Anaerobic digestion of energy crops in batch
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the optimal digester hydraulic retention time and or reactor
B0t=dt B0$k$expkt reaches it maximum (rmax) at time zero,
the k-value in (1) increasedwith the inoculum to substrate (I:S)
ratio (Fig. 1) for straw, and reached a plateau I:S ratios higher
than 2e4, meaning that the k-value in full scale is larger than
in most biogas batch experiments.
Currently, energy crops are mainly evaluated in terms of
their B0 which may i) obscure the advantage of pretreatment
In this study, 18 biogas batch experiments with six different
Based on values of the coefficient of determination (R2), the
energy crops cumulative digestion profile fitted better to
* Corresponding firstname.lastname@example.org (A. Feilberg).
Available online at www.sciencedirect.com
b i o s y s t em s e n g i n e e r i n g 1 1 2 ( 2 0 1 2 ) 2 4 8e2 5 1E-mail addresses: email@example.com (A.M. Nielsen), analthough that may not always be the case. In a study
(Hashimoto, 1989) it was shown that B(t) in Eq. (1) could be
applied to the cumulative CH4 production from straw.
An analysis of data from Hashimoto (1989) revealed that
energy crops in triplicates were conducted according to the
ISO (ISO, 1995) method and the procedure described in Moller,
Sommer, and Ahring (2004). Experiments were conducted at
35 0.5 C and at an I:S ratio of 1:1.how sigmoidal the curve is, follows a pattern described by B(t)
in Eq. (1) for manure and sugars.
Bt B01 expktc; C 1 (1)
According to this equation, the CH4 production
digester configuration when energy crops are digested.
2. Methods1. Introduction
To obtain an economically viab
Northern Europe, animal manure i
crops such as maize, wheat and rye
production as a function of time B(
(B0), degradation constant (k), and1537-5110/$ e see front matter 2012 IAgrEdoi:10.1016/j.biosystemseng.2012.03.008 2012 IAgrE. Published by Elsevier Ltd. All rights reserved.
gas production in
gested with energy
es. Cumulative CH4ate methane yield
stant (c) describing
methods, ii) complicate the choice of energy crops for specific
hydraulic retention times, and iii) reduce the perceived advan-
tages from new digester configurations. It was the purpose of
this study to investigate if another pattern of CH4 production
would fit experimental data better than (1) and furthermore to
evaluate if this other pattern would have any implications for
the evaluation of pretreatment methods for energy crops andconfiguration for the specific energy crop.into methane, andAccepted 22 March 2012
Published online 16 May 2012 evaluation of their degradation kinetics, specifically the rate of which they are convertedResearch Note
Anaerobic digestion of energy
Anders M. Nielsen*, Anders Feilberg
Aarhus University, Department of Engineering, Blichers Alle 20, DK
a r t i c l e i n f o
Received 29 November 2011
Received in revised form
19 March 2012
Batch digestion of e
anaerobic digestion o
several days for ene
degradable than sug
production pattern fr
curve shape. Implica
journal homepage: www.else. Published by Elsevier Ltgy crops at mesophilic temperature indicates that the rate of
ergy crops into methane may first reach its maximum rate after
crops such as maize, oat, ryegrass, and wheat that are less
and fats. Experiments show that the cumulative methane
energy crops rather follows a sigmoidal, and not exponential,
ns are that optimal usage of energy crops should involve anrops in batch
30 Tjele, Denmarker.com/locate/ issn/15375110d. All rights reserved.
a sigmoidal shaped form expressed by B(t) in Eq. (2) than B(t) in
Eq. (1) (Table 1). Eq. (2) predicts lower B0-values after 70 days of
digestion than Eq. (1).
Bt B0$1 expktc; C > 1 (2)
use of existing pretreatment methods is not given full credit,
B(t) cumulative methane production
B0 ultimate methane yield Vfed volume of slurry fed
to the digester each slurry shift
B0(t) differentiated B(t)k degradation constant
t time in days
c exponent quantifying how sigmoidal a curve is
rmax time at which degradation rate reaches its
b i o s y s t em s e ng i n e e r i n g 1 1 2 ( 2 0 1 2 ) 2 4 8e2 5 1 249Pretreatment of energy crops focus on improving B0 by
degrading substances that would otherwise not be degraded
(Hashimoto, 1986) and enhancing degradability. Energy crops
contain some easily degradable structures, protected by ligno-
cellulose, which are most accessible to bacteria and enzymes
when the surrounding plant structures have undergone some
degradation. For biomasses such as manure, Eq. (2) does not
apply, but the increase in biodegradability, and B0, is still beEq. (2) signifies a digestion rate of B0(t)/dt B0$k$c(1 expkt)c1. The maximum rate of CH4 production (rmax)occurs at time rmax ln(c)/k for energycrops that followanEq. (2)cumulative digestion pattern.
Fig. 2 illustrates the magnitude of the difference between
real data and Eqs. (1) and (2) for all energy crops investigated in
4. Discussion & analysis
4.1. PretreatmentFig. 1 e Analysis of data from Hashimoto (1989).as an additional advantage of pretreatment is an improved k-
value and reduced c-value in Eq. (2) signifying a faster CH4production earlier.
4.2. From batch to full scale digester
Models have been developed that correlates B0 to expected
CH4 production in digesters. However, the developed models
contain flexible dimensionless parameters such as in Chen
and Hashimoto (1978), and e.g. a stress index as in Hill
(1991). Not all such parameters are contained in data from
experiments, and can be chosen rather arbitrarily.
Future models may have to focus on a combination of the
digestion pattern of the energy crop, reactor configuration,
age distribution in the digester and other measurable
Two-stage digestion is a configuration with numerousaccompanied by an accelerated digestion (Quai et al., 2011).
Pretreatment has the potential to completely destroy plant
structures and should increase k, reduce c, and also reduce
If the effect of pretreatment could be quantified in terms of
both B0, k and c in Eq. (2) at different I:S ratios, there would be
a background for extrapolating data and using derivates of Eq.
(1) and Eq. (2), and one ormore adjustment factors, to estimate
the CH4 potential of a crop in e.g. 15 days (B15), 25 days (B25) or
for any hydraulic retention time.
Pretreatment methods aim at destroying protecting plant
structures, decreasing cellulose crystallinity, and increasing
enzyme accessibility to degraded structures. Therefore, the
(I:S) The ratio of substrate volatile solids to inoculum
Vfed volume of slurry fed to the digester each slurry
VD1 volume of digester 1 in a two-stage configuration
VD2 volume of digester 2 in a two-stage configuration
ss slurry shifts
Z positive integersx a counteradvantages (Blonskaja, Menert, & Vilu, 2003; Demirer & Chen,
2005; Ghosh, Ombregt, & Pipyn, 1985). first stage
agess Vfed=VD1$VD1 Vfed
ss; ssZ (3)
Note. The continuous version of Eq. (3) is (1 Vfed/VD1)t, wheret is time.
Algebraic derivations of age profiles of biomass in one-
stage (Eq. (3)) and second stage digester (Eq. (4)), as a func-
tion of biomass age in slurry shifts (ss), volume of inlet feed
volume (Vfed) and first and second stage reactor volume (VD1and VD2), was derived for this study. By combining the two
equations, the average age of biomass in different configura-
tions, and individual digesters, can be found. The average age
in slurry shifts is higher in a two-stage configuration, than in
a one-stage digester.
For an energy crop following a CH4 production pattern,
such as Eq. (2), Eq. (5) describes how much CH4 can be
produced in a single stage digester:
For a two-stage configuration, it is obvious that CH4
difficult degradable structures. That is, a higher I:S ratios there
will be a tendency for lower c-values.
Table 1 e Equations fitted to experimental data.
Crop B(t), Eq. (1)
Ryegrass 499 (1 e0.039 t), R2 0.97 440 (1 Maize silage 528 (1 e0.020 t), R2 0.96 385 (1 Oil radish 297 (1 e0.045 t), R2 0.97 274 (1 Wheat 515 (1 e0.051 t), R2 0.98 482 (1 Oat 527 (1 e0.025 t), R2 0.96 413 (1
b i o s y s t em s e n g i n e e r i n g 1 1 2 ( 2 0 1 2 ) 2 4 8e2 5 1250production in a digester with an age profile skewed towards
more older biomass reaching itsmaximum rate of digestion at
rmax > 0, would benefit from a rmax being reached after
a number of days.
For single stage digesters, the data presented in this study
is only relevant for tuning the HRT to best fit the specific
energy crop e e.g. a shorter HRT may be suitable for some
energy crops, but not for others.Fig. 2 e Experimental data, Eq. (1), and Eq. (2). Note that the
origins of the equations are located at five different values
on the ordinate.r e f e r e n c e s
Blonskaja, V., Menert, A., & Vilu, R. (2003). Use of two-stageanaerobic treatment for distillery waste. Advances inEnvironmental Research, 7, 671e678.
Chen, Y. R., & Hashimoto, A. G. (1978). Kinetics of methanefermentation. Biotechnology and Bioengineering Symposium, 8,269e282.
Demirer, G. N., & Chen, S. (2005). Two-phase anaerobic digestionof unscreened dairy manure. Process Biochemistry, 40,5. Conclusion
The degradation pattern of some biomasses is better
described by Eq. (2) than Eq. (1). The exact effect of I:S ratio
on the k-value in Eq. (2) remains to be established
for specific energy crops, but it likely follows a pattern
like the one that can be derived from data by Hashimoto
Future progress in pretreatment of energy crops should
focus on not only increasing B0, but increasing B0 and k, and
reducing rmax and c in Eq. (2) when it applies, and imple-
menting the knowledge in equations such as Eq. (5). Other-
wise, sufficient credit may not be given to existing4.3. I:S ratio
A study by Rapose, Banks, Siegert, Heaven, and Borja (2006)
emphasised the importance of I:S ratios when studying
batch experiments as their study showed higher B0-values at
I:S values up to 2. Indeed, the effect of I:S ratio need further
investigation. As the I:S ratio is a metric for the bacteria to
substrate ratio, the c-value may also be affected by the I:S
ratio, as more bacteria may be faster at degrading otherwise
B(t), Eq. (2) rmax Eq. (1) rmax Eq. (2)
e0.079 t)1.99, R2 0.98 0 days 8.7 dayse0.061 t)2.32, R2 0.99 0 days 13.8 dayse0.045 t)1.37, R2 0.98 0 days 7.0 dayse0.083 t)1.63, R2 0.99 0 days 5.9 dayse0.070 t)2.36, R2 0.99 0 days 12.3 days3542e3549.Ghosh, S., Ombregt, J. P., & Pipyn, P. (1985). Methane production
from industrial wastes by two-phase anaerobic digestion.Water Research, 19(9), 1083e1088.
Hashimoto, A. G. (1986). Ammonia inhibition ofmethanogenesis from cattle wastes. Agricultural Wastes, 17,241e261.
Hashimoto, A. G. (1989). Effect of inoculum/substrate ratio onmethane yield and production rate from straw. BiologicalWastes, 28, 247e255.
Hill, D. T. (1991). Steady-state mesophilic design equations formethane production from livestock. Transactions of the ASAE,34(5), 2157e2163.
ISO. (1995). Water quality: Evaluation of the ultimate anaerobicbiodegradability of organic compounds in digested sludge - methodby measurements of the biogas production. InternationalStandard. ISO/DIS 11734.
Moller, H. B., Sommer, S. G., & Ahring, B. K. (2004). Methaneproductivity of manure, straw and solid fractions of manure.Biomass and Bioenergy, 26, 485e495.
Quai, W., Yan, X., Ye, J., Sun, Y., Wang, W., & Zhang, Z. (2011).Evaluation of biogas production from different biomasswastes with/without hydrothermal pretreatment. RenewableEnergy, 36(12), 3313e3318.
Rapose,F.,Banks,C. J., Siegert, I.,Heaven,S.,&Borja,R. (2006). Influenceof inoculum to substrate ratio on the biochemical methanepotentialofmaize inbatchtests.ProcessBiochemistry,41, 1444e1450.
b i o s y s t em s e ng i n e e r i n g 1 1 2 ( 2 0 1 2 ) 2 4 8e2 5 1 251
Anaerobic digestion of energy crops in batch1. Introduction2. Methods3. Results4. Discussion & analysis4.1. Pretreatment4.2. From batch to full scale digester4.3. I:S ratio