energy balance of a two-phase anaerobic digestion process for energy crops

Research Article

Energy balance of a two-phase anaerobicdigestion process for energy crops

This article deals with the digestion of energy crops in a two-phase biogas processbased on an anaerobic leach-bed reactor combined with an anaerobic filter. Thebiogas process is a microbiological conversion of biomass into methane andcarbon dioxide. This process is carried out by different microorganisms and canbe divided into four steps which normally take place in only one digester. To beable to digest difficult energy crops by mono-digestion and to meet the differentneeds of the several bacteria, which take part in the four-step process of themethane production, the process was divided into two phases: (i) an anaerobicbatch leach-bed phase, where the leachate was produced and (ii) an anaerobicfilter, where the organic fraction of the leachate was converted into biogas.Considering the results of the experiments, the two-phase digestion of energycrops exhibited stable digestion behavior. No biological imbalance of the process,e.g. due to a sudden change of substrate, was detected either in the leach bed or inthe anaerobic filter. Variation in suitability for two-phase fermentation with ananaerobic batch leach-bed reactor was observed for various substrates. Thedifferent substrates varied in their influence on acid formation and concentrationas well as an influence on the course of the pH value.Therefore, an effect on thedistribution of energy to the phases could be observed.

Keywords: Anaerobic filter / Biogas / Energy crops / Leach bed / Two-phase

Received: March 30, 2010; revised: October 4, 2010; accepted: October 18, 2010

DOI: 10.1002/elsc.201000071

1 Introduction

A reduction in pasture utilization has been observed in theGerman state of Baden-W .urttemberg. This was attributed tothe increasing efficiency in milk production. By 2015, 26% ofexisting grasslands will not be in use for fodder production [1].In that case, either the grass could instead be used as an energycrop or a part of the unused land could be used for theproduction of other energy crops. The possibility of using thisbiomass for renewable energy production without manure is ofgreat interest. In mono-fermentation, the substrates can bedifficult to digest. Grass for example, having high-fibercontent, could cause technical problems being used incontinuously stirred tank reactors [2]. Hence, for the mono-digestion of fiber-containing energy crops, a process withoutagitation is favorable. Processes working in this way often have

bad fermentation room utilization, because in single-phasesolid-state fermentation reactors it is usual to use up to 70% ofdigested material as inoculum [3]. Studies into a two-phasesystem have shown that higher methane yields can be achievedwhen compared with one-phase systems [4].

Two-phase systems offer the possibility to optimize thebiological process, because the degradation of an organicsubstrate to methane is a four-step process [5], and each ofthese steps is performed by several different bacteria. Thesebacteria have very special demands in terms of living condi-tions, and they each require different pHs and temperaturevalues for their optimal performance. In a single-phase biogasprocess, the four steps take place in the same fermenter, withthe same temperature and pH value, which are tailored to theneeds of the methanogenic bacteria. These are the mostsensitive organisms in the chain, whereas the hydrolyticbacteria are more flexible and adapt more easily to the livingconditions of the methanogenic bacteria [6].

The optimum pH range for the first steps of the biogasformation process (hydrolysis and acidogenesis) is between 5.5and 6.5 [7]. The metabolic optimum is at a pH of about 6.5.Most of the methanogenic bacteria, however, achieve theiroptimum stability and activity in the mesophilic range

Simon Zielonka

Andreas Lemmer

Hans Oechsner

Thomas Jungbluth

University of Hohenheim,

State Institute of Agricultural

Engineering and Bioenergy,

Stuttgart, Germany

Abbreviation: COD, chemical oxygen demand

Correspondence: Simon Zielonka ([email protected],

[email protected]), Universit.at Hohenheim Landesanstalt f .ur

Agrartechnik und Bioenergie, GarbenstraXe 9, 70599 Stuttgart,

Germany.

& 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.els-journal.com

Eng. Life Sci. 2010, 10, No. 6, 515–519 515

between 35 and 401C and at pH 7–7.5 [8]. Caused by the factthat methanogenic microorganisms are not able to producemethane at a pH value below 6.8, this value has to be exceededin single-stage digesters all the time. Therefore, the conditionsunder which hydrolysis takes place in a single-stage process aresuboptimal.

This suboptimal environment for the hydrolytic bacterialimits their performance. This results in an improvableconversion rate for the digestion of energy crops. The purposeof this project is to investigate whether the optimization ofenvironmental conditions for hydrolysis could improve theefficiency of bacterial fermentation of organic material.Considering the above-mentioned facts, the use of a two-phaseprocess could optimize the growth requirements of the variousmicroorganisms [9].

For this purpose, the University of Hohenheim developed aprocess for the digestion of energy crops on the principle of ananaerobic leach-bed reactor combined with an anaerobic filter.In the test plant, the energy contribution to the phases wasresearched with respect to different energy crop silages.

2 Materials and methods

2.1 Substrates

The substrates used were whole plant silages from corn, ryeand grass and were all sourced from within Germany. The cornsilage was obtained near Cottbus and was provided by theBrandenburg University of Technology Cottbus. The rye silagecame from the Agrarbetrieb Damsdorf GbR farm in Damsdorfnear Potsdam and was provided by the Leibniz Institute forAgricultural Engineering Potsdam-Bornim. The grass silagewas obtained from an intensively used grass land (four cuts ayear) from a test station of the University of Hohenheim inStuttgart.

2.2 Experimental setup

The experiments were performed in the biogas laboratory ofthe State Institute of Agricultural Engineering and Bioenergyof the University of Hohenheim. The percolation systemconsisted of five pairs of vertical solid-phase digesters with auseable volume of 50 L per digester.

The hydrolysis and acidogenesis phases of the biomassdigestion took place in the first digester, an anaerobic leach-bed reactor, which was driven at 551C. This temperature wasselected because the variation of the temperature in the leach-bed reactors showed that the thermophilic variant (551C) hadthe fastest and highest biogas yield and the highest degree ofdegradation compared with a mesophilic (381C) andpsychrophilic variant (251C) [10, 11]. In the leach-bedreactor, the biomass is degraded to organic acids whichaccumulated in the fluid fraction (the leachate). The leachatewas then pumped into the anaerobic filter where the organicfraction is converted into methane and carbon dioxide. Theanaerobic filters were running at a temperature of 381C andwere filled with polyethylene packing as a settling bed for

microorganisms and 45 L of percolate. Initially, the liquidphase of a separated effluent of a praxis scale biogas plant wasused as an inoculum to start the anaerobic filters. At the timeof the experiments described here, the anaerobic filters whererunning for about 2 years with the leachate of grass silage. InFig. 1, a schematic diagram of the experimental biogas plant isshown.

2.3 Analytic equipment

The parameters pH and chemical oxygen demand (COD) wereanalyzed in the percolate at the outflow of the digesters. ThepH was measured with a WTW 323 handheld pH meter with aSenTix 41 probe (WTW, Weilheim, Germany). The COD wasanalyzed with the Dr. LANGE cuvette test LCK 014(1000–10 000 mg/L) and the HACH LANGE Thermostat LT200. The Substrate and the digestate were analyzed for theirheat of combustion with the bomb calorimeter PARR 6100.Therefore, the substrate was dried at 601C for 48 h and milledto 2 mm. The biogas production was measured every secondday with a gas meter converted to norm liters according to thereport line VDI 4630. The biogas was analyzed for CH4, CO2

and H2 quantities with a SICK MAIHAK S710 analyzer.

3 Results and discussion

The presented results are based on four replicates of the variantcorn silage, four replicates of rye silage and two replicates ofgrass silage.

For the experiment, the leach-bed reactors were filled withabout 1 kg VS of energy crops silage and 10 kg of tap water. Nodigestate was used to start the process. After filling the leach-bed reactors with energy crops silage, they were operatedinitially with internal recirculation for several days (startup

Figure 1. Schematic of experimental biogas plant for two phaseprocess management at the University of Hohenheim.

516 S. Zielonka et al. Eng. Life Sci. 2010, 10, No. 6, 515–519


phase). In addition to the internal percolation after the startupphase, the leach-bed reactors were connected to the anaerobicfilters and 3.25 kg of percolate was exchanged between thestages once a day. The reflux of the percolate of the anaerobicfilter with a pH of about 7.5 into the leach-bed reactor causedan increase in the leachate pH. If the pH reached about 6.5,then methane production started in the leach-bed reactors.After 25 days retention time, the gas production decreasedmarkedly and the experiment was stopped.

The substrates used – whole plant silages from corn, rye andgrass – showed an individual behavior during the fermenta-tion. As an example in Fig. 2, the energy balance of thesubstrate corn silage is shown.

From the 100% input energy of corn silage, 18% left thesystem as leach-bed reactor digestate. This also contained thebiomass which was built up in the leach-bed reactor. Briefly,22% of the energy input was converted into leach-bed reactorgas. In total, 66% of the energy input was transported with theleachate into the anaerobic filter. The reflux back into theleach-bed reactor contained 8% of the energy of the substrate.

Figure 2. Energy balance of corn silage in a two phase biogasplant with an anaerobic batch leach bed reactor and a fixed bedreactor (Leach bed reactor temp.: 551C, fixed bed reactor: 381C).

Table 1. Energy balance in a two-phase biogas plant with a leach-bed reactor and a fixed bed reactor.

Grass silage Rye silage Corn silage

Input Substratea) MJ 20.46 19.8 19.22

Output Leach-bed reactor digestatea) MJ 3.3370.30 5.3070.20 3.4670.65

Biogas leach-bed reactorb) MJ 6.2170.55 5.5270.50 4.2970.92

Biogas anaerobic filterb) MJ 7.2070.68 8.5671.09 10.1070.62

Leachatec) MJ 10.1770.51 11.4570.32 12.6370.43

Methane reactor effluentc) MJ 1.8570.08 1.6370.25 1.5070.15

Biomass production anaerobic filterc) MJ 0.00470.002 0.00470.003 �0.00270.002

Heat MJ 3.7270.94 0.4171.13 1.3870.28

a) Analyzed with a bomb calorimeter.b) Calculated from the methane and hydrogen content of the biogas.c) Calculated from the COD.

Figure 3. Distribution of thebiogas yield to the stages of atwo phase biogas plant withanaerobic leach bed reactor andfixed bed methane reactor usingdifferent energy crops.

Eng. Life Sci. 2010, 10, No. 6, 515–519 Energy balance of a two-phase biogas digestion process 517


Hence, 58% were converted in the anaerobic filter. A total of53% left the anaerobic filter as biogas. For the leach-bedreactor, there was 2% remaining and for the anaerobic filterthere was 5% remaining which did not leave the system. Theassumption was that the remainder is used for biomassproduction or it may be heat loss. Through the analysis of theCOD of the anaerobic filter before and after the experiment,the biomass production could be excluded because the CODdecreased slightly by around 0.002% which was no significantchange. Therefore, about 8% of the input energy was convertedinto heat.

Table 1 summarizes that each tested substrate had a char-acteristic energy balance.

It is clear that the distribution of energy to the biogasof the two stages varies. Using the substrate grass silage,only 54% of the energy of all the produced gas is produced inthe anaerobic filter. With rye silage as the substrate, it isalready 61% and using corn silage it is 70% (Fig. 3). This canbe attributed primarily to the different amounts of energywhich the leachate removed from the leach-bed reactor(Table 1).

The reason for this difference is the difference in pHchange. In Fig. 4, the pH values of the percolate (measured atthe effluent of the leach-bed reactors to the methane reactors)from over the whole experiment period, are shown. At thebeginning, the leachate of all substrates in the leach-bed

Figure 4. The increase of the pH-value of the percolate of differentsubstrates in a two phase diges-tion with a leach bed reactor.

Figure 5. acid equivalent of grass,rye and corn silage in leach bedreactors.

518 S. Zielonka et al. Eng. Life Sci. 2010, 10, No. 6, 515–519


reactors had pH values between 4.5 and 5. Until the percolatewas exchanged for the first time between phases, the pH valuesremained relatively stable. After the first exchange of processfluid between the stages on day 4 of the experiment, a suddenincrease in pH was observed caused by mixing of liquids withdifferent pH values. Although the pH values of corn silagequickly fell below 5 again, the values of grass and rye silagereached only a pH of 5.6 or 5.7. Then the pH value of grasssilage in the leach-bed reactor increased much faster than theones of the other substrates, so that the end pH value wasreached days earlier (Fig. 4).

The slower pH increase is due to the acid formation of thesubstrates. In Fig. 5, the acid equivalent is shown. In thestartup phase between day 0 and day 4, with internal circula-tion of the leachate in the leach-bed reactors, a slight increaseof the acids was observed. At day 4, the acid equivalents of allsubstrates decreased about 1 g/kg because of the first leachateexchange between the phases. After this decrease, a strongincrease up to about 8 g/kg was recorded until day 7. For theremaining time of the experiment, the acid equivalentsdecreased to a level of 0.25 g/kg. Although acid equivalents ofthe grass silage needed about 7 days to decrease to this level,the acid formation of rye silage and corn silage lasts about 10days longer.

4 Conclusions and prospect

The two-phase digestion of energy crops silage showed stabledigestion behavior. No imbalance of the process was detected,either in the leach-bed reactor or in the anaerobic filter, despitea sudden substrate change following long-term feeding withgrass silage. Every tested substrate showed its own character-istic digestion behavior. Compared with grass silage, corn andrye silage showed a longer ongoing acid formation in theleach-bed reactor which kept the pH on a more suitable levelfor hydrolysis. Therefore, the corn silage provided the bestallocation of energy to the phases with 53% of the energy inputin the biogas of the anaerobic filter. Hence, a difference insuitability for two-phase fermentation with a leach-bed reactoris observed for the investigated substrates.

Substrates with a longer acid formation period seem to bemore suitable for a batch process with acidification in theleach-bed reactor. Research is also needed to assess whether afed-batch process has advantages for substrates with a shortacid formation period.

Acknowledgements

In the frame of the cooperative research project ‘‘Biogas CropsNetwork,’’ the project was done in close cooperation with theLeibniz Institute for Agricultural Engineering Potsdam-Bornim (ATB) and the Brandenburg University of Technology,

Cottbus. This project was sponsored by the German federalministry of education and research (BMBF).

Conflict of interest

The authors have declared no conflict of interest.

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energy balance of a two-phase anaerobic digestion process for energy crops

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