Advanced treatment of residual nitrogen from biologically treated coke effluent by a microalga-mediated process using volatile fatty acids (VFAs) under stepwise mixotrophic conditions

•Microalgae were used to treat the residual NH4+-N from pretreated coke effluent.•Two mixotrophic cultivating modes were used to enhance NH4+-N removal.•Use of VFAs in mixotrophic mode significantly improved removal rate of NH4+-N.•Arctic Chlorella sp. had the highest removal rate and FAME production.•RDA was used to determine the significance of factors affecting NH4+-N removal.

This work describes the development of a microalga-mediated process for simultaneous removal of residual ammonium nitrogen (NH4+-N) and production of lipids from biologically treated coke effluent. Four species of green algae were tested using a sequential mixotrophic process. In the first phase—CO2-supplied mixotrophic condition—all microalgae assimilated NH4+-N with no evident inhibition. In second phase—volatile fatty acids (VFAs)-supplied mixotrophic condition—removal rates of NH4+-N and biomass significantly increased. Among the microalgae used, Arctic Chlorella sp. ArM0029B had the highest rate of NH4+-N removal (0.97 mg/L/h) and fatty acid production (24.9 mg/L/d) which were 3.6- and 2.1-fold higher than those observed under the CO2-supplied mixotrophic condition. Redundancy analysis (RDA) indicated that acetate and butyrate were decisive factors for increasing NH4+-N removal and fatty acid production. These results demonstrate that microalgae can be used in a sequential process for treatment of residual nitrogen after initial treatment of activated sludge.

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