Biomass concentration ($x$)

The biomass concentration is the mass of biological material per unit culture volume; here it is measured as dry cell mass in $\mathrm{g}{\mathrm{L}}^{-1}$. A biomass concentration differs from a cell count per unit volume because cell sizes can change. In the ideal chemostat, biomass leaves the vessel at rate $Qx$, giving a removal term $-Dx$ in the ordinary differential equation for $x$.