Bioresource Technology 124 (2012) 321–327

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Bioresource Technology

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Thermo- and mesophilic anaerobic digestion of wheat straw by the upflowanaerobic solid-state (UASS) process

Marcel Pohl ⇑, Jan Mumme, Kathrin Heeg, Edith NettmannLeibniz Institute for Agricultural Engineering Potsdam-Bornim, Max-Eyth-Allee 100, 14469 Potsdam, Germany

h i g h l i g h t s

" Anaerobic digestion of wheat straw in UASS reactors was examined." The emphasis was to observe process behavior at different temperature levels." Thermophilic temperature showed faster degradation and methane yields." Lignocellulosic substrate is degraded within a short hydraulic retention time." A significant difference in nutrient demand at different temperature levels was observed.

a r t i c l e i n f o

Article history:Received 10 June 2012Received in revised form 13 August 2012Accepted 15 August 2012Available online 24 August 2012

Keywords:Anaerobic digestionMethaneWheat strawSolid-stateUASS

0960-8524/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.biortech.2012.08.063

⇑ Corresponding author. Tel.: +49 331 5699 936; faE-mail address: mpohl@atb-potsdam.de (M. Pohl).

a b s t r a c t

In this experimental work, the feasibility of wheat straw as a feedstock for biogas production is investi-gated using the newly developed upflow anaerobic solid-state (UASS) process. With the analyticalemphasis placed on methane and metabolite production, both mesophilic and thermophilic 39 L UASSreactors were operated for 218 days at an organic loading rate of 2.5 gVS L�1 d�1 using wheat straw as solesubstrate. For improved methanization of soluble metabolites, each UASS reactor was connected to anindividual 30 L anaerobic filter (AF). During steady state thermophilic straw digestion was found to havea 36% higher methane yield (0.165 L gVS

�1) whereas the hydrolysis rate constant increased by 106%(0.066 d�1).

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Sustainable biogas production plays a decisive role in the ongo-ing fight against global warming and climate change, as it replacesfossil fuels, reduces the energy demand of waste treatment plantsand can yield valuable organic fertilizers. According to the UnitedNations by the year 2050 up to 77% of the world’s energy demandcould be supplied by renewables (IPCC, 2011). However, as bio-mass is gaining more and more economic interest further expan-sion of biogas production increasingly depends on theexploitation of new sources of biomass. Furthermore, economicbiogas production depends on high biogas yields and sustainableproduction (Amon et al., 2007).

Especially lignocellulosic wastes, such as wheat straw and greencuttings, are considered a major potential substrate for biogas pro-duction but also for other energetic and material uses. Nowadaysstraw is either used as bedding material for livestock, applied to

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x: +49 331 5699 849.

the soil as natural fertilizer or burned on the field. Anaerobic diges-tion on the other hand is providing both energy and organic fertil-izer and it delivers a carbon neutral energy source (Ward et al.,2008; Cui et al., 2011). Besides biogas production current researchefforts are placed on thermo-chemical conversion to liquid andgaseous fuels, but production costs are currently too high to beof commercial interest (Talebnia et al., 2010).

To improve the energy efficiency of anaerobic digestion, severalmethods of pretreatment for lignocellulosic biomass have beeninvestigated such as milling (Palmowski and Müller, 2000), micr-owaving (Jackowiak et al., 2011), other thermal and biological pre-treatments (Hendriks and Zeeman, 2009). Currently, extrusion as acombined process of applying temperature and pressure to ligno-cellulosic substrate in one stage is vastly investigated. However,as pretreatment is a substantial cost factor, the ideal design of ananaerobic digestion process would allow the use of raw lignocellu-losic biomass.

The reactor type used in this study is the novel upflow anaero-bic solid-state (UASS) reactor for solid-state anaerobic digestionintroduced by Mumme et al. (2010). This reactor type promotes

322 M. Pohl et al. / Bioresource Technology 124 (2012) 321–327

spontaneous solid–liquid separation through differences in densityof substrate and process liquor and the adherence of self-producedbiogas bubbles. Thereby it enables both independent removal ofsolid residues and continuous process liquor recirculation. Further-more, there is no need for a stirrer, which reduces energy con-sumption and ensures, that the digestate with the longesthydraulic retention time is removed first. Additionally, a thermo-dynamic advantage of the unstirred system is the smaller distancebetween H2-producing and H2-consuming bacteria whose separa-tion would form a thermodynamic bottleneck (Kim et al., 2002).For those reasons, the UASS promises to be a highly effective reac-tor type for anaerobic digestion of fiber-rich substrate.

An important process parameter operating biogas plants is thereactor’s temperature. Two levels prevailed, mesophilic (30–35 �C) and thermophilic (55–60 �C) temperatures. Mesophilicdigesters tend to be more robust and tolerant to stress factors suchas high ammonia concentrations and fluctuating temperatures, butthe conversation rate is lower. Thermophilic digesters on the otherhand offer a higher methane yield and shorter retention times, butdo require more technological effort (e.g. for insulation) and ahigher energy input. In practice mesophilic digesters are still dom-inating due to their higher reliability and easier plant management(Yilmaz et al., 2008; Eskicioglu et al., 2011).

In a common one-stage anaerobic digestion process, the acid-forming and the methane-forming microorganisms are kept to-gether. As these two groups have different demands in regard tovarious factors such as temperature and pH, the one-stage designrequires a compromise favoring the methane producers. Pohlandand Ghosh (1971) circumvented this problem by physically sepa-rating acidogenic and methanogenic microorganisms into tworeactors, connected by the process liquor, providing optimizedenvironmental conditions for each functional group. They reportan enhanced process stability as well as better process control ina two-stage system.

The terms ‘‘two-stage anaerobic digestion’’ and ‘‘two-phaseanaerobic digestion’’ are often used as synonyms (Blumensaatand Keller, 2005). However, in this paper the following terminol-ogy is used: ‘‘two-phase’’ describes the separation of fluid and solidphases within a single reactor’s working volume, whereas ‘‘two-stage’’ describes the separation of hydrolysis and methanizationsteps.

Single objectives of this study were to determine the feasibilityof the novel upflow anaerobic solid-state (UASS) process for anaer-obic digestion of lignocellulosic biomass as well as to investigatethe long-term stability of wheat straw digestion and the potentialneed for nutrient addition. Furthermore the process performanceunder mesophilic and thermophilic temperatures and the accord-ing degradation kinetics were examined.

2. Methods

2.1. Feedstock and nutrient supplements: origin and characteristics

The substrate used was chopped wheat straw with a cuttinglength of between 5 and 65 mm. The organic dry matter contentwas 85.9% and the crude fiber fraction at 46.3%. The concentrationof ammonium–nitrogen has been below the detection limit of2 mg kg�1. The concentration of volatile fatty acids (VFAs) in thestraw, calculated as the sum of concentrations of C2–C6 acids,has been at 1.92 g L�1. Methane building potential of straw consid-erably depends on the past weather conditions as well as the timeof harvesting, so values found in the literature are varying in awidespread range from 55 to 185 L kg�1 (Oechsner, 2005). For abetter comparability of the UASS system with other reactor config-

urations using the same substrate and to save costs, the wheatstraw did not undergo any further pretreatments.

Weiland (2010) proposes a carbon to nitrogen (C:N) ratio of20:1 for anaerobic digestion, Deublein and Steinhauser (2010) rec-ommend to be in the range of 16:1–25:1. Although nitrogen can bebound in the lignin structure, the C:N ratio in the wheat straw fedwas determined to be 112:1, so a shortage in nitrogen available formicrobial growth over time could be assumed. The low content ofnitrogen was compensated by adding ammonium carbonate to theprocess. The aim was to reach an ammonium–nitrogen concentra-tion of 500–1000 mg L�1 in the liquid phase.

Due to a poor content in wheat straw, several trace elementshad to be added along with the feeding. They are essential formicroorganisms to grow and reproduce. The trace element solutionhas been mixed according to medium No. 144 of the ‘‘Germany col-lection of microorganisms and cell cultures’’ (Brunswick, Ger-many). Among others, the solution contained iron, calcium,copper, zinc and sodium. The dosage of the fivefold concentrationused was 0.01 g gVS

�1, which multiplies to 0.25 mL d�1 at organicloading rate (OLR) of 2.5 gVS L�1 d�1 (Abdoun and Weiland, 2009).

2.2. Anaerobic digester system

The technical setup is a quadruple two-phase, two-stage reactordesign, each of the four systems consisted of an upflow anaerobicsolid-state (UASS) reactor and an anaerobic filter (AF) to preventVFA accumulation. The UASS reactor had a working volume of39 L, the AF of 30 L. Both reactors were connected by means ofthe process liquor circulation (Fig. 1). Direction of flow for UASSas well as AF reactors was from bottom to top. The volumetric flowrate of the peristaltic pumps driving the process liquor was set to1.15 L h�1.

The UASS reactors as well as the AFs were built with a heatedwater jacket using LAUDA thermostats (Lauda, Lauda-Königshofen,Germany). The UASS reactors were built from stainless steel withan inspection window made of acrylic glass, whereas the AF reac-tors were completely built from transparent acrylic glass.

All UASS reactors featured a perforated plate to hold the solid-state bed below the liquid surface. AFs were filled with polyethyl-ene biofilm carriers (‘‘Bioflow 40’’, RVT Process Equipment GmbH,Germany), 325 each, with a surface of 305 m2 m�3 for biofilmestablishment.

Online measurement included pH, liquid temperature (bothMettler-Toledo, USA) and biogas volumetric flow (Ritter, Ger-many). The biogas volume was normalized to standard conditionsof 1013 mbar, 273.15 K and 0% humidity. The gas compositionregarding CH4, CO2, O2 and H2S was analyzed on a daily basis usingan industrial biogas analyzer SSM 6000 (Pronova, Germany).

The setup of four AD systems was used to investigate the effect oftwo temperatures, 37 and 55 �C, using a fixed OLR of 2.5 gVS L�1 d�1.For a higher statistical significance, the experiments were run induplicates. Consequently, two reactors were operated under thesame conditions throughout the experiment.

Substrate was added in a daily load through the diagonal feed-ing tube. Digestate was removed manually once a week by openingthe top cover of the UASS reactor. In order to determine theremaining methane potential, the digestates were subjected to bio-chemical methane potential assays.

2.3. Analytical methods

The biochemical methane potential (BMP) of the straw and thedigestates was determined in accordance with the guideline ‘‘VDI4630’’ (2006) at 37 and 55 �C, respectively. Differing from theguideline, experiments have been carried out with a volatile solids(VSs) based inoculum to substrate ratio of 1 instead of 2, which has

pump

biogasprocess liquor

UASS AF

solid-state bed

biofilm carriers

T pH, T

T pH, T

feeding tube

Fig. 1. Scheme of the reactor system’s setup (UASS, upflow anaerobic solid-state reactor; AF, anaerobic filter).

M. Pohl et al. / Bioresource Technology 124 (2012) 321–327 323

insignificant effects on the results (Raposo et al., 2006). The trialswere conducted in 2 L glass fermentation vessels equipped withseparate gas holders. Biogas composition was determined fromthe gas collected in the gas holders. In order to obtain the finalmethane yield (yCH4 ;max) the measured course of the yield wasextrapolated using a fitted Chapman model (Linke and Mähnert,2005), as this model gives a good fit due to its sigmoidal character(Eq. 1). Another viable way would be the use of Contois kinetics, asrecommended by Vavilin et al. (2001), which assumes that thekinetics do not depend on the substrate concentration, but onthe amount of substrate per biomass unit.

yCH4¼ yCH4 ;max � ð1� expð�k � tÞÞb ð1Þ

The effluents from all UASS and AF reactors as well as the soliddigestates were analyzed for their chemical properties on a weeklybasis. Samples have been taken along with the digestate removal.Chemical analyses for all samples, liquid as well as solid, includedpH, electric conductivity, total solids, volatile solids, total ammonianitrogen, total Kjeldahl nitrogen, COD, volatile fatty acids (C2–C6),and phosphorus. Additionally, effluents and digestates have beenanalyzed for trace element concentrations every 4 weeks (Mg, K,Ca, Na, Fe, Ni, Co, Cu, Zn, Mn, Cr, Pb, Mo, P, S and B). The reactor’seffluents have been analyzed for total organic (TOC) and total inor-ganic carbon (TIC), as well. The digestates have also been assayedfor carbon, nitrogen, sulfur, hydrogen, and crude fiber contents.

Volatile fatty acids have been measured with a CP-3800 gas-phase chromatograph (Varian Inc., USA), equipped with a FFAP col-umn (30 m � 0.32 mm, film thickness 0.5 lm by Permabond, USA)and a flame ionization detector. Trace elements have been mea-sured using an iCAP 6000 Series ICP-MS (Thermo Fisher ScientificInc., USA). C, N, S, and H fractions of the digestate have beenanalyzed with a vario EL III elemental analyzer (ElementarAnalysensysteme GmbH, Germany).

2.4. Calculation and statistical methods

Statistical analyses of the data measured have been carried outto judge the quality of curve fitting as well as to rate the concor-dance of the duplicate continuous experiments. The Pearson prod-uct-moment correlation coefficient (PPMCC) was used to describe

the degree of consistency between the reactors operated as dupli-cates. PPMCC’s results range from �1 (inversely proportional) via 0(no correlation) to +1 (straight proportional).

The phases of decay, days 185–219, have been used for determi-nation of the hydrolysis rate constants (kH). Therefore the dailymethane production rates of the UASS and the associated AF havebeen summed up. The Chapman kinetics (Eq. 1) have been fitted tothe measured data using a non-linear least squares routine.

3. Results and discussion

3.1. Reactor startup and operation

The operation of the reactor systems was divided into four con-secutive stages. In the initial startup phase (days 0–33), it wasawaited for the inoculum to release remaining biogas potential.As gas release of the inoculum was negligible, the second phase(days 34–162) was initiated, in which the reactors have been fedfor 5 days a week. Afterwards, in stage three, for 7 days a week(days 163–185). In the terminal fourth stage (days 185–218), feed-ing was stopped in order to record the phases of decay. Despite thechange in the feeding regime, the organic loading rate ofOLR = 2.5 gVS L�1 d�1 was kept constant in stages two and three.A shorter start-up phase in the thermophilic systems, as reportedby Li et al. (2011), has not been observed in our experiment.

Prior to the actual experiment all systems were tested for gasand liquid tightness. Despite this, some occasional leakages inthe biogas lines were observed during operation (drops in methaneproduction rate in Fig. 2a and b, days 40–60) and sealed. The reac-tor systems have been inoculated with a 5 kg mixture of wheatstraw and digestate from a 300 L UASS reactor mesophilically fer-menting maize silage, filled up with tap water. The mixture had apH of 8.35, a COD of 185.5 g kg�1, a total solids mass fraction of165.4 g kg�1, and a volatile solids mass fraction of 151.5 g kg�1.

The height of the solid-state bed, measured from the top sieve,was held at an average height of 0.47 m, mainly influenced by themass taken out at digestate removal. Taking into account, thatmass balances for the meso- and thermophilic reactors have beenkept equal, a variance in the height of the solid-state bed was ob-served. The beds of the thermophilic reactors have generally been

days of operation

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UASS

AF

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AF

UASS

AF

UASSAF

Fig. 2. Methane production rates of the four reactor systems analyzed (a and b:thermophilic, c and d: mesophilic).

Table 1Results from BMP tests referred to fresh matter (FM) and volatile solids (VSs) of thesubstrate (arithmetic averages from duplicates, R2: coefficient of determination ofChapman-fit).

ybiogas,max [L kgFM�1] R2 yCH4 ;max [L kgFM

�1] R2

Mesophilic 385.79 0.994 216.97 0.994Thermophilic 483.33 0.995 270.44 0.995

ybiogas,max [L kgVS�1] R2 yCH4 ;max [L kgVS

�1] R2

Mesophilic 434.09 0.994 244.16 0.994Thermophilic 543.85 0.995 304.29 0.995

40

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[%]

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0

20

40

60

(a)

(b)

(c)

(d)

AF

UASS

UASS

AF

AF

UASS

UASS

AF

Fig. 3. Methane fractions in the biogas produced by the reactors analyzed (a and b:thermophilic, c and d: mesophilic).

324 M. Pohl et al. / Bioresource Technology 124 (2012) 321–327

smaller, 0.45 m compared to 0.49 m, which points to a higher de-gree of substrate decomposition. This observation is verified byhigher hydrolysis rate constants for the thermophilic process. Vol-ume lost by weekly removal of 3 kg of digestate was compensatedby refilling the system with 5 L of tap water. Water loss by evapo-ration into biogas was calculated to be 18 g m�3 on average, forchemical analysis 0.25 L of process liquor has been removed aweek, so digestate removal is the main water drain of the process.The water fraction in the biogas has been determined using theequations and parameters published by the IAPWS (1992).

By observation of marked straw chaffs, the solids retention time(SRT) of the wheat straw in the UASS was determined to be be-tween 14 and 21 days. Clogging of the reactor by degraded wheatstraw has not been observed during the experiment.

The correspondences of the experimental results are shown inthe correlation coefficients of the methane production rates ob-served from the reactors. With a PPMCC of 0.94 the results of thethermophilic UASS reactor during steady-state (days 65–202)show a positive correlation. For the mesophilic UASSs the correla-tion coefficient of 0.8 shows a linear dependence of the duplicatesas well. The coefficients for the thermo- and mesophilic AFs havebeen determined to be 0.85 and 0.88, respectively.

Table 1 shows the results of biochemical methane potentialtests carried out with the wheat straw taken as substrate for theanaerobic digestion experiments. Values for both temperatureshave been determined in duplicates. In contrast to Hegde andPullammanappallil (2007) our experiments did reveal significantdifferences between thermophilic and mesophilic methane yield

potentials. As for the continuous experiments, BMP assays showeda higher methane yield potential at thermophilic temperatures.However, the thermophilic BMP tests revealed a 24.6% highermethane yield than the mesophilic ones, while the thermophiliccontinuous experiments exceeded the mesophilic counterpartsby 36%.

3.2. Biogas and process liquor characteristics

Methane contents of the produced biogas observed in steadystate have been between 41.6% and 60.8%, the remainder beingmainly carbon dioxide. Fig. 3 shows that the process temperaturedoes not have a direct effect on the composition of the biogas pro-duced. Except for the low methane levels in AF 1 and UASS 4 at thebeginning of the experiment which occurred due to leakages, the

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el. conductivity

ammonium

el. conductivity

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ammonium

el. conductivity

ammonium

Fig. 4. UASS’ ammonium–nitrogen levels and electrical conductivity in the UASSeffluents during the experiment.

M. Pohl et al. / Bioresource Technology 124 (2012) 321–327 325

methane and carbon dioxide fractions are on a comparable level.Oxygen contents have been close to zero or zero throughout theexperiment. It is obvious though, that the AFs are specialized onmethanization, like it is commonly seen in two-stage processes.This expresses in a generally higher fraction of methane in the bio-gas compared to the UASSs outputs. Changing the feeding regimeto daily intervals (days 163–185) resulted in a smoothing in gasproduction, but did neither affect the weekly averages of biogasproduced nor the methane fractions.

VFA concentration increased during the experiment from initial467.8 mg L�1 up to 1874.7 mg L�1 as an average over all eight reac-tors’ effluents. Although VFAs are known to have an inhibitory ef-fect on biogas production (Mata-Alvarez et al., 2000), theconcentrations reached in the experiment conducted seemed notto be of a level sufficient for a measurable process inhibition.

Although there was no measurable difference in the effluents ofthe UASSs and the AFs within one system in terms of chemical oxy-gen demand (COD), a difference in the COD depending on the tem-perature level was observed. While the mesophilic liquors showeda slight increase from 3.5 to 3.6 g L�1, the thermophilic counter-parts’ COD levels rose from 3.5 up to 5.8 g L�1 throughout theexperiment. pH of the process liquor increased during the experi-ment for the thermophilic as well as for the mesophilic UASS reac-tors from 7.2 to 8.3 and from 7.0 to 7.9, respectively.

Electrical conductivity in the liquid phase, a sum parameter forthe salt concentration in the process liquor, rose in all reactors dur-ing time of the experiment from a starting value of 3 mS cm�1 upto 10.5 mS cm�1, which has been stable through the last 3 weeksof the experiment. This behavior is in line with the observationsof Nordberg et al. (2007) that two-phase anaerobic digestion sys-tems tend to enrich dissolved inorganic compounds. As knownfrom full scale plants, an electrical conductivity of 20–30 mS cm�1

is tolerable for an anaerobic digestion process, although it is diffi-cult to numeralize a potential inhibition from a sum parameter.

As Fig. 4 shows, there is no difference in both, ammonium accu-mulation in the liquid phase and in the uptake of nitrogen fromammonium by microorganisms, regardless of the temperature le-vel. Averaged over the four systems, ammonium–nitrogen rosefrom 39.75 to 1131.97 mg kg�1 in the liquid phase. The ammoniumlevels show a very good correlation with the electrical conductivity(avg. PPMCC = 0.99) measured in the same reactor’s effluent. As theammonium ion is gathered along with other salts in the electricalconductivity measurement, the good correlation indicates thatother salts are not accumulating in the process liquor.

From the literature only very inconsistent or vague values for aninhibitory effect of ammonium–nitrogen are found, as the effectappears not to be primary depending on the absolute concentra-tion. In fact, methanogens have the ability to adapt to rising levelsin ammonium, as far as adequate time for accommodation is given,but the ammonia level should not exceed 200–300 mg L�1. Duringthe experiment, the ammonium concentration in the liquid phaseslightly surmounted the desired upper limit of 1000 mg L�1, butno inhibitory effects have been observed. As inhibition by ammo-nia rises along with pH and temperature (Gallert and Winter,1997), there should have been an impact on the biogas yield ofthe thermophilic reactors first. On the other hand, Gallert et al.(1998) point out that the thermophilic flora is capable of toleratingsignificantly more ammonia than the mesophilic flora. Addition-ally, among the anaerobic microorganisms, the methanogens arethe least tolerant and the most likely to cease growth due toammonia inhibition (Chen et al., 2008).

The addition of sulfur to the process did not show a positive ef-fect on either biogas rates or methane fractions. A negative effecthas obviously been a rise in hydrogen sulfide (H2S) in the biogasproduced. A rise from 200 ppm before the addition to more than500 ppm was observed.

3.3. Characteristics of the solid residue

In contrast to the process liquor’s behavior, COD in the digestaterose almost uniformly during the first weeks of feeding for bothtemperature levels, from a primary COD of 118 g kg�1 to a valueof 160 g kg�1 in average. During the time of steady-state, theCOD of all four reactors observed stayed at a constant level of about160 g kg�1. The pH of the digestate removed was steadily in therange of 8.5 and 9.2 for all four UASS reactors.

The electrical conductivity in the weekly removed digestate in-creased along with conductivity of the process liquor but on asmaller scale, numerically from 0.23 to 0.71 mS cm�1. This indi-cates that salts stayed dissolved in the process liquor rather thanbeing held back by the solid-state bed, which is assumed to havethe function of a fixed bed filter. In contrast, VFAs have been foundin higher concentrations in the digestate, than in the process li-quor. This is explained by fermentative and acetogenic microor-ganisms adhered to the straw’s surface in the solid-state bed. Asfor this low OLR, methanogenesis takes place in the solid-statebed as well, VFAs are not leached out of the bed. This is backedby the fact, that the AFs, intended to be the methanogenic stage,produce only little amounts of biogas.

Ammonium–nitrogen in the solid-state bed continuously rosefrom 86.3 to 940.2 mg kg�1, which suggests that essential nitrogenis withdrawn from the process during digestate removal. Overallthe solid-state bed seems to act as a filter holding backmicroorganisms, rather than chemical compounds. Results fromthe microbiological analysis will be published later.

thermophilic reactors

methane rate UASS 1 [L L-1 d-1]

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y = 0.93x + 0.02R² = 0.84

y = 0.954x + 0.02R² = 0.65

Fig. A1. Correlation of thermo- and mesophilic UASS reactors additionally showingthe slope of the correlation (thermophilic: 0.93, mesophilic: 0.954).

326 M. Pohl et al. / Bioresource Technology 124 (2012) 321–327

3.4. Influence of temperature

As for the biochemical methane potential assays, Chapmankinetics (Eq. 1) were applied to identify hydrolysis rate constants(kH) of the continuous process. Hydrolysis is widely consideredthe rate limiting step in anaerobic digestion, but altering tempera-ture mainly affects acidogenesis (Donoso-Bravo et al., 2009). Curvefitting revealed that hydrolysis in thermophilic reactors is morethan twice as fast as in their mesophilic counterparts.Thermophilic hydrolysis rate constant is computed to bekH,thermo = 0.066 d�1, whereas the mesophilic hydrolysis rate con-stant is identified to be kH,meso = 0.032 d�1. Therefore the unusedmethane potential in the solid-state bed after feeding is higher inthe mesophilic reactors. While the thermophilic reactors’ solid-state beds hold an average remaining methane potential of55 L kgVS

�1, the mesophilic digestates still reveal a potential of115 L kgVS

�1. This result supports the hydrolysis rate constantsfound to be in a realistic proportion, as the faster digested thermo-philic solid-state bed holds less remaining methane potential. Asalready stated in Section 3.2, biogas composition – especially in re-gard to the methane fraction – did not noticeably differ betweenmesophilic and thermophilic processes. This advantage of the ther-mophilic degradation of biomass can be explained by the empiricalArrhenius equation, stating that chemical reaction rates double at arise in temperature of 10 �C. Additionally, thermophilic microor-ganisms have higher specific growth rates than their mesophiliccounterparts (Kim et al., 2002).

For the thermophilic UASS–AF systems an average methaneyield of 165 LCH4 kgVS

�1 was measured, whereas the mesophilicsystems yielded 121 LCH4 kgVS

�1. The higher gas rates and yields,as shown in Table 2, are mainly attributed to faster disintegrationand hydrolysis at higher temperatures – most likely due to a higherenzymatic activity at higher temperatures (Kim et al., 2012),whereas the lower rate of methanogenesis at thermophilic temper-atures (Kim et al., 2002) is of minor consequence (Donoso-Bravoet al., 2009). In contrast the methane yields of the stabilizing AFreactors are higher at mesophilic temperatures, which suggest thatthe mesophilic UASS’ effluents contain higher concentrations ofVFAs available for methanization in the AF.

Calculating the COD degradation efficiency between the wheatstraw fed and the digestate taken out, COD degradation rates of49.8% and 47.9% have been determined for the thermophilic and themesophilic reactor couples, respectively. This seems a very small dif-ference in degradation, having in mind that the thermophilic UASSreactors yielded 66.7% more methane in their steady state.Presumably, this disproportion is due to the high amount of non-degradable COD in the substrate used. This indicates that on differenttemperature levels, different microbial consortia are more or lesscapable of breakingdownthe substrate’s structure. A detailed analysisof the microbial consortia in the reactors will be published elsewhere.

As stated in Section 2.3, trace elements have been analyzedevery 4 weeks throughout the experiment. The concentrations of

Table 2Reactor performances (arithmetic averages from duplicates).

UASS Unit Thermophilic Mesophilic

Biogas production rate L L�1 d�1 0.734 0.428Methane production rate L L�1 d�1 0.403 0.239Methane yield L kgVS

�1 161 96Hydrolysis rate constant (kH) d�1 0.066 0.032AF

Biogas production rate L L�1 d�1 0.08 0.119Methane production rate L L�1 d�1 0.05 0.079Methane yield L kgVS

�1 19 31Combined system (UASS + AF)Methane yield L kgVS

�1 165 121

several trace elements have not been influenced by the tempera-ture of the process, e.g. Ca, Cr, K, Mg, Mn, Mo and Na. Others haveshown higher concentrations by accumulation during the experi-ment in the thermophilic UASSs, as there were: B, Co, Cu, Fe, Pand Zn. While moderate concentrations of trace elements act as anutrient for microbial growth and can therefore raise methaneyields, anaerobic digestion of industrial wastewater shows theirinhibitory potential at high levels (Demirel and Scherer, 2011). Cal-cium concentration for example was between 80 and 177 mg L�1,where a concentration of about 200 mg L�1 is recommended, butinhibitory effects could be seen above 2500 mg L�1 (Chen et al.,2008). Chen et al. (2008) further reported that low levels of potas-sium (<400 mg L�1) enhance the thermophilic and mesophilicanaerobic digestion, while higher concentrations can act inhibi-tory, especially in thermophilic system. For our experiment, potas-sium concentrations rose throughout the experiment from 1000 to1200 mg L�1. Altogether, a difference in trace element concentra-tions from the UASS’ effluents to the AFs effluents could in generalnot be observed. Sodium concentrated up from 50 to 65 mg L�1 onaverage on day 150 of the experiment and kept a stable levelafterwards.

4. Conclusions

Our results reveal that it is technically feasible to continuouslyferment lignocellulosic biomass in a long term process without sig-

M. Pohl et al. / Bioresource Technology 124 (2012) 321–327 327

nificant disturbances. For a good process performance, addition oftrace elements is necessary, though. At a SRT of 14–21 days, thethermo- and mesophilic UASS–AF systems yielded 38% and 50.1%of the substrate’s methane forming potential, respectively. Addi-tionally, the thermophilic systems outperformed the mesophiliccounterparts with faster hydrolysis.

Topics for future research include further temperature levels,higher organic loading rates and cofermentation. Discontinuousfeeding and digestate removal cycles may lead to an on-demandmethane production.

Acknowledgements

This work is funded by the German Federal Ministry of Educa-tion and Research (BMBF) in cooperation with Project AgencyJülich (PtJ). The authors would like to thank T. Zenke for settingup and operating the lab scale experiments. Furthermore we wouldlike to thank M. Sontag and E. Janiszewski for conducting BMP testsand lab analyses, respectively.

Appendix A

(See Fig. A1).

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