Algal photosynthetic aeration increases the capacity of bacteria to degrade organics in wastewater

Wastewater treatment is an energy-intensive process and a net emitter of greenhouse gas emissions. A large fraction of these emissions is due to intensive aeration of aerobic bacteria to facilitate break-down of organic compounds. Algae can generate dissolved oxygen at levels in excess of saturation, and therefore hold the potential to partially displace or complement mechanical aeration in wastewater treatment processes. The objective of this study was to develop an internally consistent experimental and modeling approach to test the hypothesis that algal photosynthetic aeration can speed the removal of organic constituents by bacteria. This framework was developed using a simplified wastewater treatment process consisting of a model bacteria (Escherichia coli), a model algae (Auxenochlorella protothecoides), and a single carbon source that was consumable by bacteria only. This system was then tested both with and without the presence of algae. A MATLAB model that considered mass transfer and biological kinetics was used to estimate the production and consumption of O2 and CO2 by algae and bacteria. The results indicated that the presence of algae led to 18-66% faster removal of COD by bacteria, and that roughly one-third of biochemical oxygen demand was offset by algal photosynthetic aeration.