Which are ALWAYS CORRECT about an ideal chemostat? P internal substrate concentration equals exit concentration; Q optimal dilution is below critical dilution; R biomass concentration rises with dilution; S cell recirculation permits operation beyond critical dilution.
Perfect mixing gives P. For the usual steady biomass-productivity objective, the optimum occurs below the washout boundary, supporting Q under the intended convention. Recycling retains cells and can allow hydraulic dilution beyond the corresponding no-recycle critical rate, supporting S. R is false in the ordinary Monod chemostat because residual substrate rises and biomass generally falls as washout approaches. The intended combination is P,Q,S, D; the “optimal” statement presumes the conventional productivity objective.
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