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Production of lactic acid and derivatives from grass using mixed populations

Second generation biorefinery is a facility for biofuel and material production where both fuel and high value products are produced from waste feedstock such as lignocellulosic biomass. Hydroxycarboxylic acids such as lactic acid can be produced from lignocellulosic biomass, which can serve as precursor to its polymer polylactic acid (PLA), or precursor for aviation fuel. A widely abundant biomass feedstock which comes forth for this purpose is grass. To achieve this, the intermediary processes have to be thoroughly investigated. 
Before the biomass feedstock can be utilized, it has to be pretreated to improve its biodegradability. Hence, the first objective of this thesis was to perform pretreatment on the selected biomass – grass. Thermo-mechanical pretreatment with extrusion and chemical pretreatment using calcium hydroxide were employed to enhance the biodegradability of grass. The efficiency of the pretreatment was evaluated based on the methane production and chemical oxygen demand (COD) conversion through mesophilic anaerobic digestion. 
Once the pretreatment was confirmed to be effective in improving the biodegradability of biomass, storage tests was performed to investigate the effect of pretreatment on biomass storability. Adjacent to storage, a fermentation process was performed to obtain lactic acid. The native microbiome from grass involved in the fermentation was also investigated. 
Extraction of lactic acid is required after this fermentation process. Membrane electrolysis using anion exchange membrane was used to extract lactic acid from the fermentation broth. Through this process, the pH of the fermentation broth can be controlled without extra chemicals, and lactic acid can be extracted and concentrated into a clear solution. 
Due to the hydrophilic nature of lactic acid, a possible strategy is to convert lactic acid into a more hydrophobic product – caproic acid through microbial chain elongation which is also known as reverse beta oxidation. Caproic acid has a maximum water solubility of 11 g L-1 at 20 °C, which allows an easier product separation from the fermentation broth. The extracted caproic acid can be further upgraded to fuel by Kolbe electrolysis. 
For polymerization of lactic acid into PLA, further purification processes were needed. First esterification process was used to remove acetic acid from the extracted solution. Ion exchange resin was used to further remove impurities from the broth. Finally, pure lactic acid was obtained using diethyl ether extraction. Polycondensation was performed to polymerize the lactic acid into the biodegradable plastic PLA.