(116h) Anaerobic Co-Digestion of Swine Wastewater and Agricultural Residues for Biogas Production

Sabarish Srinivasan¹, Jay J. Cheng², Qingfang Zhang³

¹ Panther Creek High School, Cary, NC 27519, USA

² Department of Biological and Agricultural Engineering, North Carolina State University, Raleigh, NC 27695, USA

³ Lanzhou University of Technology, Lanzhou, Gansu Province, China

1. INTRODUCTION

The rapid and irreversible depletion of fossil fuels accompanied by an increasing global desire for energy has led to the search for alternative energy sources. Biogas, composed mainly of methane and carbon dioxide, can provide electricity generation along with heating in place of nonrenewable energy choices. An efficient technique for producing biogas is anaerobic digestion, a microbial decomposition of organic matter that naturally occurs without oxygen. The addition of co-substrates in anaerobic digestion is known as anaerobic co-digestion: the breakdown of several organic waste varieties in the same “digestor” or bioreactor. The utilization of agricultural residues as co-substrates in the co-digestion process is especially appealing due to the reduction of agricultural waste as well as the biogas formation provided. This mechanism generally increases the amount of biogas yielded due to positive synergistic effects between microorganisms in the digestion and other factors such as optimization of the C:N ratio [1]. A wide variety of feedstocks may be utilized, but a major option is lignocellulosic biomass: plant material, composed of lignin, hemicellulose, and cellulose, that is high in carbon content, contributing to the balance of the C:N ratio for better digestion [2]. This study was designed to investigate lignocellulosic biomass as a co-substrate in order to determine the means of obtaining the highest yield in terms of biogas creation in anaerobic co-digestion.

2. MATERIALS AND METHODS

In this study, the volumetric biogas output ability of disparate substrates in the anaerobic co-digestion process was tested in four batch bioreactors operating at a temperature of 35°C in a computerized respirometer. The batch reactors were four 500 mL glass bottles that each contained two ducts with tubes attached to sustain gas collection and to sample the biogas. The gas produced was measured by 500 mL volume cylindrical liquid displacement meters over a period of 30 days. To maintain the anaerobic environment, the bioreactors were purged of oxygen through the addition of nitrogen at the beginning of each trial. Additionally, the reactors were immersed in water that reached approximately ¾ of their height to maintain their temperature at 35°C while running. All four reactors were provided with 300 milliliters of anaerobic culture and 200 milliliters of swine wastewater. The culture was derived from a 14-liter continuous stirred tank reactor undergoing anaerobic co-digestion with swine manure and switchgrass operated at mesophilic conditions at 35°C and with a hydraulic residence time of 25 days. Furthermore, 5 grams of an agricultural residue were added once initially to each reactor: switchgrass, corn stover, rice straw, or wheat straw. The amount of co-substrate in the experiment was changed after 9 days of experimentation due to unyielding, lower than expected stabilization in biogas production. In this new trial, 3 grams of corn stover, 8 grams of rice straw, and 8 grams of wheat straw, were implemented while the portion of switchgrass was not changed. For this test, the experiment was completely restarted under the same conditions as the initial trial except for the new co-substrate totals.

3. RESULTS

For the first 4 days of the experiment, the biogas production of the co-digestions of corn stover, rice straw, and wheat straw, fluctuated rapidly before leveling and maintaining a consistent output from the 4^th day to the 9^th day: 90 mL for corn stover, 170 mL for rice straw, and 155 mL for wheat straw. After the 9^th day and the alterations to the quantities of co-substrates utilized, the corn stover biogas production presented a decline of approximately 40%. The rice straw biogas yield did not demonstrate a remarkable change with a 3% increase over the 30-day period. Conversely, the gas creation in the switchgrass co-digestion exhibited an instantaneous exponential growth trend throughout the first 9 days, and this inclination continued until the termination of the study. The output for the wheat straw, on the other hand, exhibited an increase of 150% in two days and an overall increase of 50% in the second phase of the testing, passing switchgrass biogas volume in the latter half of the trial. The wheat straw biogas production maxed at 53.75 mL/g biomass while the switchgrass biogas formation maxed at 48 mL/g biomass during the second phase of the experimentation. The results indicate that biogas production from the switchgrass is steadier and has maintained a higher conversion efficiency that that of the wheat straw over the entire trial period. Moreover, the switchgrass biogas formation surpassed that of wheat straw by a large margin of 67 mL/g, demonstrating that this residue is exceptionally effective in comparison to the other substrates used in the study.

4. DISCUSSION

The outcomes of the experiment indicate that switchgrass is the most effective co-substrate because of its high biogas production rate. The chemical composition of the dry agricultural residues reveals that the cellulose, hemicellulose, and lignin (lignocellulosic elements) proportions differ among the co-substrates. The average switchgrass cellulose percentage is 39.5% which exceeds that of the other residues, and the average switchgrass hemicellulose percentage is 20.3% which undershoots that of the other residues, suggesting a correlation between cellulose, hemicellulose, and biogas generation due to switchgrass performance [3]. One factor that could explain the wheat straw production is the relatively low lignin (17%) portion which could have accelerated the digestion process because lignin cannot be decomposed in anaerobic digestion without pretreatment. Switchgrass did display more consistent biogas growth when compared to wheat straw; however, further testing must be conducted to ascertain the possibility of altering conditions to augment the biogas released in the co-digestion of wheat straw. Additionally, the usage of switchgrass may be more efficient because the switchgrass quantity does not have to be changed for optimal results, unlike the other co-substrates. The physical attributes that should be considered in the application of lignocellulosic substrates including fiber length, fiber strength, cell wall thickness, and bulk density, can also play a significant role in their utilization in agricultural practices. For example, switchgrass is generally a sturdier substance due to these characteristics than wheat straw, and, therefore, can handle compression forces and stress with greater ease, furthering its applicability to industrial settings. Another factor that may influence the scalability of this investigation is the small quantities of co-substrates wielded, ranging from 5 grams to 8 grams. Extrapolation would be necessary for execution in energy or other relevant sectors, and further studies should be organized with larger amounts of co-substrates before leveling up to industrial application.

5. CONCLUSION

The study of switchgrass, wheat straw, rice straw, and corn stover in the anaerobic co-digestion process by batch mode reveals that switchgrass is the best co-substrate of those that were tested in this experiment. The combination of chemical and physical properties that aid in the rapid degradation of this material and conversion to biogas was unmatched in the first trial, exhibiting quick exponential biogas development. Although the wheat straw biogas production did rival that of switchgrass in the second trial, the increased quantity that was necessary for this change does not justify the marginal difference in biogas output, illustrating that, overall, switchgrass is the most efficient, reliable, and productive agricultural residue in this investigation of the anaerobic co-digestion of swine manure and certain co-substrates.

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