The present study, based on a previous batch-wise experiment, investigated a lab-scale semi-continuous cultivation of green microalgae Chlorella vulgaris (UTEX 2714), as a useful means for nutrient reduction as well as production of algal biomass which can be used as potential feedstock for the production of biofuel and other commodities, on 20 x diluted dairy manures. Both undigested and digested samples were applied in parallel experiments for comparison regarding the requirements of hydraulic retention times (HRTs), removal efficiencies of nitrogen, phosphorus, and chemical oxygen demand (COD), biomass productivities, and CO₂ sequestration abilities. It was demonstrated that algae grown in undigested dairy manure achieved removal rates of 99.7%, 89.5%, 92.0%, and 75.5% for NH₄+--N, TN, TP, and COD, respectively, under a 5-day HRT, while the HRT had to extend to 20 days in order to achieve 100.0% removal of NH₄+--N in digested one with simultaneous removals of 93.6% of TN, 89.2% of TP, and 55.4% of COD. The higher organic carbon contained in undigested dairy manure helped boost the growth of mixotrophic Chlorella, thus resulting in a much shorter HRT needed for complete removal of NH₄+--N. Moreover, algae grown in digested dairy manure provided more penitential than those grown in undigested one in CO₂ sequestration per milligram of harvested dried biomass (1.68 mg CO₂/mg dry weight (DW) vs 0.99 mg CO₂/mg DW), but did not surpass in total the amount of CO₂ sequestered on a 15-day period basis because of the better productivity gained in undigested dairy manure.