Food waste fermentation for lactic acid production and recovery
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This research aimed at optimizing the lactic acid production process exploiting the organic matter present in food waste. The objective was to investigate every aspect of the industrial lactic acid production and define new strategies to improve the fermentation efficiency and the extraction phase trying to reduce the operational costs within the process. In the first part of this PhD research, a screening test of four different bacterial sources (Granularsludge, semi digested dairy sludge, primary aerobic sludge and endogenous food waste bacterial community) has been performed at different pH conditions (pH 7 with phosphate buffer and uncontrolled pH with and without thermal pretreatment of food waste) to understand how the final metabolites composition of the fermentation changed. The results highlighted that pH lower than 4 is the most suitable for lactic acid production and its accumulation within the reactors with no further bioconversion. In all the cases in which pH was higher than 4 VFAs production pathways dominated the fermentation. The second part of the research aimed to set up a semi-continuous system operating in repeated batch mode to investigate the effect of the organic loading rate and the hydraulic retention time in the LA production. Carbohydrates conversion yield to lactic acid decreased as the organic loading rate increased following a parabolic correlation. An optimum of 0.81 g LA/g CA was obtained for organic loading rate of 2.00 g VS•L-1•d-1 that decreased to 0.31 g LA/g CA for organic loading rate of 7.50 g VS•L-1•d-1. Lactobacillus sp. dominated the microbial ecology when yogurt was used as inoculum especially for low HRT values. In the third part of this research the FW thermal and ultrasonic pretreatment has been investigated. Ultrasonic pretreatment had a higher impact on the sCOD concentration with 12 % increase against the 9 % given by the thermal pretreatment. No improvements were detected, instead, in the fermentation phase after the pretreatment. Thermal pretreatment gave a maximum yield of 0.49 g LA/g COD, reached after 80 °C treatment for 90minutes, showing no difference with the no pretreatment control. In the fourth phase a continuous operation in sequencing batch reactor (SBR) mode was set up and the role of HRT and was evaluated. The liquid fraction of the food waste was fed to the reactor. In this part also the storage condition of the food waste was investigated. Results show that fermentation processes started already in the fermentation tank and long storage times have a negative impact on the fermentation since the substrate fed in the reactor was already rich in acids and ethanol with low carbohydrates concentration. In the fifth and last phase of the PhD research, solvent extraction was performed to evaluate the efficiency of this method in the lactic acid extraction. The tests were performed on real fermentation broth produced during phase four and also on a synthetic lactic acid solution. Tributylphosphate was used as solvent and ethyl acetate as diluent. The effect of the two salts addition iv was evaluated, and the influence of the other metabolites present in the real effluent on the lactic acid extraction was determined. The last chapter of this thesis analyzes the results and gives a general summary of the results highlighting how the low pH fermentation would be beneficial for LA production and stating how the use of food waste can be a risk if it is applied for a full-scale application. In the end, a zero waste industrial process is proposed with a relative cost analysis that shows how the strategies studied in this thesis result in a slightly cheaper production merging also the possibility to solve the food waste environmental problems.