Comparative Assessment of Activated Sludge, Microalgae, and Microalgal–Bacterial Consortia for Dairy Wastewater Treatment Efficiency and Methane Potential of Recovered Biomass

Abstract

Modern biological wastewater treatment technologies should not only effectively remove organic compounds and nutrients but also enable the recovery of biomass suitable for further valorization. The aim of this study was to compare activated sludge (AS), a Chlorella sp. monoculture (MA), and a microalgal–bacterial consortium (M-BC) for the treatment of model dairy wastewater and to assess the methane potential of the recovered biomass. AS provided the highest organic matter removal efficiency, reducing COD and BOD5 concentrations to 141.4 ± 7.6 mg O2/L and 32.3 ± 1.7 mg O2/L, respectively, corresponding to removal efficiencies of 94.3 ± 0.3% and 98.1 ± 0.1%. The M-BC variant achieved the highest nitrogen and phosphorus removal efficiencies, at 90.3 ± 0.6% and 90.0 ± 0.6%, respectively. The system based solely on MA was less effective from a treatment perspective, but the resulting biomass had the highest volatile solids content and the highest methane potential. In this case, cumulative CH4 production after 20 days was 255.1 ± 12.3 mL CH4/g VS. The corresponding values were 230.2 ± 15.9 mL CH4/g vs. for M-BC and 189.5 ± 6.3 mL CH4/g vs. for AS. The Gompertz model, however, indicated the most favorable digestion kinetics for M-BC biomass, with an Rmax of 26.8 ± 1.8 mL CH4/g VS·d and a lag phase of 0.53 ± 0.08 d.