  The paper considers a chemostat model describing an activated sludge process
in wastewater treatment. The model is assumed to be subject to environment
noise in terms of both white noise and color noise. The paper fully
characterizes the asymptotic behavior of the model that is a hybrid switching
diffusion. We show that the long-term properties of the system can be
classified using a value $\lambda$. More precisely, if $\lambda\leq 0$, the
bacteria in the sewage will die out, which means that the process does not
operate. If $\lambda>0$, the system has an invariant probability measure to
which the transition probability of the solution process converges
exponentially fast. One of the distinctive contributions of this paper is that
the critical case $\lambda=0$ is considered. Numerical examples are given to
illustrate our results.
