Problem definition and objectives: The increasing global demand for sustainable energy solutions and the need for effective waste management have led to a growing interest in the use of renewable energies. Biomass energy occupies the first place among renewable energies in the world's energy supply; in such a way that in 2005, about 10% of the world's primary energy supply was provided from biomass sources. According to the findings, organic biomass can be considered as one of the potentials for biogas production. One of the organic sources that is known as a water-soluble waste material is black liquor. Black liquor usually contains a high concentration of organic compounds such as volatile fatty acids and other biodegradable substances. These organic compounds can act as a substrate and provide additional carbon and energy sources for anaerobic microorganisms responsible for biogas production. The presence of these available organic compounds in black liquor can potentially increase the overall efficiency of the anaerobic digestion process and lead to an increase in biogas yield compared to the use of biological waste alone. Related articles have been reviewed in the direction of biogas production by pyrolysis, but the study of the effect of black liquor on the biogas production process by anaerobic fermentation was seen as a gap. Accordingly, the aim of this study is to provide valuable insights into the optimization of biogas production processes by investigating the synergistic effects of combining black liquor with biological waste, which ultimately helps in the development of more efficient and sustainable renewable energy systems.
Methodology: In this study, wheat straw test samples were obtained from the educational and research farm of Mohaghegh Ardabili University. In order to conduct pre-extraction experiments and cooking operations of wheat straw, they were cut into sizes of one to three centimeters by a cutting machine and placed in a laboratory environment to reach equilibrium humidity. To build the reactor in this experiment, 9 glass bottles with a fixed volume of 1 liter and an effective working volume of 0.9 liters were used, and each digester was connected to two 1.5 liter bottles to receive biogas and measure the production volume by the water displacement method. The prepared samples were entered into the reactor through the lid of these bottles, then the possibility of gas escaping around the lid was eliminated by sealing the lid, and the digesters (reactors) were placed inside a water bath. One of the important conditions for conducting the experiment is to keep the ambient temperature constant. For this purpose, a digital thermometer was used to measure the desired temperature with an accuracy of ±0.1 and the reactors were placed in a hot water bath. This study included three treatments with three replications. The first treatment was the control treatment with a carbon to nitrogen ratio of 30 as the best carbon to nitrogen ratio for producing the highest volume of biogas. In the second and third treatments, black liquor was added to the samples at 5 and 10% by weight, respectively.
Results: The highest reduction was in the third treatment. According to the modeling obtained, the second treatment increased the biogas production potential by approximately 190%. However, the biogas production potential in the third treatment decreased by about 3% compared to the second treatment. Considering the 78% reduction in biogas production delay, the 3% reduction in biogas production potential can be ignored. Also, if a short period of time is considered for biogas production, the least delay in biogas production is required. Accordingly, with the conclusions obtained, it can be stated that the third treatment (presence of 10% water-soluble black liquor) can be considered as the best treatment with the highest biogas production efficiency.
Conclusion: The findings of this study demonstrate the potential of using black liquor as a feedstock in anaerobic digestion of biological wastes to enhance biogas production. It was observed that the addition of black liquor, rich in readily biodegradable organic compounds, provided a continuous supply of readily available carbon and energy sources for the anaerobic microorganisms responsible for biogas production. This continuous supply of readily available organic matter from black liquor helped maintain high activity levels of the digesting bacteria for a long time, leading to increased and more sustainable biogas yields.