mesophilic anaerobic digestion of corn thin stillage: a technical and energetic assessment of the...

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Research ArticleReceived: 20 February 2011 Revised: 25 April 2011 Accepted: 2 May 2011 Published online in Wiley Online Library: 8 June 2011

(wileyonlinelibrary.com) DOI 10.1002/jctb.2664

Mesophilic anaerobic digestion of corn thinstillage: a technical and energetic assessmentof the corn-to-ethanol industry integratedwith anaerobic digestionPo-Heng Lee,a Jaeho Bae,a Jeonghwan Kima and Wen-Hsing Chenb∗

Abstract

BACKGROUND: The purpose of this study was to reduce the VS (volatile solid) and recover energy (methane) from thin stillagethrough mesophilic anaerobic digestion in corn–ethanol plants. The performance of a continuously stirred tank reactor (CSTR)with different hydraulic retention times (HRTs) was evaluated in this study.

RESULTS: The results show no differences in volatile solid (VS) destruction (82–83%) in the reactor with HRTs ranging from 25to 40 days. The maximum volumetric methane production rate of 1.41 L L−1 day−1 was produced at 25-day HRT, whereas themaximum methane yield of approximately 0.63 L CH4 g−1 VSfed (0.77 L g−1 VSremoved) was achieved with HRTs between 30 and40 days. Simulation results using a kinetic model indicate that the reactor needs to be operated for longer than 23 days in orderto achieve 80% of maximum methane yield. The techno-economic potential of a corn–ethanol facility to produce an estimated57% energy recovery using mesophilic anaerobic digestion has long been overlooked. A corn–ethanol plant integrated withmesophilic anaerobic digestion increases the net energy balance ratio from 1.26 to 1.80.

CONCLUSION: Mesophilic anaerobic digestion complements the corn–ethanol business so that the sustainable energy obtainedfrom corn recovery is made more lucrative and renewable.c© 2011 Society of Chemical Industry

Keywords: anaerobic digestion; methane; thin stillage; ethanol; volatile solid

INTRODUCTIONThe USA is importing petroleum fuel from overseas to satisfyover 50% of the nation’s energy demand. In recent years,concerns about national security, environmental consequencesand economics have become the major driving forces for theUSA to explore alternative energy sources. Ethanol, which is arenewable bio-fuel from corn, and is easy to transport and store,has been considered as an alternative fuel to petroleum oil. Thesteps in ethanol production include hydrolysis, saccharification,fermentation, and distillation and dehydration.1 Ethanol can befermented from sugar-based2,3 or starch-based feedstocks.4,5 Inthe USA, however, corn is a major abundant agricultural product sothat it is more readily available than all other feedstocks combined6

for producing ethanol.Corn whole stillage is the organic residue after ethanol is

distillated from the fermented corn mixture. In a traditionalprocess, the whole stillage is centrifuged to separate the liquidfraction, or thin stillage, from the solid fraction, or the wet distillers’grains (WDG). The thin stillage is usually condensed in an energy-intensive evaporator to form thick, viscous syrup. The syrup isthen added back to WDG to yield DDGS (distillers’ dried grainswith solubles) (DDGS) which is used as an animal feed. Hill et al.7

reported that the total energy input to produce DDGS was 16.86%,and the profit of the DDGS to the total revenue was 16.87%.

Hence, condensing the thin stillage is non-profitable; it can onlybe regarded as a final disposal method.

Anaerobic digestion of the thin stillage is an alternativeapproach to recover more energy from the corn1 in additionto treating the stillage. Recently published research results onthe treatment of corn thin stillage show a maximum 90% volatilesolids (VS) reduction; effluent volatile fatty acids (VFAs) reducedbelow 200 mg L−1 (as acetic acid) were achieved with a continuousstirred-tank reactor (CSTR) operated at thermophilic temperature(55 ◦C) with a 20-day hydraulic retention time (HRT).8 In that study,using the methane recovered from the anaerobic digester in a corn-derived ethanol production plant contributed to an estimated 43%to 59% reduction of natural gas consumption. The savings wouldamount to $7 to 17 million for an ethanol production plant with aproduction capacity of 360 million L year−1. In another study,9 thethin stillage was treated in a thermophilic anaerobic sequencingbatch reactor (ASBR) to remove 90% TCOD (total chemical oxygen

∗ Correspondence to: Wen-Hsing Chen, Department of EnvironmentalEngineering, National ILan University, I-Lan 260, Taiwan.E-mail: [email protected]

a Department of Environmental Engineering, Inha University, Republic of Korea

b Department of Environmental Engineering, National ILan University, Taiwan

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Mesophilic anaerobic digestion of corn thin stillage www.soci.org

demand), 89% VS, and to produce 0.25 L CH4 g−1 TCODadded with a10-day HRT. The mass and energy balances resolved by Agler et al.9

indicate that methane from the thermophilic digestion processrendered approximately 50.7% of natural gas consumption. Thiswas consistent with the observation made by Pfeffer et al.10 thatover 76% of the energy demand of the bioethanol fermentationprocess (without DDGS production) could be supplied by methanethrough anaerobic digestion. Anaerobic corn stillage digestionthus provides an attractive option for enhancing the net energygain in the existing corn-to-ethanol business.

As noted previously, effective treatment of the thin stillagefrom corn grain feedstock can be achieved by thermophilicanaerobic digestion. However, thermophilic anaerobic digestionhas several disadvantages, including producing effluent withVFAs concentrations11 and being highly sensitive to cations12

when compared with mesophilic anaerobic treatment. This mayimply that using mesophilic digestion is more advantageous thanthermophilic digestion when treating thin stillage. Stover et al.13,14

found that mesophilic digestion operated with 20-day SRT (sludgeretention time) could remove 96% soluble COD (SCOD) from adiluted influent TCOD of 26 g L−1. Ganapathi15 also demonstratedsuccessful treatment of a diluted soluble portion of corn thinstillage in a lab-scale CSTR. However, mesophilic anaerobicdigestion of corn thin stillage is not as widely implementedin industry as other industrial-agricultural processes, possiblybecause of the lack of appropriate economic analyses, whichleaves in doubt the commercial optimization of the corn ethanolindustry. Therefore, this study aimed to stabilize corn thin stillagethrough anaerobic digestion in a mesophilic CSTR. The reactorwas operated at different HRTs (20, 25, 30, and 40 days) todetermine the optimum operating conditions for the mesophilicdigestion process. In addition, the study applied an establishedkinetic model to simulate the methane yield performance so thatlaboratory results could be generalized for future field applications.Thereafter, a fair comparison of the holistic economic examinationbetween mesophilic and thermophilic conditions was performedto examine the energy balance and net energy ratio of these twoprocesses.

MATERIALS AND METHODSPreparation of sludge inoculation and substrateThe sludge inoculum was obtained from the mesophilic anaerobicdigester that receiving cattle waste daily from Alto Dairy (Waupun,Wisconsin). The corn thin stillage used as the feedstock in thisstudy was collected from the Lincolnway Energy (LE) ethanolplant, Nevada, Iowa. It contained 65.9 to 79.4 g L−1 total solids(TS), 57.4 to 70.7 g L−1 VS and 105.0 to 131.0 g L−1 TCOD. Thethin stillage sample was collected once a week, and was storedin a refrigerator at 4 ◦C to avoid degradation. The feed solutionwas prepared by adding 1 mL of the trace element solution8 toevery 20 g TCOD of thin stillage. The stock trace element solutionconsisted of the following ingredients (mg L−1): AlCl3·6H2O (90);CaCl2·2H2O (38); CoCl2·6H2O (2000); ethylenediamine tetraaceticacid (1000); FeCl3·4H2O (10 000); HCl (1.0); H3BO3 (50); MnCl2·4H2O(500); Na2SeO3 (123); (NH4)Mo7O24·6H2O (50); NiCl2·6H2O (142);Resazurin (200); and ZnCl2 (50).16

Reactor operationA completely mixed reactor with a working volume of 18 L holding6 L of seeding sludge and 12 L of degassed tap water was operated

under mesophilic condition (35 ± 1 ◦C). Methane gas was bubbledthrough the reactor content to purge oxygen prior to sealingthe reactor to maintain anaerobic condition in the reactor. Thereactor was operated in a semi-continuous mode with intermittentmixing (10 min in every 30 min). During the initial 3-day start-upperiod, the HRT of the reactor was maintained at 50 days, itwas then reduced to 40 days. The HRT would be subsequentlydecreased only when the reactor attained quasi-steady-statecondition (presumed after a minimum of three volume turnoversand less than 5% variation in biogas production on 5 consecutivedays of operation). The reactor was subject to two feed cycles perday at HRTs of 40 and 30 days, and four feed cycles per day atHRTs of 25 and 20 days. The decant cycle under complete mixingwas performed once per day at each HRT. The pH, VFAs, biogascomposition, and biogas production were monitored periodically.

Analytical methodsVFAs, pH, TS, VS, alkalinity (ALK), COD, and ammonium-nitrogenwere measured according to Standard Methods.17 The biogasproduction from the CSTR was recorded on a daily basis by usinga wet test gas meter (Schlumberger, Houston, TX). The biogascomposition was analyzed with a gas chromatograph (Gow-Macseries 350, Pittsburgh, PA) equipped with a thermal conductivitydetector. The column was a 2.43 m by 0.64 cm SS 350B Hayesep DB80/100 tubing, and the operational temperatures of the injectionport, oven, and detector were 150, 50, and 100 ◦C, respectively.Helium was used as the carrier gas at a flow rate of 140 mL min−1.

RESULTS AND DISCUSSIONPerformance of mesophilic CSTRTable 1 lists the data on the performance of the mesophilic CSTR atdifferent HRTs; the effluent VS concentrations at HRTs of 40, 30, 25,and 20 days were 10.8, 10.1, 10.9, and 34.9 g L−1, respectively. Thecorresponding VS degradations were 83%, 82%, 83%, and 51% atHRTs of 40, 30, 25, and 20 days, respectively. Meanwhile, the TCODremoval efficiencies with HRTs of 40, 30, 25, and 20 days were 85%,86%, 84%, and 57%, respectively. The mesophilic CSTR operatedwith HRT of 20 days is apparently not capable of significantlyreducing VS and TCOD. This observation clearly demonstratesthat mesophilic anaerobic digestion, if operated with HRT =20 days, is not effective in treating thin stillage with an organicloading rate (OLR) of 3.5 g VS L−1 day−1. Similar results werealso observed for biogas production. Maximum methane yieldsof approximately 0.63 L g−1 VSfed and 0.77 L g−1 VSremoved wereobserved at HRTs of 30 and 40 days, respectively. The methaneyield is noted to be reduced significantly to 0.12 L g−1 VSfed

or 0.25 L g−1 VSremoved for HRT = 20 days. This reduction isalso seen in the volumetric methane production rate and thebiogas production rate. However, both the volumetric methaneand the biogas production rates achieved maximum levels of1.41 L L−1 day−1 and 43 L day−1, respectively, at HRT = 25 days.Thus, the mesophilic CSTR treats corn thin stillage most efficientlywith a HRT of 25 days.

Figure 1 illustrates concentrations of VFAs and ALK, and theratios of VFAs to ALK for the mesophilic CSTR reactor operated atdifferent HRTs. VFAs concentrations varied slightly between 250and 330 mg L−1 as acetic acid from the 25-day HRT to the 40-dayHRT, whereas ALK concentrations remained relatively consistentbetween 7 000 and 8 700 mg L−1 as CaCO3. For HRT = 20 days,VFAs concentration dramatically increased to 3 400 mg L−1 as

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www.soci.org P-H Lee et al.

Table 1. Performance of mesophilic anaerobic digestion of corn thin stillage

HRTs (day)/organic loading rate (g VS L−1 day−1) 40/1.6 30/1.9 25/2.5 20/3.5

TCOD removal efficiency (%) 85 ± 1 86 ± 2 84 ± 2 57 ± 11

Effluent VS (mg L−1) 10, 800 ± 500 10, 100 ± 300 10, 900 ± 300 34, 900 ± 5, 300

VS destruction (%) 83 ± 0 82 ± 1 83 ± 1 51 ± 8

NH+4-N (mg L−1 as N) 2, 000 ± 120 1, 800 ± 100 1, 500 ± 70 1, 400 ± 70

Methane yield (L g−1 VSfed) 0.62 ± 0.05 0.63 ± 0.07 0.56 ± 0.04 0.12 ± 0.10

Methane yield (L g−1 VSremoved) 0.75 ± 0.06 0.77 ± 0.06 0.68 ± 0.05 0.25 ± 0.15

Biogas production rate (L day−1) 29 ± 2 35 ± 1 43 ± 1 21 ± 16

Volumetric methane production rate (L L−1 day−1) 0.93 ± 0.09 1.18 ± 0.05 1.41 ± 0.05 0.45 ± 0.35

Mean ± standard deviation (n ≥ 3).

Hydraulic Retention Time, Days

40 25

Effl

uent

VF

A o

r A

lkal

inity

, mg/

L

0

2000

4000

6000

8000

10000

VF

A :

Alk

Rat

io

0.0

0.2

0.4

0.6

0.8VFAAlkalinityVFA : Alk

30 20

Figure 1. Effluent VFAs concentration, alkalinity concentration, and VFAsto ALK ratios at different HRTs.

acetic acid, whereas ALK concentration decreased to 5 000 mg L−1

as CaCO3. Alkalinity is a pivotal parameter for stable operationof anaerobic digestion. Its consumption is accompanied byaccumulation of VFAs that lead to rising ratios of VFAs to ALK.Hence, the VFAs to ALK ratio is recognized as an indicator ofstable anaerobic digestion. Sung and Santha18 reported thatthis ratio should be less than 0.35 in the thermophilic reactor,and less than 0.10 in the mesophilic reactor to ensure that thereactors favor temperature-phased anaerobic digestion (TPAD).Experimental data published in the literature also reveals that aratio of VFAs to ALK greater than 0.4 causes a decrease in CODremoval from approximately 94% to 57% in a two-stage anaerobicdigestion process.19 In this present study, a ratio of 0.7 for the20-day HRT resulted in unstable anaerobic digestion. Moreover,the accumulation of VFAs to 3 400 mg L−1 as acetic acid may causefeedback inhibition of methanogens, and thus further deterioratemesophilic anaerobic digestion. At HRT = 20 days (Table 1), theammonium concentration of 1 400 mg L−1 (as N) was observedthat showed no profoundly inhibitory effect on methanogenesis.20

Figure 2, which shows the variations of pH and biogascomposition at different HRTs, reveals that pH values wereconsistent at approximately 7.5 for HRT = 20–40 days. It sharplydecreased to approximately 6.0 at HRT = 20 days, mainly dueto the increase in VFAs concentration.21 The poor performanceat HRT = 20 days is also evidenced by changes in biogascomposition. The methane content remained near 60% for HRT= 25–40 days, however, it drastically decreased to 38% at HRT= 20 days. Meanwhile, the carbon dioxide content increasedfrom 36% at HRT = 25 days to 56% at HRT = 20 days, and the

Hydraulic Retention Time, Days

40 25

pH

5

6

7

8

9

Bio

gas

Per

cent

age

(%)

0

20

40

60

80pHCH4CO2

2030

Figure 2. Biogas compositions and pHs at different HRTs.

biogas production rate decreased to 21 L day−1 at the 20-dayHRT (Table 1). The decline of the biogas production rate coupledwith the increase of the carbon dioxide content reflects the factthat acidogenesis prevails over methanogenesis at this stage. Thisunbalance between acidogenesis and methanogenesis causesthe accumulation of VFAs during the anaerobic digestion.21,22 Asbeen discussed earlier, the accumulation of VFAs consumes morealkalinity to decrease the pH level.

Kinetic analysisThe kinetic equations developed by Bernd23 have been used todetermine the maximum biogas yield (L g−1 VS), the ratio ofbiogas yield to the maximum biogas yield (%), and the substratedegradation constant at different OLRs or HRTs for thermophilicanaerobic digestion of potato-processing solid wastes. Theseequations were applied to mesophilic anaerobic digestion ofcorn thin stillage in this study. The equation for determining themaximum methane yield is

M = C0 · k · Mm

OLR + k · C0(1)

where M = methane yield at different hydraulic retention times,L CH4 g−1 VSremoved; Mm is the maximum methane yield, LCH4 g−1 VSremoved; k is the substrate degradation constant, day−1;and C0 is influent VS concentration of corn thin stillage, g L−1. Inthis study, the results of methane yield at different OLRs shown inFig. 3 reveal that the methane yield was negatively correlated with

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1.2

1.0

0.8

0.6

0.4

0.2

0.03.02.52.01.51.00.50.0

Organic loading rate (g VS/L/day)

Met

hane

yie

ld (

L/g

VS

rem

oved

)

Figure 3. Linear regression analysis of organic loading rate versus methaneyield (the intercept, ym = 0.74 L g−1 VS).

12

10

8

6

4

2

00.70.60.50.4

1/Organic loading rate (L-day/g VS)

M/(

Mm

-M)

Figure 4. Linear regression analysis of the reciprocal of organic loading rateversus M/(Mm − M) (Slope: k · C0 = 10.683 g L−1 day−1, C0 = 59.5 g L−1,k = 0.180 day−1).

OLR. Based on the linear regression of Equation (1), the value of Mm

was found to be 0.74 L CH4/g VSremoved, which is then substitutedinto Equation (2) to calculate the substrate degradation constantk.

M

Mm − M= k · C0

OLR(2)

giving k = 0.180 day−1 at 35 ◦C. The ratios of M to Mm − M versusthe reciprocal of OLR and the fitting results are shown in Fig. 4.

Based on Bernd’s model (Equation (1)), the ratio of methaneyield to the maximum methane yield (R) at different HRTs can beexpressed as

R = HRTs · k

HRTs · k + 1(3)

The correlation between R and HRT at k = 0.180 day−1 isthen plotted in Fig. 5. As is seen in the figure, the mesophilicanaerobic digester needs to be operated at HRTs of 23 and 50 daysin order for the reactor to produce 80% and 90%, respectively,of the maximum methane yield. At the 20-day HRT, 78% of themaximum methane yield was projected based on Equation (3).Contrary to the simulated result, the mesophilic CSTR failed tofunction properly with a 20-day HRT, presumably because of the

1.0

0.8

0.6

0.4

0.2

0.0120100806040200

35°C55°C

HRT (days)

Rat

ios

(R)

Figure 5. Variation of ratios (R) of methane yield to the maximummethane yield with different HRTs at 35 ◦C (k = 0.180 day−1) and 55 ◦C(k = 0.267 day−1).

high levels of VFAs in the thin stillage.8 These high levels of VFAs inthe feed solution to the CSTR operated at such a short HRT resultedin VFAs accumulation and subsequently methane yield reduction.This explanation is consistent with the report by Demirel andYenigun24. However, the influence of VFAs in the corn thin stillagehas not been defined in the models so that the model prediction isdifferent from the laboratory results. In general, the model resultsshow good correlation with the observed data between 25- and40-day HRTs, and hence, the model can be a valuable tool for theup-scale design.

Results obtained during 2003 to 2010 on the anaerobic digestiontreatment of corn thin stillage and other types of feedstocks indifferent system configurations are summarized in Table 2. The VSreduction of corn thin stillage was generally greater than othertypes of stillage. The maximum VS destruction (89.0%) of cornthin stillage treated in a thermophilic anaerobic sequencing batchreactor (ASBR) was achieved with a 10-day HRT.9 In the ASBR, theloss of biomass would be minimal at all HRTs because the biomassis allowed to settle down before the mixed liquor is withdrawn.Thereby, a longer sludge retention time (SRT) could be attained inthe ASBR, leading to efficient removal of VS at high organic loadingrates. Although the CSTR cannot be operated with as long an SRTas the ASBR, the 83% VS destruction achieved with a 25-day HRTin our study using CSTR is only slightly lower than that reported byAgler et al.9. The methane yield of 0.56 L g−1 VSfed and effluent VFAof 280 mg L−1 as acetic acid in our system are comparable withtheir results. In addition, from the standpoint of reactor operation,the CSTR is more accessible and easier to maintain than the ASBR.

The results reported by Schaefer and Sung8 indicate that bettersystem performance under thermophilic conditions at shorterHRTs can be obtained than under the mesophilic conditionsused in this study. In their system, VS destruction near 90% wasobtained with a 20-day HRT, which can be attributed mainly tohigher hydrolysis rate or substrate degradation constant underthermophilic conditions than under mesophilic conditions. Asdiscussed earlier, the substrate degradation rate in Equation (3) isa temperature-dependent rate constant in the biological reaction.A temperature correction equation as written in Equation (4) canbe applied to describe the effect of temperature on anaerobicdigestion of corn thin stillage.

kT = k20θ(T−20) (4)


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Table 2. A summary of comparison with other reports

Substrates Temperature DigesterHRTs(days)

VSdestruction

(%)Methane yield(L g−1 VSfed)

Organicloading rate

(g VS L−1 day−1)VFA (mg L−1 as

acetic acid) References

Corn thin stillage Mesophilic CSTRa 20 50.6 0.13 3.5 3,400 This study

Corn thin stillage Mesophilic CSTR 25 82.7 0.56 2.5 280 This study



Corn thin stillage Thermophilic CSTR 15 84.2 0.62 3.9 2,400 Schaefer and Sung8

Corn thin stillage Thermophilic CSTR 20 89.8 0.57 4.2 200 Schaefer and Sung8

Corn thin stillage Thermophilic CSTR 30 82.5 0.62 2.1 160 Schaefer and Sung8

Corn thin stillage Thermophilic ASBRb 10 89.0 0.60 3.2 <500 Agler et al.9

Corn whole stillage Mesophilic CSTR 60 82.5 0.49 1.93 4,918 Eskicioglu et al.25

Wheat 43 ◦C–45 ◦C CSTR 9 NA 0.23 11.6 NA Hutanan26

Wheat Thermophilic UASBc 2 NA 0.16d 17.1e 210 Kaparaju et al.27

Wheat Thermophilic Batch NAf NA 0.32 NA NA Kaparaju et al.27

a CSTR continuous stirred tank reactor.b ASBR anaerobic sequencing batch reactor.c UASB up-flow anaerobic sludge blanket.d Unit L g−1 CODfed.e Unit g COD L−1 day−1.f NA not available.

Table 3. Comparison of the energy production from methaneproduced from corn-thin stillage digestion under mesophilic andthermosphilic conditions for a 3.78 × 108 L ethanol per year corn-to-ethanol plant (calculation method and the background data areadapted from Agler et al.9)

Energy production from anaerobic digestion

Process Thermophilic9 Mesophilic (this study)

Methane yield (L CH4 g−1

TCOD)0.254 0.270

Methane production rate (LCH4 h−1)

4.83 × 106 5.1 × 106

Energy production frommethane (kJ h−1)

1.73 × 108 1.84 × 108

Energy reduction fromconventional Plant (%)

50.69 53.47

Energy recovery from heat exchange

Heat exchanger efficiency(%)

30 30

Corn thin stillagetemperature afterdistillation andcentrifugea (◦C/ ◦F)

70/158 70/158

Operational temperature(◦C)

55/131 35/95

Energy recoveryb (kJ h−1) 0.6 × 107 1.4 × 107

Energy reduction fromconventional plant (%)

1.52 3.59

Total energy savings

Total energy savings fromconventional plant (%)

52.21 57.06

a The temperature (70 ◦C) of the corn thin stillage after distillation andcentrifuge was assumed conservatively.b Specific heat of corn thin stillage is not available, so water specificheat (kJ L−1· ◦F) at different temperatures was assumed for use in thecalculation [4.01 (70 ◦C), 4.12 (55 ◦C), 4.15 (35 ◦C)].

where kT is the rate constant at temperature T , ◦C; k20 is the rateconstant at 20 ◦C; and θ is the temperature–activity coefficient,usually 1.02. Based on Equation (4), a k of 0.267 day−1 at 55 ◦Cwas determined. The R versus HRTs plot at k = 0.267 day−1 isshown in Fig. 5, which indicates that over 80% of the maximummethane yield was projected at HRTs exceeding 15 days. Thisresult is in agreement with those reported by Schaefer and Sung8

shown in Table 2. Table 2 also reveals that there is no significantdifference in VS destruction (83%) between the findings madeby Schaefer and Sung’s and the results obtained in this studywhen the HRT is extended to 30 days. Meanwhile, the methaneyield of 0.63 L g−1 VSfed under mesophilic conditions in this studywas superior to that under thermophilic conditions. Although thethermophilic system seems to have better performance at highorganic loading rates or low HRTs, its longer start-up period andhigher sensitivity to cation inhibition may weaken the use of thethermophilic anaerobic digester for treating corn thin stillage. Bycontrast, the advantages of mesophilic digestion over thermophilicdigestion are: shorter start-up, better effluent quality, and morestable operation.

Techno-economic overlookA techno-economic evaluation using the modified approach ofAgler et al.9 was made to compare thermophilic and mesophilicanaerobic digestion. This evaluation was carried out for acorn-to-ethanol plant with production capacity of 3.78 × 108 Lethanol per year.7,9 The methane yield of 0.254 L CH4/g TCODat the optimum 10-day HRT under thermophilic condition,9

and 0.271 L CH4 g−1 TCOD at the optimum 25-day HRT (thisstudy) under mesophilic conditions were used for the analysis.The comparison is summarized in Table 3. As revealed in thetable, the methane production rates were 5.10 and 4.83 million LCH4 h−1 at mesophilic and thermophilic conditions, respectively.This corresponds to energy recoveries of 53.47% and 50.56% frommethane through the mesophilic and thermophilic anaerobicdigestions, respectively. With 30% of heat exchange efficiency, theenergy recovery through the heat exchangers amounts to 3.6%


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Table 4. Comparison of the net energy balance between mesophilic and thermophilic conditions (calculation method and the background dataare modified or adapted from Hill et al.7 and Agler et al.9)

Energy input fromprocessing (per unitenergy in ethanol)

Conventional corn ethanol plantintegrated withoutanaerobic digestion

Corn ethanol plant integratedwith thermophilic

anaerobic digestion

Corn ethanol plant integratedwith mesophilic anaerobic

digestion

Processing 0.359 0.287 0.258

Other categoriesa 0.599 0.599 0.599

Total energy input 0.958 0.646 0.617

Energy output from ethanol 1 1 1

Energy output from feed credit 0.203 0.112 0.112

Total energy output 1.203 1.112 1.112

Net energy balance ratio 1.26 1.72 1.80

a Other categories includes Farm, Construction, Labor, and Transport7,9.

and 1.5% in this 378 million L year−1 ethanol plant integrated withmesophilic and thermophilic digestions, respectively. Overall, theenergy from both methane and heat exchange will be 57.06%for the mesophilic anaerobic digestion, and 52.21% for thethermophilc anaerobic digestion.

The holistic energetic assessment, however, should include thereduced stream of corn thin stillage, which results in reducingrevenue from DDGS.9 Accordingly, the net energy balance isestimated to obtain an overall assessment of the corn ethanolplant integrated with either mesophilic or thermophilc anaerobicdigestion. Table 4 shows the net energy balance of the cornethanol plant integrated without and with anaerobic digestion atmesophilic or thermophilic temperatures. Current operation withthin stillage evaporation requires 0.958 unit of energy input inthe ethanol plant. The system integrated with anaerobic digestionunder msophilic or thermophilic temperatures only requires 0.617(this study) or 0.6469 units of energy input. Therefore, the energyoutput per unit ethanol produced in the ethanol plant willbe reduced from 1.203 without anaerobic digestion to 1.112with anaerobic digestion integrated. For an overall economicassessment of the ethanol corn plant with and without anaerobicdigestion, a net energy balance ratio, which is defined as theratio of total energy output to total energy input, is employedto express the net energy increase or decrease. The net energybalance ratios are 1.26 for the conventional corn–ethanol plant,1.80 for the corn–ethanol plant incorporated with mesophilicanaerobic digestion, and 1.72 for the plant incorporated withthermophilic anaerobic digestion. Thus, the assessment ascertainsthat the corn ethanol plant coupled with anaerobic digestionunder mesophilic or thermophilic conditions will increase the netenergy gain, primarily due to methane production.

CONCLUSIONSignificant reductions of VS and TCOD from corn thin stillagecould be achieved in a mesophilic CSTR with 25 to 40 days ofHDTs. The efficiencies of removal of VS and TCOD exceeded82% and 84%, respectively. The maximum volumetric methaneproduction rate of 1.41 L L−1 day−1 occurred with a 25-dayHRT. The maximum methane yields of approximately 0.63 LCH4 g−1 VSfed and 0.77 L g−1 VSremoved were achieved at HRTsbetween 30 and 40 days. Mesophilic anaerobic digestion failed totreat thin stillage satisfactorily if the system was operated with a20-day HRT, corresponding with an organic loading rate of 3.5 g VSL−1 day−1. Such a low HRT or high organic loading rate resulted in

accumulation of VFAs to a concentration of 3400 mg L−1 as aceticacid, causing a feedback inhibition of the methanogens, and thusfurther deteriorated the mesophilic anaerobic digestion. Results ofthe kinetic analyses show that the substrate degradation constantwas 0.180 day−1 for corn thin stillage digested under mesophiliccondition. The analyses also project that the reactor HRT shouldexceed 23 days in order for the system to achieve 80% of themaximum methane yield. The model used in this study presentsa satisfactory fit to the experimental data under steady-stateoperational conditions. Based on the model simulation, the 25-day HRT is recommended for future operation of the mesophilicreactor for good effluent quality and stable performance.

The corn-to-ethanol plant integrated with anaerobic digestionand heat exchanger to extract heat from the corn thin stillagewill recover 57.06% energy with mesophilic anaerobic digestionand 52.21% with thermophilc anaerobic digestion. The net energybalance ratio is raised from 1.26 for a conventional corn–ethanolplant, to 1.80 for the plant integrated with mesophilic anaerobicdigestion or to 1.72 for the plant incorporated with thermophilicanaerobic digestion. Mesophilic anaerobic digestion, whichrequires longer HRTs than thermophilic anaerobic digestion, hasthe merits of higher energy recovery and relatively better stabilityof methane and heat exchanger so that it is a better choicethan the thermophilic anaerobic digestion as a complementaryalternative for making the corn ethanol business more profitableand renewable.

ACKNOWLEDGEMENTSThis study was supported by grants from Taiwan National ScienceCouncil (NSC 99-2324-B-197-002) and Inha University.

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