Persistence criteria for a chemostat with variable nutrient input and variable washout with delayed response in growth

We study a general chemostat model with a discrete-time delay between consumption and growth. The goal of this article is to provide sufficient and necessary conditions for persistence. This extends previous works in the matter. Furthermore, in the particular case of periodic nutrient input and the same periodic washout with delayed response in growth, the persistence criteria are average criteria. We remark in no case we need to impose any restrictions on the size of the delay.