The slurry from anaerobic digestion process (biogas slurry) is now well known to potentially cause water
eutrophication and even large-scale algal blooms. The Chlorella vulgaris utilized in this research is able to
purify the biogas slurry effectively under optimum light quality and light intensity supply strategy. The
descending order of dry cell weight for C. vulgaris reproduction and nutrient removal efficiency of biogas
slurry both are red > white > yellow > purple > blue > green, indicating that the red light is the optimum
light quality. Furthermore, this study was among the first attempt to analyzed the economic
efficiency for nutrient removal under various light intensity supply strategies, and found out that the
optimal light intensity supply strategy used for microalgae culture should be incremental intensity
strategy, rather than constant intensity strategy, to avoid light energy wastage and photoinhibition. The
optimal light intensity supply strategy is: Phase 1, 0e48 h with 800 mmol m2 s1; Phase 2, 48e96 h with
1200 mmol m2 s1; and Phase 3, 96e144 h with 1600 mmol m2 s1. In addition, the optimal cultivation
time is 144 h.