A hetero-photoautotrophic two-stage cultivation process to improve wastewater nutrient removal and enhance algal lipid accumulation

► Effective coupling municipal wastewater treatment with algae-based biofuel production. ► Development of a hetero-photoautotrophic two-stage cultivation process. ► The maximal algal biomass concentration reached above 1.1g/L with lipid content around 30%. ► High wastewater nutrient removal efficiencies for phosphorus, ammonia, total nitrogen and COD. ► The two-stage model developed can be used to treat organic-rich wastewaters. A hetero-photoautotrophic algal growth model was studied for improved wastewater treatment and low cost algal biofuel feedstock production. The microalga, Auxenochlorella protothecoides UMN280, was grown heterotrophically on concentrated municipal wastewater and then autotrophically with CO2 supplementation (air, 1% and 5%, respectively). Strain UMN280 was harvested by self-sedimentation after the heterotrophic stage and the supernatant was aerated with different levels of CO2 to facilitate autotrophic growth in the second stage. The maximal biomass concentration and lipid content at the first and second stages reached 1.12g/L and 28.90%, and 1.16g/L and 33.22%, respectively. The nutrient removal efficiencies for total phosphorus, ammonia, nitrogen and chemical oxygen demand at the end of the two-stage cultivation were 98.48%, 100%, 90.60% and 79.10%, respectively. The above process can be used to treat organic-rich wastewaters (e.g. industrial and animal manure wastewaters) to achieve the dual purpose of low-cost wastewater treatment and biofuel feedstock production.