Washout occurs when $D > \mu_{max}$: cells cannot grow fast enough to replace those lost in the effluent, leading to exponential decline in biomass until the fermenter is sterile. The steady-state biomass concentration is: $x = Y_{X/S}(S_0 - S)$, where $S = K_S D/(\mu_{max} - D)$. As $D \rightarrow \mu_{max}$, $S \rightarrow S_0$ and $x \rightarrow 0$, confirming washout. In practice, fermenters are operated at $D = 0.5$–$0.9 \times \mu_{max}$.