Photosynthetic aeration in biological wastewater treatment using immobilized microalgae-bacteria symbiosis

Chlorella vulgaris encapsulated in alginate beads were added into a bioreactor treating synthetic wastewater using Pseudomonas putida. A symbiotic CO2/O-2 gas exchange was established between the two microorganisms for photosynthetic aeration of wastewater. During batch operation, glucose removal efficiency in the bioreactor improved from 50 % in 12 h without aeration to 100 % in 6 h, when the bioreactor was aerated photosynthetically. During continuous operation, the bioreactor was operated at a low hydraulic retention time of 3.3 h at feed concentrations of 250 and 500 mg/L glucose. The removal efficiency at 500 mg/L increased from 73 % without aeration to 100 % in the presence of immobilized microalgae. The initial microalgae concentration was critical to achieve adequate aeration, and the removal rate increased with increasing microalgae concentration. The highest removal rate of 142 mg/L-h glucose was achieved at an initial microalgae concentration of 190 mg/L. Quantification of microalgae growth in the alginate beads indicated an exponential growth during symbiosis, indicating that the bioreactor performance was limited by oxygen production rates. Under symbiotic conditions, the chlorophyll content of the immobilized microalgae increased by more than 30 %. These results indicate that immobilized microalgae in symbiosis with heterotrophic bacteria are promising in wastewater aeration.