MEMBRANE BIOREACTOR FOR LACTIC-ACID PRODUCTION

A new type of membrane bioreactor is proposed for the transformation o f lactose into lactic acid by Lactobacillus rhamnosus. The reactor is tubular and contains two coaxial porous alumina tubes. The internal on e supports an alpha alumina membrane (mean pore size: 2.0 x 10(-7) m) on its internal wall, the external one has the same membrane on its ex ternal face. The bacteria are fixed into the support macroporosity and confined in the annular space defined by the two separating walls. Th e substrate solution is fed into the reactor inner compartment whereas the liquid percolates in the radial direction across the two membrane s. This original configuration allows the transformation of lactose in to lactic acid in the porous space contained between the two microfilt ration layers. A model is developed in cylindrical coordinates and tak es into account the mass transfer phenomena coupled with biological re action in the membrane annulus. The continuous decrease of permeate fl ow rate is introduced into the theoretical calculation by a Michaels t ype law relating the permeate flux evolution to the bacteria populatio n growth. The theoretical results are in good agreement with experimen ts and the bacteria population growth evolution could be simulated in order to choose the experimental conditions for the best productivity. This membrane reactor can be operated for 90 h, producing a total sub strate conversion at high contact time. However, a continuous decrease of permeate flux was observed as a result of membrane plugging by the bacteria population growth.