NOVELTY - Treating wastewater and cultivating biomass comprises: providing aerated bioreactor (202); supporting phototactic heterotrophic microorganism population; supplying wastewater influent (206) having nutrients and organic carbon substrate; cultivating the population, which together with the influent forms bioreactor effluent; transferring the effluent to a light clarifier; concentrating the effluent to form a low-solids (216) and a high-solids effluent (214); dewatering high-solids effluent to form concentrated biomass paste (220); and processing low-solids effluent to form treated water stream.    USE - The methods: are useful for treating wastewater and cultivating biomass; and producing ethanol (all claimed).    ADVANTAGE - The methods: enable treatment of high-strength wastewater having high BOD, and high nitrogen and phosphorus concentrations; produce a valuable biomass by-product that can be used as a feedstock for production of ethanol or other bioenergy; optimize microorganism biomass harvesting; serve as pre-treatment for industrial wastewater producers or in municipal wastewater treatment plants to permanently remove BOD, nitrogen, and phosphorus from the wastewater stream; and utilize population of heterotrophic microorganisms that exhibit a negative phototactic response, in place of conventional bacteria. The phototactic heterotrophic microorganisms: remove the nutrients and convert the organic carbon found in wastewater into a higher value biomass that can be further processed into products e.g. biofuels, bioenergy, bioplastics, biofertilizers, animal feed and nutritional/pharmaceutical ingredients; grow heterotrophically on the organic carbon in the wastewater and are not dependent on sunlight for growth, which allows a much higher biomass density to accumulate in the bioreactor; and are capable of growing heterotrophically in nutrient-laden conditions. The increased biomass density permits a much higher efficiency in harvesting a biomass formed from the phototactic heterotrophic microorganisms.    DETAILED DESCRIPTION - Treating wastewater and cultivating biomass comprises: providing an aerated bioreactor (202) supporting a population of a phototactic heterotrophic microorganism; supplying a wastewater influent (206) (having nutrients and an organic carbon substrate for growth of the population) to the aerated bioreactor; cultivating the population to convert at least a portion of the nutrients and the organic carbon substrate into a higher value biomass, where the wastewater influent and the population together form a bioreactor effluent; transferring the bioreactor effluent from the bioreactor to a light clarifier; concentrating the bioreactor effluent in the light clarifier to form a low-solids (216) and a high-solids effluent (214); dewatering the high-solids effluent to form a concentrated biomass paste (220); and processing the low-solids effluent to form a treated water stream. The step of concentrating the bioreactor effluent is performed by inducing the population to phototactically self concentrate by exposing the population to a source of light having at least one of an intensity and a wavelength sufficient to cause the population to migrate away from the source of light, where the population is concentrated to form the high-solids effluent. INDEPENDENT CLAIMS are also included for:    (1) a system for cultivating biomass, comprising the aerated bioreactor configured to receive an influent stream having nutrients and an organic carbon substrate for growth of the population, where the influent stream and the population together form a bioreactor effluent; and    (2) cultivating biomass comprising: providing the aerated bioreactor; supplying the influent stream to the aerated bioreactor; cultivating the population; and at least one of (a) concentrating the bioreactor effluent in the light clarifier to form the low-solids and the high-solids effluents, (b) treating the bioreactor effluent with a base to form the low-solids and the high-solids effluents, where the base lyses at least a portion of the population, and where the lysed population precipitates to form the high-solids effluent, (c) introducing a flocculent to the bioreactor effluent to induce a rapid settling of at least a portion of the population to form the low-solids and the high-solids effluents, (d) filtering the bioreactor effluent through a membrane filter to remove at least a portion of the population to form the low-solids and the high-solids effluents, or (e) separating the bioreactor effluent into the low-solids and the high-solids effluents by dissolved air flotation.    DESCRIPTION OF DRAWING(S) - The figure shows a flow diagram illustrating a wastewater treatment system, and a method of treating wastewater and cultivating biomass.    Bioreactor (202)    Wastewater influent (206)    High-solids effluent (214)    Low-solids effluent (216)    Concentrated biomass paste (220)