Thierry Mounir
To address the problem of insucient wastewater treatment and eutrophication of lakes and rivers, the symbiotic association of microalgae and bacteria has shown great potential and numerous advantages. First, co-cultures can exhibit ecient treatment of domestic wastewater, with the combination of high nutrients (nitrogen and phosphorus) removal abilities of the microalgae and ecient degradation of organic matter by bacteria. Second, the symbiosis between microalgae and bacteria exchanging CO2 and O2 makes aeration not necessary, thus signicantly reducing the operating costs of the wastewater treatment plant. Third, recovered biomass of algae could be used as highpro t products like biofuels and food source. Fourth, the xation of the atmospheric CO2 by the microalgae through photosynthesis would contribute to greenhouse gas reduction in the atmosphere. Finally, the decayed microalgal biomass could serve as organic carbon source for denitrication by bacteria. However, this technology is not well-known yet, and further research on the in uence of the operating parameters have to be conducted. Furthermore, its main drawback is the harvesting of the microalgae. Due to their small size, microalgae demonstrate a low settling rate of 3.6 x 103 m/h. A solution is to enhance aggregation of the biomass through occulation, followed by granulation to reach fully-developed microalgal-bacterial consortia. These granules exhibit high treatment performances and high settleability. The rst objective of this project was to identify the optimal microalgae:bacteria ratio for COD and nutrients removal. The second objective was to form fully-developed granules, starting from a co-culture, and assess the in uence of dierent agitation speeds on the granulation. The photobioreactor with a microalgae:bacteria ratio of 75:25 expressed the most promising results in COD removal eciency (95% after 8 days), nitrogen removal eciency (94% after 9 days), phosphorus removal eciency (93% after 7 days), and biomass productivity (144 mg/L/d). In this photobioreactor, nitrogen was removed by a combination of assimilation into microalgal biomass and nitrication/denitrication by the bacteria. The COD was removed by degradation by bacteria and mixotrophic microalgae that used the organic matter as carbon source. Finally, phosphorus was removed by assimilation into microalgal biomass. Although obvious aggregation between microalgae and bacteria could be observed, no fully-developed granules were formed after 89 batches of 24 hours. Because of low COD and nitrogen removals, no starvation period was experienced by the co-cultures. Therefore, extracellular polymeric substances were not released in sucient amounts to enhance the granulation. Furthermore, excessive stirring (200 rpm) may have lead to microalgal cells damage, thus promoting bacterial growth over microalgal growth. On the other hand, slower agitation speeds (80 and 120 rpm) seemed to enhance microalgal growth. Overall, the agitation speed appeared to have an indirect impact on the treatment performances and the settleability of the co-cultures, by signicantly in uencing the microalgal and bacterial growths.
2020
