Starvation enhances phosphorus removal from wastewater by the microalga Chlorella spp. co-immobilized with Azospirillum brasilense

In synthetic wastewater, growth and phosphorus absorption by two species of microalgae, Chlorella sorokiniana and Chlorella vulgaris, and in domestic wastewater by C. sorokiniana significantly enhanced after a starvation period of 3 days in saline solution, combined with co-immobilization with the microalgae growth-promoting bacterium (MGPB) Azospirillum brasilense Cd in alginate beads. Starvation of 5 days negatively affected the subsequent growth of C. vulgaris, but not of C. sorokiniana in fresh wastewater. Starvation of immobilized cultures of microalgae separately or microalgae with bacteria, followed by returning the immobilized cultures to the same wastewater did not enhance phosphorus absorption. However, a starvation period followed by subsequent submersion of the cultures in fresh wastewater allowed the continuation of phosphorus absorption. The best phosphorus removal treatment from a batch of synthetic or domestic wastewater was with tandem treatments of wastewater treatment with pre-starved, co-immobilized microalgae and replacement of this culture, after one cycle of phosphorus removal, with a new, similarly starved culture. This combination treatment with two cultures was capable of removing up to 72% of phosphorus from the wastewater. There was a direct correlation between the initial load of phosphorus in the domestic wastewater and the efficiency level of removal, being highest at higher phosphorus loads in co-immobilized cultures. This occurred for both immobilized and co-immobilized cultures. Further, the results showed that negative effects of starving the microalgae were mitigated by the application of the MGPB A. brasilense Cd. This is the first report of this capacity in Azospirillum sp. on a single-cell plant. This study showed that starvation periods, combined with co-immobilization with MGPB, have synergistic effects on absorption of phosphorus from wastewater and merits consideration in designing future biological treatments of wastewater.