Influence of supplementary carbon on reducing the hydraulic retention time in microalgae-bacteria (MaB) treatment of municipal wastewater

Real municipal wastewater was treated using microalgae and bacteria (MaB) flocs with the goal to achieve the European standards (EUS91) in less than 2 days of hydraulic retention time (HRT). The pH in the control series (CS) was allowed to increase freely. In the inorganic carbon supplement (ICS) series, the pH was maintained below 8.0 by means of CO2 injection. The presence of CO2 regulated the nitrogen profiles, but the total removal did not improve. Nitrification was stronger and advanced more when CO2 was applied during 2 days of HRT with an average ammonium oxidation rate of 40.5 +/- 4.9 %. The residual phosphate and nitrogen oxides were assimilated on the third day of treatment. Balancing the organic carbon (COD:NH4-N ratio of 15.9) suppressed the autotrophic activities (nitrification) and improved the biomass assimilation significantly. Batch experiments with balanced organic carbon could remove nitrogen and phosphorus with rates of 43.1 +/- 5.1 mg N L-1 d-1 (88.3 +/- 13.9 %) and 6.21 +/- 0.07 mg PO4-P L-1 d-1 (98.2 +/- 1.6 %) after 28 hours. Our results show that balanced MaB culture can satisfy the EUS91 limits within approximately 24 hours of treatment.