Lactic acid recovery from grass silage : optimising the downstream processing through micro and nanofiltration membrane technologies

Abstract

This research, part of the AgRefine project, aims to optimise the downstream processing of lactic acid derived from grass silage and other feedstocks using membrane technologies with a focus on microfiltration, nanofiltration and electrodialysis in the separation process within a biorefinery context. The optimal conditions for the downstream processing were investigated at a laboratory scale. The considered evaluated parameters were membrane type, temperature, pressure, and pH to enhance the recovery and purity of lactic acid (Publication #1). This study investigates the production and purification of lactic acid, a crucial compound for the pharmaceutical, food, and plastic industries. By evaluating four nanofiltration membranes (NF270, MPF-36, Toray NF, Alfa Laval NF), the research aims to optimise lactic acid yield through advanced membrane technology. Experiments conducted under varying pH and temperature conditions revealed that the NF270 membrane provided the highest LA rejection at 71%, whereas the MPF-36 membrane showed the lowest at 7% when the pH of the solution was lower than the LA pKa. Additionally, Publication #3 includes the treatment of novel feedstock, fermented candy waste with digestate, using lactic acid bacteria to increase the production of lactic acid and purify it under optimised conditions regarding pH and temperature. A configuration of electro-driven (electrodialysis and bipolar electrodialysis) and pressure-driven membranes (microfiltration and nanofiltration) resulted in lower energy consumption and a high-concentrated solution of lactic acid from the innovative fermented substitute feedstock by the proposed optimised downstream processing. During the downstream processing, retentates are generated, which are rich in compounds like sugars, organic acids, phosphate, nitrogen andVII minerals. As part of this optimisation, it is proposed that retentates could be treated as a culture medium for Chlorella vulgaris (S. Publication #4). This approach aims to prevent new waste generation, promote biomass production, and add value to the effluent stream. The integrated method showcases the potential for sustainable resource utilisation and highlights the importance of innovative strategies in biorefinery processes. Moreover, the successful cultivation of microalgae in less contaminated streams, such as nanofiltration permeates, demonstrates new insights into bio-purification methods. Specifically, these streams contain high concentrations of lactic acid and low concentrations of acetic acid, which Chlorella vulgaris could use acetic acid as a carbon source. Moreover, the process of monitoring and quantifying the analytes in the grass silage juice during downstream processing was carefully considered. Standard methods like high-performance liquid chromatography and ion chromatography, as well as novel techniques like whole-cell biosensors, were tested for their effectiveness in monitoring the analytes. Finally, a discussion about the biorefinery technologies developed in the AgRefine project were suggested as a simplified model (Publication #2) to be applied in extreme conditions environments, including the Atacama and Sonoran deserts, demonstrating their broad applicability and potential benefits.