The aim of this work was to investigate the productivity of continuous lactate fermentations at increased cell densities on simulated lignocellulosic hydrolyzate and wheat straw hydrolyzates using Ca(OH)(2) suspensions as base. To this end, a semi-automated membrane bioreactor prototype with external tubular membranes was designed and constructed. Long-term continuous fermentations were run in membrane bioreactor mode and compared to batch and continuous mode. Fouling of the membranes proved to be the major bottleneck in the process. The semi-automated membrane module switch elegantly circumvented this problem, allowing constant operating conditions throughout extended periods of time. Maximal productivities of 30.5 g kg(-1).h(-1) could be reached in the prototype during 30.5 h combined with an average lactate concentration of 85.4 g kg(-1) and complete consumption of C5 and C6 carbohydrates. In this fermentation zone, the (liquid) residence time was 2.8 h. These numbers demonstrate significant performance improvements in comparison with batch and (regular) continuous conditions.