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Anaerobic digestion of grain stillage at high organic loading rates in three different reactor systems
b i o m a s s a n d b i o e n e r g y 5 5 ( 2 0 1 3 ) 2 8 5 e2 9 0
Available online at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
Anaerobic digestion of grain stillage at high organic loading rates in three different reactor systems Thomas Schmidt a,b,*, Ju¨rgen Pro¨ter a, Frank Scholwin b, Michael Nelles a,b a
Deutsches Biomasseforschungszentrum gemeinnu¨tzige GmbH, Department Biochemical Conversion, Torgauer Straße 116, 04347 Leipzig, Germany b Faculty of Agricultural and Environmental Sciences, Chair of Waste Management, University of Rostock, Justus-von-Liebig-Weg 6, 18059 Rostock, Germany
article info
abstract
Article history:
In this study the anaerobic digestion of grain stillage in three different reactor systems
Received 18 January 2012
(continuous stirred tank reactor, anaerobic sequencing batch reactor, fixed bed reactor)
Received in revised form
with and without immobilization of microorganisms was investigated to evaluate the
6 February 2013
performance during increase of the organic loading rate (OLR) from 1 to 10 g of volatile
Accepted 11 February 2013
solids (VS) per liter reactor volume and day and decrease of the hydraulic retention time
Available online 11 March 2013
(HRT) from 40 to 6 days. No significant differences have been observed between the performances of the three examined reactor systems. The changes in OLR and HRT caused a
Keywords:
reduction of the specific biogas production (SBP) of about 25% from about 650 to 550 L kg1
Biogas
of VS but would also diminish the necessary digester volume and investment costs of
Ethanol
about 75% compared to the state of the art. ª 2013 Elsevier Ltd. All rights reserved.
Stillage Digester Trace elements
1.
Introduction
The production of ethanol has increased enormously within the last years due to the political aim to reduce greenhouse gas (GHG) emissions in the transportation sector. The ethanol production in Germany, where mainly grain and especially wheat is used as input substrates increased from 20 Gg in 2004 to 590 Gg in 2009 [1]. The European Union directive 2009/28/EC defines the target that at least 20% of the total energy consumption and a share of 10% in the transport sector shall be obtained from renewable sources in 2020 [2]. Based on this directive, Germany has implemented requirements on sustainable biofuels into a national law which stipulates a
reduction of GHG emissions of at least 35%, increasing to 50% in 2017 and to 60% in 2018 [3]. The GHG emissions from grain ethanol production differ a lot depending on factors as the input substrate production and transport, the source of energy used within the process, the production of Dried Distillers Grains with Soluble (DDGS), and many others. Changing existing processes could lead to GHG emission savings between 16 and 49% [3,4]. Therefore not all production facilities reach the reduction targets at the moment or at least have to improve their reductions in GHG emissions in the future. Within the production process of ethanol from grain large amounts of the byproduct stillage are generated. Depending on the production process and the recirculation rate, about
* Corresponding author. Department Biochemical Conversion, Deutsches Biomasseforschungszentrum gemeinnu¨tzige GmbH, Torgauer Straße 116, 04347 Leipzig, Germany. Tel.: þ49 341 2434 516. E-mail address: [email protected] (T. Schmidt). 0961-9534/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biombioe.2013.02.010
286
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5e10 L of stillage emerge per liter of ethanol produced and are often processed into the feed concentrate DDGS by an energy intensive drying process. The utilization of grain stillage as substrate for biogas production is an alternative to reduce energy consumption and GHG emissions in the ethanol production process by the provision of electricity and heat [5]. Furthermore the biogas can be upgraded to methane and fed into the natural gas grid. The anaerobic digestion (AD) of grain stillage by different technologies has been reported in literature by many authors and an extensive review on the technical evaluation of stillage treatment was published by Wilkie, Riedesel, and Owens [6]. The suitability of the Upflow Anaerobic Sludge Blanket (UASB) Reactor for the anaerobic treatment of stillage was proved [7] but a diminishment of the sludge bed [8] could prevent a long term operation. A two stage system with separation of acidogenesis and methanogenesis in semi-continuous fed CSTR reactors did not positively affect the process compared to a single stage operation [9]. Anaerobic baffled reactor systems were tested with good results [10,11]. Methane yields from grain stillage varied between 302 and 449 L kg1 of VS in different publications (see Table 1). Scientific data on large scale AD systems using grain stillage are very limited. In Germany up to now industrial applications are implemented at two ethanol production plants. Such an ethanol plant produces e.g. about 100 Gg of ethanol from 270 Gg of rye [16] and utilizes the liquid fraction of decanted stillage for biogas production in six 8000 m3 CSTR digesters at hydraulic retention times of 20e30 days [17]. The biogas is upgraded to 2500 m3 methane per hour and fed into the natural gas grid. This study focuses on the AD of grain stillage in three different reactor systems with and without immobilization of microorganisms to evaluate the performance at increasing OLR’s and decreasing HRT’s. Some authors recommend a minimum HRT of 10e12 days in CSTR-systems to prevent a washout of acetoclastic methanogens and synthrophic bacteria, particularly Syntrophobacter and Syntrophomonas [18e22]. A washout can occur when the HRT is lower than the generation time of the microorganisms. Compared to previously published experiments, all systems were operated with the same substrate and therefore differences in the performance could be directly related to the technical and operational aspects. The objective of this study was to achieve high OLR’s and low HRT’s by an optimization of process parameters, to
compare the different systems, to analyze effects of biomass immobilization, and thus to minimize the necessary volume and investment costs for biogas digesters for the AD of grain stillage.
2.
Materials and methods
2.1.
Substrate and additives
DDGS was obtained from CropEnergies AG in Zeitz which is selling it as concentrated feed for animal husbandry with the brand name Protigrain. According to the producer the input materials wheat, barley, molasses, triticale, and corn [23] are used and it contains 92,8% TS and per kg of TS’s 327 g crude protein, 79 g crude fibre, 77 g crude lipids, 78 g starch and sugar, and 62 g crude ash [24]. The DDGS was taken directly after the production process and stored in sealed plastic drums to prevent humidification. It was used as raw material and approximately once in a week substrate for the experiments was prepared by diluting it with water at a weight ratio of 1:3.75 and then mixing it for 24 h. The mixture was separated with an oscillating sieve (0.4 mm mesh) to simulate decanting procedure and to get the liquid fraction which was stored in a refrigerator at 4 C. Only this simulated thin stillage (STS) was used as substrate in the experiments. STS was diluted with tap water to adjust the total solids content. An iron additive (IA) was added for chemical precipitation of sulfur as iron sulfide and to prevent H2S inhibition to the substrate at a dosage of 0.0372 g of TS per g of TS and at a dosage of 0.0186 g of TS per g of TS between day 627 and 643. A trace element solution was added daily to the STS with dosages of 20 mg Ni, 5 mg Co, 40 mg Zn, and 5 mg Mo per kg of TS in the STS added according to the recommendations published by Kayhanian and Rich [25] from day 288.
2.2.
Experimental setup and procedures
The three reactor systems Continuous Stirred Tank Reactor (CSTR), Fixed Bed Reactor (FBR), and Anaerobic Sequencing Batch Reactor (ASBR) were compared in these experiments. In all reactors the temperature was controlled by thermostats to obtain stable mesophilic conditions (38 C). Substrate addition was automatized by timer controlled peristaltic pumps (Medorex TL). The OLR in this work always refers to the VS
Table 1 e Biogas yield, methane content, methane yield, organic loading rate (OLR) and hydraulic retention time (HRT) in the anaerobic digestion of grain stillage. Origin of stillage Wheat Wheat Wheat Rye Triticale Grain Grain
Biogas yield (L kg1)
Methane (%)
Methane yield (L kg1)
OLR (g L1 d1)
HRT (d)
Reference
766 650 708a 680 625 700 387
54.7 e 63.4 e 54.4 55 78
419 e 449 e 340 390 ca. 302b
e e 6.96a e e e ca. 3.38b
e e 9.25 e e e 20
[12] [13] [9] [13] [14] [15] [11]
a Calculated from a biogas yield of 425 L kg1 of COD and an OLR of 11.6 kg m3d1 of COD. b Calculated from a methane yield of 390 L kg1 of COD and an OLR of 4.28 kg m3d1 of COD.
b i o m a s s a n d b i o e n e r g y 5 5 ( 2 0 1 3 ) 2 8 5 e2 9 0
287
added. The CSTR (5 L) was operated for 758 days and agitated continuously with a blade agitator. The OLR was increased from 2 to 10 g L1 d1, the HRT was decreased from 33 to 6 days, and substrate was added once a day until day 432 and 12 times a day afterwards. The FBR (12.9 L) was operated for 294 days and was mixed by recirculating 3.5 L of reactor content every 2 h until day 633 and from day 678e700 by a circulating pump. Apart from that there was no mixing. ENVIPAC (Size1) biofilm carriers from ENVIMAC Engineering GmbH with a diameter of 20 mm, a surface of 0.123 m2 L1 and a pore volume of 94% were used for biomass immobilization. The OLR was increased from 5 to 10 g of L1 d1, the HRT was decreased from 13 to 6 days, and substrate was added 12 times a day. The ASBR (13 L) was operated for 712 days and agitated for 1 h after substrate addition. The OLR was increased from 1 to 10 g L1 d1, the HRT was decreased from 40 to 6 days, and substrate was added 4 times a day every 6 h until day 572 and 12 times a day every 2 h afterwards. EvU-Pearl biofilm carriers from EvU-Innovative Umwelttechnik GmbH were used, but no specific data were available. Schematics of the reactors are presented in Fig. 1.
presented in norm litres per kg of volatile solids. Norm conditions are 273.15 K and 101.325 kPa. The biogas composition was monitored twice a week prior to substrate addition by gas analyser GA 94 until day 527 in the CSTR and day 371 in the ASBR. Therefore only the actual biogas composition was analyzed. Afterwards and in the FBR the composition was analyzed two times a day by AWIFLEX gas analyzer and daily average values were calculated. The infra-red landfill gas analyzer GA 94 (Ansyco, Karlsruhe, Germany) was connected to the reactors via a bypass. In the AWIFLEX gas analyzer system (Awite Bioenergy GmbH) the biogas was collected in bags and CH4 and CO2 were analyzed by optical infrared sensors and O2, H2, and H2S by electrochemical sensors. For the determination of major and trace elements dried samples were pretreated with a mixture of HNO3/H2O2/HF followed by neutralization with H3BO3 and the resulting clear solution was analyzed by inductively coupled plasma atomic spectrometry (ICP-OES, ThermoFisher iCAP 6200) according to standard procedures [27e30].
2.3.
3.
Results and discussion
3.1.
Substrate and iron additive
Analytical methods
Each new STS batch was analyzed for the content of TS and VS to calculate OLR’s and amounts of substrates. Effluent samples were taken 2 times a week. Acid capacity (AC) and volatile fatty acids (VFA) were analyzed in the liquid phase of the samples after removal of the solids by centrifugation at 20,000 g for 20 min. The AC, which represents the sum of organic acids, of the samples was analyzed by the titration method according to Kapp [26] with 0.025e0.1 M H2SO4 in a pH range of 5.0 and 4.0 using a Titration Excellence T90 titrator (Mettler-Toledo GmbH, Switzerland). The concentrations of the VFA (acetic, propionic acid) were determined by using a 5890 series II gas chromatograph (Hewlett Packard, USA) equipped with a HS40 automatic headspace sampler (Perkin Elmer, USA) and an Agilent HP-FFAP column (30 m 0.32 mm 0.25 mm). Ritter TG 05 drum-type gas meters and Milligascounters were used for the measurement of the daily gas volume. Gas yields were
Table 2 presents the elemental composition of STS and IA. The C:N:P:S-ratio in STS is 132:13:3:3 and compared to an optimum ratio of 600:15:5:3 recommended by Weiland [20] the contents of N and S are too high. The C:N ratio of 10.4:1 is at the lower limit compared to the recommended ratios of 10:1e30:1 [31] and shows also the high N content in the substrate. A comparison of the essential trace elements in STS with the recommended concentrations shows, that all elements are below the recommendation or at the lower limit. The addition of the IA at a dosage of 0.0372 g of TS per g of TS strongly increased the concentration of Fe, Co, Mn, and Ni but still most trace elements were not contained in sufficient amounts. Therefore Ni, Co, Zn, and Mo were supplemented additionally.
Fig. 1 e Schematic of the reactors; left to right: CSTR, ASBR, FBR; a: substrate storage; b: peristaltic pump; c: agitator; d: thermostat; e: effluent outlet; f: gas meter; g: gas analyzer; h: biofilm carriers; i: overflow; j: sampling point; k: circulating pump.
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Table 2 e Elemental composition of STS, IA, STS and IA mixture, and recommended concentrations of trace elements in mg kgL1 of TS.
C N P S Co Cu Fe Mn Mo Ni Se W Zn
STS
IA
STS þ IA
Recommended concentrationsa
450,000 43,400 11,100 10,200 0.17 10.6e10.9 175e244 79.5e82.2 1.57e1.65 0.92e3.52 0.1e0.23 0.41e0.75 87.6e88.4
32,300e35,100 1800e2500 681 1200e10,270 36e45 6.2e30 368,100e396,200 7362e17,370 0e12 54e79 0 0 120e162
1.7 11.3 14,542 530.6 1.8 4.7 0.2 0.6 93.4
0.4e10 10e80 750e5000 100e1500 0.05e16 4e30 0.05e4 0.1e30 30e400
Fig. 3 e Concentrations of VFA, AC, and OLR in the ASBR.
a According to Refs. [25,32].
3.2.
Process stability
The accumulation of VFA is an indicator for an unstable anaerobic process and was observed in the CSTR and the ASBR. Two parameters express such an accumulation, the concentration of single VFA and the AC. As illustrated in Fig. 2, three major peaks of both parameters have been identified in the CSTR. The peaks (day 201e221, day 265e294, and day 719e728) resulted from an accumulation of acetic acid up to 320 and 3697 mg L1 and propionic acid up to 651 mg L1 respectively. In the same periods the AC increased to 4.1, 6.4, and 1.6 g L1. As shown in Fig. 3, two major peaks have been identified in the ASBR. The AC increased to 3.8 and 3.5 g L1 at day 229 and 565, respectively. For the first peak no analysis of VFA was available but the second peak was caused by accumulation of propionic acid to values up to 2527 mg L1. In both reactors a decrease of the OLR (CSTR: days 210e218 and 273; ASBR: days 230e231) effected a degradation of the VFA to normal levels, but as shown in Fig. 2, VFA and AC accumulated again when a certain OLR was reached. After supplementation of the trace element solution from day 288
Fig. 2 e Concentrations of VFA, AC, and OLR in the CSTR.
on, the OLR could have been increased to 10 g L1 d1. Therefore a deficiency of essential trace elements seemed to be the cause for the accumulation of VFA in the process (see also Table 2) but another cause was identified for the second peak in the ASBR. After the feeding interval was changed from six to twelve additions per day (day 572) according to the procedure in the CSTR, the values of propionic acid were reduced by this adaptation to normal levels. Therefore it can be concluded, that this accumulation of propionic acid was caused by overloading of the process and accompanying shocks by pH droppings after STS addition. In the FBR no significant accumulations of VFA were observed.
3.3.
Biogas quantity
The absolute biogas production in all reactor systems increased with increasing OLR’s. To compare the efficiency it is more meaningful not to use the absolute biogas production but the specific biogas production (SBP), which expresses the biogas production per gram of VS added to the reactor in L kg1. Due to the influence of the HRT on the degradability, SBP and HRT of the three reactor systems are illustrated in Fig. 4. In general the SBP is decreasing with shorter HRT’s but not in direct correlation. At long HRT’s of 25e40 days the SBP in
Fig. 4 e SBP and HRT in the CSTR, FBR and ASBR.
b i o m a s s a n d b i o e n e r g y 5 5 ( 2 0 1 3 ) 2 8 5 e2 9 0
4.
Fig. 5 e Biogas composition in the three reactor systems at selected OLR’s and HRT’s.
the CSTR and the ASBR reached values between 600 and 700 L kg1. At a HRT of 6 days the SBP values in all reactors fluctuated between 500 and 600 L kg1 with slightly higher values in the CSTR compared to FBR and ASBR. The low values of 408 and 526 L kg1 observed at day 209 and 286 in the CSTR can be explained directly by the process instability. Accordingly the high values of 696 and 711 L kg1 at day 223 and 293 are the result of the transformation of the previously accumulated VFA’s into biogas.
3.4.
Biogas quality
The average values of the biogas compositions in the three reactor systems are presented in Fig. 5 for selected OLR’s and HRT’s when all systems were operated under the same conditions, because an entire illustration of the gas composition for all systems is difficult due to the differences in duration of operation and in levels of OLR and HRT. It should be considered, that the measured values have been influenced by change of the analyzer, calibration of the analyzers, and change of feeding intervals. Thus high methane concentrations of 64.5% and 64.4% in the CSTR and 66.0% and 68.4% at OLR’s of 3.0 and 4.4 g L1 d1 respectively results from sampling of the biogas before semicontinuous substrate addition and are not representing daily average values. In further progress methane contents varied between 57.3% and 62.3% without any significant differences between the three reactors. H2S concentrations reached values up to about 500 mL m3 up to an OLR of 9.5 g L1 d1 and a HRT of 6 days but increased to values from 1937 to 2530 mL m3 as a consequence of halving the dosage of the iron additive. At an OLR of 10 g L1 d1 desulfurization was not as efficient as in the beginning, even though the same dosage of additive was supplemented to the substrate. H2 concentrations varied significantly between the reactors. Lowest values of 45e104 mL m3 were observed in the CSTR and highest in the FBR, where concentrations increased up to 1031 mL m3. These differences can be explained by the influence of agitation on the interspecies electron transfer.
289
Conclusions
Grain stillage can be used as sole substrate in different reactor systems at high OLR’s up to 10 g L1 d1 and low HRT’s of 6 days when trace elements are supplemented and substrate is added continuously. No significant differences have been observed between the three examined reactor systems. Astonishingly even in the CSTR system without biomass immobilization a stable process at HRT’s below 10e14 days was achieved in contrast to the recommendations given in literature. The shortening of the HRT from 25 to 6 days caused a reduction of the SBP of about 25% from about 650 to 550 L kg1 which is not favorable considering GHG emissions from the effluents. Regarding to the digester volume this shorting would lead to a diminishment of about 75%. With reference to the 48,000 m3 industrial biogas plant mentioned before, a reduction of the HRT to 6 days would minimize the necessary digester volume to 11,520 m3. Assuming investment costs of 647 Euro per m3 [33] the investment for the digesters at a biogas plant for the anaerobic digestion of grain stillage could be minimized from 31,056,000 to 7,453,440 Euro and therefore could improve the cost effectiveness considerably.
Disclosure statement All authors affirm, that there is no actual or potential conflict of interest including any financial, personal or other relationships with other people or organizations within three (3) years of beginning the work submitted that could inappropriately influence (bias) their work.
Role of the funding source This work was funded by the Federal Ministry of Economics and Technology and the Federal Ministry of Education and Research. The sponsors did not influence the study design, the collection, analysis, and interpretation of data, the writing of the report, and the decision to submit the paper for publication.
references
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[6] Wilkie AC, Riedesel KJ, Owens JM. Stillage characterization and anaerobic treatment of ethanol stillage from conventional and cellulosic feedstocks. Biomass Bioenerg 2000;19:63e102. [7] Gao M, She Z, Jin C. Performance evaluation of a mesophilic (37 C) upflow anaerobic sludge blanket reactor in treating distiller’s grains wastewater. J Hazard Mater 2007;141: 808e13. [8] Laubscher ACJ, Wentzel MC, Le Roux JMW, Ekama GA. Treatment of grain distillation wastewaters in an upflow anaerobic sludge bed (UASB) system. Water SA 2001;27: 433e44. [9] Hutnan M, Hornak M, Bodik I, Hlavacka V. Anaerobic treatment of wheat stillage. Chem Biochem Eng Q 2003;17: 233e41. [10] Akunna JC, Clark M. Performance of a granular-bed anaerobic baffled reactor (GRABBR) treating whisky distillery wastewater. Bioresour Technol 2000;74:257e61. [11] Bozadzhiev OE, Valchevski I, Lalov IG. Biomethanation of distillery wastewater in an anaerobic baffled reactor system. Bioautomation 2007;8:146e53. [12] Bauer A, Leonhartsberger C, Bo¨sch P, Amon B, Friedl A, Amon T. Analysis of methane yields from energy crops and agricultural by-products and estimation of energy potential from sustainable crop rotation systems in EUe27. Clean Technol Environ Policy 2009;12:153e61. [13] Gleixner AJ. Biogas und Brennerei e eine synergetische Beziehung. Available at: http://www.innovas.com/ templates/innovas_standard/pdf/biogas/Biogas_und_ Brennerei_eine_Synergetische_Beziehung.pdf; 2004 [accessed 12.04.11]. [14] Schumacher B. Untersuchungen zur Aufbereitung und Umwandlung von Energiepflanzen in Biogas und Bioethanol. 1st ed. Berlin: Mensch-und-Buch-Verlag; 2008. [15] KTBL. Gasausbeute in landwirtschaftlichen Biogasanlagen. 2nd ed. Darmstadt: Kuratorium fu¨r Technik und Bauwesen in der Landwirtschaft; 2010. [16] Wiedemann K, Schmidt A. Raffiniertes Konzept. Neue Energie 2010;9:70e4. [17] Rudolph W. Ritterschlag fu¨r Bioraffineriekonzept. Biogas J 2011;3:46e9. [18] Eder B, Schulz H. BiogasePraxis: Grundlagen, Planung, Anlagenbau, Beispiele, Wirtschaftlichkeit. 4th ed. Staufen bei ¨ kobucheVerlag; 2007. Freiburg: O [19] Scholwin F, Liebetrau J, Edelmann W. Grundlagen. In: Kaltschmitt M, Hartmann H, Hofbauer H, editors. Energie aus Biomasse: Grundlagen, Techniken und Verfahren.
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Available online at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
Anaerobic digestion of grain stillage at high organic loading rates in three different reactor systems Thomas Schmidt a,b,*, Ju¨rgen Pro¨ter a, Frank Scholwin b, Michael Nelles a,b a
Deutsches Biomasseforschungszentrum gemeinnu¨tzige GmbH, Department Biochemical Conversion, Torgauer Straße 116, 04347 Leipzig, Germany b Faculty of Agricultural and Environmental Sciences, Chair of Waste Management, University of Rostock, Justus-von-Liebig-Weg 6, 18059 Rostock, Germany
article info
abstract
Article history:
In this study the anaerobic digestion of grain stillage in three different reactor systems
Received 18 January 2012
(continuous stirred tank reactor, anaerobic sequencing batch reactor, fixed bed reactor)
Received in revised form
with and without immobilization of microorganisms was investigated to evaluate the
6 February 2013
performance during increase of the organic loading rate (OLR) from 1 to 10 g of volatile
Accepted 11 February 2013
solids (VS) per liter reactor volume and day and decrease of the hydraulic retention time
Available online 11 March 2013
(HRT) from 40 to 6 days. No significant differences have been observed between the performances of the three examined reactor systems. The changes in OLR and HRT caused a
Keywords:
reduction of the specific biogas production (SBP) of about 25% from about 650 to 550 L kg1
Biogas
of VS but would also diminish the necessary digester volume and investment costs of
Ethanol
about 75% compared to the state of the art. ª 2013 Elsevier Ltd. All rights reserved.
Stillage Digester Trace elements
1.
Introduction
The production of ethanol has increased enormously within the last years due to the political aim to reduce greenhouse gas (GHG) emissions in the transportation sector. The ethanol production in Germany, where mainly grain and especially wheat is used as input substrates increased from 20 Gg in 2004 to 590 Gg in 2009 [1]. The European Union directive 2009/28/EC defines the target that at least 20% of the total energy consumption and a share of 10% in the transport sector shall be obtained from renewable sources in 2020 [2]. Based on this directive, Germany has implemented requirements on sustainable biofuels into a national law which stipulates a
reduction of GHG emissions of at least 35%, increasing to 50% in 2017 and to 60% in 2018 [3]. The GHG emissions from grain ethanol production differ a lot depending on factors as the input substrate production and transport, the source of energy used within the process, the production of Dried Distillers Grains with Soluble (DDGS), and many others. Changing existing processes could lead to GHG emission savings between 16 and 49% [3,4]. Therefore not all production facilities reach the reduction targets at the moment or at least have to improve their reductions in GHG emissions in the future. Within the production process of ethanol from grain large amounts of the byproduct stillage are generated. Depending on the production process and the recirculation rate, about
* Corresponding author. Department Biochemical Conversion, Deutsches Biomasseforschungszentrum gemeinnu¨tzige GmbH, Torgauer Straße 116, 04347 Leipzig, Germany. Tel.: þ49 341 2434 516. E-mail address: [email protected] (T. Schmidt). 0961-9534/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biombioe.2013.02.010
286
b i o m a s s a n d b i o e n e r g y 5 5 ( 2 0 1 3 ) 2 8 5 e2 9 0
5e10 L of stillage emerge per liter of ethanol produced and are often processed into the feed concentrate DDGS by an energy intensive drying process. The utilization of grain stillage as substrate for biogas production is an alternative to reduce energy consumption and GHG emissions in the ethanol production process by the provision of electricity and heat [5]. Furthermore the biogas can be upgraded to methane and fed into the natural gas grid. The anaerobic digestion (AD) of grain stillage by different technologies has been reported in literature by many authors and an extensive review on the technical evaluation of stillage treatment was published by Wilkie, Riedesel, and Owens [6]. The suitability of the Upflow Anaerobic Sludge Blanket (UASB) Reactor for the anaerobic treatment of stillage was proved [7] but a diminishment of the sludge bed [8] could prevent a long term operation. A two stage system with separation of acidogenesis and methanogenesis in semi-continuous fed CSTR reactors did not positively affect the process compared to a single stage operation [9]. Anaerobic baffled reactor systems were tested with good results [10,11]. Methane yields from grain stillage varied between 302 and 449 L kg1 of VS in different publications (see Table 1). Scientific data on large scale AD systems using grain stillage are very limited. In Germany up to now industrial applications are implemented at two ethanol production plants. Such an ethanol plant produces e.g. about 100 Gg of ethanol from 270 Gg of rye [16] and utilizes the liquid fraction of decanted stillage for biogas production in six 8000 m3 CSTR digesters at hydraulic retention times of 20e30 days [17]. The biogas is upgraded to 2500 m3 methane per hour and fed into the natural gas grid. This study focuses on the AD of grain stillage in three different reactor systems with and without immobilization of microorganisms to evaluate the performance at increasing OLR’s and decreasing HRT’s. Some authors recommend a minimum HRT of 10e12 days in CSTR-systems to prevent a washout of acetoclastic methanogens and synthrophic bacteria, particularly Syntrophobacter and Syntrophomonas [18e22]. A washout can occur when the HRT is lower than the generation time of the microorganisms. Compared to previously published experiments, all systems were operated with the same substrate and therefore differences in the performance could be directly related to the technical and operational aspects. The objective of this study was to achieve high OLR’s and low HRT’s by an optimization of process parameters, to
compare the different systems, to analyze effects of biomass immobilization, and thus to minimize the necessary volume and investment costs for biogas digesters for the AD of grain stillage.
2.
Materials and methods
2.1.
Substrate and additives
DDGS was obtained from CropEnergies AG in Zeitz which is selling it as concentrated feed for animal husbandry with the brand name Protigrain. According to the producer the input materials wheat, barley, molasses, triticale, and corn [23] are used and it contains 92,8% TS and per kg of TS’s 327 g crude protein, 79 g crude fibre, 77 g crude lipids, 78 g starch and sugar, and 62 g crude ash [24]. The DDGS was taken directly after the production process and stored in sealed plastic drums to prevent humidification. It was used as raw material and approximately once in a week substrate for the experiments was prepared by diluting it with water at a weight ratio of 1:3.75 and then mixing it for 24 h. The mixture was separated with an oscillating sieve (0.4 mm mesh) to simulate decanting procedure and to get the liquid fraction which was stored in a refrigerator at 4 C. Only this simulated thin stillage (STS) was used as substrate in the experiments. STS was diluted with tap water to adjust the total solids content. An iron additive (IA) was added for chemical precipitation of sulfur as iron sulfide and to prevent H2S inhibition to the substrate at a dosage of 0.0372 g of TS per g of TS and at a dosage of 0.0186 g of TS per g of TS between day 627 and 643. A trace element solution was added daily to the STS with dosages of 20 mg Ni, 5 mg Co, 40 mg Zn, and 5 mg Mo per kg of TS in the STS added according to the recommendations published by Kayhanian and Rich [25] from day 288.
2.2.
Experimental setup and procedures
The three reactor systems Continuous Stirred Tank Reactor (CSTR), Fixed Bed Reactor (FBR), and Anaerobic Sequencing Batch Reactor (ASBR) were compared in these experiments. In all reactors the temperature was controlled by thermostats to obtain stable mesophilic conditions (38 C). Substrate addition was automatized by timer controlled peristaltic pumps (Medorex TL). The OLR in this work always refers to the VS
Table 1 e Biogas yield, methane content, methane yield, organic loading rate (OLR) and hydraulic retention time (HRT) in the anaerobic digestion of grain stillage. Origin of stillage Wheat Wheat Wheat Rye Triticale Grain Grain
Biogas yield (L kg1)
Methane (%)
Methane yield (L kg1)
OLR (g L1 d1)
HRT (d)
Reference
766 650 708a 680 625 700 387
54.7 e 63.4 e 54.4 55 78
419 e 449 e 340 390 ca. 302b
e e 6.96a e e e ca. 3.38b
e e 9.25 e e e 20
[12] [13] [9] [13] [14] [15] [11]
a Calculated from a biogas yield of 425 L kg1 of COD and an OLR of 11.6 kg m3d1 of COD. b Calculated from a methane yield of 390 L kg1 of COD and an OLR of 4.28 kg m3d1 of COD.
b i o m a s s a n d b i o e n e r g y 5 5 ( 2 0 1 3 ) 2 8 5 e2 9 0
287
added. The CSTR (5 L) was operated for 758 days and agitated continuously with a blade agitator. The OLR was increased from 2 to 10 g L1 d1, the HRT was decreased from 33 to 6 days, and substrate was added once a day until day 432 and 12 times a day afterwards. The FBR (12.9 L) was operated for 294 days and was mixed by recirculating 3.5 L of reactor content every 2 h until day 633 and from day 678e700 by a circulating pump. Apart from that there was no mixing. ENVIPAC (Size1) biofilm carriers from ENVIMAC Engineering GmbH with a diameter of 20 mm, a surface of 0.123 m2 L1 and a pore volume of 94% were used for biomass immobilization. The OLR was increased from 5 to 10 g of L1 d1, the HRT was decreased from 13 to 6 days, and substrate was added 12 times a day. The ASBR (13 L) was operated for 712 days and agitated for 1 h after substrate addition. The OLR was increased from 1 to 10 g L1 d1, the HRT was decreased from 40 to 6 days, and substrate was added 4 times a day every 6 h until day 572 and 12 times a day every 2 h afterwards. EvU-Pearl biofilm carriers from EvU-Innovative Umwelttechnik GmbH were used, but no specific data were available. Schematics of the reactors are presented in Fig. 1.
presented in norm litres per kg of volatile solids. Norm conditions are 273.15 K and 101.325 kPa. The biogas composition was monitored twice a week prior to substrate addition by gas analyser GA 94 until day 527 in the CSTR and day 371 in the ASBR. Therefore only the actual biogas composition was analyzed. Afterwards and in the FBR the composition was analyzed two times a day by AWIFLEX gas analyzer and daily average values were calculated. The infra-red landfill gas analyzer GA 94 (Ansyco, Karlsruhe, Germany) was connected to the reactors via a bypass. In the AWIFLEX gas analyzer system (Awite Bioenergy GmbH) the biogas was collected in bags and CH4 and CO2 were analyzed by optical infrared sensors and O2, H2, and H2S by electrochemical sensors. For the determination of major and trace elements dried samples were pretreated with a mixture of HNO3/H2O2/HF followed by neutralization with H3BO3 and the resulting clear solution was analyzed by inductively coupled plasma atomic spectrometry (ICP-OES, ThermoFisher iCAP 6200) according to standard procedures [27e30].
2.3.
3.
Results and discussion
3.1.
Substrate and iron additive
Analytical methods
Each new STS batch was analyzed for the content of TS and VS to calculate OLR’s and amounts of substrates. Effluent samples were taken 2 times a week. Acid capacity (AC) and volatile fatty acids (VFA) were analyzed in the liquid phase of the samples after removal of the solids by centrifugation at 20,000 g for 20 min. The AC, which represents the sum of organic acids, of the samples was analyzed by the titration method according to Kapp [26] with 0.025e0.1 M H2SO4 in a pH range of 5.0 and 4.0 using a Titration Excellence T90 titrator (Mettler-Toledo GmbH, Switzerland). The concentrations of the VFA (acetic, propionic acid) were determined by using a 5890 series II gas chromatograph (Hewlett Packard, USA) equipped with a HS40 automatic headspace sampler (Perkin Elmer, USA) and an Agilent HP-FFAP column (30 m 0.32 mm 0.25 mm). Ritter TG 05 drum-type gas meters and Milligascounters were used for the measurement of the daily gas volume. Gas yields were
Table 2 presents the elemental composition of STS and IA. The C:N:P:S-ratio in STS is 132:13:3:3 and compared to an optimum ratio of 600:15:5:3 recommended by Weiland [20] the contents of N and S are too high. The C:N ratio of 10.4:1 is at the lower limit compared to the recommended ratios of 10:1e30:1 [31] and shows also the high N content in the substrate. A comparison of the essential trace elements in STS with the recommended concentrations shows, that all elements are below the recommendation or at the lower limit. The addition of the IA at a dosage of 0.0372 g of TS per g of TS strongly increased the concentration of Fe, Co, Mn, and Ni but still most trace elements were not contained in sufficient amounts. Therefore Ni, Co, Zn, and Mo were supplemented additionally.
Fig. 1 e Schematic of the reactors; left to right: CSTR, ASBR, FBR; a: substrate storage; b: peristaltic pump; c: agitator; d: thermostat; e: effluent outlet; f: gas meter; g: gas analyzer; h: biofilm carriers; i: overflow; j: sampling point; k: circulating pump.
288
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Table 2 e Elemental composition of STS, IA, STS and IA mixture, and recommended concentrations of trace elements in mg kgL1 of TS.
C N P S Co Cu Fe Mn Mo Ni Se W Zn
STS
IA
STS þ IA
Recommended concentrationsa
450,000 43,400 11,100 10,200 0.17 10.6e10.9 175e244 79.5e82.2 1.57e1.65 0.92e3.52 0.1e0.23 0.41e0.75 87.6e88.4
32,300e35,100 1800e2500 681 1200e10,270 36e45 6.2e30 368,100e396,200 7362e17,370 0e12 54e79 0 0 120e162
1.7 11.3 14,542 530.6 1.8 4.7 0.2 0.6 93.4
0.4e10 10e80 750e5000 100e1500 0.05e16 4e30 0.05e4 0.1e30 30e400
Fig. 3 e Concentrations of VFA, AC, and OLR in the ASBR.
a According to Refs. [25,32].
3.2.
Process stability
The accumulation of VFA is an indicator for an unstable anaerobic process and was observed in the CSTR and the ASBR. Two parameters express such an accumulation, the concentration of single VFA and the AC. As illustrated in Fig. 2, three major peaks of both parameters have been identified in the CSTR. The peaks (day 201e221, day 265e294, and day 719e728) resulted from an accumulation of acetic acid up to 320 and 3697 mg L1 and propionic acid up to 651 mg L1 respectively. In the same periods the AC increased to 4.1, 6.4, and 1.6 g L1. As shown in Fig. 3, two major peaks have been identified in the ASBR. The AC increased to 3.8 and 3.5 g L1 at day 229 and 565, respectively. For the first peak no analysis of VFA was available but the second peak was caused by accumulation of propionic acid to values up to 2527 mg L1. In both reactors a decrease of the OLR (CSTR: days 210e218 and 273; ASBR: days 230e231) effected a degradation of the VFA to normal levels, but as shown in Fig. 2, VFA and AC accumulated again when a certain OLR was reached. After supplementation of the trace element solution from day 288
Fig. 2 e Concentrations of VFA, AC, and OLR in the CSTR.
on, the OLR could have been increased to 10 g L1 d1. Therefore a deficiency of essential trace elements seemed to be the cause for the accumulation of VFA in the process (see also Table 2) but another cause was identified for the second peak in the ASBR. After the feeding interval was changed from six to twelve additions per day (day 572) according to the procedure in the CSTR, the values of propionic acid were reduced by this adaptation to normal levels. Therefore it can be concluded, that this accumulation of propionic acid was caused by overloading of the process and accompanying shocks by pH droppings after STS addition. In the FBR no significant accumulations of VFA were observed.
3.3.
Biogas quantity
The absolute biogas production in all reactor systems increased with increasing OLR’s. To compare the efficiency it is more meaningful not to use the absolute biogas production but the specific biogas production (SBP), which expresses the biogas production per gram of VS added to the reactor in L kg1. Due to the influence of the HRT on the degradability, SBP and HRT of the three reactor systems are illustrated in Fig. 4. In general the SBP is decreasing with shorter HRT’s but not in direct correlation. At long HRT’s of 25e40 days the SBP in
Fig. 4 e SBP and HRT in the CSTR, FBR and ASBR.
b i o m a s s a n d b i o e n e r g y 5 5 ( 2 0 1 3 ) 2 8 5 e2 9 0
4.
Fig. 5 e Biogas composition in the three reactor systems at selected OLR’s and HRT’s.
the CSTR and the ASBR reached values between 600 and 700 L kg1. At a HRT of 6 days the SBP values in all reactors fluctuated between 500 and 600 L kg1 with slightly higher values in the CSTR compared to FBR and ASBR. The low values of 408 and 526 L kg1 observed at day 209 and 286 in the CSTR can be explained directly by the process instability. Accordingly the high values of 696 and 711 L kg1 at day 223 and 293 are the result of the transformation of the previously accumulated VFA’s into biogas.
3.4.
Biogas quality
The average values of the biogas compositions in the three reactor systems are presented in Fig. 5 for selected OLR’s and HRT’s when all systems were operated under the same conditions, because an entire illustration of the gas composition for all systems is difficult due to the differences in duration of operation and in levels of OLR and HRT. It should be considered, that the measured values have been influenced by change of the analyzer, calibration of the analyzers, and change of feeding intervals. Thus high methane concentrations of 64.5% and 64.4% in the CSTR and 66.0% and 68.4% at OLR’s of 3.0 and 4.4 g L1 d1 respectively results from sampling of the biogas before semicontinuous substrate addition and are not representing daily average values. In further progress methane contents varied between 57.3% and 62.3% without any significant differences between the three reactors. H2S concentrations reached values up to about 500 mL m3 up to an OLR of 9.5 g L1 d1 and a HRT of 6 days but increased to values from 1937 to 2530 mL m3 as a consequence of halving the dosage of the iron additive. At an OLR of 10 g L1 d1 desulfurization was not as efficient as in the beginning, even though the same dosage of additive was supplemented to the substrate. H2 concentrations varied significantly between the reactors. Lowest values of 45e104 mL m3 were observed in the CSTR and highest in the FBR, where concentrations increased up to 1031 mL m3. These differences can be explained by the influence of agitation on the interspecies electron transfer.
289
Conclusions
Grain stillage can be used as sole substrate in different reactor systems at high OLR’s up to 10 g L1 d1 and low HRT’s of 6 days when trace elements are supplemented and substrate is added continuously. No significant differences have been observed between the three examined reactor systems. Astonishingly even in the CSTR system without biomass immobilization a stable process at HRT’s below 10e14 days was achieved in contrast to the recommendations given in literature. The shortening of the HRT from 25 to 6 days caused a reduction of the SBP of about 25% from about 650 to 550 L kg1 which is not favorable considering GHG emissions from the effluents. Regarding to the digester volume this shorting would lead to a diminishment of about 75%. With reference to the 48,000 m3 industrial biogas plant mentioned before, a reduction of the HRT to 6 days would minimize the necessary digester volume to 11,520 m3. Assuming investment costs of 647 Euro per m3 [33] the investment for the digesters at a biogas plant for the anaerobic digestion of grain stillage could be minimized from 31,056,000 to 7,453,440 Euro and therefore could improve the cost effectiveness considerably.
Disclosure statement All authors affirm, that there is no actual or potential conflict of interest including any financial, personal or other relationships with other people or organizations within three (3) years of beginning the work submitted that could inappropriately influence (bias) their work.
Role of the funding source This work was funded by the Federal Ministry of Economics and Technology and the Federal Ministry of Education and Research. The sponsors did not influence the study design, the collection, analysis, and interpretation of data, the writing of the report, and the decision to submit the paper for publication.
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