Anaerobic digestion of cattle manure, corn silage and sugar beet pulp mixtures after thermal pretreatment and kinetic modeling study

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Anaerobic digestion of cattle manure, corn silage and sugar beet pulp mixtures after thermal pretreatment and kinetic modeling study ⁎

Halil Şenola, , Ünsal Açıkelb, Serkan Demirc, Volkan Odad a

Giresun University, Genetic and Bioengineering Department, Giresun, Turkey Cumhuriyet University, Department of Chemical Engineering, Sivas, Turkey c Giresun University, Industrial Engineering Department, Giresun, Turkey d Giresun University, Finance Program Department, Giresun, Turkey b

A R T I C LE I N FO

A B S T R A C T

Keywords: Biogas Cattle manure Corn silage Sugar beet pulp Thermal pretreatments Modified Gompertz model Modified Bertalanffy model

In this study, biogas production was investigated from cattle manure (CM), corn silage (CS) and sugar beet pulp (SBP) mixtures under mesophilic conditions. In anaerobic digestion (AD), CM, CS and SBP were mixed in different ratios and the optimum mixture ratio was determined as 2:1:1, w/w/w respectively. In this mixture, biogas production was 180.5 mL/g TS. After the optimum mixing ratios of CM, CS and SBP were determined, thermal pretreatments were applied to this mixture ratio. Thermal pretreatments were performed at 100, 120, 150 and 180 °C with 10, 20, 30, 60 and 120 min for each temperature. Considering biogas production after thermal pretreatment, the best thermal pretreatment time was determined as 60 min for all pretreatment temperatures. The highest biogas yield was 362.1 mL/g TS in the reactor which is pretreated at 180 °C for 60 min. After thermal pretreatment at 180 °C for 60 min, the SCOD value in the reactor increased by 124.6% compared to the control. This reactor produced 100.6% higher biogas production compared to the control. In addition, the solubilization of cellulose, hemicellulose and lignin in this reactor was 38.2%, 32.9% and 23.2%, respectively. Cumulative biogas production (CBP) fitted to modified Gompertz and modified Bertalanffy models.

1. Introduction With the rapid growth of countries during industrialization and population growth, energy demands increase day by day. The increase in energy demand leads to a reduction of fossil fuels, i.e. natural energy sources such as coal, petroleum coke, lignite and natural gas [1]. Excessive attention has been paid to the search for alternative energy owing to the emergence of environmental problems associated with fossil fuels and the depletion of fossil fuels [2,3]. One of the alternative energy sources is biogas energy. Biogas produced by anaerobic digestion (AD) of by-products of livestock and crop attracts worldwide attention in terms of being clean and renewable energy [2,4] Biogas is currently used for vehicle fuel, heat and electricity production [5]. CS is an energy plant which is formed by the corn plant after being matured and stored in an airless medium [6]. AD of CS and animal manures have been emphasized with good results [7]. SBP is a byproduct of sugar production. 170 kg of wet sugar beet pulp is produced after 1 ton of sugar production [8]. According to Boe and Angelidaki [9], low biogas production yields could be a result of the low biodegradability of fibrous matters, which can account for 40–50% of the

⁎

total solids (TS) in CM. CM is one of the most widely used substrates in AD [6]. Approximately 20 m3 of methane was obtained from 1 ton of CM [10]. Hydrolysis, one of the AD steps, is known as the rate limiting step [11]. Organic matter disintegration methods have been investigated as pretreatment processes to increase the efficiency of AD and eliminate this step (hydrolysis) [12]. These pretreatments are physical, chemical and chemical pretreatments. Pretreatments increase biogas yield. The pretreatments are remarkable in terms of low cost and considerably increase biogas/methane yield [13]. However, chemical pretreatments produces secondary pollution and biological pretreatments are also difficult to control. Therefore, physical pretreatment and thermal pretreatment are frequently used due to operational convenience and low investment [14]. Pretreatments are applied to ensure high content cellulose, lignin and hemicellulose in wastes such as SBP and CS attractive [15]. Rajpur et al. [14] performed thermal pretreatment of wheat straw at different temperatures (120–180 °C) for constant time, and reported that methane yield was increased by 53%. Ferreira et al [16] performed thermal pretreatment of wheat straw at 150–220 °C for 1–15 min and then methane yield was increased by 20%. Ennouri et al

Corresponding author. E-mail addresses: [email protected] (H. Şenol), [email protected] (S. Demir), [email protected] (V. Oda).

https://doi.org/10.1016/j.fuel.2019.116651 Received 14 May 2019; Received in revised form 9 November 2019; Accepted 12 November 2019 0016-2361/ © 2019 Published by Elsevier Ltd.

Please cite this article as: Halil Şenol, et al., Fuel, https://doi.org/10.1016/j.fuel.2019.116651

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[17] reported that biogas yield increased by 27–37% after thermal pretreatment of urban solid wastes at 60–120 °C for 30 min. In the literature, thermal pretreatment was applied to different organic wastes at different temperatures and durations. However, the pretreatment results on organic matter were not stable and varied according to the type of organic matter. In addition, each pretreatment temperature was not optimized over a wide period of time. In order to better understand the specific results of thermal pretreatments, a wider range of temperatures and application times should be studied. Further, pretreatment temperatures and pretreatment times must be optimized for costs. The aim of this study was to investigate the effect of thermal pretreatments at different temperatures for different times on organic matter and biogas yield. Biogas yields from CM, CS and SBP were investigated with and without thermal pretreatment under mesophilic conditions. The optimum mass mixing ratio of CM, CS and SBP was found for AD. Then, thermal pretreatments were applied to the optimum mixing ratio. Thermal pretreatments were performed at 100, 120, 150 and 180 °C. Each pretreatment temperature was applied for 10, 20, 30, 60 and 120 min, respectively. In addition, changes in lignocellulosic structure resulting from thermal pretreatment were also investigated. Moreover, biogas production rate was evaluated for all thermal pretreatment temperatures based on experimental data with the help of two different kinetic models.

Table 1 Characteristics of organic raw materials. Parameters

Cattle manure

Corn silage

Sugar beet pulp

TS (% w/w) VS (TS %) Ash (% w/w) Moisture (% w/w) C (% w/w) N (% w/w) SCOD (mg O2/Lslurry) C/N pH Cellulose (% w/w) Hemicellulose (% w/w) Lignin (% w/w)

18.80 82.80 3.23 81.20 32.12 1.65 22,680 19.46 6.88 22.30 18.92 12.85

32.10 92.54 2.39 77.90 44.92 1.39 12,320 32.31 3.78 25.90 19.49 4.01

84.62 86.62 11.32 15.38 43.25 1.09 5800 39.68 4.08 30.55 25.92 3.91

pretreatments were applied at temperatures of 100 °C, 120 °C, 150 °C and 180 °C each with 10, 20, 30, 60 and 120 min. Thermal pretreatments were applied in the incubator (DOL-EKO brand ILW-115-STD model, Germany). As a result of the pretreatment, the solubility was determined by filtering the slurry with glass cotton. Increase in SCOD % value after thermal pretreatments was calculated from the difference of final SCOD thermal pretreated and initial SCODuntreated, ralative to the initial SCOD (SCOD untreated) (Eq. (1)) [20].

2. Experimental

incrementalSCOD% =

(SCODthermalpretreated − SCODuntreated)

2.1. Preparation of raw materials and products The CM was collected from Giresun, Turkey. The raw CM was transformed into pure form by distinguishing it from impurities such as gravel, straw and plant seed. CS and SBP were collected from Samsun, Turkey. Each of the organic materials was milled to a particle size of 0.5–1 mm. They were stored at −18 °C for later use in AD experiments.

SCODuntreated

× 100

(1)

2.4. Analytical method In Table 1, total solids (TS), volatile solids (VS), ash, moisture, carbon (C), nitrogen (N), SCOD, pH, cellulose, hemicellulose and lignin analysis were performed before applying AD to organic waste. TS and VS were analyzed according to APHA standards [21]. The carbon to nitrogen ratio (C:N) of the lignocellulosic substrates was determined by the COSTEC elemental analyzer (Elemental Analyzer NA 2500). The cellulose, hemicellulose and lignin contents were measured using fiber analyzer (ANKOM A2000i, US) [22]. A Hitachi SU-1510 (Hitachi, Ltd. Tokyo, Japan) scanning electron microscope (SEM) was used to scan the surfaces of the organic waste. Liquid samples were centrifuged at 10,000 rpm for 10 min at room temperature and filtered with a 0.25 μm membrane filter. SCOD analyzes were performed according to closed reflux titrimetric method [21]. Physicochemical properties and analysis results of CM, CS and SBP are given in Table 1.

2.2. Anaerobic digestion AD experiments were performed in 500 mL batch reactors. The heating temperature was set to 39 ± 2 °C. Heating was provided by a magnetic stirrer heater with continuous stirring of each reactor at 100 rpm during digestion. The optimum pH for AD is neutral [6]. The pH of reactors was adjusted to 7.0 using 1 M solution of H2SO4 or NaOH at the beginning of the experiments. At the beginning of each AD experiment, the batch reactors were flushed with N2 gas for 5 min to ensure anaerobic conditions and eliminate oxygen from the reaction medium. Biogas yield was determined according to the water displacement method [18,19].The dry matter ratio was 10% w/w in all reactors. Initially, optimum mixing ratio determination studies were performed. Mixture ratios of CM:CS:SBP with the ratios of 1:0:0, 0:1:0, 0:0:1, 1:1:1, 2:1:1, 2:2:1, 2:1:2, 1:1:2, 1:2:1 w/w/w were adjusted in each batch reactors. For each set of experiment, one control reactor was used. The experiments were carried out with 3 replications. Biogas yields were measured every 5 days. The AD time was approximately 30 days for the batch reactors. AD was terminated when the last measured value was less than 2% of the previously measured value. The biogas production was provided in terms of volume per gram of total solid (TS) (mL/g TS).

2.5. Kinetic study The estimated values of cumulative biogas production (CBP) rate measured every 5 days were obtained using modified Gompertz model [23,24] and Modified Bertalanffy model [25]. For the transformation to the mechanical model, biologically significant parameters can be obtained with the help of the first and second derivatives [25]. In addition, for the transformation to the mechanical model, biologically significant parameters were obtained with the help of the first and second derivatives. The Bertalanffy function defined for growth functions was first developed by Von Bertalanffy in 1934 and later changed by Beverton and Holt (1957) [26]. Table 2 shows the modified Gompertz and modified Bertalanffy models. Where; y is the estimated methane yield (mL/g TS), with respect to time t (day); λ is lag phase (day); A is ultimate methane yield at t = ∞ (mL/g TS) and e is a Euler's function equal to 2.71828. In this study, Statistical Package for the Social Sciences (SPSS 23.0) program was used to calculate kinetic constants (λ, µm, A) from growth curves.

2.3. Thermal pretreatments Thermal pretreatments are widely used in biogas production [14]. The optimum ratio was chosen for the reactor with the highest biogas production and then thermal pretreatments were applied to this reactor. Thermal pretreatments were performed in autoclave bottles. 50 g of the optimum mixture from each reactor was added to the autoclave bottle. 10 g of distilled water was added to prevent burning of the dry biomass during thermal pretreatment. In this study, thermal 2

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biogas production and incremental SCOD % value are given in Table 4. The reactors at different thermal temperatures and times were labeled from R10 to R29. In one study, it was reported that thermal pretreatments were generally carried out between 50 and 240 °C and pretreatments below 100 °C should be applied for longer periods [14]. In another study, it was emphasized that pretreatments are expensive when the temperature exceeds 180–200 °C [13]. Therefore, in this study, thermal pretreatments were performed between 100 and 180 °C. Table 1 shows the lignocellulosic component in the highest SBP and in the lowest CM. Pretreatment technologies are needed to benefit from these lignocellulosic components. But these technologies are fertile as well as they are cost effective [13]. Therefore, the optimization of pretreatment technologies was investigated in this study. The thermal pretreatment temperatures and times took place great differences on biogas production. Except for 120 °C, the highest biogas production at other pretreatment temperatures occurred after pretreatment time of 60 min. Among the reactors that were pretreated at 120 °C, the highest biogas yield was achieved in the reactor with a pretreatment time of 120 min. Among 120 °C pretreated reactors, the difference between the reactor pretreated for 120 min and the reactor pretreated for 60 min is 2.6 mL/g TS. In terms of pretreatment costs, the most ideal reactor at all thermal pretreatment temperature is the reactor pretreated for 60 min. The highest biogas production among these reactors was 362.1 mL/g TS. Biogas yields of organic matter was affected significantly (p < 0.05) from thermal pretreatment as shown by two-way ANOVA results. It was concluded that the data in all pretreatment temperatures exhibited normal distribution (p > 0.05). Therefore, correlation coefficients among reactors were calculated. In pretreated reactors of 100 °C, the strongest correlation was found between R10–R13 and the weakest correlation was between R12 and R13. Biogas yields of reactors pretreated at 100 °C ranged from 195.5 to 242.1 mL/g TS due to different pretreatment times. Biogas production increased as the pretreatment time increased from 10 min to 60 min, but it decreased after 120 min pretreatment. This result can be attributed to the excess organic solubility in the medium after 120 min of pretreatment. The reactors pretreated at 100 °C produced 8.3–41.5% higher biogas production compared to the control. After 100 °C pretreatments, the optimum pretreatment time was determined as 60 min. Biogas yields of reactors pretreated at 120 °C ranged from 201.4 to 288.1 mL/g TS. Biogas yield increased as pretreatment time increased. The reactors pretreated at 120 °C has 11.5–59.6% higher biogas yield than the control. According to the two-way ANOVA results, in pretreated reactors at 120 °C, the strongest correlation was found between R17–R18 and the lowest correlation between R15 and R18. Biogas yields of pretreated reactors at 150 °C ranged from 222.5 to 280.0 mL/g TS. In pretreated reactors at 150 °C, the strongest correlation was found between R20 and R24 and the lowest correlation between R21 and R22. Pretreated reactors at 150 °C produced 23.2–55.1% higher biogas yield than the control. After pretreatment at 150 °C, the optimum pretreatment time was 60 min. Significant increases in biogas production were observed when the pretreatment temperature increased from 150 °C to 180 °C. In pretreated reactors at 180 °C, the strongest correlation was found between R26 and R29 and the lowest correlation between R25 and R26. Biogas yields of pretreated reactors at 180 °C ranged from 232.2 − 325.5 mL/g TS due to different pretreatment times. Incremental SCOD values were also controlled to better observe the stability and effects of thermal pretreatments. Incremental SCOD % values after thermal pretreatments are shown in Table 4. After thermal pretreatment in different conditions, the SCOD values varied among 10.1–155.4. SCOD % values increased as pretreatment temperature and time increased. However, there was no significant change in biogas yields when the pretreatment time for all pretreatment temperatures changed from 60 min to 120 min. This phenomenon can be attributed to the presence of volatile fatty acids due to high SCOD values.

Table 2 Modified Gompertz and modified Bertalanffy models. Models

Equations

Modified Gompertz

⎡ μm e (λ − t ) + 1⎤ A ⎢ ⎥ ⎦

y = Ae (e⎣ Modified Bertalanffy

y=−

1 ⎡ A −3 27 ⎢

2

+ e3 +

3 9μm .(λ − t ) ⎤ 4A

⎣

⎥ ⎦

2.6. Statistical analysis All statistical analyses were carried out using SPSS 23.0. Normal distribution was assumed based on the agreement of parametric and non-parametric testing and thus all inference was carried out using twoway ANOVA. Significance level (p) was determined as 0.05. Therefore, Pearson correlation coefficients between reactors were calculated. The statistical analysis was applied separately to the reactors at each pretreatment temperature (100, 120, 150 and 180 °C). 3. Results and discussion 3.1. Optimum mixing ratio determination studies Table 1 presents the physicochemical properties of CM, CS and SBP and the amount of lignocellulosic components. SBP has the highest lignocellulosic component ratio. In order to find the appropriate mixing ratio of CM, CS and SBP in AD, 9 reactor types in Table 3 are given. These reactors were labeled R1 to R9 according to different mixing ratios. Biogas yields varied depending on the amount of lignocellulosic components. Reactors with C/N ratios between and close to 20–30 achieved high yield, while reactors outside this range showed low yield. When this ratio rises above 20, low biogas yield was reported by Dioha et al. [27]. At reactor R1 containing only CM, biogas yield was 162.0 mL/g TS. In one study, methane yield of CM was reported to be 210 mL/g VS under thermophilic conditions [28]. The possible reason for this difference originates from different production temperatures. The highest biogas production from these reactors was 180.5 mL/g TS (R4). Thus, all thermal pretreatments were applied to the mixing ratio (2:1:1, w/w/w) in R4. AD of CM, CS and SBP has not been studied in the literature. Therefore, the reactor R4 was selected as the control. Turkey, especially in Central Anatolia cattle, corn and sugar beet are grown together [29]. Therefore, the appropriate mixture of the wastes of these three organic substances according to C/N ratios can be evaluated in the production of biogas in the regions where the source is. 3.2. Thermal pretreatment results After the thermal pretreatment applied at different temperature and different time intervals, biogas yields were provided under anaerobic conditions. As a result of this process, biogas yield, % incremental Table 3 Optimum mixing ratio determination studies for AD. Reactor

CM:CS:SBP Mixing ratio (w/w)

C/N ratio

Total amount of lignocellulosic component (% w/w)

Biogas production (mL/g TS)

R1 R2 R3 R4 R5 R6 R7 R8 R9

1:0:0 0:1:0 0:0:1 2:1:1 1:1:2 1:2:1 1:1:1 2:2:1 2:1:2

19.46 32.31 39.68 27.73 32.76 30.94 30.48 28.64 30.12

54.07 49.40 60.38 54.48 56.06 53.31 54.62 53.46 55.66

162.0 105.5 95.2 180.5 145.8 140.5 170.8 175.8 149.9

3

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Table.4 Thermal pretreatment results. Pretreatment temperature (°C)

Pretreatment time (min)

Reactor

Biogas yield (mL/g TS)

Incremental biogas yield (%)

Incremental SCOD value (%)

100

10 20 30 60 120

R10 R11 R12 R13 R14

195.5 215.5 235.5 255.5 242.1

± ± ± ± ±

9.2 11.4 10.8 9.7 11.5

8.3 19.3 30.4 41.5 34.1

10.1 22.4 34.3 70.0 78.9

± ± ± ± ±

2.4 3.5 4.7 5.5 4.8

120

10 20 30 60 120

R15 R16 R17 R18 R19

201.4 212.0 242.9 285.5 288.1

± ± ± ± ±

8.7 9.5 7.8 9.1 8.8

11.5 17.4 34.5 58.1 59.6

15.1 21.1 43.0 79.6 85.5

± ± ± ± ±

2.9 3.7 4.6 5.5 5.1

150

10 20 30 60 120

R20 R21 R22 R23 R24

222.5 235.8 245.0 280.0 275.6

± ± ± ± ±

10.2 11.5 12.1 11.8 12.5

23.2 30.6 35.7 55.1 52.6

22.0 34.1 45.2 73.0 77.3

± ± ± ± ±

3.8 4.5 4.8 5.8 4.9

180

10 20 30 60 120

R25 R26 R27 R28 R29

232.2 245.0 355.0 362.1 325.5

± ± ± ± ±

11.1 9.8 14.1 12.2 10.7

28.6 35.7 96.6 100.6 80.3

30.1 ± 5.1 44.2 ± 4.9 105.0 ± 3.4 124.6 ± 2.7 155.4 ± 3.5

Table 5 Comparison of the increments in biogas production with current literature values as a result of thermal pretreatment. Thermal pretreatment conditions

Results of AD after pretreatment

Type of organic matter

Reference

at 100 °C for 1 h at 180 °C for 1 h at 200 °C for 15 min at 120 °C for 1 h at 121 °C for 1 h at 121 °C for 1 h at 180 °C for 1 h at 120 °C for 1 h at 120 °C for1 h at 120 °C for 1 h 125 °C for 37.5 min

41.5% incremental biogas 100.6% incremental biogas 27% incremental biogas 37% incremental biogas 29% incremental biogas 11% incremental biogas 53% incremental biogas 64% incremental biogas 32% incremental methane 41% incremental methane 34% incremental biogas

2:1:1 mixtures of CM:CS:SBP 2:1:1 mixtures of CM:CS:SBP wheat straw urban solid waste wheat straw sugar cane wheat straw wheat straw rice straw barley straw cattle manure

This study This study [16] [17] [31] [14] [30] [30] [30] [30] [32]

and lignin solubilization at 100 °C was 14.8%, 22.2% and 13.4% respectively. There was no significant difference between the amount of lignocellulosic solubilization after 60 min and 120 min at 100 °C. A similar situation occurred at temperatures of 120, 150 and 180 °C. Therefore, the optimum pretreatment time for thermal pretreatments was 60 min. In 120 °C pretreatment, the cellulose solubilization was determined as 15.9% in the ideal reactor. However, in the reactor applied for 60 min, the cellulose value was reduced to 15.7%. Similarly, the hemicellulose was removed by 22.9% in 60 min, while it was removed by 22.9% in 60 min. After pretreatment at 120 °C, the amount of lignin solubilization in the R28 and R29 reactors was close to each other. Thus, when thermal pretreatment at 120 °C was evaluated in terms of lignin, the ideal rector was R18. Cellulose solubilization after thermal pretreatments at 150 °C and 120 min was 22.9%. However, in the reactor at which thermal pretreatment of 60 min was applied at the same temperature, the cellulose removal was 20.8%. Hemicellulose was removed at 32.4% in 60 min, while it decreased by 33.5% in the pretreatment period of 120 min. In R23 and R24, the amounts of cellulose, hemicellulose and lignin were close to each other. At 180 °C of thermal pretreatment, cellulose, hemicellulose and lignin solubilization started to increase significantly after 30 min. Table 7 shows the comparison of cellulose, hemicellulose and lignin solubilizations according to different thermal pretreatment conditions. According to Jiang et al. [35], the highest lignin removal in the literature was obtained as 57.5% at 230 °C. In this study, 13.4% lignin solubilization occurred after 100 °C and 1 h. According to Kim et al.

Table 5 shows the comparison for the incremental biogas production and methane production in literature after thermal pretreatment. In this study, after pretreatment at 100 °C for 1 h and at 180 °C for 1 h incremental biogas yields were 41.5% and 100.6% respectively. According to Ferreira et al.[16], 27% higher methane production was achieved after pretreatment at 200 °C for 15 min. Although the thermal pretreatment was applied at a higher temperature, the lower application time did not increase the biogas yield [30]. In the current literature, the biogas production increased as the pretreatment times increased at constant pretreatment temperature [14,16,30]. However, different biogas yields of different organic wastes have been reported in the literature, despite pretreatment under the same conditions [30]. Thermal pretreatment results in this study support this situation.

3.3. Effects of thermal pretreatments on lignocellulosic solubilization When the recent studies in the literature and the results of this study are taken into consideration, the presence of different biogas yields under the same thermal pretreatment conditions can be attributed to the difference in lignocellulosic content of organic matter. Cellulose, lignin and hemicellulose are disintegrated by thermal pretreatment. Thus, higher yields are obtained in AD [33,34]. For this reason, solubilization of cellulose, hemicellulose and lignin after thermal pretreatment was also investigated. Table 6 gives the solubilization percentages of the cellulose, hemicellulose and lignin after thermal pretreatments at different temperatures and times. The highest cellulose, hemicellulose 4

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Table 6 Lignocellulosic solubilization component in raw material after thermal pretreatments. Pretreatment temperature (°C)

Time (min)

Reactor

Cellulose solubilization (% w/w)

Hemicellulose solubilization (% w/w)

Lignin Solubilization (% w/w)

100

10 20 30 60 120

R10 R11 R12 R13 R14

5.1 ± 1.1 9.8 ± 2.1 14.1 ± 2.0 14.5 ± 1.9 14.8 ± 1.5

2.3 ± 0.8 15.7 ± 1.7 21.8 ± 1.8 21.9 ± 1.9 22.2 ± 2.5

2.4 ± 2.1 10.1 ± 2.2 13.5 ± 2.9 13.5 ± 3.5 13.4 ± 3.2

120

10 20 30 60 120

R15 R16 R17 R18 R19

4.2 ± 1.0 12.6 ± 1.5 14.6 ± 1.9 15.7 ± 2.7 15.9 ± 3.0

2.5 ± 1.5 18.0 ± 1.9 20.2 ± 2.5 22.9 ± 2.9 23.2 ± 2.7

2.9 ± 2.0 10.5 ± 1.9 12.1 ± 2.5 13.6 ± 3.1 13.9 ± 2.4

150

10 20 30 60 120

R20 R21 R22 R23 R24

8.3 ± 1.5 10.2 ± 2.1 17.6 ± 2.3 20.8 ± 2.0 22.9 ± 1.9

3.2 ± 1.8 5.8 ± 1.5 30.1 ± 3.8 32.4 ± 2.5 33.5 ± 3.1

6.5 ± 2.2 9.2 ± 2.8 17.1 ± 1.9 19.9 ± 2.8 20.2 ± 3.1

180

10 20 30 60 120

R25 R26 R27 R28 R29

17.0 25.2 35.1 38.2 39.1

18.4 22.4 30.0 32.9 33.1

6.5 ± 1.8 11.2 ± 2.7 20.1 ± 2.9 23.2 ± 3.1 23.4 ± 3.5

± ± ± ± ±

2.4 2.2 1.8 1.9 1.8

Pretreatment results

Type of organic matter

Reference

100 °C and 1 h

Cellulose = 14.8% Hemicellulose = 22.2% Lignin = 13.4% Cellulose = 38.2% Hemicellulose = 32.9% Lignin = 23.2% Cellulose = 14.7% Hemicellulose = 20.1% Lignin = 13.6% Cellulose = 22.7% Hemicellulose = 14.6% Lignin = 39.9% Cellulose = 15.0% Hemicellulose = – Lignin = 24.8% Cellulose = 10.9% Hemicellulose = – Lignin = 32.9% Cellulose = 14.3% Hemicellulose = % – Lignin = 49.1% Cellulose = 15.4% Hemicellulose = % – Lignin = 57.5%

2:1:1 mixtures of CM:CS:SBP

This study

2:1:1 mixtures of CM:CS:SBP

This study

Rice straw

[36]

Rice straw

[37]

Giant reed

[35]

Giant reed

[35]

Giant reed

[35]

Giant reed

[35]

180 °C and 1 h

100 °C and 1 h

55 °C and 8 day

170 °C and 10 min

190 °C and 10 min

210 °C and 15 min

230 °C and 5 min

1.1 1.8 2.4 2.7 3.0

After the thermal pretreatment, each pretreatment periods and rates were examined during the anaerobic process. Biogas volume was measured every 5 days during the anaerobic process. Fig. 1 shows the CBPs of organic matter applied at different thermal pretreatment temperatures. Total digestion time varied between 40 and 45 days depending on lignocellulosic content. Rajput and Visvanathan [14] reported digestion time of 45 days of lignocellulosic material. The reactors with the highest biogas yield after pretreatment at 100, 120, 150 and 180 °C and with different time intervals were R13, R19, R23 and R28, respectively. The change between TS, VS and SCOD values of these reactors after pretreatments is given in Fig. 2. Initially, the SCOD value of control reactor was 15,870 mg/L. This value was 26,990 mg/L after thermal pretreatment at 100 °C for 60 min. The SCOD values for reactors R19, R23 and R28 were 28,500 mg/L, 27,450 mg/L and 35,650 mg/L, respectively. After the pretreatments, SCOD values of the reactors and biogas yields were consistent. TS and VS values decreased depending on the degree of thermal pretreatment. This mass loss can be attributed to lignocellulosic dissolution during pretreatments. The change in VS and TS amounts after the thermal pretreatment in this study was consistent with the results of the Rajput and Visvanathan [14]. SEM images give information about the surface morphology of organic lignocellulosic material. Fig. 3 shows SEM images of different post-treatment and untreated organic materials. In Fig. 3, according to 1 and 2, it is observed that there is no porous structure with the surface crystal of the organic sample and the surface consists of a hard layer. According to 3 and 4, the surface sample of the organic sample has little fractures and has a porous structure. The pore size appears to be around 10–20 µm. It appears that these pores did not open upon pretreatment at 100 °C for 60 min. According to 5 and 6, the surface sample of the organic sample was broken and approximately 1–10 µm cracks were observed. These cracks were not observed at the pretreatment temperature of 100 °C, 120 °C and 120 min pretreatment conditions. It was observed that surface crystal began to decrease and cracks started to form. According to 7 and 8, the surface crystal of the organic sample was broken and appeared to have cracks of around 1–5 µm. As the pretreatment temperature increased, the number of cracks also increased according to 9 and 10. It is seen that the fractures and reductions of the surface sample of the organic sample have reached the maximum level compared to the other lower pretreatments. Patowary and Baruah [38] observed that the rice straw, which was kept at 90 °C thermal pretreatment temperature for 10 h, did not pass into a porous

Table 7 Comparative analysis of lignocellulosic solubilization values with current literature values after thermal pretreatment. Thermal pretreatment conditions

± ± ± ± ±

[36], similar lignin removal values were obtained. In the literature, the maximum cellulose removal was 22.7% in organic samples kept at 55 °C for 8 days [37]. In this study, 38.2% cellulose solubilization occurred at 180 °C for 1 h of pretreatment. This showed that the cellulose solubility at high temperatures was time dependent. In the literature, maximum hemicellulose solubilization was 20.1% at 100 °C and 1 h [36]. In the same conditions, hemicellulose solubilization in this study was 22.2%. When the current literature is considered, lignocellulosic solubilization values have changed depending on the temperature values and application times of thermal pretreatments. However, pretreatment under the same conditions resulted different lignocellulosic solubilization values of different organic substances. This phenomenon can be attributed to the content of cellulose, hemicellulose and lignin present in different amounts in the organic material. 5

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Fig. 1. Cumulative biogas productions after thermal pretreatments. VS TS sCOD

130

(R4, R13, R19, R23 and R28). In previous studies, CBP was evaluated by Gompertz model [41]. However, in the literature, there are limited applications of the modified Bertalanffy model, which is an alternative sigmoidal model. Table 8 shows the maximum biogas potential, specific biogas production rate, lag phase and determination coefficients (R2) of the optimum reactors in which the untreated and thermal pretreatments were applied according to the Gompertz and Bertalanffy model. In R4 and R13, the biogas yields were 180.5 and 255.5 mL/g TS respectively. However, the estimated values for the maximum biogas production of the modified Gompertz model were 194.998 and 290.760 mL/g TS, respectively. Similarly, the maximum biogas production rates estimated by the modified Bertalanffy model were 220.154 and 351.580 mL/g TS. The maximum biogas production rates estimated by modified Gompertz and modified Bertalanffy models were determined as 427.440 and 538.728 mL/g TS, respectively. The most fitted reactors for the modified Gompertz and modified Bertalanffy models were R4 and R13 respectively. Fig. 4 shows the time-dependent value of the CBP for the R4, R13, R19, R23 and R28 reactors. In all reactors, the R2 value of the modified Bertalanffy model is higher than the R2 value of the modified Gompertz model. Based on these values, it can be said that modified Bertalanffy model was more fitted than modified Gompertz model. In modified Gompertz model, the lag phase for the untreated reactor was lower than that in modified Bertalanffy model. Although the modified Bertalanffy model was more fitted for all reactors than the modified Gompertz model, it found the maximum biogas production estimates higher. In previous studies, CBP was examined for its applicability for modified Gompertz model [42-45]. In some studies, the modified Gomperz model was compared with the modified Logistic model [14,46-48]. However, there is no comparison of modified Gompertz and modified Bertalanffy models in the literature. In this study, modified Bertalanffy model values for biogas production for the first time were found to be quite compatible in CBP.

35000

110 30000

25000 70

60 min (R23 )

60 min (R13)

50

120 min (R19)

60 min (R28 )

15000

30

10000

10

-10

20000

SCOD (mg/L)

TS and VS (g/L)

90

control

100

120

150

180

5000

Thermal pretreatment temperature (ºC) Fig. 2. The changes in the amounts of TS, VS and SCOD of organic matter after thermal pretreatment.

structure and only breakages began to occur. Similarly Momayez et al. [39] determined has a rigid structure after thermal pretreatment at 130 °C for 30 min and the surface crystallinity decreases after pretreatment at 190 °C for 60 min. SEM images of cassava anaerobic residue were examined after thermal pretreatment. According to these images, it was observed that the surface morphology of organic matter was disrupted and passed from a crystal structure to a soft structure as a result of thermal pretreatment applied at 160 °C [40]. 3.4. Applications of modified Gompertz and modified Bertalanffy models Modified Gompertz and modified Bertalanffy models were applied to the optimum reactors which have the maximum biogas production 6

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Fig. 3. SEM images of orgaic samples as a result of thermal pretreatment.

4. Conclusion

among 41.5% and 100.6%. Thermal pretreatments played a role in the lignocellulosic solubilization. After thermal pretreatments, cellulose, hemicellulose and lignin solubilizations were 14.8–39.1%, 22.2–33.5% and 13.5–23.4%, respectively. As the pretreatment times for all pretreatment temperatures increased, the SCOD values increased accordingly. At all thermal pretreatment temperatures, the thermal pretreatment time increased biogas yield up to 60 min, but after 60 min there

In the anaerobic process, optimum mixing ratio of CM, CS and SBP was found in order to evaluate waste together. Thus, it was experimentally proposed that CM, CS and SBP form a good mixture in AD. Thermal pretreatments were applied from 100 °C to 180 °C for optimum mixing ratio and biogas production in these temperature ranges varied

7

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Table 8 Kinetic constants of Modified Gompertz and modified Bertalanffy models with experimental data. Modified Bertalanffy model

Λ(day)

µm (mL/g TS.d)

A (mL/g TS)

R

2.786 4.537 4.738 3.976 2.778

4.017 8.771 9.817 9.607 11.322

194.998 290.760 333.583 318.484 427.440

0.996 0.997 0.996 0.995 0.990

Cumulative biogas production (mL/gTS)

R4 R13 R19 R23 R28

Modified Gompertz model

250

2

R4-Gompertz R4-Bertalanffy R4

200

Cumulative biogas production (mL/gTS)

Reactors

150 100 50 0 0

5

10 15 20 25 30 35 40 45 50

R2

4.112 3.654 3.901 3.054 2.098

3.620 8.281 9.099 8.928 10.535

220.154 351.580 417.783 386.302 538.728

0.998 0.999 0.997 0.997 0.993

250 200 150 100 50 0 6 11 16 21 26 31 36 41 46 51 56 61 66 71

Day

450 400 350 300 250 200 150 100 50 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

Day Cumulative biogas production(mL/g TS)

A (mL/g TS)

300

1

Cumulative biogas production (mL/gTS)

Cumulative biogas production (mL/gTS)

µm (mL/g TS.d)

350

Days

0

Λ(day)

400 350 300 250 200 150 100 50 0 1

6 11 16 21 26 31 36 41 46 51 56 61 66 71

Day 600 500 400 300 200 100 0 0

5 10 15 20 25 30 35 40 45 50 55 60 65 70

Day

Fig. 4. Cumulative biogas productions fitted curves of modified Gompertz and modified Bertalanffy model curves according to different pretreatment temperature conditions. 8

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was no significant increase in biogas yield. After the thermal pretreatment, experimental data successfully fitted to modified Gompertz and Bertalanffy models.

[20]

[21]

CRediT authorship contribution statement

[22]

Halil Şenol: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration. Ünsal Açıkel: Writing - original draft. Serkan Demir: Writing review & editing. Volkan Oda: Supervision, Validation, Visualization.

[23]

Declaration of Competing Interest

[25]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[26]

[24]

[27] [28]

Acknowledgement

[29]

This research was supported by the Sivas Cumhuriyet University Scientific Research Projects Unit (CUBAP) under grant no M-665. The authors wish to thank this institution for their support.

[30]

[31]

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