Minimization of biomethane oxygen concentration during biogas upgrading in algal–bacterial photobioreactors
Introduction
The total electricity produced in the European Union in 2013 through primary biogas production accounted for 52.3 TWh [1]. Biogas from the anaerobic digestion of renewable feedstocks (such as agroindustrial or municipal organic solid wastes) constitutes a potential biofuel source able to reduce the current fossil fuel dependence of our society [2]. Biogas is a gas rich in CH4 (40-75%) and CO2 (25–60%), with other components such as H2S (0.005–2%), N2 (0–2%), O2 (0–1%) and NH3 (< 1%) present at significantly lower concentrations [3]. CO2 removal from biogas would entail a decrease in its transportation and compression costs, while the removal of H2S would reduce its toxic, corrosive and malodorous nature [4]. In this context, a removal of CO2 and H2S to achieve CH4 concentrations over 80–96% and H2S levels below 5 mg m− 3 is required for biomethane injection into natural gas grids and use as a vehicle fuel [5]. Conventional physical/chemical or biological technologies often tackle CO2 or H2S removal into two sequential steps [3], [6]. Otherwise, processes such as water/chemical scrubbing and membrane separation, which allow for a simultaneous CO2 and H2S removal from biogas, exhibit high environmental impacts and operating costs, respectively [3], [7]. In this context, algal–bacterial symbiosis allow for a simultaneous CO2 and H2S removals in an innovative, environmentally friendly and low-cost process compared to conventional methods [8].
Microalgae-based processes for biogas upgrading are characterized by the simultaneous photosynthetic CO2 consumption by microalgae in the presence of light and the oxidation of H2S to sulfate by sulfur oxidizing bacteria using the O2 produced from microalgal photosynthesis [8]. The economic and environmental sustainability of this biotechnology can be enhanced with the use of wastewaters as a free water and nutrient source for microalgae and bacteria growth [9]. Despite the promising results obtained so far in terms of biogas upgrading and wastewater treatment performance, the desorption of the photosynthetically produced O2 from the algal cultivation broth to the upgraded biogas severely challenges the application of this novel biotechnology [10]. Thus, while the upper O2 concentration limit for injection of the upgraded biogas into natural gas networks stands at 0.2–1% in most international legislations, O2 levels ranging from 2–24% have been typically reported in biogas-upgrading photobioreactors [8], [11]. In this context, Posadas et al. [10] recorded CO2 removals from synthetic biogas of 99% and O2 concentrations in the upgraded biogas of ≈ 20% in a 180 L open photobioreactor treating diluted centrates, while Serejo et al. [12] recorded CO2 and H2S removals of ≈ 80% and 100%, respectively, and O2 concentrations in the upgraded stream ranging from 2 ± 1% to 1 ± 0% in a similar photobioreactor by treating diluted anaerobically digested vinasse (ADV). Likewise, Converti et al. [13] reported O2 concentrations ranging from 10 to 24% during the upgrading of real biogas in a bubble column photobioreactor. These high O2 levels entail a potential explosion hazard and prevent the injection of the upgraded biogas into natural gas networks. Membranes or low temperature pressure swing adsorption (PSA) are typically used for O2 removal from biogas, but these technologies present very high operating costs [3]. Unfortunately, the number of studies focused on the reduction of O2 concentrations in biomethane during the simultaneous removal of CO2 and H2S in algal–bacterial processes is scarce.
The present study assessed the effectiveness of different operational strategies to reduce the O2 concentration in the upgraded biogas in a 180 L high rate algal pond (HRAP) treating ADV and interconnected to an external CO2/H2S absorption column (AC). The removal of CO2 and H2S from a synthetic biogas and the potential of this novel biotechnology for carbon and nutrient removal from ADV were also evaluated. Finally, the dynamics of the structure of microalgae and bacteria populations in the HRAP were investigated. This study constitutes, to the best of our knowledge, the first evaluation by molecular techniques of the bacterial assemblage dynamics in this innovative photosynthetic biogas upgrading process.
Section snippets
Biogas and vinasse wastewater
A synthetic biogas mixture, composed of CO2 (29.5%), H2S (0.5%) and CH4 (70%), was purchased from Abello Linde (Spain). ADV and raw vinasse (RV) wastewaters were periodically collected from the anaerobic wastewater treatment line of a food industry located in Valladolid (Spain) and stored at 4 °C prior to use. The final composition of the feed wastewaters was subjected to the variations of the received wastewaters depending on the seasonal period. ADV and RV were diluted twelve times with tap
Environmental conditions in the HRAP
The temperature of the algal–bacterial broth remained constant at 24 ± 1 °C regardless of the operational stage, which was optimum for the cultivation of microalgae and bacteria in wastewaters [16]. Larger variations occurred in the average water evaporation losses recorded along the different operational stages, which ranged from 4.4 ± 1.4 to 7.3 ± 0.2 L m− 2 d− 1 (Table 2). These values were comparable to those estimated by Guieysse et al. [24] in outdoors HRAPs operated at 7 d of HRT and located in Arid,
Conclusions
This research work confirmed the potential of algal–bacterial symbiosis to support a simultaneous wastewater treatment and biogas upgrading. A process configuration based on the direct injection of raw wastewater into the biogas absorption column successfully depleted most of the O2 present in there cycling cultivation both, which decreased the biomethane O2 content below permissible levels in most international legislations. Unfortunately, nitrogen desorption resulted in average biomethane N2
Acknowledgments
This research was supported by the Regional Government of Castilla y León (Project VA024U14 and GR76), INIA (Project RTA2013-00056-C03-02) and MINECO (Red Novedar). A. Crespo, S. Santamarta and C. Mongil are gratefully acknowledged for their practical assistance.
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Current address: The Institute of the Environment. La Serna, 58, 24007 Leon, Spain.