the effect of waste paper on the kinetics of biogas production
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the effect of increasing waste paper on biogas production from the co-digestion of water hyacinth and cow dung was studied in this work.TRANSCRIPT
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The effect of waste paper on the kinetics of biogas yield fromthe co-digestion of cow dung and water hyacinth
Momoh O.L. Yusuf*, Nwaogazie L. Ify
University Of Port Harcourt, Dept. of Civil & Environmental Engineering, PMB 5323, Choba, Rivers State, Nigeria
a r t i c l e i n f o
Article history:
Received 26 August 2009
Received in revised form
6 December 2010
Accepted 21 December 2010
Available online 12 January 2011
Keywords:
Anaerobic
Biogas yield
Ultimate methane yield
First order kinetics
Waste paper
a b s t r a c t
The effect of waste paper on biogas yield produced by co-digesting fixed amount of cow
dung and water hyacinth in five digesters AeE was studied at room temperature. Waste
paper was observed to improve biogas yield in digesters BeE with digester A acting as the
control. However, as the amount of waste paper increased the biogas yield was observed to
decrease. Kinetic model based on first order kinetic was derived to estimate the maximum,
ultimate, biogas yield and also the ultimate methane yield from these biomass mixtures.
The maximum biogas yield estimated using this model for digesters BeE were 0.282, 0.262,
0.233, and 0.217 lg�1 VS fed with goodness of fit (R2) of 0.995, 0.99, 0.889, and 0.925
respectively, which were obtained by fitting the experimental biogas yield ( yt) against (exp
(kt)�1)/exp(kt). The ultimate biogas and methane yield at very low batch solid load were
extrapolated to be 0.34 and 0.204 lg�1 VS fed respectively. In essence, the addition of waste
paper in the co-digestion of cow dung and water hyacinth can be a feasible means of
improving biogas yield and also alternative means of recycling waste paper. Furthermore,
the kinetic model developed can compliment other models used in anaerobic digestion of
agricultural and solid waste.
ª 2010 Elsevier Ltd. All rights reserved.
1. Introduction
The anaerobic digestion of solid waste has the potential of not
only treating solid waste, but also generating useful bio-fuel
which can be used for diverse purposes like cooking, powering
internal combustion engines, etc. The process of biogas
generation has been established to comprise four major pha-
ses that include hydrolysis, acidogenesis, acetogenesis and
methanogenesis. The hydrolysis phase involves the conver-
sion of complex organics into sugars; acideogenesis involves
the conversion of these sugars into organic acids; acetogenesis
involves the conversion of these organic acids into acetic acid.
Finally, the conversion of acetic acid intomethane and carbon
dioxide consists the methanogenic phase [1].
A number of factors can affect the reaction process leading
to the ultimate formation ofmethane and carbon dioxide. The
particulate nature, lignin, cellulose and hemicelluloses
content of biomass may affect the overall reaction kinetics
leading to biogas formation. Other factors that may affect the
biogas yield include, low pH due to accumulation of by-prod-
ucts formed during biodegradation, temperature and loading
rate. Knowledge about the biodegradability of biomass
employed in anaerobic digestion can be useful in selecting
suitable biomass for anaerobic process.
Many authors have developed kinetic models to describe
the biodegradability of organic material in order to charac-
terize the biodegradability process. Authors [2e6] have
employed models to study biodegradability of organic mate-
rials inanaerobicdigestion.However, thesemodelswerebased
on maximum specific growth rate of bacteria and required
short retention time which may not be applicable to energy
biomass [7]. Hence, models that describe the process of biogas
* Corresponding author. Tel.: þ234 8035386779.E-mail addresses: [email protected] (M.O.L. Yusuf), [email protected] (N.L. Ify).
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production from biomass are essentially absent. Recently,
works of authors [8,9] showed that a simple model can be
developedbasedon thefirst orderkinetics to relate biogasyield
and loading rate for completely mixed stirred reactors.
In this work, a similar approach was used to develop
simple kinetic model based on first order kinetics for deter-
mining biogas yield and the maximum biogas yield attainable
for co-digested substrates at room temperature in batch
reactors. Some specific feature of the batch process such as its
simple design and process control, robustness toward coarse
and heavy contaminants and lower investment cost canmake
them particularly attractive for developing countries [10].
2. Materials and method
The materials used for this experiment were cow dung, waste
paper, andwater hyacinth. Pre-treatment operations involved
weighing about 500 g of freshly harvested water hyacinth and
allowing it to sun-dry for a period of 30 days, after which they
were oven-dried. This oven-dried water hyacinth was then
ground to fine particles using a grindingmill. Similar operation
was applied to the waste paper. Standard methods were used
for waste paper and water hyacinth measurements [11] with
respect to the total and volatile solids. The cow dung was sun
dried foraperiodof 20days topreserve itsmicrobialpopulation
and then crushedmechanically using a mortar and pestle.
2.1. Preparation of digesters
Asetoffivebatch reactorswereusedasdigesters. Eachdigester
contained fixed amount of cow dung and water hyacinth, but
an increasing amount of waste paper. These digesters were
labeled A, B, C, D, and E, respectively. The digester labeled, had
nowaste paper, 5 g of cowdung and 5 g ofwater hyacinth. This
digester acted as the control.Wastepaper is biodegradable and
readily available in the environment. Compositions of other
batch reactor digesters BeE contain waste paper in increasing
order as described below. The digester material was made of
glass material of 500 mL capacity,
(i) Digester-B consisted of 4 g of waste paper, 5 g of cow dung
and 5 g of water hyacinth.
(ii) Digester-C consisted of 8 g of waste paper, 5 g of cow dung
and 5 g of water hyacinth.
(iii) Digester-D consisted of 12 g of waste paper, 5 g of cow
dung and 5 g of water hyacinth.
(iv) Digester-E consisted of 20 g of waste paper, 5 g of cow
dung and 5 g of water hyacinth.
The volatile solids of the biomasses were determined
before digestion commenced according to APHA [11] using
a muffle furnace-Carbolite model LMF 4 manufactured in
England. These biomasses were weighed using a weighing
balance Mettler model PN163, manufactured in Switzerland
with specification range between 0.1 mg and 160 g. The
biomasses were mixed with 250 mL of water respectively and
then corked to exclude air. Subsequent connections were as
depicted in Fig. 1. The digesters content were allowed to
ferment for a period of 62 days and agitated twice daily, the
morning and evening hours, respectively. After digestion, the
volatile solid content of the digested slurry was determined
according to APHA [11]. Ambient temperature measurements
were determined using a thermometer. The pH of the digester
mixture was determined before and after experiment using
pH meter PN 209. Biogas measurement was carried out using
water displacement method [12].
3. Results and discussion
The data collected for pH values determined before and after
experiment, total solids composition and corresponding
biogas produced in each digester are presented in Table 1. It
was observed that the pH before experiment commenced lie
within the optimum range for biogas production that is
6.6e7.6 [13]. After the experiment, pH values were observed to
increase slightly which is consistent with work of Shoeb et al.
[14]. The average temperature for the period of study was
observed to be 26 �C.The plots of the biogas yieldwith time for the digesters AeE
are presented in Fig. 2. Digester Awith nowaste paper had the
lowest biogas yield. The reduced biogas yield obtained here
could be attributed to the composition of biomass undergoing
degradation. Cow dung and water hyacinth are known to
contain cellulose and hemicelluloses which are not easily
susceptible to biodegradation. However, the addition of water
paper to this biomass led to improvement in biogas yield. In
Digester B, biogas yield progressed almost in a linear manner
indicative of and efficient conversion process at work. This
digester composition seemed just suitable for co-digestion
purposesbecausebeyondthis amountofwastepaperallocated
toDigesterB, thebiogasyieldwasobserved to generally decline
as shown in digesters C, D and E. The improved biogas yield in
the digesters containing waste paper can be attributed to the
pre-treated nature of waster paper (physical and chemically)
during its manufacture that makes cellulose present in waste
paper easily susceptible to biodegradation [15].
3.1. Kinetic model development
The first order rate equation can provide an empirical
approach in studying the biodegradability of organic material
by observing changes in the volatile solids influent and
effluent concentration. Table 1 showed the first order kinetic
constants and the corresponding biogas production for the
five digesters. There exists some degree of closeness in the
first order kinetic constant obtained in digesters B, C, D and E
which contained certain amount of waste paper as opposed to
what was obtained in digester A. Despite the high kinetic
constant of Digester A, the biogas yieldwas small in relation to
Fig. 1 e Experimental set-up.
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other digester with waste paper. It does mean therefore, that
the addition of waste paper can improve biogas yield by
modifying composition of the biomass mixtures. Pavan and
Mata-Alvarez [16] reported that the composition of waste can
affect biogas yield. Thus, the biomass composition of digesters
B, C, D, and E could be considered similar due to the presence
of certain amount of waste paper.
Because it has been established that waste paper increased
biogas yield in digesters B, C, D and E, subsequent kinetic
studies were limited to these four digesters. The estimation of
the maximum biogas yield and the ultimate biogas yield
attainable using these biomass types was established in this
study. The maximum biogas yield ( ym) is the biogas yield
obtainable if biomass is allowed to undergo biodegradation for
very long period of time in batch reactors (Fig. 3), while the
ultimate biogas yield ( yL) is the maximum biogas yield
equivalent to the ultimate anaerobic biodegradability that
results at total solid loading or organic loading rate very close
to zero [8]. Though, it’s possible to determine these values
experimental through long period of anaerobic digestion, it
may also be possible to estimate these values through curve
fitting as developed here.
The development of the model describing biogas produc-
tion process in batch reactorswith volume (VR) by co-digesting
cow dung and water hyacinth with waste paper was based on
mass balance approach by observing changes in the volatile
solids concentration (C ) i.e.,
VRdCdt
¼ QoCo � QoCþ VRrC (1)
However for a batch system flow of input (Qo) ¼ 0, (where Co
and C are the influent and effluent volatile solids) so that the
equation can be written as
VRdCdt
¼ VRrC (2)
where rC is the substrate removal rate as a function of (C ). At
any time (t) the first order ratemodel can bewritten as (3) with
first order kinetic constant (k) i.e,
dCdt
¼ �kC (3)
This equation can be written in the analytical form as,
lnCo
Ct¼ kt: (4)
This equation generally relates to substrate (biomass) biode-
gradabilitywithnoinformationaboutthebiogasyield.However,
a correlation between substrate biodegradability and biogas
yield at any time ( yt) can be developed assuming all substrate
(biomass) are converted into biogas as shown in Fig. 3 [8],
although, in reality all substratemaynot be converted tobiogas.
From the correlation it can be deduced that,
Co � Ct
Co¼ yt
ym(5)
and
Co
Ct¼ ym
ym � yt(6)
Substituting Co/Ct in equation (4) with ym/( ym � yt) we
obtained,
ln
�ym
ym � yt
�¼ kt (7)
This can be rearranged to obtain,
yt ¼ ym
�1� e�kt
�(8)
Table 1 e Digester characteristics.
Digester Totalsolids (%)
Volatile solids (g) in250 mL water
pHb pHa Cumulativebiogas (L)
k (day�1)
A 3.846 7.49 7.18 7.96 0.320 0.00795
B 5.303 10.91 6.81 7.57 0.720 0.00434
C 6.716 14.53 6.71 7.35 0.842 0.00401
D 8.088 17.79 6.69 7.40 1.052 0.00390
E 10.70 24.67 6.41 7.35 1.110 0.00336
a After experiment.
b Before experiment.
Fig. 2 e Plot biogas yield against time.
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This equation can be used to estimate maximum biogas
yield if experimental data on the daily biogas yield and first
order kinetic constant are available. In essence a plot yt against
(1 � e�kt) would produce a linear curve fit with slope ym.
Sometimes, studies on anaerobic degradation may require
that certain percentage or proportion ( p) of the maximum
biogas yield at any time (t) be determined, especially when the
time for maximum biogas yield (tmax) requires long period of
biodegradation. In such cases, the time to attain certain
percentage or proportion of the biogas yield for example 80%
(0.8) or 90% (0.9) of maximum biogas yield can be of great
importance for design purposes.
In order to obtain a model that can predict the time for
certain percentage or proportion ( p) of the maximum biogas
yield, Equation (7) was employed.
By letting yt ¼ pym we obtain,
ln
�ym
ym � pym
�¼ kt (9)
This reduced to,
ln
�1
1� p
�¼ kt (10)
So that
p ¼ 1� e�kt (11)
Thus, by plotting ( p) against different time (t) for known
value of k, would provide charts which can be employed for
estimating the time at which certain percentage or proportion
of themaximumbiogas yield canbeobtained inbatch reactors.
3.2. Application of kinetic model in the estimation ofmaximum biogas yield
The estimation of the maximum biogas yield in digesters B, C,
D and E employing Equation (8) are presented in Figs. 4e7. The
model seemed to follow a linear curve fit as expected. The
maximum biogas yield of 0.282, 0.262, 0.233, and 0.217 lg�1 VS
fed were obtained for digesters B, C, D, and E with goodness of
fit 0.995, 0.99, 0.889 and 0.925 respectively. Thus, it can be
inferred that as waste paper increased in the mixture, the
maximum biogas yield decreased. Linke [8] obtained similar
decrease in the maximum biogas yield as organic loading rate
dleiysagoi
B
etartsbuS
Co
Co
C(t)
yt
- Ct
ym - yt ym
t time (t)
Fig. 3 e Substrate transformation into biogas during
anaerobic degradation.
Fig. 4 e Plot to estimate maximum biogas yield ( ym) in
Digester B.
Fig. 5 e Plot to estimate maximum biogas yield ( ym) in
Digester C.
Fig. 6 e Plot to estimate maximum biogas yield ( ym) in
Digester D.
Fig. 7 e Plot to estimate maximum biogas yield ( ym) in
Digester E.
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increased from anaerobic digestion of solid waste from potato
processing, though for a continuous stirred tank reactor.
The kinetic model is also significant in determining the
time required to obtain certain proportion or percentage of the
maximum biogas yield. The application of Equation (11) was
used to obtain Fig. 8. It was observed that to obtain 50% or 0.5
of the maximum biogas yield for the digesters B, C, D and E
required160, 175, 180 and 205 days retention time respectively,
under room temperature condition. Hence performance of
digester can be assessed through this approach.
3.3. Application of kinetic model in the estimation ofultimate biogas ( yL) yield
The ultimate biogas yield which is the maximum biogas yield
obtainable at solid loading very close to zero was determined
by plotting the total solids (%) and/or waste paper addition
(% of total solids) against their corresponding maximum
biogas yield observed in the various digesters (Fig. 9).
The relationship between the percent of total solids fed
into the reactor and the maximum biogas yield for reactor B,
C, D and E could be described by Equation (12) with a goodness
of fit 0.973.
y ¼ �76:50xþ 26:71 (12)
where y represents percent total solids fed in the digesters and
x represent the corresponding maximum biogas yield. The
ultimate biogas yield was estimated by assuming total solids
of 0.5% which is close to zero. Substituting into Equation (12)
yields a value of x that is 0.3426 lg�1 VS.
Similarly, the relationship between the waste paper as
percent of total solid fed into the digester and the maximum
biogas yield for digester B, C, D and E can be described by the
Equation (13) with goodness of fit 0.936.
y ¼ �547:2xþ 184:5 (13)
where y represents waste paper as percent of total solids fed
and x represents maximum biogas yield. Again, by assuming
0.5%waste paper (% total solids), the ultimate biogas yieldwas
estimated to be 0.336 lg�1 VS. In essence, the ultimate biogas
yield attainable from these biomass mixture comprising
cow dung, water hyacinth and waste paper in the manner
Fig. 8 e Plot of proportion of ym against time.
Fig. 9 e Plot to determine the ultimate biogas yield attainable.
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described here would approximately be 0.34 lg�1 VS fed into
the digester.
Though the methane composition of biogas was not deter-
mined in this research, a basic assumption is that biogas
comprises 60%methane and 40% carbon dioxide [17]. Based on
this assumption theultimatemethaneyield attainable by these
biomass mixtures is 0.204 lg�1 VS fed. This value is so close to
the biochemicalmethane potential of related biomass reported
in literature [15,18]. The biochemical methane potential assay
relies on very low substrate loading to ensure that the batch
digester does not suffer imbalance through products inhibition
[15]. An average ultimate methane yield of 0.22 lg�1 VS fed for
freshwateraquaticsand0.24 lg�1VS fed for forage/grasseshave
been reported [18], while [15] estimated the ultimate methane
yield forwaste paper to range between 0.28 and 0.37 lg�1 VS fed.
Thus, the ultimate methane yield at low total solids concen-
tration extrapolated here through this approach may as well
correspond to theultimatemethane yield obtained through the
typical biochemical methane potential assay approach
employed by researchers in biogas studies.
3.4. Application of kinetic model in batch reactor design
Most models for reactor design are based on bacteria growth
e.g. Monod, Contois etc. However, design approaches based on
the yield of product are few with works of authors [8]
contributing to the list of available works in this area. A sim-
ilar approach based on yield of products has been employed in
the design and sizing of batch reactors for anaerobic digestion.
A ratio of 1:3 was used by [19] to establish a relationship
between the volumeof the gas chamber (which is proportional
to the volume of biogas produced) and the volume of the
anaerobic digester.Hence for a givenmassof volatile solids (m)
fed into the digester, the following deductions can be obtained.
Vgc ¼ 13Vdigester (14)
Rearranging,
3Vgc ¼ Vdigester (15)
But
Vgc ¼ yt$m (16)
and
yt ¼�ekt � 1ekt
�ym (17)
Substituting Equation (17) into (16)
Vgc ¼�ekt � 1ekt
�ym$m (18)
rThus,
Vdigester ¼ 3
�ekt � 1ekt
�ym$m (19)
Hence, for a given value for k and ym, the volume of the
batch reactor can be estimated for any retention time
required. For example the plot of Vdigester against various
retention times for digesters B, C, D and E (Fig. 10) showed that
2.2, 2.4, 2.7 and 3 L reactors would be required to contain the
reaction process and biogas produced for retention period of
62 days respectively. Similar, capacity in cubic meter is
possible depending on the solid loading.
This means, that 0.73, 0.8, 0.9 and 1.0 L of biogas was
producedbydigesterB,C,DandErespectively.Theseestimated
values approximate reasonably with the volume of biogas
obtained experimentally (Table 1). A safety factor between 1.05
and 1.2 may be used for final correction when designing batch
reactor for these biomass types at room temperature.
4. Conclusion
The anaerobic biodegradation of biomasses comprising cow
dung, water hyacinth and waste paper is feasible at room
temperature. The addition of waste paper to fixed amount of
cow dung andwater hyacinth was observed to improve biogas
production. However, biogas yield was observed to decrease
with increase in waste paper concentration. The ultimate
biogas yield which can be determined from very long periods
of anaerobic batch reaction was alternatively estimated
through curve fitting. Themaximum biogas yield for digesters
B, C, D and E was estimated to be 0.282, 0.262, 0.233 and
0.2176 lg�1 VS fed respectively, while the ultimate biogas and
methane yield attainable from thesemixtures were estimated
to be 0.34 and 0.204 lg�1 VS fed respectively, at 0.5% total solids
concentration in which waste paper comprised 0.5% of the
total solids. This would correspond to 5 g each of cow dung
Fig. 10 e Plot of digester sizes against retention time.
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and water hyacinth co-digested with 0.05 g of waste paper in
2 L of water or inoculums solution. The result obtained
through thesemethods of curve fitting could help compliment
biochemical methane potential assay.
In addition, the kinetic model was employed in batch
reactor design for given value of k and ym. The batch reactor
volume required was estimated to be 2.2, 2.4, 2.7 and 3 L for
digesters B, C, D and E respectively for retention period of 62
days. Again, the use of first order kinetics and maximum
biogas yield in reactor design may compliment other design
approach available in literature.
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