Methane Enhancement through Liquid Ammonia Fractionation of Corn

Oct 26, 2016 - Department of Food Engineering, University of Agriculture, Faisalabad 38000, Pakistan. § Biogas Institute of Ministry of Agriculture (...
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Methane Enhancement through Liquid Ammonia Fractionation of Corn Stover with Anaerobic Sludge Muhammad Hassan,† Weimin Ding,*,† Muhammad Umar,‡ Xu Chen,† and Libin Wu§ †

College of Engineering, Nanjing Agricultural University, Nanjing, Jiangsu Province 210031, China Department of Food Engineering, University of Agriculture, Faisalabad 38000, Pakistan § Biogas Institute of Ministry of Agriculture (BIOMA), Chengdu, Sichuan Province 610041, China ‡

ABSTRACT: Two novel pretreatment methodologies, namely, ammonia recyclable percolation (ARP) and liquid ammonia (LA), were introduced for corn stover (CS) in the present study to enhance methane production. In the past, ARP and LA fractionation were mostly utilized as pretreatments to enhance the enzymatic digestibility of CS to produce fermentable sugars and ethanol. The CS was pretreated by the ARP and LA pretreatment methods at two concentration levels, and their effects on the lignocellulosic characteristics and resultant methane enhancement were evaluated. All pretreatments were found to be significant (P < 0.05) in enhancing xylan solubilization, lignin removal, and methane production. The ARP-2 pretreatment was found to be the optimum pretreatment with 50.69%, 44.15%, and 57.52% xylan removal, lignin removal, and C/N reduction, respectively, as compared to the untreated CS. To optimize the most suitable pretreatment for CS, process stability and biochemistry parameters such as total volatile fatty acids (TVFA), chemical oxygen demand (COD), pH, and alcohol production were deeply monitored during the anaerobic digestion period.

1. INTRODUCTION Today, providing energy to the rapidly developing world has become a significant challenge. Energy shortages are a serious issue, primarily in the distant villages of developing countries. About 1500 million people on this planet do not have access to electrical energy, and about 3000 million people rely on traditional renewable fuels such as fire wood, agricultural straw, coal, forest residues, and shredded waste to meet their kitchen heating requirements.1 Because of these human activities, natural resources such as forests and natural habitats are depleted day by day. In this critical situation, biomass is considered to be a promising source for bioenergy production.2 Biomass contains energy equivalent to meet 10−14% of the total world energy demand,3 which constitutes about 90% of the total household energy demand in developing countries. As a major agricultural country, China produced about 270 million tonnes of corn stover (CS) in 2013,4 40% of which was burned just after being harvested within the fields.5 The burning of agricultural residue and straw in developing countries is a common tradition that causes serious environmental hazards and makes a clear contribution to greenhouse gas (GHG) emissions.6 Anaerobic digestion is basically a microbiological waste degradation process by which a mixture of gases, primarily CH4 and CO2, are produced in the absence of oxygen. Anaerobic digestion technology is widely utilized for treating the bulk quantity of organic agricultural solid waste7 and waste sludge having high organic contents8 that ultimately leads to the mitigation of greenhouse gas (GHG) production.9−11 Anaerobic digestion consists of four steps, namely, hydrolysis, acidogenesis, acetogenesis, and methanogenesis,12,13 and in the case of agricultural lignocellulosic biomass, hydrolysis is considered as the rate-determining step because of high contents of lignin.6,9,14−16 © 2016 American Chemical Society

Anaerobic digestion is well developed for animal waste such as hog, chicken, and cow manure, but for lignocellulosic biomass, reluctance is still observed on an industrial scale because of its complex lignocellulosic structure.17,18 To reduce this barrier, two novel pretreatment methods, namely, ammonia recyclable percolation (ARP) and liquid ammonia (LA), are introduced in the present study. Agricultural straw is mostly considered to have a low anaerobic digestibility and enzymatic hydrolysis as compared with animal manures because of its high lignin content and the complex plant tissue structure. To overcome this problem, different physical, chemical, biological, and thermochemical pretreatments have been employed for CS to increase its anaerobic digestibility.14 The main objective of these pretreatments is to reduce the lignin and cellulosic crystallinity from the CS.5 Ammonia recyclable percolation (ARP) and liquid ammonia (LA) pretreatments have mostly been utilized for the enzymatic hydrolysis of CS to produce fermentable sugars and ethanol. To investigate the significance of these pretreatments and ease of availability of ammonia reagents in the market, these methods were utilized for the anaerobic digestion of CS. The ARP method was found to provide high delignification and significant swelling in the morphological structure of CS with about 50% xylan solubilization, leading to about 93% enzymatic digestibility.19 In another study, the ARP method was employed for CS in ethanol production and was revealed to be an efficient delignifying process at up to 70−85% with about 40% xylan solubilization within the first 20 min of treatment.20 In the case of liquid ammonia (LA) pretreatment, the ammonia molecule is converted into ammonium hydroxide, and after its Received: July 18, 2016 Revised: October 25, 2016 Published: October 26, 2016 9463

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Energy & Fuels dissociation, the hydroxyl ion (OH−) separated and attacks the alkaline-labile bonds that hold xylan and lignin together. Hydroxyl ion also dislocates the hydrogen bonds in the cellulosic structure; overall, this biochemical reaction results in the swelling of fibers that ultimately leads to an enhancement of digestibility.21 The basic objective of the present study was to analyze the compositional changes in CS due to the ARP and LA pretreatments, specifically an increase in its anaerobic digestibility and an evaluation of its methane enhancement. Anaerobic digestion process biochemistry parameters such as pH, total volatile fatty acids (TVFA), alcohol production, and soluble chemical oxygen demand (COD) were deeply studied to optimize the most feasible pretreatment for CS.

thermostatted water bath was used for this first-stage pretreatment without any ammonia concentration. The second stage of the pretreatment was performed in sintered glass crucibles using a raw fiber extractor (VELP Scientifica, Usmate, Italy). Two grams of the dried CS sample was placed in each crucible at a time, and a flow rate of 0.2 mL of NH3 per gram of CS was maintained, as shown in the schematic diagram in Figure 1A. This concentration of ammonia was

2. MATERIALS AND METHODS 2.1. Materials. Fresh corn stover (CS) was collected from a commercial agricultural farm in Pukou, Nanjing, China. The CS was first sun-dried and then oven-dried at 105 °C; it was then ground with a vertical straw grinding machine (desk-type laboratory-scale continuous feeding miller, LH-08B). The ground CS was sieved by the throw action method through a 1-mm circular laboratory-scale sieve; afterward, the CS was refrigerated at −20 °C. The chemical composition of the CS was determined and is presented in Table 1.

Table 1. Chemical Characteristics of Corn Stover (CS) and Anaerobic Sludge parametera

units

corn stover

seed sludge

TS VS TOC OM TN TP TK C/N pH cellulose hemicellulose lignin COD TVFA

% % % % % % % − − % % % mg/L mg/L

98.96 97.89 41.95 72.32 0.75 0.30 0.16 55.93 − 39.05 27.88 9.46 − −

2.50 70.57 27.36 41.17 4.32 1.54 1.24 6.33 7.35 − − − 9376 188.54

Figure 1. Schematic explanations of the (A) ARP and (B) LA pretreatments used for corn stover.

diluted with hot water, and the CS boiling process was carried out at 120 °C for 15 and 20 min for ARP-1 and ARP-2, respectively. The boiling solution was removed from the bottom of each crucible with the help of a suction pump, and it was recycled with a recirculation pump for the next batch of CS. The treated CS was washed five times with distilled water to remove the residual ammonia within the crucibles. The pretreatment details for the ARP method are provided in Table 2. For the LA pretreatment, an autoclave (vertical pressurized chamber autoclave, BXM-30R, Boxun Industry & Commerce Co. Ltd., Shanghai, China) was used. The first-stage pretreatment was carried out in a thermostatted water bath at 80 °C for 15 and 20 min for treatments LA-1 and LA-2, respectively, without any ammonia content. The second-stage pretreatment was carried out in an autoclave at 120 and 100 °C for 10 and 20 min for LA-1 and LA-2, respectively. An Erlenmeyer flask was used for the LA pretreatment process, and 25 g of dried CS sample was placed in it. A schematic diagram of the LA pretreatment process is presented in Figure 1B. The other pretreatment details are provided in Table 2 with severity factors. The severity factor22 for each pretreatment was calculated using the equation

a

TS, total solids; VS, volatile solids; TOC, total organic carbon; OM, organic matter; TN, total nitrogen; TP, total phosphorus; TK, total potassium; C/N, carbon-to-nitrogen ratio; COD, chemical oxygen demand; and TVFA, total volatile fatty acids.

2.2. Pretreatment Method. To enhance the anaerobic digestibility of the CS, the ARP and LA pretreatment method were carried out. Two levels for each treatment were selected, and the details are presented in Table 2. To obtain perfect homogenization, the dried CS was mixed with distilled water in a 1:6 ratio and heated at 120 °C for 15 and 20 min for treatments ARP-1 and ARP-2, respectively. A

Table 2. Pretreatment Details with the Severity Factors first-stage pretreatment

second-stage pretreatment including ARP and LA pretreatment process

treatment

NH3 (%)

heating temperature (°C)

heating duration (min)

ARP boiling temperature (°C)

heating duration (min)

autoclave temperature (°C)

autoclave pressure (MPa)

severity factor log Ro

ARP-1 ARP-2 LA-1 LA-2

5 7.5 5 7.5

120 120 80 80

15 20 10 15

120 120 0 0

15 20 10 20

0 0 120 100

− − 0.20 0.15

2.33 2.03 1.30 1.59

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Energy & Fuels Table 3. Chemical Characteristics of Corn Stover (CS) Determined after Each Pretreatment pretreatment ARP-1 ARP-2 LA-1 LA-2 control

cellulose (%) 39.90 41.31 41.40 41.46 39.05

⎡ ⎛ T − R ⎞⎤ ⎟ log R o = log⎢time exp⎜ ⎝ 14.75 ⎠⎥⎦ ⎣

± ± ± ± ±

2.11 2.88 0.66 1.04 0.03

a b b b a

xylan (%) 17.90 13.75 20.21 16.48 27.88

± ± ± ± ±

lignin (%)

0.65 0.36 0.31 0.31 0.87

e d c b a

5.73 5.29 6.49 7.23 9.46

COD change (%) = (1)

± ± ± ± ±

0.13 0.05 0.08 0.09 0.13

C/N ratio e d c b a

26.32 23.64 27.03 34.90 55.65

CODi − CODf × 100 CODi

± ± ± ± ±

0.32 0.86 0.48 0.70 0.38

c d c b a

(2)

where the subscripts i and f indicate the initial and final COD concentrations, respectively, during the anaerobic digestion process. 2.6. Data Analysis. For statistical analysis of the data, Statistics 8.1 software (Statistix 8.1, Tallahassee, FL) was used. A complete randomized design (CRD) was followed with the significance level of P < 0.05. An analysis of variance (ANOVA) was built to determine the significant differences between the chemical characteristics of the pretreated and untreated CS and their respective cumulative methane yields. Comparisons between the treatments were built using the leastsignificant-difference (LSD) pairwise comparison test. OriginPro 8.1 (OriginLab, Northampton, MA) software was used for the graphical representation of the data.

where time is in minutes, T is the pretreatment temperature in °C, and R is the reference temperature (100 °C). After the ARP and LA pretreatment processes, the CS was left in a fume hood overnight to remove its ammonical odor. Afterward, the pretreated CS was kept in the refrigerator at 4 °C for batch anaerobic digestion experiments and chemical composition determination. 2.3. Sludge Activation Process. Seed sludge was collected from a wastewater treatment plant of the Yangzi Petrochemical Company Limited, Nanjing, Jiangsu Province, China. For activation, the seed sludge was kept in the anaerobic digester for about 1 month at 37 °C, and 1.5 g/L of glucose was fed daily into the digester to improve the microbial growth. After the activation process, the seed sludge was removed from the digester, thoroughly mixed, and filtered through a 20-mesh screen to avoid feeding any foreign material into the anaerobic digesters.14 The chemical properties of the activated sludge are included in Table 1. 2.4. Batch Anaerobic Digestion Experiments. To evaluate the total methane production potentials of the treated and untreated CS, a batch mesophilic (37 ± 1 °C) anaerobic digestion process was carried out. One-liter batch digesters having an 800 mL working volume were used, and a 5% TS level was maintained into each digester. Each digester was provided with two ports: The first was connected to the gas holder, whereas the second was used to collect samples for analyzing process biochemistry parameters such as pH, soluble chemical oxygen demand (COD), total volatile fatty acids (TVFA), and alcohol production during the anaerobic digestion period. Between the digester and the gas holder, there was a gas collection port for collecting gas samples through syringes. The total produced biogas was measured by the brine-saturated water displacement method.14 The total biogas production and the biogas composition were determined every day with the digestate being withdrawn through the digesters at 3-day intervals to measure the pH, COD, TVFA, and alcohol production performance. Daily manual agitation was employed to avoid stratification in the digesters. All experiments were done in triplicates, and only activated sludge was also digested to determine its anaerobic digestibility performance and the exact methane production from the pretreated and untreated CS. 2.5. Analytical Procedures. Parameters such as TS, VS, TN, TK, TP, TOC, OM, and COD were measured according to the standard methods prescribed by APHA.23 For the daily biogas contents, gas chromatography (GC 9890A, Renhua, China) was used. This instrument has a thermal conductivity detector (TCD) with column specifications of Φ 4 mm × 1 m (Shimadzu, Kyoto, Japan). A gas sample 0.5 mL was injected into the gas chromatograph to analyze the gas composition, and hydrogen was used as the carrier gas. For the determination of TVFAs and alcohol production (GC-2014; Shimadzu, Kyoto, Japan) was used having a TCD and column specifications of DA, 30 m × 0.53 mm × 1 μm Stabilwax. The injector and detector temperatures of the GC instrument were 150 and 240 °C, respectively. To measure the chemical compositions of pretreated and untreated CS, a raw fiber extractor (VELP Scientifica, Usmate, Italy) was used, and Van Soest’s standard protocol was followed.24 For pH measurements, a digital pH meter (FE20K, Mettler-Toledo, Greifensee, Switzerland) was used. The percentage change in COD concentrations was calculated using the equation

3. RESULTS AND DISCUSSION 3.1. Effects of Pretreatment on the Chemical Composition of Corn Stover. The ARP and LA pretreatment modalities were employed in the present study for CS with two levels, differing in pretreatment conditions and ammonia concentration, and all pretreatments were found to be very effective in fractionating the CS and increasing its anaerobic digestibility. In all pretreatment processes, a 30% aqueous solution of ammonia was used because it causes swelling when in contact with fiber-based materials. The most prominent reaction of ammonia with the lignin is C−O−C bond cleavage and rupture in the lignin−carbohydrate complex (LCC).20 Although complete lignin removal is not feasible, the ARP process was found to remove up to 70−85% of the lignin content,20 whereas 70% delignification and 25% xylan removal were observed by Zhang et al.25 In the past, the ARP process has been used for hard chips,26 agricultural biomass,27 paper mill waste, and CS19,28 to study the enzymatic digestibility characteristics of these materials. The ARP process was found to be the most effective in increasing the anaerobic digestibility of CS because hot boiling water was also used before application of the ammonia dose (0.2 mL of NH3/g of CS) to CS in the crucibles, as shown in Figure 1A. The pretreatment details for the ARP process are provided in Table 2, and the pretreatment aftereffects are presented in detail in Table 3. Both levels of the ARP process were found to be significant in producing favorable conditions with enhanced anaerobic digestibility. The xylan removal efficiencies of the ARP-1 and ARP-2 processes were 35.82% and 50.69%, respectively, and the lignin removal efficiencies were 39.41% and 44.15%, respectively, as presented in Figure 3f below. The ARP-2 process was found to be most effective, with a severity factor of 2.03, and ultimately, this higher lignocellulosic degradation resulted in the highest methane production during the anaerobic digestion period. Another most important parameter for the anaerobic digestion period is the C/N ratio, which reflects an important transportation route in providing food for the microbial community during the anaerobic digestion period. The optimum value of the C/N ratio for anaerobic digestion is in the range of 20−30.20 The 9465

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Figure 2. (a) Daily methane production and (b) cumulative methane yield during whole anaerobic digestion cycle.

enhancement.35 All of the results of the present study are in accordance with those of these previous studies. The ARP and LA pretreatment methods are well suited to enhancing anaerobic lignocellulosic digestibility, but hardwoods and softwoods are more resistant to these pretreatments.36 The ammonia recycling process (ARP) significantly affects the biomass composition by achieving lignin depolymerization and the cleavage of lignin−carbohydrate linkages.36 Ultimately, this phenomenon strongly supports the conclusion that, after ARP pretreatment, more easily accessible biomass is generated for microbial feedings. Another pretreatment method, called ammonia fractionation of biomass, was reported to enhance the delignification of biomass,28 because lignin is the major hindrance in enzymatic digestibility36 and it also affects the hydrolysis step of the anaerobic digestion process. Because the hydrolysis step is the rate-determining step for lignocellulosic biomass, the ARP and LA pretreatment methods are employed to maximize xylan solubilization and lignin removal. To disrupt the reaction kinetics of the hydrolysis step, the ARP and LA processes could be promising options for achieving higher hydrolysis rates that could possibly lead to the production of more methane from CS. Figure 2a presents a chart of daily methane production, and Figure 2b illustrates the cumulative methane production during the whole anaerobic digestion period upon application of the ARP and LA pretreatment processes, along with the results for a control sample with no pretreatment. The methane production was recorded for about 40 days after almost negligible methane production was observed. For the ARP-1and ARP-2-pretreated CS, the cumulative methane productions were 298.42 and 319.78 mL/(g of VS), respectively, resulting in enhancements of 53.46% and 64.45%, respectively, compared with that of the untreated CS. The percentage methane enhancement capability of the each pretreatment is also presented in Figure 3e (below) with respect to that of the untreated CS. Peak methane production was found in the first week of the digestion period and overall in the first 3 weeks, and the maximum peak values were predominant, as shown in Figure 2a. The ARP-2-pretreated CS produced the maximum methane among all of the tested pretreatments; these results also confirm its highest xylan solubilization and lignin removal efficiency as reported in Table 3 as compared with the results for the control. The maximum methane productions of 18.01 and 17.15 mL/(g of VS) were recorded for the ARP-1- and ARP-2-pretreated CS samples, respectively, on the third day of the anaerobic digestion period.

aqueous ammonia pretreatment played an effective role in this regard: For the untreated CS, the C/N ratio was 55.65, whereas after pretreatment by the ARP process, 52.70% and 57.52% reductions were recorded in the C/N ratios for the ARP-1 and ARP-2 processes, respectively, as depicted in Figure 3f below. In the present study, the LA pretreatment process was carried out using process parameters such as temperature, ammonia loading, severity factor, and residence time as presented in Table 2 that were derived from previous studies.29−32 Both levels of LA pretreatment were found to be significant in enhancing the lignocellulosic characteristics of the CS. The overall percentage xylan and lignin removals for the LA-1 and LA-2 pretreatment processes were 27.50 and 40.90, respectively, and the percentage C/N reductions were 51.43% and 37.29%, respectively, compared to the value for the untreated CS, as shown in Figure 3f below. The LA-2 process was found to be effective in xylan solubilization, but the LA-1 process performed better than the LA-2 process in terms of lignin removal. Both pretreatment processes ARP and LA were thus found to be significantly effective in lignin and xylan removal for the lignocellulosic-based CS biomass, whereas the ARP-2 process was found to be the best for CS in terms of increasing its anaerobic digestibility and favorably contributing to the methane enhancement. 3.2. Effects of Pretreatment on the Daily and Total Methane Yields. In the present study, all of the pretreatments were found to be significant (P < 0.05) in enhancing the methane production under anaerobic digestion from 30.36% to 64.45% as compared to that of the untreated CS. In previous studies, the ARP and LA pretreatment processes were employed mostly for the production of fermentable sugars that accounted for ethanol production; to the best of our knowledge, this is the first time that the ARP and LA pretreatment processes have been carried out for CS to enhance the methane production. In the literature, an aqueous ammonia presoaking pretreatment was reported for switch grasses to produce methane. It was found to be effective in providing xylan and lignin removals of up to 35% and 41%, respectively, and 1.4 g of dry material/g of NH3 was provided after 5 days of pretreatment time.33 Another study reported that the pretreatment of rice straw with aqueous ammonia in a 1:3 ratio at room temperature led to a 17−100% biogas enhancement, when the NH3 concentration ratio was increased from 1% to 4%.34 Song et al. also reported an aqueous ammonia pretreatment for CS carried out at 2−8% in which all concentration levels were found to be significant in methane 9466

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Figure 3. Process biochemistry during the anaerobic digestion period: (a) Total volatile fatty acid concentration, (b) alcohol production profile during the anaerobic digestion period, (c) pH profile during the whole digestion cycle, (d) soluble COD profile during the anaerobic digestion period, (e) percentage increase in the methane yield due to different pretreatments and their respective COD removal efficiencies, and (f) percentage xylan and lignin removals due to different pretreatments and their respective C/N reductions.

recorded on the 13th day of the digestion period, and the overall digestion behavior of the LA-pretreated CS was found to be about the same as that of the ARP-pretreated CS, as shown in Figure 2a. All of the results of the present study clearly agree with those of previous research.34,35 One of the prominent advancements of the present study is the reduction in the overall anaerobic digestion period, with 75−80% of the methane production being recorded in the first 20 days of the anaerobic digestion period. The ARP-2 pretreatment was found

The LA-pretreated CS was also found to provide significantly enhanced methane production. The cumulative methane productions for the LA-1- and LA-2-pretreated CS were found to be 273.36 and 253.49 mL/(g of VS), respectively, which were 40.57% and 30.36% higher), respectively, than that of the untreated CS. The percentage methane enhancements for all of the pretreatments are elaborated in Figure 3e (below). The peak methane production for the LA-1 and LA-2 processes, 12.74 and 13.65 mL/(g of VS), respectively, were 9467

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100% consumption was found for all pretreatments and untreated CS. Figure 3b presents the alcohol production behavior of the treated and untreated CS samples during the anaerobic digestion period. The maximum values for alcohol production were 517, 623, 791, and 1105 mg/L for the ARP-1-, ARP-2-, LA-1-, and LA-2-pretreated CS samples, respectively. All peak productions of the alcohol were found on 13th day of the experiments for all pretreatments, whereas the maximum alcohol production was 1105 mg/L for the LA-2 pretreatment, as shown in Figure 3b. The trend in alcohol production for the anaerobic sludge was largely similar to that of the TVFAs, and minor changes were observed throughout the digestion period. The foremost requirement for methanogenic bacteria is an optimum pH value because fermentative bacteria are highly pH-specific. If there is a minor fluctuation in the pH value, the microorganisms are greatly affected, and their colonies start to decline. The optimum pH range for methanogenic bacteria is between 5.0 and 8.5.35 Figure 3c presents a pH chart for the whole anaerobic digestion period for the pretreated and control CS samples. The pH is mostly affected by any foreign basic or acidic substance such as detergent, soaps, oil, and grease and the lipid concentration of the waste sludge fed into the digester or because of the generation of intermediate products such as TVFAs. The TVFA production and pH were found to be inversely related to each other, and TVFA accumulation reduced the anaerobic digestion efficiency. This phenomenon occurs sometimes in large-scale biogas plants and can possibly stop biogas production. The pH value was found to be highly dependent on the initial pH value of the feeding materials and the pretreatment type used. In our pretreatment, ammonia, a basic reagent, was used, but different concentrations of ammonia were first checked to evaluate the optimum pH value before starting pretreatment. A maximum pH value of 7.96 was found for the LA-1 process after pretreatment, as shown in Figure 3c, whereas all of the other values were found to be within the range of 7.8−7.9. As the anaerobic digestion process started, a decreasing trend of pH was observed for all pretreated and untreated CS samples. The initial pH value for the untreated CS was about 7.51 because of the absence of any pretreating agent, and its final value was about 6.20. This change can easily be explained as resulting from TVFA production, as the two parameters are inversely related to each other. Minor pH fluctuations were observed for the anaerobic sludge and untreated CS, and almost all values were found to lie within the narrow ranges of 7.3−7.5 and 6.2−6.5, respectively. All initial pH values were about 8 for the ARP and LA pretreatments; these higher pH values provided the highest buffering capacity and higher pH resistivity during TVFA accumulation and activated the anaerobic digestion process that ultimately resulted in the methane enhancement. These results were also found to be in agreement with those of previous research.14,15 3.4. COD Profile during the Anaerobic Digestion Process. Soluble chemical oxygen demand (COD) is the most important characteristics of the waste anaerobic sludge and wastewater effluents and provides a measure of environmental pollution. In the present study, the waste anaerobic sludge was codigested with the pretreated CS; therefore, the COD profile of the overall digestion period is also significat in the study of the COD variation during the whole digestion period. To obtain better insight into the COD variations, the COD values were determined at each 3-day interval. The COD is a measure of the degradation factors and the stability of the performance

to be the most effective because it provided the highest xylan solubilization, lignin removal ability, and resultant methane enhancement. 3.3. TVFA, Alcohol, and pH Profiles during the Anaerobic Digestion Process. In the anaerobic digestion process for the lignocellulosic biomass, four stages are considered to control the overall digestion process. These stages are hydrolysis, acidogenesis, acetogenesis, and methanogenesis. As a result of hydrolysis of the biomass monomers, glucose, amino acids, peptides, and long carbon fatty acids are produced, and these intermediate products37 of the anaerobic digestion process are further disintegrated into volatile fatty acids (VFAs) and alcohols by the acidogenesis process.19 The total VFA and alcohol productions are the governing parameters for the anaerobic digestion process because they are responsible for pH fluctuations, the methanogenic population during the anaerobic digestion process, the buffering capacity of the sludge, and the overall anaerobic digestion efficiency.14,15 To obtain a better understanding of TVFA production, all of the possible forms of the volatile fatty acids such as acetic acid, propionic acid, butyric acid, valeric acid, isobutyric acid, and isovaleric acid were measured to optimize the whole anaerobic digestion process. Figure 3a presents a clear picture of TVFA generation with respect to the anaerobic digestion period. At the start of the digestion process, limited quantities of longchain fatty acids such as valeric acid, isovaleric acid, and isobutyric acids were found, but as the digestion process proceeded , the production of these long-chain fatty acids rose, whereas at the end of the digestion process, a declining behavior was again observed. The VFA with the most predominant production was acetic acid, and its production pattern was found to be almost the same as that of long-chain fatty acids for all pretreatments. In Figure 3a, the VFA chart presents production of TVFAs (sum of all of the types of VFAs) with respect to the digestion period. At the start of the digestion process, TVFA production was found to be very low. In the case of the ARP-pretreated CS, the peak TVFA production was recorded on the 16th day of the experiment for both levels of pretreatment. The peak TVFA production for the ARP-1 pretreatment was 8316 mg/L, and that for the ARP-2 pretreatment was 8599 mg/L. For the LA-1 pretreatment, the maximum TVFA production was 10835 mg/ L, and for the LA-2 pretreatment, it was 8294 mg/L, and both peaks were found in the second week of the digestion period. The TVFA production pattern for the untreated CS was found to be different from those of the pretreated CS samples, and its peak production was recorded on the seventh day of the experiment, as shown in Figure 3a. The maximum TVFA stabilization and consumption were observed for the ARP-2 pretreatment, as is clear in Figure 3a, which also shows its role in the maximum methane production. The TVFA production pattern for the anaerobic sludge was found to be the same at about 200−300 mg/L throughout the whole anaerobic digestion period, and this phenomenon can be well explained because of lack of any foreign substrate. Alcohol or ethanol production is also a side product of the produced TVFAs because of the acidogenesis process. These TVFA and alcohol molecules further disintegrate into simple acetates, hydrogen, and carbon dioxide.14 As a consequence, the maximum alcohol and TVFA production and consumption during the anaerobic digestion process were directly related to the maximum methane production. In the case of alcohols, 9468

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throughout the whole anaerobic digestion process, and it is a function of all of the intermediate products of the anaerobic digestion process such as monomers, long-chain fatty acids (LCFAs), total volatile fatty acids (TVFAs), alcohols, peptides, amino acids, and microbial communities. All of these intermediate byproducts are directly or indirectly responsible for the methane production and organics removal.15 A reduction in the COD value also means a reduction in the organic substrate availability for the microorganisms and the VS removal. Figure 3d presents the complete COD variations with respect to the anaerobic digestion period, and Figure 3e shows the percentage COD removals due to different pretreatments of the CS. At the beginning of the anaerobic digestion process, all of the COD values were within the range of 5500−7000 mg/L, but as the digestion period proceeded, the COD values started to increase for all of the pretreatments, as well as for the untreated CS. The peak COD values were observed during the 22nd day of the experiments, and values of 14016, 12512, 12352, and 11776 mg/L were recorded for the ARP-1-, ARP-2-, LA-1-, and LA-2-pretreated CS samples, respectively. The maximum COD value was recorded for the ARP-1-pretreated CS, whereas the lowest value of 11595 mg/L was found for the untreated CS, as shown in Figure 3d. The maximum COD value of 13088 mg/L was found for the anaerobic sludge on the 22nd day of the experiment. During the fourth week of the digestion period, the COD values started to decrease sharply, and they reached about the initial values at the end of the fourth week, as shown in Figure 3d. In the fifth and sixth weeks of the digestion period, the lowest COD values were recorded, at 3019 mg/L for the ARP-2-pretreated CS and 4032 mg/L for the LA-2-pretreated CS. The final COD value for the activated sludge was 1803 mg/L, as presented in Figure 3d. The maximum COD stabilization of 67.93% was recorded for the anaerobic sludge, whereas overall COD removal efficiencies of 42.46%, 49.46%, 39.16%, and 39.62% were calculated using eq 2 for the ARP-1, ARP-2, LA-1, and LA-2 pretreatments, respectively. For the untreated CS, the lowest COD removal efficiency of 23.40% was determined, which reflects its improper anaerobic digestion. The highest COD removal was observed for the ARP-2 pretreatment, which also verifies its maximum suitability for methane enhancement. In the present study, the organic waste removal, COD removal, and VS reduction were found to be significant and consistent with the results of previous research,14,15 thus demonstrating the suitability of these pretreatments for CS.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-25 5860 6502. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present study was supported by the National Science and Technology Support Program, P. R. China (2013BAD08B04). The principal/first author of the manuscript is grateful to the HEC Pakistan for providing an M.S. leading to a Ph.D. scholarship (HRDI-UESTPs/UETs Batch III).

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4. CONCLUSIONS During the present study, the ARP-2 pretreatment was found to be the leading pretreatment technology for CS because of its capability of 50.69% xylan solubilization, 44.15% lignin removal, and 64.45% methane enhancement as compared with the control. After the ARP-2 pretreatment, the ARP-1 and LA-1 processes were the best pretreatment methods, providing 39.41% and 31.38% lignin removals, respectively, and 53.46% and 40.57% methane enhancements, respectively, as compared with the results for the untreated CS. Anaerobic sludge supported the maximum methane production along with the highest COD removal efficiency of 67.93%. The ARP and LA pretreatment processes also reduced the carbon-to-nitrogen ratio of the CS, leading to significant methane enhancements. 9469

DOI: 10.1021/acs.energyfuels.6b01745 Energy Fuels 2016, 30, 9463−9470

Article

Energy & Fuels

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DOI: 10.1021/acs.energyfuels.6b01745 Energy Fuels 2016, 30, 9463−9470