Pretreatment study of food waste using sonication for enhancement of methane production

The application of low-frequency ultrasonic pretreatment to the organic fraction municipal solid waste (OFMSW), particularly food waste (FW), is able to overcome slow hydrolysis rate by disintegrating the substrate into a biodegradable substrate. In addition, the process promotes solubilization of organic matter resulting in high amount of substrate readily available within the digestion process and thus enhancing high methane production. The main objective of this study was to obtain high methane production through anaerobic digestion of sonicated food waste. To accomplish this aim, the following objectives were pursued namely correlation between ultrasonication with the physicochemical property changes and anaerobic biodegradability, evaluation of the digester performance and system stability of the anaerobic digestion of the sonicated food waste at different organic loading rate (OLR) and the determination of the kinetic parameters for the performance of the anaerobic digestion of sonicated food waste. The anaerobic digestion experiment on sonicated food waste was conducted in two phases, where in Phase 1, the ultrasonic pretreatment was performed by sonicating 200 mL of food waste slurry at different sonication duration of 2, 4, 6, 8 and 10 minutes at 20 kHz frequency and specific energy input ranged from 5,396 to 25,997 kJ/kg TS, and then anaerobic batch tests were carried out on the sonicated food waste samples. While in Phase 2, the anaerobic digestion was conducted on the sonicated food waste (SFW) and non-sonicated food waste (NSFW) or control in a 13 L laboratory scale anaerobic digester (working volume of 10 L) with increasing organic loading rate (OLR) ranging from 1.5-3.5 gCOD/L.day. The experimental works were carried out in two stages (batch start-up and then continued with semi-continuous feeding). The performance of all digesters was evaluated based on the methane composition, methane production rate as well as methane yield. In the ultrasonic pretreatment (Phase 1), when the specific energy inputs increased from 0 to 25,997 kJ/kg TS, analysis of chemical properties found that the percentage of soluble COD (SCOD) values have increased to 34-40 %. The range of CODsolubilization and degree of disintegration (DD) values were between 11.4-13.4% and 57.15-71.08%, respectively. A linear relationship with an R² of 0.907 was obtained for the correlation between CODsolubilisation and DD. The application of sonicated food waste in the anaerobic digestion process has increased the cumulative CH4 production about 40.3-70.5%. The volume of methane produced was increased from 4.2 liters (non-sonicated food waste) to 7.9 - 14.5 liters in the sonicated food waste. While for Phase 2, the methane composition generated during the process was higher in SFW digester at OLR of 1.5 g COD/L.day and 3.5 g COD/L.day compared to NSFW digester. There was an increase of 9.54-41.28% in methane composition when SFW digester was operated at OLR of 3.5 g COD/L.day. Methane production rate was enhanced by 20.8 -75.7% or 1.26-4.12 times in SFW digester when operated at an OLR of 3.5 g COD/L.day. Methane yields did not show significant changes at the OLR of 1.5 and 2.5 g COD/L.day in both digesters, but started to show a significant increase when SFW digester was operated at OLR of 3.5 g COD/L.day. The methane yield was enhanced by 42.87-82.83% or 1.75-5.82 times at the stated OLR. The process performance as well as acceptable stability in SFW digester provided satisfactory predictions with Monod, Modified Stover-Kincannon, Grau second-order multicomponent substrate removal and Contois kinetic models. The experimental data were well fitted with the models with high correlation coefficients (R²) ranging from 0.914-0.996. The kinetic coefficients such as yield coefficient (Y), maximum specific growth rate (μmax), death rate (kd), half velocity coefficient (Ks), saturation value constant (KB), maximum utilization rate (Umax), multicomponent Grau second- order substrate removal coefficient (ks), dimensionless Grau second order constant (a and b), µmax and B (for Contois model) recorded 1.580 gVSS/gCOD, 1.219 day⁻¹, 0.06 day⁻¹, 617.59 g/L, 1.928 gCOD/L.day, 1.667 gCOD/L.day, 0.103 day⁻¹, 48.121, 1.156, 10.76 day⁻¹ and 361.31 gCOD/gVSS, respectively. Furthermore, a significant relationship was observed between the predicted and experimental data with correlation coefficients (R²) ranging from 0.893-0.996. In this study, the Monod model with R² = 0.996 indicates the most appropriate model for interpreting the kinetic parameters of the anaerobic system in the CSTR treating sonicated food waste (SFW) slurry. Overall, the implementation of ultrasonic pretreatment prior to anaerobic digestion of food waste can increase the production of methane by recording overall enhancement in methane production rate of 18.99%. The overall enhancement of methane yield was 23.86% with the highest methane composition of 79.6%. Therefore, the kinetic model used in this study can be applied to predict the performance of the anaerobic digestion system treating sonicated food waste slurry.

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