen concentration of R1, R2 and R3 were
determined to be 467.33±31.65, 41.09±5.90 and
14.52±4.49 mg L
-1
, respectively.
The volatile solid
removal efficiency slightly decreased with decreasing
nitrogen adding, from 64.37±0.40% at excess nitrogen
adding to 57.88±2.61% at the maintained nitrogen
concentration of 10 mg L
-1
. The specific biogas yields
of R1, R2 and R3 were 517.15±155.24, 536.76±50.65
and 546.93±53.66 mL g
-1
TS, respectively, while the
specific methane yields were 281.95±99.01,
294.25±103.19 and 288.88±35.76 mL g
-1
TS,
0
2
4
6
8
10
12
14
16
C:N20
C:N25
C:N30
C:N40
C:N>200
mM
ETOH AA PA I-BA BA I-VA VA
0
500
1,000
1,500
2,000
2,500
3,000
3,500
4,000
4,500
Biogasaccumulation (ml)
0
500
1,000
1,500
2,000
2,500
3,000
3,500
4,000
4,500
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Day
CH
4
accumulation (ml)
