Performance ambiental da produção de biogás de microalgas cultivadas em esgoto doméstico

Abstract

The phenomenon of global warming, driven by the use of conventional fuels and manufacturing activities, results in significant emissions of carbon dioxide (CO2) and other greenhouse gases (GHG) from industrial facilities and other anthropogenic activities. The increasing energy consumption, imminent fossil fuel scarcity, and growing interest in renewable sources motivate research in biofuels. Biogas production through anaerobic digestion (AD) of microalgae, integrated with domestic sewage treatment, emerges as a promising technique for waste management and reducing the carbon footprint. However, operational, political, and public acceptance challenges limit the full potential of this approach. In this context, research efforts should be dedicated to improving the efficiency and performance of this process. This study evaluates the environmental performance of biogas production from microalgae cultivated in domestic sewage, employing mass and energy balance analyses, along with a life cycle assessment. The modeling, based on secondary data, considered a baseline scenario with cultivation in a high-rate algal pond, biomass harvesting by gravitational sedimentation, liquid AD, and biogas upgrading by water scrubbing. Additional scenarios were explored to minimize input consumption and environmental impact. The carcinogenic human toxicity category was the most affected, highlighting the importance of AD heating and cultivation water. The AD step was the most impactful, contributing 35.28% to potential environmental impacts. Scenario S3, incorporating techniques such as thermal pretreatment, heat recovery, co-digestion, photosynthetic biogas upgrading, and potential use of digestate as biofertilizer, demonstrated better environmental performance. Sensitivity analysis emphasized methane productivity as a crucial factor in reducing or increasing potential impacts (e.g., 11% reduction in ionizing radiation potential impacts after a 10% increase in this parameter). Comparing to natural gas production, S3 reduced damages to natural resource by up to 6 times but increased damages to human health by 23 times. Analyzing carbon dioxide equivalent (CO2-eq) emissions and the net energy ratio (NER) in scenario S3, positive emissions (0.1795 kg CO2-eq) and a NER greater than 1 (1.71) were observed. The AD step stood out as the main contributor to these results, representing 98% of CO2-eq emissions and 94% of the total system energy consumption, suggesting that this scenario is not advantageous for biogas production due to high energy consumption for digester heating. However, sensitivity analysis revealed that a 20% reduction in AD temperature could result in a more favorable scenario for biogas production. In this context, negative CO2-eq emissions (-0.1226 CO2-eq) and a positive energy balance with a NER of 0.69 were observed. These results indicate that AD at ambient temperatures can positively impact the environmental and energy efficiency of the process. This study provides valuable insights into biogas production from microalgae biomass, emphasizing the importance of optimizing operational parameters to achieve a more favorable environmental and energy performance. These findings have the potential to guide future research and practices in the field of harnessing microalgae for sustainable biogas production.