Improvement of Exhausted Sugar Beet Cossettes Anaerobic Digestion

Jan 20, 2015 - Exhausted sugar beet cossettes (ESBC) are one of the main byproducts generated in the sugar industries. The pellets of dried pulp of ES...
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Improvement of Exhausted Sugar Beet Cossettes Anaerobic Digestion Process by Co-Digestion with Pig Manure Kaoutar Aboudi,* Carlos José Á lvarez-Gallego, and Luis Isidoro Romero-García Department of Chemical Engineering and Food Technology, Faculty of Sciences, Agrifood Campus of International Excellence (CeiA3), University of Cádiz, 11510 Puerto Real, Cadiz, Spain ABSTRACT: Exhausted sugar beet cossettes (ESBC) are one of the main byproducts generated in the sugar industries. The pellets of dried pulp of ESBC used in this study are composed of 85% beet pulp and 15% molasses and have a high lignocellulosic-type organic matter content (91% volatile solids) and a nitrogen deficiency. Mesophilic anaerobic co-digestion in batch of dried pellets of ESBC and pig manure (PM) was studied in this paper for five mixture ratios. The mixtures were selected on the basis of the carbon/nitrogen ratio, leading to ESBC:PM percentages of 0:100, 32:68, 48:52, 72:28, and 100:0. Codigestion increased the methane yield in all cases relative to the digestion of ESBC alone. Acetic and propionic acids were the main volatile fatty acids observed in the tests. Initial accumulation of propionic acid in the reactors with higher ESBC content (and especially for the 100:0 reactor) indicated the difficulties associated with ESBC degradation. The best results were obtained for co-digestion of ESBC and PM in a 32:68 mixture, leading to 103% and 30% improvement in the specific methane production relative to the digestion of ESBC and PM, respectively. Furthermore, this mixture ratio was also optimal to minimize the lagphase period for methane production.

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INTRODUCTION Today, energy crisis is a key issue all over the world. Security of the energy supply, especially sustainable energy, and reduction of CO2 emissions are considered to be the most important priorities. Biogas coming from anaerobic digestion (AD) of organic wastes is one of the most efficient and effective options among the various alternative sources of renewable energy currently available. Biogas can be produced from a variety of substrates, such as energy crops, animal manure, municipal solid waste (MSW), sewage sludge, industrial waste, etc. Lignocellulosic agri-food wastes, such as exhausted sugar beet cossettes (ESBC), can be used as a raw material for biogas production since they are composed mainly of carbohydrates, cellulose, hemicelluloses, and lignin material, which are potential sources of biogas as a clean energy.1 ESBC are a byproduct of the beet sugar industry, formed from the fibrous residues of sugar beets (Beta vulgaris) after several extraction processes. Usually, the pulp and molasses remaining from sugar beet extraction are mixed together, dried, and sold as livestock feed or directly burned as fuel. Animal feeding is the main current use of ESBC (due to the highly digestible cellulose content), but it has a low added value because it can only be used as a nutritional supplement mixed with other animal feed components. The commercialization of sugar beet byproducts as animal feedstock is critical to achieve economic viability. However, an interesting alternative to valorize sugar beet byproduct is its use for energy production, i.e., methane production by AD. This option could reduce the potential for air pollution and contribute to minimizing the energy requirements of sugar beet processing companies, making it worthwhile ecologically as well as economically. A major disadvantage related to AD of lignocellulosic-type wastes is their low biodegradability.1−3 It has been reported that co-digestion can be an interesting option for improvement in biogas yields during AD of lignocellulosic wastes due to the © XXXX American Chemical Society

positive synergisms established in the digestion process by providing a better nutritional balance.4−7 Co-digestion combines two or more organic substrates in order to improve process performance.8−14 Recently, several studies have shown that co-digestion of agricultural wastes with livestock wastes permits the beneficial complementarily in wastes’ characteristics and enhances the methane productivity of biomethanization process.5,6,8,11−14 Livestock wastes such as pig manure (PM) not only contribute nitrogen and alkalinity but also provide a high microbiological activity, which is able to degrade vegetal fiber.15,16 Several reasons explain the growing interest and justify the development of biogas production by anaerobic co-digestion of agro-industrial wastes. First, there is a good availability and diversity of a large amount of biodegradable organic waste that could be used for biogas production. Moreover, restrictions imposed by European environmental regulations (European Directive 99/31/EC) have limited the disposal in landfills of waste with high organic matter content.17 Additionally, from a technological point of view, there have been improvements in the viability of agro-biogas plants due to the development of codigestion processes. Therefore, biogas from co-digestion of agro-industrial waste has a large market in the future energy scenario. No previous study has been found in the literature on codigestion of ESBC as dried pulp (pellets) with PM. However, co-digestion of other similar mixtures, such as agricultural waste with livestock manure, has been recently reported. MataAlvarez et al. have published a review about the anaerobic codigestion studies carried out during the past 5 years, focusing Received: August 29, 2014 Revised: January 9, 2015

A

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The characteristics of substrates and inoculum used in the experiments are shown in Table 1.

on sewage sludge, biowaste, and animal manures as the main substrates. In this latter case, the agro-industrial wastes have been widely used as co-substrates (with percentages of 54% and 39%) and co-digested with pig manure and cow manure, respectively.14 Molinuevo-Salces et al. investigated the effect of the co-digestion of vegetable wastes from a vegetable processing factory, composed of green peas, maize, carrots, and leeks (25:25:25:25 dry weight), with swine manure in a semi-continuous anaerobic digester. They found that the mixture of these types of substrates had a positive effect on methane production due to the reciprocal benefits of the high alkalinity of swine manure and the high carbon content supplied by vegetable wastes, which improved the process performance in comparison with the treatment of both wastes separately.18 Yangin-Gomec and Ozturk also found that daily methane production and total energy production increased about 1.2-fold when maize silage was co-digested with cattle and chicken manures.19 Kafle and Kim worked on co-digestion of apple waste and swine manure at different mixing ratios. They determined that the mixture of the wastes at a ratio of 1:2 improved the biogas yield by approximately 16% and 48% when operating at mesophilic and thermophilic temperatures, respectively.5 Similarly, Alvarez and Lidén reported that mesophilic co-digestion of manure (cattle and swine), solid slaughterhouse wastes, and fruit and vegetable wastes gave a high performance of the treatment process, which cannot be achieved when treating the wastes separately. They have determined that methane yield of semi-continuous co-digestion of these three wastes can reach up to 0.3 m3/kg VSadded using organic loading rates (OLRs) up to 1.3 kg VS/m3·day, with a reduction in volatile solids (VS) content around 65%.6 The above-discussed studies indicated that the addition of livestock manure to the agri-food organic wastes enhanced the viability and performance of the AD. It should be noted that a few researcher have studied the AD of sugar beet byproducts like tops, leaves, or wet pulp.20−30 Nevertheless, there is no study about co-digestion of dried pulp (pellets) of ESBC with animal manure. The objective of this paper was to evaluate the viability and feasibility of the anaerobic co-digestion of ESBC with PM. All the batch experiments were conducted in duplicate, operating at mesophilic temperature (35 °C) and testing five mixtures of the substrates selected. The criteria to determine the optimum conditions of the co-digestion process were the yield of methane, the process stability, and the removal of organic matter reached (in terms of VS and DOC).

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Table 1. Main Physico-chemical Characteristics of Substrates and Inoculum Used in the Experiments pH TS VS sCOD DOC TVFA alkalinity N-NH4 TKN C/N ratio

units

ESBC

PM

inoculum

− g/kg g/kg g/L g/L g/L g/L g/L g/L −

5.8 ± 0.2 880 ± 0.1 800.8 ± 0.1 46 ± 0.2 35 ± 0.1 4.2 ± 0.8 3.9 ± 0.1 0.4 ± 0.2 1.3 ± 0.1 33.5 ± 0.2

6.2 ± 0.2 240 ± 0.11 177.6 ± 0.1 10.2 ± 0.1 8.2 ± 0.1 5.1 ± 0.4 47.8 ± 0.5 2.2 ± 0.1 3.3 ± 0.1 14 ± 0.1

7.3 ± 0.1 40 ± 0.1 22 ± 0.1 35 ± 0.2 4.9 ± 0.1 1.8 ± 0.1 54.8 ± 0.3 0.1 ± 0.2 nd nd

Experimental Design. The five ESBC:PM mixtures tested were 0:100, 32:68, 48:52, 72:28, and 100:0, based on the weight percentage of each substrate in the feedstock. The mentioned substrates mixtures correspond to carbon/nitrogen ratios of 14, 18.5, 23.5, 28.5, and 33.5, respectively. In general, a C/N ratio in the range of 20−30 has been suggested as the most favorable C/N ratio for anaerobic treatment.32 On the other hand, Nyns proposed the range of 16−19 as the best C/ N ratio.34 Kivaisi and Mtila proposed the range of 16.8−18 as adequate for lignocellulosic material treatment.35 The C/N ratios at different ESBC:PM mixtures studied in this paper ranged from 14 (100% PM) to 33.5 (100% ESBC). All the tests were conducted in duplicate. However, due to a failure in one of the 0:100 mixture reactors, data for this mixture come from only one of duplicates. Reactors were inoculated 50% (on a weight basis), and the solids content in the reactors was adjusted to 8% TS.30 It was not possible to operate the reactors at higher TS percentages, because the rheological behavior of the ESBC pellets is detrimental to a proper mixing and homogenization of the reactor content. Batch Experimental Set-Up. A series of 10 stainless steel batch bioreactors with a total volume of 3 L (working volume, 2 L) were used in this study (Figure 1). The reactors have a glass cover with several input/output ports, which were used for gas outlet, feed inlet, and temperature control. Temperature was maintained in the mesophilic range (35 °C) by a heating plate located at the base of each reactor. Furthermore, each reactor had an independent agitation system, and the stirring rate was maintained at 18 rpm during the entire study period. At the start of the batch assay, the digesters were filled as outlined below. First, adequate proportions of ESBC and PM were mixed with deionized water to obtain the required 8% TS content, as commented before. The weight proportion of each substrate in the mixture depended on the mixture ratios established at the beginning of the assay. Then, 1 kg of the mixture was added to the reactor and heated to the conditions required by the mesophilic process (35 °C). Next, 1 kg of the mesophilic (35 °C) active inoculum previously described was added to complete the working volume of the digester (i.e., the inoculation was performed at 50% on a weight basis). The initial pH was measured and was adjusted to the required pH of 7.5 by adding 8 M NaOH.31 The reactors were purged with N2 for 5 min to remove oxygen prior to the beginning of the test to ensure anaerobic conditions. Each reactor had an individual agitation system (motor and stirring blade) to ensure the homogenization of the reactor content during the whole assay. Stirring was maintained at 18 rpm. Finally, the temperature of the reactors (35 °C) was maintained by a PID controller. Thus, each reactor had an individual heating jacket, and temperature was monitored and controlled continuously by an inner temperature sensor. The maximum temperature fluctuations with this heating device were 0.5 °C.

MATERIALS AND METHODS

Collection and Characterization of Substrates. The pellets of the ESBC used in this study were collected from a sugar beet processing plant located at Jerez de la Frontera (Cádiz) in the South of Spain. The PM was collected directly from a semi-intensive livestock at Puerto de Santa Mariá (Cádiz) in the same county. ESBC used in the present study contained 15% molasses and 85% pulp. The dried pellets, 6 mm in diameter, had total solids (TS) contents in the range of 80−90%. PM was a mixture of solid and liquid manure deposited on the solid floors of the farm without any liquid/solid separation system. Samples of both wastes were collected and stored at 4 °C. The reactor from which the inoculum was taken was a laboratoryscale mesophilic anaerobic semi-continuous stirred tank reactor (10 L) fed with ESBC and working at 20 days hydraulic retention time (HRT). Specific methane production (SMP) of the inoculum used was 279.4 mL of CH4/g VSfed. B

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Total solids (TS), volatile solids (VS), alkalinity, pH, soluble chemical oxygen demand (sCOD), dissolved organic carbon (DOC), and ammonium were monitored three times a week according to standard methods (APHA, 2005).36 For sCOD and DOC analysis, the samples were previously lixiviated with deionized water during 2 h and filtrated by 0.47 mm.37 The DOC analysis was carried out in an Analytic-Jena multi N/C 3100 carbon analyzer with chemiluminescence detection (CLD) by the combustion-infrared method (5310B). The oxidizer was oxygen 5.0 at pressure of 4−6 bar, and the method detection limit of the analyzer was 4 μg C/L. For total volatile fatty acidity (TVFA) analysis, samples from the previous lixiviation and filtration were filtered again through a Teflon filter of 0.22 μm and analyzed by using a gas chromatograph (Shimadzu GC-2010) equipped with a flame ionization detector (FID) and capillary column filled with Nukol (polyethylene glycol modified by nitroterephthalic acid; part no. 221-72658-92). The detection limits for the VFAs were 15, 45, 27, 33, 37, 42, 48, 54, and 68 ppm for acetic, propionic, isobutyric, butyric, isovaleric, valeric, isocaproic, caproic, and heptanoic acids, respectively.

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RESULTS AND DISCUSSION

Substrate Characteristics. The characterization of the substrates is summarized in Table 1. As shown, ESBC has higher organic matter content than PM in terms of VS and solubilized organic matter (sCOD and DOC). Otherwise, ESBC presents a clear deficiency of nitrogen relative to PM. The nitrogen content of the agricultural wastes consists mainly of organic nitrogen. However, the chief nitrogen content in livestock wastes was N-NH4, which is important to maintain the buffering capacity of the system. The ammonia nitrogen concentration was found to be less than the inhibitory concentration.32,37 Effect of Co-digestion on the Anaerobic Process Performance Physico-chemical Parameters pH, VFA, and Alkalinity. During the initial days of operation, the hydrolytic and acidogenic stages occurred, and all reactors showed a decreasing trend for pH. Therefore, neutralization with 8 M NaOH was necessary once a day during the first days of the assay to avoid an irreversible destabilization of the process. However, the neutralization requirements of reactors can be different, depending on the mixtures tested. Thus, stable pH values were recuperated quickly (third day of the assay) for mixtures of 0:100 and 32:68. However, 10 days were required

Figure 1. Schematic diagram of the batch reactor used in the experiment: (1) reactor vessel, (2) heating plate, (3) temperature and mixing device, (4) biogas Tedlar bag, (5) pH probe, (6) motor agitation, (7) stirring blade, and (8) reactor tap. Figure 1 shows a schematic diagram of the reactor in which the mentioned elements are shown. Analytical Methods. Biogas was collected in a 10 L Tedlar gas bag (SKC made in UK), and its volume was measured daily using a highprecision drum-type gas meter (Ritter TG/5). The gas composition was determined by using a gas chromatograph (Shimadzu GC-2014) with a stainless steel column packed with Carbosieve SII (diameter of 3.2 mm and 3.0 m length; part no. 201-48386) and a thermal conductivity detector (TCD). The injected sample volume was 1 mL, and the operational conditions were as follows: 7 min at 55 °C; ramped at 27 °C/min until 150 °C; detector temperature, 255 °C; injector temperature, 100 °C. Helium was used as carrier gas with a flow rate of 30 mL/min. The method detection limits for H2, CO2, N2, O2, and CH4 were 3.1%, 3.4%, 1.5%, 3.2%, and 3.9%, respectively.

Figure 2. Evolution of TVFA in the reactors (as mg AcH/L). C

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Figure 3. Comparison of detailed VFA (acetic, propionic, and butyric acids) evolutions in the reactors.

to achieve a stable pH ∼7.5 for 48:52 mixture reactors. Finally, stable pH values were not reached until the day 15 (pH ∼8) for 72:28 and 100:0 mixture reactors. Figure 2 shows the evolution of total acidity (the total amount of the VFA expressed as acetic acid), calculated as the weighted average, based on the molecular weights of the volatile fatty acid concentrations from C2 to C7, and Figure 3 depicts the evolution of the main individual VFA. As can be seen from the profile of the TVFA in Figure 2, the maximum of total acidity reached in each test increased with the ESBC percentage in the mixture, which is reasonable because the organic matter concentration is higher (more carbohydrates from ESBC).24,28−30 However, the increases in maxima of TVFA are not directly related to the organic matter concentration since the maxima of TVFA for the mixtures 48:52, 72:28, and 100:0 are very similar, whereas maxima of TVFA for the mixtures 0:100 and 32:68 are very much lower. In addition, the time to reach maximum TVFA concentrations follows the same trend. For reactors with ESBC:PM mixtures of 0:100 and 32:68, the maxima of TVFA were reached quickly

(around the third day of the test), while for reactors with mixtures of 48:52, 72:28, and 100:0, the maxima of TVFA were reached later. Biogas productions are in accordance with these trends for TVFA, as shown can be seen in Figure 6 (below). In short, for 0:100 and 32:68 mixtures, the maxima of TVFA are related to the organic matter content in the tests. In this case, the linking between the different stages of the anaerobic process is reached quickly, and simultaneous production/ degradation of VFAs occurs. However, the increase of the ESBC content in the mixtures leads to decoupling of acidogenic and methanogenic stages, and VFAs are only generated until the maximum admissible concentration is reached in the medium.Thus, as illustrated in Figure 2, the three mixtures with a higher ESBC content reach a similar maximum value of TVFA despite their different organic matter content. Later, when methanogenic microorganisms adapted to this high TVFA concentration, the VFA degradation started although the removal rate depends on the ratio of waste in the mixtures. D

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Figure 4. Evolution of the acidity/alkalinity ratio in the reactors.

Figure 5. DOC evolution in the reactors.

caused a decrease in methanogenic activity during the digester start-up period.5 On the other hand, an interesting aspect is the evolution of the propionic acid concentration in the different reactors. It is not usual, in a stable process, that propionic acid is higher than butyric acid. This situationin most of the cases and for different substratesis related to an inhibition process leading to further problems in removing the accumulated VFA. In the literature, it has been reported than propionic acid removal is very difficult, and the activity of specific microorganisms is required.40 In this study, successful removal of propionic acid has been attained in all reactors but more slowly for reactors with higher ESBC proportions. Degradation of propionic acid has been possible in this work by using an adapted inoculum coming from a semicontinuous reactor for the treatment of ESBC. Nevertheless, it was also observed that propionic acid could be assimilated only when acetic acid has been completely removed from the system. Tian et al. also found the same behavior studying AD of sugar beet tailing.26 Furthermore, ESBC has a low alkalinity relavtive to PM, so the alkalinity of the 100:0 mixture reactors was lower than for the rest of the reactors. However, for this test, alkalinity values

Acetic, propionic, and butyric acids were the main components in total acidity (C2−C7). It is noticeable than the trend for each of the VFA is the same than that observed previously for TVFA. From Figure 3, it can be deduced that the reactor with the ESBC:PM mixture of 0:100 has the lowest content for all the individual VFA relative to the rest of the reactors during the entire assay. On the other hand, as the content of ESBC was increased in the mixtures, a higher concentration of VFA was obtained (especially evident for propionic acid), and the VFA degradation was retarded. Therefore, the increase of ESBC in the mixtures can be related with a certain inhibition of the methanogenic stage.25,26 This aspect will be confirmed with the delays in the production of biogas. Panichnumsin et al. reported a similar trend for the treatment of cassava pulp with PM. The authors indicated that high ratios of cassava pulp produce VFA accumulation and pH reduction, affecting the removal efficacy for VS and COD.39 Kafle and Kim have also observed the same behavior for the co-digestion of apple waste with swine manure. In that case, a high content of apple waste quickly produced acidification of the reactor, and it E

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Table 2. Summary of Performance Parameters for the Co-digestion of Dried Pellets of ESBC and PM under Mesophilic Conditions ESBC:PM mixture

C/N ratio

total CH4 yield (mL/kg VSconsumed)

0:100 32:68 48:52 72:28 100:0

14 18.5 23.5 28.5 33.5

521.2 677.8 ± 8.7 603.9 ± 6.5 363.2 ± 16.7 334.6 ± 64.3

total CH4 yield (mL/kg VSadded) 494 482 282 295

468 ± 19.3 ± 13.4 ± 6.42 ± 37.1

VS removal (%)

lag-phase (days)

biogas quality (% CH4)

54.5 75.2 ± 3.2 81.6 ± 1.4 73.3 ± 6.2 74 ± 2.3

10 19 32 37

8 ± ± ± ±

86.2 72.6 ± 3.5 71.9 ± 2.8 73.9 ± 4.2 69.2 ± 3.8

1.4 2.1 0.1 0.1

Figure 6. Cumulative specific methane production expressed as mL CH4/g VSconsumed.

Evolution of the Organic Matter and Methane Production. The evolution of solubilized organic matter in terms of DOC is shown in Figure 5. The main results obtained in this study regarding the organic matter removal are summarized in Table 2. From Figure 5, it can be observed that the hydrolysis phase for the 0:100 and 32:68 mixture reactors is quickly coupled with the acidogenic and methanogenic phases. In fact, the increase in the organic matter in the system is very low and restricted to only the first days of the test for the 32:68 mixture reactors. Moreover, biogas production started promptly with a short adaptation phase, as illustrated in Figure 6. This coupling between the generation and the consumption of the organic matter indicates that equilibrium between the different phases of the AD process was reached, and the complete conversion of hydrolyzed material to the final products was achieved by reducing the lag-phase period. As it was commented for the evolution of TVFA, for the rest of the reactors the process is unbalanced, and it can be observed that the duration of hydrolysis and acidogenic stages increases proportionally to ESBC proportion in the mixtures. Thus, as the ESBC content increased in the mixtures, more time was required to degrade the organic matter solubilized as a consequence of the appearance of an adaptation phase for methanogenic microorganisms. This fact could be related to distortions in the required microbiological equilibrium in anaerobic processes due to the accumulation of intermediate products, VFA mainly, generated from ESBC.42 The above-mentioned trends for the different reactors can be analyzed by comparison of the reactors with each substrate individually at the 0:100 and 100:0 mixtures, since clearly different behaviors with respect to the organic matter removal

were maintained at 20−40 g CaCO3/L throughout the process, which was suitable for AD.41 For the rest of the reactors, the system alkalinity was higher. In fact, from day 18 on, all reactors had reached high and stable alkalinity values in the range of 80−100 g CaCO3/L. One of the main criteria for checking the anaerobic reactors’ stability is the acidity/alkalinity ratio. In the literature, three different ranges have been defined for this parameter. If acidity/ alkalinity ratio is lower than 0.4, the process will operate properly, whereas when this ratio is between 0.4 and 0.8, the digester is affected by a problem that can lead to destabilization and malfunctioning. Finally, when this ratio exceeds the value of 0.8, the process fails because of the acidification of the digester and the inhibition of methanogenic activity.42 Acidity/alkalinity ratios obtained in this study are shown in Figure 4. The acidity/alkalinity ratios for 0:100 and 32:68 mixture reactors were lower than 0.4 during the entire process. For the 48:52 mixture reactors, the ratio was higher than 0.4 during the first 11 days of the assay, reaching about 0.7, but soon declined and remained within the optimum range, while for the 72:28 mixture reactors, the ratio was higher than 0.8 during the first days of the test, though it later dropped suddenly and remained below 0.4. Finally, for the 100:0 mixture reactors, the acidity/alkalinity ratio was above the limit value for failure, exceeding 0.8. Later, during a large period of the assay, the ratio was within the range of 0.4−0.7, and finally stabilized at 0.4 around day 43. From the analysis of both the process alkalinity and the acidity/alkalinity ratio, it can be concluded that co-digestion of ESBC and PM has beneficial effects on the process stability and the anaerobic biodegradability of ESBC. F

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Table 3. Comparison of Methane Production Obtained from Sugar Beet Byproducts and Similar Substrates from Different Studies ref 24

reactor type/operation

21

batch continuous (OLR, 6.75 g VS/L·d) batch

44 22

batch batch

10

continuous (HRT, 20 d; OLR, 3.9 g TS/L·d) co-substrate (HRT, 20 d; OLR, 2 kg VS/m3·d)

26 39

batch co-substrate (HRT, 15 d; OLR, 3.5 kg VS/m3·d)

20

continuous (HRT, 68.5 d; OLR, 4 g VS/L·d) continuous (HRT, 54.8 d; OLR, 5 g VS/L·d) − batch

45 present study

substrates sugar beet wet pulp sugar beet wet pulp, cow manure sugar beet wastewater, sugar beet wet pulp sugar beet wet pulp spent sugar beet pulp sugar beet (tops and roots fractions) dairy manure sugar beet tops, cow manure sugar beet tailing Cassava pulp(50−60% starch), pig manure sugar beet pulp sugar beet byproducts ESBC ESBC, PM

and the methane production can be observed. Thus, 100:0 mixture reactor was characterized by a slower degradation of organic matter than the 0:100 mixture reactor. This behavior can be related to the higher content in non-easily degradable material (specially, lignocellulosic material) in ESBC. 16 However, for the 0:100 mixture reactor a perfect synchronization between production and consumption of organic matter was obtained, and the soluble organic matter concentration remained practically constant. The organic matter removal percentages obtained in the different reactors have shown that the organic matter removal for the 0:100 mixture reactor is lower than for the other reactors, which can be related with the low organic matter content of PM. Thus, the VS removal (which refers to the total organic matter in the reactors) was around 55% for the reactor with 100% of PM, while it remained in the range of 73−82% for the rest of the reactors, with the highest value for the 48:52 mixture reactors. Furthermore, for the DOC removal (which represents the reduction of soluble organic matter in the medium), no significant differences were observed except for the 0:100 mixture reactor which was also lower and around 43%. Comparison among the four reactors containing ESBC should take into account the time necessary to reach a determined organic matter removal percentage, since no significant differences can be established for total reduction of organic matter. In fact, to reach the organic matter removal percentage of 70−80%, the times required were about 23, 34, 45, and 56 days for the reactors with mixtures of 32:68, 48:52, 72:28, and 100:0, respectively. From these data it is evident than the increase in the proportion of ESBC in the mixture (increase of C/N ratio) implies higher times to attain the required removal percentage. The evolution of cumulative methane yields based on the VS consumed during the co-digestion of ESBC and PM under mesophilic conditions is shown in Figure 6. The reactors were operated until no significant biogas production was detected for several consecutive days. As shown in Figure 6, SMPs for the co-digestion reactors with the 32:68 and 48:52 mixtures were

temperature range

results/best conditions

thermophilic thermophilic mesophilic

240 mL CH4/g VSadded 280 mL CH4/g VSadded (ratio 1:1) 311.9 mL CH4/g CODadded

mesophilic thermophilic thermophilic

296.4 mL CH4/g CODadded 336 mL CH4/g VSadded 10 L CH4/Ldigester

mesophilic thermophilic mesophilic mesophilic thermophilic − mesophilic

24.6 L biogas/d (40% beet tops) 229 L CH4/kg VSadded (up to 40% manure) 1.3 m3/m3·d 306 mL CH4/g VSadded (40% CP) 292 mL CH4/g VS·d 355 mL CH4/g VS·d 236−381 m3biogas/ton of substrate 295 mL CH4/g VSadded 494 mL CH4/g VSadded (18.5 C/N ratio, 32% ESBC)

higher than for the digestion of the wastes separately or with higher ESBC content. As can be observed in Table 2, the total methane production in mL CH4/g VSconsumed for each reactor follows this sequence: 32:68 mixture > 48:52 mixture > 0:100 mixture ≫ 72:28 mixture > 100:0 mixture. Therefore, for the two reactors with higher ESBC content, the productivities of methane were lower than for the rest of the reactors, indicating the low biodegradability of ESBC waste. Besides, the highest methane production has been obtained for co-digestion reactors. This fact supports the hypothesis that a mixture of both wastes has a synergic effect on AD. Figure 6 shows that the starting-up period for the methanogenesis increases with the ESBC proportion in the mixtures. Thus, from these data, it can be concluded that the 32:68 mixture reactor, which corresponds to a C/N ratio of 18.5, was the most convenient since it combines a fast startingup of the process and the highest cumulative SMP. As shown in Table 2, the highest methane percentage in biogas was reached for the PM reactor (i.e., the mixture 0:100). In contrast, the ESBC reactor (i.e., the mixture 100:0) showed the lowest methane content in biogas. This fact can be related with the high alkalinity of PM, in contrast to ESBC. The mixtures of both substrates presented similar methane percentages, likely due to the alkalinity provided by PM. Several references about specific methane production by AD of sugar beet byproducts or similar wastes are shown in Table 3. Typically the SMP for the AD of sugar beet byproducts without co-substrate is 240−355 mL CH4/g SVadded or 296.4 mL CH4/g CODadded. The addition of some co-substrate, like cow or dairy manure and sugar beet wastewater, in different proportions improves the SMP by about 12%. No works on anaerobic codigestion of dried pulp sugar beet byproducts (ESBC) with PM were found. As can be seen in Table 3, in the present study, the reactors containing ESBC alone (C/N ratio of 33.5) show SMP similar to that reported in the literature (295 ± 5.2 mL CH4/g VSadded). G

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Energy & Fuels Moreover, an improvement of 67% in methane production (expressed as mL CH4/g VSadded) was obtained in the present study by means of co-digestion of ESBC and PM at 32:68 mixture, which corresponds to 18.5 C/N ratio. Alkaya and Demirer-Göksel have studied the anaerobic mesophilic codigestion of sugar beet wastewater and beet pulp in batch reactors and the digestion of the wastes separately, as shown in Table 3. Those authors found that beet pulp (wet pulp) was more difficult to degrade relative to wastewater in terms of methane yield and COD removal, and they mentioned that this result is related with the lignocellulosic composition of beetpulp, which causes difficulties for degradation when compared to wastewater.21 Koppar and Pullammanappallil44 and Umetsu et al.22 have also reported that a high addition of sugar beet byproducts to dairy manure can stop the biogas production and inhibit the process. Panichnumsin et al. have investigated the co-digestion of cassava pulp with different concentrations of PM. They found that an excess of cassava pulp relative to PM had a negative effect on methane production and process stability.39 The results discussed above show that ESBC was co-digested successfully with PM, and the best mixture tested was 32:68, leading to higher productions than obtained in the literature for similar wastes.

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REFERENCES

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CONCLUSIONS The following conclusions can be put forward on the basis of the findings of this study: • ESBC and PM were co-digested successfully in mesophilic anaerobic conditions. • Degradation of ESBC was more difficult than that of PM. However, co-digestion had a favorable effect on process performance, offering a high buffering capacity in the medium compared with the digestion of EBSC individually. • The ESBC:PM mixture ratio of 32:68 was the best codigestion mixture studied because the lag-phase for methane production was reduced 3 times relative to the 100% of ESBC mixture reactors. Moreover, the highest methane yield of 677.8 ± 8.7 mL of CH4/g VSconsumed was achieved in this case.

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ESBC = exhausted sugar beet cossette PM = pig manure C/N ratio = carbon/nitrogen ratio VFA = volatile fatty acid TVFA = total volatile fatty acidity acidity/alkalinity ratio = the ratio of total volatile fatty acidity to the alkalinity in the medium TS = total solids VS = volatile solids sCOD = soluble chemical oxygen demand DOC = dissolved organic carbon TKN = total Kjeldahl nitrogen N-NH4 = ammonia nitrogen SMP = specific methane production HRT = hydraulic retention time PID = proportional integral derivative nd = not determined

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +34956016474. Fax: +34956016411. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Spanish Ministry of Science and Innovation (PROBIOGAS Project PS-120000-2007-6 and UNCA08-1E-035 Project) and co-funded by the Spanish Ministry of Economy and Competitiveness (CTM201343938-R) and the European Regional Development Fund (ERDF). The authors acknowledge the University of Cadiz (Spain) for the Scholarship UCA-2010-063PU/EPIF-FPI-A/ BC and the Agrifood Campus of International Excellence (Ceia3).

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NOMENCLATURE AD = anaerobic digestion H

DOI: 10.1021/ef502502a Energy Fuels XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/ef502502a Energy Fuels XXXX, XXX, XXX−XXX