A process for purifying a composition comprising water, a target substance, impurities and optionally cells, the process comprising the steps (A) and (B): (A) preparing a liquid feedstock by performing step (Ai) and/or (Aii) on the composition: (Ai) removing at least some of the cells from the composition; (Aii) concentrating the composition by removing water therefrom; and (B) passing the liquid feedstock through an apparatus comprising at least two processing units, each such unit producing a product stream containing purified target substance and optionally a waste stream comprising at least some of the impurities, wherein each unit comprises specified components (i) to (v). The units may be essentially the same except for a device they contain, leading to advantages in terms of simplicity, cost and ease of operation, lower risk of operator error, easier maintenance and lower inventory of spare parts.