The removal of nitrogen (ammonium ion (NH4+ )) is shown in Fig. 4. During the lag phase, the nitrogen removal efficiency varied from 107.63±4.84 mg/L to 92.86±5.36 mg/L matched with no growth of the Chlorella vulgaris, but then sharply decreased from 107.63±4.84 mg/L to 3.48±0.47 mg/L during the rapid growth-phase. It is interesting to note that the total biomass weight gained during the entire cultivation period is well balanced by nitrogen removed from the culture medium as clearly shown in Fig. 1.