NOVELTY - Selecting (M1) useful material from a mixed solid, liquid or gaseous material stream, comprises: defining a value range requirement for at least one parameter of interest of useful material to be selected from the material stream; passing the material stream through at least one detector adapted to measure the parameter of interest of the material stream; and separating the material stream into useful material and residue based on the measured parameter.    USE - The methods are useful: for selecting useful material from a mixed solid, liquid or gaseous material stream; to grade algal fuel and selecting individual organisms in the algal culture that have superior energy storing performance; for enhancing exergetic efficiency (all claimed); to detect and grade the desirable hydrocarbon material in bulk streams of mixed material; to mine useful material from post-consumer waste; and to regulate the liberation of energy from algae producers.    ADVANTAGE - The method provides and tests quantifiable standard, in an automatic manner.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) a system for selecting a useful material from the mixed solid, liquid or gaseous material stream passing through the system, comprising: at least one detector (10) adapted to measure a parameter of interest of the material stream passing through it; and at least one separator (20) for separating the material stream into useful material and residue based on the measured parameter and a value range requirement after passing through the detector; and    (2) enhancing the exergetic efficiency comprises: measuring the Z(eff); measuring density; determining energy content; measuring actual performance of a fuel; feeding data of density, energy content into at least one of algorithms; and making adjustments as to the membership functions that define desirable fuel.    DESCRIPTION OF DRAWING(S) - The figure illustrates a schematic view of a material handling system.    Detector (10)    Separator (20)    Treater (30)    Processor (40)    Controller (50)