Influence of acoustic cavitation and its combination with H2O2, acid, and alkali as a pre-treatment technique on lactic acid production from rice as a model food waste

Acoustic cavitation was utilized as a pretreatment technique to modify a model food waste (rice) as the substrate of fermentation, enhancing the production of lactic acid. The comparison of a low-frequency (24 kHz) reactor with a high-frequency (120 kHz) reactor demonstrated that the low-frequency reactor remarkably enhanced soluble chemical oxygen demand (sCOD), while the enhancement achieved by the high-frequency one was negligible. Increasing power density from 100 W/L to 400 W/L in the low-frequency reactor showed a continuous rise in sCOD and total lactic acid production. The application of ultrasound at a frequency of 24 kHz and a power density of 400 W/L raised sCOD by 555 %, L-lactic acid by 92 %, D-lactic acid by 43 %, and total volatile fatty acids (VFAs) by 15 %. Acid (HCl) and alkali (NaOH) were combined with ultrasound (24 kHz, 400 W/L) in the pretreatment stage, and they elevated sCOD by 750 % and 625 %, respectively. The pretreatment performed by the combination of ultrasound and acid enhanced L-lactic acid, D-lactic acid, and total VFAs by 71 %, 100 %, and 29 %, respectively. The combination of ultrasound and alkali in the pretreatment stage also increased L-lactic acid and D-lactic acid by 62 % and 104 %, respectively H₂O₂ was also combined with ultrasound (24 kHz, 400 W/L) and it raised L-lactic acid and D-lactic acid by 65 % and 109 %, respectively. Microbiological analyses demonstrated that Limosilactobacillus sp. formed the highest percentage of species in digestate samples. Economic assessment demonstrated that sole ultrasound (24 kHz and 400 W/L) and its combination with H₂O₂ were the most economically viable pretreatment methods, resulting in net savings of €153 and €156 per ton of substrate treatment, respectively.