Anaerobic Codigestion of Alkali-Pretreated Prosopis juliflora Biomass

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Biofuels and Biomass

Anaerobic Co-digestion of Alkaline Pretreated Prosopis juliflora biomass with Sewage Sludge for Biomethane production Amudha Thanarasu, Karthik Periyasamy, Jason Thamizhakaran Stanley, Kubendran Devaraj, Premkumar Periyaraman, Anuradha Dhanasekaran, and Sivanesan Subramanian Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00836 • Publication Date (Web): 24 Jul 2019 Downloaded from pubs.acs.org on July 25, 2019

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Anaerobic Co-digestion of Alkaline Pretreated Prosopis juliflora biomass with Sewage Sludge for Biomethane production

Amudha Thanarasua, Karthik Periyasamya, Jason Thamizhakaran Stanleyb, Kubendran Devaraja, Premkumar Periyaramanc, Anuradha Dhanasekaranc, Sivanesan Subramaniana*

aDepartment

of Applied Science and Technology, Anna University, Chennai, Tamil Nadu, India

bDepartment

of Chemical Engineering, Anna University, Chennai, Tamil Nadu, India

cDepartment

of Biotechnology, Anna University, Chennai, Tamil Nadu, India

Corresponding author E-mail: [email protected]; Phone No: +91-4422359168

ACS Paragon Plus Environment

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ABSTRACT The sustainable waste management and energy generation from the lignocellulosic biomass is the predominant methodology for meeting the energy requirement globally. Lignocellulosic biomass of Prosopis juliflora (PJ) can be utilized to produce biomethane with the co-digestion of activated sludge, a promising alternative energy source. PJ is a perennial, deciduous shrub and an invasive weed in most of the countries and a source of abundant organic material. Organic waste utilized in Anaerobic Digestion (AD) process becomes the need of the hour for environmental protection and for energy generation. In this study, NaOH pretreatment of PJ was performed to remove maximal lignin content and increases the substrate porosity with the least sugar loss. The batch anaerobic digestion of carbon-rich substrate PJ study was conducted at a mesophilic temperature of 37 ± 2 ᵒC under six different feedstock (PJ) proportion. The physicochemical properties of the feedstock revealed the presence of nutrients needed by methanogenic bacteria. The maximum biogas yield from 6 different ratios of feedstocks were 15.0±4.5, 24.5±7.3, 45.0±13.5, 32.5±9.7, 21.5±6.4 and 15.5±4.6 mL/g VS for 100:0, 80:20, 60:40, 40:60, 20:80 and 0:100 respectively. Highest cumulative methane yield of 587.3 mL/g VS was seen in the feedstock ratio 60:40 (PJ:SS) as a result of an increase in the microbial activity due to the presence of optimal C/N ratio. Scanning Electron Microscope (SEM), X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Thermogravimetric (TGA) analysis were used to study the structural and biochemical changes of pretreated biomass. Furthermore, the modified Gompertz model fitted with the experimental data and used to determine the kinetic constants. The correlation coefficient of optimized feed concentration (60:40) R2 was = 0.997.


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KEYWORDS Prosopis juliflora, Alkali Pretreatment, Anaerobic digestion, Sewage sludge, Modified Gompertz model and Biomethane.

1. INTRODUCTION Energy generation from renewable and synergistic waste products are one of the prominent needs in the current sustainable energy scenario. It has been discussed and was instigated to reach the Global net production of the alternative energy fuel by the year 2035 especially for most of the transportation sector. The renewable energy sector attains its golden era because of the stringent regulations and laws implemented by the global summits, apropos to reduce the major problems like global warming, greenhouse gas emission and climatic drift. The lignocellulosic biomass such as agricultural residues (sugarcane bagasse, wheat straw, corn stover), dedicated crops (miscanthus, switchgrass) and forest product (hardwood and softwood) are abundant renewable source and presumed to be a sustainable feedstock for biogas production by anaerobic codigestion. Prosopis juliflora (PJ) is the source of lignocellulosic biomass used as a key substrate in the present study. The tree is broadly distributed over the Middle East, India, Nigeria, Sudan, Somalia, Senegal and South Africa and also in the southeastern United States regions1. PJ engrosses more than 4 litres of water to attain 1kg of biomass and makes a hostile attack in the cultivable land2 and it also absorbs humidity from the air when there is less availability of groundwater. One of the main reasons for water dearth and consumption of groundwater is this PJ and this tree is also not suitable for birds to shelter. It was reported that the investment required to remove all PJ trees from Tamil Nadu, India, is around 15 billion USD which is way far for a state to effect the change3. Burning of PJ as fuelwood is not an eco–friendly process, so PJ considered


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to be an intimidating ecological threat4. The composition of PJ, however, consists primarily of cellulose (60-65%), hemicellulose (15-20%) and lignin (20-25%) content3. For this reason, the main objective of the current study is to determine the feasibility of converting the lignocellulosic biomass into sustainable production of biogas by anaerobic digestion (AD) process. AD is a sequential biological process which involves biodegradation of organic matter in the absence of oxygen through microorganisms5,6 converting all biomass waste such as Municipal Solid Waste (MSW), agricultural crop waste, tannery fleshing waste, livestock manure and other organic waste into biogas which can be converted into bioenergy (e.g. heat and electricity)7. Biogas from biomass is the leading renewable source of energy fulfilling about 10% of energy requirement globally8,9. Pretreatment was employed to reduce the biomass recalcitrance prior to the AD process. The pretreatment of lignocellulose biomass (PJ) consisting of cellulose, hemicellulose, and lignin was used in increasing the surface area of substrates by removing the lignin content and also acts as a stimulating step for the conversion of lignocellulose biomass to biofuels10. Pretreatment of lignocellulose is important to attain a high biogas yield in the degradation process11. Due to its complexity and different chemical structure, efficient pretreatment is required for easy hydrolysis process making cellulose and hemicelluloses more accessible to the hydrolytic microorganisms6,12. Moreover, pretreatment is used to speed up the hydrolysis process and subsequently reduces the hydraulic retention time (HRT) for the anaerobic digestion11. Generally, various pretreatment techniques are executed to alter properties like the removal of lignin, reduction of crystallinity, etc. to improve the biodegradability of lignocellulosic biomass 1. The pretreatment of lignocellulosic biomass has been carried with various methods such as physical13, chemical14, physico-chemical16, biological18 and combined pretreatments like mechanical and enzymatic with inclusive of different techniques15. Here, as mentioned a combination of two pretreatment techniques has been done i.e.


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of mechanical comminution in which the size reduction was employed with milling and also of chemical pretreatment using NaOH. This combined pretreatment which falls under the combined pretreatment is way better to other pretreatments method in the aspect of its operation cost, reduction in crystallinity index to a greater extent and also of effective delignification on consideration to time, whereas in other pretreatments anyone of the mentioned aspects or criteria is lacking to wider aspect. Thus, with respect to the above-mentioned features, the combined pretreatment of comminution and chemical pretreatment is carried for the delignification of PJ. Pretreated PJ can be co-digested with methanogen rich activated sludge obtained from a sewage treatment plant (STP) in order to increase the biogas production19. The largest by-product of wastewater treatment plants is the sludge and its disposal/reuse is the most challenging environmental problems in wastewater treating processes20. AD improves stabilization of the sewage sludge and in the end, all easily accessible biomass has been degraded by microorganisms which in turn lowers the biological activity 21. Furthermore, it was observed that there is a reduction in the pathogens count and weed seeds present in the sludge (Lukehurst et al., 2010). It was observed that there is a significant decrease in the odour emission once the sludge is stabilized, which is an advantage in the biomass utilization from agricultural sources 22. The aim of the current work is to study the influence of the alkaline pretreatment process over the biomass to improve the feedstock characteristics and its effects on the Biochemical Methane Potential (BMP) and also to validate the importance of co-substrate addition in improving the methane production and the outcome of different ratios of PJ:SS for methane production on batch AD reactors. Finally, a kinetic study was carried out using a modified Gompertz model from experimental data.


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2. MATERIALS AND METHODS 2.1 Raw Material Procurement Prosopis juliflora was collected from Anna University campus (13.0103° N, 80.2318° E) located at Tamil Nadu, India. Subsequently, the biomass PJ was crushed in the ball mill in order to reduce the particle size around 5-7 mm. These sizes were considered for extraction of lignin content in an intact form after the alkaline pretreatment procedures and were used as a substrate for the biogasification. The fresh sludge was obtained from the Sewage Treatment Plant (STP), College of Engineering, Anna University, and it contained rich methanogenic bacteria and was used in inoculum preparation. Methanogens activated sewage sludge (MAS) was developed by the addition of newly prepared micronutrients (g/L) such as ((NH)4HPO4-0.2; CaCl2.2H2O-0.1; NH4Cl-0.25; MgCl2.6H2O-0.2; KCl-0.08; MnCl2.4H2O-0.1;COCl2.6H2O-0.05; H3BO3–0.05, CuCl2.2H2O-0.04, NH4MoO4-0.01, ZnCl2-0.05, FeCl2.4H2O-0.2 and Na2S.9H2O- 0.05 onto the obtained sludge21. All the reagents used were of analytical grade and procured from Merck. The pH of AD system plays a substantial role for the metabolism and growth of microorganisms. Though the methanogenic bacteria can tolerate wide range of pH, however the optimum pH of 6.87.3 to attain more growth. Thus, the inoculum prepared was maintained in optimum pH range (6.8 to 7.2) by the addition of sodium bicarbonate to keep the stable pH condition throughout the AD process. Then this biomass was characterized by using standard methods of APHA (2005), various parameters such as volatile fatty acids (VFA), pH, alkalinity, total Kjeldahl nitrogen (TKN), soluble chemical oxygen demand (SCOD), volatile solids (VS), total solids (TS) and moisture content (MS) were analyzed before and after fermentation process.


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2.2 Chemical Pretreatment of PJ Generally, alkaline pretreatment process was carried out by using KOH, Ca (OH)2 and NaOH to remove cellulose, hemicellulose and lignin so as to degrade the lignocellulosic biomass for better utilization by the microorganisms and enzymes in the AD 6. In the present work, sodium hydroxide (NaOH) was used in the pretreatment of PJ. Pretreatment (delignification) consisted in mixing of 100 g of dried ball milled PJ in 1 L of 2.5 % NaOH (w/v) in a closed bottle and was autoclaved at 121°C (at saturated water pressure) for 30 min, afterward it was cool down to room temperature and then the cooked PJ was washed with distilled water until it attains to a neutral pH (7.0 ±0.2). Then the mixture was filtered on a sintered glass crucible of fine porosity (porosity index of 4). Finally, the biomass was then pressed and air dried at 70°C until it obtains a constant weight and was stored at 4 ºC. The carbohydrate (cellulose & hemicelluloses) and lignin fractions of the untreated and NaOH treated PJ samples were analyzed according to the National Renewable Energy Laboratory (NREL) analytical procedure23, as follows, the dried PJ biomass (0.3g) sample was mixed with 3 mL of 72 % (w/w) sulfuric acid at room temperature, the mixture was stirred occasionally with the glass rod for one hour and then 84 mL of de-ionized water was added to the mixture and autoclaved at 121 °C for 1 h. After cooling, the mixture was filtered on a sintered glass crucible of fine porosity (porosity index of 4). Monomeric sugars such as glucose, xylose content was analyzed by High-Performance Anion-Exchange Chromatography with Pulsed Amperometric Detection (HPAEC-PAD, Dionex ICS 5000) equipped with a CarboPac PA 10 (250x4 mm, Dionex) column.

2.3 Experimental setup and design Anaerobic digester of laboratory studies was conducted in a batch system using Schott Duran


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GL 45 glass bottle with the working volume of 400 mL (Fig.S1. “Electronic supplementary data”). The bottles were hermetically sealed with a rubber septum and equipped with a capillary silicone tube. Six different feed ratios were used for PJ: SS to carryout methane productions and are 100:0, 80:20, 60:40, 40:60 and 20:80, 0: 100 (see. Table 1.), eventually based on the volatile solids (VS), nitrogen gas was purged into the reactor to maintain the initial anaerobic condition. Table 1: Feedstock ratio Mixing ratio

Prosopis juliflora (gm)

Sewage sludge (gm)

water (mL)

100:0

300

0

100

80:20

320

80

0

60:40

240

160

0

40:60

160

240

0

20:80

80

320

0

0:100

0

400

0

2.4 Biochemical Methane Potential (BMP) Test The Biochemical methane potential (BMP) test was carried out under mesophilic temperature (35 ± 2 ºC) for 45 days of retention time. All reactors were shaken manually twice a day. Biogas production was measured by Alkaline displacement method24 for every 2 days. Each bottle was connected to the aspirator bottle which contains 1M NaOH through the silicone tube. The outlet tube was immersed in 1000 mL beaker to quantify the displacement of NaOH. Thus, the displaced amount of NaOH was the same as the amount of biomethane produced. The pH of the substrate in the reactors was measured using a pH meter for every 2 days. These processes were done very quickly to maintain the same condition for methanogenic bacteria in the reactor.


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Triplicates were maintained for the statistical implication of all the reactors. The VS removal and VFA concentration were calculated using the following equation 1 and 221. VSr(%) = VFA =

VSinitial ― VSfinal

(1)

VSr(g)

𝐴 × 0.05 𝑚𝑜𝑙/𝐿 × 40 𝑉 × 𝐷𝐹

(2)

× 1000

Where Sr = removal efficiency of volatile solids in the feedstock; VSinitial = initial concentration of Volatile Solid(g); final = final concentration of VS (g); followed by A is the volume of NaOH spent (mL), 40 is the molecular weight of NaOH, V is the volume of sample taken (mL) and DF is the dilution factor. 2.5.Characterization of untreated and alkali-treated PJ 2.5.1. Scanning Electron Microscopy analysis The surface morphology of alkaline treated and untreated PJ was studied using a Scanning Electron Microscope (TESCAN VEGA3) using 20 kV accelerating voltage. Samples of untreated and pretreated PJ were prepared by drying it at 60 °C until a constant weight was attained. The dried samples were then placed on the carbon tape of the SEM stub and coated with gold.

2.5.2. Fourier Transform -Infrared Spectroscopy analysis The chemical and structural changes of pretreated and untreated PJ were characterized by FTIR analysis. Initially, the samples were dried at 75 ᵒC in an oven and then placed on the sample holder of FTIR spectrometer (BRUKER ALPHA) over a wavelength range of 4000 cm-1 to 400 cm-1.


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2.5.3. X-ray diffraction (XRD) analysis The crystallinity index (CrI) of untreated and alkali-treated PJ biomass samples was analyzed by XRD (X’Pert PRO MPD®, PAN analytical, Netherlands). The samples were analyzed in a step-scan mode with 2θ (Bragg angle) between 2° and 55°, step size 0.067°, counting time 90 s and Cu Kα radiation. The crystallinity index (CrI) was determined by the following equation25.

CrI(%) =

(

I002 ― Iam I002

) × 100

(3)

Where 𝐼𝑎𝑚 represents the amorphous portion at 2θ = 18° and I002 is the maximum intensity at 2θ = 22° 2.5.4. Thermogravimetric Analysis (TGA) Thermogravimetric analysis was carried out by using Thermogravimetric analyserSTA6000 (Perkin-Elmer Instruments, England). About 15 mg of samples were taken and heated to a temperature of about 105 °C for 15 min for the complete removal of water. A fine powder of 5-10 mg of untreated and alkali treated PJ sample was taken in an aluminum crucible. The analysis and dynamic TG scans were recorded in a temperature range from 30°C to 600°C at a heating rate of 10°C/min. A nitrogen atmosphere was supplied at the flow rate of 20 mL/min. 2.6. Statistical Analysis The bio-kinetics constants such as M and Rm were determined by using the modified Gompertz model through MATLAB R2015a. One-Factor analysis of variance (ANOVA) was used to check the consequence of experimental results using p≤0.03 using Origin 2018. Triplicate was maintained for all the experiments and the outcomes were specified as a mean standard deviation.


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2.7. Kinetic study for biogas production Kinetic study of the AD process generally represents the specific growth rate of methanogenic bacteria in anaerobic digesters during the degradation process. Modified Gompertz kinetic model is the widely used model to predict methane production using the BMP process. The equation of the modified Gompertz model was given below26: y (t) = ym.exp { ― exp

[

U.e ym

]

((λ ― t) + 1 }, t ≥ 0

(4)

Where, y(t) = specific methane yield at a given time (mL/g VSadded) ym = maximum methane potential (mL/g VSadded) U = maximum methane production rate (mL/g VSadded) e = mathematical constant (2.71828) t = digestion time since the startup of BMP test (d) 𝛌 = lag phase time (h)

3. RESULTS AND DISCUSSION 3.1. The influence of NaOH pretreatment on PJ The composition of untreated and pretreated PJ was analyzed for cellulose, hemicelluloses, lignin, and ash contents on a dry basis and results are tabulated in Table.2. The results suggesting that, around 15% of acid insoluble lignin was removed when treated with 2.5% NaOH at 121°C


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for 30 min. As shown in Table 2, pretreatement could cause loss in cellulose and hemicelluloses by 3% and 5% respectively. Previous works done by Iberahim N I et al, 2013 also agree that NaOH (6%) at 70°C gave the best result in hydrolysis of oil palm mesocarp fibre pretreatment and yielded increased total reducing sugar prodyction during hydrolysis process27. Similarly, 49% of cellulose, 25% of hemicelluloses were preserved and 14 % acid insoluble lignin were removed when the corn stover treated with 5% of NaOH at 116 °C for 48 min28. Table. 2. Composition of Untreated and NaOH treated PJ Contents (%) *

Composition

Untreated PJ

NaOH treated PJ

Cellulose

45 ± 1.5

42 ± 1.5

Hemicelluloses

25 ± 1.0

20 ± 1.0

Acid Insoluble Lignin (AIL)

23 ± 1.2

8 ± 0.8

Ethanol extractive

2 ± 0.5

6 ± 0.5

Ash

5 ± 0.5

4 ± 0.5

*Standard deviation is representative of triplicates.

3.2.Characterization of untreated and alkali-treated PJ 3.2.1 FTIR Analysis of Prosopis juliflora FTIR spectrum of the treated and untreated PJ was shown in Figure.1. Comparing the FTIR spectra of the untreated wood and the pretreated PJ indicates the structural change in its hardness. The wavenumber at 3330 cm-1 and 3340 cm-1 is recognized as the stretching of –OH groups. While there is only a slight variation in the wavenumber range of 2355 cm-1 which represents the presence of cellulose. The band between 1400 cm-1 to 1500 cm-1 and 1650 cm-1 to 1700 cm-1 are generally


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attributed to the stretching of C-H and C=O in carbonyl functions, which indicated the presence of aldehydes, ketones, carboxylic acids and esters. The strong variation observed at 1740 cm-1 refers to the acetyl group in hemicelluloses, which appears only in the untreated PJ wood. Treated PJ indicates that heating in alkaline medium eliminates all esters. The region between 1100 cm-1 and 1400 cm-1 shows much variation between treated and untreated PJ. The presence of aromatic C=C stretching (1430 cm-1) and different C–C and C–O bonds in lignin tend to indicate that delignification takes place by the alkaline cooking medium. The biodegradability of lignocellulosic biomass is depending on the amount of lignin present in the biomass. The aromatic (C–H) stretching vibration in lignin is obtained at 890 cm-1 and 920 cm-1 which decreased significantly after cooking of crushed PJ11. The results are evidenced that NaOH pretreatment was efficient in removing lignin. The volatility of the biomass was determined by the presence of amines (3000-3300cm-1) carboxylic acid (3300-2500 cm-1) and hydroxyl-substituted compounds present in the sample29.

2355

NaOH Pretreated

1740 1436 1603

892 918

3338

Untreated PJ

2355

1500

892

3330

918 1410

Wavelength (cm-1)

Figure. 1. FTIR of Pretreated and Untreated Prosopis juliflora wood


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3.2.2. SEM of Prosopis juliflora The Scanning Electron Microscope (SEM) analysis revealed the changes in the internal morphological structure of treated PJ as a result of alkaline pretreatment method. The SEM images of treated and raw PJ is depicted in Fig.S2 (a) and (b). From the study, it was clearly found that the untreated PJ was highly compact and has a smooth surface structure. However, the pretreated PJ appeared pores and separated cellulose fibrils, resulting in the effective pretreatment process and the lignin removal. 3.2.3. XRD analysis of PJ The X-ray diffractogram pattern of the untreated and alkali treated PJ is shown in Figure.2. The peaks showed at 2θ =15.9° and 2θ = 22° was attributed to the amorphous (Iam) and crystalline (I002) part of the cellulose, respectively. The minimum intensity peaks showed at 2θ=15.9 is the indication of the presence of lignin, hemicellulose and amorphous cellulose. The CrI value of untreated and alkali treated PJ were found to be 31% and 36.5% respectively. The curve fits well that of pure cellulose I. Thus, it attributes that NaOH pretreatment has eliminated much of the amorphous polymers, like lignin and hemicelluloses, and probably also a lot of other small molecules like oligomers and soluble extractives. During the NaOH pretreatment, removal of amorphous substances, and also possibly, partial rearrangement or recrystallization of cellulose molecular chains, the crystallinity of the remained cellulose becomes quite high. This leads to a much significant increase of the heat resistance to degradation, as shown in the TGA analysis (Fig. 3 (a) & (b)). It implied that NaOH believed to disrupt glycosidic bonds of polysaccharides and hydrolysable linkage in lignin which causes the increase in the crystallinity index. The higher CrI values of pretreated PJ improved the biodegradability by the bacterial communities and increasing the biogas yield30.


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Figure.2. XRD of untreated and alkali-treated PJ 3.2.4. Thermogravimetric analysis of untreated and alkali-treated PJ Thermogravimetric analysis was used to study the thermal behavior of untreated and alkali treated PJ wood. The thermograms (TGA) and (DTG) are shown in Fig.3 (a) and (b) as weight loss and derivative of the thermogravimetric as a function of temperature, respectively. The pyrolysis process of untreated and alkali treated PJ indicated four different regions at different temperature rates. Indeed, in the untreated PJ, progressive degradation of lignin and hemicelluloses are well shown, by the progressive weight loss, since about 200°C (meaning progressive degradation) of the sample, before the final step of cellulose degradation at the highest temperature of about 340°C. At this final temperature, the content of organic matter keeps rather high (about 30%), probably due to highly resistant high molar mass cellulosic chains in the highly crystallized part of the original uncooked cellulosic sample. It is interesting to note that alkali treated PJ highly stable until 300 °C, the temperature at which cellulose starts to degrade. Then most of the degradation is continued until 350°C, the higher likely for the more resistant chains (highly


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ordered, long and crystalline). Such a small range of temperature at which thermal degradation occurs (about 10°C between 340 and 350°C), and the rather low value of residual matter (20% by weight) after cellulose degradation shows that thermal degradation is, in this case, a rather homogeneous process, meaning that the sample is made of rather ordered and homogeneous matter of homogeneous molar mass (probably much reduced compared to that of the longer chains in the original uncooked sample), and homogeneous level of crystallinity. The major components of PJ cell structure such as lignin and cellulose were dematerialized quickly at this temperature range whereas hemicellulose was degraded during the first region. The derivative weight loss was utmost same in both untreated and alkali treated PJ in the third region. In the fourth region, no weight loss has occurred. Hence, the complete delignification of untreated and alkali treated PJ occurred at 200-300 ᵒC and 260-340 ᵒC. So approximately 43% of lignin in the untreated PJ was unrefined/unprocessed after 300 ᵒC. At low temperature, the high heating resistance of lignin precludes the further degradation process31.

Figure.3. TGA (a) and DTG (b) of untreated and alkali-treated Prosopis juliflora


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3.3 Initial characterization of feedstock The physicochemical characteristic of feedstock (PJ and SS) before degradation and after alkaline pretreatment process are presented in Table 3. From the data, the substrate PJ had high solid content (96%) and that did not exceed 97%. The high VS content in the PJ (88 ± 2.6%) and SS (85 ± 2.5%) indicates that it has a significant influence in biogas production by AD process. The biogas production is directly proportional to the reduction in biomass and that was indicated by VS reduction rate32. The total amount of lignin, cellulose and hemicellulose present in the PJ was 85 ± 2%. Lignin is not easily biodegradable material, unlike cellulose which is highly degradable under anaerobic environment conditions33. Extracellular enzymes and multiple cellulolytic enzymes produced from cellulose degrading bacteria are the main sources to achieve efficient degradation13. The amount of carbohydrate of the substrates PJ and SS are 65 ± 2% and 23.8±1.7% respectively, which specify PJ and SS have a high concentration of carbon resource and thus it contributes more carbon to methane (CH4) production. The substrate PJ contains less protein value and therefore an external source of nitrogen is essential for effective digestion. So, the PJ was digested with sewage sludge which contains 25.6% nitrogen. The maximum methane production is obtained by optimum carbon to nitrogen (C: N) ratios ranging from 25 to 3034. The major process parameters which influence the biogas production are alkalinity, VFA, pH, sCOD. Treated PJ had a good value of alkalinity and pH as well as less value of VFA. Pretreated PJ data was showing the ability of feedstock for biogas production. The initial value of these parameters for sewage sludge was noted. The results are expressed as the mean ± standard deviation and N/A represents non-measurable result was obtained in some parameters.


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Table 3. Initial characterization of treated Prosopis juliflora and Sewage sludge S.NO

Parameter

Prosopis juliflora

Sewage sludge

1.

TS (%)

32 ± 0.9

25 ± 0.7

2.

VS (%)

88 ± 2

85 ± 2.5

3.

Ash (%)

2.1± 0.1

8 ± 0.1

4.

Carbohydrate (%)

65 ± 1.9

53.8

5.

Protein (%)

5 ± 0.3

25.6

6.

Lignin (%)

23 ± 0.7

N/A

7.

Hemicellulose (%)

25 ± 0.8

N/A

8.

Cellulose (%)

45 ± 1.4

N/A

9.

COD (mg/L)

N/A

21817 ± 54

10.

Alkalinity (mg/LCaCO3)

2000 ± 60

4000 ± 120

11.

TKN (mg/L)

4.5 ± 0.1

780 ± 23

12.

VFA (mg/L)

85 ± 2.6

17.1 ± 0.5

326 ± 9.8

224 ± 6.7

8 ± 0.6

8.9 ± 0.4

ORP (Oxidation Reduction 13. Potential) (mV) 14.

pH

3.4 Parameter variation in all reactors during the AD process The variation of process parameters such as pH, VFA, SCOD, VS and alkalinity during the degradation process after alkaline pretreatment of PJ was illustrated in Figure.4. pH is one of the important parameters for the stability of the AD process because its variation affects the


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microorganism growth during the degradation process35. The variation in pH of all ratios (100:0 0:100) during the AD process was indicated in Figure.4 (a). The pH of activated sludge was adjusted to 7.3 ± 0.3 before the fermentation process. The changes in pH trend were observed for 45 days of HRT and it ranges between pH 6.3 to 7.0. The pH range should be within 6.0-8.5 because higher alkalinity of methanogens is toxic in nature36. At the early stage, the pH drop was observed in all concentration of feedstock which might be due to the rapid accumulation of organic acid during the starting period of the digestion process. During the initial days, the activity of fermentative microbes is greater than methanogenic and acetogenic microorganisms37. At this point, the acid production was high as represented in Figure.4 (d). The feed ratio 100:0 and 60:40 had a high VFA of 2605 ± 78 and 2487 ± 74 mg/L respectively than another ratio at the initial stage. At this same stage, the alkalinity was reduced and SCOD was increased due to the activity of hydrolytic enzymes and the variation are shown in Figure.4 (c) and (e). The initial sCOD range of all feed ratio of 100:0 to 0:100 (PJ: SS) was range between 24506 to 35986 mg/L. Then the pH reached a stable condition due to the production of alkalinity and utilization of VFA during the intermediate stage of the AD process. The methane-producing bacteria consume the volatile acids and produce methane and CO2 38. The highest VFA reduction was obtained in the ratio of 60:40 and 40:60. That was due to the complete conversion of substrates to methane production. The biogas production is directly proportional to the amount of biomass loss from the anaerobic process39. The final value of sCOD of all feedstock lay between 4600 to 15230 mg/L with deduction of about 84 and 75%. The feed ratio 60:40 had reached the highest alkalinity value of 4250 mg CaCO3/L and followed by the ratio of 40:60 (3125 mg CaCO3/L). The feed ratio 60:40 and 40:60 are the optimum source of carbon and nitrogen than the other ratios. The nitrogen and carbon contents are significant nutrients for new bacteria formation40. The amount of


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biodegradation of biomasses was indicated by VS reduction rate (on the basis of %TS) and the trend was shown in Figure.4(b). The feed ratio 60:40 was achieved highest VS reduction rate of 93% and the lowest VS reduction rate was obtained in the feed ratio of 100:0. (a)

(b)

(d)

(e)

(c)

Figure.4. Variation of process parameters in relation to methane production

3.5. Daily and cumulative methane production from untreated PJ:SS vs Alkaline pretreated PJ:SS Figure.5 (a) and (b) represents the daily and cumulative methane production from untreated PJ:SS, respectively. From the observation, it can be concluded that untreated PJ:SS found high methane yield of 2.3 ± 0.1 mL/g VS from the feed ratio of 60:40. The cumulative methane production of all ratios such as 100:0, 80:20, 60:40, 40:60,20:80 and 0:100 were yields of 11.72 ±


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0.7, 10.4± 0.6, 12.7 ± 1.2, 6.93 ± 1.01, 8.05 ± 0.5 and 6.75 ± 0.8mL/g VS respectively. It is worth mentioning that methane production from the untreated PJ:SS was very low due to the presence of lignin and it gives protective sheathing and hydrophobic nature. Moreover, lignin detains cellulose accessibility to microbes and enzymatic attacks, thus leading to low cellulose and hemicelluloses degradation. On the other hand, experiments carried out with alkaline pretreated PJ:SS showed improved methane production than untreated one and the results are shown in Figure.5 (c) and (d). The highest methane production of 15.0±0.4, 24.5±0.7, 45.0±1.3, 32.5±0.9, 21.5±0.6 and 15.5±0.5 mL/g VS was obtained from alkaline pretreated PJ: SS with the ratios of 100:0, 80:20, 60:40, 40:60, 20:80 and 0:100, respectively. As shown in Figure.5 (d) the cumulative methane production was reached 349.4 ± 10.4(100:0), 369.4 ± 11.0(80:20), 587.3 ± 17.6(60:40), 446.3 ± 13.4(40:60), 412.6 ± 12.4(20:80) and 340.1 ± 10.2(100:0) mL/g VS respectively. This result suggests that more than 90% of enhanced methane production was observed with respect to untreated PJ:SS. The NaOH pretreatment of PJ bonds between cellulose, hemicellulose and lignin. The porosity of lignocellulosic biomass was increased due to the partial removal of lignin. Rich carbon substrates of PJ are low in nutrient sources such as nitrogen, phosphorus and trace elements and that was balanced by the co-substrate of sewage sludge (SS). These balanced nutrient ratios present in the substrate helped to achieve an effective AD process. It was noticed that the ratio 60:40 (PJ:SS) showed the highest methane yield followed by the ratio 40:60. However, at the initial stage, the methane production was inhibited (Fig.5(c)) by the dominant activity of hydrolytic microbes and they have produced more volatile acids. The methane production was increased by consumption of volatile acids by acetogenic and methanogenic bacteria after the 10th day of the degradation process. The pretreated PJ with feed ratio of 100:0 and 80:20 had low nitrogen source, therefore the anaerobic bacteria present in the biomass could not form their cell structures and the cell growth


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was very less. In the meantime, the feed ratio of 20:80 and 0:100 contained a low C/N ratio and was not in the optimum range. Therefore, the feed ratio of 100:0, 80:20, 20:80 and 0:100 were produced less biomethane yield of 15, 24, 21 and 15 mL/g VS respectively. At the same time, the feed ratio of 60:40 and 40:60 had an optimum C/N ratio and generated high biomethane yield of 45 and 32 mL/g VS respectively. The optimal range of the C/N ratio in the anaerobic degradation process was influenced by the optimum feed ratio. The methane production from alkali pretreated PJ wood had a higher value than other alkali pretreated plants such as Birch wood (25mL/g VS) and pine wood (14 mL/g VS). Similar studies have been reported on alkaline pretreatment of lignocellulosic biomasses improved the methane production. Pretreatment of birch and Spruce by using 7% of NaOH at the temperature variation of 80 ᵒC for 2h enhanced the methane production by 57% (0.23 to 0.36 L/g VS) and 600% (0.21 to 0.03 L/g VS) respectively41. The fallen leaves pretreated with 3.5% NaOH enhanced the methane by 24 fold more than untreated leaves42. NaOH pretreatment of corn stover (2%, w/v) under the conditions of 120ᵒC for 3 h obtained more than 73% biogas than that of untreated corn stover43. In the present study, NaOH pretreatment has also been shown to increase biogas production from PJ wood by 20 folds higher value compared to that of untreated PJ.


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Energy & Fuels

(a)

(c)

(b)

(d)

Figure. 5. Daily and Cumulative methane production for untreated and treated PJ and SS 3.6 Kinetic study using a modified Gompertz model Gompertz model was used to predict the cumulative biomethane production curve for all ratio, based on digestion time and the results are shown in Figure.6. The predicted parameters are mentioned in Table 4. The model predicts that methane production is directly proportional to the specific growth rate of methanogenic organisms as calculated by using equation 4. From the result, it was observed that high value of ym was obtained in the substrate ratio of 60:40 (233.4 mL/g VS) and 40:60 (220.86 mL/g VS), respectively. The ideal percentage of carbohydrate and protein content present in the mixing ratio (60:40, 40:60) increases the microbial activity. That means the


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feed ratio of 60:40 and 40:60 (45.0±1.3 and 32.5±0.9 mL/g VS) produced the maximum biomethane in higher amount compared to other feed ratios of 100:0, 80:20, 20:80 and 0:100. Due to anaerobic microorganisms were positioned in the optimal growth zone supported with mixing substrates of 60:40 and 40:60. Higher the value of ym reflects the higher value of U and vice versa. The 𝛌 value indicates the time taken for the anaerobic bacteria adopt in the substrates before the degradation process5. The substrate concentration 100:0, 80:20, 20:80 and 0:100 had a high 𝛌 value of 3.54, 2.68, 2.78 and 2.72 respectively, needed more time to produce biogas than other feed ratio. On the other side, the substrate concentration 60:40 and 40:60 took less time to produce biogas due to the perfect ratio of nutrients present in the feedstock. High percentage of carbon source in the substrate need less time to produce biogas than a high percentage of nitrogen source44.


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Energy & Fuels

(a)

(b)

R2 = 0.975 100:0

R2 = 0.982 80:20

(d)

(c)

R2 = 0.998 40:60

R2 = 0.997 60:40

(e)

(f)

R2 = 0.983 20:80

R2 = 0.986 0:100

Figure.6. Comparison graph between Simulations vs. Experimental data


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Table 4. Kinetic Parameters obtained from Modified Gompertz model Different ratios of Prosopis juliflora: Sewage Sludge (PJ:SS) Parameter 100:0

80:20

60:40

40:60

20:80

0:100

𝛌 (days)

3.54

2.68

1.75

1.93

2.78

2.72

U (mL/g VS)

0.68

3.35

4.65

6.85

4.83

6.54

ym(mL/g VS)

4.32

88.4

233.4

220.86

146.4

186.3

R2

0.958

0.97

0.988

0.982

0.968

0.973

4. CONCLUSION The present study evaluated the feasibility of utilizing P. juliflora (PJ) as the main substrate for the production of biogas in the AD process. From the experimental study, the pretreated PJ and SS achieved high methane yield of 45.0±1.3 mL/g VS on the 15th day of the retention period having a feedstock ratio of 60:40. NaOH (2%, w/v) treatment of PJ prior to the AD was evidenced to enhance biogas production and VS reduction via improved hydrolysis and the effect of pretreatment was proved by SEM, FTIR and XRD. The Alkali pretreated PJ increased methane production by 20 folds when compared to untreated PJ (control). Modified Gompertz model replicated the experimental data. These findings could represent some fundamentals for further improvement of anaerobic digestion from intimidating weed that would create harms to the environment.


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ASSOCIATED CONTENT Author Information Corresponding Author E-mail: [email protected] (Sivanesan); Phone: +91-44-22359168.

ORCID Amudha Thanarasu: 0000-0001-9394-0111 Karthik Periyasamy: 0000-0001-6956-5999 Sivanesan Subramanian: 0000-0002-2103-4862

ABBREVIATIONS VFA, volatile fatty acids; TKN, total Kjeldahl nitrogen; SCOD, soluble chemical oxygen demand; VS, volatile solids; TS, total solids; MS, moisture content; AD, Anaerobic digestion; PJ, Prosopis juliflora; SS, Sewage sludge; MAS, Methanogens activated sewage sludge.


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Energy & Fuels


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Graphical Abstract


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Anaerobic Codigestion of Alkali-Pretreated Prosopis juliflora Biomass

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