It was examined carefully that the bacterial die-off between Chlorella vulgaris and E. coli. W3110 was tested through adding TOC (total organic carbon) with the lab-scaled continuous river water flow system (CRWFS). Artificial synthetic wastewater was applied at two levels of organic carbon concentration; 1,335 mg/l in treatment type 1 and 267 mg/l in type 2. In both types, the population densities of Chlorella vulgaris were similar in a maximum 8.25 ${ imes}$ 10$^$6/ cells/ml (type 1) and 6.925 ${ imes}$ 10$^$6/ cells/ml (type 2). The maximum densities of E. coli. W3110 were 2.0 ${ imes}$ 10$^$8/ colony forming unit (CFU)/ml in type 1 and 3.9 ${ imes}$ 10$^$8/ CFU/ml in type 2. The densities increased for 11 days in type 1 and 4 days in type 2, then decreased rapidly till the 35th day, then slightly increased again. This trend was prominent in type 2. It implied that a wider range of nutrients was required in the growth of heterotrophic bacteria in type 2 than in type 1. We could not expect successful bacterial die-off if the wastewater retention time was not furnished sufficiently.