Characterisation of microalgae-based monocultures and mixed cultures for biomass production and wastewater treatment

Microalgae-based systems have been increasingly considered to remediate wastewater, while producing valued biomass. While microalgal monocultures are difficult to maintain in wastewater, mixed cultures have the potential to offer improved system stability and robustness. This research aims to provide a detailed comparison of microalgal monoculture and mixed culture photobioreactors (PBRs) in terms of microalgae growth, biomass harvesting potential, nutrient removal and organic characteristics. Some of the PBRs were further tested for their responses to the presence of wastewater-derived bacteria. Monocultures (containing species from Chlorophyta, Cyanophyta and Eustigmatophyta, respectively) and mixed cultures (mixing two or three species) were evaluated using advanced characterisation techniques including flow cytometry, liquid chromatography – organic carbon detection and excitation−emission matrix fluorescence spectroscopy. The impact of bacteria on system performance was explored by co-culturing some of the cultures with activated sludge. In general, mixed culture systems showed more stable biomass growth and improved nutrient removal uptake (by 20%), as compared to monocultures. However, overyielding (higher biomass production) was not observed in the mixed cultures. Mixing different microalgae tended to increase the concentrations of soluble extracellular materials (e.g. biopolymers) that were able to retain water within the biomass, leading to a lower biomass harvesting potential, as indicated by the capillary suction time. Adding activate sludge significantly increased reduction of measurable dissolved organic carbon concentration (particularly biopolymers and low molecular weight compounds) and nitrogen by >250%. However, wastewater-derived bacteria reduced the yield of all the cultures substantially (by 63–76%) and lowered the dewaterability of the cultures.