Effect of the Substrate to Inoculum Ratios on the Kinetics of Biogas Production during the Mesophilic Anaerobic Digestion of Food Waste

Aakash Khadka,Sheila Dangol,Alessandro A Carmona-Martínez,Bijay Thapa,Anmol Parajuli,Lokesh Sapkota,Anish Ghimire

doi:10.3390/en15030834

Abstract

This study evaluates the effects of the varying substrate to inoculum ratios (S:I) of 0.5, 1, 2, 3, 4, 5, and 6 (volatile solids/VS basis) on the kinetics of biogas production during batch mesophilic (35 ± 1 °C) anaerobic digestion (AD) of simulated food waste (FW), using anaerobic digestate as the inoculum. Kinetic parameters during biogas production (scrubbed with NaOH solution) are predicted by the first-order and the modified Gompertz model. The observed average specific biogas yields are in descending order corresponding to the S:I ratios 1, 2, 4, 6, 3, 5, and 0.5, respectively, and the significant effect of the S:I ratio was observed. The tests with the S:I of 1 have the maximum average biogas production rates of 88.56 NmL/gVS.d, whereas tests with the S:I of 6 exhibited the lowest production rates (24.61 NmL/gVS.d). The maximum biogas yields, predicted by the first order and the modified Gompertz model, are 668.65 NmL/gVS (experimental 674.40 ± 29.10 NmL/gVS) and 653.17 NmL/gVS, respectively. The modified Gompertz model has been proven to be suitable in predicting biogas production from FW. VS removal efficiency is greater in higher S:I ratios, with a maximum of 78.80 % at the S:I ratio of 6, supported by the longer incubation time. Moreover, a significant effect of the S:I ratio is seen on kinetics and energy recovery from the AD of FW.

Highlights

Anaerobic digestion (AD) is a mature technology with substantial application in sludge and organic waste management [1]
The lower values of end pH seen in the higher S:I ratio test suggests the accumulation of volatile fatty acids (VFAs) in a greater degree compared to the tests with lower S:I ratios (Table 2)
The results show that the modified Gompertz model gives a better prediction

Summary

Introduction

Anaerobic digestion (AD) is a mature technology with substantial application in sludge and organic waste management [1]. AD of organic waste could yield renewable energy (as methane) and biofertilizers (as digestate). AD is vital for reducing greenhouse gas emissions from waste sectors, combating climate change, and sustaining life on earth [1,2]. AD plays an integral role in managing organic waste. Food waste (FW) is abundantly available waste biomass that contains high moisture and readily degradable organic matter. Both characteristics make it an attractive feedstock for energy recovery from the AD process [3]

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