Valorisation of the under-sieve fraction of mixed municipal solid waste by hydrothermal carbonization

Hydrothermal carbonization (HTC) was investigated as an innovative route to valorise a waste stream from mechanical biological treatment of mixed municipal solid waste, namely the under-sieve fraction (USF), a high-moisture stream typically landfilled. This study explores HTC as an alternative to USF disposal, leveraging its ability to process wet organic feedstocks into value-added products. The main product of HTC, a carbonaceous solid called hydrochar, shows interesting potentials in various applications as a solid biofuel, soil amendment, and adsorbent material. Using Design of Experiments (DoE) and Response Surface Methodology (RSM), optimal HTC operating conditions (temperature, residence time, and solid load) were identified to maximise process yields. Experimental results demonstrated hydrochar yields up to 76 %, carbon yields up to 96 %, and energy yields up to 94 %. Maximum yields were achieved under mild conditions (190 °C, 60 min), indicating reduced process energy requirements. Hydrochar suitability as a soil amendment was also assessed, highlighting its potential as a soil improver or growing medium. These findings confirm HTC as a promising alternative for managing USF, offering environmental and resource recovery benefits compared to traditional landfill disposal.