ARTICLE pubs.acs.org/EF
Enhanced Methane Formation through Application of Enzymes: Results from Continuous Digestion Tests Teresa Suarez Qui~nones,*,† Matthias Pl€ochl,‡ J€orn Budde,† and Monika Heiermann† † ‡
Leibniz Institute for Agricultural Engineering Potsdam-Bornim, Max-Eyth-Allee 100, 14469 Potsdam, Germany BioenergieBeratungBornim GmbH, Max-Eyth-Allee 101, 14469 Potsdam, Germany ABSTRACT: Continuous bio-methanization of different feedstocks (rye grain silage, maize silage, feed residue (mix of silages), solid cattle manure, and grass silage) was investigated in a long-term laboratory-scale experiment with and without enzyme application. Ten-liter reactors were operated simultaneously in a two-step digestion mode for the continuous production of biogas from different feedstocks over 354 days. One set of reactors was operated as main digester, while the second set was used for the second step. The daily input of feedstock was increased from an organic loading rate of 1 to 3 kg ODM 3 m3 3 d1. All digesters were run under stable conditions, indicated by the ratio of volatile fatty acids to the total inorganic carbon, ranging around 0.2 in the first step and 0.15 in the second step. The hydraulic retention time was maintained between 80 and 90 days during the experiment. The application of enzymes was able to enhance biogas production by 1015% and increase the methane content of biogas by an increment of 510% for the investigated materials except for feed residue. The increase in biogas yields was also reflected in the change in the ratios of heating values of the methane produced to the dry materials. These ratios ranged between 0.43 and 0.71 for the untreated feedstock, increasing to 0.440.88 after enzyme application.
1. INTRODUCTION In a previous study the authors investigated the effect of hydrolytic enzymes on feedstock for anaerobic digestion in batch digestion tests.1 They found that the hydrolysis of the investigated materials increased depending on temperature, pH values, and enzyme concentrations. These results confirmed the findings of other authors26 that hydrolytic enzymes increase the degradability of ligno-cellulosic material, e.g., for use in bioethanol production. Further, their results showed increased biogas yields and methane contents for the enzymatically treated materials: rye grain silage, maize silage, grass silage, feed residue, and solid cattle manure. Hydrolysis of lignocellulose complexes is assumed to be a rate-limiting step in anaerobic digestion,79 so applying hydrolytic enzymes is expected to be a way of enhancing not only the yield, but also the rate of biogas production. Although an accelerated biogas production rate with increased biogas yields was indicated in batch digestion tests, due to the complex nature of agricultural feedstocks and the digestion process, kinetic measurements in continuous-flow experiments are advisable. In this study we explored several periods with and without application of hydrolytic enzymes to different feedstocks in longterm experiments. As in the previous study1 we used rye grain silage, maize silage, feed residues, solid cattle manure, and grass silage as feedstock. Following the design of most current fullscale plants, we applied two-step digestion, i.e., a main digester and a post-digester. As the entire range of microbiological conversions is present in both digesters, this is a one-stage process. An additional (pre-)step as well as a first stage is introduced by applying enzymes, as component hydrolysis now occurs in a separate step. To determine the degree of digestion within the particular steps, we conducted batch digestion tests with the digestates of main digester and post-digester. r 2011 American Chemical Society
In addition, the effect of enzyme application on the availability of trace elements was studied. Applying enzymes may lead to the decomposition of lignocellulose structure which can increase the availability of trace elements necessary for enhanced bacterial growth and thus also contribute to an accelerated biogas production rate. Analysis of feedstock and digestates provides information on increased availability of particular trace elements.
2. MATERIALS AND METHODS 2.1. Feedstock for Anaerobic Digestion Test. Rye grain silage, maize silage, grass silage, feed residues (random mixture of silages not eaten by the cattle and gathered from the floor of the stable), and solid cattle manure were used together with cattle slurry (liquid cattle manure) as input during experiments. The characteristics and origin of feedstock and cattle slurry are summarized in Table 1. Samples of feedstock material were stored at 34 °C with carbon dioxide snow in sealed bins for analysis and continuous anaerobic digestion tests. All materials were analyzed for their chemical and physical properties using standard analytical methods of the Association of German Agricultural Investigation and Research Institutions (VDLUFA10) and the Leibniz Institute for Agricultural Engineering Potsdam-Bornim (ATB) described elsewhere.1 Values of parameters analyzed are shown in Table 2. The enzyme preparation was a commercially available product. It is based on fungal fermentation and further described in a previous experiment.1 The ability of enzymes to degrade the selected feedstock was determined, and the activity was calculated as a percentage Received: October 8, 2010 Revised: September 21, 2011 Published: September 22, 2011 5378
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of polysaccharide utilization in relation to the polysaccharide content of the untreated feedstock.1113 2.2. Prediction of Biogas Production in Continuous Digestion Test from Batch Test Results. In general the results from batch tests can be used to predict the biogas production in continuous digestion. The mass change in the digester with time equals the mass inflow min minus the mass outflow mout minus the mass loss due to the conversion to biogas mr m_ ¼ m_ in ðm_ out þ m_ r Þ
ð1Þ
The mass flow due to biogas production in continuous digestion equals the concentration of organic dry matter c(t) multiplied by the rate constant k of an assumed first order reaction (from batch digestion test) and multiplied by the volume of the digester V ð2Þ m_ r ¼ k 3 cðtÞ 3 V In a steady state the mass change m is set to 0. Further the mass inflow equals the organic loading rate multiplied by the volume of the digester, the mass outflow equals the mass at time t (c(t) 3 V) divided by the retention time tR, thus one obtains for c(t) cðtÞ ¼
OLR 1 k þ tR
ð3Þ
The rate constant k can be obtained from biogas yield y(t) at the maximum conversion in batch digestion test compared to the maximum biogas yield of batch test ymax and by integrating the Table 1. Characteristics of Feedstock and Cattle Slurry Originating from the Fehrbellin Biogas Plant feedstock
characteristics
rye grain silage
var. Pollino, ensiled with Biosila 100 g per 200 t grain
maize silage
var. Flavi ensiled with Biosil 100 g per 200 t whole crop
feed residue solid cattle
mixture of silages listed in this table straw, liquid, and solid cattle excreta
harvest harvest
manure
a
grass silage
grass, ensiled with Biosil 100 g per 200 t harvest
cattle slurry
fresh slurry from dairy cattle
Biosil - biological silage additive.
first order reaction (eq 2) over time yðtÞ ¼ ymax 3 ek 3 t
ð4Þ
The solution for k then is yðtÞ ln ymax k¼ tmax t
ð5Þ
Between gas yield yF and mass (flow) mr can be converted using the average density of biogas F = 1.35 kg 3 m3 yr ¼ F 3 m_ r
ð6Þ
2.3. Continuous Digestion Test. Continuous anaerobic digestion tests were conducted according to German Standard Procedure VDI 4630.14 Eight 10-L lab-scale anaerobic digesters (named R1R8) were operated simultaneously in a two-step digestion mode for the continuous production of biogas from different feedstocks for a period of 354 days of investigation. Each of the four parallel lines of two reactors in series received one of the four feedstocks (rye grain silage, maize silage, feed residues, or solid cattle manure) over the entire 354-day period. In addition, a fifth line with two further reactors (R9 and R10) was used with grass silage as feedstock beginning in week 32. The digesters R1, R3, R5, R7, and R9 were operated as main digester (first step), while the digesters R2, R4, R6, R8, and R10 were used as post-digesters (second step) during the experiment. The experiment was divided into two series, each series had a period without enzyme, followed by a period with enzyme application. The enzymes were added to the feedstock in a prestep, so that digestion in experiments with enzyme can be considered as three-step and two-stage digestion (Figure 1). The distribution of feedstock per digester and the time course of experiments are shown in Table 3 and Table 4. The 10-L reactors were filled up to 8 L, and the contents were incubated under mesophilic conditions at a temperature of 38 °C. Feeding was carried out manually on six days per week. Mixing was performed with a central paddle impeller at a speed of about 100 rpm for 15 min every hour. The daily input of feedstock was increased from an organic loading rate level (OLR) of 1 kg ODM 3 m3 3 d1 (level 1) to
Table 2. Chemical and physical properties of feedstock and cattle slurry as basic feedstock for continuous digestion testsa feedstock
pH
EC mS 3 cm1
DM g 3 kgFM1
ODM g 3 kgFM1
VOA g 3 kgFM1
NH4N g 3 kgFM1
Ntot g 3 kgFM1
rye grain silage
6.2
0.8
808.1
766.2
1.2
0.1
15.4
maize silage
3.8
1.5
308.8
285.1
3.4
0.3
4.2
feed residue
4.7
2.9
415.4
385.2
2.2
0.5
9.7
solid cattle manure grass silage
8.8 5.3
2.1 3.7
250.7 366.3
227.7 321.4
0.8 5.1
0.4 0.9
3.4 9.5
cattle slurry 1
6.9
12.3
63.1
83.6
5.2
1.2
2.6
cattle slurry 2
6.6
9.0
104.1
80.7
8.1
1.4
3.8
cattle slurry 3
7.2
11.5
59.2
79.8
5.1
0.9
2.3
cattle slurry 4
6.9
12.0
100.3
81.9
6.1
1.3
4.0
cattle slurry 5
7.0
14.7
57.2
79.8
7.45
2.1
4.8
cattle slurry 6
6.9
14.7
52.7
78.6
6.2
1.4
2.7
cattle slurry 7
7.0
14.6
56.5
78.0
7.2
1.34
2.8
EC = electric conductivity; DM = dry matter; ODM = organic dry matter; VOA = volatile organic acids; NH4N = ammonium nitrogen; Ntot = total nitrogen; FM = fresh matter. a
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Figure 1. Design of the continuous digestion tests regarding process steps, feeding, and components.
Table 3. Distribution of Feedstock Per Digester main digester
feedstock
second step
(+ cattle slurry)
digester
feedstock
R1
rye grain silage
R2
digestate from R1
R3
maize silage
R4
digestate from R3
R5
feed residue
R6
digestate from R5
R7
solid cattle manure
R8
digestate from R7
R9
grass silage
R10
digestate from R9
3 kg ODM 3 m3 3 d1 (level 3; see Table 5), and remained at this level for another 6 weeks, considered as a stabilization period as well as a control variant, before the addition of enzyme. The feeding volume was adjusted to a total of approximately 100 mL with cattle slurry in order to achieve a constant hydraulic retention time of 8090 days. Because of the long time span of the entire experiment we used seven different lots of cattle slurry (Table 2). In the case of solid cattle manure it was necessary to reduce the OLR to 2 kg ODM 3 m3 3 d1 because of difficulties in mixing the digester content. Ligno-cellulose-rich feedstock like solid cattle manure, with a crude fiber share of almost 46% DM, easily forms swimming layers due to gas bubbles attaching themselves to larger fibrous particles and lifting them upwards, thus accelerating formation of the swimming layer. During periods with enzyme application the feedstocks were hydrolyzed using the fungal hydrolytic enzyme mixture in 250-mL stoppered Erlenmeyer flasks for 3 h. The enzyme concentration was 0.04 g enzyme 3 gODM1 substrate, and temperature was kept at 40 °C. No pH control was applied due to the usual acidic pH value of the substrate. The biogas produced was continuously measured with a milligascounter type TGC1/5 (Ritter, Bochum, Germany). The composition of the digester biogas was measured online using
a gas analyzer SSM 6000 (Pronova, Germany). Air temperature and air pressure were continuously recorded. The digester temperature was controlled by water bath. The ratio of volatile fatty acids (VFA) to total inorganic carbon (TIC) in reactor contents was measured weekly by titration with sulfuric acid and resembles the values of sulfuric acid consumption at pH = 5 (TIC) and pH = 3 (VFA). Values of this ratio below 0.3 are considered to show stability of the process. Quantitative evaluation of the results gained in continuous anaerobic digestion tests included normalizing the volume of biogas to standard conditions: dry gas, t0 = 273 K, p0 = 1013 hPa. 2.4. Trace Element Analyses of Digestate. An atomic absorption spectrometer AAS 6 Vario (Analytik Jena AG, Jena, Germany) was used to determine the content of trace elements in the selected feedstock and digestate (in the last week of the experiment). Element analysis was conducted in a 3-fold replicate using VDLUFA methods. 2.5. Batch Digestion Test. Batch anaerobic digestion tests were made on samples from digesters R1 to R8 on completion of period 3 with enzyme addition. These tests were conducted in 2-L vessels under mesophilic conditions (at a temperature of 35 °C). The vessels were shaken once a day to re-suspend sediments and scum layers. The biogas was collected in scaled wet gas meters over a period of approximately 90 days and measured daily. The duration of these batch digestion tests was longer than standard tests on undigested feedstock in order to account for the low production rate of already digested material. Methane (CH4), carbon dioxide (CO2), oxygen (O2), and hydrogen sulfide (H2S) content were determined at least eight times during the batch fermentation test using infrared and chemical sensors ANSYCO GA 2000 Plus. Quantitative evaluation of the results gained in batch anaerobic digestion tests included the following steps: 5380
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Table 4. Overview of Experiments Conducted in Continuous Digestion Tests under Mesophilic Conditions series
period
approach
duration
material tested
1
1
increase of OLRa to level 3
5 weeks
R1R8
2
without enzyme
6 weeks
R1R8
3
with enzyme
18 weeks
main digesters R1, R3, R5, R7 with enzyme; post digesters
4
without enzyme
15 weeks
R1R10
5
with enzyme
6 weeks
main digesters R1, R3, R5, R7, R9 with enzyme; post digesters
R2, R4, R6, R8 without enzyme 2
R2, R4, R6, R8, R10 without enzyme a
OLR = organic loading rate.
Table 5. Daily Input of Feedstock at Different Organic Loading Ratesa feedstock in g FM 3 d1 OLR kg ODM 3 m3 3 d1
a
rye grain silage
maize silage
feed residue
solid cattle manure
grass silage
1.0
5.3
13.6
10.5
17.8
12.1
1.5
11.4
29.3
22.5
38.2
25.9
2.0
17.5
44.9
34.6
58.7
39.8
2.5
23.5
60.6
46.6
79.1
53.7
3.0
29.6
76.2
58.7
99.5
67.5
OLR = organic loading rate; ODM = organic dry matter; FM = fresh matter.
• normalizing the volume of biogas to standard conditions: dry gas, t0 = 273 K, p0 = 1013 hPa • correcting the methane and carbon dioxide content to 100% (headspace correction, VDI 4630)
3. RESULTS AND DISCUSSION 3.1. Continuous Digestion Tests. 3.1.1. Predicted Biogas Yields. The predicted biogas yields for continuous digestion are
generally lower than these from batch tests1 (Table 6). The application of enzymes leads to higher biogas yields. The highest increase is with solid cattle manure. The rate constant decreases with enzyme application for rye grain silage, solid cattle manure, and grass silage. This effect reappears in the mass flow, which ranges in the reaction without enzymes from 2.36 (feed residue) to 2.56 (grass silage). After enzyme application these values have a range of 2.30 to 2.55 (cf Table 6). The prediction of the biogas production in continuous digestion from batch test results has only limited value. There are several reasons for this: • a complex reaction chain is reduced to one first order reaction • two final products are reduced to a sum of products • the time steps for determining the maximum velocity during batch tests are too coarse and irregular. 3.1.2. Measured Biogas Yields. Reports of biogas yields from continuous digestion tests are very rare in literature. One exception is the work of Thamsiriroj and Murphy15 who investigated the monodigestion of grass silage in a two step pilot-scale digestion system. The methane yields they obtained lie between 455 and 460 LN 3 kgODM 1 and are very high, even compared with batch digestion tests. Reasons for these high methane yields may be extremely low organic loading rates of 0.5 kgODM 3 m3 3 d1 and corresponding very long hydraulic retention times of 221 days.
During the first twelve weeks, the input up to reactors R1, R3, R5, and R7 was increased to an organic loading rate of 3 kg ODM 3 m3 3 d1 and the processes in all reactors were almost stabilized. Reactors R2, R4, R6, and R8 were used as second-step reactors and fed with the digestate from the corresponding first step reactors. During that period the weekly average biogas yield (Figure 2) increased to values of approximately 600650 LN 3 kgODM 1 for maize silage and rye grain silage, reaching approximately 85% of the values obtained in batch digestion tests in the previous study.1 The other feedstock, solid manure and feed residue, showed weekly average biogas yields of approximately 400470 LN 3 kgODM 1, which in the case of feed residue were close to values obtained from batch digestion tests and in the case of solid manure1 were clearly above the values from batch digestion tests. To obtain interpretable values we took the weekly sum of biogas production divided by seven days rather than the weekly average biogas yields (Figure 2). Application of enzymes started after 12 weeks and continued for 19 weeks. During that period there was an increase in biogas production for rye grain silage, maize silage, and solid manure, whereas feed residue showed a decrease in the first step reactor and a slight increase in the second-step reactor which did not, however, compensate the decrease in the first step (Figures 2 and 3). The increases in biogas production after enzyme application in continuous-flow experiments were much lower than the increases in batch digestion tests.1 Whereas in the latter we observed increases of 25100% after enzyme application, the increases during the continuous experiments did not exceed 15%. Especially in the case of solid cattle manure there was a significant increase of almost 100% in biogas yield in batch digestion tests, while in the present experiment an increase below 10% was detected. After week 31 a second series started with the same input but without enzyme application for reactors R1R8, and with grass silage as input for reactors R9 and its digestate for R10. Enzyme application was interrupted during weeks 3245, a time span 5381
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Table 6. Predicted Values for Biogas Production in Continuous Digestion Compared to Values from Batch Tests (ymax) and Continuous Digestion Experiment parameter
rye grain silage
maize silage
feed residue
solid cattle manure
grass silage
k
d
0.070
0.059
0.062
0.071
0.097
kb c(t)a
d1 kgODM m3
0.055 34.54
0.072 39.43
0.071 38.25
0.055 34.39
0.095 26.45
c(t)b
kgODM m3
42.11
33.88
34.07
41.58
26.86
a
kgODM m3d1
2.42
2.34
2.36
2.43
2.56
mr b
kgODM m3d1
2.30
2.44
2.43
2.31
2.55
yFa
l d1
14.37
13.88
14.00
14.38
15.17
yFb
l d1
13.62
14.43
14.41
13.67
15.12
y(t)a
LN kgODM1
601.87
660.28
531.88
268.36
517.63
y(t)b ymaxa
LN kgODM1 LN kgODM1
662.45 645.63
709.03 700.71
704.57 565.77
516.15 287.98
538.26 570.23
ymaxb
mr
a
units 1
a
LN kgODM1
699.61
761.88
756.70
545.60
591.92
a,c
y(t)
LN kgODM1
630.77
599.18
467.19
390.23
383.60
y(t)b,c
LN kgODM1
667.01
661.30
412.52
401.86
457.48
Without enzyme application. b With enzyme application. c y(t).
Figure 2. Biogas yield (LN 3 kgODM 1) produced from different feedstock without and with application of enzyme (weekly average).
slightly longer than one residence time to reverse the enzyme effect. During this fourth period, biogas production decreased in all cases to values even below the values of the second period (Figures 2 and 3). In the case of grass silage biogas yield was stabilized at approximately 400 LN 3 kgODM 1, comparable with values obtained in batch digestion tests.1 After these 14 weeks, enzyme application was restarted and continued for another 8 weeks. During this fifth period, rye grain silage showed a clear increase in biogas production, even higher than during the third period. This was mainly caused by peak production during the first week of enzyme application (Figure 2), exceeding even the theoretical maximum of 1390 LN 3 kgODM 1 which can be calculated from Buswell equation.16 Therefore enzyme application was reduced by half. We suppose that biogas was adsorbed to suspended particles and released by a decrease in viscosity caused by enzyme application.17 Maize silage and solid manure showed a considerable increase in biogas production with enzyme application, but this was lower than during the second period. The biogas production of solid manure decreased even further compared to the periods before. One can conclude that there was a significant interaction between the solid manure and the cattle slurry added to the
Figure 3. Biogas yield (LN 3 kgODM 1) (daily values derived from weekly sums) of step one and step two from different feedstock without (w/o) and with application (w) of enzyme.
feedstock shown clearly during the first period and decreasing over the entire experimental period. Grass silage, which was included in the fourth period, showed a clear increase in biogas production with enzyme application, contrary to the findings in the batch digestion tests, where an increase in biogas yield was only detected with concurrent enzyme and buffer application.1 3.1.3. Methane Content. Without enzyme application, the methane content ranged from 55% for rye grain silage to 57% for maize silage and 58% for feed residue and solid cattle manure in periods 12. During the fourth period without enzyme application, the methane content of grass silage actually reached 62%. Enzyme application increased the methane content of biogas production for each feedstock. In addition, the methane content was higher in all cases in the second series than in the first. Methane content values ended at 6168% (Figure 4), increasing by an average increment of 45%. Feed residue showed an extraordinary increase in methane content from the second to third period, from 58% to 68%. It is interesting that reversing the enzyme application in the fourth period did not lead to a decrease 5382
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Figure 4. Averaged methane content of biogas from first step reactors for selected feedstock without (w/o) and with application (w) of enzyme.
Table 7. Energy Efficiency η Calculated Using the Methane Yield (m3 kgODM1), the Heating Value of Methane (39.96 MJ 3 m3; DIN 5185018), the Heating Value of Dry Biomasses (18.1 MJ 3 kgDM1 for Rye Grain and Solid Manure and 18.5 MJ 3 kgDM1 for the Other Materials; Kaltschmitt and Hartmann19) and the ODM/DM Ratio η 1. series without enzyme
2. series
with enzyme
without enzyme
with enzyme
first step reactors R1 R3 R5 R7 R9
0.72 0.66 0.54 0.43
R2 R4 R6 R8 R10
0.056 0.055 0.066 0.077
0.79 0.75 0.49 0.43
0.77 0.69 0.45 0.45 0.44
0.88 0.71 0.51 0.41 0.45
0.053 0.057 0.033 0.068 0.035
0.046 0.049 0.033 0.056 0.075
second step reactors 0.061 0.054 0.073 0.088
ηLHV 1. series
2. series
without enzyme with enzyme without enzyme with enzyme
R1+ R2 R3 + R4 R5 + R6 R7 + R8 R9 + R10
sum first step reactors with second step reactors 0.79 0.88 0.86 1.02 1.14 1.06 0.75 0.68 0.64 0.85 0.87 0.80 0.62
0.96 1.07 0.57 0.88 0.70
Figure 5. Experimental course of the VFA/TIC-ratio (a) in the first step digestion without (w/o) and with added (w) hydrolytic enzymes, and (b) in the second step digestion without (w/o) and with added (w) hydrolytic enzymes.
in methane content, but instead to a further increase—except in the case of feed residue. Nevertheless, its methane content was increased further during the fifth period by resumed enzyme 5383
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Table 8. Concentration of Trace Elements in Feedstock and Effluents from the First and Second Step Reactors (Trace Elements in mg kg1) feedstock
Mg
Ca
Cu
Zn
Al
Cr
Fe
K
Na
Ni
Cd
Mn
rye grain silage
1213
137
0.24
24.1
172.0
9.1
133.1
5064
110.2
0.023
0.0408
26.7
maize silage feed residues
448 1344
65 363
0.18 0.25
5.4 47.5
27.1 78.7
4.2 6.5
13.2 87.0
3635 5798
11.1 1942.3
0.009 0.024
0.0145 0.0302
8.7 37.2
solid manure
1095
278
0.29
27.9
42.4
2.7
51.4
5644
1193.8
0.009
0.0256
23.1
grass silage
934
389
0.34
11.4
104.9
2.1
87.9
9155
777.6
0.030
0.0021
26.3
R1
468
256
9.4
31.5
24.3
1.4
62.2
2679
505.6
0.58
b.d.l.a
17.7
R3
473
266
7.8
25.9
21.8
0.2
49.5
3210
481.6
0.34
b.d.l.
15.5
R5
643
738
12.1
43.7
30.6
1.3
79.6
3001
1018.4
0.39
b.d.l.
24.5
R7
549
412
11.0
39.2
31.9
0.6
67.9
2917
839.5
0.66
b.d.l.
22.2
R9
638
764
8.6
31.0
66.5
3.8
136.8
3788
711.6
2.05
b.d.l.
21.9
R2
435
251
13.6
33.3
23.7
2.3
67.2
2736
543.0
1.34
b.d.l.
18.5
R4
409
300
10.3
27.6
17.5
0.8
54.3
3162
496.3
0.93
b.d.l.
15.4
R6
566
717
14.8
45.2
30.8
1.4
83.0
3043
1025.2
0.81
b.d.l.
24.5
R8
527
551
13.8
41.5
26.0
0.4
67.9
2930
787.7
0.80
b.d.l.
22.5
R10
471
590
8.8
30.8
41.0
2.4
91.3
3077
673.6
1.42
b.d.l.
19.5
effluent first step reactors
effluent second step reactors
a
b.d.l. = below detection limit.
Figure 6. Results from batch digestion test of the digestates of first and second step reactors. (Filled symbols show values for step one and open symbols show values for second step).
application. The changes in methane contents measured in these continuous-flow experiments agree well with the values obtained in batch digestion tests,1 where the increases also ranged from 5% to 10%. 3.1.4. Heating Values of Biogas Yields. An interesting question here is how energy efficient is biogas production compared to the direct combustion of the feedstock. The energy efficiency η for the various reactors are listed in Table 7. They were calculated using the methane yield (m3 kgODM1), (39.96 MJ 3 m3; DIN 5185018), the heating value of dry biomasses (18.1 MJ 3 kgDM1 for rye grain and solid manure and 18.5 MJ 3 kgDM1 for the other materials; Kaltschmitt and Hartmann19), and the ODM/DM ratio. The heating values of the biogas produced per unit of feedstock were calculated as an average of each series with and without enzyme. The energy efficiencies were increased by enzyme application from values between 0.43 and 0.71 to between 0.44 and 0.88,
reflecting the increases in biogas yield for the particular feedstock. The highest energy efficiencies were achieved with rye grain. In the direct combustion of these materials, considerable amounts of water would have to be evaporated and the latent heat of vaporization of water (ηLHV = 2440 kJ 3 kg1) taken into consideration. In comparison, the biogas process provides up to 14% more energy (in the form of a high-value energy carrier) than direct combustion of these materials (cf Table 7). 3.1.5. VFA/TIC of Digestate. A general indicator for stable operation of digestors is the ratio of volatile fatty acids to total inorganic carbon which should not exceed a value of 0.3.20 Otherwise over-acidification of the digester substrate can occur and process instability can be expected. Throughout the experiment the VFA/TIC-ratio varied between 0.15 and 0.25 for all first-step reactors (Figure 5a), with the exception of R1 (rye grain silage), where the ratio exceeded 0.3 in week 20 (Figure 5a). At the same time a small drop in biogas yield was observed, but the system recovered fast. R7 (solid cattle manure) showed a very unstable start phase and the VFA/ TIC ratio correlated with relatively low biogas yields during this period. The VFA/TIC ratio as well as the biogas production of R7 became stable with the beginning of enzyme application (Figure 5a). The VFA/TIC ratio in the second-step reactors revealed a very stable behavior with values between 0.10 and 0.15 (Figure 5b). Only R10 (grass silage) had a singular peak value of 0.3 after 5 weeks of enzyme application. Regardless of the particular values, the course of VFA/TIC ratios illustrated that the experiment was conducted under stable conditions. 3.1.6. Concentrations of Trace Elements in Non-digested and Digested Material. The analyses of digestates for trace elements reveal very similar values, independent of the concentration of these elements in the particular feedstock (Table 8). This implies 5384
dx.doi.org/10.1021/ef2009343 |Energy Fuels 2011, 25, 5378–5386
Energy & Fuels that the concentrations of trace elements mainly originate from the liquid cattle manure in the feedstock mixtures. The electric conductivity levels of the liquid cattle manure (above 10 mS 3 cm1) indicate the presence of considerable amounts of minerals. This is also shown by the difference in dry matter and organic dry matter content (Table 2). The concentrations of trace elements tended to rise from firststep reactors to second-step reactors, except in grass silage reactors R9 and R10 where the trend was reversed. As the reactors R9 and R10 only ran for a short time, it can be assumed that a certain depletion of trace elements was possible in the firststep reactors. In general, the results confirm the thesis that enzyme application and hence improved hydrolysis of ligno-cellulose structures improve the availability of trace elements. The results of the grass silage experiment indicate further that this is a long-term effect accompanied by depletion of trace elements through the export of these with microbes with the regular digestate removal. The values in our own experiment lie within the range of values obtained in full-scale biogas plants.21 They provide various hints for essential element concentrations, e.g., of Se, Ni, and Co, but there is still a large gap in knowledge. 3.1.7. Batch Digestion Test of Digestate. To determine the degree of anaerobic digestion that took place during continuousflow mode, batch digestion tests were conducted with samples of digestate from reactors R1R8. Tests with digestate of R9 and R10 had to be discarded for technical reasons. After 90 days the biogas yield of digestates from the first-step reactors R3, R5, and R7 (maize silage, feed residue, and solid manure) reached values of 180230 LN 3 kgODM 1 (Figure 6), which is almost a quarter to a third of the potential derived in batch digestion tests of the feedstock.1 Rye grain silage (R1) was almost 85% digested already in the first step. The results of the batch digestion tests show that a second step in continuous-flow digestion is of great advantage. Here the remaining biogas potential was decreased to values between 80 and 100 LN 3 kgODM 1 (Figure 6), which is less than 15% of the initial biogas potential determined in the batch digestion tests for the feedstock.1
4. CONCLUSIONS The results obtained here demonstrate that 10-L lab-scale reactors can be used as a model for full-scale continuous digestion. Coupling two of these reactors provides a good model for two-step digestion as is common in full-scale practice. With a daily input of approximately 100 mL a hydraulic retention time of 8090 days is achieved, which is also close to full-scale practice. The lab-scale digestion was conducted under very stable conditions as is indicated by the average VFA/TIC ratios of 0.2 for the first step and 0.15 for the second step. Regarding the results of solid cattle manure, one can conclude that in this case a longer process settling period would have been advantageous. Comparing the results of periods with and without enzyme application demonstrates that in general these enzymes can enhance biogas production. The enhancement affects both rate of biogas production and biogas yield related to input mass. The latter was shown in batch digestion tests by boosting the maximum methane yield of the selected feedstock by 15% to above 50%. The results of batch digestion tests were confirmed here, as can be seen from the increases in the total biogas production rate averaged for each of the four periods respectively.
ARTICLE
That the biogas formation rate is enhanced by enzyme application can be detected from the fact that the remaining biogas potential of the digestate in first step reactors is less during periods of enzyme application than during periods without application of enzymes. Comparing the results of the second and the third periods reveals that the application of enzymes has a rather immediate effect which diminishes if the application is reversed. This leads to two conclusions: firstly that the enzymes are subject to normal turn-over and do not have any longer retention time, and secondly that the lifetime of the enzymes does not exceed more than a few days. Hydrolytic enzymes have a positive effect on anaerobic digestion and thus on the biogas production from various feedstocks. They enhance the hydrolysis of complex compounds to simpler structures and seem to have an impact on lignocellulosic complexes. The latter leads to higher biogas yields of the particular feedstock, i.e., less material is necessary to obtain the same amount of energy. This was verified in lab-scale experiments, in both batch digestion tests and long-term continuous-flow experiments. In general the results obtained confirm the observations made in practice. Nevertheless, performing large-scale experiments in full-scale biogas plants using the methods developed and applied here in laboratory is recommended.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors wish to express their appreciation to Anka Thoma, Ines Ficht, Jonas Nekat, Miriam Felgentreu and Giovanna Rehde for the technical support, Rhinmilch GmbH (Fehrbellin) for the provision of feedstock material, and Bioreact GmbH for the provision of enzyme preparation. The work underlying this publication was supported by the European Commission FP 6, Contract No TREN/06/FP6EN/S07.64183/ 019884. ’ REFERENCES (1) Suarez Qui~ nones, T.; Pl€ ochl, M.; Budde, J.; Heiermann, M. Do hydrolytic enzymes enhance methane formation of agricultural feedstock? Lecture. Biogas Science 2009, Internationale Wissenschaftstagung, Band 1, ISSN 1611-4159, 137149. (2) Palmqvist, E.; Hahn-H€agerdal, B. Fermentation of lignocellulosic hydrolysates. II: Inhibitors and mechanisms of inhibition. Bioresour. Technol. 2000, 74, 25–33. (3) Schwarz, W. H. The cellulosome and cellulose degradation by anaerobic bacteria. Appl. Microbiol. Biotechnol. 2001, 56, 634–649. (4) Kim, S.; Dale, B. E. Global potential bioethanol production from wasted crops and crop residues. Biomass Bioenergy 2004, 26, 361–375. (5) Saha, B. C.; Cotta, M. A.; Iten, L. B.; Wu, Y. V. Dilute acid pretreatment, enzymatic saccharification, and fermentation of rice hulls to ethanol. Biotechnol. Prog. 2005, 21, 816–822. (6) Karimi, K.; Kheradmandinia, S.; Taherzadeh, M. J. Dilute-acid hydrolysis of rice straw. Biomass Bioenergy 2006, 30, 247–253. (7) Noike, T.; Endo, G.; Chang, J.; Yaguchi, J.; Matsumoto, J. Characteristics of Carbohydrate Degradation and the Rate-limiting Step in Anaerobic Digestion. Biotechnol. Bioeng. 1985, 27, 1482–1489. (8) Zhang, B.; He, P.-j.; L€u, F.; Li-ming, S.; Pei, W. Extracellular enzyme activities during regulated hydrolysis of high-solid organic wastes. Water Res. 2007, 41, 4468–4478. 5385
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