b i om a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 1 3 4 5e1 3 5 1

Avai lab le a t www.sc iencedi rec t .com

ht tp : / /www.e lsev ier . com/ loca te /b iombioe

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

* Corresponding author. Tel.: þ234 803538677E-mail addresses: yusuf_mom@yahoo.co

0961-9534/$ e see front matter ª 2010 Elsevdoi:10.1016/j.biombioe.2010.12.033

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 particulate nature, lignin, cellulose and hemicelluloses

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

9.m (M.O.L. Yusuf), ifynwaoier Ltd. All rights reserved

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

gazie@yahoo.com (N.L. Ify)..

Fig. 1 e Experimental set-up.

b i om a s s an d b i o e n e r g y 3 5 ( 2 0 1 1 ) 1 3 4 5e1 3 5 11346

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

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.

b i om a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 1 3 4 5e1 3 5 1 1347

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

Fig. 2 e Plot biogas yi

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)

eld against time.

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. 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.

b i om a s s an d b i o e n e r g y 3 5 ( 2 0 1 1 ) 1 3 4 5e1 3 5 11348

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.

Fig. 4 e Plot to estimate maximum biogas yield ( ym) in

Digester B.

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

Fig. 7 e Plot to estimate maximum biogas yield ( ym) in

Digester E.

Fig. 8 e Plot of proportion of ym against time.

b i om a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 1 3 4 5e1 3 5 1 1349

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,

Fig. 9 e Plot to determine the ultim

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

ate biogas yield attainable.

Fig. 10 e Plot of digester sizes against retention time.

b i om a s s an d b i o e n e r g y 3 5 ( 2 0 1 1 ) 1 3 4 5e1 3 5 11350

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 � 1

kt

�ym (17)

e

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

b i om a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 1 3 4 5e1 3 5 1 1351

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.

r e f e r e n c e s

[1] Veeken A, Kalyuzhnyi S, Scharff H, Hamelers B. Effect of pHand VFA on hydrolysis of organic solid waste. J Environ Eng2000;126(2):1076e81.

[2] Ghosh S, Klass DL. Two phase anaerobic digestion. ProcessBiochem 1978;13:15e24.

[3] Lin GY. Anaerobic digestion of landfill leachate. Water SA1991;17:301e6.

[4] Hobson PN. The kinetics of anaerobic digestion of farmwaste. J Chem Tech Biotechnol 1983;333:1e20.

[5] Henze M. Anaerobic wastewater, in wastewater treatment.Berlin, Germany: Springer Verlag; 1995.

[6] Chen YR, Hashimoto AG. Substrate utilization kinetic modelfor biological treatment process. Biotechnol Bioeng 1980;22:2081e95.

[7] Mahnert P. Kinetik der biogasproduktion aus nachwachsendenrohstoffen und g}ulle. Dissertation. Berlin Germany: Faculty ofagriculture and horticulture, Homboldt University; 2007.

[8] Linke B. Kinetic study of thermophilic anaerobic digestion ofsolid wastes from potato processing. Biomass Bioenergy;2006:892e6.

[9] Mahert P, Linke B. Kinetic study of biogas production fromenergy crops and animal waster slurry: effect of organicloading rate and reactor size. Environ Technol 2009;30(1):93e9.

[10] Ouedraogo A. Pilot scale two-phase anaerobic digestion ofthe biodegradable organic fraction of Bamako districtmunicipal solid waste. In: Mata-Alvarec J, Tilche A,Cecchi F, editors. II int sympo. Dig. solid water, held inBarcelona, June 15e17, 1999. Assoc Wat Qual, vol. 1; 1999.p. 65e74.

[11] APHA, AWWA, WPCE. Standard methods for theexamination of water and wastewater. 16th ed. Washington,DC: APHA; 1985.

[12] Itodo IN, Lucas EB, Kucha EI. The effect of media materialand its quality on biogas yield. Nigeria J Renew Energy 1992;3:45e9.

[13] Natural Research Council. Food, fuel and fertilizer fromorganic wastes. Washington, DC: National Academy Press;1981.

[14] Shoeb F, Singh, JH. Kinetic of biogas evolved from waterhyacinth. 2nd International Symposium on NewTechnologies for Environmental Monitoring and Agro-Application. Turkey; 2000.

[15] Owen JM, Chynoweth DP. Biochemical methane potential ofmunicipal solid waste components. Water Sci Tech 1993;27:1e14.

[16] Pavan P, Mata-Alvarez J. Performance of thermophilic semi-dry anaerobic digestion process changing the feedbiodegradability. In: Mata-Alvarec J, Tilche A, Cecchi F,editors. II Int. sympo. dig. solid water, held in Barcelona, June15e17, 1999. Assoc Wat Qual, vol. 1; 1999. p. 65e74.

[17] Myles RN. Practical guide to Janata biogas plant technology.In: AFPRO; 1985. New Delhi, India.

[18] Shiralipour A, Smith PH. Conversion of biomass intomethane gas. Biomass 1984;5:85e92.

[19] Igoni AH, Abowei MFN, Ayotamuno JM, Chibuogwo LE. Effectof total solids concentration of municipal solid waste inanaerobic batch digestion on the biogas produced. J FoodAgric Environ 2007;5(2):333e7.