Interaction between Chlorella vulgaris and nitrifying-enriched activated sludge in the treatment of wastewater with low C/N ratio

Wastewater treatment based on algae-bacteria consortia is expected to find use among other biological processes. In this report, the effects of the inoculation ratios of a Chlorella vulgaris and nitrifier-enriched-activated-sludge (NAS) consortium on nutrient removal, carbon capture, and metabolite generation were investigated under autotrophic conditions. The maximum NH4+-N removal (100%) was observed within 7 days in a photo-bioreactor containing 10% C. vulgaris: 90% NAS (w/w); this consortium is referred to as B10. The maximum PO43--P removal (87.5%) was observed in a pure culture of C. vulgaris after 14 days. Due to the fast nitrification achieved using B10, this system showed the highest nitrate accumulation value of 19.73 mg NO3−-N/L at the end of the experiment. The DO concentration in B10 reached 4 mg/L due to the photosynthetic activity of C. vulgaris, providing appropriate conditions for the NAS. Among all the tested consortium ratios, B90 (90%:10% C. vulgaris/NAS, w/w) showed the highest carbon capture (156 mg), while B10 captured only 43.2 mg of carbon. Furthermore, metabolite analysis showed a positive correlation between the proportion of C. vulgaris and the generation of metabolites in the cultures. A trade-off was observed between nutrient removal and carbon capture in C. vulgaris and NAS co-cultures. Overall, our study shows that the aeration system in conventional nitrification processes could be replaced by a cleaner process based on microalgae, resulting in enhanced nutrient removal, increased carbon capture, reduced metabolite generation, and a decrease in excess sludge production.