single-stage, batch, leach-bed, thermophilic anaerobic digestion of spent sugar beet pulp

9
Single-stage, batch, leach-bed, thermophilic anaerobic digestion of spent sugar beet pulp Abhay Koppar, Pratap Pullammanappallil * Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL 32611, USA Received 26 March 2007; received in revised form 15 June 2007; accepted 15 June 2007 Available online 12 September 2007 Abstract Spent sugar beet pulp as received was digested in a single-stage, batch, unmixed, leach-bed, laboratory scale thermophilic anaerobic digester. Biogasification of each 0.450 kg (wet weight) batch of spent pulp was initiated by inoculating with anaerobically digested liquor from previous run. The average methane yield was 0.336 m 3 CH 4 at STP (kg VS) 1 , the maximum methane production rate was 0.087 m 3 CH 4 at STP (kg VS) 1 d 1 , average lag time to initiate methanogenesis was only 0.44 days and time required to achieve 95% methane yield was 8 days. The pH in the digesters ranged between 8.0 and 9.5. High rates of methane generation were sustained even at high pH values. The equivalent organic loading rate in the batch digesters was 4 kg COD m 3 d 1 . The digestion process used here offers signif- icant improvements over one-stage and two-stage systems reported in the literature with comparable performance as it is a single-stage system where the feedstock does not require size reduction, and mixing is not required in the digester. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Anaerobic digestion; Spent sugar beet pulp; Leach-bed; Thermophilic; Biogas 1. Introduction During the process of extraction of sugar from sugar beets large amounts of spent pulp is produced. The pro- cessing of 1 ton of beet produces about 250 kg of pressed pulp, with water content of 75–80% (Spagnuolo et al., 1997). Most sugar beet mills dry and pelletize the spent beet pulp (sometimes molasses is added to increase the protein content) and it is sold as a livestock feed. Increases in oil and natural gas prices over the last decade have caused the cost of drying to increase substantially to the extent that sale of dried pulp is yielding diminished returns. For example, at American Crystal Sugar Company (East Grand Forks) about 900,000 MMBtu/year of natural gas is used for drying the spent pulp at a cost of $75/ton of dried pulp produced. Therefore, more profitable avenues for utilization of spent pulp need to be developed. Sugar manufacturing from sugar beets is an energy intensive pro- cess that requires about 7 MMBtu of fuel in boilers and 135 kWh of electrical power to produce 1 ton of sugar (data from American Crystal Sugar Company). Use of spent pulp as fuel would alleviate the use of fossil fuels and guarantee a supply of fuel that is not affected by fluc- tuating fuel process. Biogasification of spent sugar beet pulp using the anaerobic digestion process would produce fuel in the form of methane and additionally eliminate or mitigate disposal issues. Several processes have been developed to effectively bio- gasify spent sugar beet pulp. These processes typically involve size reduction (Frostell et al., 1984; Lane, 1984; Weiland, 1993) and/or pretreatment by hydrolyzing enzymes (Garcia et al., 1984) before being anaerobically digested in continuously agitated single-stage (Weiland, 1993; Frostell, 1984; Lane, 1984; Ghanem et al., 1992) or two-stage (Stoppok and Buchholz, 1985; Hutnan et al., 2000, 2001) systems. A biogasification technology that has been recently used for the treatment of solid wastes is a batch, leach-bed process. This technology has been 0960-8524/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2007.06.051 * Corresponding author. Tel.: +1 352 392 1864x203; fax: +1 352 392 4092. E-mail address: pcpratap@ufl.edu (P. Pullammanappallil). Available online at www.sciencedirect.com Bioresource Technology 99 (2008) 2831–2839

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Page 1: Single-stage, batch, leach-bed, thermophilic anaerobic digestion of spent sugar beet pulp

Available online at www.sciencedirect.com

Bioresource Technology 99 (2008) 2831–2839

Single-stage, batch, leach-bed, thermophilic anaerobic digestionof spent sugar beet pulp

Abhay Koppar, Pratap Pullammanappallil *

Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL 32611, USA

Received 26 March 2007; received in revised form 15 June 2007; accepted 15 June 2007Available online 12 September 2007

Abstract

Spent sugar beet pulp as received was digested in a single-stage, batch, unmixed, leach-bed, laboratory scale thermophilic anaerobicdigester. Biogasification of each 0.450 kg (wet weight) batch of spent pulp was initiated by inoculating with anaerobically digested liquorfrom previous run. The average methane yield was 0.336 m3 CH4 at STP (kg VS)�1, the maximum methane production rate was 0.087 m3

CH4 at STP (kg VS)�1 d�1, average lag time to initiate methanogenesis was only 0.44 days and time required to achieve 95% methaneyield was 8 days. The pH in the digesters ranged between 8.0 and 9.5. High rates of methane generation were sustained even at high pHvalues. The equivalent organic loading rate in the batch digesters was 4 kg COD m�3 d�1. The digestion process used here offers signif-icant improvements over one-stage and two-stage systems reported in the literature with comparable performance as it is a single-stagesystem where the feedstock does not require size reduction, and mixing is not required in the digester.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Anaerobic digestion; Spent sugar beet pulp; Leach-bed; Thermophilic; Biogas

1. Introduction

During the process of extraction of sugar from sugarbeets large amounts of spent pulp is produced. The pro-cessing of 1 ton of beet produces about 250 kg of pressedpulp, with water content of 75–80% (Spagnuolo et al.,1997). Most sugar beet mills dry and pelletize the spent beetpulp (sometimes molasses is added to increase the proteincontent) and it is sold as a livestock feed. Increases in oiland natural gas prices over the last decade have causedthe cost of drying to increase substantially to the extentthat sale of dried pulp is yielding diminished returns. Forexample, at American Crystal Sugar Company (EastGrand Forks) about 900,000 MMBtu/year of natural gasis used for drying the spent pulp at a cost of �$75/ton ofdried pulp produced. Therefore, more profitable avenuesfor utilization of spent pulp need to be developed. Sugar

0960-8524/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.biortech.2007.06.051

* Corresponding author. Tel.: +1 352 392 1864x203; fax: +1 352 3924092.

E-mail address: [email protected] (P. Pullammanappallil).

manufacturing from sugar beets is an energy intensive pro-cess that requires about 7 MMBtu of fuel in boilers and135 kWh of electrical power to produce 1 ton of sugar(data from American Crystal Sugar Company). Use ofspent pulp as fuel would alleviate the use of fossil fuelsand guarantee a supply of fuel that is not affected by fluc-tuating fuel process. Biogasification of spent sugar beetpulp using the anaerobic digestion process would producefuel in the form of methane and additionally eliminate ormitigate disposal issues.

Several processes have been developed to effectively bio-gasify spent sugar beet pulp. These processes typicallyinvolve size reduction (Frostell et al., 1984; Lane, 1984;Weiland, 1993) and/or pretreatment by hydrolyzingenzymes (Garcia et al., 1984) before being anaerobicallydigested in continuously agitated single-stage (Weiland,1993; Frostell, 1984; Lane, 1984; Ghanem et al., 1992) ortwo-stage (Stoppok and Buchholz, 1985; Hutnan et al.,2000, 2001) systems. A biogasification technology thathas been recently used for the treatment of solid wastes isa batch, leach-bed process. This technology has been

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2832 A. Koppar, P. Pullammanappallil / Bioresource Technology 99 (2008) 2831–2839

successfully applied to digest feedstocks like, municipalwaste, yard wastes, biosolids and vegetable and fruit wastes(Chynoweth et al., 1992; Chugh et al., 1999; Hegde andPullammanappallil, 2007). The process offers severaladvantages as it does not require fine shredding of waste,does not require mixing or agitation of digester contents,does not require bulky, expensive, high-pressure vesselsas it can be operated at low (ambient) pressures and canbe operated stably at both mesophilic and thermophilictemperatures (Pullammanappallil et al., 2005).

The leach-bed process can be operated in a single-stageor dual-stage mode depending on the characteristics of thebiomass feedstock. Originally, it was devised to be oper-ated in the dual-stage mode (Chynoweth et al., 1992) wherefresh biomass as received (or after coarse shredding) isloaded into a vessel and wetted. The leachate that is pro-duced during the wetting process is flushed throughanother vessel containing previously stabilized anaerobi-cally digested residue. The effluent generated in this vesselduring the flushing operation is returned back to the freshbiomass. This mode of leachate recirculation is calledsequencing. Sequencing is repeated until methanogenesisis initiated in the fresh biomass bed and pH of this bed isclose to neutral, after which sequencing was terminatedand the leachate recirculated directly within the bed. Deg-radation of biomass feedstocks like municipal solid wastes,yard trimmings, water hyacinth and sorghum wereachieved within 20–30 days in this process (Chynowethet al., 1992; Chugh et al., 1999). This process was modifiedto a single-stage design by flooding the fresh biomass bedwith liquid drained from the previous digestion of the feed-stock (Hegde and Pullammanappallil, 2007). It was shownthat wastes from fruit and vegetable markets can bedigested within ten days using such a single-stage process.

Anaerobic digestion is typically carried out either in themesophilic (28–40 �C) or thermophilic (50–57 �C) tempera-ture range. At thermophilic temperature the rates of degra-dation and biogasification are faster, and have greaterpotential to destroy weed seeds and plant pathogens, whichis especially beneficial for reapplying the undigested residuewith little post treatment back on to the fields to recyclenutrients. Even though the latter benefit may not be appli-

Fig. 1. Digester setup for biogas

cable to spent beet pulp; it should be taken into consider-ation if the pulp were to be digested together with otherorganic residues like sugar beet trimmings or tailings.Sugar beet pulp (Frostell et al., 1984) and similar feed-stocks have been biogasified at thermophilic temperatureusing both slurry and leach-bed reactors. For example,shredded (<7 mm size) orange peel waste was digested ina continuously fed stirred tank reactor (Kaparaju andRintala, 2006), shredded and ensiled sorghum crop wasdigested in a solids concentrating reactor (Biljetina et al.,1987) and vegetable waste as received was digested in a sin-gle-stage leach-bed digester (Hegde and Pullammanappal-lil, 2007).

The aim of this work was to investigate the effectivenessof employing a single-stage leach-bed process to biogasifyspent sugar beet pulp and evaluate the performance ofthe process in terms of its kinetics, methane yield andextent of degradation. The performance of this processwas then compared to other processes that have been devel-oped for spent sugar beet pulp.

2. Methods

2.1. Anaerobic digester

Three digesters (Digesters 1, 2 and 3), each 5 L(0.005 m3), (working volume of 4 L, (0.004 m3), were con-structed by modifying Pyrex glass jars. The height andinner diameter of the digesters were 0.406 m (16 in.) and0.0610 m (2.4 in.), respectively. The digesters were sealedwith a top lid, outer diameter of 0.0965 m (3.8 in.), usingan O-ring fitted for gas and liquid tightness and clampedwith a stainless steel clamp. Three ports were provided atthe top of the lid, one for gas outlet, and others for samplewithdrawal. At the bottom of the digesters an outlet portwas provided for draining. No additional external/internalmixing device was employed. The digester set-up is shownin Fig. 1. Gas production from the digesters was measuredusing a positive displacement gas meter. The device con-sisted of a clear PVC U-tube filled with anti-freeze solution,solid state time delay relay (Dayton OFF Delay Model6X153E), a float switch (Grainger), a counter (Redington

ification of sugar beet pulp.

Page 3: Single-stage, batch, leach-bed, thermophilic anaerobic digestion of spent sugar beet pulp

Table 1Loading and unloading data for digesters

Loading

Wet weight (kg) 0.45Dry matter (kg) 0.099Volatile matter (kg) 0.093Inoculum added (L) 2.0Packing density (kg/m3), (wet weight basis) 225Packing density (kg/m3), (dry weight basis) 25Digestion temperature (�C) 55

Unloading

Dry matter (kg) 0.0396Volatile matter (kg) 0.0214Dry matter reduction (%) 92Volatile matter reduction (%) 96

A. Koppar, P. Pullammanappallil / Bioresource Technology 99 (2008) 2831–2839 2833

Inc.) and a solenoid valve (Fabco Air). The U-tube gasmeter was calibrated in-line to determine volume of biogasper count. A count was considered as that amount of gasread on syringe (in milliliters) for which the gas meter com-pletes one whole number count (e.g. one count = 0.045 L,then two counts = 0.09 L and continued on). The pH inthe digester was measured daily using pH meter (AccumetpH meter, Model 805 MP).

2.2. Feedstock

The feedstock was provided by American Crystal SugarCompany, Minnesota. Aliquots of 0.45 kg (wet weight asreceived) of spent pulp were taken in Ziploc airtight plasticbags and stored in deep freezer. Content of one bag (i.e.,0.45 kg) was loaded into a digester for each experimentalrun. To prevent compaction of the solids, 2 kg of a bulkingmaterial (lava rocks from landscaping supplier, 0.025 m (inaverage size) was also mixed with the spent pulp. The liquidvolume held in the digester was 2 l (0.002 m3) after theaddition of bulking material.

2.3. Anaerobic digestion protocol

The bags were removed from the freezer 6 h prior to theloading of the digester to allow sufficient time for thawing.The experiments were carried out in triplicate using Digest-ers 1, 2 and 3 and in each digester five runs were carried outserially. All digestion experiments were carried out at ther-mophilic temperature by placing the digesters in an incu-bating chamber set at 55 �C. The first run in eachdigester was inoculated with 2 L (0.002 m3) of inoculumtaken from thermophilic digester that had been digestingsugar beet tailings for over a year. In addition, 10 g L�1

of sodium bicarbonate was also added to buffer againstpH changes. Once the gas production from first experimen-tal run tapered down, the digesters were opened and nextbatch (0.45 kg) of pulp charged from top. The second runin each digester was initiated by the digester liquor remain-ing from first run, no further inoculum or sodium bicar-bonate was added. The digesters were then closed, placedin the incubator and digestion allowed to proceed. Theexperimental runs were repeated three more times to com-plete five runs in each digester.

2.4. Analysis

Total solids (TS) were determined gravimetrically afterdrying overnight at 105 �C. Volatile solids (VS) contentwas determined by ashing a dried sample at 550 �C for2 h and determining the ash-free dry weight. Methaneand carbon dioxide content in the biogas was determinedwith a gas chromatograph (Fisher Gas Partitioner, Model1200). The GC was calibrated with an external standardcontaining N2:CH4:CO2 in volume ratio 25:45:30. Gaschromatograms were processed and recorded using an inte-grator (SP 4290 Integrator, Spectra Physics, Inc.). Methane

volume was reported at standard temperature and pressure(STP) conditions. For soluble COD (SCOD) analysis theleachate samples withdrawn from the digesters were centri-fuged (Fisher Marathon micro H centrifuge), filtered usingWhatman filter paper (45 lm), pipetted (2 ml) in COD vials(range: 2–150 ppm, HACH) and placed in COD reactor(HACH) for 2 h. The COD of the leachate was measuredby using colorimeter (HACH DR/890 colorimeter).

The performance of the digesters (five runs each) wasevaluated by fitting the cumulative methane productiondata to the modified Gompertz equation (Lay et al.,1998). The Gompertz equation describes cumulative meth-ane production from batch digesters assuming that meth-ane production is a function of bacterial growth. Themodified Gompertz equation is presented below:

M ¼ P � exp � expRm � e

Pk� tð Þ þ 1

� �� �ð1Þ

where M is the cumulative methane production, m3

(kg VS)�1 at any time t, P is the methane yield potential,m3 (kg VS)�1, Rm is the maximum methane productionrate, m3 (kg VS)�1 d�1, k is the duration of lag phase, d,and t is the time at which cumulative methane productionM is calculated, d. The parameters P, k and Rm were esti-mated for each of the 15 data sets by using the ‘Solver’ fea-ture in MS-Excel. The value of parameters whichminimized the sum of the square of errors between fitand experimental data were determined.

3. Results

3.1. Characteristics of feed and digested residue

The dry matter content of the spent beet pulp as receivedwas 22% and 96% of the dry matter was volatile. Table 1lists the quantities of pulp (in terms of wet, dry and volatilematter) loaded into the digesters for each run and the load-ing characteristics. Even though 99 g (0.099 kg) dry matterwas loaded into the digester, the packing density was only25 kg dry matter/m3 due to the addition of lava rocks as

Page 4: Single-stage, batch, leach-bed, thermophilic anaerobic digestion of spent sugar beet pulp

2834 A. Koppar, P. Pullammanappallil / Bioresource Technology 99 (2008) 2831–2839

bulking agent. At the beginning of each run 450 g (0.45 kg)wet weight of spent pulp was added without removing sol-ids residue from the previous run. At the end of Run 5 thedry matter remaining in the digester was measured to be39.6 g (0.0396 kg) and 54% of the dry matter was volatile.The dry matter and volatile solids reduction achieved bybiogasification was 92% and 96%, respectively.

3.2. Biogasification of spent sugar beet pulp

Profiles of cumulative methane yield and methane frac-tion in gas phase are shown in Figs. 2 and 3, respectively,for Digester 2 from all five runs. Data from Digesters 1and 3 have not been plotted here as these were similar tothat from Digester 2. However, data from all digester runsare summarized in Table 2. Run 1 was initiated by inocu-lum taken from thermophilic digester that had been digest-ing sugar beet tailings for over a year. Three days after startup the methane production rate peaked at 1.7 m3 m�3 d�1,indicating a quick onset of methanogenesis. After 5 days,the methane rate dropped to 1.3 m3 m�3 d�1 by which timethe cumulative experimental methane yield was 0.240 m3

CH4 at STP kg VS�1. Run 1 reached completion in 16 dayswhen the daily methane production rate dropped to0.01 m3 m�3 d�1. The cumulative experimental methaneyield was 0.350 m3 CH4 at STP kg VS�1 at end of Run 1.

The next digestion run (Run 2) was initiated by floodingwith liquor from Run 1. No further alkalinity was added tothe inoculum. It can be seen in Fig. 2 that methanogenesiswas initiated quicker when compared to Run 1. Three daysafter start-up the methane production rate was2.15 m3 m�3 d�1 and the methane yield was 0.251 m3

CH4 at STP kg VS�1. Run 2 reached completion in just 8days. The cumulative experimental methane yield was0.350 m3 CH4 at STP kg VS�1 at end of Run 2. Peak meth-ane gas productivity of 4.36 m3 m�3 d�1 was recorded onjust the second day after startup for Run 2. The mean (five

Time Ela0 5 10 15C

umul

ativ

e M

etha

ne Y

ield

(m3 C

H4 a

t STP

/ kg

VS)

0.0

0.1

0.2

0.3

0.4

Fig. 2. Methane yield in Dige

runs) cumulative methane yield from Digester 2 was0.342 m3 CH4 at STP kg VS�1. Similar, trends for cumula-tive methane yield and methane productivity were observedin Digesters 1 and 3. The mean cumulative methane yieldfor Digesters 1 and 3 was 0.329 m3 CH4 at STP (kg VS)�1

and 0.338 m3 CH4 at STP (kg VS)�1, respectively. Theoverall mean methane yield (Digesters 1, 2 and 3, five runseach) was 0.336 m3 CH4 at STP (kg VS)�1 (standard devi-ation = 0.0063 m3 CH4 at STP (kg VS)�1; standarderror = 0.0036 m3 CH4 at STP (kg VS)�1), indicating uni-formity in feedstock and even operating condition at alltimes in the three digesters. The values for experimentalmethane yield are listed in Table 2.

To analytically quantify parameters of batch growthcurve, a modified Gompertz equation was fit to cumulativemethane production data (Digesters 1, 2 and 3; five runseach). Values of parameters lag time, k, maximum methaneproduction rate, Rm, and ultimate methane yield, P arelisted in Table 2. For these values of parameters, the Gom-pertz equation yielded an excellent fit to all sets of cumula-tive methane data. The mean (five runs) ultimate methaneyield (P) from experiments in Digester 2 was0.344 m3 CH4 (kg VS)�1. The lag time for first run was just0.8 days. The lag time was shortened to 0.4 days in Run 2(inoculating with contents from the previous digestion)and further to 0.15 days in Run 3. The mean (five runs)lag time (k) for Digester 2 was 0.44 days. The maximummethane production rate (Rm) for first run was0.059 m3 d�1, increased to 0.115 m3 d�1 for Run 2 anddropped to 0.086 m3 d�1 for Run 3. The mean (five runs)maximum methane production rate (Rm) for Digester 2was 0.065 m3 d�1. Similar trends were observed for Digest-ers 1 and 3. The mean (five runs each) ultimate methaneyield (P) for Digesters 1 and 3 was 0.340 m3 CH4 (kg VS)�1

and 0.328 m3 CH4 (kg VS)�1, respectively. The average ulti-mate methane yield (Digesters 1, 2 and 3) was 0.338 m3 CH4

(kg VS)�1 (standard deviation = 0.0081 m3 CH4 (kg VS)�1;

psed (Days)20 25 30 35

Run 1: Digester 2 Run 2: Digester 2 Run 3: Digester 2 Run 4: Digester 2 Run 5: Digester 2 95% Confidence intervalMean

ster 2 from the five runs.

Page 5: Single-stage, batch, leach-bed, thermophilic anaerobic digestion of spent sugar beet pulp

Time Elapsed (Days)0 5 10 15 20 25 30

Met

hane

gas

frac

tion

in B

ioga

s

0.0

0.2

0.4

0.6

0.8

1.0

Run 1: Digester 2 Run 2: Digester 2

Run 3: Digester 2 Run 4: Digester 2Run 5: Digester 2 Mean Curve95% Confidence interval

Fig. 3. Methane gas content in biogas from Digester 2 for all five runs.

A. Koppar, P. Pullammanappallil / Bioresource Technology 99 (2008) 2831–2839 2835

standard error = 0.0047 m3 CH4 (kg VS)�1). The mean lagtime (k) for Digesters 1 and 3 was 0.78 days and 0.59 days,respectively. The overall mean lag time (Digesters 1, 2 and3) was 0.62 days (standard deviation = 0.17 days; standarderror = 0.098 days). The mean maximum methane produc-tion rate (Rm) for Digesters 1 and 3 was 0.043 m3 d�1 and0.051 m3 d�1, respectively. The overall mean maximummethane production rate (Digesters 1, 2 and 3) was0.053 m3 d�1 (standard deviation = 0.011 m3 d�1; standarderror = 0.0063 m3 d�1). The maximum methane productionrate (Rm) from Digesters 1, 2 and 3 dropped sharply afterRun 3 indicating some inhibition.

Time taken to produce 95% of methane yield potentialcan be used as a parameter to compare overall durationof digestion. Since the cumulative methane productioncurve only asymptotically approaches the methane yield,a digester would take infinite time to produce 100% ofmethane potential. Therefore, the 95% value was arbi-trarily chosen. In Run 1, Digester 2, it took approximately9 days to reach 95% methane yield potential, the durationwas further reduced to 5 days and 6 days in Runs 2 and 3,respectively. This showed that introducing leachate from apreviously digested material inoculated the fresh feedstockwith appropriate microorganisms to carry on the digestionprocess, besides improving the kinetics of digestion. Thetime required to achieve 95% methane potential increasedfor succeeding runs. The overall mean duration to achieve95% methane potential (Digesters 1, 2 and 3, five runseach) was 12 days (standard deviation = 1.5 days; standarderror = 0.86 days).

A profile of methane percentage in biogas for Digester 2(five runs) is shown in Fig. 3. In Run 1, the methane per-centage after 3 days of start up was 49%, rose to 55% byday 6 and was above 60% by the end of 11 days. The peakmethane percentage for Run 1 was 69%. In Run 2, themethane percentage after 3 days was 57%, dropped to

50%, but reached 60% in 8 days. Similar trend wasobserved in Run 3. In Run 4 though, methane percentageafter 3 days was 46% (significantly lower than previousruns), but reached 60% in 5 days. The peak methane per-centage for Run 4 was 77%. In Run 5, methane contentof biogas increased more slowly reaching only 27% byday 3. It took about 10 days for the methane content toreach 60%. However, peak methane content was 80%,much higher than previous runs.

In Run 1, the pH dropped from 8.2 to about 7.0 on thesecond day of digestion. The pH increased gradually andstabilized between 7.5 and 8.0 at the end of Run 1. InRun 2, the pH increased from 8.4 to above 8.5. The pHat the end of Runs 3, 4 and 5 was above 9.0. Alkalinityaddition was not required because of high pH values afterRun 1. The pH was not corrected but allowed to evolve.

4. Discussion

4.1. Biogasification efficiency

Biogasification efficiency was calculated as a percentageof the feed chemical oxygen demand (COD) converted tomethane. Using a specific COD value of 1.295 gCOD/gdry matter (Hutnan et al., 2000) for spent beet pulp, theaverage biogasification efficiency obtained in experimentshere was 77%, i.e. 77% of COD of the feedstock was cap-tured as methane during biogasification. This value is com-parable to that obtained by Frostell et al. (1984), Stoppokand Buchholz (1985), Hutnan et al. (2000). This extent ofbiogasification resulted in dry matter reduction of 92%and volatile solids reduction of 96% and is comparable tothe values obtained by Hutnan et al. (2001) and higher thanthat reported by Frostell (1984). Biogasification was effi-cient at high pH and the leach bed system attained the

Page 6: Single-stage, batch, leach-bed, thermophilic anaerobic digestion of spent sugar beet pulp

Table 2Summary of performance of anaerobic leach bed digesters

Digester(Runs)

Temperature(�C)

FinalpH

Final cumulative methaneyield (experimental)(m3 CH4 STP kg VS�1)

Gompertz parameters (model)a Duration to produce95% methane yieldpotential (Days)

Pb

(m3 CH4 kg VS�1)Rm

b

(m3 CH4 kg VS�1 day�1)kb (Days)

Digester 1

Run 1 8.69 0.337 0.344 0.066 1.08 7.87Run 2 9.27 0.302 0.296 0.045 1.12 11.20Run 3 9.44 0.364 0.342 0.066 0.69 7.98Run 4 55 9.13 0.321 0.339 0.021 0.25 18.83Run 5 9.71 0.323 0.381 0.019 0.77 20.58Average values for Digester 1 0.329 0.340 0.059e 0.78 9.02e

Digester 2

Run 1 7.91 0.350 0.342 0.059 0.8 9.19Run 2 8.69 0.350 0.340 0.115 0.4 4.68Run 3 9.17 0.374 0.350 0.086 0.15 5.71Run 4 55 8.92 0.332 0.325 0.047 0.54 10.92Run 5 9.66 0.302 0.362 0.017 0.3 21Average values for Digester 2 0.342 0.344 0.087e 0.44 6.53e

Digester 3

Run 1 8.78 0.330 0.315 0.058 0.49 8.80Run 2 9.05 0.370 0.345 0.069 0.17 6.89Run 3 9.24 0.349 0.344 0.058 0.68 9.22Run 4 55 9.50 0.302 0.302 0.025 0.1 16.94Run 5 9.64 0.336 0.336 0.048 1.70 17.32Average values for Digester 3 0.337 0.328 0.062e 0.59 8.3e

Final mean values 0.336 0.338 0.053 0.62 8.0Standard Error 0.0036 0.0047 0.0063 0.098 0.86Standard deviation 0.0063 0.0081 0.011 0.17 1.5Final ranged 0.336 ± 0.0063 0.338 ± 0.0081 0.053 ± 0.011 0.62 ± 0.17 11.81 ± 1.5F-criticalc 7.71 – – – –F0

c 0.05Confidence level (%) 95

a Gompertz parameters were derived by fitting the experiment data into modified Gompertz model.b Symbols have their usual meaning.c Single-factor ANOVA analysis (for comparison between experimental and Model yield values).d Mean ± standard deviation (for five runs each).e Average for three runs only.

2836 A. Koppar, P. Pullammanappallil / Bioresource Technology 99 (2008) 2831–2839

maximum methane potential albeit significant increase inlag time after Run 3.

Since not all organic matter is mineralized to methaneduring biogasification, there was significant residualorganic matter remaining in the digester liquor at the endof each run. Residual organic matter was measured as vol-atile solids and soluble COD (SCOD) of liquor. The SCODin the inoculum used to start up the digester was4800 mg L�1. The residual SCOD was 1650 mg L�1 (33 gCOD/kg VS, volatile matter added basis) at the end ofRun 1, which increased to 4780 mg L�1 (50 g COD/kgVS) after Run 2 then rose sharply to 14,750 mg L�1

(103 g COD/kg VS) at the end of Run 3 and furtherincreased to 21,590 mg L�1 (114 g COD/kg VS) at theend of Run 4. In the current experiments, solids were leftin the digester during serial digestions, thus increasing itsretention time. For example, by the end of Run 4 solidsloaded in Run 1 would have resided for at least 50 daysin the digester. This prolonged exposure could have causedconsiderable breakdown of solids increasing the solubleCOD and viscosity of the leachate. The accumulation of

residual COD can be decreased by incorporating a solidsseparation step, decreasing the number of times the inocu-lum is reused or diluting the inoculum.

4.2. Inhibition of biogasification

The maximum methane production rate (Rm) was0.086 m3 d�1 after Run 3, dropped sharply to 0.047m3 d�1 after Run 4 and dropped further to 0.017 m3 d�1

at the end of Run 5. Similarly, the time required to achieve95% of ultimate methane potential for Run 3 was 5.71days, increased to 10.92 days by the end of Run 4 and fur-ther rose sharply to 21 days. Clearly runs 4 and 5 wereinhibited. Based on measurements made during the exper-iments inhibition was initially attributed to two reasons, (1)high pH and (2) free ammonia accumulation. The pH atthe end of each run was between 8 and 9.5. However,despite the high pH values there was no effect on the rateof methanogenesis in Runs 1, 2 and 3. Methane productionrates observed in Runs 1, 2 and 3 correlate to high maxi-mum growth rates for aceticlastic methanogenic organisms.

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A. Koppar, P. Pullammanappallil / Bioresource Technology 99 (2008) 2831–2839 2837

Simulations showed that maximum specific growth ratesneed to be around 0.36 d�1 to be able to achieve methaneproduction rates measured in the digesters. Maximumgrowth rates for aceticlastic methanogenesis in literaturerange between 0.206 and 0.357 d�1 (see for example, Law-rence and McCarty, 1969; Wandrey and Aivasidis, 1983;Ahring and Westermann, 1987). Given that methanogenicrate was high even at pH values greater than 9, inhibitiondue to pH was discounted. The pH of runs 4 and 5 weresimilar to that measured in Runs 1–3.

It was then investigated whether inhibition could be dueto free ammonia. The total ammonia concentration at theend of Run 2 was 145 mg L�1 (0.145 g L�1) and175 mg L�1 (0.175 g L�1) at the end of Run 3. Free ammo-nia concentration was calculated from the total ammoniaconcentration using the formula developed by Hansenet al. (1998) for a pH of 9.0. This value was 95 mg L�1

(0.095 g L�1) at the end of Run 2 and 149 mg L�1

(0.149 g L�1) at the end of Run 3. It has been shown thatanaerobic digestion is inhibited at a free ammonia concen-tration of 150 mg L�1 (0.15 g L�1) (McCarty and McKin-ney, 1961), while the acetate uptake is reduced by 50% at128 mg L�1 (0.128 g L�1) (Heinrichs et al., 1990). Moreimportantly biogasification process becomes more sensitiveto ammonia when the pH value increases (Koster and Lett-inga, 1988) due to increase in free ammonia concentration.Increase of pH from 7 to 8 will actually lead to an 8-foldincrease of free ammonia concentration (Hansen et al.,1998). The free ammonia concentration level measured inthe current study after Run 2 was approaching inhibitorylevels. Free ammonia concentration increased to150 mg L�1 (0.15 g L�1) by the end of Run 3. Therefore,it appears that decrease in methane production rateobserved in Run 4 and Run 5 may be due to toxicity fromfree ammonia accumulation.

Table 3Comparison of performance of present biogasification study to that reported

Parameters Frostell et al. (1984) Stoppo(1985)

Reactor configuration One-stage with aerobic reactordownstream

Two-st

Reactor type STR STR

Scale of operation Pilot Lab-scFeeding mode Semi-continuous Contin

Temperature (�C) 55 35Pretreatment Size reduction-milling NoMethane yield (m3 CH4 kg VS�1) 0.358 0.346–Biogasification efficiency (%) 82 81Solids/hydraulic retention time

(days)27 ± 8 2.4 –7

Organic loading rate (kg CODCOD m�3 d�3)

5.7 ± 1.7 0.9–2.7

Residual SCOD (g COD g VS�1) 0.33 NAVS reduction (%) 81 ± 2 NA

‘‘NA’’ denotes: not available.* Denotes value reported at STP.

4.3. Comparisons of high solids leach bed process to other

processes

Table 3 compares the biogasification performance of thepresent study to that reported in the literature. Much of theresearch concerning biogasification of spent sugar beetpulp has been carried out in two-stage systems (i.e., sepa-rate acidogenic and methanogenic stages) under mesophilicconditions (Weiland, 1993; Stoppok and Buchholz, 1985;Hutnan et al., 2000, 2001). Sugar beet pulp was first hydro-lyzed and acidified in a stirred tank reactor and then onlythe liquid portion from this acidification stage was pumpedto the methanogenic reactor. The methane yield for two-stage processes (Stoppok and Buchholz, 1985; Hutnanet al., 2000) was about 0.350 m3 CH4 kg VS�1. In the workdone by Hutnan et al. (2001) at pilot scale, a combinationof STR (stirred tank hydrolysis reactor)-UASB (upflowanaerobic sludge blanket methanogenesis reactor) recordeda lower methane yield of about 0.235 m3 CH4 kg VS�1.They attributed this to the unstable conditions (possiblybuild up of volatile organic acids at higher loadingrates) in the operation of pilot plant. The organic loadingrates (OLR) achieved were between 0.9 and 2.7 kgCOD m�3 d�1 (Stoppok and Buchholz, 1985) and 2–6.7 kg COD m�3 d�1 (Hutnan et al., 2000, 2001). The sig-nificant difference between these two studies was the modeof feeding. Stoppok and Buchholz (1985) fed the system ina continuous mode at hydraulic retention time between 2.5and 7 days (HRT) whereas Hutnan et al. (2000, 2001)opted for a semi-continuous mode of feeding at HRTbetween 13 and 17 days. Weiland (1993) reported a meth-ane yield of 0.298 m3 CH4 kg VS�1 for two-stage systemsunder mesophilic conditions. The system was operated atOLR of 10 kg COD m�3 d�1 in the hydrolysis/acidificationstage and 6 kg COD m�3 d�1 in the methanogenesis stage

in the literature

k and Buchholz Hutnan et al.(2000)

Hutnan et al.(2001)

Present work(2007)

age Two-stage Two-stage One-stage

Non-stirrredtanks

STR-UASB Non-stirred

ale Lab-scale Pilot Lab-scaleuous Semi-

continuousSemi-continuous

Batch

35 35 55No No No

0.355 0.352 0.235 0.336*

81 54 7713–17 13 7

2.5–6.7 2 4

NA NA 0.114NA 92 96

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2838 A. Koppar, P. Pullammanappallil / Bioresource Technology 99 (2008) 2831–2839

at an overall HRT of 13 days. Importantly, pretreatmentwas achieved by maceration (80% of solids were less than0.63 mm) of beet pulp before feeding to the hydrolysis reac-tor. Other studies deal with one-stage process under ther-mophilic conditions (Frostell et al., 1984) and mesophilicconditions (Lane, 1983; Garcia et al., 1984; Ghanemet al., 1992; Weiland, 1993). Frostell et al. (1984) operateda one-stage stirred tank reactor under thermophilic condi-tions at pilot scale. The system was fed in a semi-continu-ous mode and size reduction was done using a millingoperation before feeding to the digester. OLR was5.7 ± 1.7 kg COD m�3 d�1 and the HRT of the systemwas between 27 ± 8 days. Lane (1984) operated a one-stage, semi-continuously fed stirred tank reactor undermesophilic conditions (36 ± 1 �C) that was fed hammer-milled feedstock (12 mm particle size) and obtained a meth-ane yield of 0.263 m3 CH4 (kg VS)�1 at OLR of 3.05 kgCOD m�3 d�1. Weiland (1993) reported a methane yieldof 0.272 m3 CH4 (kg VS)�1 when working with a one-stagemesophilic digester. Pretreatment was achieved by macera-tion (80% of solids were less than 0.63 mm) of beet pulpbefore feeding to the digester. The HRT and OLR forthe system were 10 days and 8 kg COD m�3 d�1,respectively.

In comparison, the biogasification system investigated inthis study offers significant improvements over one-stageand two-stage systems reported in literature. It is a one-stage system where the feedstock is not size reduced andmixing is not required in the digester. Also, requirementof an additional vessel for acidification/hydrolysis is com-pletely eliminated in this design. Values of methane yield(0.336 m3 at STP (kg VS�1)), retention time (7 days) toachieve the methane yield and equivalent loading rate(4 kg COD m�3 d�1) obtained from this system was com-parable with the one-stage and two-stage systems (Table2). Another important feature of leach-bed system is thereuse of inoculum from previous run. This allowsthe growth of robust microbial population in digester.The time required for building the inoculum is approxi-mately 15 days and is significantly lower than most systemsmentioned in the literature. For example, it took up to 1year for Frostell et al. (1984) to fully adapt inoculum. Onthe downside the effluent released from one-stage processhere has high organic matter content as it contains all undi-gested suspended solids. An additional solids separationand aerobic treatment step might be required to furtherpolish effluent from anaerobic stage to meet local regula-tions for water discharge. Perhaps due to milling of feed-stock Frostell et al. (1984) produced an effluent that wasmuch higher in soluble COD (0.33 g COD g VS�1 com-pared to 0.114 g COD g VS�1 produced here) and thereforeindicated the need for a combined anaerobic–aerobic treat-ment system to treat effluent because of high COD andBOD concentration (25.2 ± 1.9 kg COD m�3 and7.35 ± 2.33 kg BOD m�3). The effluent quality after treat-ment was 5.14 ± 1.47 kg COD m�3 and 2.00 ± 0.59 kgBOD m�3).

The process developed here can be scaled up either to asemi-continuous system which is fed once daily or to amodular batch system that will receive one day productionof spent pulp. The modular system will contain eightdigesters, inoculum holding tank, a lamella separator anda screw conveyor. The beet pulp from the plant will befed to the digesters via screw conveyor. One digester willbe charged daily. Once the digester is charged, the inocu-lum from holding tank will flooded into the digester anddigestion allowed to proceed. This procedure will berepeated everyday till all the digesters are filled. Each diges-ter will be emptied after every eight days. The residue alongwith the liquid will pass through a lamellar separator toseparate the solids. The solids can be land-applied. Theliquid will be taken to the inoculum holding tank. Sinceliquid will be lost with the solids, there will be a need foradding make up water to the inoculum which will diluteany inhibitory agents like ammonia.

5. Conclusions

Spent sugar beet pulp (dry matter 22% and volatile mat-ter 96% of dry matter) was biogasified in a single-stage,batch, unmixed, leach-bed, 4-L-thermophilic (55 �C) anaer-obic digester without any pretreatment. Approximately77% of COD of the beet pulp was converted to methaneresulting in an average methane yield of 0.336 m3 CH4 atSTP (kg VS)�1, 92% dry matter reduction and 96% volatilematter reduction. Methanogenesis was initiated in eachbatch within half day, the methane content of gas phasereached 50% within 2 days and 95% of the theoreticalmethane potential was produced within 8 days. The pHin the digesters ranged between 8.0 and 9.5. Each batchof spent pulp was inoculated with digested liquor fromthe previous run. However, after reusing the digester liquorthree times, inhibition of methanogenesis was observed inthe fourth run. This was attributed to accumulation ofammonia and the resulting toxicity. The leach-bed systeminvestigated here offers significant benefits with comparableperformance over other systems as it eliminated the needfor (1) separate hydrolysis/acidification and methanogene-sis reactors; (2) pretreatment of solids and (3) mixingwithin the digester.

Acknowledgements

The authors gratefully acknowledge Xcel Energy, Min-nesota (Contract No. RD-34) and American Crystal SugarCompany, Minnesota for financially supporting theproject.

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