Autotrophic microalgae require CO2 to perform photosynthesis, and produce O2 as a by-product. On the other hand, heterotrophic bacteria catalyzing aerobic bioprocesses require O2 and release CO2 as the by-product. Therefore, it is possible to establish a symbiotic relationship between the microalgae and the bacteria, wherein CO2 and O2 can be freely exchanged between the microorganisms, obviating the need for any external gas supply for their growth and metabolism.
The suitability of such a symbiotic interaction is most obvious in a biological wastewater treatment process, where aeration of the wastewater is considered an energy intensive step. Bacterial/algal symbiosis has the potential to reduce the cost of aeration, and it can use inorganic pollutants from wastewater as fertilizers for growth, reducing the cost of microalgae cultivation. However, co-cultivation of bacteria and microalgae has limitations, especially regarding light penetration in a dense culture. Besides, nutrient-rich microalgae can be food for some microorganisms.
One approach to alleviate this challenge is through the use of microalgal biofilm on a transport support. While the microalgal cell wrapped in extracellular polymeric substances will not be in direct contact with the bacteria, they can still exchange CO2 and O2 with the heterotrophic microorganisms in symbiotic interactions. Besides, by regulating the thickness of the biofilms, it would be possible to supply sufficient light to the microalgae through the transparent support.
In this research, photosynthetic aeration of biological wastewater treatment was performed by using Chlorella vulgaris as the photoautotroph. The effects of bioreactor operating parameters on aeration were examined and the biofilms were characterized. The results suggest photosynthetic aeration can be an alternative to mechanical aeration in wastewater treatment.
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