MaB-flocs for a more sustainable wastewater treatment

Conventional wastewater treatment with activated sludge has a large carbon footprint, high aeration rates are combined with CO2 emission from bacteria. By using micro-algal bacterial flocs (MaB-flocs), CO2 could be captured within the biomass and oxygen could be produced in situ. In order to maximize this photosynthetic aeration and CO2 mitigation, we investigated whether inorganic carbon could alter the algae/bacteria ratio while keeping a good removal performance and settleability of the MaB-flocs. Therefore, three illuminated sequencing batch reactors with MaB-flocs were fed with synthetic wastewater enriched with 84.2, 42.1 and 0 mg L-1 C-KHCO3 supplemented with 0, 42.1, 84.2 mg L-1 C-sucrose, respectively, representing the inorganic carbon source compared to an organic carbon source. Bicarbonate significantly increased the chlorophyll a concentration of the MaB-flocs, but only poor settling flocs could be obtained causing a high turbidity of the effluent. Moreover, significant lower nitrogen removal efficiencies were measured feeding bicarbonate compared to sucrose and the pH increased (9.5). Sucrose benefited a good reactor performance and showed a good settleability of MaB-flocs. Despite the lower chlorophyll a concentration of the biomass and the lower in situ oxygen concentration, average soluble COD removal efficiencies of 95 % were achieved with sucrose. Furthermore sucrose was successful in containing the optimal pH at 7. This study shows the importance of the carbon source for a good reactor performance. As a consequence, the inorganic/organic carbon ratio of the wastewater should be taken into account when algal bacterial reactors are used for wastewater treatment