Bioresource Technology 141 (2013) 177–183

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Bioresource Technology

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Feasibility of anaerobic digestion from bioethanol fermentation residue

0960-8524/$ - see front matter Crown Copyright � 2013 Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.biortech.2013.03.068

⇑ Corresponding author. Tel.: +82 41 589 8445; fax: +82 41 589 8580.E-mail address: jjyoon@kitech.re.kr (J.-J. Yoon).

Jeong-Hoon Park a,b, Sang-Hyoun Kim c, Hee-Deung Park b, Dong Jung Lim d, Jeong-Jun Yoon a,⇑a Green Materials Technology Center, Korea Institute of Industrial Technology (KITECH), 35-3 Hongcheon-ri, Ipjang-myeon, Cheonan, Chungnam 330-825, Republic of Koreab Department of Civil, Environmental and Architectural Engineering, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-714, Republic of Koreac Department of Environmental Engineering, Daegu University, Jillyang, Gyeongsan, Gyeongbuk 712-714, Republic of Koread Biolsystems Co. Ltd., Joong Pyung B/D 6F 64-1, Umyeon-dong, Seocho-gu, Seoul 137-900, Republic of Korea

h i g h l i g h t s

�Maximum CH4 conversion rate of fermentation residue is 84.8% at 5 g COD/L of residue.� Appropriate F/M ratio was significant on anaerobic digestion of fermentation residue.� Inhibitory effects on anaerobic digestion can be overcome by increasing cell.� Formic acid is strong inhibitor than levulinic acid on anaerobic digestion.

a r t i c l e i n f o

Article history:Available online 16 March 2013

Keywords:Red algaeAnaerobic digestionLevulinic acidFormic acidInhibition

a b s t r a c t

The focus of this study was the reuse of red algal ethanol fermentation residue as feedstock for anaerobicdigestion. Levulinic acid and formic acid, the dilute-acid hydrolysis byproducts, inhibited methanogene-sis at concentrations over 3.0 and 0.5 g/L, respectively. However, the inhibition was overcome by increas-ing inoculum concentration. A series of batch experiments with the fermentation residue showedincreased methane yield and productivity at higher inoculum concentration. The maximum methaneconversion rate of 84.8% was found at 5 g COD/L of fermentation residue at 0.25 g COD/g VSS of food-to-microorganism (F/M) ratio. The red algal ethanol fermentation residue can possibly be used as a feed-stock in anaerobic digestion at appropriate concentration and F/M ratio.

Crown Copyright � 2013 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Marine algal biomass has various advantages as a sustainablefeedstock for bioenergy production (Kim et al., 2010). Marine algaegrow faster and have greater carbon dioxide fixation ability thanland plants (Lüning and Pang, 2003; Packer, 2009). Also, it can becultivated easily without extra addition of nutrient or fertilizersuch as nitrogen source (Buck and Buchholz, 2004). Furthermore,the lignin-free chemical structure would allow obtaining sugarswithout complex and expensive pretreatment such as lignin re-moval (Levin et al., 2004; Mosier et al., 2005).

More than 70% of red algal biomass, however, remains as fer-mentation residue in ethanol production (Park et al., 2012b). Ifthe residue can be converted to methane by anaerobic digestion,ethanol could be supplied at lower prices. Furthermore, the netenergy yield of ethanol fermentation would be enhanced. Already,numbers of research studies have investigated the feasibility ofenergy recovery from ethanol fermentation residue by anaerobicmicrobes. When used as feedstock for methane fermentation,

the fermentation residue had a chemical oxygen demand (COD)of around 50 g COD/L (Hunter, 1988). In 1983–1985, Stover inves-tigated the anaerobic digestion from 64 g COD/L of corn stillagewith suspended-growth and fixed-film digester (Stover et al.,1983, 1984, 1985). In 2008, Schaefer used 100 g COD/L and 60 gVS/L of ethanol waste under thermophilic condition (Schaeferand Sung, 2008); however, most of these studies did not investi-gate in detail the effect of inhibition on anaerobic digestion.Inhibitory effect should be investigated because potential to de-crease COD level can circumvent the expensive costs associatedwith directly processing water high in COD concentrations.Reducing COD levels will lower the cost of treatment; therefore,pretreatment via anaerobic digestion is a possible solution toachieve this reduction. Furthermore, this method will not only re-duce cost but it can recover energy as well – further offsetting nettreatment expenses.

This study investigated the feasibility of the red algal ethanolfermentation residue as a feedstock for anaerobic digestion. The ef-fect of levulinic acid and formic acid as the potential inhibitors inthe residue was examined. In addition, the residue was used asthe feedstock for batch anaerobic digestion at various concentra-tions and food-to-microorganism (F/M) ratios.

178 J.-H. Park et al. / Bioresource Technology 141 (2013) 177–183

2. Methods

2.1. Inoculum sludge for anaerobic batch test

Granular sludge was taken from an anaerobic digester in a localbrewery wastewater treatment plant. The pH, total suspended sol-ids (TSS) and volatile suspended solids (VSS) concentration ofsludge were 7.1, 52.6 and 49.2 g/L, respectively. Solids were ana-lyzed by standard method from APHA (Eaton and Franson, 2005).

2.2. Red algal ethanol fermentation residue

Gelidium amansii obtained from a littoral area in Morocco wasused as the feedstock for ethanol production. The G. amansii com-position was as follows (%, dry base): cellulose (glucose) 14.9, gal-actose 23.1, 3,6 anhydrogalactose (3,6-AHG) 29.3, protein 15.6, ash5.7 and others 11.4 (Park et al., 2011). The red algal biomass waswashed using tap water to remove salt and milled to a size of<300 lm. Red algae powder was added with 1.0% (w/v) H2SO4 toa S/L ratio of 10% into a 30 L high-pressure reactor (depth; 50 cm,inner diameter; 28 cm). The slurry was pretreated at 150 �C and2.5 ± 0.3 kgf/cm2 for 15 min and neutralized using CaCO3 at a pHrange of 6.0. Subsequently, the hydrolyzate was obtained by centri-fuging at 3500 rpm for 10 min and used for the following ethanolfermentation (Park et al., 2012a). After the ethanol distillation pro-cess, fermentation residue was separated into liquid and yeast bycentrifuging at 8000 rpm for 10 min to obtain the fermentationresidue. The characteristics of the residue were as follows: COD,levulinic, and formic acid concentration were 74.0, 2.96, and0.99 g/L, respectively.

2.3. Anaerobic batch test

Acetic acid (Acros, USA) and propionic acid (Acros, USA) of 3 gCOD/L was used as the substrate of the feedstock in the inhibitionstudy. The examined range of levulinic acid and formic acid were0.5–5.0 g/L and 0.1–2.0 g/L, respectively. Two different levels ofinoculum (4.5 and 20 g VSS/L) were tested. The equilibriumamounts of the substrate, the potential inhibitor, and the inoculumwere added to a 160-mL serum bottle. The bottle was filled to100 mL using distilled water, purged for 3 min with nitrogen gas,sealed, and then incubated at 35 ± 1 �C and 150 rpm.

Methane production from fermentation residue was performedin batches and each test had a working volume of 100 mL in a160 mL serum bottle. The batch test was operated in an incubatorin anaerobic conditions with the temperature maintained at35 ± 1 �C and agitation at 150 rpm.

Sludge was filled up to 4.5 g/L. Subsequently, the serum bottlewas purged for 3 min with nitrogen gas at 1 L/min to remove air.The effect of levulinic acid and formic acid on methane fermenta-tion study was performed by using two different concentration(4.5 and 20 g/L) of granular sludge with a working volume of300 mL in 500 mL medium bottles.

The initial acetic acid and propionic acid concentration was3.0 g COD/L in all inhibitory batch tests using levulinic acid and for-mic acid. Other operation conditions such as temperature and rpmwere the same as the conditions used in the previous fermentation.

2.4. Analysis

The methane contents in the biogas production was measuredby gas chromatography (GC, Gow Mac series 580) using a thermalconductivity dectector (TCD) and a 1.8 m � 3.2 mm stainless-steelcolumn packed with porapak Q (80/100 mesh) with helium as acarrier gas. The temperatures of the injector, detector, and column

were kept at room temperature, 90 and 50 �C, respectively (Parket al., 2012b).

Methane correction was calculated as follows:

VCH4 ðSTPÞ ¼ VCH4 ðat 35 �CÞ � 273ð273þ 35Þ �

ð760� 42:2Þ760

ð1Þ

where, 42.2: water vapor pressure at 35 �C (mmHg).Levulinic acid, and formic acid were analyzed by high perfor-

mance liquid chromatography (HPLC, YL9100 series, Korea) usinga refractive index (RI) detector, an ultraviolet (UV) detector(210 nm), and a 300 mm � 7.8 mm Aminex HPX-87H (Bio-Rad,USA) ion exclusion column with H2SO4 of 5 mM as the mobilephase. The liquid samples were pretreated with a 0.45 lm mem-brane filter before injection to HPLC. The chemical oxygen demand(COD) and VS were measured according to Standard Methods (Ea-ton and Franson, 2005).

2.5. Assay

The methane production curve was fitted to a modified Gom-pertz equation (Lay et al., 1999) (2), which provides a suitablemodel for describing the methane production in batch tests:

M ¼ P � exp �expRm

P� ðk� tÞ � e

� �þ 1

� �ð2Þ

where M is the cumulative methane production (mL), P is the meth-ane production potential (mL), Rm is the maximum methane pro-duction rate (mL/day), k is the lag-phage time (day), t is time(day) and e is the exponential 1.

3. Results and discussion

3.1. Effect of levulinic acid and formic acid on methane fermentation

The main components of fermentation residue were protein andinhibitory substrates (Fujishima et al., 2000; Park et al., 2011). Su-gar was among the initial substrates, but it was completely fer-mented into ethanol. The chemical composition of G. amansii wasanalyzed in Section 2.2. Fermentation residue at high concentra-tions was shown to be a more efficient feedstock than at low con-centrations for anaerobic digestion in the view of energy balance(Fujishima et al., 2000; Van Velsen, 1979). However, increase ofthe concentration of fermentable residue raised the concentrationsof potential inhibitors. During the dilute-acid hydrolysis with highpressure and temperature, sugar is converted to 5-HMF, which isfurther degraded to levulinic acid and formic acid (Larsson et al.,1999). The inhibitory effect of the byproducts should be investi-gated to guarantee efficient and robust anaerobic digestion(Hashimoto, 1986; Koster and Lettinga, 1984, 1988). In a previousstudy, the effect of 5-HMF was investigated (Park et al., 2012b). Theresult of the study showed that concentrations of 5-HMF less than5 g/L resulted in methane production by anaerobic digestion; how-ever, concentrations above 5 g/L yielded no methane production. Inthese conditions, 5-HMF was generally removed by physicochem-ical method such as activated carbon adsorption (Chandel et al.,2011). On the other hand, levulinic acid and formic acid were noteasily removed by physical methods; even though, these inhibitorsare produced at a relatively lower concentrations than 5-HMF. Forexample, a previous study showed that fermentation residue werefound to have concentrations of levulinic acid and formic acid be-low 2.0 and 0.5 g/L, respectively. However, these inhibitors couldnot be easily removed by physical method. Therefore, biologicalmethod should be investigated to overcome inhibitory substratessuch as levulinic acid and formic acid. All batch experiments in thismanuscript were conducted in duplicate. As a result, experimental

Fig. 1. Cumulative methane production profile at various levulinic acid concentrations (high F/M ratio): (a) acetic acid 3 g COD/L with inoculum volume 4.5 g VSS/L and (b)propionic acid 3 g COD/L with inoculum volume 4.5 g VSS/L.

Fig. 2. Cumulative methane production profile at various levulinic acid concentrations (low F/M ratio): (a) acetic acid 3 g COD/L with inoculum volume 20 g VSS/L and (b)propionic acid 3 g COD/L with inoculum volume 20 g VSS/L.

Fig. 3. Cumulative methane production profile at various formic acid concentrations (high F/M ratio): (a) acetic acid 3 g COD/L with inoculum volume 4.5 g VSS/L and (b)propionic acid 3 g COD/L with inoculum volume 4.5 g VSS/L.

J.-H. Park et al. / Bioresource Technology 141 (2013) 177–183 179

values (Figs. 1–4) were very similar (error range <2%); therefore,these values were averaged, and error bars or standard deviationswere not included in figure.

Inoculum concentration (4.5 or 20 g VSS/L) produced differentmethanogenic events. Fig. 1 showed that levulinic acid at over3.0 g/L delayed the starting point of methane production to 4.5 gVSS/L. When levulinic acid is present methane production is moresensitive to propionic acid substrate than to acetic acid. Acetic acidlag phase time was almost 5 days at levulinic acid concentrationsof 5 g/L. However, propionic acid needed two times longer toproduce methane than did acetic acid at 4.5 g VSS/L. At levulinicacid concentration from 0.5 to 5 g/L, the lag phase time of methane

production showed differing results for acetic acid and propionicacid conditions. The acetic acid condition showed a lag phase timeof 1.69 days at 0.5 g/L and 5.05 days at 5 g/L concentration of levu-linic acid, demonstrating a threefold lag time increase. When pro-pionic acid was added as the substrate, lag phase time was2.54 days at 0.5 g/L and 10.12 days at 5.0 g/L, showing a fourfoldincrease in lag time. A summary of the statistical analysis resultsby Gompertz equation is presented in Table 1. Since propionic acidcontains much more carbon than does acetic acid, it needs moreenzymes or metabolic steps to it into convert biogas. However,raising the inoculum concentration to 20 g VSS/L clearly overcameinhibition from levulinic acid up to 5 g/L (Fig. 2). At these

Fig. 4. Cumulative methane production profile at various formic acid concentrations (low F/M ratio): (a) acetic acid 3 g COD/L with inoculum volume 20 g VSS/L and (b)propionic acid 3 g COD/L with inoculum volume 20 g VSS/L.

Table 1The characteristics of methane production at various concentrations of levulinic acid with 4.5 g VSS/L of inoculum by the modified Gompertz equation.

Parameters Acetic acid (3 g COD/L) Propionic acid (3 g COD/L)

Levulinic acid (g/L)

Control 0.5 1.0 2.0 3.0 5.0 Control 0.5 1.0 2.0 3.0 5.0

Pa (mL) 354.4 418.4 528.9 619.5 697.7 875.8 350.54 443.2 545.1 611.2 734.5 635.5Rm

b (mL/day) 68.27 59.71 48.90 45.13 43.97 43.75 49.43 45.45 43.47 60.55 60.80 44.32kc (day) 1.74 1.69 1.85 1.93 2.60 5.05 2.16 2.54 3.45 4.63 5.34 10.1R2d 0.9937 0.9900 0.9973 0.9938 0.9960 0.9932 0.9960 0.9973 0.9932 0.9925 0.9840 0.9921

a P, ultimate methane production potential.b Rm, maximum methane production rate.c k, lag phase time.d R2, the coefficient of determination.

Table 2The characteristics of methane production at various concentrations of levulinic acid with 20.0 g VSS/L of inoculum by the modified Gompertz equation.

Parameters Acetic acid (3 g COD/L) Propionic acid (3 g COD/L)

Levulinic acid (g/L)

Control 0.5 1.0 2.0 3.0 5.0 Control 0.5 1.0 2.0 3.0 5.0

Pa (mL) 298.4 353.2 359.3 447.2 551.0 791.6 358.5 399.3 491.5 822.8 786.0 911.1Rm

b (mL/day) 114.4 108.5 111.2 90.54 56.89 51.36 119.3 97.93 90.59 69.37 60.65 49.37kc (day) N/De N/D N/D N/D N/D N/D 0.62 0.58 0.47 0.62 1.18 1.19R2d 0.9368 0.9049 0.9382 0.8870 0.9244 0.9440 0.9951 0.9951 0.9964 0.9936 0.9882 0.9853

a P, ultimate methane production potential.b Rm, maximum methane production rate.c k, lag phase time.d R2, the coefficient of determination.e N/D, not detected.

Table 3The characteristics of methane production at various concentration of formic acid with 4.5 g VSS/L of inoculum by the modified Gompertz equation.

Parameters Acetic acid (3 g COD/L) Propionic acid (3 g COD/L)

Formic acid (g/L)

Control 0.1 0.2 0.5 1.0 2.0 Control 0.1 0.2 0.5 1.0 2.0

Pa (mL) 343.2 349.8 354.1 358.7 393.2 388.1 319.2 319.0 334.8 343.3 477.0 416.3Rm

b (mL/day) 110.3 107.9 78.31 53.34 32.27 49.36 58.91 58.75 57.49 41.51 16.12 15.74kc (day) 0.36 0.21 0.36 1.16 1.34 9.77 1.61 1.38 1.34 1.49 6.81 14.82R2d 0.9837 0.9802 0.9914 0.9980 0.9968 0.9974 0.9963 0.9968 0.9964 0.9969 0.9851 0.9955

a P, ultimate methane production potential.b Rm, maximum methane production rate.c k, lag phase time.d R2, the coefficient of determination.

180 J.-H. Park et al. / Bioresource Technology 141 (2013) 177–183

conditions, methane was instantly produced without an adaptionperiod. When the inoculum is sufficient, methane production is

not significantly inhibited by levulinic acid of concentrations rang-ing up to 5.0 g/L. In addition, methane production was higher than

Table 4The characteristics of methane production at various concentration of formic acid with 20.0 g VSS/L of inoculum by the modified Gompertz equation.

Parameters Acetic acid (3 g COD/L) Propionic acid (3 g COD/L)

Formic acid (g/L)

Control 0.1 0.2 0.5 1.0 2.0 Control 0.1 0.2 0.5 1.0 2.0

Pa (mL) 301.6 324.8 321.6 336.7 406.0 445.9 325.3 323.5 332.9 346.0 400.6 438.4Rm

b (mL/day) 220.0 241.1 217.0 187.3 142.5 65.76 126.5 122.8 125.6 126.0 117.5 85.82kc (day) 0.2803 0.0877 0.1248 N/Dd N/D N/D 0.3097 0.3079 0.2500 0.2307 0.0327 N/DR2e 0.9471 0.9273 0.9335 0.9349 0.9546 0.9811 0.9732 0.9740 0.9740 0.9804 0.0789 0.9848

a P, ultimate methane production potential.b Rm, maximum methane production rate.c k, lag phase time.d N/D, not detected.e R2, the coefficient of determination.

J.-H. Park et al. / Bioresource Technology 141 (2013) 177–183 181

in the control of all levulinic acid containing experiments (Table 2).This implies that levulinic acid in concentrations up to 5 g/L couldbe used as an energy source for methane fermentation.

Formic acid has higher inhibition potential than 5-HMF and lev-ulinic acid. Although formic acid was produced in smaller propor-tion than other inhibitors, it was the strongest inhibitor of methanefermentation when dissolved at the same concentration as theother inhibitors. As shown in Fig. 3, only low concentration(0.5 g/L) of formic acid could retard methane production. The lagphase time sharply increased over 0.5 g/L of formic acid. The

Fig. 5. Statistical analysis of cumulative methane production by Gompertz equation: (methane production.

investigation of the effect of formic acid on methane fermentationhad almost the same trend as the previous levulinic experiments.Propionic acid resolution was more sensitive than acetic acid reso-lution to the inhibition of formic acid (Fig. 3). At 2.0 g/L of formicacid concentration, the lag phase time between acetic acid and pro-pionic acid were 9.77 days and 14.82 days, respectively (Table 3).With the increase of inoculum concentration from 4.5 to 20 gVSS/L, the lag phase time sharply decreased (Fig. 4). In addition,the maximum methane production rate increased 1.33-fold from49.36 mL/day at 4.5 g VSS/L of inoculum to 65.76 mL/day at

A) effect of levulinic acid on methane production and (B) effect of formic acid on

Fig. 6. Methane conversion rate from various concentration of fermentationresidue at 4.5 and 20 g VSS/L of inoculum concentration.

182 J.-H. Park et al. / Bioresource Technology 141 (2013) 177–183

20.0 g VSS/L when acetic acid substrate was used with formic acidinhibitor (at concentration of 2.0 g/L). In the case of propionic acidsubstrate used in the same conditions, maximum methane produc-tion rate was 15.74 at 4.5 g VSS/L and 85.82 at 20 g VSS/L – a 5.43-fold increase. In the presence of levulinic acid concentration lowerthan 5.0 g/L or formic acid at concentrations lower than 2.0 g/L, theincrease of inoculum did not affect methane production (Table 4).The cumulative methane production data was analyzed using theGompertz equation to quantify the methane yield, the methaneproductivity and the lag-phase time at each experimental condi-tion (Tables 1–4). Then, the effects of the inhibitors were assayedwith the value found at the modeling (Fig. 5). The cumulativemethane production had no specific correlation to F/M ratio, butthe case of maximum methane production rate and lag-phase timewas different results. At inhibitory levels, methane production ratesignificantly increased with decreasing F/M ratio for levulinic acidconcentrations up to 5.0 g/L and for formic acid concentrations upto 2.0 g/L. With the decrease of F/M ratio, the lag phase time alsosharply decreased. This result signifies that methane accumulatedrapidly even in the presence of inhibitor as F/M ratio decreased.

3.2. Methane production from fermentation residue of red algalbioethanol

In previous study, Park’s research group strongly emphasizedthat anaerobic digestion of fermentation residue would have enor-mous energy potential. Energy production by anaerobic digestionis 2.24-fold higher than the energy produced by the main ethanolproduction process (Park et al., 2012b). Fig. 6 shows the relativepercentage of methane conversion, which is calculated based onthe theoretical maximum, according to fermentation residue con-centration. Methane production from various concentrations (5–30 g/L) of ethanol fermentation residue were investigated to studythe effect of inhibitor concentration on methane production at 4.5and 20 g VSS/L. First, 4.5 g VSS/L of inoculum was used as seed inthis experiment. Increasing fermentation residue concentrationgradually attenuated methane yield. Maximum methane conver-sion rate was observed as 84.8% at 5 g COD/L of fermentation res-idue. When fermentation residue concentration exceeded 5 g COD/L, methane yield gradually decreased. We investigated what ac-counted for this decrease in methane yield; our initial hypothesiswas excessive food to microorganism ratio or inhibitory substrate.However, levulinic acid and formic acid did not reach critical level(3.0 and 0.5 g/L) until 30 g/L of fermentation residue. At that resi-due concentration, levulinic acid and formic acid concentrationwere 1.2 and 0.4 g/L, respectively. The F/M ratio was a more signif-icant factor affecting methane production than inhibitor concen-tration was in anaerobic digestion. This study was focused onmethanogene resistance or the feasibility of methane production

in the presence of the inhibitory compounds. Therefore, low F/Mratio (0.25 g COD/g VSS) was chosen. There are several previous re-ports that used F/M ratios around 0.25 (Montalvo et al., 2012;Stroot et al., 2001).

4. Conclusions

Sustainable treatment and reuse of fermentation residue isessential to warrant red algal bioethanol use as a promising biofueltechnology. This study showed that the inhibitory effects of formicacid and levulinic acid on anaerobic digestion can be prevented byincreasing cell biomass concentration. The maximum methaneconversion rate of 84.8% was found at 5 g COD/L of fermentationresidue and at 0.25 g COD/g VSS of food-to-microorganism (F/M)ratio. Anaerobic digestion of the red algal ethanol fermentationresidue has the potential to enhance net energy yield of biofuelproduction as well as to alleviate water pollution.

Acknowledgements

This work was supported by a Grant (09-FN-1-0014) from Kor-ea Institute of Energy Technology Evaluation and Planning, Minis-try of Knowledge Economy, Republic of Korea and a Grant (JA-12-0001) from Korea Institute of Industrial Technology, Republic ofKorea, and Basic Science Research Program through the NationalResearch Foundation of Korea (NRF) funded by the Ministry of Edu-cation, Science and Technology (MOST) (2011-0014666).

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