Stirring and biomass starter influences the anaerobic digestion of different substrates for biogas production

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  • Research Article

    Stirring and biomass starter influences theanaerobic digestion of different substrates forbiogas production

    Here, we present the results of lab-scale experiments conducted in a batch modeto determine the biogas yield of lipid-rich waste and corn silage under the effect ofstirring. Further semi-continuous experiments were carried out for the lipid-richwaste with/without stirring. Additionally, it was analyzed how the starter used forthe batch experiment influences the digestion process. The results showed asignificant stirring effect on the anaerobic digestion only when seed sludge from abiogas plant was used as a starter. In this case, the experiments without stirringyielded only about 50% of the expected biogas for the investigated substrates. Theaddition of manure slurry to the batch reactor as part of the starter improved thebiogas production. The more diluted media in the reactor allowed a better contactbetween the bacteria and the substrates making stirring not necessary.

    Keywords: Anaerobic process / Biogas plant / Biogas yield / Starter / Stirring

    Received: December 3, 2009; revised: May 25, 2010; accepted: June 8, 2010

    DOI: 10.1002/elsc.200900107

    1 Introduction

    The aim of any biogas plant is the production of energythrough the digestion of organic waste. The biogas plantsoperated with manure run generally with the addition ofdifferent cosubstrates. The addition of degradable cosubstrateachieves a higher biogas yield compared with the monodigestion of manure [13].

    The biogas yield of the individual substrates variesconsiderably depending on their origin, the content of organicsubstance and the substrate composition. The parameterscommonly used to characterize the cosubstrate such as organicdry mass and chemical oxygen demand cannot be used directlyto determine the biogas potential, due to different biode-gradabilities of the various cosubstrates [1, 4, 5]. It is thereforeessential that the substrates to be used in biogas plants aretested first in lab-scale batch to estimate the final biogas yieldin the plant. It has been assumed that the results obtained fromthe batch experiments describe the behavior of the substrate inthe anaerobic process of a continuous reactor (a normal biogasplant), assumed to be completely mixed [6].

    However, our experiments show that the results of the batchexperiment are not always transferable to a continuous process.

    There may be different reasons, depending on the setup and theperformance of the batch experiment. One reason might be thebioactive population of the bacteria provided for the anaerobicprocess in the batch reactor. To start the anaerobic process in abatch, an inoculum (starter substrate) containing a biodiversityof bacteria similar to those in the biogas plant, where thesubstrate shall be used, is necessary. This is usually done in theway that the outlet biomass sludge of the biogas plant is used asstarter material in the batch process (inoculum). This materialcontains the typical bioactive population of bacteria but only alow amount of digestible organic matter [7].

    For example, Carruci et al. [8] determined the biogaspotentials for different organic wastes with and in the absenceof mixing. Due to the low concentration of the inoculatedbacteria, the digestion is rather slow and not affected bytransport-related phenomena and thus by the mixing process.

    Another reason for the discrepancy between the batchresults and the behavior of the substrate in the continuousprocess reactor might be the stirring and the solubility/ordispersion of the substrate in the liquid suspension, influen-cing the bioavailability of the substrate in the anaerobicprocess. This had been observed in reactors operated withdifferent mixing methods [911].

    In practice, the biomass in a biogas plant is continuouslystirred, but it cannot be assumed that we have an ideal stirredtank reactor with a perfect mixing. Adequate mixing isessential to transfer substrate to the microorganisms, tomaintain uniformity and to prevent solids deposition [6].Therefore, it is an essential question as to how the stirring

    Christian Rojas

    Sheng Fang

    Frank Uhlenhut

    Axel Borchert

    Ingo Stein

    Michael Schlaak

    Institut f .ur Umwelttechnik

    EUTEC, Fachbereich

    Technik, Fachhochschule

    Emden/Leer, Emden

    Abbreviations: oTS, organic total solids; TS, total solid

    Correspondence: Christian Rojas (rrcj10@gmail.com), Institut f .ur

    Umwelttechnik EUTEC, Fachbereich Technik, Fachhochschule Emden/

    Leer, Constantiaplatz 4, D-26723 Emden.

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

    Eng. Life Sci. 2010, 10, No. 4, 339347 339

  • influences the results obtained by the batch experiment andhow a transfer of these results to the continuous reactor ispossible.

    For this reason, we have performed experiments in a batch[12] and in a continuous lab reactor (7 L) with stirring andwithout stirring using different starting materials (inoculum)and different substrates.

    1.2 Theoretical background

    1.2.1 The anaerobic process

    Biomass can be converted to methane through a biologicalgasification process known as anaerobic digestion. Anaerobicdigestion is a low-temperature process which involves thebiological breakdown or conversion of organic material tomainly methane (CH4), carbon dioxide (CO2) and H2O in theabsence of oxygen. The anaerobic decomposition is a complexprocess including intermediate molecules such as sugars,hydrogen and acetic acid, before biogas is produced finally[13].

    The anaerobic digestion process involves a large numberof microorganisms which convert the feedstock to themethane and carbon dioxide-rich biogas through a number ofdifferent processes. These microorganisms include hydrolyticbacteria, acetic acid-forming bacteria (acetogens) and metha-nogenic bacteria (methanogens). Anaerobic digestion which isa complex biochemical reaction is, in general, described byfour steps: hydrolysis, acidogenesis, acetogenesis and metha-nogenesis.

    In the first step, the hydrolysis, the big molecules ofcarbohydrates, proteins and lipids are split into smallercomponents (sugars, fatty acids and amino acids) by the helpof extra-cellular enzymes secreted by microorganisms. Acomplex consortium of microorganisms participates in thehydrolysis and fermentation of organic material. Most of thebacteria are strict anaerobes such as Bacteriocides, Clostridiaand Bifidobacteria [6]. In general, the amount of methaneproduced in the anaerobic digestion of organic compounds islimited by the hydrolytic reactions.

    The following step, the acidogenesis, involves the break-down of the various products from the hydrolysis into acids.The resulting hydrolytic products are immediately fermentedto short-chain fatty acids, CO2 and H2 during the subsequentfermentative acidogenesis by another group of facultative orstrictly anaerobic bacteria (e.g. Bacteroides, Clostridium,Butyribacterium, Propionibacterium, Pseudomonas and Rumi-nococcus). The major short-chain fatty acids formed includeacetate, propionate, butyrate, formate, lactate, isobutyrate andsuccinate, with acetate predominating [14].

    These acids are used in the third step, the acetogenesis, forthe formation of mainly acetate by the actions of the aceto-genic bacteria. The acetate is the major end product of theacetogenesis step in all anaerobic digesters [15]. It should benoted that because acetogenic bacteria cannot be cultured assingle cultures, acetogens are not well studied.

    Finally, the products of the acetogenesis will then beconverted to methane, carbon dioxide and water by the

    methane-forming bacteria (Archaea domain) in the final stage,the methanogenesis. Hydrogenotrophic methanogens producearound 30% of the CH4 via the reduction of CO2 by H2 or bythe conversion of other C1 substrates (e.g. methanol andmethylamines), whereas acetoclastic methanogens convertacetate to CH4, around 60% of the total amount [6, 14].

    1.2.2 Characteristics of substrates as coferments

    In most agricultural biogas plants, liquid manure is the mainsubstrate. Seeing as the manures total solids concentration israther low (around 57% for pigs and 79% for cows) and itslignocelluloses content is quite high, it is a substrate that whentreated alone gives low yields of methane. The fraction of fibersis difficult to digest and often passes undigested through theprocess. Hence, the potential for manure is about 1020m3

    CH4/ton of manure [1]. For this reason, manure is currentlybeing combined with other cosubstrates in order to improvethe biogas yield.

    However, as a carrier substrate that supports the anaerobicdigestion of other organic waste (cosubstrates or coferment)which is not easy to treat alone, manure as an additive is agreat option. The qualities that make manure a great carriersubstrate are as follows:

    (i) its high content of water (supporting to solve or dispersethe cosubstrate, often having a low water content),

    (ii) its content of a bacterial diversity (supporting thedigestion process),

    (iii) its capacity as pH buffer protecting the process against apH drop, which might influence the process negativelyand

    (iv) its supply of nutrients needed for an optimal bacterialgrowth

    Furthermore, the mixture of manure (with its high watercontent) and the concentrated cosubstrate allows the influenceof the retention of the dispersion in the reactor or the flow ratethrough the reactor [1].

    By adding cosubstrates to the manure, the organic fractionis increased and therefore the biogas yield [1, 4, 16]. The drymatter content in the total substrate formed by the manureand the cosubstrate should not exceed 12% in order to ensurecorrect pumping and mixing, crucial factors for any technicaltransformation process [4, 17, 18].

    A major agricultural waste, crop residues (as bagasse-sugarcane, corn stove and corn silage), is often used as acosubstrate (coferment). It is obtained from food processingafter harvesting [7].

    An incorporation of easily degradable matter can increasethe methane yield of the anaerobic process, especially when thefat fraction is considerable. Many off-farm substrates andespecially food-processing byproducts can be used, for exam-ple, kitchen waste, oily waste, contaminated phospholipidsfrom biodiesel manufacture, etc. [3, 5].

    The coferments that are used to produce biogas frequentlyare tested through batch experiments to determinate the biogaspotential. At the beginning of the batch process, the reactor is

    340 C. Rojas et al. Eng. Life Sci. 2010, 10, No. 4, 339347

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

  • fed with a starter biomass (the inoculum), introducing asuitable population of anaerobic microorganisms. The inocu-lums may come from sewage sludge or digested substrate of abiogas plant. Then the batch system is sealed and fermentationtakes place. The ultimate volume of biogas produced duringanaerobic degradation divided by the amount of organic drymatter initially introduced is known as the biogas yield and isexpressed as (m3/(t oTS)) (oTS, organic total solids). Theexperimental conditions can affect this value and the digestionvelocity, among them the stirring and the selection of theinoculum are between the most significant [1921].

    2 Materials and methods

    2.1 The substrates and coferments

    The inoculum and the manure for the experiments describedwere provided by the biogas plant in Wittmund, Lower Saxony.The inoculum is the digested biomass from this biogas plantconsisting of a mixture of manure and kitchen waste mainlyused as coferment.

    Before filling into the laboratory reactor, the inoculum(seed sludge from the Wittmund biogas plant) and the manurewere sieved (3.0mm mesh size) to eliminate big residues. Theprepared solutions were stored at 41C until use. Before startingthe anaerobic digestion experiments, samples of the assignedsubstrates were analyzed according to the standard methods:dry mass at 1051C, organic dry mass at 5501C. The Ammo-nium and TKN (Total Kjeldahl Nitrogen) were determinedwith Lange Test 303 and 338, respectively. The results for thesubstrates are summarized in Table 1.

    This study is related to the study of different substrates usedin the anaerobic digestion, in particular with the use of agri-cultural and food industry-related substrate. Differentsubstrates (lipid-rich substrate, crab meat-based substrate,corn silage and manure) were tested before these particularexperiments in batch reactors and in the continuous reactor,with a mixture of manure and digested substrate from a biogasplant. From those, two cosubstrates were selected for thefollowing reasons: their different digestion time and substrateparticle size. The selected coferments, lipid-rich waste (wastefrom kitchen, meat and milk-processing companies) and cornsilage were tested by the batch process. The biogas potential for

    these two coferments was measured through comparativebatch experiments.

    Only the lipid-rich waste was tested in the continuousreactor, the corn silage being difficult to pump under labora-tory conditions.

    The lipid-rich waste is widely used in biogas productionbecause of its high biogas yield. Around 70% of the organicmass comprises herbal and faunal lipids. This substrate is easyto pump and normally is collected in the grease separator fromkitchens, meat markets, slaughterhouses, margarine produc-tion lines, etc.

    Corn silage is an agricultural feedstock (cud-chewinganimals like cattle and sheep) rich in cellulose and easilyaccessible to the bacteria, a quality that provides a highmethane yield potential when used as a cosubstrate. Thecontent of total solids (% TS) for usual applications is around30%.

    Previous experiments under different loads with thesesubstrates gave a methane concentration of 6471 and57.471.2% for the lipid-rich substrate and corn silage,respectively, normal values for such substrates [16].

    The characteristics for both coferments, used in thisexperiment, are summarized in Table 2 together with the setupof the starter.

    2.2 Experimental setup

    (i) Stirring effect (batch reactor): A comparative study of thebiogas production with and without stirring was carriedout by a batch process operating in the mesophilictemperature range (371C) according to [12]. The batchreactors (digesters) have a total volume of 500mL. Biogasgenerated in each reactor was collected in a Eudiometer,values being recorded every 24h (Fig. 1A). The controlconsisted of inoculum (digested biomass from Wittmund),without any co-substrate. The setups for the digesters withco-substrates are summarized in Table 3. Half of the bottleswere stirred, with a magnet stirrer (approx. 60 rev/min).The difference between the biogas from the control digesterand from the digesters with co-ferments results in thebiogas production for the tested co-ferment. All the testswere realized as duplicates, the average results arepresented.

    (ii) Stirring effect (continuous reactor): A test in a continuousreactor with lipid-rich waste was additionally performed,where the stirring was similar to the b...

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