membrane being potted into plastic inserts 14 16 with epoxy 18 comprising a ceramic insert it should be appreciated that alternative resins or mechanical sealing technology may also be used a housing or reactor shell 20 of glass is arranged coaxially with the capillary membrane 12 and is provided with end caps 22 and 23 which thread onto the glass housing 20 the housing 20 includes a feed inlet 26 for introducing a liquid nutrient feed solution for microorganisms into the space 24 between the porous substrate and the housing and an inoculation inlet 25 for introducing an inoculum into the housing for attachment to the membrane 12 the housing further includes a feed outlet 27 for discharging the feed solution from the housing the ceramic insert 12 14 16 includes a product outlet 28 in fluid communication with the capillary lumen 34 for discharging permeate from the housing in use a biofilm 32 is established on an external surface 30 of the capillary membrane 12 this is achieved by filtering a spore or vegetative inoculum of the desired microorganism through the capillary membrane 12 and draining any permeate out of the lumen 34 and through the outlet 28 the lumen 34 is thus in flow communication with the outlet 28 the inoculum is thus immobilised on the membrane surface 30 the membrane 12 has a relatively thin porous skin 36 on the inside and a relatively thick finger like externally unskinned void structure 38 radiating outwardly from the skin 36 typically the membrane 12 has but is not limited to an outside diameter of about.