Hydrothermal carbonization vs. anaerobic digestion to valorize fruit and vegetable waste: A comparative technical and energy assessment

Herein, the valorization of vegetable and fruit waste was assessed via hydrothermal carbonization (HTC) and anaerobic digestion (AD) in terms of product characterization and energy requirements. HTC was conducted at reaction temperatures between 150 & DEG;C and 190 & DEG;C, and residence times between 20 min and 40 min. The increase in the process severity resulted in hydrochars with higher carbon contents and higher energy densification ratios. AD was performed in two different ways. i.e., batch and semi-continuous reactions. From the batch experiments a methane yield of 300 L CH4/kg VS was obtained, while for the semi-continuous, the average specific methane production estimated (for HRTs from 75 to 50 days) was 213 & PLUSMN; 32 L CH4/kg VS. To estimate the energy re-quirements, mass and energy balances were performed considering the basic stages of each process to obtain a suitable biofuel material. In this sense, it was concluded that for this specific waste, AD was a more suitable process with a positive energy net balance. On the contrary, HTC presented a negative energy net balance being required 1.29 MJ/kg of fresh food waste. A combined HTC-AD treatment may be an efficient method to take advantage of both technologies leading to higher energy efficiencies and other valuable products.

Automated summary

In this study, the valorization of vegetable and fruit waste was evaluated through hydrothermal carbonization (HTC) and anaerobic digestion (AD), focusing on product characterization and energy requirements. HTC at higher temperatures and longer residence times yielded hydrochars with higher carbon contents and energy densification ratios. AD experiments showed that batch and semi-continuous reactions both produced methane, with batch experiments yielding a methane yield of 300 L CH4/kg VS and semi-continuous experiments estimating an average specific methane production of 213±32 L CH4/kg VS. Energy requirements were assessed through mass and energy balances, and it was concluded that AD was a more suitable process with a positive energy net balance, while HTC had a negative energy net balance, requiring 1.29 MJ/kg of fresh food waste. A combined HTC-AD treatment could be an efficient method to maximize the energy efficiency and value of the waste.