Effects of operational parameters on fluidised cultivation of flocculated microalgal–bacterial consortia and removal of dissolved organic matter and nutrients from municipal wastewater

This study investigates the effects of operational parameters, particularly carrier packing ratios and hydraulic retention time (HRT), on microalgal–bacterial consortia (MBC) cultivated using primary treated municipal wastewater and polyurethane foam carriers. Two experiments (A and B) were conducted in four reactors. In A, reactors A1 and A2 operated under shortened HRT with different packing ratios. In B, reactor B1 was started at shortened HRT and subsequently at extended HRT, whereas reactor B2 was continuously operated at extended HRT. The MBC distribution, settleability of the suspended MBC, wastewater-treatment performances and requirement of carbon dioxide (CO2) gas injection during the treatment were observed. A large proportion of the total MBC biomass was attached to carriers and detached as clusters in A1 and A2. Under shortened HRT, suspended MBC was flocculated and exhibited enhanced settleability (3-h sedimentation ratios: 0.82 and 0.83), regardless of carrier packing ratios, whereas higher packing ratios improved the removal of dissolved organic matter. Regardless of the HRT shift, B1 and B2 operated under extended HRT revealed a trade-off, i.e. improved dissolved total nitrogen removal was accompanied by reduced settleability of suspended MBC (3-h sedimentation ratios: 0.49 and 0.67), decreased dissolved organic matter removal and increased CO2 gas-injection demand.