Unsterilized sewage treatment and carbohydrate accumulation in Tetradesmus obliquus PF3 with CO₂ supplementation

A mixed gas with 10% CO₂ was supplemented in unsterilized sewage to promote Tetradesmus obliquus PF3 for sewage treatment and nutrient recovery. The sewage was purified with higher removal efficiency of COD (90 ± 3%), TN (93 ± 3%) and TP (99 ± 0%) in the unsterilized condition with 10% CO₂ supplementation than that with 0.038% CO₂ (air) supplementation (COD: 65 ± 2%; TN: 81 ± 1%; TP: 95 ± 0%). The coexistence of bacteria and microalgae in unsterilized sewage had a significant effect on COD removal, while microalgae played a dominant role in nutrients (N and P) removal. Quality of effluent water complied with the class I-A criteria specified in Chinese Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant. Supplementation with 10% CO₂ promoted the recovery of nitrogen (84 ± 10%) and phosphorus (90 ± 9%) from sewage compared to that of 0.038% CO₂ (air) supplementation (N: 58 ± 3%, P: 59 ± 4%), which is due to the improved carbon supply and more favorable pH of 6.8–7.8 adjusted by high CO₂ concentration. In addition, the CO₂ fixation rate reached up to 551 ± 1 mg L^{− 1} d^{− 1} by Tetradesmus obliquus PF3 and the carbohydrate content in the obtained biomass was as high as 60 ± 2%. This study indicates that the supplementation with 10% CO₂ in unsterilized sewage water has remarkable promotion effects on wastewater treatment and nutrient recovery. Therefore, waste gas containing high concentration of CO₂, such as flue gas was suggested as carbon supplementation in the algal wastewater treatment.