Abstract
Photosynthetic biogas upgrading using two-stage systems allows the absorption of carbon dioxide (CO2) in an absorption unit and its subsequent assimilation by microalgae. The production of microalgae requires large amounts of nutrients, thus making scale-up difficult and reducing economic feasibility. The photosynthetic process produces oxygen (O2) (1 mol per mol of CO2 consumed), which can be desorbed into purified biogas. Two-stage systems reduce its impact but do not eliminate it. In this study, we test the use of landfill leachate as a nutrient source and propose a viable and economical strategy for reducing the O2 concentration. First, the liquid/gas (L/G) ratio and flow mode of the absorber were optimized for 20% and 40% CO2 with COMBO medium, then landfill leachate was used as a nutrient source. Finally, the system was inoculated with nitrifying bacteria. Leachate was found to be suitable as a nutrient source and to result in a significant improvement in CO2 absorption, with outlet concentrations of 0.01% and 0.6% for 20% and 40% CO2, respectively, being obtained. The use of nitrifying bacteria allowed a reduction in dissolved oxygen (DO) concentration, although it also resulted in a lower pH, thus making CO2 uptake slightly more difficult.
Highlights
The use of biogas as a renewable energy source is strongly encouraged by international organizations and states
This study aimed to describe the impact of landfill leachate as a source of nutrients and the use of a culture of nitrifying bacteria on the efficiency of a two-stage system for biogas upgrading
The photobioreactor was first operated with the COMBO synthetic medium for 55 days
Summary
The use of biogas as a renewable energy source is strongly encouraged by international organizations and states. In this sense, the European Union has established an objective of reducing total greenhouse gas emissions by between 80% and 95% compared to 1990 [1]. In its report “A roadmap for moving to a competitive low carbon economy in 2050”, the EU has established a target of an 80% reduction in emissions by 2050 [2]. Methane (CH4) and carbon dioxide (CO2) are the main gases present in biogas, with the potential effect of CH4on global warming being 24.5times higher than that of CO2. To obtain biomethane, according to the standard specifications of each region or country, biogas must be purified to eliminate minor compounds and upgraded to vary the methane content [2]
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