Chemical Papers 64 (2) 127–131 (2010)DOI: 10.2478/s11696-009-0108-5

ORIGINAL PAPER

Co-digestion of agricultural and industrial wastes‡

Anna Kacprzak*, Liliana Krzystek, Stanis�law Ledakowicz

Department of Bioprocess Engineering, Faculty of Process and Environmental Engineering,

Technical University of Lodz, Wolczanska Str. 213, 90-924 Lodz, Poland

Received 19 May 2009; Revised 24 September 2009; Accepted 30 September 2009

The efficiency of anaerobic digestion process is dependent on the type and composition of thematerial to be digested. This work examines the co-digestion of corn silage, beet pulp silage, carrotresidues, and cheese whey in different configurations together with a glycerin fraction - the wasteproduct of transestrification of oils (biodiesel production) in a 25 L bioreactor operated mesophicallyin a quasi-continuous mode. Co-digestion of corn silage with carrot residues appeared to be moreeffective than that with cheese whey resulting in the gas production rate equal to 5.9 L L−1 d−1

and 1.4 L L−1 d−1, respectively. The performed experiments showed that a combination of threesubstrates: corn silage, cheese whey, and glycerin fraction resulted in the highest methane contentequal to 61 % and the biogas production rate of 1.8 L L−1 d−1.c© 2009 Institute of Chemistry, Slovak Academy of Sciences

Keywords: biogas, biomass, waste glycerin utilization, co-digestion, sugar beet pulp, corn silage,carrot residues, cheese whey

Introduction

There is high energy potential in both unutilizedagricultural and industrial residues. Biogas productionfrom agricultural biomass is of growing importance asit offers considerable environmental benefits such asenergy savings, recycling of nutrients within agricul-ture and reduced CO2 emissions. Anaerobic digestiontechnology has evolved quickly and, at present, cancompete with aerobic systems, especially for treatingindustrial wastewater and organic solid wastes withhigh organic loading (Fernández et al., 2005).Many kinds of organic waste have been efficiently

digested anaerobically, such as sewage sludge, indus-trial waste, slaughterhouse waste, fruit and vegetablewaste, manure, and agricultural biomass. The wasteshave been treated both separately and in co-digestionprocesses (Yen & Brune, 2007).Co-digestion is a technology increasingly applied

for simultaneous treatment of different solid and liq-uid organic wastes (Bouallagui et al., 2009). It im-

proves biogas yields due to the positive synergism es-tablished in the digestion medium and the supply ofmissing nutrients by the co-substrates. Sometimes theuse of a co-substrate can also help to establish therequired moisture contents of the digester feed (Mata-Alvarez, 2000). Several studies have shown that mul-ticomponent mixtures of agro-wastes and industrialwastes can be digested successfully; although withsome mixtures, a degree of both synergism and an-tagonism occurred (Alvarez & Lidén, 2008; Ma etal., 2008; Bouallagui et al., 2009). In the agriculturalsector, a possible solution to process crop biomassis its co-digestion with animal manures, the largestagricultural waste stream (Lehtomäki et al., 2007).Co-digestion of agro-industrial residues with animalmanures has been reported previously (Callaghan etal., 2002; Kaparaju & Rintala, 2003), with particu-lar interest being shown in the co-digestion of ani-mal manures with whey (Gelegenis et al., 2007; Ghaly,1996), corn silage (Amon et al., 2007), or sugar beets(Umetsu et al., 2006). However, there is a little pub-

*Corresponding author, e-mail: annakacprzak81@tlen.pl‡Presented at the 36th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare,25–29 May 2009.

128 A. Kacprzak et al./Chemical Papers 64 (2) 127–131 (2010)

Table 1. Characteristics of substrates used in the experiments

Parameter Corn silage Beet pulp Carrot Whey Glycerin

pH 3.7 4.0 – 4.48 7.23Total solids [%] 27.5 19.93 14.03 6.1 11Volatile solids [%] 26.14 18.87 5.06 5.26 7.2COD [g L−1]a 376 – – 66.7 –Ash [%] 1.36 10.63 8.97 0.84 3.8VFA [mg L−1]b 1950 – – 2220 1296C to N mole ratio 60 : 1 20 : 1 30 : 1 21 : 1 25 : 1

a) g of oxygen per liter; b) mg of acetic acid per liter.

lished data on the co-digestion of energy crops withindustrial waste alone.Sugar beet pulp, a by-product of the sugar beet

industry, is produced annually in large quantities. Inthe EU alone, about 108 tons of roots per year areprocessed (Spagnuolo et al., 1997). Sugar beet pulpcan be an important renewable resource and its bio-conversion appears to be of great biotechnological im-portance (Hutnan et al., 2000). There is only very lit-tle available information on sugar beet pulp anaerobicdigestion and methane production. One of the refer-ences was found in the paper of Lane (1984), wherea 10 L continuously stirred tank was implemented.Pilot-scale anaerobic digestion of beet pulp was alsopresented by Weiland (2003).Several possibilities for whey exploitation have

been assayed over the last 50 years. Nevertheless,cheese producing units usually do not proceed withinvestments for the recovery of valuable constituents(like casein) contained in whey and so approximatelyhalf of the world whey production is not treated, butdiscarded as waste effluent (Gelegenis et al., 2007).Although anaerobic treatment of cheese whey for bio-gas production has been reported by several authors(Ghaly & Pyke, 1991; Lo & Liao, 1986; Yan et al.,1989), low biogas productivity and methane yield havebeen associated with the low pH of whey. Gelegeniset al. (2007) examined co-digestion of whey with di-luted poultry manure and concluded that whey wasquantitatively degraded to biogas when co-digestedwith diluted poultry manure; chemical oxygen demand(COD) removal increased from 70 % to 77 % whenwhey was added to the manure.Previous studies have shown, in lab-scale reactors,

that co-digestion of whey with other substrates can beadvantageous, optimization of the co-digestion processhas been hardly attempted until now. In this study,optimization of co-digested mixtures of different sub-strates: cheese whey, corn silage, beet pulp, carrotresidues, and glycerine fraction was investigated in or-der to obtain maximum productivity of biogas withhigh methane content.

Experimental

Five experiments were conducted in this study.They all were carried out in a bioreactor with the

working volume of 25 L, operated mesophically insemi-continuous mode at the hydraulic retention time(HRT) of 25 days. Temperature was maintained at37◦C by a thermostating system. Inoculum (sludge af-ter anaerobic digestion) came from the anaerobic di-gestion chambers of the Municipal Wastewater Treat-ment Plant in Lodz, Poland. The mixture of substratewith water was fed manually to the digester once a dayafter the withdrawal of the same amount of fermenta-tion broth. Feed influent was prepared daily.Experiment 1 consisted in fermentation of corn

silage alone. In experiment 2 and 3, co-digestion pro-cesses were conducted. They were performed in orderto compare the effect of cheese whey or carrot residuesaddition to corn silage on biogas yield. The fourthand fifth experiments consisted in co-fermentation ofthree substrates (corn silage or sugar beet pulp, whey,glycerin fraction). The feeding mode was determinedbased on the previous experiments. On the first day,all three substrates were fed in specified mass ratios tothe bioreactor (organic loading rate OLR = 3.02 g d−1

with corn silage and OLR = 3.66 g d−1 with beetpulp); on the next two days, only glycerin fraction(OLR = 1.43 g d−1) was being added. This three-dayfeeding cycle was repeated until the highest yield ofbiogas and methane content together with a steadystate were obtained. Characteristics of all substratesare shown in Table 1.Mixed samples were drawn from the bioreactor

daily to determine: total suspended solids (TSS),volatile suspended solids (VSS), volatile fatty acids(steam distillation – BUCHI B-324), elemental con-tent (C, N) of inoculum and individual experimen-tal feeds (Elemental Analyzer NA 2500, CE Instru-ments), chemical oxygen demand (COD) on cen-trifuged samples (Hach-Lange, method 435). Biogasflow rate (flowmeter Ritter) and pH (pH-meter elec-trode WTW pH 540 GLP) were measured continu-ously together with the biogas content (gas contentanalyzer LMS GAS DATA). All analytical procedureswere performed in accordance with Standard Methods(APHA – AWWA, 1992).

Results and discussion

To optimize the co-digestion process as well as tocompare co-digestion of different substrates (cheese

A. Kacprzak et al./Chemical Papers 64 (2) 127–131 (2010) 129

52 56 60 64 68 72 760

1

2

3

4

5

6

Time [d]

GP

R [L

L-1 d

-1]

44

46

48

50

52

54

56

Bio

gas

com

posi

tion

[mol

e %

]

Fig. 1. Gas production rate ( ), methane (�), and carbon diox-ide (•) content in biogas produced in experiment 1.

30 40 50 60 700

5

10

15

20

25

44

48

52

56

60

GP

R [L

L-1 d

-1]

Time [d]

Met

hane

con

tent

[mol

e %

]

Fig. 2. Gas production rate (full symbols) and methane con-tent (opened symbols) in experiments 1 (squares), 2(circles), and 3 (triangles).

whey, corn silage, carrot residues, beet pulp, and glyc-erine fraction) combinations, and to obtain maximumproductivity of biogas with high methane content, fiveexperiments were performed.In the first experiment, corn silage was digested

alone (OLR = 3.2 g d−1). In Fig. 1, changes of the gasproduction rate (GPR), methane and carbon dioxidecontent in steady state (measured parameters were notchanging more than 10 %) are presented. Gas produc-tion rate (GPR) was expressed as the so-called spacetime yield: gas volume to reactor volume per time inL L−1 per day.Under steady state conditions (from the 57th day),

GPR was in the range of 3.5–3.6 L L−1 d−1. Methanecontent of the digester biogas varied between 51.3 %and 53.9 %, and that of carbon dioxide was in therange from 47.3 % to 48.6 % (Fig. 1). The mean resultsof all experiments corresponding to the steady stateperiods are presented in Table 2.In the second experiment, the addition of a sec-

ond substrate was considered. In this case it was thecheese whey (OLR = 3.6 g d−1). The co-digestion pro-cess was performed. The biogas production rate at thebeginning of this experiment was in the range of 0.5–1.1 L L−1 d−1 and the methane content was less than

Table 2. Operational parameters for all experiments understeady state

Experiment No.

1 2 3 4 5

HRT [d] 25 25 25 25 25Operation days 57 29 44 33 35GPR [L L−1 d−1]a 3.4 1.4 5.9 1.8 2.2Methane content [%]a 53.1 60.1 53.8 61 52

a) Mean value.

0

200

400

600

800

1000

1200

0 10 20 30 40 50

30

40

50

60

Time [d]

Met

hane

con

tent

[mol

e %

]

1

2

3

4

5

GP

R [L

L-1 d

-1]

VF

A [m

g L-1

]

Fig. 3. Gas production rate (�), volatile fatty acids concentra-tion (•), and methane content ( ) in experiment 4.

60 % (data not presented). Under steady state con-ditions, from the 29th day, the mean value of GPRwas 1.4 L L−1 d−1 while the methane content of thedigester biogas varied between 56 % and 62 %, withan average of 60.1 % (Fig. 2).In the third experiment, co-digestion of corn silage

and carrot residues (OLR = 3.11 g d−1) was per-formed. Under steady state conditions (from the 44thday), GPR was in the range of 5.7–5.8 L L−1 d−1.Methane content of the digester biogas in steady statevaried between 51.08 % and 53.59 % (Fig. 2). Com-paring the results of experiments 1 and 3, a successfulco-digestion process can be observed. The addition ofa co-substrate (carrot residue) improved the biogasproduction rate by 75 %.In experiment 4, all three substrates (cheese whey,

corn silage, and glycerin fraction) were fed to thebioreactor (OLR = 3.02 g d−1) in specified mass ra-tio; on the next two days, only the glycerin fraction(OLR = 1.43 g d−1) was being added. The biogas pro-duction rate at the beginning of experiment 4 was inthe range of 1.0–1.5 L L−1 d−1. During the next daysit was steadily increasing reaching 1.6–2.2 L L−1 d−1

(Fig. 3). On each third day, when all three substrateswere fed to the digester, GPR was considerably higherin comparison to GPR with only the glycerin fractionbeing fed.In experiment 5, beet pulp, cheese whey and glyc-

erin fraction were fed to the bioreactor in specified

130 A. Kacprzak et al./Chemical Papers 64 (2) 127–131 (2010)

20 25 30 35 40 45 5030

40

50

60

70

0

1

2

3

4

5

6M

etha

ne c

onte

nt [m

ole

%]

Time [d]

GP

R [L

L-1 d

-1]

Fig. 4. Gas production rate (full symbols) and methane con-tent (opened symbols) in experiments 4 (squares) and5 (circles).

20 25 30 35 40 45 5020

30

40

50

60

0

2

4

6

8

10

12

200

400

600

800

1000

1200

1400

Met

hane

con

tent

[mol

e %

]

Time [d]

GP

R [L

L-1 d

-1]

VF

A [m

g L-1

]

Fig. 5. Gas production rate (circles), methane content (tri-angles), and volatile fatty acids (VFA) concentration(squares) in experiments 2 (full symbols) and 4 (openedsymbols).

mass ratio (OLR = 3.66 g d−1); on the next two days,again only the glycerin fraction (OLR = 1.43 g d−1)was being added. Under steady state conditions (fromthe 35th day), GPR was in the range of 1.9–2.7 L L−1

d−1. Methane content of the digester biogas varied be-tween 51.03–52.9 %. Here, analogously to experiment4, on each third day, when all three substrates werefed to the digester, GPR was considerably higher incomparison to GPR with only thee glycerin fractionbeing fed.Comparing experiments 4 and 5, a difference in

the co-digestion of corn silage and sugar beet pulpwith two other co-substrates (cheese whey and glyc-erin fraction) can be observed. Co-digestion with sugarbeet pulp resulted in higher gas production rate, upto 2.2 L L−1 d−1; however, the methane content waslower by approximately 10 %.Comparing experiments 2 and 4, it can be con-

cluded that the addition of the third co-substrate(glycerin fraction) to the silage and cheese whey mix-ture and the change of the bioreactor feeding modelowered the VFA concentrations from 780–1200 mg ofacetic acid per L to 400–920 mg of acetic acid per L(Fig. 5). In experiment 4, concentration of VFA was

less than 1000 mg of acetic acid per L, indicating highstability of the operation conditions, which was con-firmed by a steady pH profile. Co-digestion of threesubstrates appeared to be more effective. Addition ofa glycerin fraction improved both the gas productionrate and the methane content (Fig. 5).

Conclusions

Five co-digestion experiments with different sub-strate combinations were conducted in order to ob-tain maximum productivity of methane rich biogas.Anaerobic digestion of corn silage alone in experiment1 resulted in GPR equal to 3.4 L L−1 d−1 and in bio-gas with the methane content of 51 %. The addition ofa second substrate (cheese whey or carrot residue) tothe corn silage changed the biogas productivity andthe methane content. Experiments 2 and 3 showedthat carrot residue is a better co-substrate for cornsilage than cheese whey, resulting in GPR equal to5.9 L L−1 d−1. The methane content was reported toremain on a similar level in the range from 51 % to53.8 % in all tree experiments.Additionally, as co-digestion of corn silage with

cheese whey appeared not to be very effective, theaddition of a third substrate (glycerin fraction) in ex-periment 4 improved the biogas productivity as well asthe methane content. The combination of these threesubstrates allowed obtaining 61 % of methane rich bio-gas.Experiment 5 showed that beet pulp in combina-

tion with cheese whey and glycerin fraction resulted ina higher GPR than corn silage; however, the methanecontent was lower by 10 %.

Acknowledgements. The work was supported by the grantNo. PBZ-MNiSW-1/3/2006 founded by the Ministry of Science& Higher Education, Poland.

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