Improved Anaerobic Digestion by the Addition of Paper Tube Residuals

Article pubs.acs.org/EF

Improved Anaerobic Digestion by the Addition of Paper Tube Residuals: Pretreatment, Stabilizing, and Synergetic Effects Anna Teghammar,*,†,‡ Maria del Pilar Castillo,§ Johnny Ascue,§ Claes Niklasson,‡ and Ilona Sárvári Horváth† †

School of Engineering, University of Borås, Allégatan 1, 50190 Borås, Sweden Department of Chemical and Biological Engineering, Chalmers University of Technology, 41296 Göteborg, Sweden § Swedish Institute of Agricultural and Environmental Engineering (JTI), 750 07 Uppsala, Sweden ‡

ABSTRACT: This study deals with the addition of paper tube residuals to a nitrogen-rich mixture of organic waste obtained from industrial and municipal activities. This nitrogen-rich mixture, called buffer tank substrate (BTS) in the following text, is used in a large-scale biogas plant. The effects were investigated in semi-continuous co-digestion processes, and variations in operational conditions were studied. The addition of paper tubes had stabilizing effects, prevented the failure of the process, and made it possible to decrease the hydraulic retention time from 25 to 20 days. Furthermore, synergetic effects were found, with 15−34% higher methane yields, when paper tubes were co-digested with BTS. Moreover, steam explosion pretreatment of the paper tube waste with the addition of 0−2% NaOH was evaluated by batch digestion experiments. Increasing the NaOH concentrations used in the pretreatment resulted in increasing methane yields, with the highest of 403 N mL of CH4 g−1 of volatile solids (VS) corresponding to an increase by 50% compared to that when untreated paper was digested (268 N mL of CH4 g−1 of VS). The long-term effects of this best pretreatment were further investigated by continuous co-digestion experiments, leading to a higher methane yield when pretreated paper tubes were used in the co-digestion process compared to untreated.

1. INTRODUCTION The European Commission has set the goal that, by 2020, 20% of the energy consumed should come from renewable energy sources, as well as 10% of the energy consumed within the transport sector.1 To be able to reach this goal, especially in the transport sector, the production and use of biogas provides a good alternative. New substrates, however, are needed to be able to produce larger amounts of biogas than what is possible today. Lignocellulosic-rich waste materials have a high potential for biogas production. They are produced in abundant amounts worldwide, in the form of agricultural waste streams, such as straws, stalks, and manure, as well as wood and paper. These materials offer the additional advantage of not competing with land usage for food and feed cultivation compared to energy crops. A significant drawback with lignocellulosic materials, however, is that they are hard for the microorganisms in the anaerobic digester to digest.2 The three main biopolymers in lignocellulosic-rich materials are cellulose, hemicellulose, and lignin, which are present in a complex and cross-linked structure.2,3 To make these materials degradable by the bacteria, different pretreatments are often needed. Pretreatments can be carried out by mechanical, physical, chemical, and/or biological methods,3−5 which are able to open up the structure of the lignocellulose, improve the accessibility of the enzymes to the material, and sometimes combine with a degradation of the indigestive lignin.3 One of the most promising pretreatment methods is steam explosion. This method uses a combination of high-pressure steam (15−20 bar) and high temperature (up to 250 °C) followed by a © 2012 American Chemical Society

pressure-drop explosion. Steam explosion pretreatment has previously been studied on lignocelluloses prior to anaerobic digestion on paper tube residuals,6 wheat straw,7 biofibers from digested manure,8 salix,9,10 oat straw,11 and bulrush.12 Studies on semi-continuous co-digestion assays have previously been performed on manure, sewage sludge, and/or industrial food waste.13−21 However, only a few studies have been found on the use of lignocelluloses in co-digestion processes. One study was found on dry continuous anaerobic co-digestion, where paper waste was added to food and livestock waste.22 In another study, wheat straw hydrolysate from an ethanol pilot plant with hydrothermal pretreatment was co-digested with cow and pig manure in continuous stirredtank reactors (CSTRs).23 Powdered rice and wheat straw added to cattle dung in semi-continuous reactors resulted in increased methane production in a study by ref 24. However, no studies have been found on continuous co-digestion on a lignocellulosic-rich material, which has been pretreated to improve the biogas production, according to the authors’ knowledge. The addition of lignocellulosic-rich materials to anaerobic codigestion processes can have additional positive effects, leading to better nutritional balance. Feedstocks that are rich in proteins can lead to ammonia inhibition when degraded, if care is not taken.25 In Sweden, slaugtherhouse waste or other protein-rich substrates are often added together with municipal solid waste into the anaerobic digester, which can lead to high protein levels.26 Ammonia inhibition can lead to the Received: October 4, 2012 Revised: December 11, 2012 Published: December 11, 2012 277

dx.doi.org/10.1021/ef301633x | Energy Fuels 2013, 27, 277−284

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accumulation of volatile fatty acids (VFA), with a possible failure of the digestion process as a consequence.25 The carbon/nitrogen (C/N) ratio should ideally be something between 10:1 and 30:1 for the digester to work at its full potential. 27 The addition of carbon-rich materials, as lignocelluloses, to the anaerobic digestion process with nitrogen-rich feedstocks will result in an increased C/N ratio, which would stabilize the process.13,25 The specific objectives of this study were to investigate the effect of using paper tube residuals as a co-substrate in the continuous digestion of a nitrogen-rich substrate. Pretreated and untreated paper tubes were used, and the biochemical methane potential of both pretreated and untreated paper was also determined. The addition of untreated paper tubes was studied at two different operational sets, while the treated paper was studied at one operational set.

Table 1. Feedstock Streams in BTS1 and BTS2 substrate

BTS1 (%)

BTS2 (%)

OFMSWa industrial biosludgeb industrial wastewater sludge slaughterhouse waste industrial food waste citrus wastes fish sludge total

35.2 28.7 2.9 13.0 4.0

47.6 30.0 4.8 6.1 6.4 5.1

16.2 100

100

a

OFMSW = organic fraction of municipal solid waste. bSludge from a slaughterhouse wastewater treatment plant.

amount of substrate corresponded to the VS content mentioned above. Finally, water was added to each flask to reach a final total volume of 600 mL. The headspace of each flask was flushed with a mixture of 80% nitrogen and 20% carbon dioxide. The cultures were run in triplicate for each substrate, with three control flasks detecting the gas production of only inoculum running in parallel. Gas analysis of methane and carbon dioxide was performed according to a previous method by ref 28. Gas samples of 0.25 mL were withdrawn regularly from the headspace, using a pressure-tight syringe (VICI, Precision Sampling, Inc., Stockton, CA) to analyze the composition of the gas. The composition of the gas was then analyzed by a gas chromatograph (Auto System Perkin-Elmer, Waltham, MA) equipped with a packed column (6 ft × 1.8 in. outer diameter, 80−100 mesh, Perkin-Elmer, Waltham, MA). The injection temperature was 150 °C, and the carrier gas was nitrogen, operated with a flow rate of 20 mL/min at 60 °C. To avoid overpressure in the glass flasks, excess gas was released through a needle after the gas analysis, and gas analysis was performed again. All methane volumes are presented at normal conditions (1 atm and 0 °C). One-way analysis of variation (ANOVA) was used to quantify the treatment impact of NaOH on methane production. The ANOVA calculation was performed with the software package Minitab. 2.5. Continuous Co-digestion Assay. The continuous experiments were performed in 10 L CSTRs, with a culture volume of 5 L. The assays were performed in three different operational sets. In the first set, three reactors were run in parallel. The first reactor was used as a control using BTS1 as the only substrate. Untreated paper tubes (UP) were added together with BTS1 in the second reactor, while pretreated paper tubes (PP) were added together with BTS1 in reactor 3. The pretreatment of the paper tubes was selected as the treatment with the highest methane potential according to the batch experiment. Inoculum 1 was used in this first set of experiments. All three reactors were run with an organic loading rate (ORL) of 1.3 g of VS L−1 day−1 from BTS1, with the addition of 0.4 g of VS L−1 day−1 from paper tubes in reactors 2 and 3, corresponding to a VSBTS/VSpaper (g/g) ratio of 3:1, and all three reactors were run at a hydraulic retention time (HRT) of 25 days (Table 3). The reactors were run for a period of three HRTs after a stable process was reached. In the second and third operation sets of continuous experiments, two reactors were run in parallel, with the first reactor with BTS2 only and the second reactor with BTS2 + UP. In the second operational set, the reactors were run with an OLR of 1.5 g of VS L−1 day−1 from BTS2, with the addition of 0.5 g of VS L−1 day−1 from untreated paper tubes in reactor 2, corresponding to the VSBTS/VSpaper (g/g) ratio of 3:1, and both reactors were run at a HRT of 25 days. After a stable process was reached, and the reactors acted stationary, the reactors were run for an additional period of three HRTs. This was followed by a third operational set on the same reactors but with a change of the HRT to 20 days and an OLR of 1.3 g of VS L−1 day−1 from BTS2 with the addition of 0.7 g of VS L−1 day−1 from untreated paper tubes in reactor 2, corresponding to the VSBTS/VSpaper (g/g) ratio of 2:1 (Table 3). The reactors were operated under these conditions again for a period of three HRTs. All reactors were fed daily, and the incubation temperature was 55 °C. The gas composition was analyzed by gas chromatography (PerkinElmer ARNEL, Clarus 500) equipped with a

2. EXPERIMENTAL SECTION 2.1. Materials. The paper tube residuals used as a substrate in this study were the discarded material obtained from Nordens Pappersindustri AB (Borås, Sweden). These residuals consisted of a mixture of 10% raw paper tubes without glue, 35% tubes with the glue sodium silicate, and 55% tubes with the glue polyvinyl alcohol. This mixture was used for all pretreatments and digestions. The paper tubes were milled to 1−2 mm particle size with a knife-milling Warring commercial blender. The composition of the mixture in dried form was 53% cellulose, 10% hemicellulose, 23% lignin, and 10% glue (mixture of sodium silicate and polyvinyl alcohol). The total solids (TS) content was 95%, and the volatile solids (VS) content was 85%. All compositional analyses were performed as reported previously in ref 6, except the analysis of total C (43%) and total N (0.13%), which was performed by SP Technical Research Institute of Sweden. The elementary analysis resulted in a C/N ratio of 330:1. 2.2. Pretreatment Conditions. The paper tubes were subjected to thermochemical steam explosive pretreatments in an explosive pilotscale reactor (Process och Industriteknik AB, Sweden), with a reaction chamber of 10 L and an expansion tank for the explosive flushing. The pretreatments were performed at 15−20 bar and 190 °C for 10 min and with the addition of 0−2% sodium hydroxide. The reactor was heated with 60 bar steam provided by a power plant (Borås Energi och Miljö AB, Borås, Sweden). The substrate was mixed with water to achieve a concentration of 10% TS and an appropriate amount of added sodium hydroxide prior to the pretreatment. During the pretreatment, the paper tube solution was diluted by the condensed steam to a concentration of about 5% TS. 2.3. Inoculum and Buffer Tank Substrate. The inoculums and the buffer tank substrate (BTS) used were obtained from the largescale biogas plant at Borås Energi och Miljö AB (BEMAB), Borås, Sweden. The inoculums and BTS were collected on two different occasions, 5 months apart (inoculums 1 and 2 and BTS1 and BTS2). The composition of different feedstock streams in BTS1 and BTS2 is presented in Table 1, and the characteristics of the inoculums 1 and 2 and BTS1 and BTS2 can be found in Table 2. The BTS2 substrate was selected at a time when the process was unstable and, thus, faced operating problems because of the characteristics of the feedstock streams. 2.4. Batch Anaerobic Digestion Assay. Batch anaerobic digestion assays were performed to evaluate the effect of the sodium hydroxide concentration in steam explosion pretreatment on the methane yield of paper tube samples. The assays were performed at thermophilic conditions (55 °C), and the experiment was run for 50 days. The cultures were prepared in serum glass bottles with a working volume of 2.1 L and closed with butyl rubber seals and aluminum caps. Before digestion, the pretreated and untreated samples were neutralized to pH 7 with either NH4OH or H3PO4. Each flask contained a VS ratio of 2:1 inoculum/substrate, with a total of 3 g of VS from each substrate. The volume of the added inoculum was 260 mL; the ammonium phosphate buffer was 200 mL (50 mM); and the 278


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Table 2. Characteristics of the Two Different Inoculums and Buffer Tank Substrates Used substrate inoculum 1 inoculum 2 BTS1 BTS2 a

TS (%) 2.16 2.05 20.5a 26.0a

a

VS (%) 1.43 1.37 18.1a 23.4a

a

pH a

7.83 7.77a 5.42c 4.41c

VFA (g/L) a

3.55 3.72a 15.4c 8.0c

NH4+ (g/L)

Ntot (g/L)

fats (g/L)

proteins (g/L)

carbohydrates (g/L)

C/N

COD (g/L)

2.54a 2.52a 2.62a 1.76a

nab nab 12.5a 15.7a

nab nab 76.3a 78.4a

nab nab 67.3a 78.2a

nab nab 59.8a 92.9a

nab nab 6.20a 5.98a

nab nab 225a 288a

Data from Borås Energi och Miljö AB. bna = not available. cData from SP Technical Research Institute of Sweden.

packed column (7 ft HayeSep N 60/80, 1/8 in. SF), a flame ionization detector (FID) at 250 °C, helium as a carrier gas (31 mL/min), and the injection temperature at 60 °C. All of the injections were performed with a headspace sampler TurboMatrix 110. Gas samples were taken every 24 h. All methane volumes are presented at normal conditions (1 atm and 0 °C). 2.6. Analytical Methods. The TS and VS were measured according to the standard method SS 028113.29 The ammonia concentration was measured according to ISO 1173230 and SIS 028134.31 The organic nitrogen and total nitrogen were measured according to SS-ISO 13878.32 Total carbon was analyzed according to SS-ISO 10694,33 and the pH was analyzed according to SS 028122.34 The concentrations of VFA were analyzed by high-performance liquid chromatography (HPLC, Agilent 1100 system, Agilent, Sweden) equipped with a refractive index detector. An ion-exchange column (Rezex ROA organic acid H+) was used for the separation operating at 60 °C, and 5 mM H2SO4 with a flow rate of 0.6 mL/min was used as an eluent.

digested exclusively; in reactor 2, untreated paper tubes (UP) were co-digested with BTS1; and in reactor 3, pretreated paper tubes (PP) were digested with BTS1. The pretreatment of the paper tubes were selected according to the best methane yield obtained in the batch digestion assay (steam explosion at 190 °C for 10 min with 2% NaOH; Table 4). The pretreatment of the paper in co-digestion with BTS1 showed a higher methane potential (521 N mL of CH4 g−1 of VS) compared to that when untreated papers were digested together with BTS1 (498 N mL of CH4 g−1 of VS; Figure 1 and Table 5). The BTS1 alone produced 470 N mL of CH4 g−1 of VS. This means a total increase of 6 or 11% of the methane yields when co-digesting BTS1 with UP or PP, respectively, compared to that obtained during the digestion of BTS1 alone. The concentration of methane in the biogas was 70% in reactors 1 and 2 and 69% in reactor 3, while the remaining part was carbon dioxide. The C/ N ratio of the digested mixture changed from 6.20 to 8.46 after the addition of paper, for both untreated and pretreated paper. The characteristics of effluents, obtained after 3 HRTs, are shown in Table 6. The NH4+ level and the level of total VFA were lower in the reactors where BTS1 was co-digested with untreated or pretreated paper tubes, while the pH was stable at around 8 for all three reactors. Besides the higher methane yields followed by the codigestion of paper tubes and BTS, synergetic effects could be found (Table 5). The estimated theoretical methane yield (ETMY) for BTS1 + UP was calculated as the addition of the methane yield per BTS1 only (470 N mL of CH4 g−1 of VS; Table 5), and the methane yield of untreated paper tubes was available from the batch assay (268 N mL of CH4 g−1 of VS; Table 4). According to the VS ratio of 3:1, the expected ETMY value for BTS1 + UP was calculated as being 420 N mL of CH4 g−1 of VS (Table 5). The obtained value of BTS1 + UP was 498 N mL of CH4 g−1 of VS, and in comparison to the ETMY value, it is an increase of 19%. This synergy effect could also be seen for BTS1 + PP. The theoretical value of BTS1 + PP was calculated as 453 N mL of CH4 g−1 of VS, and because the obtained value of BTS1 + PP reached 521 N mL of CH4 g−1 of VS, there is an increase of 15% (Table 5). Consequently, synergy effects could be detected, because the co-digestion of different substrate streams resulted in elevated methane yields compared to those from the single streams. Furthermore, the effects of the pretreatment were evaluated by a comparison between the addition of untreated and pretreated paper tubes in co-digestion with BTS1. BTS1 + PP produced 521 N mL of CH4 g−1 of VS compared to 498 N mL of CH4 g−1 of VS for BTS1 + UP, corresponding to an increase in the total methane yield of 5%. The energy recovery calculated as the heating value of the produced methane35 divided by the heating value of the substrate36,37 is presented in Table 5. The energy balance of 61% for the digestion of BTS1 only is improved to 70 and 73% during the co-digestion process of BTS1 with UP and PP, respectively.

3. RESULTS Pretreatments with steam explosion were performed on the paper tubes and evaluated by batch assays measuring the biomethane potential. Pretreated samples showing the best performance in batch experiments were used in the continuous co-digestion experiments and were co-digested with a nitrogenrich feedstock (BTS1). Furthermore, the untreated paper tube residuals were also co-digested with an unstable nitrogen-rich substrate (BTS2) to study possible stabilizing effects. 3.1. Biomethane Potential of Untreated and Treated Paper Tubes: Batch Assays. Steam explosion pretreatments with four different setups with the addition of 0−2% NaOH at 190 °C for 10 min were performed on the paper tubes, and the effects were evaluated by batch digestion assays (Table 4). Steam-exploded paper tubes with 0% NaOH resulted in about the same methane production as untreated paper tubes (about 270 N mL of CH4 g−1 of VS compared to 250 N mL of CH4 g−1 of VS), while 2% NaOH gave the highest methane output of approximately 400 N mL of CH4 g−1 of VS, improving the methane potential by 50%. Samples treated with lower concentrations of NaOH gave higher yields of methane compared to that of untreated paper but less than the sample that was treated with 2% NaOH. Moreover, not only was the accumulated methane production increased, but the initial methane production rate was also enhanced as a result of the pretreatment. The digestion rate increased with an increasing NaOH concentration, from 25 N mL of CH4 g−1 of VS day−1 for untreated paper tubes to 41 N mL of CH4 g−1 of VS day−1 when 2% NaOH was added, which is an increment of 64% (Table 4). One-way ANOVA analysis showed that the concentration of NaOH is a significant factor (p value of 0.048, with a 5% significance level), affecting the accumulated methane yield. 3.2. Addition of Pretreated Paper Tubes in Continuous Co-digestion. Semi-continuous co-digestion assays were performed with three reactors in parallel to evaluate the longterm effects of the pretreatment. In reactor 1, BTS1 was 279


0.7

yield (N mL g−1 of VS)

SDb

digestion ratec (N mL g−1 of VS day−1)

untreated 0% NaOH 0.5% NaOH 1% NaOH 2% NaOH

268 251 295 352 403

±48 ±70 ±57 ±66 ±20

25 22 26 32 41

VS ratio = VSBTS/VSpaper (g/g). bBTS, buffer tank substrate; UP, untreated paper; PP, pretreated paper.

0.4 0.4 1.3 1.3 1.3 BTS UP + BTS PP + BTS 1 2 3

Figure 1. Accumulated methane production from the co-digestion of untreated paper tubes (UP) and pretreated paper tubes (PP) with BTS1 at the HRT of 25 days. The OLR from BTS1 was 1.3 g of VS L−1 day−1 in all three reactors with the addition of 0.4 g of VS L−1 day−1 UP or PP, corresponding to the VSBTS/VSpaper (g/g) ratio of 3:1, in reactors 2 and 3, respectively.

3.3. Paper Tubes as a Stabilizer. In the second and third operation sets, a different buffer tank substrate was used (BTS2) in the co-digestion with untreated paper tubes. The content of BTS2 can be found in Table 1. Figure 2 shows the methane production obtained in each reactor after a stable process was reached. In reactor 1, untreated paper tubes (UP) were added to BTS2, while in the control reactor, BTS2 was digested exclusively. Both reactors were operated at two different operation sets. At OLR of 2 g of VS L−1 day−1, HRT of 25 days, and a mixing ratio of 3:1 corresponding to VSBTS/VSpaper (g/g), the co-digestion of BTS2 and UP produced 444 N mL of CH4 g−1 of VS (operation set 2). The control reactor with only BTS2 resulted in 354 N mL of CH4 g−1 of VS at the same conditions. Consequently, a 25% higher methane yield (per gram of VS) could be achieved when co-digesting BTS2 with UP compared to that of the digestion of BTS2 alone. When the HRT was decreased to 20 days and the VSBTS/VSpaper (g/g) ratio was changed to 2:1, the methane production of BTS2 + UP increased to 482 N mL of CH4 g−1 of VS. However, the control reactor with only BTS2 collapsed after a period of two retention times, and no methane yield could be obtained (Figure 2). The methane content in the produced biogas was 70% in all reactors, except reactor 2 in operation set 3. Prior to collapse, this reactor produced 58% methane. The effluent characteristics of the reactors, after 3 HRT, can be found in Table 6. At the second operation set, the process in reactors 1 and 2 was stable and showed a similar pH of 7.9 and

a

0.5 1.5 1.5

PP (g of VS L−1 day−1) UP (g of VS L−1 day−1)

pretreatment conditions

Accumulated methane yield after 43 days of digestion. bOne standard deviation on the accumulated yield. cDigestion rate calculated from the first 4 days of digestion.

substrateb

BTS1 (g of VS L−1 day−1)

a

a

reactor

25 HRT, VS ratio of 3:1

methane production

1.3 1.3

UP (g of VS L−1 day−1)

Table 4. Accumulated Methane Production and Digestion Rate from Batch Anaerobic Digestion after Pretreatment with Different NaOH Concentrations

UP (g of VS L−1 day−1)


Article


20 HRT, VS ratioa of 2:1 25 HRT, VS ratio of 3:1

a a

operation set 1

Table 3. Operational Parameters in the Three Different Continuous Co-digestion Experiments

operation set 2

operation set 3

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Table 5. Synergy Effects of Co-digestion of BTS and Untreated Paper Tubes Expressed in Methane Yields Per Gram of VS operation seta

substrateb

obtained (N mL of CH4 g−1 of VStotal)

C/N ratio

ETMYc (N mL of CH4 g−1 of VS)

increased CH4 yield in co-digestion (%)

energy recoveryd (%)

1 1 1

BTS1 BTS1 + UP BTS1 + PP

470 498 521

6.20 8.46 8.46

420 453

19 15

61 70 73

a Operation set 1: HRT of 25, 1.3 g of VS L−1 day−1 from BTS1 and 0.4 g of VS L−1 day−1 from UP or PP respectively, and VS ratio of 3:1 VSBTS/ VSpaper (g/g). bUP, untreated paper; PP, pretreated paper; BTS, buffer tank substrate. cETMY = estimated theoretical methane yield. dEnergy recovery calculated as the heating value of produced methane divided by the heating value of the different substrates.35−37

Table 6. Effluent Characterization in the Continuous Operation for All Three Sets after 3 HRTsa operation setb 1 1 1 2 2 3 3

substrate BTS1 BTS1 BTS1 BTS2 BTS2 BTS2 BTS2

+ UP + PP + UP + UP

pH

TS (%)

8.0 8.0 8.1 7.9 7.9 8.01 6.98

1.3 2.4 2.3 2.0 1.7 2.7 2.8

VS (%)

NH4+ (g/L)

total VFA (g/L)

acet (g/L)

prop (g/L)

but (g/L)

3.65 2.47 3.25 1.92 4.49 1.81 5.98

1.08 0.97 1.02 1.29 2.30 1.24 1.98

2.42 1.48 2.17 0.34 0.97 0.44 2.25

0.10

1.5 1.2 2.0 2.0

3.1 2.5 2.1 1.92 1.92 1.68 1.66

ibut (g/L)

isov (g/L)

val (g/L)

0.054 0.019 0.061

Improved Anaerobic Digestion by the Addition of Paper Tube Residuals

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