Nutrients recovery from anaerobic digestion effluent for the production of microalgae biomass

The efficient management of anaerobic digestion effluents (ADEs) from biogas plants has raised increasing environmental concerns, as a result of their high content in nutrients, mostly in the form of NH4- and PO4-. Their discharge to farmlands is restricted due to irrigation guidelines, while nutrients represent valuable sources for microalgae cultures. In this study, the green microalgae Chlorella sorokiniana, isolated from anaerobic digestate, was cultivated in diluted ADE and BG-11 as reference medium, both in flasks and in a 20 L tubular airlift photobioreactor (PBR). Biomass productivity and pollutants removal rate were examined under different CO2 atmospheres and sample dilution in lab-scale experiments, in order to identify the impact of the culture conditions οn nitrogen assimilation by microalgae and to determine the critical variables for process scale-up. The culture growth was further studied in the PBR system, in batch and continuous operation mode, using an ultrafiltration membrane for efficient biomass harvesting. As expected, a higher growth rate was achieved in BG-11 than ADE due to the chemical nature of the effluent. However, efficient ammonia nitrogen removal was observed in the wastewater sample, reaching up to 90%, proving that microalgae cultivation represents a challenging method, combining the simultaneous nutrients recovery and excess biomass production.