Continuous Ethanol Production with a Membrane Bioreactor at High Acetic Acid Concentrations

The release of inhibitory concentrations of acetic acid from lignocellulosic raw materials during hydrolysis is one of the main concerns for 2nd generation ethanol production. The undissociated form of acetic acid can enter the cell by diffusion through the plasma membrane and trigger several toxic effects, such as uncoupling and lowered intracellular pH. The effect of acetic acid on the ethanol production was investigated in continuous cultivations by adding medium containing 2.5 to 20.0 g·L-1 acetic acid at pH 5.0, at a dilution rate of 0.5 h-1. The cultivations were performed at both high (~25 g·L-1) and very high (100-200 g·L-1) yeast concentration by retaining the yeast cells inside the reactor by a cross-flow membrane in a membrane bioreactor. The yeast was able to steadily produce ethanol from 25 g·L-1 sucrose, at volumetric rates of 5-6 g·L-1·h-1 at acetic acid concentrations up to 15.0 g·L-1. However, the yeast continued to produce ethanol also at a concentration of 20 g·L-1 acetic acid but at a declining rate. The study thereby demonstrates the great potential of the membrane bioreactor for improving the robustness of the ethanol production based on lignocellulosic raw materials.