Coupling photosynthetic biogas upgrading with the astaxanthin production using Neochloris sp

Abstract

The feasibility of coupling photosynthetic biogas upgrading with astaxanthin production by Neochloris sp. using real centrate as a feedstock was herein evaluated. To maximize astaxanthin production, this study focused on optimizing the cultivation conditions such as light intensity, culture medium, and composition of biogas. A light intensity of 300 μmol m-2 s-1 showed the highest specific growth rate (0.34 d-1) and total suspended solid of biomass (0.33±0.03 g L-1) of microalgae compared to other light intensity (0, 100, 500, 800 μmol m-2 s-1). When the headspace in the bottles was filled with air, mineral salt medium showed higher growth (0.78±0.007 g L-1) than other medium conditions (10% diluted centrate from domestic wastewater, 10% diluted centrate adjusted initial pH of 7, and 10% diluted centrate with activated sludge aliquot). On the other hand, when biogas was injected into the headspace in the bottles, 10% diluted centrate showed a higher specific growth rate (0.58-1.1 d-1) and TSS (2.57±0.92~6.98±0.029 g L-1) because pH control by CO2 dissolution prevented growth inhibition by free ammonia. In terms of biogas upgrading, CO2 of the headspace in the bottles was removed by 82.8-99.8% and decarbonization of biogas was achieved. Interestingly, biogas containing 600 ppm H2S revealed a two-fold higher biomass yield than biogas without H2S. When 5000 ppm of H2S, growth inhibition occurred, but addition of activated sludge aliquot was effective to remove H2S by sulfur oxidizing bacteria, and microalgae grew (4.36±0.057 g L-1). Thus, this study indicated that Neochloris sp. could validated the proof of concept with biogas containing 600 ppm H2S. Comparison of the maximum biomass yield with different dilution centrate under a headspace of biogas with 600 ppm H2S showed no significant differences. Astaxanthin yield were 0.072% of dry biomass (MSM), 0.024% (10% diluted centrate), 0.028% (50% diluted centrate), and 0.177% (raw centrate (100% centrate)) on day 10. From these results, 100% centrate can be utilized for Neochloris sp. cultivation. Thus, photosynthetic biogas upgrading using Neochloris sp. can support the valorization of residual effluents such as digestate and biogas in the form of high added value pigments and biomass.