Effects of monospecific and mixed-algae culture on performance of algae-sludge membrane bioreactors

[Display omitted] •Mixed algae strains inoculation in MBR provides better results than pure strains.•An increase in number of nitrifier bacteria enhanced NH4 removal in mixed algae-MBR.•Algal O2 enhanced nitrification and diminished membrane fouling in the cake layers.•More fatty acids exist in batch algae-sludge reactors than in the sludge reactor.•Mixed algae-MBRs had a greater diversity of bacteria than solo algae MBRs. To increase wastewater treatment efficiency and biofuel production, seven microalgae were mixed with activated sludge in batch bioreactors. Based on batch results, two microalgae (Chlamydomonas and Selenastrum) and their mixture were inoculated into conventional-membrane-bioreactors (CMBRs) to evaluate effects of monospecific and mixed-algae culture on the performance of algae-sludge-MBRs. The best nutrient removal, highest chlorophyll-a, and lowest membrane fouling were achieved by the mixed-algae membrane bioreactor. In comparison to activated sludge, the algae-sludge mixture had fivefold higher lipid contents during batch experiments. Additionally, using confocal microscopy, autofluorescence and staining were combined to distinguish algae from bacteria on membrane surfaces, revealing a greater role for bacteria in membrane fouling. Furthermore, sequencing analysis showed that the microbial community (e.g. Nitrospira and Falavobacterium) changed by inoculating algae which benefits CMBRs. Consequently, the stimulation or inhibition of different species might be the reason that the mixed-algae-MBR achieves superior performance compared to CMBR and single-algae-MBRs.