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Bioprocess and Biosystems Engineering, 35, 3, pp. 341-349, 2011-07-22
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Anaerobic co-digestion of dairy manure with mulched switchgrass for
improvement of the methane yield
Frigon, Jean-Claude; Roy, Caroline; Guiot, Serge R.
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O R I G I N A L P A P E R
Anaerobic co-digestion of dairy manure with mulched switchgrass
for improvement of the methane yield
Jean-Claude Frigon• Caroline Roy•
Serge R. Guiot
Received: 11 February 2011 / Accepted: 3 July 2011 / Published online: 22 July 2011 ÓCrown Copyright as represented by the National Research Council Canada 2011
Abstract The owners of farm-scale anaerobic digesters are relying on off-farm wastes or energy crops as a co-digestion feedstock with animal manure in order to increase their production of methane and thus revenues. Switchgrass represents an interesting feedstock for Cana-dian digesters owners as it is a high-yielding low-mainte-nance perennial crop, well adapted to northern climate. Methane potential assays in batch tests showed methane production of 19.4 ± 3.6, 28.3 ± 1.7, 37.3 ± 7.1 and 45.7 ± 0.8 L kg-1, for raw manure, blended manure, manure and mulched switchgrass, manure and pretreated switchgrass, respectively. Two 6-L lab-scale anaerobic digesters were operated for 130 days in order to assess the benefit of co-digesting switchgrass with bovine manure (digester #2), at a 20% wet mass fraction, compared with a manure-only operation (digester #1) The digesters were operated at an hydraulic retention time of 37 ± 6 days and at loads of 2.4 ± 0.6 and 2.6 ± 0.6 kg total volatile solids (TVS) L-1day-1 for digesters #1 (D1) and #2 (D2), respectively. The TVS degradation reached 25 and 39%, which resulted in a methane production of 1.18 ± 0.18 and 2.19 ± 0.31 L day-1 for D1 and D2, respectively. The addition of 20% on a wet mass ratio of switchgrass to a manure digester increased its methane production by 86%. The co-digestion of switchgrass in a 500 m3 manure digester could yield up to 10.2 GJ day-1 of purified methane or 1.1 MWh day-1 of electricity.
Keywords Anaerobic digestion Switchgrass Panicum vergatum Digester Methane
Introduction
The use of anaerobic digestion for methane production from animal manure in farm-scale digesters is well established in Asia and Europe while North America has still small industries that are just developing for the past 10 years. In the USA, there are around 150 operating biogas systems com-pared with a dozen in Canada, which makes both countries similar in term of installations per capita. The first operating anaerobic digesters for manure digestion are less than 10 years old in Canada [1] although the new Biogas Systems Financial Assistance Program in the province of Ontario will see an additional 15 projects operational in 2010 alone. The Green Energy Act provides the Feed-in tariff which is by far the best price for biogas in North America. The numbers of systems installed should then be increasing over the next years, with these new incentives put in place at the province level. Biomethane obtained from anaerobic digestion is the most efficient and clean-burning biofuel which is available today [2]. Furthermore, there is flexibility since the energy produced can then be transformed into heat, combined heat and power (electricity), or purified as natural gas.
Various co-substrates can be added to manure, in order to increase the methane production of an anaerobic diges-ter. For example, a 2.5% volumetric addition of cooking grease increased by 124% the methane production from a 250-L digester fed with swine manure [3]. The addition of energy crops to increase the methane yield and the result-ing net renewable energy production in farm-scale anaer-obic digesters treating manure is also widely spread in Europe [4, 5]. Specifically, there is an addition of crops, mainly corn silage, in more than 90% of the on-farm digesters in Germany [6].
The production of renewable energy from wastes can be appealing, as credits can be derived from the reduction of J.-C. Frigon C. Roy S. R. Guiot (&)
National Research Council Canada, 6100 Royalmount, Montreal H4P 2R2, Canada
e-mail: serge.guiot@cnrc-nrc.gc.ca DOI 10.1007/s00449-011-0572-5
green house gas (GHG) in addition to the income from the sale of energy. The operators of farm-scale digesters are allowed to use off-farm wastes to increase the production of methane of their process. However, the supply of such wastes can be problematic, as well as other concerns such as odors and pathogens. The use of energy crops grown on-site by the producers owning anaerobic digesters would allow for a stable feed of substrate to their digester, without the problems associated with off-farm wastes. Although corn silage is widely used in Germany and Austria as a feedstock for co-digestion with manure, this seems to be an unrealistic option considering the price of corn in Canada ($160-$176/ton) [7]. What is needed is the culture of an energy crop that requires very few investments, in equip-ment and time. Switchgrass is a strong candidate, as it only needs seeding in the first years and no nitrogen or herbicide application. It is basically a ‘‘let it grow and har-vest’’ crop. Furthermore, it can be grown on poor soils, thus not requiring the best land of the producers.
When choosing a crop for methane production, the net energy yield per hectare is often mentioned as the most important factor [8]. However, crop availability, as well as the cost of seeding, maintaining, harvesting and condi-tioning the crop prior to digestion are key economic factors which will influence crop selection. Panicum vergatum, or switchgrass, is a high-yielding perennial grass that is already perceived as a leading candidate for energy crop production in the USA [9]. It is well adapted for growth in Canada as it is resistant to naturally occurring pest and diseases and requires low fertilizer applications [10]. Fur-thermore, it is relatively inexpensive to grow and harvest. Switchgrass is composed of around 12–19% lignin, 31–37% hemicellulose and 29–45% cellulose, hence suggesting a high potential conversion of the plant into biofuel [11,12]. The methane potential of a variety of cellulosic crops has been recently evaluated [13–15]. However, the litera-ture for the methane potential of switchgrass remains scarce [16–18]. The purpose of this study was therefore to evaluate the effect of adding switchgrass as a co-digestion substrate to dairy bovine manure, in order to confirm the increase in methane produced by an anaerobic digester.
Materials and methods Origin of materials
The digesters were inoculated with biomass from a full-scale anaerobic digester treating fruit processing wastewater (Lassonde inc, Rougemont, Qc, Canada). The dairy bovine manure used from day 0 to day 63 of operation of the digesters was collected at the farm ‘‘Ferme Wallu’’ (Ste-Julienne, Qc, Canada). The second sample of dairy
bovine manure, used until the end of the experiment, was collected at the farm ‘‘Ferme Larose’’ (Verche`res, Qc, Canada). Both samples of manure were stored at 4 °C until use.
The switchgrass, a Kanlow variety, was obtained at the farm ‘‘Ferme Norac’’, from an 11-year-old field yielding 10–12 tons per hectare (t ha-1). It was harvested during August 2008 while it was mature and still fully green and cut in pieces of 2 cm or less and compressed in 10-L pails to minimize oxygen and initiate silaging. An aliquot of switchgrass was collected once a week from one of the pail as required for the feeding of the digesters. The switchgrass was then mulched (finely chopped) for 20 s with a knife mill Grindomix model GM200 (Retsche, Newton, PA, USA), with the blades spinning at 5,000 rpm.
Methane potential assays
The preparation of the methane potential assays was based on the Biochemical Methane Potential (BMP) assay for wastewater from Cornacchio et al. [19] and modified as described in [18]. The inoculum was starved for 48 h prior to the start-up of the assays, by being incubated at 35 °C and 125 rpm agitation without substrate. The ratio of inoculum to sample was set at 1:1 on a volatile solids (VS) concen-tration basis as suggested by Lehtomaki et al. [20] for grass silage methane potential assays. Each bottle received 20 grams of inoculum, 6 mL of defined media, 8 mL of bicarbonate buffer and 1 mL of reducing agent [18].
The amount of added substrates was 8 and 2 wet grams of manure and switchgrass, respectively. The volume of the bottles was completed to 100 mL with deoxygenated dis-tilled water. The bottles were prepared anaerobically in triplicates in 500 mL serum bottles and filled under a constant nitrogen flow. The pH was adjusted if necessary when the bottles were completed, before sealing. During the filling of the bottles, aliquots of the added inoculum were sampled and analyzed for VS concentration, to insure that a constant amount of inoculum was added through the experiment. The bottles were incubated at 35 °C with an agitation of 150 rpm. Control bottles were prepared to subtract endogenous methane production generated by the inoculum from the tests assays. The control bottles were identical to the test bottles, except that the manure and switchgrass suspensions were replaced with the same vol-ume of deoxygenated water. The assays were conducted until the methane production became negligible (\5 mL CH4day-1) as performed elsewhere [20].
The dairy manure was blended using the same procedure as for the switchgrass with the knife mill, to obtain an homogenous substrate and to facilitate the feeding of the digesters in the next phase of the study. Pretreatment of the mulched switchgrass was performed by mixing 10 g of
switchgrass with 90 mL of water and 0.7 g of NaOH and then incubate the mixture at room temperature for 3 h. Then, the preparation was autoclaved (121 °C, 20 psi, 15 min) and cooled before addition to the assay.
Lab-scale digesters Description
Two continuously stirred tank reactors (CSTR) were operated in parallel for the study, equipped as shown in Fig.1. The total volume of each digester was 9.5 L, with 6 L of working volume. The digesters were made in glass, with a jacket to allow water to circulate and maintain the temperature at 35 °C. The digesters were stirred using a top-mounted high-torque Bellco Biotechnology motor working at 60 rpm. Each digester was equipped with one large feeding and sampling port (3/4’’ ID) and a biogas exit port.
Start-up
Each digester was inoculated with two L of granular bio-mass (Lassonde inc) for a total of around 240 gVS, with 800 mL of tap water and 200 mL of bicarbonate buffer
(50 g L-1 NaHCO3). The digesters were then
progres-sively filled by adding 100 g of dairy manure each week-day (mixed with 140 mL of tap water), until a volume of 6 L was reached in the digesters. Around 170 gVS of manure was thus added between days 0 and 18 of opera-tion. On day 18, the digesters were considered full, and the differentiated operation began.
Operation
A retention time of 35 days was applied to the digesters, based on the results of the methane potential assays. The hydraulic retention time (HRT) was estimated according to Helffrich [21] as a quotient of the daily feedstock mass fresh matter (FM) and the total digester volume, since the volumetric calculation is not useful in solid substrate digestion.
The digesters were fed daily during weekday (5 times per week). First, 240 g of the digester’s content was removed. Then, the preparation of a feeding for the dairy manure digester (D1) was as follows: 100 g of manure was mixed with 140 mL of water, then blended in the Grindomix for 20 s at 5,000 rpm. The feeding of the manure and switchgrass digester (D2) was prepared as follows: 20 g of switchgrass was mulched in the
Fig. 1 Schematic of the digesters set-up
Grindomix for 20 s at 5,000 rpm; and added to 80 g of manure mixed with 70 mL of water. Finally, both aliquots were mixed with an additional 70 mL of water and fed to the digester. The blending of the manure and mulching of the switchgrass prior to their addition in the digesters allowed for a homogenous filling and drawing of the material from the digesters.
A buffer solution (42 g L-1 NaHCO
3 and 100 g L-1
KHCO3) was added to the digesters when the pH dropped
below 6.6, which happened only a few times over the 130 days of operation. 50 mL of buffer was added on days 70 and 77 for both digesters, while 10 mL was added on days 106, 107 and 119 for digester #2.
Analytical methods
All gas or methane volume presented in this paper are described at standard temperature and pressure, e.g., 273.15 K and 100 kPa pressure. The biogas production was measured in the serum bottles with a water-displace-ment system built from a volumetric glass burette, with graduation every 0.2 mL. After equilibrium of the bottle headspace to 1 atm, a gas sample (0.3 mL) was taken with a model 1750 gas-tight syringe (Hamilton, Reno, USA) and analyzed for H2, N2, CH4and CO2by gas chromatography
(GC). The GC was a Agilent 6890 (Agilent Technologies, Wilmington, USA) coupled to a thermal conductivity detector (TCD). The gas sample was injected on a 11-m X 2 mm I.D. Chromosorb 102 packed column (Supelco, Bellafonte, USA). The column was heated at 50 °C and maintained for 3.9 min. Argon was used as the carrier gas. The injector and detector were maintained at 125 and 150 °C, respectively.
Liquid samples (1.5 mL) were taken from the serum bottles with 3 mL syringes equipped with a number 26-gauge needle and analyzed for pH, soluble chemical organic demand (COD) and volatile fatty acids (VFA), namely acetate, propionate and butyrate. Total volatile solids (TVS) and volatile suspended solids (VSS) were also determined at the beginning of the assays and at the end for each bottle. The pH was measured on an Accumet AP61 portable pH meter equipped with a micro probe (Fisher, Fairlawn, USA) directly on the recuperated sample, within 1 min of sampling. The COD and the VSS were deter-mined according to Standard Methods [22].
The samples for the VFAs were centrifuged at 12 100 g in a J2-21 M centrifuge (Beckmann, Canada) equipped with a JA-20 rotor for 10 min. A volume of 350 lL of the supernatant was then mixed with an internal standard containing 6% formic acid and injected in the GC. The analysis was performed on a Perkin–Elmer Sigma 200 GC (Norwalk, USA) equipped with a flame-ionization detector (FID). A sample volume of 0.5 lL was injected on a 3 ft X
2 mm 60/80 Carbopack C column with 0.3% Carbowax 20 M and 0.1% H3PO4 (Supelco, Bellafonte, USA). The
oven, injector and detector temperatures were 120, 200 and 200 °C, respectively, and the method lasted 4 min. Nitro-gen was used as the carrier gas at a flow rate of 50 mL min-1.
Results and discussion
Characterization of the manure and switchgrass
The Table1 is showing a characterization of the manure and switchgrass samples used in this study. The manure solid concentration was higher than what can be found in the literature, although the sCOD, VFA and ammonium concentration were in the normal range [23–25]. The TVS concentration of both manure was similar, although the Wallu manure displayed higher concentration of VFA, possibly suggesting a more degraded sample. Acidification level were thus different as well, with 39 and 22% of the soluble COD accounted for by VFA in the Wallu and Larose manure, respectively. The C:N and C:P ratios were analyzed only for the Larose manure and reached 21:1 and 65:1, respectively. The summer-harvested switchgrass (SHS) contained on a dry matter basis 28.1, 37.4 and 24.6% of lignin, hemicellulose and cellulose, respectively [18]. Therefore, a significant fraction of the organic solids, e.g., lignin, could be considered anaerobically non biode-gradable. The C:N and C:P ratios of 67:1 and 197:1 were clearly a sign of an excess of carbon in SHS. The mixing of manure and switchgrass as applied to the digester #2 resulted in combined C:N and C:P ratio of 40:1 and 119:1, respectively, which was in the proper range for optimal anaerobic degradation [26,27].
Methane potential assays
Methane potential assays were performed prior to the experiment in digester, as a proof of concept to demon-strate the increased methane production from the co-digestion of switchgrass with manure. The cumulative methane production reached 19.4 ± 3.6, 28.3 ± 1.7, 37.3 ± 7.1 and 45.7 ± 0.8 L kg-1, for the raw manure, blended manure, manure and mulched switchgrass, manure and pretreated switchgrass, respectively (Fig. 2). The manure was blended to facilitate the feeding of the digesters, although this assay showed that the blending consisted in a form of pretreatment that increased the methane production of the manure by 46%. Nevertheless, the methane produced from the manure only assays were in the same range as what is generally presented in the liter-ature for manure anaerobic digestion [27, 28]. The
replacement of 20% of the manure by switchgrass (wet weight) in the assay yielded 32% more methane, although the gain was 92% if compared with the raw manure. The amount of methane was then nearly doubled with the addition of a blending pretreatment to the manure and switchgrass. The alkaline and autoclaving pretreatment of switchgrass further increased the methane production gain, from 32 to 61%.
A conversion of the methane production for the assays showed 216 ± 40, 316 ± 19, 262 ± 50 and 322 ± 5 L kg-1TVS for the raw manure, blended manure, manure and mulched switchgrass, manure and pretreated switch-grass, respectively. This is an indication that the methane production in the co-digestion assays is increased (when expressed by wet kg) because of the bigger mass of TVS in
the assay due to the addition of switchgrass (three times more volatile solids per kg than manure) and not from a higher methane potential of switchgrass on a volatile solids basis. Indeed, based on a methane potential of 233 L kg-1
TVS for switchgrass alone [18], the methane potential from a mix of switchgrass and manure (51 and 49% dry weight) could be predicted at 275 L kg-1TVS by weighted sum of
the aforementioned separate values, which is within the range of the co-digestion observed yield of 262 ± 50 L kg-1 TVS.
This comparatively low specific methane yield from switchgrass anaerobic digestion could be improved in digester operation compared with single-batch incubation such as a BMP, provided growth and adaptation of com-petent biomass is occurring in the digester. Nevertheless, there was a net gain in the methane produced from the same wet mass of substrate when co-digesting manure and switchgrass in comparison with manure only digestion.
From Fig.2, it could be interpolated that 90% of the methane potential from the co-digestion of manure and switchgrass would be obtained in 35 days of incuba-tion, which leads to the targeted HRT for the digester experiment.
Experiments with the continuous digesters
The Table2is presenting the operational parameters of the digesters during the 133 days of operation. The pH was kept around neutral with values around 7.2 at the effluent for both digesters. The feeding of digester #1 (manure only) was stable as shown by the small standard deviation of the influent TVS concentration. The TVS and VSS degradation averaged 25 and 30% for digester #1, respec-tively. This performance was within the range of thermo-philic digesters with blended bovine manure [24], although being much lower than the 83% reported by Lindorfer et al. [4]. Even though most of the organic material was under solid form (87% of TVS as VSS), there was still a Table 1 Characterization of
the dairy bovine manure and summer-harvested switchgrass used during this study
Concentrations in g kg-1
Ratio C:N and C:P [18] TStotal solids, TVS total volatile solids, ND not determined, NA not applicable (no soluble fraction)
Parameters Wallu manure Larose manure Switchgrass
TS 152.6 ± 5.8 143.5 ± 2.6 379.9 ± 21.0 TVS 137.2 ± 11.6 127.2 ± 5.4 350.6 ± 34.5 Ratio C:N ND 21:1 67:1 Ratio C:P ND 65:1 197:1 pH 6.0 ± 0.1 6.6 ± 0.2 NA sCOD 24.7 ± 1.7 17.8 ± 3.8 NA Acetate 2.51 ± 6.21 1.80 ± 0.22 NA Propionate 0.97 ± 0.15 0.60 ± 0.09 NA Butyrate 2.99 ± 0.49 0.64 ± 0.21 NA VFA in COD-equiv. 9.6 ± 1.6 4.0 ± 0.6 NA Ammonium 0.28 ± 0.05 0.26 ± 0.04 NA
Fig. 2 Methane production during batch assays: impact of the co-digestion of pretreated switchgrass with bovine manure
significant concentration of soluble COD in the influent, with a fraction of 29% as VFA (acetate, propionate, butyrate). This soluble organic matter was mostly degraded during digestion, with 71% soluble COD removal and no VFA were recovered in the effluent. The ammonium con-centration increased after digestion but remained below 4 g L-1, a potential inhibition concentration [29]. Thus, the operation of digester #1 displayed a stable operation over the course of the study.
The feeding of digester #2 (manure and mulched switchgrass) was not kept as stable, as the concentration of TVS decreased over time, from 80.6 ± 1.8 g kg-1(weeks
2–6) to 62.6 ± 0.3 g kg-1(weeks 16–19) for an average of
73.9 ± 7.5 g kg-1 (Table 2). This decrease in the TVS concentration resulted from the partial degradation of the switchgrass during storage and will be discussed later on. Nevertheless, the average TVS and VSS reduction reached 39% for the co-digestion of manure and switchgrass. This is much less than the 59% VS reduction obtained from Lehtoma¨ki and Bjo¨rnsson [30] with grass silage. This could be partly due to the much lower lignin content of their substrate at 5.4%.
The influent and effluent sCOD and VFA concentration were similar than that for digester #1 and indicated that most of the soluble organic mater was degraded as well in digester #2. The ammonium concentration was lower in the D2 effluent compared with D1 (0.40 vs 0.56 g L-1). It can be hypothetized that the ammonium generated during the digestion of the substrates was partly used by the micro-organisms for the digestion of the switchgrass, deficient in N. This would add an emphasis on co-digesting substrates in order to equilibrate their C:N ratio for an optimal deg-radation [31].
The organic loading rate (OLR) averaged 2.4 ± 0.6 and 2.6 ± 0.6 gTVS L-1 day-1 for digesters #1 and #2,
respectively. As mentioned earlier, the progressive deteri-oration of the switchgrass resulted in an OLR starting at 3.0 ± 0.5 gTVS L-1 day-1 during the first weeks of
operation, to 2.4 ± 0.5 gTVS L-1day-1 during the last weeks of operation. The varying concentration in TVS resulted in an organic loading of digester #2 superior to digester #1 by 52% at the beginning of the study compared with 18% at the end. This was reflected in the methane production from both digesters over the course of the experiment.
The methane production from both digesters over the course of the study is displayed in Fig.3. Each value represents the sum of the daily produced methane during each week of operation. Some weeks are not shown here, as some of the daily values were missing, due to leak repairs or maintenance of the digester (opening of the lid). Nevertheless, it can be seen that an average methane production of 2.19 ± 0.31 L day-1 was produced in
D2 over the course of the experiment, compared with 1.18 ± 0.18 L day-1 for D1, a 86% increase in methane production. However, it was noted during the course of the experiment that the switchgrass could not be preserved and some fungi growth would start appearing. During the last 4 weeks of operation, the switchgrass was significantly attacked by fungi and appeared partially degraded. There-fore, there was a decrease in the methane production from D2 at the end of the experiment. In effect, the methane production was 1.15 ± 0.02 and 2.45 ± 0.29 L day-1 for
D1 and D2 during weeks 2–6, respectively. While the methane production remained stable for the manure fed D1, at 1.15 ± 0.08 L day-1 during weeks 16–19, it decreased
significantly for the manure and switchgrass fed digester, at 1.86 ± 0.26 L day-1. Hence, the methane production was 113% superior in D2 at the beginning of the study, com-pared with only 62% at the end. Furthermore, the increase in methane production was less significant when comparing specific methane yields, with an average of 88 ± 17 and 125 ± 14 L kg-1 TVS
in for D1 and D2, respectively, a
42% difference. Although the increase in the methane production from the digester operating with switchgrass as a co-substrate compared with manure-only feeding is Table 2 Operating conditions of the digesters
Parameters Digester #1 Digester #2
In Out In Out pH 6.3 ± 0.5 7.2 ± 0.2 6.2 ± 0.6 7.2 ± 0.2 TVSa 53.1 ± 3.9 39.4 ± 5.3 73.9 ± 7.5b 45.0 ± 5.0 VSSa 46.5 ± 1.2 32.4 ± 2.2 65.7 ± 9.8 40.0 ± 1.2 sCODa 14.3 ± 3.3 4.2 ± 1.0 15.0 ± 3.2 4.1 ± 1.1 VFAa 4.2 ± 2.1 \0.1 3.5 ± 1.5 \0.1 Ammoniuma 0.19 ± 0.03 0.56 ± 0.05 0.22 ± 0.05 0.40 ± 0.04 a Concentrations in g kg-1 b
Concentrations varied from 80.6 ± 1.8 g kg-1(day 21–35) to 62.6 ± 0.3 g kg-1(day 126–133)
TVStotal volatile solids, VSS volatile suspended solids, sCOD soluble chemical oxygen demand, VFA volatile fatty acids
substantial, it was still lower than that with other co-sub-strates. For example, a yield of 280 mL methane per gVS was obtained from the co-digestion of manure and sugar-beet byproducts [32] and 249 mL methane per gVS for crop silage mix [33].
A methane potential assay was performed in serum bottles with the switchgrass attacked by fungi at the end of the study to verify if the decrease in methane production in the digester #2 during the last weeks of operation could be linked to a decrease in methane production from the switchgrass feedstock. The TVS concentration of the switchgrass feedstock was only 293 ± 5.4 g kg-1 at the
end of the study. The bottles were prepared with 5 g of switchgrass, as for the assays presented in Fig.2, and incubated for 6 weeks. The final methane production reached 0.08 ± 0.02 L kg-1 TVS. This represents only
35% of the expected yield from fresh switchgrass [18] and would indeed explain the lower methane production during the last weeks of operation of digester #2.
A last set of methane potential assay was launched at the end of the experiment, on the digestate from both digesters, in order to quantify the residual methane production. The methane produced reached 45.2 ± 2.7 and 75.0 ± 2 mL g-1VSdigestate addedafter 181 days of incubation for the
digestate of D1 and D2, respectively. This is much lower than the 133–197 mL g-1VS of digestate added obtained by Lehtoma¨ki et al. [23] from co-digestion of manure with grass silage, oat straw or sugar beet tops after 100 days of incubation. It can be concluded that the HRT of 35 days
applied to D2 was sufficient to release most of the methane potential from the mix of manure and switchgrass. Case figure for a 500 m3farm-scale anaerobic digester
There is no pilot-scale or full-scale anaerobic digesters co-digesting manure and switchgrass, to our knowledge. The feasibility of co-digesting switchgrass with manure could be evaluated at pilot-scale at first, such as the work presented by Lehtoma¨ki and Bjornsson [30] where they used a 7.6 m3 leech bed and a 2.6 m3UASB for the co-digestion of sugar beets and grass silage. In effect, there are some potential challenges that would need to be resolved before a practical scale-up of manure and switchgrass co-digestion. Specifically, at laboratory scale, there was a small portion of the mulched switchgrass floating on the surface of the bulk liquid in the digester. Proper mixing is then required to prevent poor degradation performance and the formation of a crust on the surface of the digester.
Meanwhile, it can be an interesting exercise to extrap-olate the results obtained above and apply them on a the-oretical 500 m3farm-scale anaerobic digester, as a case of figure. A 500-m3 anaerobic digester would manage the manure of around 200 cows and fit the need of an average dairy farm in Canada. To obtain a 35-day HRT, around 15 tons day-1 of bovine manure would be required for a manure-only digester, compared with 12 tons of manure and 3 tons of fresh switchgrass for a co-digestion process. Considering a yield of 11 tons ha-1 for switchgrass, approximately 100 ha would be needed to supply switch-grass to the digester for the year.
The extrapolation of the optimal methane yield from the laboratory-scale digesters to a 500-m3 digester would represent 59 et 122.5 m3day-1 for D1 and D2,
respec-tively (Table3). These methane yields were obtained with diluted manure (1:2.4, wet wt.) in order to insure proper homogeneity and feeding in the digesters. Considering that the actual loading rate was quite low, it can be presumed that raw manure could be directly fed to the farm-scale digester as performed in typical operations, yielding 142 and 294 m3day-1for D1 and D2, respectively. This would translate in significant production of purified methane (10.2 GJ day-1) or net electricity (1.1 MWh day-1) for the
operational conditions of D2 as shown in Table3. The methane production achieved with the co-digestion of manure and switchgrass was comparable to the 500 m3of biogas per day generated from a full-scale system digesting ensiled sugar beets at an OLR of 1.67 kg VS m-3day-1
and HRT of 52.5 days [34].
The feasibility of co-digesting manure and switchgrass was demonstrated and is yielding more renewable energy than anaerobic digestion of dairy manure alone. A more detailed analysis is needed to verify if the scenario would Fig. 3 Monitoring of the methane production over the course of the
be economically feasible, although this is outside the scope of this study.
Conclusions
A demonstration of the co-digestion of switchgrass with dairy manure was made in laboratory-scale digesters, showing an increase of 86% in the methane production compared with a digester fed only with manure at a similar loading of fresh material. However, the specific methane production (125 ± 14 L kg-1TVSin) was lower compared
with co-digestion systems processing sugar crops or grass silage, partly because of the higher lignin concentration of the switchgrass. There was a trend that co-digestion slightly increased the methane yield per unit solid added, likely due to a synergistic effect between manure and switchgrass. An efficient storage will be required for the switchgrass, in order to keep its full potential for year round feeding of the digester. Although the ensilaging of switchgrass has not been tried to our knowledge, since it is not used for animal feed, it should provide an adequate stabilization of the substrate. This is critical, as it was demonstrated in this study that switchgrass was attacked by fungi and partially degraded within 4 months of conventional storage, resulting in a loss of approximately 17% of its organic solids.
The extent of organic solid degradation, 39% for the co-digestion of manure and switchgrass, is leaving room for improvement of the methane yield that can be obtained. In this regard, future work focusing on the development and application of an effective and cost-efficient pretreatment for switchgrass would be required.
Acknowledgments The contribution of Mr. A. Abella for the initial methane potential assay and Mrs. F. Matteau-Lebrun during the
operation of the digesters is strongly acknowledged. The authors also wish to thank MM. A. Corriveau et S. Deschamps for analytical assistance with the VFA and sugar determination and Mr. A. Mig-neault for the preparation of the schematic of the digesters. The financial support from the AAFC-NRCan-NRC National Bioproduct Program (Project #3) and the ecoENERGY Technology Initiative from Natural Resources Canada is gratefully acknowledged. NRC paper #53369.
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