High-efficient removal of ammonium and co-production of protein-rich biomass from ultrahigh-NH4+ industrial wastewater by mixotrophic Galdieria sulphuraria

Microalgal cultivation system is regarded as an effective bioremediation system for NH4+-containing wastewater. However, ultrahigh NH4+ concentration in wastewater impeded the normal growth of most microalgal species leading to low NH4+ removal efficiency or even algal death, while indigenous bacteria and other biological contaminants in wastewater are negative for algal growth and eventually crash the microalgal-based wastewater treatment system. If maintaining sterile condition throughout the treatment process, the high consumption of energy must result in a high cost of operation with less valuable output. Aiming to achieve the goal of cost-effective NH4+ removal and protein-rich biomass co-production, a novel strategy of NH4+ reutilization from ultrahigh-NH4+ industrial wastewater was developed through photo-fermentation of the thermoacidophilic red alga Galdieria sulphuraria (G. sulphuraria) as cell factory. Results indicated that the highest protein content (71.66 % DW) was attained under optimal conditions with inoculum density of 1 × 108 cells mL−1, pH 3.5, 3380 mg NH4+ L−1 and light intensity of 230 μmol m−2 s−1 in shake flasks system for 4-day culture. Subsequently, repeated fed-batch culture under sterile and non-sterile mode was performed in ca. 5000 mg NH4+ L−1 wastewater medium in 5 L photo-fermenters, achieving a 98 % of NH4+ removal efficiency with a higher NH4+ removal rate (1705.67 mg L−1 d−1), biomass concentration (64.65 g L−1) and protein productivity (8.75 g L−1 d−1) under non-sterile culture mode. This work provided a new solution for high-efficient NH4+ removal coupled with protein-rich biomass production by a lower cost, high-value biomass output and environment-friendly way.