The influence of pH (7, 8 and 9) and CO2 source (pure CO2 or CO2 from flue gas) on both the performance of secondary domestic wastewater treatment and biomass productivity and composition in three outdoors pilot raceways was evaluated for 6 months. Average COD, TN, TP and E. coli removal efficiencies of 84±7%, 79±14%, 57±12% and 93±7%, respectively, were recorded. The influence of pH on wastewater treatment was negligible, while the supply of CO2 from flue gas supported higher COD, TOC and TP removals. Biomass productivities ranged from 4±0 g m-2 d-1 in December to 17±1 g m-2 d-1 in July. The highest C, N and P biomass contents (64.8%, 12.6 % and 2.4 %, respectively) were recorded when flue gas was supplied. Finally, while the protein content in the biomass remained constant (38.2±3.3%), the lipid and carbohydrate contents ranged from 5.8% to 23.0% and from 38.0% to 61.2%, respectively.

16 2 The influence of biogas flow rate (0, 0.3, 0.6 and 1.2 m 3 m -2 h -1 ) on the elemental and 17 macromolecular composition of the algal-bacterial biomass produced from biogas upgrading in a 18 180 L photobioreactor interconnected to a 2.5 L external bubbled absorption column was 19 investigated using diluted anaerobically digested vinasse as cultivation medium. The influence of 20 the external liquid recirculation/biogas ratio (0.5 < L/G < 67) on the removal of CO2 and H2S, 21and on the concentrations of O2 and N2 in the upgraded biogas was also evaluated. A L/G ratio of 22 10 was considered optimum to support CO2 and H2S removals of 80% and 100%, respectively, at 23 all biogas flow rates tested. Biomass productivity increased at increasing biogas flow rate, with a 24 maximum of 12±1 g m -2 d -1 at 1.2 m 3 m -2 h -1 , while the C, N and P biomass content remained 25 constant at 49±2%, 9±0% and 1±0%, respectively, over the 175 days of experimentation.

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