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

Abstract 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.