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Evaluation of methane production from anaerobic digestion of different agro-industrial wastes
Vitanza R., Cortesi A., Gallo V., Colussi I., Rubesa Fernandez A. S.
University of Trieste, Department of Engineering and Architecture,
Piazzale Europa 1, I-34127, Trieste, Italy
Corresponding author: Rosa Vitanza
University of Trieste, Department of Engineering and Architecture, Piazzale Europa 1, I-34127,
Trieste, Italy
(0039) 040 5583254
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Abstract
The production of methane via anaerobic digestion of biomass, such as energy crops, agro-
industrial wastes and OFMSW, would provide a clean fuel from renewable feedstock and would
replace the fossil fuel derived energy. Because of this, the prediction of methane yield (as regards
to gas volume and rate of production) of residual and waste materials is gaining increasing interest.
The biochemical methane potential (BMP) test is widely used for anaerobic process feasibility and
design purpose, providing information about the biodegradability of high solid content substrates.
In this work, methane yield coefficients and first-order disintegration rates for five different
substrates (apple waste, brewery spent grain, brewery yeast waste, maize silage and red chicories
waste) are evaluated. BMP tests are performed in 5 L fed-batch stirred reactors at several
inoculum/substrate ratios. All runs are performed without the addition of chemicals.
Keywords: BMP test, biomethane, anaerobic digestion, anaerobic biodegradability
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1. Introduction
The European Union is promoting the use of energy from renewable sources in
replacement of fossil fuels. According to the Directive 2009/28/EC (EU Directive, 2009), 20 % of
the final energy consumption have to be provided by renewable sources by 2020.
In this context, the biogas production via anaerobic digestion of biomass is gaining
importance. Biogas is an energy carrier with several possible applications: raw biogas may be
used for heating and electricity production, whereas upgraded biogas (with a content of methane of
95 – 99%) may be used as vehicle fuel or injected in a natural gas network (Olsson and Fallde,
2014). The three main biogas production routes are: direct recovery from landfill, anaerobic
digestion of wasted sludge from WWTPs, and purpose-designed biogas plants (van Foreest,
2012). With respect to the latter, many feedstock can be processed to produce the collectively
known “other biogas”: manure, energy crops (e.g. maize silage) and agro-industrial waste (e.g.
remains of breweries, fruit processing and slaughter houses).
In 2012 the estimated primary production of biogas in the EU was of 12,016 ktoe
(Eurobserv’er, 2013). The sector is dominated by the three main countries, Germany, United
Kingdom and Italy, accounting for three quarters of the installed capacity (Eurobserv’er, 2013).
In recent years, Italy is witnessing a proliferation of biogas energy plants. According to
Fabbri et al. (2013), the biogas plants operating in Italy at the end of 2012 were 994, with an
increase of 91% compared with 2011. The management of these plants is not trivial: it involves the
achievement of the proper OLR, the balance of the C/N ratio, the maintenance of the proper pH
values, and the mitigation of the inhibitory effects. Additionally, another important point is that the
feedings of the agricultural biogas plant (usually managed by the farmers) are subject to the
growing periodicity. In this context, it’s clear that laboratory experiments and process modeling are
indispensable tools for supporting the plants design and management.
In the present paper, results of biomethanization tests performed with five different
substrates (apple waste, brewery spent grain, brewery yeast waste, maize silage and red chicories
waste) are reported. The experiments were carried out in different time intervals. The specific
methane production for each substrate was related to its own anaerobic biodegradability by
merging the chemical composition data (taken from literature) with the results of laboratory BMP
tests. The obtained methane production curves are then employed to calculate the CH4 yield of
each substrate and to estimate a first-order disintegration/hydrolysis rate constant.
2. Materials and methods
2.1 Substrates
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Tested substrates came from farms and food processing industries located in the North-
East of Italy.
Apple wastes (AW) originated from an apple juice manufacture in Friuli Venezia Giulia
(FVG) region. The apple juice process production creates residuals ranging from 25% to 35% of
the fresh fruit mass with a high nutritional content.
Brewery wastes were collected from a local (FVG region) brewery and consisted of spent
grains and exhausted yeast. Brewer’s spent grains (BSG), the residual solid fraction of the final
mash process of malting, are a main waste fraction of beer production (Mussatto et al., 2006;
Thomas et al., 2006; Lorenz et al., 2013), corresponding to around 85% of total by-products
generated. Brewer's yeast (BY) is produced by the well-known one-celled fungus Saccharomyces
cerevisiae. In the brewing process, brewer's yeast is added to hops and malted barley to ferment
them into alcohol. During alcoholic fermentation, the cells reproduction (gemmation) takes on and,
at the end of the process, the residual yeast is about twofold – threefold of the added quantity.
The red radicchio (a type of chicory) wastes (RR) ,were acquired from a farm located in the
Veneto region. In the Italian North-East horticulture, the production of red radicchio (Treviso
variety) is one of the leading cultivation. The RR production involves many steps, during which
several amounts of by-products are discarded and left to waste.
The maize silage (MS) was obtained from a silo after approximately six months of ensiling.
The samples were tested in order to study the efficiency of a local farm plant biogas.
All substrates were analyzed in order to determine the total and volatile solids (Standard
Methods, 2005) and the chemical oxygen demand (Raposo et al., 2008). The results of the
substrates characterization are presented in table 1.
2.2 Experimental set – up
The biomethanization tests were carried out in the home made equipment (Colussi et al.,
2014) here used in a single-stage arrangement. The anaerobic reactors are glass bottles, with 5 L
of volume each, placed in a controlled temperature environment (water bath) of 35 °C (± 0.1 °C)
and mixed continuously with magnetic stirrers to suspend the sludge solids. Pressure transducers
were connected to the bioreactors to outline the pressure changes during the test. The volumetric
method (acidic water displacement) was used to measure the biogas produced, with a composition
achieved by a gas analyzer. All the data were finally recorded by a PC.
As mentioned earlier, the results originated from trials carried out in different periods. The
duration of each experimentation (with single substrate) ranged from one month to two months
during which several feeds were done. Each new feed took place when no appreciable biogas
production was observed. A summary of the experimental feeds is reported in table 2.
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3. Results and discussion
31. Specific methane production
When organic material is degraded anaerobically, the end result is carbon in its most
oxidized form (CO2) and in its most reduced form (CH4) (Angelidaki and Sanders, 2004). If the
substrate composition is known, the theoretical methane yield potential can be obtained from the
Buswell’s equation (Buswell and Neave, 1930):
CaHbOc+(a−b4− c2 )H 2O❑
→ (a2+ b8− c4 )CH 4+( a2−b8+ c4 )CO2 (1)
Then the theoretical specific methane yield, usually expressed as CH4 volume per mass
volatile solids added or COD added, might be calculated as:
ThC H 4=( a2 + b
8− c4 )22.4
(a+ b4−a2 )32[STP LC H 4
gCOD ] (2)
ThC H 4=( a2 + b
8− c4 )22.4
12a+b+16 c [STP LC H 4
gVS ] (3)
where 22.4 (L) is the volume of 1 mole of gas at STP conditions and 32 (g·mol-1) is the molar mass
of O2.
Several factors usually lower the previous theoretical yield in actual anaerobic digesters
(Angelidaki and Senders, 2004), among which the un-degradability of lignin in anaerobic
conditions.
The theoretical methane yield of the tested substrates was calculated according to
experimental COD and chemical composition (taken from literature). The chemical oxygen demand
of each component was calculated based on the reaction of organic compound oxidation (Koch et
al., 2010):
CaHbOcN d+(a+ b4− c2−34d )O2❑
→aCO2+(b2−32 d )H2O+dN H3 (4)
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The anaerobic biodegradability of each substrate was calculated by dividing the theoretical
methane yield in COD units by the stoichiometric production of 0.350 LCH4·gCOD-1 at STP
conditions.
The results of calculations are summarized in tables 3 and 4.
As it can be seen from table 4, the anaerobic biodegradability of substrates containing lignin
is lower than 100%.
Theoretical methane yield was expressed both as COD units and VS units except for
brewery yeast. For this substrate, the correlation between COD and VS was impossible to
calculate, because the brewery yeast slurry is rich in alcohols (mainly ethanol) derived from
fermentation, which are likely to volatilize during solids determination. For this kind of wastes,
containing a significant proportion of highly volatile compounds, the organic content is represented
more accurately by COD (Nieto et al., 2012).
The average methane yield coefficient YCH4 was estimated plotting the final cumulative
methane production versus the added load (for each substance), as proposed by Raposo et al.
(2006): the slope of the line represents the requested YCH4 (figure 1).
Methane yield of apple waste was 0.284 LCH4 (STP)·gCODadd-1, i.e. 0.309 LCH4
(STP)·gVSadd-1, when expressed in VS units. This value is comparable with those found by Nieto et
al. (2012). Brewery spent grains revealed an average methane yield of 0.284 LCH4·gCOD-1
(expressing the substrate as COD) or 0.429 LCH4·gVS-1 (expressing the substrate as VS), in
agreement with data recounted by Lorenz et al. (2010). The average methane yield of brewery
yeast was of 0.255 LCH4·gCOD-1, a value difficult to compare with literature because previous
studies considered the biomethanization of BY mixed with wastewater (Neira and Jeison, 2010;
Zupančič et al., 2012). Maize silage methane yield was 0.218 LCH4·gCOD-1 (in COD units) or
0.327 LCH4·gVS-1 (in VS units), value that was within the range of methane yields typically found in
literature (Herrmann et al., 2011). Red radicchio gave a production of 0.313 LCH4·gCOD-1 or 0.403
LCH4·gVS-1.
The efficiency of digestion process was calculated comparing the actual specific
productions with the theoretical ones. Efficiency values resulted in 90.4 % for apple waste, 93,4%
for brewer’s spent grains, 78.3 % for brewer’s yeast, 83% for maize silage and 92.6 % for red
radicchio.
3.2 Disintegration and hydrolysis phase
The anaerobic digestion of a complex organic substrate is a non-linear bioprocess assumed
to pass several stages, starting from complex organic material to monomers to gaseous
compounds (Biernacki et al., 2013).
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The extracellular breakdown of complex organic substrates to soluble substrates is
expressed as disintegration and hydrolysis phase ((Biernacki et al., 2013). Several Authors agree
that this starting phase is the rate limiting step of the anaerobic degradation (Biernacki et al.,
2013).
Results from BMP tests can be used to obtain information on the disintegration/hydrolysis
rate (Angelidaki et al., 2009): in fact, when there is no accumulation of intermediary products,
methane production can be represented by a first-order kinetic for the hydrolysis of particulate
organic matter (Veeken and Hamelers, 1999).
The kh first-order hydrolysis rate (for each substrate) was evaluated using non-linear least
squares curve fitting on the net cumulative specific methane production (SMP):
SMP(t )=SMP0 ∙ (1−e(−kh ∙t ) ) (5)
where SMP(t) is the specific methane production (LCH4·gCOD-1) at time t at standard conditions
(STP) and SMP0 represents the theoretical specific methane yield above calculated. In literature
the same first-order rate equation was introduced, estimating the SMP0 value as the maximum
methane yield of the substrate (Veeken and Hamelers, 1999; Galì et al., 2009).
Figure 2 shows the comparison between experimental and simulated cumulative methane.
Estimated values of disintegration/hydrolysis rate constant are reported in table 5. The kh values,
ranging from 0.180 d-1 for brewery spent grains to 0.877 d-1 for apple waste, resulted of the same
order of magnitude of those reported in literature (Veeken and Hamelers, 1999; Vavilin et al. 2008).
4. Conclusions
The aim of this work was to increase the database concerning the biomethanization of
agro-industrial wastes and energy crops with regard to the Italian agriculture. Due to the
government subsidies, in the last years Italy has witnessed a proliferation of biogas energy plants,
management of which is not trivial, involving several scientific and technological aspects. The
studied substrates (coming from farms and food processing industries located in the North-East of
Italy) were subjected to BMP tests in order to calculate the methane yield of each waste material
and to estimate their first-order disintegration/hydrolysis rates. The obtained average methane
yield ranged from 0.218 LCH4·gCOD-1 for maize silage to 0.313 LCH4·gCOD-1 for red radicchio,
achieving more than the 80% of the theoretical production. The estimated kh values ranged from
0.180 d-1 for brewery spent grains to 0.877 d-1 for apple waste.
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Figure 1
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Figure 2
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Figure captions:
Fig. 1. Average methane yield estimation
Fig. 2. SMP (L-CH4·gCOD-1) profiles (* experimental; - simulated)
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Table 1
Substrates characterization
Substrate Total Solids[mg TS· g-1]
Volatile Solids[mg VS· g-1]
Total COD[mg COD· g-1]
Apple waste (AW) 147.1 135.2 174.0
Brewery spent grains (BSG) 187.0 180.5 276.2
Brewery yeast waste (BY) 158.9 147.8 341.5
Maize silage (MS) 333.0 325.0 493.0
Red chicory waste (RC) 53.0 47.0 65.0
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Table 2
Trials summary
Substrate Trials duration[d]
N. of feed
S/I ratio[gCOD·gCOD-1]
AW 60 7 0.03÷0.06
BSG 60 6 0.03÷0.08
BY 60 6 0.05÷0.08
MS 30 3 0.08
RR 30 4 0.02÷0.03
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Table 3
Theoretical oxygen demand (Th OD) and methane yield of typical substrate components
SubstrateComponent Composition
Th OD CH4 yield
[gO2·gVS-1] [STP LCH4·gCOD-1] [STP LCH4·gVS-1]
Carbohydrate (C6H10O5)n 1.19 0.350 0.415
Lignin C10.92H14.24O5.76 1.56 --- --
Protein C5H7O2N 1.42 0.350 0.496
Lipid C57H104O6 2.90 0.350 1.014
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Table 4
Theoretical methane yield (Th CH4 yield) of tested substrates
Parameter Units AW BSG BY MS RR
Carbohydrates % dry wt 75.04 (a) 26.5 (b) 32.86 (c) 66.30 (d) 53.33 (e)
Lignin % dry wt 13.16 (a) 22.7 (b) 0.00 (c) 11.60 (d) 0.00 (e)
Proteins % dry wt 3.02 (a) 12.3 (b) 56.03 (c) 10.30 (d) 23.33 (e)
Lipids % dry wt 3.31 (a) 25.0 (b) 3.44 (c) 5.10 (d) 1.67 (e)
Th. CH4 yieldSTP LCH4·gCOD-1 0.292 0.304 0.350 0.300 0.350
STP LCH4·gVS-1 0.388 0.458 - 0.405 0.452
Anaerobicbiodegradability % 83.4 86.9 100.0 85.7 100.0
(a) Galì et al., 2009; (b) Kanauchi et al., 2001; (c) Pacheco et al., 1997; (d) Biernacki et al., 2013;
(e) Bettio, 2008
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Table 5
Estimated disintegration/hydrolysis rate constants
AW BSG BY MS RR
kh [d-1] 0.877 0.180 0.400 0.185 0.496
18
