Performance of Chlorella sorokiniana-activated sludge consortium treating wastewater under light-limited heterotrophic condition

Light limitation often occurs in algae-bacteria consortium. Chlorella sorokiniana is special for heterotrophic growth in the dark, autotrophic or mixotrophic growth in the light. Wastewater treatment by Chlorella sorokiniana-activated sludge consortium under dark heterotrophic conditions (24 h dark, dark period of 12 h/12 h light/dark) was systematically evaluated for the first time. The performance closely depended on sludge/algae ratio with best initial ratio of 1:2 (R₂). Compared to activated sludge (R₀), R₂ showed enhanced NH₄⁺-N and P removal (by 6% and 10%, respectively), similar COD removal, and better settleability. Notably, less O₂ consumption of R₂ than activated sludge made energy-saving possible. Further analysis found that interaction made sludge/algae ratio reversal to be 3:1. The promoting interaction between algae and bacteria was associated with up-regulated cofactors and vitamins, while defensive interaction came from secondary metabolites of terpenoids and polyketides. Despite oxidative stress in the dark consortium, photosynthesis of algae reactivated when switched into light. The performance order was light consortium > dark consortium > activated sludge. Nitrosomonas and Dechloromonas were enriched for nutrients removal. The results reveal the superiority of algae-bacteria consortium over activated sludge whether with or without light.