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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 [1–3].

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 [9–11].

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 ([email protected]), 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, 339–347 339

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 manure’s total solids’ concentration israther low (around 5–7% for pigs and 7–9% 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 10–20 m3

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, 339–347


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 [19–21].

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 (3.0 mm 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 500 mL. Biogasgenerated in each reactor was collected in a Eudiometer,values being recorded every 24 h (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 batch experiments.Later, a second test in the continuous reactor was carriedout without stirring (Table 4). The reactor was inoculatedwith the same seed sludge (inoculum) as for the batchexperiments. The reactor can be regarded as a CSTR with athermal jacket, having a capacity of 7 L of working volume(Fig. 1B). The biogas which was discharged from thereactor was measured continually using a milligascounterand an infrared-gas sensor device. The feed was introducedevery 2 days to assure that the substrate of the feed wastotally digested before the next charge was introduced(with stirring). Therefore, the feed without stirring had the

Table 1. Characterization of the inoculum (seed sludge from theWittmund biogas plant) and manure used for the anaerobicdigestion.

Substrate Test 1: 100%

seed sludge

Test 2: 40% seed

sludge, 60% manure

Seed sludge Seed sludge Manure

Total solids (% w/w) 5.19 5.47 1.38

Organic total solids

(% TS)

60.15 62.80 61.08

TKN (% TS) 7.1 8.4 6.0

Ammonium (% TS) 5.7 6.3 5.2

pH 8.0 8.0 7.6

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same feed frequency to make the comparison between bothcases. In this case, the low-substrate feed (2.5% reactorvolume) allows a complete fermentation in comparison

with larger reactors. The operation was indeed semi-continuous and the reactor presented a short mixing afterfeeding in both cases, to assure a good mixture of theaggregated substrate in the reactor.

(iii) Change of starter by addition of manure for the batch setup:A mixture of seed sludge (digested biomass fromWittmund) and manure was used as inoculum to findout the effect of starter composition on the biogasproduction in the batch process. The digesters were

Table 2. Characterization of the coferments used for the research of the stirring effect on the anaerobic digestion.

Substrate Test 1: 100% seed sludge Test 1: 100% seed sludge Test 2: 40% seed sludge, 60% manure Reactor test

Lipid-rich waste Corn silage Lipid-rich waste Corn silage Lipid-rich waste

Total solids (% w/w) 13.46 30.55 13.46 31.01 6.71

Organic total solids (% TS) 78.26 95.74 78.26 96.38 81.1

NTK (% TS) 1.5 0.5 1.5 0.5 1.5

Ammonium (% TS) 0.5 0.2 0.5 0.2 0.5

pH 4.5 4.5 4.5 4.5 4.5

Figure 1. Diagram of theequipment. (A) Batch experi-ments and (B) continuousreactor.

Table 3. Experimental setup for the batch tests with/withoutstirring using starter as inoculum.

Sample Inoculum

(mL)

Co-substrate

(mL)

Total

volume

(mL)

Agitation

Control 390 � 400a) Yes

Control 390 � 400a) No

Lipid-rich

waste

390 10 400 Yes

Lipid-rich

waste

390 10 400 No

Corn silage 390 5b) 400 Yes

Corn silage 390 5b) 400 No

a) Filled up with water.b) Grams.

Table 4. Experimental setup for the continuous test with lipid-rich waste with/without stirring.

Coferment Feed (mL) Feed frequency Agitation

Lipid-rich waste 150 Every 2 days Yes

Lipid-rich waste 150 Every 2 days No

Table 5. Experimental setup for the batch tests with/withoutstirring using inoculum and manure as starter.

Sample Seed

sludge

(mL)

Manure

(mL)

Coferments

(mL)

Total Agitation

Control 160 240 � 410a) Yes

Control 160 240 � 410a) No

Lipid-

rich waste

160 240 10 410 Yes

Lipid-rich

waste

160 240 10 410 No

Corn silage 240 5b) 410 Yes

Corn silage 160 240 5b) 410 No

a) Filled up with water.b) Grams.



inoculated with 160 mL anaerobic seed sludge and theremaining working volume was filled with freshly preparedmanure slurry having a 1.38% dry solid concentration(100 mL of manure contains 1.38 g of dry solids). Theeffect of stirring was tested by setups with and withoutagitation (stirring) (Table 5).

3 Results and discussion

The biogas produced in the different batch experiments wasrecorded daily during the digestion period and plotted asbiogas rate and accumulated biogas production (values at

standard conditions: 01C, 1.013 mbar) related to the aggre-gated amount of coferments expressed as oTS. The presentedresults are the average values for each case; the standarddeviation of the measured values is plotted in the graphics.

3.1 Influence of stirring

The results for the anaerobic digestion in batch test for thelipid-rich waste, using as inoculum the digested biomass fromthe Wittmund plant, are shown in Fig. 2. To determine thepossible effect of stirring, all other conditions were maintainedas similar as possible for the different experiments.

Figure 2. Biogas rate and accumulated biogas (01C, 1013 mbar)for the lipid-rich waste with stirring (– –) and no stirring (—)using 100% digested biomass from Wittmund as starter.(A) Daily biogas production and (B) accumulative curve.

Figure 3. Biogas rate and accumulated biogas (01C, 1013 mbar)for the corn silage with stirring (– –) and no stirring (—) using100% digested biomass from Wittmund as starter. (A) Dailybiogas production and (B) accumulative curve.

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In Fig. 2A, the daily biogas production rate is shown. Themaximum biogas rate was already achieved during the first dayfor both cases: stirring and no stirring, 400 and 250 m3/t oTS,respectively. More than 90% of the produced biogas wasobtained after 3–4 days of anaerobic digestion. The behavior ofthe cumulative biogas is shown in Fig. 2B. After a steepincrease, biogas production rate decreased, resulting in aplateau of the cumulative curve. There is a discrepancybetween the biogas yield for the batch process with andwithout stirring. For the experiments with stirring, the calcu-lated biogas yield is 699 m3/t oTS after 7 days digestion time.Therefore, the measured biogas yields lay in the range of theliterature values for this kind of coferment: 700 m3/t oTS [16].The batch experiment without stirring shows a considerablylower biogas yield (318 m3/t oTS) compared with the processwith stirring: only 45% of the available organic substrate wasdigested.

As already known, lipids need a considerable time to bedegraded, because of the nature and size of the molecules ofthis substrate. The big molecules of lipids need to be brokenbefore the anaerobic bacteria can be used for the fermentation.However, in our case the ‘‘lipid-rich substrate’’ is the result ofalready degraded lipid leftovers from the industry, which alsohas a large ‘‘storage’’ time being used, making the degradationprocess faster.

The contents of solid of the substrate is around 7–13%dry solids (Table 2). That means, the lipids were verydiluted at the time of digestion, benefiting the digestion in thereactor.

In Fig. 3, the results for the second studied coferment, cornsilage, are shown. In this case, the coferment was not in liquidform like the lipid-rich waste. It consisted of particles up to1 cm diameter. The maximum biogas rate for the cornsilage was achieved on the third day of the digestion (130 and55 m3/t oTS with and without stirring, respectively), which wasexpected, due to the slower digestion rate for these type ofsubstrate (Fig. 3A). After 5 days, the biogas production forboth cases was almost finished. The average value for the

biogas yield for the corn silage in the case with stirring was437 m3/t oTS. Literature values for this coferment rangebetween 450 and 700 m3/t oTS [7].

In the same way as for the lipid-rich waste, the experimentswithout stirring presented a considerable lower biogas yields(175 m3/t oTS). That means, the biogas production withoutstirring was reduced to about 60% compared with the processwith stirring for this substrate.

The reactor in both cases was filled with inoculum from thebiogas plant, and hence it can be assured that the bacterialconcentration and variety were not limited. Otherwise, thedigestion would start slower but eventually the final biogasyield would be obtained, this was not the case.

The difference between the two cases may be attributed to areduced contact between the active biomass (the anaerobicbacteria) and the available substrates. A part of the substratemight have been sedimented at the bottom of the reactor andwas not available for the bacteria. In the case of the corn silage,the substrate seemed to float partly on the surface of the liquid.

To confirm this hypothesis, the continuous reactor wasoperated with similar conditions (inoculum, temperature andfeed) with/without stirring. Although the biogas productioncannot be compared with the batch reactors, a qualitativecomparison is possible. The consequence of the absence ofstirring should be similar if the bacteria are not in contact withthe total amount of feed substrate. As shown in Fig. 4, theresults of the continuous reactor for the lipid-rich waste showthe same behavior, when the reactor is not stirred. Theresulting values range between about 700 and 20 m3/t oTS� d.

Under ‘‘normal’’ conditions (stirring 60 rev/min), thebiogas production in the continuous reactor reaches maximumvalues directly after the feeding step (700 m3/t oTS� d). Afterabout 2 days, the biogas production is completed (withmixing). The results are given for three feeding cycles. Theresults for the process without stirring give values at thebeginning of the feeding cycle which are around 50% lowerthan for the experiment with stirring (Fig. 4). However, thedigestion in this case is not finished after 2 days. It may be

Figure 4. Biogas rate from semi-continuousoperating with lipid-rich substrate at 371Cwith/without stirring in reactor.



assumed that the total amount of biogas finally produced inboth cases is about the same, but in the case of no stirring thedigestion process is slower. Without stirring the methaneconcentration was lower, when the amount of substrate wasdegraded (3rd day), the values for both was similar. In thiscase, there is an accumulation effect due to the feeding.

3.2 Effect of the starter biomass

(i) Lipid-rich waste

The batch experiments with lipid-rich waste and corn silageusing 60% manure slurry as part of the starter are shown inFigs. 5 and 6. The total solids’ concentration of the manureused was very low (1.38% TS, Table 1). As a carrier substratethat supports the anaerobic digestion of concentratedsubstrates, which are not easy to treat directly, manure mayaffect the solubility of the added cosubstrate due to its highamount of water and other components.

The results show that the impact of the stirring is notsignificant with respect to the biogas yield when manure isaggregated as part of the starter. For the lipid-rich waste, thebatch experiments without stirring present a similar digestionas the experiments with stirring (Fig. 5). Furthermore, in bothcases the biogas yield at the end of the anaerobic digestion isquite similar, around 700 m3/t oTS, in agreement with thevalue of the stirred batch without manure in the inoculums(Fig. 2B. It can be deduced that the more diluted conditions inreactors allow a better contact between the bacteria and thesubstrates, so that the stirring is not essential.

Comparing the results of the lipid-rich waste as cofermentin the stirred batch reactor for the different kinds of startermaterials (seed sludge of the Wittmund biogas plant withoutand with manure) (Figs. 2A and 5A) shows a shift in thereaction time for the maximum rate from the end of the firstday to the end of the second day. The final accumulated biogasamount in both cases is about the same: 700 m3/t oTS.

The reason for the influence on the reaction time may bedue to the exchange of a part of the seed sludge by manureresulting in a decreasing of the amount of bioactive material isdecreased. The seed sludge contains 5 oTS, the manure 1.38%[22], which indicates that the oTS of fresh manure slurry isabout 1% of the particulate chemical oxygen demand. It can beassumed that the higher concentration of digested biomass,connected with a higher concentration of bacteria affects thebiogas rate, especially at the beginning. As was mentionedbefore, the manure may contain bacteria but its concentrationis quite low compared with the seed sludge.

(ii) Corn silage as co-ferment

The results presented for the corn silage show no significantdifference in biogas production between the stirred andnonstirred batch reactor when manure is added as part of thestarter (Fig. 6). However, the discrepancy for the maximumbiogas rate (on second day) is higher in this case compared with

the lipid-rich waste (Fig. 5). The agitated corn silage reaches amaximum biogas rate of approximately 185 m3/t oTS� d,without stirring this value is 125 m3/t oTS� d (Fig. 6A). Incomparison, the lipid-rich substrate gives similar values with/without stirring when manure was added. The different particlesizes of the substrate could be the reason for this. The lipid-richsubstrate has a major homogeneity and small particle sizebenefiting the digestion [3, 23]. In the corn silage, the size of theparticles is rather high and in homogenous and it is found asagglomerations on the surface of the dispersion in the batchreactor. The larger the corn silage particles are, the bigger the

Figure 5. Biogas rate and accumulated biogas (01C, 1.013 mbar)for the lipid-rich waste with stirring (– –) and no stirring (—)using 40% digested biomass from Wittmund and 60% manureas starter. (A) Daily biogas production and (B) accumulativecurve.

Eng. Life Sci. 2010, 10, No. 4, 339–347 345


tendency to float on the liquid. This could explain the moreimportant influence of the mixing. In any case, since the sameconditions were applied to all reactors, the different substratesused are the determinant factor for the biogas yield.

With the addition of manure, the contact between thebacteria and the substrate makes the stirring effects notconsiderable for the biogas yield. At the end of the experiment,both cases for the corn silage, with stirring and without stirringgive similar biogas yields, about 450 and 400 m3/t oTS,respectively. Thus, the biogas yield obtained using as starter adigested biomass–manure mixture without stirring can beconsidered equivalent to the biogas yield using a pure digestedbiomass as starter with stirring (437 m3/t oTS).

4 Concluding remarks

Based on the results for the different substrates, the effect ofthe stirring and of the different starters in a continuous reactorand a batch reactor, the following conclusions can be made.

The availability of bacteria in the reactor does not assurethat the substrate will be totally digested, if mass transferbetween the bacteria and the substrate is not efficient. Whenonly digested substrate is used as inoculum, the stirring isneeded to obtain the normal biogas yield with the batchexperiments.

The absence of stirring has a similar effect on the biogasproduction in batch reactor and continuous reactor, when thestarter comes from digested material of a biogas plant.

The addition of manure to allow the digestion in theabsence of stirring. However, the biogas rate at the beginning islower, due to the dilution of the bacterial concentration.

Regarding full-scale equipment, it can be said that thebenefit of mixing effect (mechanical) also depends on otherfactors such as the dilution and concentration of bacteria inthe reactor. Like the experiments showed, the main factor for abetter fermentation is the contact between the substrate andthe bacteria. If the dilution in the reactor is enough to allow agood mass transfer process, the mechanical mixing becomesless important. However, the properties of the substrate shouldalso be taken into consideration.

Acknowledgements

The authors are grateful to Dipl. Ing. M. Beyer of the biogasplant in Wittmund, Germany, for her support and fruitfuldiscussions.

Conflict of interest

The authors have declared no conflict of interest.

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