Enhanced Volatile Fatty Acids Production During Anaerobic Digestion
Enhanced Volatile Fatty Acids Production During Anaerobic Digestion
Enhanced Volatile Fatty Acids Production During Anaerobic Digestion
Bioresource Technology
journal homepage: www.elsevier.com/locate/biortech
h i g h l i g h t s
The effects of micro-oxygenation on volatile fatty acids (VFAs) production during anaerobic digestion was studied.
Inoculum type, incubation time, and oxygen dosage showed significant interactions.
Anaerobically digested cattle manure (ADCM) was the ideal inoculum for VFAs production.
VFAs yield showed a quadratic correlation with oxygen dosage.
a r t i c l e i n f o a b s t r a c t
Article history: A series of batch experiments were conducted to investigate the effects of inoculum type, oxygen (O2)
Received 31 December 2016 dosage, and incubation time on volatile fatty acids (VFAs) production during anaerobic digestion (AD)
Received in revised form 6 February 2017 of Napier grass (Pennisetum purpureum), a high yielding energy crop. The results showed that anaerobi-
Accepted 8 February 2017
cally digested cattle manure (ADCM) as an inoculum generated significantly higher VFAs compared to
Available online 10 February 2017
that of anaerobically digested waste activated sludge (ADWAS) as an inoculum. Additionally, the incuba-
tion time of 3 days and O2 dosage of 15 mL/g volatile solidsadded showed the highest VFAs production
Keywords:
when ADCM was used as an inoculum. Moreover, the VFAs production had a quadratic correlation with
Micro-oxygenation
Volatile fatty acids
O2 dosage with R2 of 0.86. The Scanning Electron Microscopy (SEM) images of the digested fiber showed
Anaerobic digestion rough and crumbled surface structures as opposed to that of the undigested fiber, which was further con-
Lignocellulosic biomass firmed by changes in structural composition of the digested fiber.
Energy crop 2017 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.biortech.2017.02.029
0960-8524/ 2017 Elsevier Ltd. All rights reserved.
140 C. Sawatdeenarunat et al. / Bioresource Technology 237 (2017) 139145
et al., 2010). With ADWAS as an inoculum, acetic acid concentra- 3.5. Changes in structural composition of biomass
tion increased with increasing incubation time and contributed
over 80% of VFAs at an incubation time of 5 days in the micro- The best, the median, and the worst operating conditions for
oxygenated batches. This was mainly due to conversion of higher VFAs productions using both ADCM and ADWAS inocula were
carbon VFAs into acetic acid by the acclimated acetogenic microor- selected (Table 2) to investigate the changes in structural composi-
ganisms in ADWAS. Under non-oxygenated conditions, the pro- tion of biomass during AD.
duced acetic acid was not significantly different. Conversely, the The structural carbohydrates (i.e., cellulose and hemicellulose)
amount of acetic acid produced with ADCM as an inoculum and lignin (ADL) contents of Napier grass before and after AD are
decreased at longer incubation time. Acetic acid, which is the only summarized in Table 3. Compared with raw Napier grass, the fibers
VFAs that methanogens can consume, was converted into methane obtained from the batch tests with ADCM as inoculum showed sig-
during long incubation time, which facilitated the growth of nificantly lower amount of hemicellulose (i.e., 2534%) and cellu-
methanogens as evident from the increasing methane yield as lose (i.e., 218%). Conversely, for the fibers obtained from batch
shown in Fig. 2. Thus, the distribution of individual VFAs strongly tests using ADWAS as inoculum, there were no significant differ-
depended on the types of inoculum used. ence in both hemicellulose and cellulose contents in the biomass
before and after AD (Table 3 and Table S2 in SI). The higher cellu-
lose and hemicellulose degradation of fibers obtained from the
3.4. Statistical prediction model for VFAs yield
batch tests using ADCM as inoculum thus resulted significantly
higher VFAs yields compared with the batch tests using ADWAS
As seen from Fig. 1, the optimum condition for high VFAs yield
as an inoculum. Gu et al. (2014) also presented a similar discussion
was obtained when ADCM was used as an inoculum for the incuba-
when digested dairy manure and digested municipal sludge, and
tion time of 3 days. The correlation between VFAs yield and O2
rice straw were used as the inoculum and substrate, respectively
dosage under the selected condition was further studied by apply-
for methane production. The authors reported an increase in cellu-
ing quadratic regression model. The VFAs yield showed a close fit
lose and hemicellulose degradation resulting in higher specific
to the quadratic model with the coefficient of determination (R2)
methane yield. For the batch tests using ADWAS as an inoculum,
of 0.86. The predicted model was found to be YVFAs = 0.06X2-
there were no significant differences in biomass composition after
ox + 2.65Xox + 81.20, where YVFAs and Xox represent VFAs yield
AD among all operating conditions. This further showed that
and O2 dosage, respectively. The optimum O2 dosage was then cal-
ADWAS was the least effective inoculum in the AD of Napier grass.
culated from the first-order differentiation of the equation. The
Surendra and Khanal (2014) also observed high methane yield
predicted optimum O2 dosage in this study was found to be
from Napier grass, when ADCM was used as an inoculum. Typi-
22 mL/g VSadded, which resulted in the maximum VFAs yield of
cally, the macromolecules, such as lignocellulose are degraded dur-
110.05 5.46 mg/g VSadded. To further prove that the predicted
ing hydrolysis step of AD to produce monomeric sugars, which are
value truly reflects the actual value, a series of batch experiments
subsequently converted into VFAs and then to methane. Thus, the
was conducted in triplicate using the O2 dosage of 22 mL/g VSadded,
higher the rate of degradation of these compounds, the higher the
incubation time of 3 days, and ADCM as inoculum. The experimen-
product yield (i.e., VFAs and/or methane) would be. Based on the
tal results showed that the VFAs yield of 112.70 5.15 mg/gVSadded,
compositional analysis, it was apparent that the microorganisms
was not significantly different from the predicted value (i.e.,
preferably consumed hemicellulose while leaving cellulose and lig-
110.05 5.46 mg/gVSadded) obtained using above equation. Thus,
nin in the digestate during AD. Approximately 2534% and 2123%
the proposed model was precise enough to effectively predict the
of hemicellulose were degraded from the raw Napier grass using
VFAs yield during AD of Napier grass using ADCM as inoculum dur-
ADCM and ADWAS as inoculum, respectively. However, the respec-
ing incubation time of 3 days. Since using ADCM as inoculum and
tive cellulose degradations were only 218% and 918% of total
incubation time of 3 days was the optimum operating condition in
cellulose using ADCM and ADWAS as inoculum. Yue et al. (2010)
this study, the constructed model could effectively predict the
studied the composition change of cow manure fiber during AD
required O2 dosage to obtain the expected VFAs production with-
and reported that hemicellulose was the favorable component of
out performing additional experiment.
Table 2
The selected conditions to examine the changes in structural composition of biomass.
Table 3
The characteristics and structural carbohydrates under the selected conditions.
Sample NDF (% TS) ADF (% TS) Lignin (ADL) (% TS) Hemicellulose (% TS) Cellulose (% TS)
Raw Napier grass 75.0 1.0 49.7 0.6 10.2 0.3 25.3 1.6 39.5 0.9
ADCM,15,3 73.4 0.6 55.4 1.8 23.0 1.0 19.0 1.3 32.3 1.0
ADCM,0,1 73.0 0.6 55.3 2.9 18.9 0.6 17.7 2.2 36.3 0.6
ADCM,30,1 73.3 1.1 56.5 0.9 17.8 2.4 16.8 1.0 38.6 2.4
ADWAS,30,1 66.6 2.4 46.7 0.2 11.0 1.3 19.9 1.4 35.7 1.3
ADWAS,30,3 69.2 4.0 49.6 0.4 13.7 4.7 19.5 1.4 36.0 4.7
ADWAS,30,5 66.7 1.5 46.8 0.2 14.3 3.1 19.9 2.8 32.5 3.1
the fiber to be converted to methane. The digested fiber contained allowing to use his equipment for processing biomass, and Dr.
11% less hemicellulose compared with raw manure (Yue et al., Halina Zaleski for her valuable guidance during data analysis.
2010). The cellulose-rich fiber has undergone partial biological pre-
treatment during AD and could directly be subjected to enzymatic Appendix A. Supplementary data
hydrolysis to release monomeric sugars as a-potential feedstock
for producing other bio-based chemicals via anaerobic biorefinery Supplementary data associated with this article can be found, in
approach (Sawatdeenarunat et al., 2016). Ethanol is one of such the online version, at http://dx.doi.org/10.1016/j.biortech.2017.02.
products that could be obtained from cellulose-rich fiber. The etha- 029.
nol production from low-hemicellulose feedstock could avoid the
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