AbstractVarious aspects of process water recycle in a continuous flow fermentation process are analyzed. Simple mass balance equations in terms of product and feed components for a single‐stage reactor producing biomass are developed. Constraints on the recycle ratio, imposed by the efficiency of the dewatering stage, are examined. The recycle analysis is extended using a kinetic growth model incorporating water soluble product formation and growth inhibition. The potential effect of recycle on substrate conversion and product accumulation is also examined and the concept of a critical recycle ratio in fermentation processes is develope