In this paper, a membrane lifetime model is developed and experimentally validated. The lifetime model is based on the Weibull probability density function. The lifetime model can be used to determine an unambiguous characteristic membrane lifetime. Experimental results showed that membrane lifetime shortens if the average membrane fouling status increases. The lifetime modeling results are then used to determine the economic lifetime of membranes. Subsequently, the economic lifetime of a membrane is used to optimize membrane lifetime, i.e. minimizing the total costs. Based on the experimental results it can be concluded that the total costs are minimal if the average membrane fouling status is approximately 1.7× the membrane resistance.