Cultivation and selection of cyanobacteria in a closed photobioreactor used for secondary effluent and digestate treatment.

The main objective of this study was to select and grow wastewater-borne cyanobacteria in a closed photobioreactor (PBR) inoculated with a mixed consortium of microalgae. The 30L PBR was fed with a mixture of urban secondary effluent and digestate, and operated in semi-continuous mode. Based on the nutrients variation of the influent, three different periods were distinguished during one year of operation. Results showed that total inorganic nitrogen (TIN), inorganic phosphorus concentration (PO), phosphorus volumetric load (L-P) and carbon limited/non-limited conditions leaded to different species composition, nutrients removal and biomass production in the culture. High TIN/PO concentrations in the influent (36mg N L/3mg P L), carbon limitation and an average L-P of 0.35mg P Ld were negatively related to cyanobacteria dominance and nutrients removal. On the contrary, cyanobacteria predominance over green algae and the highest microbial biomass production (averaging 0.084g Volatile Suspended Solids (VSS) Ld) were reached under TIN/PO concentrations of 21mg N L/2mg P L, no carbon limitation and an average L-P of 0.23mg P-PO Ld. However, although cyanobacteria predominance was also favored with a L-P 0.15mg Ld, biomass production was negatively affected due to a P limitation in the culture, resulting in a biomass production of 0.0.39g VSS Ld. This study shows that the dominance of cyanobacteria in a microalgal cyanobacterial community in an agitated PBR using wastewater as nutrient source can be obtained and maintained for 234days. These data can also be applied in future biotechnology applications to optimize and enhance the production of added value products by cyanobacteria in wastewater treatment systems.