Valorisation of organic residues through hydrothermal carbonization

Hydrothermal carbonization (HTC) is a thermochemical process which can directly convert wet organic materials producing a carbon-rich solid (hydrochar, HC) and a liquid phase (process water, PW). Hydrochar has chemical and physical properties that make it similar to natural peats and coals. Depending on the process conditions, mostly temperature (180-250°C) and residence time (few hours), this material can be enriched in its carbon content, modifying its structure and providing it interesting characteristics that make it possible to be used for several applications, like energy production, as a soil ameliorant, adsorbent material for different pollutants, and some others reported in literature. In this work, the HTC process was applied to different organic materials such as hemp, grape marc, spent coffee grounds, AD (anaerobic digestion) digestate from hemp, from cow manure, and from agricultural residues, cigarette butts, surgical masks and gloves. Feedstocks and resulting products were physically and chemically characterised to be tested for different applications (e.g., energy production by combustion or AD, soil ameliorant, peat substitute in growing media or fertigation, or to be biologically treated in aerobic wastewater treatment plant). Compared to the feedstocks, hydrochars showed for all the tested materials strong physical and chemical modification (e.g., carbon content increased, and ash reduced). Due to the increased HHV, hydrochar may be used as solid fuel, which entails a higher energy recovery compared to the feedstock. Following HTC, all the tested materials showed a reduced volume (over 70%, in some cases) and an increased density. The possibility to use HC as peat substitute in growing media was tested for cow manure digestate, spent coffee grounds, and grape marc, showing that low amount of HC can partially substitute peat with similar or even better results. However, the addition of higher amount showed inhibition. Pre- and post-treatments tested (extraction, drying, washing) seem to have positive effect on seed germination and plant growth, removing some phytotoxic compounds. However, since these steps can be expensive, further tests should be carried out to understand which compounds cause inhibition, how to remove them or whether it is possible to destroy or avoid producing them by adequately setting the HTC process parameters. Process water (PW) should be properly treated since it contains many different compounds which can become an environmental problem. In this work, the PW valorisation was tested through characterising the liquid. The high amount of nutrient in PW may suggest the use in fertigation; however, germination tests showed inhibition effect. The aerobic treatability was tested through the bioassay on nitrifying bacteria. The high amount of VFAs may make PW a suitable substrate for AD to produce biogas. This work demonstrated that hydrothermal carbonization is a suitable process for the treatment of a wide range of organic materials. Depending on the feedstock, the resulting products may have different properties and characteristics which make them feasible for diverse applications. In general, all the feedstock tested may be used as solid fuel with several advantages, such as higher energy content, reduced volume, possible reduced emission during combustion, ease of dewatering, etc. The use on soil is promising when low amount of HC is used. However, pre- or post-treatments may increase the concentrations. Process water may be biologically treated: anaerobically, to produce biogas and remove some organic compounds and aerobically, but an acclimatation phase may be necessary. HTC proved to be a promising process in the biorefinery concept, especially when integrated with other processes (AD, aerobic degradation, etc.). Materials and energy can be recovered by HTC and re-introduced in the productive cycle and in the market, promoting the circular economy and the zero-waste concept.