Influence of pH and CO2 source on the performance of microalgae-based secondary domestic wastewater treatment in outdoors pilot raceways

• Effect of pH on wastewater treatment and biomass productivity/composition was null.
• CO2 from flue gas supported a superior wastewater treatment performance.
• Carbon, nutrients and E. coli were efficiently removed from domestic wastewater.
• Maximum biomass productivity of 17 ± 1 gm−2d−1 was recorded in the outdoor pilot RWs.

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 Escherichia 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.