Methane enhancement and asynchronism minimization through co-digestion of goose manure and NaOH solubilized corn stover with waste activated sludge

Energy xxx (2016) 1e8

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Methane enhancement and asynchronism minimization through codigestion of goose manure and NaOH solubilized corn stover with waste activated sludge Muhammad Hassan a, Weimin Ding a, *, Muhammad Umar b, Kunlun Hei c, d, Jinhua Bi c, d, Zhendan Shi e a

College of Engineering, Nanjing Agricultural University, Nanjing, Jiangsu 210031, China Department of Food Engineering, University of Agriculture, Faisalabad 38000, Pakistan College of Resource and Environmental Science, Nanjing Agricultural University, Nanjing, Jiangsu 210095, China d Institute of Agricultural Resources and Environment, Jiangsu Academy of Agricultural Science, Nanjing, Jiangsu 210014, China e Institute of Animal Science, Jiangsu Academy of Agricultural Science, Nanjing, Jiangsu 210014, China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 March 2016 Received in revised form 29 October 2016 Accepted 2 November 2016 Available online xxx

Anaerobic co-digestion of corn stover (CS) and goose manure (GM) was carried out in the present study at four composition levels. Corn stover was pretreated to enhance its lignocellulosic digestibility. The NaOH pretreatment effect on the chemical composition of the corn stover was also determined and the methane production from all the composition levels was found significant (P < 0.05) as compared with the control. The cumulative methane production of treatment C2 (0.6 CS: 0.4 GM), C3 (0.4 CS: 0.6 GM) and C4 (0.2 CS: 0.8 GM) were 86.1%, 92.1% and 83.1% enhanced as compared with the control respectively. On the basis of the experimental results, it was concluded that a C/N ratio between 20 and 30 was found optimum to enhance methane production. Asynchronism minimization was observed for all the treatments. Process chemistry of the whole co-digestion process like total volatile fatty acids (TVFAs), alcohol production pattern, pH, soluble chemical oxygen demand (CODs), total available ammonia (TAN) and free available ammonia (FAN) were deeply monitored. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Alkali solubilized corn stover Asynchronism minimization Carbon to nitrogen ratio optimization Methane enhancement

1. Introduction With the growing population of the world and huge urbanization in the developing countries, waste generation trends have multiplied during the past decades. In addition to agricultural activities, commercial poultry production is also a common rural life style within the rural areas worldwide. Goose meat is considered one of the most favorite protein sources in China and almost 93% of the world goose is reared in China [1], resulting in the annual production of millions of tons of goose manure. As the leader of the agricultural production, China also produces 216 million of tons of corn stover annually, and about half the straw production is burnt in the fields in standing conditions [2]. Burning of these crop residues and direct use of livestock manure as fertilizer in the agricultural fields emits a high concentration of methane, carbon

* Corresponding author. E-mail address: [email protected] (W. Ding).

dioxide and nitrogen oxides that contribute greatly to greenhouse gas (GHG) emissions [2,3]. In such a situation anaerobic digestion technology emerges as a promising option with useful by product as methane [3,4] and is gaining more importance than the other conventional energy resources used on a commercial scale. Anaerobic digestion is basically a microbial and biochemical process [5,6] where a mixture of gases consisting mostly of methane and carbon dioxide is produced [7]. Anaerobic digestion process consists of four steps; hydrolysis, acidogenesis, acetogenesis and methanogenesis while hydrolysis is considered as AD rate limiting step in agricultural biomass utilization [6,8]. All types of agricultural biomass are rich in organic contents, protein, fats, carbohydrates and cellulose that can be a suitable feedstock for anaerobic digestion and hydrolyzed further to produce methane as a final product with the help of different microbial activities [9]. Anaerobic digestion is the most suitable and environmental friendly option [4] to reduce the risk of carbon and nitrous oxide emissions from organic waste along with the energy and bio-fertilizer production [7,10,11].

http://dx.doi.org/10.1016/j.energy.2016.11.007 0360-5442/© 2016 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Hassan M, et al., Methane enhancement and asynchronism minimization through co-digestion of goose manure and NaOH solubilized corn stover with waste activated sludge, Energy (2016), http://dx.doi.org/10.1016/j.energy.2016.11.007

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M. Hassan et al. / Energy xxx (2016) 1e8

Corn stover is considered as a high lignin product, which is a major hindrance by reducing anaerobic efficiency and enzymatic hydrolysis. If lignocelluloses are heated at a higher temperature, the first hemicellulosic structure breakdown would occur and then shortly lignin starts solubilizing and may result in the production of phenolic acids but overall, thermal pretreatment of the lignocellulosic biomass has resulted in a promising technology to enhance the digestibility of the corn stover [12]. Alkaline pretreatment could enhance solubilization, lignin condensation, redistribution and cellulosic crystallinity modification [13]. However, the anaerobic digestion process was found to be very sensitive to change in pH, temperature, C/N ratio, and process efficiency by possibly maximizing within the narrow band of these parameters. Lignocellulosic biomasses are considered to have high lignin and carbon contents while livestock manures are found to be rich in nitrogen contents [14]. Because of an unbalanced C/N ratio and poor microbial growth, the mono-digestion of corn stover and goose manure resulted in poor anaerobic digestion efficiency. Due to a low C/N ratio of goose manure, it can possibly inhibit the anaerobic digestion process while high carbon content in the corn stover resulted in improper digestion. Corn stover consists of complex plant structure and high lignin content which are a major hindrance during cellulosic hydrolysis [4]. The carbon to nitrogen (C/N) ratio is one of the most important parameters used for the proper microbial community growth, and its optimum range of 20e30 has been reported by many researchers [15,16]. Co-digestion of corn stover with goose manure provided a feasible platform to reduce the risk of process inhibition, buffering capacity, and pH fluctuation during the AD process [4]. Different research was carried out in past experiments with codigestion in order to optimize the C/N ratio of livestock manure. Yang et al. [4] reported dry anaerobic co-digestion of livestock waste with an organic fraction of household waste and agricultural biomass and about 150% methane enhancement was recorded. Zhou et al. [17] reported solid state anaerobic co-digestion of stackpretreated corn stover with cow dung and 40.7% enhanced methane yields was recorded as compared with the control. In another study, 99% improved methane production was found when alkaline pretreated wheat straw was co-digested with a different composition of the cow manure [10]. Another study based on the optimized feeding composition for wheat straw and chicken manure resulted in a maximum methane production at C/N of 27.2 [16]. Chicken manure was co-digested with three crop residues to avoid ammonia inhibition during the AD process with 93% improved methane production [18]. However, almost no research was found based on the co-digestion of thermal-NaOH pretreated corn stover with goose manure. The basic objective of the present study was to enhance methane production and C/N optimization with the co-digestion of thermal-NaOH pretreated corn stover and goose manure. A significant research gap was found in existing literature which explained the co-digesting substrate interaction; therefore, substrates asynchronism was studied in detail. To enhance the anaerobic efficiency of the corn stover, it was thermally pretreated with NaOH and pretreatment effects were also evaluated. All process parameters like TVFAs, alcohol production pattern, TAN, FAN, pH, CODs, VS removal percent and CODs removal in percent were monitored during the AD period. 2. Material and methods 2.1. Experimental feedstock collection and sludge activation Fresh harvested corn stover was collected from a commercial agricultural farm in Pukou, Nanjing Jiangsu province, China and

fresh goose manure was provided by Changzhou goose production farm, Changzhou, Jiangsu province China for experimentation. Seed sludge was provided by a commercial biogas plant located in the vicinity of Pukou, Nanjing China. Corn stover was air dried in the open sunshine and further it was oven dried at 105
C for about 24 h afterwards it was chopped into 2e3 cm pieces. These chopped pieces were further milled by desk type continuous grinder (LH08B, China) and less than 1 mm size of the corn stover was achieved. This mechanical pretreatment helped to obtain a homogeneous size of the corn stover in order to get the maximum surface area in contact with NaOH during the thermal pretreatment. The homogenized corn stover was utilized for thermal pretreatment process and chemical composition of the corn stover and goose manure was determined before storing in the refrigerator at 4
C and is shown in Table 1. The fresh collected seed sludge was kept in 20 L anaerobic digester and activated with daily glucose dose of 2 g L1$day1 for 15 days [13]. After the activation process, the waste sludge was withdrawn from the digester and sieved through 0.5 mm polyester screen in order to remove all the grits and foreign substances. The activated sludge was further used to determine the chemical compositions and batch anaerobic digestion tests. The chemical composition of the waste activated sludge is shown in Table 1. 2.2. Pretreatment of the corn stover In order to enhance the anaerobic digestion efficiency of the corn stover, it was thermally pretreated with 7.5 g NaOH/100 g CS. 50 g of the oven dried corn stover was homogenized with distilled water in a ratio of 1:6 respectively and heated at 80
C for 15 min. After adding NaOH into the mixture, it was autoclaved for 20 min at 100
C, 0.10 MPa with a severity factor of 1.30 (BXM 30R, vertical chamber, steam autoclave, China). The thermochemical pretreatment procedure was previously developed [13]. The chemical compositional changes after the pretreatment were determined and are shown in Table 1. 2.3. Experimental setup and batch AD tests Four different compositions of the pretreated corn stover and goose manure were designed; 0.80:0.20, 0.60:0.40, 0.40:0.60 and 0.20:0.80 on m/m basis, respectively. Resulting C/N ratios due to different compositions were evaluated and are described in Table 2. Mono-digestion of the pretreated corn stover and goose manure were also adopted and untreated corn stover termed as control during the whole experiment. Each experiment was run in triplicate and 1 L laboratory scale fabricated digesters were used for the batch anaerobic digestion tests at mesophilic condition (37 ± 1)
C and 5% TS level was maintained in each digester. Each digester was connected with the gas holder and a sampling port was installed between digester and gas holder to collect biogas samples for further analyses. Each digester contained sampling port through which digestate was withdrawn at three days interval to determine the anaerobic digestion characteristics and process biochemistry parameters of the experiment. The activated sludge was also digested without introducing any substrate to calculate the exact methane production from the different compositions and check the biochemical process profile of the waste activated sludge. Brine saturated water displacement principle was utilized to measure daily biogas production. Each digester was agitated once daily to reduce the stratification phenomena. Biogas production and contents were determined on the daily basis while other process parameters were measured at three days interval from the liquid withdrawn digestate.

Please cite this article in press as: Hassan M, et al., Methane enhancement and asynchronism minimization through co-digestion of goose manure and NaOH solubilized corn stover with waste activated sludge, Energy (2016), http://dx.doi.org/10.1016/j.energy.2016.11.007

M. Hassan et al. / Energy xxx (2016) 1e8

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Table 1 Chemical compositions of the goose manure (GM), pretreated and untreated corn stover (CS) and waste activated sludge (mean values ± standard deviation). Parameters

Units

GM

CS untreated

CS Pre-treated with NaOH

Activated sludge

TS VS TN TP TAN FAN TOC OM C/N Cellulose Hemicellulose Lignin pH CODs TVFAs Protein

% % % % mg/L mg/L % % e % % % e mg/L mg/L %

29.9 ± 3.1 66.0 ± 6.6 2.3 ± 0.0 1.0 ± 0.0 1732.5 ± 80 90.2 ± 15.2 37.7 ± 1.9 65.1 ± 3.3 16.3 ± 1.1 e e e 7.7 ± 0.0 8360 ± 990 e 13.9 ± 0.3

98.7 ± 0.5 97.6 ± 0.2 0.8 ± 0.0 0.3 ± 0.1 e e 42.5 ± 2.0 73.2 ± 0.0 56.3 ± 2.5 44.0 ± 0.0 26.9 ± 0.9 9.9 ± 0.5 e e e e

97.6 ± 0.4 98.9 ± 0.3 0.8 ± 0.0 e e e 30.7 ± 4.6 52.9 ± 0.1 37.6 ± 4.5 49.9 ± 0.2 18.5 ± 0.5 5.7 ± 0.2 e e e e

2.1 ± 0.3 83.8 ± 0.6 3.2 ± 0.1 1.0 ± 0.3 1284 ± 155 59.8 ± 14.7 37.9 ± 2.0 65.3 ± 0.0 8.8 ± 0.3 e e e 7.8 ± 0.1 3605.3 ± 530 345 ± 23.7 e

Table 2 Anaerobic digestion characteristics and performance details. Treatment

Composition ratio CS:GM

Respective C/N ratio

Cumulative CH4 production ml/ g.VS

% CH4 enhancement

% VS removal

% CODs removal

CS C1 C2 C3 C4 GM Control

1:0 0.8:0.2 0.6:0.4 0.4:0.6 0.2:0.8 0:1 Untreated CS

37.6 33.3 29.1 24.8 20.6 16.3 56.3

293.1 353.4 380.4 392.6 374.2 328.0 204.4

43.4 72.9 86.1 92.1 83.1 60.5 0.00

49.9 54.1 57.3 63.4 58.9 48.7 32.2

68.4 59.6 75.6 73.6 66.9 63.6 43.7

2.4. Sampling and analytical methods The TS (total solids), VS (volatile solids), TN (total nitrogen), TP (total phosphorous), TOC (total organic carbon), OM (organic matter) and CODs (soluble chemical oxygen demand) were determined according to the standard protocols defined by APHA [19]. Methane contents were determined by GC (Renhua China; GC9890A,) equipped with (TCD) thermal conductivity detector with column specification of (Ф 4 mm  1 m, Shimadzu, Japan). The 0.5 ml of the biogas samples was injected for methane content determination. Hydrogen was used as the carrier gas during biogas content analysis. For volatile fatty acids and alcohol production (GC-2014; Shimadzu, Japan) equipped with thermal conductivity detector (TCD) of column specification (DA- Stabilwax; 30 m  0.53 mm  1 mm) was used. The detector and injector temperature of the GC was kept at 240
C and 150
C respectively. The pH values of the digestate were determined by a digital pH meter (Mettler-Toledo Switzerland; Model-FE20K). Chemical characteristics of the corn stover, were measured by standard Van Soest's method [20] by using Raw Fiber Extractor of (VELP Scientifica, Italy). The corn stover was boiled with neutral detergent fiber (NDF) solution, acidic detergent fiber (ADF) solution and 72% H2SO4 for cellulose, hemicellulose and lignin content determination of the corn stover respectively as prescribed by Van Soest, et al. [20]. During experimentation, the total ammonia nitrogen (TAN) concentration was measured with total nitrogen analyzer (Lianhua Technological Company, Ltd, China) while free ammonia nitrogen FAN was estimated by using Equation (1) [21,22].

" FAN ¼ TAN 1 þ

#

10pH 10ð0:09018þ

Þ

2729:92 Tk

1

(1)

where, FAN (g/L) is the free ammonia contents and TAN (g/L) is the total ammonia nitrogen contents. The CODs removal was calculated by the following equation referred by Hassan et al. [13] and VS removal (%) was estimated by Equation (2) [21].

3  VSdigestate  100  VSfeed 5  100  VS removal ð%Þ ¼ 41  VSfeed  100  VSdigestate 2

(2)

2.5. Data analysis ANOVA (Analysis of Variance) was performed by using Statistix 8.1 (Statistix 8.1; Tallahassee, Florida, USA) to estimate the significant differences between pretreated and untreated corn stover and their respective methane production. Complete randomized design (CRD) was carried out at P < 0.05 while comparison of treatments means was developed by using least significant difference pairwise comparison analysis. For graphical representation of the research data, OriginPro 8.5 of (Originlab, USA) software was used. 3. Results and discussion 3.1. Asynchronism minimization and AD performance of the pretreated corn stover Thermal-NaOH pretreatment was proven significant (P < 0.05) to reduce the lignin contents, asynchronism and hemicelluloses solubilization of the corn stover. Chemical characteristics of the pretreated corn stover were measured and results are presented in Table 1. Thermo-NaOH pretreatment was found effective enough to

Please cite this article in press as: Hassan M, et al., Methane enhancement and asynchronism minimization through co-digestion of goose manure and NaOH solubilized corn stover with waste activated sludge, Energy (2016), http://dx.doi.org/10.1016/j.energy.2016.11.007

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M. Hassan et al. / Energy xxx (2016) 1e8

reduce the lignin contents by up to 42.4% and hemicelluloses solubilization by up to 31.0% with 24.6% shifts of cellulose from denser fibers to the modified cellulosic network. Another advantage of the pretreatment was 33.3% reduction of C/N ratio as presented in the Table 1. In a previous research, lignin removal up to 42% was observed by 7.5% thermal-NaOH pretreatment [13] and this was confirmed by the present study. NaOH pretreatment proved effective to cleave the lignin carbohydrate linkages, structural swelling, depolymerization, reduced crystallinity index and destruction of the lignin matrix [13], thus lead to enhanced methane production. The results of this research were consistent with those by He et al., who determined a 54% hemicelluloses solubilization and 45% lignin removal when rice straw was pretreated with 10% NaOH [23]. Asynchronism should be minimized to enhance the anaerobic efficiency of the corn stover and pretreatment where NaOH proved helpful in enhancing methane production [24]. After pretreatment, a significant reduction in C/N ratio was also observed that provided a pivot role in changing anaerobic digestion behavioral characteristics of the corn stover. If comparison was made between the digestion curves of the untreated and the pretreated corn stover, a clear contrast could be seen as shown in Fig. 1. The daily methane production curve for control was found more fluctuating than the CS curve, and on the 3rd, 7th and 11th day of the digestion period mostly methane generation peak resulted. The distance between the curves was found to be 4 days in both peak values but in case of pretreated corn stover, the highest methane production of 22 ml/g VS was observed in the first day of the digestion period. Furthermore, minor fluctuations in methane production were observed between the 5th and 9th day of the experiment but overall asynchronism was reduced to a significant level due to thermal-NaOH pretreatment of the corn stover. The narrow gap between the curve peaks meant the enhanced anaerobic digestibility of the corn stover and significant reduction in asynchronism [24]. This phenomenon was also observed and verified in the present study. 3.2. Detailed asynchronism investigation and effects of C/N optimization on the daily and cumulative methane production of the different compositions During the anaerobic digestion experiment, it was revealed that the mono-digestion of goose manure resulted in higher methane yields than obtained with the pretreated corn stover. The phenomenon could be attributed to the fact that the livestock manure had higher biodegradability because of the readily convertible proteins and carbohydrate contents than the complex lignin

hemicellulosic matrix of the corn stover [24]. As conclusion, different compositions of both substrates were introduced for anaerobic digestion as detailed in Table 2 to reduce the asynchronism. Considering the anaerobic digestion of goose manure, a low C/N ratio was found that possibly could inhibit the anaerobic digestion process by production of residual and free ammonia which produces toxic agents for the methanogens. Therefore, a proper nutrient balance for microorganism balance was considered very important to enhance the anaerobic digestion efficiency; hence, co-digestion played a vital role in this scenario. It was concluded from the different previous studies that the optimum value of C/N should be between 20 and 30 to achieve higher anaerobic digestion efficiency [15,16,25]. The C/N ratio optimization due to co-digestion also proved a helpful technique in asynchronism minimization. Daily and cumulative methane productions along with the digestion period are shown in Fig. 1. Four different compositions of the corn stover and goose manure were designed having C/N ratio of 33.3:1, 29.1:1, 24.8:1 and 20.6:1. In the case of pretreated corn stover, almost three peaks were observed on the 3rd, 7th and 11th day of the anaerobic digestion period with four day interval between peak curves. While, in the case of pretreated corn stover, a lower peak difference was noticed and in the case of treatment C1, the daily methane production curve was found to be more fluctuating than the control but the overall methane production was higher than obtained from the pretreated corn stover. In discussing the AD behavior of the treatment C1, it should be noted that almost 5 methane production peaks were recorded on the 4th, 10th, 13th, 16th and 18th day of the experiment with peak gaps of 6 days, 3 days, 3 days and 2 days respectively. While considering the treatment of C2, only two peaks were observed on the 5th and 18th day of the digestion experiment with a peak gap of 13 days. The longest peak gap was observed with this composition but cumulative methane production was found higher than treatment C1. Furthermore, while considering the asynchronism of the treatment C3, it should be noted that an almost gradual and symmetrical digestion curve was found as compared with all the other treatments along with the highest cumulative methane production. When compared to the treatment C4, three methane production peaks were recorded on the 5th, 13th and 16th day of the experiment with an interval gap of 8 and 3 days respectively and the overall curve was found less fluctuated as compared with the control. Generally speaking, co-digestion and pretreatment were found to be very effective techniques in reducing the asynchronism of the corn stover and goose manure and all of the experimentation of this research was found inconsistent with that of previous researchers [24,26,27].

Fig. 1. Daily (a) and cumulative (b) CH4 production (ml/g.VS) profiles with respect to the anaerobic digestion period.

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Daily and cumulative methane production curves are shown in Fig. 1 (a) and (b) respectively, and relative % methane enhancement, cumulative methane production, and respective C/N ratio are described in Table 2. The treatment C3 remained prominent with 92.1% methane enhancement criteria followed by the treatment C2, C4 and C1 with 86.1%, 83.1% and 72.9% enhanced methane production as compared with the control. Cumulative methane production for pretreated corn stover and goose manure was found to be 43.4% and 60.5% more enhanced than the control respectively. In conclusion, co-digestion not only proved effective to reduce asynchronism but also had a significant role in methane enhancement due to C/N ratio optimization. It was also concluded that a wide range of C/N ratio between 20 and 30 was found significant for methane enhancement. While, a C/N ratio of 24.8:1, proved the most effective to increase methane production by 92.1% as compared with the control. The highest daily methane production peaks were mostly found on the 5th day of the experiment. The methane production curve for the treatment C2 was found more fluctuated but a smooth methane production that was observed for the treatment C3 as shown in Fig. 1 as compared with control. Maximum daily methane production of 25.4 ml/g VS was recorded for the treatment C4 and the lower methane production of 22.7 ml/ g VS was observed for treatment C1 within the optimized compositions. Additionally, more than 80% methane was produced in the first 20 days of the digestion period and it was considered to be another advantage of the present work, with a reduced digestion time. This trend was found more significant within the optimized composition rather than for the mono-digestion of the corn stover or goose manure. After the 30th day of the experiment less than 3 ml/g VS of methane were generated for all the treatments. The cumulative methane production of all the co-digestion compositions were found higher than the mono-digestion of corn stover and the goose manure, and this phenomena was also attributed to the enhanced buffering capacity of the co-digestion system [28]. Furthermore, in explanation of the asynchronism behavior during anaerobic co-digestion, it was found very complex and not only dependent on the composition ratio but also dependent on many inter-process parameters like TVFAs, alcohol production trends, interaction of the substrates and process biochemistry of the digestion period that needs further exploration. Therefore, it is recommended to further study the asynchronism of the different substrates that could be co-digested to enhance the anaerobic digestibility performance. 3.3. TVFAs and alcohol production pattern during the AD process Anaerobic inter process production of the volatile fatty acids and alcohol production behavior during the anaerobic digestion period are very important to this study, and were found to be responsible for the smooth production of methane and carbon dioxide. As a result of the hydrolysis of fats and proteins of the substrate monomers and long chain fatty acids produced further disintegrated to short chain fatty acids and alcohol molecules during the acidogenesis step of the anaerobic digestion. These fatty acids and alcohol molecules produced further converted into methane, carbon dioxide and traces of water vapor during the acetogenesis and the methanogenesis step of anaerobic digestion [13,16]. As a result, higher TVFAs were found during consumption by the methanogens which resulted in enhanced methane production. In the present study, TVFAs and alcohol production trends during the whole anaerobic digestion period were studied in detail every three day interval and production and consumption trends are presented in Fig. 2 (a) and (b) respectively. The TVFAs were composed of six types of low chain volatile fatty acids methanoic acid, propionic acid, butyrate, iso-butyrate, pentanoic acid and

5

delphinic acid. Acetic acid was found as predominant specie during the startup of experiment, and the other acidic species production also rose significantly as the AD proceeded. During the third week of the anaerobic digestion period, almost the maximum TVFAs production was observed for all the compositions and the mono digested CS and GM. The sudden rise in the TVFAs concentration was predominant during the second week of the anaerobic digestion period, and maximum TVFAs consumption appeared during the fourth week of the anaerobic digestion process. In the sixth week of the anaerobic digestion period, the lowest values of TVFAs, between 3000 mg/L and 4000 mg/L were observed for all the treatments. Graphical rise and fall characteristics of TVFAs graph had shown maximum similarity with their respective methane production curves as can be observed in Fig. 1 (a) and 2 (a). During the experiment, treatment C4 was found having the maximum TVFAs production of 15202 mg/L on the 19th day of the experiment followed by the GM, C2, C1 and C3 with 14196 mg/L, 14091 mg/L, 13818 mg/L and 13309 mg/L respectively. During the first week of the digestion period, the lowest values of about 500e700 mg/L were observed for all the treatments. The maximum TVFAs consumption of 74.8% was found for the treatment C1 followed by C4, C2 and C3 with 72.9%, 65.4% and 60.7% respectively. These results also had justified by their respective role in methane production during the co-digestion [13]. Furthermore, the maximum TVFAs production of 12335 mg/L, 12980 mg/L and 14196 mg/L were recorded for the pretreated, untreated corn stover and goose manure with about 70% TVFA consumption by the microbial communities respectively. In considering the waste activated sludge, it was observed that the lower TVFAs production and consumption were observed along the digestion period and this phenomenon also verified the lack of substrate within the activated sludge. The maximum TVFAs production of 2238 mg/L was found on the 16th day of the experiment with 81% TVFAs conversion efficiency. During the acetogenesis of the monomers and syntrophic bacterial activities, alcohol molecules were also being produced and they were also considered as the intermediate products of the anaerobic digestion process. These syntrophic bacterias readily converted the easily digestible glucose or ethanol molecules into methane, carbon dioxide, hydrogen and water molecules by the homo-acetogenesis process and the syntrophic acetate oxidation process [13]. Like TVFAs, the alcohol production trends were deeply studied for all the treatments and results are shown in Fig. 2 (b). The maximum alcohol production of 1904 mg/L was observed for the treatment C3 followed by 1873 mg/L, 1633 mg/L and 1611 mg/L for C4, C2 and C1 treatments respectively. The alcohol and TVFAs production trends were found almost similar for all the treatments. In the second week of the digestion period, a sudden rise in the productions of alcohol was observed with almost all peaks in the third week while at the end of the experiment all alcohol peak values were found within the range 200e250 mg/L. While discussing the waste activated sludge results, it was concluded that limited values were found due to substrate non-availability. The explanation for the alcohol production and consumption was found very complex; therefore, further research should be focused in these areas. 3.4. TAN, FAN, CODs and pH behavior of the different compositions during the AD process Livestock manures were considered consisting of the high total ammonia nitrogen contents because of the higher uric acids production and inter convertible undigested proteins during the anaerobic digestion process [16,29]. Goose manure was utilized in the present study and the manure was found to have a lower C/N

Please cite this article in press as: Hassan M, et al., Methane enhancement and asynchronism minimization through co-digestion of goose manure and NaOH solubilized corn stover with waste activated sludge, Energy (2016), http://dx.doi.org/10.1016/j.energy.2016.11.007

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Fig. 2. Anaerobic co-digestion process parameter details of the different compositions (a) Total volatile fatty acids in mg/L(b) Alcohol production profile in mg/L (c) Total ammonia nitrogen contents (TAN) in mg/L (d) Free ammonia nitrogen (FAN) contents in mg/L (e) pH variations and (f) Chemical oxygen demand (COD) in mg/L along with the digestion time.

Please cite this article in press as: Hassan M, et al., Methane enhancement and asynchronism minimization through co-digestion of goose manure and NaOH solubilized corn stover with waste activated sludge, Energy (2016), http://dx.doi.org/10.1016/j.energy.2016.11.007

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ratio, therefore, there was a significant chance that ammonia inhibition could take place during anaerobic digestion. Thus, an external carbon source, corn stover having high C/N ratio, was introduced with co-digestion methods in order to reduce the risk of ammonia accumulation during the anaerobic digestion period. Total ammonia concentration varied between 1500 mg/L and 3000 mg/L which could inhibit the methanogenic activity while a concentration of more than 3000 mg/L could cease methane production during anaerobic digestion. The most dangerous and unwanted form of nitrogen was free ammonia nitrogen (FAN) produced during the digestion period that could inhibit microbial activity and the optimum range was found to be between 100 mg/L and 150 mg/L and higher concentration of FAN could cease methane production during the anaerobic digestion period [16,22]. The TAN and FAN profile are illustrated in Fig. 2 (c) and (d) respectively. The TAN was determined every three days while FAN values were estimated by using Equation (1). Co-digestion played a vital role in reducing the TAN and FAN generated during the digestion period and this phenomenon was verified from our experimental results. Maximum TAN and FAN values of 3274 mg/L and 181 mg/L were observed, respectively, for the goose manure on 16th day of the experiment. During the first two weeks, a slight increase in the TAN and FAN were noticed, but in the third week of the digestion period, a rapid increase in TAN and FAN were found with almost all peak values. Afterwards, decreasing behavior was predominant in the next weeks of digestion and at the end; all values were slightly higher than the initial values of TAN. But in the case of FAN, the decreasing trend was found more significant than for TAN values in the last three weeks of the digestion period. The maximum TAN values for treatments CS, C1, C2, C3, C4, GM and control were 1364, 2175, 2365, 2730, 2798, 3274 and 1783 mg/L respectively. As the concentration of the goose manure rises, TAN and FAN values also increase as shown in Fig. 2 (c) and (d); therefore, co-digestion is recommended and suggested for the industrial scale digesters. Methanogenic bacteria are highly pH sensitive and they could function between 6.5 and 7.8 [13,30]. The pH values are shown in Fig. 2 (e). The TVFAs and pH values were proved to have an inverse relationship with each other and this theory was also confirmed from the present study [13]. Co-digestion not only improved cumulative methane production but it also provided a better buffering capacity of the whole anaerobic digestion system. The initial pH values for all the four compositions were between 7.7 and 7.8 and the end values were 7.2 and 7.3 respectively. The lowest pH value of 7.0 was observed as the end value of the untreated corn stover. Thus the whole co-digestion system provided an excellent approach in maintaining the buffering capacity during the anaerobic digestion period. Chemical oxygen demand is also an important process parameter during anaerobic digestion. In the early stages of the digestion period, the goose manure and corn stover were hydrolyzed and different products like proteins, peptides, glucose, alcohols, long and short chain fatty acids and monomers were produced. All of these by products were directly or indirectly involved in the methane production. Soluble chemical oxygen demand (CODs) was basically the measurement used to determine the existence of all these products and their transformation into other products [13,30]. The CODs production profile is shown in Fig. 2 (f). At the start of the second week of the digestion period, a rapid rise in the CODs values was observed and almost all peak values were found in the 10th day of the experiment. Afterwards, a gradual decrease in the CODs was found with respect to the digestion period, and at the end of the experiment, all values were found within the range of 3000 mg/L. The maximum CODs of the CS, C1, C2, C3, C4, GM and control were 12000, 18213, 16227, 18147, 14707, 16333 and

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13800 mg/L respectively. The lowest CODs were measured for the waste activated sludge because no substrate was added to it. In conclusion, the higher CODs and the respective stabilization confirmed the theory that the volatile solids of the substrate were hydrolyzed and degraded by methanogenic activities [13,30]. 3.5. The VS and CODs removal efficiencies for the different compositions Anaerobic digestion is basically a phenomenon of the organic solids reduction and conversion into methane and carbon dioxide with the help of microbial activities. The corn stover and goose manure consisted of highly volatile solids, and with the help of microbial and biochemical processes, these volatile solids were further disintegrated to methane and other gases [13,30,31]. Codigestion and pretreatment technologies basically helped microbes to increase the rate of volatile solids consumption that ultimately accounted for higher methane production. The volatile solids and CODs removal are shown in Table 2. The CODs removal also meant the consumption of organic and volatile solids through the utilization of the microbial activities. In the present work, the highest volatile solids removal of 58.4% was observed for treatment C3. This phenomenon also confirmed its highest methane production behavior. The compositions C1, C2 and C4 originated 54.1%, 54.3% and 55.9% VS removal while CS, GM and control had 49.9%, 48.7% and 42.2% VS removal as presented in Table 2. The lowest VS removal was observed in the control. The maximum CODs of 75.6% was reached in the treatment C2, hence it was confirmed the suitability of the co-digestion for enhanced methane production. As the waste activated sludge, the CODs removal was found to be 51.4% which supported and justified the adoptability of the co-digestion of substrates with waste activated sludge. All of our experimental results were consistent with previous studies [16,25,31e33]. 4. Conclusions Considering that during the AD of goose manure, high TAN was present; therefore, a C/N optimization was highly recommended. Adding an external carbon source like CS was introduced in the present study. Co-digestion of the GM with CS provided an excellent approach in minimizing the asynchronism, and risk of ammonia inhibition with enhanced methane production observed. All compositions were found significant (P < 0.05) to enhance the methane production. The maximum methane production was reported for treatment C3, which was of 92.1% higher, as compared with the control at C/N ratio of 24.8. Co-digestion of agricultural waste with the waste activated sludge or high strength waste water can be a possible option to treat the bulk of agricultural wastes in rural areas, with enhanced methane production. Acknowledgements The present work was financially supported by Jiangsu Agriculture Science and Technology Innovation Fund (CX(15)1008) and the National Science and Technology Support Program (2013BAD08B04). We are grateful to the HEC Pakistan for providing Master leading to PhD scholarship (HRDI-UESTPs/UETs Batch III) to the first author of this manuscript. References [1] Pingel H. World Poult 2004;20(8):26e8. [2] Fu SF, Wang F, Yuan XZ, Yang ZM, Luo SJ, Wang CS, et al. The thermophilic (55
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Nomenclature

Abbreviations CS: corn stover GM: goose manure AD: anaerobic digestion TS: total solids [%] VS: volatile solids [%] TVFAs: total volatile fatty acids [mg/L] TAN: total available ammonia [mg/L] FAN: free available ammonia [mg/L] CODs: soluble chemical oxygen demand [mg/L] TN: total nitrogen [%] TP: total phosphorous [%] TOC: total organic carbon [%] OM: organic matter [%] C/N: carbon to nitrogen ratio VELP: a scientific instruments company of Italy ANOVA: analysis of variance CRD: complete randomized design

Please cite this article in press as: Hassan M, et al., Methane enhancement and asynchronism minimization through co-digestion of goose manure and NaOH solubilized corn stover with waste activated sludge, Energy (2016), http://dx.doi.org/10.1016/j.energy.2016.11.007