A bioreactor for microbial conversion of at least one conversion substrate, which has a treatment zone to accommodate when in use a solution of at least one conversion substrate, a culture holding zone to accommodate when in use a microbial culture capable of metabolizing at least one conversion substrate, a source of primary growth substrate for the microbial culture, a first permeable membrane forming an interface between the treatment zone and the culture holding zone, and a second permeable membrane forming an interface between the source of primary growth substrate and the culture holding zone, the first permeable membrane being of a material which will allow passage of the at least one conversion substrate from the treatment zone to the culture holding zone whilst being impermeable to the microbial culture, the second permeable membrane being of a material permeable to the primary growth substrate but substantially impermeable to water; and a process of operating said bioreactor.