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Enhancement of methane production with horse manure supplement and pretreatment in a full-scale biogas process
Energy 73 (2014) 523e530
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Enhancement of methane production with horse manure supplement and pretreatment in a full-scale biogas process € nch-Tegeder*, Andreas Lemmer, Hans Oechsner Matthias Mo University of Hohenheim, State Institute of Agricultural Engineering and Bioenergy, Garbenstraße 9, 70 599 Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 January 2014 Received in revised form 15 May 2014 Accepted 13 June 2014 Available online 10 July 2014
The increased demand for renewable energy resources worldwide has lead to a strong interest in biomass for energy and heat production. However, the use of energy crops competes with human food production for limited available arable land. Therefore, it is necessary to develop alternate feedstocks for anaerobic digestion and increase the use of agricultural residues and by-products. In this work, the usability of straw-based horse manure was investigated in a full-scale biogas plant over a period of 160 days. Additionally, for the improvement of the methane production, a mechanical disintegration device was tested. The results of this long-term study indicate that the digestion of horse manure is not sufficient without further disintegration. The pretreatment of the substrates caused an increase in specific methane production of approximately 26.5%. The determination of the degradation efficiency resulted in an almost complete degradation of the disintegrated substrates during the theoretical hydraulic retention time of 80 days. Regarding these results, the energy demand for the pretreatment is negligible. Therefore, the anaerobic digestion of lignocellulosic materials with an appropriate pretreatment is the suggested method for a sustainable and economically viable energy production. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Mechanical disintegration Full-scale Horse manure Degradation efficiency Biogas Anaerobic digestion
1. Introduction The microbial conversion of organic waste and by-products from agriculture and the food industry to biogas is well known and the process has been implemented for many years [1]. The anaerobic processing of these materials and generation of renewable energy offers many environmental benefits [2,3]. There is a fundamental consensus that this is the most favorable pathway of waste processing worldwide [4]. Due to legislatives regulations, the production and utilization of biogas in the European Union increased rapidly during the past years [5,6]. The German Renewable Energy Sources Act has played a particular role in the wide application of biogas technology in the agricultural sector [7]. Triggered by the highest volumetric methane yields, the digestion of energy crops increased dramatically and has replaced residues and wastes [8]. This shift in utilization of resources for biogas production leads to a strong competition for available arable land between energy production and the human food supply [9]. This
* Corresponding author. Tel.: þ49 711 459 22685; fax: þ49 711 459 22111. €nchE-mail addresses: [email protected] (M. Mo Tegeder), [email protected] (A. Lemmer), [email protected] (H. Oechsner). http://dx.doi.org/10.1016/j.energy.2014.06.051 0360-5442/© 2014 Elsevier Ltd. All rights reserved.
results in rising prices for agricultural products and therefore affects the economic efficiency of biogas production [10]. Furthermore, it cannot be expected that the feed-in tariffs for electrical energy produced by renewable resources will increase. Therefore, the future challenge for the biogas branch is to prove its economic viability by optimizing the methane yield per unit input and decrease the acquisition costs for the substrates. Hence, a swift return to the use of agricultural residues and wastes is the key. In Germany, the leading country for biogas utilization, only 12% of the produced animal residues are processed by anaerobic digestion [11]. The application of liquid manures for biogas production is a commonly practiced technology. However, most agricultural biogas plants were not designed for the utilization of solid manures and other residues with higher solid contents [12,13]. The conversion of large fibrous particles result in swim layers inside the digester and cause procedural problems like clogging of pumps and pipes [14]. For this reason, only a small percentage of solid manure is utilized in conventional biogas plants [15,16]. The biodegradability of these substrates is also limited based on the recalcitrant of the biofibers and the high proportion of nondegradable materials [17]. Therefore, an appropriate treatment of the substrates is essential for achieving a sufficient degradation rate and methane yield [18]. The pretreatment of lignocellulosic materials for biogas production is the substance of a wide range of
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Fig. 1. Flow scheme of the research biogas plant “Unterer Lindenhof” [37].
scientific publications [19]. The investigated methods can be classified into physical, biological, chemical and physicochemical pretreatments [20]. Depending on the treated substrate, all methods resulted in an increase in methane yield between 5 and 20% in batch digestion tests [21,22]. The results of the enzymatic treatments are contradictory [23,24]. Additionally, the feasibility of practical application must be taken into account. Although the chemical and thermo-chemical pretreatments resulted in the highest methane yield increase, the feasibility for application at existing agricultural biogas plants is questionable [19]. A promising technology for pretreatment in full-scale application is mechanical disintegration [25]. Due to the change in structural features and decrease in particle size, the accessibility of substrate surface area increases [26], thereby increasing the specific methane yields of the substrates and decreasing the degradation time [18]. This also reduces mechanical problems, substrate viscosity and thereby avoids floating layers [27,28]. In addition, the mechanical pretreatment offers the opportunity to use alternate substrates with a higher fibrous content like solid manure and agricultural residues for biogas production [29,30]. An abundant potential resource for anaerobic digestion in Germany is horse manure. More than one million horses produce approximately twelve million tons of manure per year [31,32]. Normally, the bedding material dominates the composition of the horse manure. In Germany, straw is the most commonly used bedding material for horse stalls [33]. The disposal of the manure became increasingly difficult due its low fertilizer quality for crop production. Therefore, the digestion of the horse manure in agricultural biogas plant is an interesting alternative. The methane yields and digestibility of horse manure in lab scale was reported in literature [16,34]. Nevertheless, the usability of straw-based horse manure in a continuous full-scale biogas plant is not yet reported. Further results indicate that the processing of such fibrous materials is not possible in continuously stirred tank reactors. Therefore, the aim of this study was to investigate the feasibility of horse manure in agricultural biogas plants and to determine the effects of the lignocellulosic materials on the biogas process over a period of approximately 160 days. Additionally, there is a lack of information about the necessity and effects of mechanical disintegration. Thus, a cross-flow grinder was installed at the research biogas plant for the mechanical substrate pretreatment and to determine the effects of the disintegration on the fullscale biogas process. The alteration of the degradation efficiency of the substrates due to the pretreatment was also studied.
2. Materials and methods 2.1. Full-scale investigations In this work, the full-scale investigations were performed during a period of 160 days at the research biogas plant “Unterer Lindenhof” of the University of Hohenheim. The research biogas plant was previously described by Naegele et al. [35]. The biogas plant consists of two main digesters and one secondary digester with a working volume of 800 m3 each (Fig. 1). To ensure a constant process temperature (40.0 ± 1.0 C), each continuous stirred tank reactor possesses a heating system. Every digester is equipped with a separate solid feeding system, consisting of a vertical mixer and the feeding screws. A cross-flow grinder (Bio-QZ, MeWa, Gechingen, Germany) was set up and integrated between the vertical mixer and digester 1. The cross-flow grinder is patented as a decomposing device for the disintegration of recycling materials [36]. The Bio-QZ consists of a cylindrical working chamber with two rotating staggered steel chains located on the bottom of the working chamber. For the pretreatment of biomass, the working chamber is filled with a portion of substrate and the disintegration runs for a definable time span. During this working mode, the rotating chains cause radial and vertical material flow in the working chamber. The high flow velocity and the particle collisions of the substrates lead to a significant increase in particle surface area and defibration of lignocellulosic materials. For altering the pretreatment intensity, the bulk of the substrate portion and the retention time in the working chamber can be modified. In this trial, the treatment time was set to 15 s and the filling of the crossflow grinder finished when the current draw reached 65% of the maximum. To estimate the electric energy demand of the crossflow grinder, an electronic three-phase transformer connected meter (DAB 13000, ABB, Zürich, Suisse) was installed. For the appropriate mixing of the digester content, each digester is equipped with a submersible motor mixer (4670, ITT Flygt AB, Sweden). Additionally, a propeller incline shaft agitator (Biogator HPR I, REMA, Germany) is installed in digester 1 and digester 2 incorporates a paddle incline agitator (Biobull, Envicon, Germany). In general, the mixing of the digester slurry takes place every 25 min for 5 min. The research biogas plant is in operation since 2008. To ensure a constant and sufficient gas production for the combined heat and power unit with an electrical power of 192 kW, each digester was fed with a total amount of 8.8 ± 2.0 t FM (fresh matter) per day.
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2.2. Determination of the degradation efficiency
Table 1 Process setup of the full-scale research biogas plant “Unterer Lindenhof”. Parameter
Unit
Digester 1 x
Total daily input Daily input vertical mixer Total daily input VS HRT OLR
[t/d] [t/d] [t/d] [d] [kg/m3 d]
10.5 6.0 2.3 78.8 2.9
± ± ± ± ±
Digester 2 STD
x
2.0 1.2 0.4 14.1 0.5
10.4 5.9 2.3 79.5 2.8
STD ± ± ± ± ±
525
2.3 1.4 0.5 15.6 0.6
Normally, the mixture of the daily substrate input consists of a mixture of 40% liquid manure, 10% solid manure, 15% maize silage, 20% grass silage, 10% grain silage and 5% crushed grain. During this experiment, digester 1 was fed with 10.5 ± 2.0 t FM per day and digester 2 was fed with nearly an identical amount of 10.4 ± 2.3 t FM (Table 1). The mixture of the daily input into the digesters is shown in Fig. 2. The liquid and solid manure consists of a mixture of cattle and pig manure from livestock at the agricultural research facility. The horse manure originates from a horse farm in the neighborhood of the research station and consists mainly of straw used as bedding material. The silages and the crushed grain were produced on the arable land and grassland of the research station. The OLR (organic loading rate) related to the VS (volatile solids) of the substrates averages 2.9 ± 0.5 kg/m3*d in digester 1, with a theoretical HRT (hydraulic retention time) of 78.8 ± 14.1 d and a OLR of 2.8 ± 0.6 kg/ m3*d with a HRT of 79.5 ± 15.6 d in digester 2. The feeding of solid materials to the digesters takes place twelve times per day and the feeding of liquid manure twice per day. The amount of substrate input was recorded by the system control unit of the biogas plant. The TS (total solids) and VS content of the feeding substrates were analyzed biweekly. The TS, VS, pH and VFA (volatile fatty acids) content of the digester substrates were also analyzed in the same manner. The analyses were conducted according to the guidelines of the Federation of German Agricultural Investigation and Research Institutes [38] in the laboratory of the State Institute of Agricultural Engineering and Bioenergy (Stuttgart, Germany) as described by Vintiloiu et al. [39]. The gas production of each digester was recorded continuously with a gas flowmeter (GD 300, Esters Eletronik GmbH, Rodgau, Germany). The gas yields were corrected to standard conditions (0 C, 1013 hPa). To determine the gas quality, a multisensor analyzing system (INCA 4000, Union Instruments GmbH, Karlsruhe, Germany) was used. The gas quality of the produced biogas of each digester was determined every hour.
To determine the degradation efficiency of the digesters at the research biogas plant, the methane yield potential and the chemical composition of the feedstock (liquid manure, solid manure, horse manure, maize silage, grass silage, grain silage and grain grid) were analyzed. Substrate samples were collected at three times during the second HRT (beginning, middle and end). To guarantee homogenous samples for the analyses, the solid samples were dried for 48 h at 60 C in a drying chamber and afterwards ground with a cutting mill to a particle size of 1 mm. 2.3. Chemical composition analyses The TS and VS values of the fresh substrate samples were analyzed in the laboratory of the State Institute of Agricultural Engineering and Bioenergy. The chemical composition of the substrates was analyzed by the State Institute of Agricultural Chemistry (Stuttgart, Germany). The concentrations of XP (crude protein), XL (crude fat), XF (crude fiber)and NfE (nitrogen free extracts) of the dried and grinded samples were analyzed according to the European regulations for the Weender feed analysis [40]. Additionally, the contents of cell-wall fractions, NDF (neutral detergent fiber), ADF (acid detergent fiber) and ADL (acid detergent lignin) were analyzed as described by the VDLUFA [38]. 2.4. Batch digestion test To determine the specific methane yields of the feedstock, the HBT (Hohenheimer Biogas Yield Test) was carried out. The HBT is a patented [41] and highly reproducible [42] batch digestion test according to the guidelines of the VDI 4630 [43]. The digestion of the dried and ground samples and the liquid manure took place in 100 ml glass syringes in a motor-driven rotor, which is located in an incubation chamber. Generally, the HBT is conducted for 35 days at 37.0 ± 0.5 C. The glass syringes were prepared with 0.5 g of the dried and ground sample and 30 g standard inoculum from the State Institute of Agricultural Engineering and Bioenergy. The analysis of each sample was conducted in triplicate. For the correction of the gas production, the inoculum was tested as a reference. The biogas production was recorded periodically with an accuracy of 1 ml. The methane content of the produced gas was measured with a gas transducer (AGM, Pronova Analysetechnik, Berlin, Germany). The gas amounts of the tested samples were corrected to standard conditions (0 C, 1013 hPA) and calculated to the specific yields, relating to kg VS. Additionally, a hay and
Digester 1
Digester 2 39.1 ± 11.2 % liquid manure
20.4 ± 7.7 % horse manure
38.6 ± 15.0 % liquid manure
3.8 ± 1.3 % crushed grain
19.8 ± 7.8 % horse manure
3.7 ± 1.6 % crushed grain
6.9 ± 4.0 % grass silage 8.0 ± 2.2 % solid manure 10.9 ± 4.6 % maize silage
11.0 ± 4.4 % grain silage
7.8 ± 3.8 % grass silage 7.8 ± 2.4 % solid manure 11.4 ± 3.1 % maize silage
Fig. 2. Composition of the daily feeding materials during the 160 days of investigation.
10.9 ± 3.2 % grain silage
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STD x
31.5 27.8 38.1 63.5 31.7 48.8 80.1
[% TS]
NfE
± ± ± ± ± ± ±
1.5 7.7 3.1 1.1 1.8 6.2 0.4
526
concentrate standard were used to quantify the test conditions of the HBT. The results of the batch digestion test were used to calculate the expected daily specific methane production per kg VS depending on the substrates added to the digesters of the research biogas plant.
1.8 7.8 2.2 0.1 0.8 0.9 0.1
3.1. Feedstock characteristic
7.7 15.5 10.2 1.8 4.9 4.2 0.8 27.8 43.8 54.3 24.5 46.3 35.8 3.1 40.9 49.8 75.0 42.2 58.9 54.2 12.2 19.7 25.7 38.6 21.7 35.4 29.1 2.4 20.5 18.2 6.9 8.7 7.5 11.3 12.0
The chemical compositions of the feeding substrates at the fullscale biogas plant during this trial are shown in Table 2. As expected, the manures contain the highest contents of recalcitrant organic matter because the animal digestion leads to an accumulation of lignin in the dung [44]. Furthermore, in this trial, the horse and solid manure showed the highest variations in substrate composition. In contrast to the plant materials, it is quite difficult to store the horse and solid manure in a horizontal silo and provoke degradation before use in the biogas process. Therefore, these materials were collected weekly from the livestock farm to avoid further unregulated decomposition. Additionally, the solid manure consists of pig, cow and poultry manure and increases the variations in the composition. The silages and the grain grid used show a good forage quality and the compositions are comparable with the €hler et al. [45]. results of Do Fig. 3 shows the estimated specific methane yields of the ground feeding substrates during the second HRT in the batch digestion test. The highest methane yield was determined for the crushed grain (0.395 ± 0.009 Nm3 CH4/kg VS). Slightly lower are the methane values for the maize silage with 0.376 ± 0.006 Nm3 CH4/kg VS. For the grass silage 0.351 ± 0.014 Nm3 CH4/kg VS were detected after 35 days digestion in the HBT. The digestion of the grain silage resulted in the lowest methane yields of the energy crops with 0.336 ± 0.007 Nm3 CH4/kg VS. These results are in accordance with Amon et al. [3] and underlines the suitability of energy crops for anaerobic digestion and demonstrates the good substrate quality at the research biogas plant. A considerable decrease in methane potential was observed in the case of the manures. The specific methane yield of the liquid manure is 0.265 ± 0.008 Nm3 CH4/kg VS. The observed methane values of the horse manure after 35 days of digestion are in the range of 0.224 ± 0.010 Nm3 CH4/kg VS and are marginally higher than previous results [34]. In consideration of the large variations in the composition of the solid manure, the detected methane yields show the largest differences during the trial (0.180 ± 0.071 Nm3 CH4/kg VS). Regarding these results, the large differences in methane potential between the energy crops and the manure clarifies the popularity of energy crops instead of manure for anaerobic digestion. Nevertheless, the manure is a lowcost substrate with a beneficial affect on the digestion process [46].
1.7 26.8 28.2 26.7 22.6 34.2 84.6 1.3 19.5 13.5 1.8 12.1 7.3 0.6 2.4 31.9 32.4 27.8 26.7 36.9 86.2 Liquid manure Solid manure Horse manure Maize silage Grass silage Grain silage Crushed grain
x
± ± ± ± ± ± ±
STD
x
± ± ± ± ± ± ±
1.1 16.3 13.5 1.8 11.1 6.6 0.3
x STD
[% TS] [% FM]
± ± ± ± ± ± ±
2.7 9.7 0.9 0.4 3.5 3.5 1.1
6.7 1.8 1.5 2.7 2.6 3.2 3.7
± ± ± ± ± ± ±
1.1 0.6 0.1 0.2 0.1 1.6 1.2
x x STD
[% TS]
XL XP VS
[% FM]
TS Substrate
Table 2 Chemical characteristics of the feeding substrates at the research biogas plant “Unterer Lindenhof”.
STD
[% TS]
XF
± ± ± ± ± ± ±
4.3 10.3 1.9 0.8 6.8 6.4 0.3
x STD
[% TS]
NDF
± ± ± ± ± ± ±
6.2 16.2 2.7 1.7 10.2 10.8 1.6
x STD
[% TS]
ADF
± ± ± ± ± ± ±
5.0 19.2 1.9 0.9 9.2 6.7 0.3
x STD
[% TS]
ADL
± ± ± ± ± ± ±
STD
3. Results and discussion
3.2. Process stability To estimate the effects of the horse manure and the mechanical disintegration on the biogas process, samples of the digester content were taken biweekly directly at the outlet of the digesters. The results of the laboratory analysis are listed in Table 3. For the identification of differences, the results were split in the first and second HRT. The feeding of horse manure resulted in a shift of TS and VS contents in both digesters. Interestingly, digester 2 had slightly lower TS and VS values than digester 1 with the mechanical pretreatment despite nearly identical feeding. Therefore, it might be that the mixing in digester 2, in respect of the larger particle sizes, is less efficient than in digester 1. Although, due to a considerable increase in the mixing intensity (mixing interval with 15 min working mode and 15 min break), no swim layers could be
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0.265
0.180
0.224
0.376
0.351
0.336
0.395
Liquid manure
Solid manure
Horse manure
Maize silage
Grass silage
Grain silage
Crushed grain
specific methane yield [Nm³ / kg VS]
0.4
0.3
0.2
0.1
0.0
3
Fig. 3. Cumulative specific methane yield [Nm /kg VS] of the feeding substrates after 35 days of digestion in the HBT.
Digester 1 produced 0.575 ± 0.073 Nm3/kg VS during the first 40 days of this trial. The production of digester 2 was approximately 16% lower than digester 1 with the mechanical disintegration (0.484 ± 0.075 Nm3/kg VS). There was a drop in specific biogas production in the second half of the first HRT for digester 1 of approximately 19.3% (0.464 ± 0.098 Nm3/kg VS) and 18.6% (0.394 ± 0.087 Nm3/kg VS) for digester 2. This dramatic decrease in biogas production resulted from the enrichment of the slowly degradable lignocellulosic horse manure in the process and the replacement of the easy digestible energy crops. The difference in biogas production between the digester with the Bio-QZ and the digester without further substrate pretreatment during this time was still 15.1%. In the second HRT, the specific biogas production in digester 1 rose to 0.503 ± 0.072 Nm3/kg VS and indicates an adaptation of the microbial community to the lignocellulosic materials. Otherwise, a further decrease in specific biogas production was observed at digester 2 for 10.1% (0.354 ± 0.050 Nm3/kg VS) and clearly underlines that the untreated straw particles are not sufficiently accessible for the hydrolytic enzymes. This leads to a significant difference between digester 1 and 2 of approximately 29.6% in the specific biogas production. These results indicate that the use of horse and solid manure is a good option for an environmentally friendly and sustainable energy production through anaerobic digestion. Additionally, the digestibility of horse manure in a common agricultural biogas plant has been proven. The mechanical pretreatment of such fibrous substrates is the key for a sufficient and stable digestion process. Furthermore, it has been shown that the variations in specific biogas production during the periods resulted in an adaption of the microbes to the lignocellulosic materials.
detected in the digester, but the formation of sink layers and substrate agglomerations cannot be excluded. The required mixing intensity for substrates with a smaller particle size is definitely lower and the risk of substrate decomposition will be minimized. As expected, the monitoring of the pH-values during this investigation showed no variations between the digesters and the enduring use of horse manure. Similarly, no destabilization of the digestion process could be observed regarding the accumulation of VFA. In fact, the contrary occurred. As the test duration increased, the concentrations of VFA in the digesters decreased. Therefore, it can be assumed that the risk of process inhibition due to the enrichment of acids is not the limiting factor by the usage of horse manure. However, the increase of solids in the digester can result in a decrease of substrate pumpability. The high solid content of the digester substrate also disables the gas lift. Hence, the process limitation of the digestion of horse manure is not the acid accumulation, but the increase in solid concentration.
3.3. Biogas quality and quantity The daily average values of methane content of the digesters are presented in Fig. 4. During the first 50 days of the experiment, the methane content of the produced biogas was slightly higher in the digester with the pretreated substrates. Afterwards the methane contents of the biogas are similar for both digesters. Overall the methane content of digester 1 is 54.2 ± 0.6% and 53.7 ± 0.9% for the produced biogas in digester 2. Due to the possible differences of daily feeding of the digesters, it is necessary to consider the specific gas production, which expresses the biogas production per kg of VS added to the digester, to assess the degradation efficiency. The absolute gas production is therefore not an appropriate tool. Before this trial with horse manure began, the differences in gas production between the digesters were negligible (0.598 ± 0.051 Nm3/kg VS in digester 1 and 0.588 ± 0.054 Nm3/kg VS in digester 2). Fig. 5 shows the specific biogas production in the digesters during approximately 160 days.
3.4. Degradation efficiency To determine the degradation efficiency of the digesters at the research biogas plant, the maximum specific methane potential of the added VS was calculated on the basis of the daily input to the
Table 3 Total and volatile solids, pH-value and volatile fatty acids concentrations of the digester contents during the first and second hydraulic retention time. Parameter
Unit
Digester 1
Digester 2
1. HRT
2. HRT
x TS VS pH VFA
[% FM] [% FM] [g/kg]
10.7 7.7 7.8 0.4
± ± ± ±
STD
x
0.8 0.7 0.1 0.4
11.3 8.4 7.8 0.1
1. HRT
± ± ± ±
STD
x
0.5 0.3 0.1 0.1
10.0 7.1 7.9 0.3
2. HRT
± ± ± ±
STD
x
0.8 0.8 0.1 0.4
10.3 7.6 7.9 0.3
STD ± ± ± ±
0.5 0.5 0.1 0.2
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methane content [%]
58
digester 1 digester 2
56
54
52
50 0
50
100
150
day Fig. 4. Methane content of the produced biogas of digester 1 with a mechanical disintegration unit and digester 2 without substrate pretreatment.
specific biogas production [Nm³ / kg VS]
digesters. Therefore, the results of the substrates analysis and the specific methane yields of the HBT are reliable. It must be mentioned that according to the guidelines of the VDI 4630, the batch digestion test determines the specific methane potential in lab scale and these values tend to overestimate the methane potential for practical application [43]. Additionally, the grinding of the substrates for the HBT leads to optimum test conditions and substrate degradation. Nonetheless, the calculated specific methane production offers a well-grounded basis for the assessment of the degradation efficiency in this study. The calculated maximum specific methane production and the measured specific methane production from the digesters during the second HRT are presented in Fig. 6. The results show that the calculated specific methane yield for both digesters is absolutely identical (0.306 ± 0.015 Nm3 CH4/kg VS). The mechanical disintegration of the substrates leads to a specific methane production of 0.272 ± 0.038 Nm3 CH4/kg VS. Even with a HRT of approximately 80 days, the full-scale actual methane production at digester 1 was only 11.1% lower than the calculated methane production of the lab scale values. Regarding the scale effects, these narrow differences are negligible and it can be assumed that with an increase in HRT the differences between the theoretical and practical values will be eliminated. The observed differences between the calculated specific methane production and the measured methane production were
considerably higher at digester 2 (34.6%). The methane production of digester 2 reached only 0.200 ± 0.040 Nm3 CH4/kg VS and was significantly lower (26.5%) than in digester 1 during this trial. These large differences in the degradation efficiency were not expected, but show the effectiveness of a mechanical disintegration unit at a full-scale biogas plant. The additional energy requirement of the cross-flow grinder for the pretreatment of the substrates at digester 1 was 11.3 ± 1.3 kWh/t FM during this trial. Regarding the large differences in specific methane production of digester 1 and 2, the energy demand for the disintegration is negligible when considering these benefits. Overall, the results of this study show that the use of alternate lignocellulosic substrates, like horse manure, in biogas plants is possible. However, the digestion of horse manure without a sufficient pretreatment in a continuous stirred tank reactor leads to an unacceptable degree of degradation and is not feasible for practical application. 4. Conclusion The purpose of this work was to investigate the digestibility of straw-based horse manure in a full-scale continuous stirred tank reactor. Additionally, the effects of mechanical disintegration with a cross-flow grinder were evaluated in this full-scale experiment. The pretreatment of the substrates resulted in an increased methane production of 26.5% in comparison to the untreated variant. The
1.0
digester 1 digester 2
0.8
0.6
0.4
0.2
0.0 0
50
100
150
day 3
Fig. 5. Specific biogas production [Nm /kg VS] of digester 1 with the Bio-QZ and digester 2 without further substrate treatment.
specific methane production [Nm³ / kg VS]
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0.4
0.3
0.2
0.1
0.0 Digester 1 calculated
Digester 1 measured
Digester 2 calculated
Digester 2 measured
3
Fig. 6. Calculated and measured specific methane production [Nm /kg VS] of the digesters at the research biogas plant “Unterer Lindenhof” during the second HRT.
differences between the batch digestion test, which determined maximum methane potential of the grinded substrates and the fullscale actual methane production, are negligible in the case of the digester with the pretreatment device. The results of the full-scale setup prove the suitability of horse manure for biogas production. However, an appropriate pretreatment of the substrates is necessary to achieve an acceptable degradation rate and gas production. These results show that the available mechanical decomposing devices have positive effects on the methane production without the risk of forming inhibitory or toxic by-products due to disintegration. In addition, the mechanical disintegration units have the advantage of easy implementation at existing agricultural biogas plants. Given this information, it seems that mechanical pretreatment is key for an increase in anaerobic digestion of recalcitrant degradable agricultural residues, solid manures and alternate lignocellulosic materials and therefore is an environmentally friendly, economically viable and socially acceptable pathway for renewable energy generation. This work also shows the high reliability of the HBT for determination of the methane potential and the application of these results on the full-scale continuous biogas process.
Acknowledgments This research was funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (grant no. 03KB064) as part of the project “Horse manure e further development of technologies for the efficient use of horse manure”.
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[9] Menardo S, Balsari P. An analysis of the energy potential of anaerobic digestion of agricultural by-products and organic waste. Bioenerg Res 2012;5(3): 759e67. [10] Schievano A, D'Imporzano G, Adani F. Substituting energy crops with organic wastes and agro-industrial residues for biogas production. J Environ Manag 2009;90(8):2537e41. [11] Daniel-Gromke J, Denysenko V, Sauter P, Naumann K, Scheftelowitz M, Krautz A, et al. Stromerzeugung aus Biomasse 03MAP250-Zwischenbericht. Leipzig: Deutsches Biomasseforschungszentrum; 2013. [12] Cui Z, Shi J, Li Y. Solid-state anaerobic digestion of spent wheat straw from horse stall. Bioresour Technol 2011;102(20):9432e7. €fer W. Methane yield of oat husks. [13] Kusch S, Schumacher B, Oechsner H, Scha Biomass Bioenerg 2011;35(7):2627e33. [14] Hashimoto AG. Conversion of straw-manure mixtures to methane at mesophilic and thermophilic temperatures. Biotechnol Bioeng 1983;25(1): 185e200. [15] Fischer E, Powrosnik AM, Beil C. Assessment of process stability and biogas yield for the anaerobic digestion of horse dung in lab-scale. Landtechnik 2013;68(4):248e51. [16] Kusch S, Oechsner H, Jungbluth T. Biogas production with horse dung in solidphase digestion systems. Bioresour Technol 2008;99(5):1280e92. [17] Bruni E, Jensen AP, Angelidaki I. Comparative study of mechanical, hydrothermal, chemical and enzymatic treatments of digested biofibers to improve biogas production. Bioresour Technol 2010a;101(22):8713e7. [18] Appels L, Lauwers J, Degrve J, Helsen L, Lievens B, Willems K, et al. Anaerobic digestion in global bio-energy production: potential and research challenges. Renew Sustain Energy Rev 2011;15(9):4295e301. [19] Angelidaki I, Ahring BK. Methods for increasing the biogas potential from the recalcitrant organic matter contained in manure. Water Sci Technol 2000;41(3):189e94. [20] Agbor VB, Cicek N, Sparling R, Berlin A, Levin DB. Biomass pretreatment: fundamentals toward application. Biotechnol Adv 2011;29(6):675e85. [21] Schwarz B, Friedrich E, Friedrich H, Jobst K, Lincke M. Desintegrationsverfahren e Aufwand und Nutzen für die Biogaserzeugung. In: FNRe V, editor. Gülzower Fachgespr€ ache: Biogas in der Landwirtschaft e Stand und Perspektiven. Weimar: Fachagentur Nachwachsende Rohstoffe e.V; 2009. pp. 278e88. [22] Carlsson M, Lagerkvist A, Morgan-Sagastume F. The effects of substrate pretreatment on anaerobic digestion systems: a review. Waste Manag 2012;32(9):1634e50. M, Lemmer A, Oechsner H, Jungbluth T, Schimpf U. Effect of adding [23] Brule fibrolitic enzymes to the methane yields of rye silage. Landtechnik 2008;63(3):178e9. ~ ones TS, Plo € chl M, Budde J, Heiermann M. Results of batch anaerobic [24] Quin digestion test e effect of enzyme addition. Agric Eng Int CIGR J 2012;14(1): 38e50. €nitz K, Adamsen APS, Møller HB. Extrusion as a pretreatment to [25] Hjorth M, Gra increase biogas production. Bioresour Technol 2011;102(8):4989e94. [26] Kratky L, Jirout T. Biomass size reduction machines for enhancing biogas production. Chem Eng Technol 2011;34(3):391e9. [27] Haeffner B, Nied C, Voigt J, Sommer K. Verweilzeitverkürzung bei der Biogasentstehung durch Zerkleinerung von lignocellulosehaltigen Substraten. Chem Ing Tech 2010;82(8):1261e4. [28] Hartmann H, Angelidaki I, Ahring BK. Increase of anaerobic degradation of particulate organic matter in full-scale biogas plants by mechanical maceration. Water Sci Technol 2000;41(3):145e53. [29] Sharma SK, Mishra IM, Sharma MP, Saini JS. Effect of particle size on biogas generation from biomass residues. Biomass 1988;17(4):251e63. [30] Menardo S, Airoldi G, Balsari P. The effect of particle size and thermal pretreatment on the methane yield of four agricultural by-products. Bioresour Technol 2012;104(1):708e14. [31] Fuchs C, Steinmetz AK. Pferdehaltung: Planen und kalkulieren. Darmstadt: Kuratorium für Technik und Bauwesen in der Landwirtschaft e. V; 2012.
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[32] Deutsche Reiterliche Vereinigung eV. Zahlen, Daten, Fakten ferdesport 2012; 2012. [33] Hess C, Beck J, Jungbluth T. Problematik der Mistverwertung in der Pferde€t Hohenheim; 2004. p. 121. haltung Baden-Württembergs. Universita €nch-Tegeder M, Lemmer A, Oechsner H, Jungbluth T. Investigation of the [34] Mo methane potential of horse manure. Agric Eng Int CIGR J 2013;15(2):161e72. [35] Naegele HJ, Lindner J, Merkle W, Lemmer A, Jungbluth T, Bogenrieder C. Effects of temperature, pH and O2 on the removal of hydrogen sulfide from biogas by external biological desulfurization in a full scale fixed-bed trickling bioreactor (FBTB). Int J Agric Biol Eng 2013;6(1). [36] Wabnig H. Decomposing device, European Patent EP 1 479 441 A2; 2004. [37] Naegele H-J, Lemmer A, Oechsner H, Jungbluth T. Electric energy consumption of the full scale research biogas plant “Unterer Lindenhof”: results of longterm and full detail measurements. Energies 2012;5(12):5198e214. [38] VDLUFA. Method book III - the chemical analysis for feedstuffs. Darmstadt, Germany: VDLUFA Verlag; 2007. [39] Vintiloiu A, Lemmer A, Oechsner H, Jungbluth T. Mineral substances and macronutrients in the anaerobic conversion of biomass: an impact evaluation. Eng Life Sci 2012;12(3):287e94. [40] Commission Regulation 2009/152/EC. Laying down the methods of sampling and analysis for the official control of feed. Off J Euro Union L54/1 2009.
€t und Quantita €t des [41] Helffrich D. Laborverfahren zur bestimmung der Qualita beim anaeroben Abbau organischer Substanzen entstehenden Biogases im atchverfahren, German Patent DE 10227685 B4; 2005. [42] Mittweg G, Oechsner H, Hahn V, Lemmer A, Reinhardt-Hanisch A. Repeatability of a laboratory batch method to determine the specific biogas and methane yields. Eng Life Sci 2012;12(3):270e8. [43] VDI-Society Energy and Environment. VDI 4630: fermentation of organic materials e characterization of the substrate, sampling, collection of material data, fermentation tests; 2006. [44] Triolo JM, Sommer SG, Møller HB, Weisbjerg MR, Jiang XY. A new algorithm to characterize biodegradability of biomass during anaerobic digestion: influence of lignin concentration on methane production potential. Bioresour Technol 2011;102(20):9395e402. €hler H, Eckel H, Fro €ba N, Grebe S, Hartmann S, Ha €ußermann U, et al. [45] Do Faustzahlen Biogas. Darmstadt: Kuratorium für Technik und Bauwesen in der Landwirtschaft; 2007. [46] Oechsner H, Lemmer A, Preissler D. Spurenelemente in NawaroBiogasanlagen zum Ausgleich substratbedingter Mangelerscheinungen und €rprozesses. In: FNR/KTBL-Kongress, editor. Biogas in zur Stabilisierung des Ga €ttingen: KTBL-Schrift; 2011. der Landwirtschaft: Stand und Perspektiven. Go pp. 48e61.
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Enhancement of methane production with horse manure supplement and pretreatment in a full-scale biogas process € nch-Tegeder*, Andreas Lemmer, Hans Oechsner Matthias Mo University of Hohenheim, State Institute of Agricultural Engineering and Bioenergy, Garbenstraße 9, 70 599 Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 January 2014 Received in revised form 15 May 2014 Accepted 13 June 2014 Available online 10 July 2014
The increased demand for renewable energy resources worldwide has lead to a strong interest in biomass for energy and heat production. However, the use of energy crops competes with human food production for limited available arable land. Therefore, it is necessary to develop alternate feedstocks for anaerobic digestion and increase the use of agricultural residues and by-products. In this work, the usability of straw-based horse manure was investigated in a full-scale biogas plant over a period of 160 days. Additionally, for the improvement of the methane production, a mechanical disintegration device was tested. The results of this long-term study indicate that the digestion of horse manure is not sufficient without further disintegration. The pretreatment of the substrates caused an increase in specific methane production of approximately 26.5%. The determination of the degradation efficiency resulted in an almost complete degradation of the disintegrated substrates during the theoretical hydraulic retention time of 80 days. Regarding these results, the energy demand for the pretreatment is negligible. Therefore, the anaerobic digestion of lignocellulosic materials with an appropriate pretreatment is the suggested method for a sustainable and economically viable energy production. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Mechanical disintegration Full-scale Horse manure Degradation efficiency Biogas Anaerobic digestion
1. Introduction The microbial conversion of organic waste and by-products from agriculture and the food industry to biogas is well known and the process has been implemented for many years [1]. The anaerobic processing of these materials and generation of renewable energy offers many environmental benefits [2,3]. There is a fundamental consensus that this is the most favorable pathway of waste processing worldwide [4]. Due to legislatives regulations, the production and utilization of biogas in the European Union increased rapidly during the past years [5,6]. The German Renewable Energy Sources Act has played a particular role in the wide application of biogas technology in the agricultural sector [7]. Triggered by the highest volumetric methane yields, the digestion of energy crops increased dramatically and has replaced residues and wastes [8]. This shift in utilization of resources for biogas production leads to a strong competition for available arable land between energy production and the human food supply [9]. This
* Corresponding author. Tel.: þ49 711 459 22685; fax: þ49 711 459 22111. €nchE-mail addresses: [email protected] (M. Mo Tegeder), [email protected] (A. Lemmer), [email protected] (H. Oechsner). http://dx.doi.org/10.1016/j.energy.2014.06.051 0360-5442/© 2014 Elsevier Ltd. All rights reserved.
results in rising prices for agricultural products and therefore affects the economic efficiency of biogas production [10]. Furthermore, it cannot be expected that the feed-in tariffs for electrical energy produced by renewable resources will increase. Therefore, the future challenge for the biogas branch is to prove its economic viability by optimizing the methane yield per unit input and decrease the acquisition costs for the substrates. Hence, a swift return to the use of agricultural residues and wastes is the key. In Germany, the leading country for biogas utilization, only 12% of the produced animal residues are processed by anaerobic digestion [11]. The application of liquid manures for biogas production is a commonly practiced technology. However, most agricultural biogas plants were not designed for the utilization of solid manures and other residues with higher solid contents [12,13]. The conversion of large fibrous particles result in swim layers inside the digester and cause procedural problems like clogging of pumps and pipes [14]. For this reason, only a small percentage of solid manure is utilized in conventional biogas plants [15,16]. The biodegradability of these substrates is also limited based on the recalcitrant of the biofibers and the high proportion of nondegradable materials [17]. Therefore, an appropriate treatment of the substrates is essential for achieving a sufficient degradation rate and methane yield [18]. The pretreatment of lignocellulosic materials for biogas production is the substance of a wide range of
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Fig. 1. Flow scheme of the research biogas plant “Unterer Lindenhof” [37].
scientific publications [19]. The investigated methods can be classified into physical, biological, chemical and physicochemical pretreatments [20]. Depending on the treated substrate, all methods resulted in an increase in methane yield between 5 and 20% in batch digestion tests [21,22]. The results of the enzymatic treatments are contradictory [23,24]. Additionally, the feasibility of practical application must be taken into account. Although the chemical and thermo-chemical pretreatments resulted in the highest methane yield increase, the feasibility for application at existing agricultural biogas plants is questionable [19]. A promising technology for pretreatment in full-scale application is mechanical disintegration [25]. Due to the change in structural features and decrease in particle size, the accessibility of substrate surface area increases [26], thereby increasing the specific methane yields of the substrates and decreasing the degradation time [18]. This also reduces mechanical problems, substrate viscosity and thereby avoids floating layers [27,28]. In addition, the mechanical pretreatment offers the opportunity to use alternate substrates with a higher fibrous content like solid manure and agricultural residues for biogas production [29,30]. An abundant potential resource for anaerobic digestion in Germany is horse manure. More than one million horses produce approximately twelve million tons of manure per year [31,32]. Normally, the bedding material dominates the composition of the horse manure. In Germany, straw is the most commonly used bedding material for horse stalls [33]. The disposal of the manure became increasingly difficult due its low fertilizer quality for crop production. Therefore, the digestion of the horse manure in agricultural biogas plant is an interesting alternative. The methane yields and digestibility of horse manure in lab scale was reported in literature [16,34]. Nevertheless, the usability of straw-based horse manure in a continuous full-scale biogas plant is not yet reported. Further results indicate that the processing of such fibrous materials is not possible in continuously stirred tank reactors. Therefore, the aim of this study was to investigate the feasibility of horse manure in agricultural biogas plants and to determine the effects of the lignocellulosic materials on the biogas process over a period of approximately 160 days. Additionally, there is a lack of information about the necessity and effects of mechanical disintegration. Thus, a cross-flow grinder was installed at the research biogas plant for the mechanical substrate pretreatment and to determine the effects of the disintegration on the fullscale biogas process. The alteration of the degradation efficiency of the substrates due to the pretreatment was also studied.
2. Materials and methods 2.1. Full-scale investigations In this work, the full-scale investigations were performed during a period of 160 days at the research biogas plant “Unterer Lindenhof” of the University of Hohenheim. The research biogas plant was previously described by Naegele et al. [35]. The biogas plant consists of two main digesters and one secondary digester with a working volume of 800 m3 each (Fig. 1). To ensure a constant process temperature (40.0 ± 1.0 C), each continuous stirred tank reactor possesses a heating system. Every digester is equipped with a separate solid feeding system, consisting of a vertical mixer and the feeding screws. A cross-flow grinder (Bio-QZ, MeWa, Gechingen, Germany) was set up and integrated between the vertical mixer and digester 1. The cross-flow grinder is patented as a decomposing device for the disintegration of recycling materials [36]. The Bio-QZ consists of a cylindrical working chamber with two rotating staggered steel chains located on the bottom of the working chamber. For the pretreatment of biomass, the working chamber is filled with a portion of substrate and the disintegration runs for a definable time span. During this working mode, the rotating chains cause radial and vertical material flow in the working chamber. The high flow velocity and the particle collisions of the substrates lead to a significant increase in particle surface area and defibration of lignocellulosic materials. For altering the pretreatment intensity, the bulk of the substrate portion and the retention time in the working chamber can be modified. In this trial, the treatment time was set to 15 s and the filling of the crossflow grinder finished when the current draw reached 65% of the maximum. To estimate the electric energy demand of the crossflow grinder, an electronic three-phase transformer connected meter (DAB 13000, ABB, Zürich, Suisse) was installed. For the appropriate mixing of the digester content, each digester is equipped with a submersible motor mixer (4670, ITT Flygt AB, Sweden). Additionally, a propeller incline shaft agitator (Biogator HPR I, REMA, Germany) is installed in digester 1 and digester 2 incorporates a paddle incline agitator (Biobull, Envicon, Germany). In general, the mixing of the digester slurry takes place every 25 min for 5 min. The research biogas plant is in operation since 2008. To ensure a constant and sufficient gas production for the combined heat and power unit with an electrical power of 192 kW, each digester was fed with a total amount of 8.8 ± 2.0 t FM (fresh matter) per day.
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2.2. Determination of the degradation efficiency
Table 1 Process setup of the full-scale research biogas plant “Unterer Lindenhof”. Parameter
Unit
Digester 1 x
Total daily input Daily input vertical mixer Total daily input VS HRT OLR
[t/d] [t/d] [t/d] [d] [kg/m3 d]
10.5 6.0 2.3 78.8 2.9
± ± ± ± ±
Digester 2 STD
x
2.0 1.2 0.4 14.1 0.5
10.4 5.9 2.3 79.5 2.8
STD ± ± ± ± ±
525
2.3 1.4 0.5 15.6 0.6
Normally, the mixture of the daily substrate input consists of a mixture of 40% liquid manure, 10% solid manure, 15% maize silage, 20% grass silage, 10% grain silage and 5% crushed grain. During this experiment, digester 1 was fed with 10.5 ± 2.0 t FM per day and digester 2 was fed with nearly an identical amount of 10.4 ± 2.3 t FM (Table 1). The mixture of the daily input into the digesters is shown in Fig. 2. The liquid and solid manure consists of a mixture of cattle and pig manure from livestock at the agricultural research facility. The horse manure originates from a horse farm in the neighborhood of the research station and consists mainly of straw used as bedding material. The silages and the crushed grain were produced on the arable land and grassland of the research station. The OLR (organic loading rate) related to the VS (volatile solids) of the substrates averages 2.9 ± 0.5 kg/m3*d in digester 1, with a theoretical HRT (hydraulic retention time) of 78.8 ± 14.1 d and a OLR of 2.8 ± 0.6 kg/ m3*d with a HRT of 79.5 ± 15.6 d in digester 2. The feeding of solid materials to the digesters takes place twelve times per day and the feeding of liquid manure twice per day. The amount of substrate input was recorded by the system control unit of the biogas plant. The TS (total solids) and VS content of the feeding substrates were analyzed biweekly. The TS, VS, pH and VFA (volatile fatty acids) content of the digester substrates were also analyzed in the same manner. The analyses were conducted according to the guidelines of the Federation of German Agricultural Investigation and Research Institutes [38] in the laboratory of the State Institute of Agricultural Engineering and Bioenergy (Stuttgart, Germany) as described by Vintiloiu et al. [39]. The gas production of each digester was recorded continuously with a gas flowmeter (GD 300, Esters Eletronik GmbH, Rodgau, Germany). The gas yields were corrected to standard conditions (0 C, 1013 hPa). To determine the gas quality, a multisensor analyzing system (INCA 4000, Union Instruments GmbH, Karlsruhe, Germany) was used. The gas quality of the produced biogas of each digester was determined every hour.
To determine the degradation efficiency of the digesters at the research biogas plant, the methane yield potential and the chemical composition of the feedstock (liquid manure, solid manure, horse manure, maize silage, grass silage, grain silage and grain grid) were analyzed. Substrate samples were collected at three times during the second HRT (beginning, middle and end). To guarantee homogenous samples for the analyses, the solid samples were dried for 48 h at 60 C in a drying chamber and afterwards ground with a cutting mill to a particle size of 1 mm. 2.3. Chemical composition analyses The TS and VS values of the fresh substrate samples were analyzed in the laboratory of the State Institute of Agricultural Engineering and Bioenergy. The chemical composition of the substrates was analyzed by the State Institute of Agricultural Chemistry (Stuttgart, Germany). The concentrations of XP (crude protein), XL (crude fat), XF (crude fiber)and NfE (nitrogen free extracts) of the dried and grinded samples were analyzed according to the European regulations for the Weender feed analysis [40]. Additionally, the contents of cell-wall fractions, NDF (neutral detergent fiber), ADF (acid detergent fiber) and ADL (acid detergent lignin) were analyzed as described by the VDLUFA [38]. 2.4. Batch digestion test To determine the specific methane yields of the feedstock, the HBT (Hohenheimer Biogas Yield Test) was carried out. The HBT is a patented [41] and highly reproducible [42] batch digestion test according to the guidelines of the VDI 4630 [43]. The digestion of the dried and ground samples and the liquid manure took place in 100 ml glass syringes in a motor-driven rotor, which is located in an incubation chamber. Generally, the HBT is conducted for 35 days at 37.0 ± 0.5 C. The glass syringes were prepared with 0.5 g of the dried and ground sample and 30 g standard inoculum from the State Institute of Agricultural Engineering and Bioenergy. The analysis of each sample was conducted in triplicate. For the correction of the gas production, the inoculum was tested as a reference. The biogas production was recorded periodically with an accuracy of 1 ml. The methane content of the produced gas was measured with a gas transducer (AGM, Pronova Analysetechnik, Berlin, Germany). The gas amounts of the tested samples were corrected to standard conditions (0 C, 1013 hPA) and calculated to the specific yields, relating to kg VS. Additionally, a hay and
Digester 1
Digester 2 39.1 ± 11.2 % liquid manure
20.4 ± 7.7 % horse manure
38.6 ± 15.0 % liquid manure
3.8 ± 1.3 % crushed grain
19.8 ± 7.8 % horse manure
3.7 ± 1.6 % crushed grain
6.9 ± 4.0 % grass silage 8.0 ± 2.2 % solid manure 10.9 ± 4.6 % maize silage
11.0 ± 4.4 % grain silage
7.8 ± 3.8 % grass silage 7.8 ± 2.4 % solid manure 11.4 ± 3.1 % maize silage
Fig. 2. Composition of the daily feeding materials during the 160 days of investigation.
10.9 ± 3.2 % grain silage
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STD x
31.5 27.8 38.1 63.5 31.7 48.8 80.1
[% TS]
NfE
± ± ± ± ± ± ±
1.5 7.7 3.1 1.1 1.8 6.2 0.4
526
concentrate standard were used to quantify the test conditions of the HBT. The results of the batch digestion test were used to calculate the expected daily specific methane production per kg VS depending on the substrates added to the digesters of the research biogas plant.
1.8 7.8 2.2 0.1 0.8 0.9 0.1
3.1. Feedstock characteristic
7.7 15.5 10.2 1.8 4.9 4.2 0.8 27.8 43.8 54.3 24.5 46.3 35.8 3.1 40.9 49.8 75.0 42.2 58.9 54.2 12.2 19.7 25.7 38.6 21.7 35.4 29.1 2.4 20.5 18.2 6.9 8.7 7.5 11.3 12.0
The chemical compositions of the feeding substrates at the fullscale biogas plant during this trial are shown in Table 2. As expected, the manures contain the highest contents of recalcitrant organic matter because the animal digestion leads to an accumulation of lignin in the dung [44]. Furthermore, in this trial, the horse and solid manure showed the highest variations in substrate composition. In contrast to the plant materials, it is quite difficult to store the horse and solid manure in a horizontal silo and provoke degradation before use in the biogas process. Therefore, these materials were collected weekly from the livestock farm to avoid further unregulated decomposition. Additionally, the solid manure consists of pig, cow and poultry manure and increases the variations in the composition. The silages and the grain grid used show a good forage quality and the compositions are comparable with the €hler et al. [45]. results of Do Fig. 3 shows the estimated specific methane yields of the ground feeding substrates during the second HRT in the batch digestion test. The highest methane yield was determined for the crushed grain (0.395 ± 0.009 Nm3 CH4/kg VS). Slightly lower are the methane values for the maize silage with 0.376 ± 0.006 Nm3 CH4/kg VS. For the grass silage 0.351 ± 0.014 Nm3 CH4/kg VS were detected after 35 days digestion in the HBT. The digestion of the grain silage resulted in the lowest methane yields of the energy crops with 0.336 ± 0.007 Nm3 CH4/kg VS. These results are in accordance with Amon et al. [3] and underlines the suitability of energy crops for anaerobic digestion and demonstrates the good substrate quality at the research biogas plant. A considerable decrease in methane potential was observed in the case of the manures. The specific methane yield of the liquid manure is 0.265 ± 0.008 Nm3 CH4/kg VS. The observed methane values of the horse manure after 35 days of digestion are in the range of 0.224 ± 0.010 Nm3 CH4/kg VS and are marginally higher than previous results [34]. In consideration of the large variations in the composition of the solid manure, the detected methane yields show the largest differences during the trial (0.180 ± 0.071 Nm3 CH4/kg VS). Regarding these results, the large differences in methane potential between the energy crops and the manure clarifies the popularity of energy crops instead of manure for anaerobic digestion. Nevertheless, the manure is a lowcost substrate with a beneficial affect on the digestion process [46].
1.7 26.8 28.2 26.7 22.6 34.2 84.6 1.3 19.5 13.5 1.8 12.1 7.3 0.6 2.4 31.9 32.4 27.8 26.7 36.9 86.2 Liquid manure Solid manure Horse manure Maize silage Grass silage Grain silage Crushed grain
x
± ± ± ± ± ± ±
STD
x
± ± ± ± ± ± ±
1.1 16.3 13.5 1.8 11.1 6.6 0.3
x STD
[% TS] [% FM]
± ± ± ± ± ± ±
2.7 9.7 0.9 0.4 3.5 3.5 1.1
6.7 1.8 1.5 2.7 2.6 3.2 3.7
± ± ± ± ± ± ±
1.1 0.6 0.1 0.2 0.1 1.6 1.2
x x STD
[% TS]
XL XP VS
[% FM]
TS Substrate
Table 2 Chemical characteristics of the feeding substrates at the research biogas plant “Unterer Lindenhof”.
STD
[% TS]
XF
± ± ± ± ± ± ±
4.3 10.3 1.9 0.8 6.8 6.4 0.3
x STD
[% TS]
NDF
± ± ± ± ± ± ±
6.2 16.2 2.7 1.7 10.2 10.8 1.6
x STD
[% TS]
ADF
± ± ± ± ± ± ±
5.0 19.2 1.9 0.9 9.2 6.7 0.3
x STD
[% TS]
ADL
± ± ± ± ± ± ±
STD
3. Results and discussion
3.2. Process stability To estimate the effects of the horse manure and the mechanical disintegration on the biogas process, samples of the digester content were taken biweekly directly at the outlet of the digesters. The results of the laboratory analysis are listed in Table 3. For the identification of differences, the results were split in the first and second HRT. The feeding of horse manure resulted in a shift of TS and VS contents in both digesters. Interestingly, digester 2 had slightly lower TS and VS values than digester 1 with the mechanical pretreatment despite nearly identical feeding. Therefore, it might be that the mixing in digester 2, in respect of the larger particle sizes, is less efficient than in digester 1. Although, due to a considerable increase in the mixing intensity (mixing interval with 15 min working mode and 15 min break), no swim layers could be
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527
0.265
0.180
0.224
0.376
0.351
0.336
0.395
Liquid manure
Solid manure
Horse manure
Maize silage
Grass silage
Grain silage
Crushed grain
specific methane yield [Nm³ / kg VS]
0.4
0.3
0.2
0.1
0.0
3
Fig. 3. Cumulative specific methane yield [Nm /kg VS] of the feeding substrates after 35 days of digestion in the HBT.
Digester 1 produced 0.575 ± 0.073 Nm3/kg VS during the first 40 days of this trial. The production of digester 2 was approximately 16% lower than digester 1 with the mechanical disintegration (0.484 ± 0.075 Nm3/kg VS). There was a drop in specific biogas production in the second half of the first HRT for digester 1 of approximately 19.3% (0.464 ± 0.098 Nm3/kg VS) and 18.6% (0.394 ± 0.087 Nm3/kg VS) for digester 2. This dramatic decrease in biogas production resulted from the enrichment of the slowly degradable lignocellulosic horse manure in the process and the replacement of the easy digestible energy crops. The difference in biogas production between the digester with the Bio-QZ and the digester without further substrate pretreatment during this time was still 15.1%. In the second HRT, the specific biogas production in digester 1 rose to 0.503 ± 0.072 Nm3/kg VS and indicates an adaptation of the microbial community to the lignocellulosic materials. Otherwise, a further decrease in specific biogas production was observed at digester 2 for 10.1% (0.354 ± 0.050 Nm3/kg VS) and clearly underlines that the untreated straw particles are not sufficiently accessible for the hydrolytic enzymes. This leads to a significant difference between digester 1 and 2 of approximately 29.6% in the specific biogas production. These results indicate that the use of horse and solid manure is a good option for an environmentally friendly and sustainable energy production through anaerobic digestion. Additionally, the digestibility of horse manure in a common agricultural biogas plant has been proven. The mechanical pretreatment of such fibrous substrates is the key for a sufficient and stable digestion process. Furthermore, it has been shown that the variations in specific biogas production during the periods resulted in an adaption of the microbes to the lignocellulosic materials.
detected in the digester, but the formation of sink layers and substrate agglomerations cannot be excluded. The required mixing intensity for substrates with a smaller particle size is definitely lower and the risk of substrate decomposition will be minimized. As expected, the monitoring of the pH-values during this investigation showed no variations between the digesters and the enduring use of horse manure. Similarly, no destabilization of the digestion process could be observed regarding the accumulation of VFA. In fact, the contrary occurred. As the test duration increased, the concentrations of VFA in the digesters decreased. Therefore, it can be assumed that the risk of process inhibition due to the enrichment of acids is not the limiting factor by the usage of horse manure. However, the increase of solids in the digester can result in a decrease of substrate pumpability. The high solid content of the digester substrate also disables the gas lift. Hence, the process limitation of the digestion of horse manure is not the acid accumulation, but the increase in solid concentration.
3.3. Biogas quality and quantity The daily average values of methane content of the digesters are presented in Fig. 4. During the first 50 days of the experiment, the methane content of the produced biogas was slightly higher in the digester with the pretreated substrates. Afterwards the methane contents of the biogas are similar for both digesters. Overall the methane content of digester 1 is 54.2 ± 0.6% and 53.7 ± 0.9% for the produced biogas in digester 2. Due to the possible differences of daily feeding of the digesters, it is necessary to consider the specific gas production, which expresses the biogas production per kg of VS added to the digester, to assess the degradation efficiency. The absolute gas production is therefore not an appropriate tool. Before this trial with horse manure began, the differences in gas production between the digesters were negligible (0.598 ± 0.051 Nm3/kg VS in digester 1 and 0.588 ± 0.054 Nm3/kg VS in digester 2). Fig. 5 shows the specific biogas production in the digesters during approximately 160 days.
3.4. Degradation efficiency To determine the degradation efficiency of the digesters at the research biogas plant, the maximum specific methane potential of the added VS was calculated on the basis of the daily input to the
Table 3 Total and volatile solids, pH-value and volatile fatty acids concentrations of the digester contents during the first and second hydraulic retention time. Parameter
Unit
Digester 1
Digester 2
1. HRT
2. HRT
x TS VS pH VFA
[% FM] [% FM] [g/kg]
10.7 7.7 7.8 0.4
± ± ± ±
STD
x
0.8 0.7 0.1 0.4
11.3 8.4 7.8 0.1
1. HRT
± ± ± ±
STD
x
0.5 0.3 0.1 0.1
10.0 7.1 7.9 0.3
2. HRT
± ± ± ±
STD
x
0.8 0.8 0.1 0.4
10.3 7.6 7.9 0.3
STD ± ± ± ±
0.5 0.5 0.1 0.2
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528
methane content [%]
58
digester 1 digester 2
56
54
52
50 0
50
100
150
day Fig. 4. Methane content of the produced biogas of digester 1 with a mechanical disintegration unit and digester 2 without substrate pretreatment.
specific biogas production [Nm³ / kg VS]
digesters. Therefore, the results of the substrates analysis and the specific methane yields of the HBT are reliable. It must be mentioned that according to the guidelines of the VDI 4630, the batch digestion test determines the specific methane potential in lab scale and these values tend to overestimate the methane potential for practical application [43]. Additionally, the grinding of the substrates for the HBT leads to optimum test conditions and substrate degradation. Nonetheless, the calculated specific methane production offers a well-grounded basis for the assessment of the degradation efficiency in this study. The calculated maximum specific methane production and the measured specific methane production from the digesters during the second HRT are presented in Fig. 6. The results show that the calculated specific methane yield for both digesters is absolutely identical (0.306 ± 0.015 Nm3 CH4/kg VS). The mechanical disintegration of the substrates leads to a specific methane production of 0.272 ± 0.038 Nm3 CH4/kg VS. Even with a HRT of approximately 80 days, the full-scale actual methane production at digester 1 was only 11.1% lower than the calculated methane production of the lab scale values. Regarding the scale effects, these narrow differences are negligible and it can be assumed that with an increase in HRT the differences between the theoretical and practical values will be eliminated. The observed differences between the calculated specific methane production and the measured methane production were
considerably higher at digester 2 (34.6%). The methane production of digester 2 reached only 0.200 ± 0.040 Nm3 CH4/kg VS and was significantly lower (26.5%) than in digester 1 during this trial. These large differences in the degradation efficiency were not expected, but show the effectiveness of a mechanical disintegration unit at a full-scale biogas plant. The additional energy requirement of the cross-flow grinder for the pretreatment of the substrates at digester 1 was 11.3 ± 1.3 kWh/t FM during this trial. Regarding the large differences in specific methane production of digester 1 and 2, the energy demand for the disintegration is negligible when considering these benefits. Overall, the results of this study show that the use of alternate lignocellulosic substrates, like horse manure, in biogas plants is possible. However, the digestion of horse manure without a sufficient pretreatment in a continuous stirred tank reactor leads to an unacceptable degree of degradation and is not feasible for practical application. 4. Conclusion The purpose of this work was to investigate the digestibility of straw-based horse manure in a full-scale continuous stirred tank reactor. Additionally, the effects of mechanical disintegration with a cross-flow grinder were evaluated in this full-scale experiment. The pretreatment of the substrates resulted in an increased methane production of 26.5% in comparison to the untreated variant. The
1.0
digester 1 digester 2
0.8
0.6
0.4
0.2
0.0 0
50
100
150
day 3
Fig. 5. Specific biogas production [Nm /kg VS] of digester 1 with the Bio-QZ and digester 2 without further substrate treatment.
specific methane production [Nm³ / kg VS]
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529
0.4
0.3
0.2
0.1
0.0 Digester 1 calculated
Digester 1 measured
Digester 2 calculated
Digester 2 measured
3
Fig. 6. Calculated and measured specific methane production [Nm /kg VS] of the digesters at the research biogas plant “Unterer Lindenhof” during the second HRT.
differences between the batch digestion test, which determined maximum methane potential of the grinded substrates and the fullscale actual methane production, are negligible in the case of the digester with the pretreatment device. The results of the full-scale setup prove the suitability of horse manure for biogas production. However, an appropriate pretreatment of the substrates is necessary to achieve an acceptable degradation rate and gas production. These results show that the available mechanical decomposing devices have positive effects on the methane production without the risk of forming inhibitory or toxic by-products due to disintegration. In addition, the mechanical disintegration units have the advantage of easy implementation at existing agricultural biogas plants. Given this information, it seems that mechanical pretreatment is key for an increase in anaerobic digestion of recalcitrant degradable agricultural residues, solid manures and alternate lignocellulosic materials and therefore is an environmentally friendly, economically viable and socially acceptable pathway for renewable energy generation. This work also shows the high reliability of the HBT for determination of the methane potential and the application of these results on the full-scale continuous biogas process.
Acknowledgments This research was funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (grant no. 03KB064) as part of the project “Horse manure e further development of technologies for the efficient use of horse manure”.
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