Feasibility study of biogas upgrading coupled with nutrient removal from anaerobic effluents using microalgae-based processes

Abstract

The present research was conducted in order to simultaneously optimize biogas upgrading and carbon and nutrient removal from centrates in a 180 L high rate algal pond interconnected to an external CO 2 absorption unit. Different biogas and centrate supply strategies were assessed in order to increase biomass lipid content. Results showed 99% CO 2 removal efficiencies from simulated biogas at liquid recirculation rates in the absorption column of 9.9 m 3 m -2 h -1 , concomitant with nitrogen and phosphorus removal efficiencies of 100% and 82%, respectively, using a 1:70 diluted centrate at a hydraulic retention time of 7 days. The lipid content of the harvested algal-bacterial biomass remained low (2.9-11.2%) regardless of the operational conditions, with no particular trend over the time. The good settling characteristics of the algal-bacterial flocs resulted in harvesting efficiencies over 95%, which represented a cost-effective alternative for algal biomass reutilization compared to conventional physical-chemical techniques.Finally, high microalgae biodiversity was found regardless of the operational conditions. 2

Minimization of biomethane oxygen concentration during biogas upgrading in algal–bacterial photobioreactors

“…The presence of this microalga was not detected during stage 2B, when the filamentous cyanobacterium Pseudanabaena minima accounted for 53% of the total number of photosynthetic microorganisms. The high microalgae diversity found in our research was in agreement with that reported in outdoors and indoors pilot HRAPs treating diluted piggery and centrate wastewater [10,38].…”

“…The higher O 2 requirements for complete organic matter oxidation during RV feeding into the AC (stage 3) resulted in biomethane O 2 concentrations in compliance with the requirements of most international legislations for injection of the upgraded biogas in natural gas networks (0.2-1%) [5]. This biomethane O 2 concentration was significantly lower than that recorded by Posadas et al [10], which ranged from 2.0 ± 1.0% to 20.7 ± 0.1%, and by Serejo et al [12], which averaged 2 ± 1%. Likewise, the O 2 concentrations recorded here were significantly lower than those achieved by Mann et al [30] in a 1 L enclosed tubular photobioreactor supporting CO 2 and H 2 S-REs of 97% and 100%, respectively, with biomethane O 2 concentrations ranging from 18-23%.…”

“…Under these operational conditions, the carbon mass balance calculation revealed that 65 ± 9% of the TC removed from the wastewater and biogas was recovered in the harvested biomass during stages 1, 2A and 2B, while this value decreased to 29 ± 7% in stage 3. This result could be explained taking into account the higher TOC oxidation during stage 3, which resulted in higher IC concentrations in the cultivation medium and, therefore, in higher CO 2 removals by stripping [10].…”

“…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 O 2 from the algal cultivation broth to the upgraded biogas severely challenges the application of this novel biotechnology [10]. Thus, while the upper O 2 concentration limit for injection of the upgraded biogas into natural gas networks stands at 0.2-1% in most international legislations, O 2 levels ranging from 2-24% have been typically reported in biogas-upgrading photobioreactors [8,11].…”

“…Thus, while the upper O 2 concentration limit for injection of the upgraded biogas into natural gas networks stands at 0.2-1% in most international legislations, O 2 levels ranging from 2-24% have been typically reported in biogas-upgrading photobioreactors [8,11]. In this context, Posadas et al [10] recorded CO 2 removals from synthetic biogas of 99% and O 2 concentrations in the upgraded biogas of ≈20% in a 180 L open photobioreactor treating diluted centrates, while Serejo et al [12] recorded CO 2 and H 2 S removals of ≈ 80% and 100%, respectively, and O 2 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 O 2 concentrations ranging from 10 to 24% during the upgrading of real biogas in a bubble column photobioreactor.…”