NOVELTY - The system comprises a continuous stirred tank reactor (CSTR) (14) and a first plug flow reactor (PFR) (16) of a sterile first stage, a second non-sterile PFR (18) of a non-sterile second stage, an adding unit, a removing unit, and a processor (36). The CSTR includes a chamber for mixing a full load nutrient with algae cells under sterile conditions to create a culture. The culture is transferred from the CSTR to the first PFR, and where the algae cells in the culture are grown in the first PFR as a selected nutrient. The first PFR includes a chamber for receiving the culture from the CSTR.    USE - The system is useful for implementing non-sterile heterotrophic algae growth, which is useful for producing biofuel.    ADVANTAGE - The system ensures economical, rapid and easy implementation of the non-sterile heterotrophic algae growth.    DETAILED DESCRIPTION - The system comprises a continuous stirred tank reactor (CSTR) (14) and a first plug flow reactor (PFR) (16) of a sterile first stage, a second non-sterile PFR (18) of a non-sterile second stage, an adding unit, a removing unit, and a processor (36). The CSTR includes a chamber for mixing a full load nutrient with algae cells under sterile conditions to create a culture. The culture is transferred from the CSTR to the first PFR, and where the algae cells in the culture are grown in the first PFR as a selected nutrient. The first PFR includes a chamber for receiving the culture from the CSTR and for growing the culture under sterile conditions during a first residence time, where a selected nutrient is depleted from the culture during the first residence time of 4-10 hours to create an effluent. The second PFR includes a chamber for receiving the effluent from the first PFR and for holding the effluent in the second PFR during a second residence time of 12-120 hours. The adding unit is designed for adding organic carbon to the effluent in the chamber of the second PFR during the second residence time to stimulate rapid formation of lipids within the algae cells, and comprises a source of the organic carbon and a conduit connecting the source of organic carbon in communication with the chamber of the second PFR for adding organic carbon to the effluent. The removing unit is designed for removing grown algae from the second PFR. The processor is configured for converting grown algae from the second PFR into a biofuel. The selected nutrient inhibits growth of contaminants that compete with algae cells for food. The organic carbon is added to maintain a predetermined carbon to nitrogen ratio of 14:1-25:1. The first stage and second stage comprise a bioreactor (12), and have first residence time and the second residence time respectively. An INDEPENDENT CLAIM is included for a method for non-sterile heterotrophic algae growth.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a system for implementing non-sterile heterotrophic algae growth.    Bioreactor (12)    CSTR (14)    First PFR (16)    Second PFR (18)    Processor. (36)