Regular articleLactic acid production from recycled paper sludge: Process intensification by running fed-batch into a membrane-recycle bioreactor

•L. rhamnosus cultivation is product-inhibited at lactic acid concentrations above 58 g L−1.•A dynamic filtration unit was built for high-concentrated solids suspensions.•Product is removed while retaining bacterial cells and enzymes.•SSF process was capacity and yield-intensified into the membrane-recycle bioreactor.•Model towards best operation of SSF processes into membrane-recycle bioreactors.

Production of lactic acid from recycled paper sludge by simultaneous saccharification and fermentation (SSF) has previously been implemented under a pulsed fed-batch mode. However, lactic acid concentrations above 58 g L−1 inhibit cultivation of the Lactobacillus rhamnosus strain used. Thereby, the present work targeted process intensification by running it into a membrane-recycle bioreactor, providing product removal together with reuse of enzymes and bacterial cells.A shear-enhanced flat sheet cross-flow filtration system was built, working properly with the high-solids concentration suspension. Based on product inhibition and solids concentration constraints, a model was proposed for operation of the membrane-recycle bioreactor. SSF should be run in batch mode into the fermentor for 48 h, and then switch-on recirculation through the filtration module, with pulsed fed-batch addition of recycled paper sludge.This innovative approach can be applied to improve other SSF processes dealing with high-solids concentrations, towards cost-effective lignocellulosic feedstock biorefineries.

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