At chemostat steady state, washout of cells equals growth: $\dfrac{dx}{dt} = 0 = (\mu - D)x$. Since $x \neq 0$, we must have $\mathbf{\mu = D}$. The dilution rate $D = F/V$ (h⁻¹) therefore directly sets the specific growth rate. This is the fundamental principle of the chemostat — the operator controls $D$, which fixes $\mu$, which in turn fixes the steady-state substrate concentration via the Monod equation.