Many environmental crises are threatening to collapse human societies, but also life on the Earth as we may know. Climate change due to anthropogenic global emissions is one of the main issues, but also soil degradation, and the management of the enormous amount of organic wastes that are harmfully released into the environment by human activities. On the one hand, direct spreading of biowastes onto the land is often limited due to eutrophication or pollution by toxic compounds. On the other hands, these biomasses may contain nutrients, such as phosphorus (listed as a critical raw material) and nitrogen, which could be recovered to sustain the high demand in expensive soil amendments and fertilizers. Thus, finding new solution to close the loop towards circular economy and sustainable processes is crucial, nowadays, to convert the global trends and restore the delicate equilibrium in the water-food-energy nexus, with soil ‘under special surveillance’. In this framework, many researchers are focusing their attention on hydrothermal carbonization (HTC) as a possible candidate, in particular to treat moist biowastes and obtain useful solid (hydrochar) and liquid (HTC liquor) products. Nutrients contained in biowastes are sensitive to HTC process parameters and initial feedstock properties, thus it is crucial to analyze their distribution and (im)mobilization among the HTC products for their reuse in soil. On the other hand, since some organic compounds are generated during the thermochemical process, which may be responsible for toxicity to plants and other organisms, analysis of their possible toxicity towards the biosphere becomes crucial before implementation at a large scale. This thesis collects our efforts to explore new processes and deepen the knowledge about the possibility to produce amendments suitable for soil application from hydrochar. The core is from a process engineering perspective, focusing on the production phase and the characterization of the products, never forgetting any possible limits or the implications on ecotoxicological issues. In the first part of the thesis, ‘How to make hydrochar a soil amendment?’ is the main question. Based on few pioneer studies about composting of hydrochar, we assessed a complete analysis of hydrochar co-compost, produced from the 25-day aerobic stabilization of digestate of organic fraction of municipal solid waste, together with a fraction of its hydrochar, and green waste. Under the umbrella of C2Land project (funded by The European Institute of Innovation & Technology -EIT), we produced this new amendment in specifically designed bioreactors, then we characterized the products from the point of view of physico-chemical properties, nutrients distribution, toxicity to plants and mammalian cells. We are confident that the crucial result of toxicity removal from hydrochar through composting will attract the interest of many stakeholders, since implementing HTC in anaerobic/aerobic plants is almost ready to be applied at a large scale. Moreover, an experimental campaign performed at the University of Leeds further explored the effect of some other post-treatments on hydrochar composition and phytotoxicity, depending on the initial feedstocks (manure, sewage digestate, water hyacinth, and grass). The second part of the thesis was dedicated to nutrients recovery, necessary when some harmful matrixes - such as the municipal sewage sludge digestate - are not allowed to be reused for agricultural purposes, due to law limitations or pollution issues. In this case, nutrients recovery (N, P) could be achieved chemically, via HTC and struvite precipitation, being struvite recognized as a good fertilizer. Firstly, we performed a techno-economic assessment of the implementation at a district level of HTC-centered sewage sludge management. The mass reduction of waste streams may have some important effects on transportation costs, while the recirculation of HTC liquor back to the anaerobic digestors can imply more biomethane production with increased revenues. A mass balance for the potential of N and P recovery through struvite precipitation in the centralized facility was also assessed. Secondly, a near-zero waste treatment process for municipal sewage sludge via HTC was designed according to the available literature and preliminarily analyzed at lab level. The fractionation of the different types of phosphorus was analyzed through the STM-P protocol before and after HTC, confirming the mineralization of organic phosphorus during the process. Citric acid was validated as a good green solvent for phosphorus extraction and high-quality struvite precipitation, even if the process was not still optimized in terms of yields. Future works may identify the best process parameters for final metal recovery from liquid byproducts in order to internally recirculate them back to water line, and to characterize the leached hydrochar as purified renewable fuel or soil amendment. Thus, the proof-of-concept chain would be confirmed and could be applied to the sustainable HTC-centered biorefineries of the future.

The present work regards the development of a methodology for the study of atmospheric particulate matter (PM) which is alternative to instrumental measurements. The methodology developed exploits the surfaces already present in the field as samplers of PM. In particular, conifer needles and building facades are employed to investigate different temporal ranges: conifer needles potentially retain particles circulating in the atmosphere from the recent past up to now, while building facades could retain particles from an older period up to know. The field of application of the approach developed are the situations in which a wide territory must be monitored, eventually including remote locations, or information on past pollution scenario must be reconstructed in the absence of monitoring stations. For instance, the evaluation of the improved efficiency of off-gas abatement systems of industrial plants is a typical case of application. These pollution sources affect large areas and might have been active before regulation on air quality required constant monitoring of their emissions. Typically in such a case the methodology could assist in evaluating how large was in the past and it is nowadays the area of impact of the plant. In general, such an approach could be valuable whenever relying on instrumental measurements is cost and time consuming in terms of installing a large network of monitoring stations to study the dispersion of pollutants from a single or few sources. To have a detailed description of the spatial distribution of pollutant particles, they are studied individually with subsequent higher magnification. Where no traces of a source are detected by scanning electron microscopy coupled with energy dispersive x-ray spectroscopy (SEM-EDXS), the samples are analyzed with the higher resolution of transmission electron microscopy coupled with energy dispersive x-ray spectroscopy (TEM-EDXS) and selected area electron diffraction (SAED) in order to make sure that no smaller particles, able to travel farther from their source, are present at a certain site. All data provided by electron microscopy analysis of particles collected by conifer needles are placed in the context of elemental concentrations measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES), which is a bulk analytical technique. The same is not possible for the data on single particles present on building facades given the inorganic matrix of the substrate, especially in the case of metal oxide paints, which does not allow the bulk measurement. Both the preparation of the samples for bulk analytical techniques and single particle analysis by electron microscopy were optimized. For method development and evaluation, the analytical protocol was applied to estimate spatial and temporal trends of accumulation of inorganic pollutants that can be related with changes in the emissions of atmospheric pollutants by an electric arc furnace (EAF) steelmaking plant located in a test site. The benefits of combining the single particle and bulk analytical techniques emerged especially for the discrimination of the emissions from different sources.

Hydrothermal carbonization (in acronym, HTC) is a thermochemical conversion process through which it is possible to directly transform wet organic substrates into a carbonaceous material, referred as hydrochar. Hydrochar has chemical and physical characteristics that make it similar to fossil peats and lignite. Depending on the process conditions, mostly temperature and residence time, 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, such as for energy production, as a soil conditioner and improver, for carbon dioxide sorption and sequestration, and some others reported in literature. HTC is a different process, if compared to other common thermochemical processes, such as pyrolysis, torrefaction, gasification, etc., because it works in wet conditions (humidity content higher than 60%). As a matter of fact, biomass is transformed into hydrochar because of the properties of hot pressurized water, that acts both as a reactant and as a catalyst. The HTC process has been studied from many years, although at present not all the chemical reactions that occur during the process are completely known. Moreover, the application of this quite new process to different substrates can bring to different results. Even though HTC can be applied to any kind of organic material (of both animal and vegetable derivation), the possible uses of hydrochar can strongly be influenced by the characteristics of the feedstock. This, for example, can be due to legislative constraints. In Chapter 1, an overview of the existing literature is presented. To get insights on this process, a small bench scale batch reactor has been designed and built at the Department of Civil, Environmental and Mechanical engineering of the University of Trento, Italy. This reactor has been tested, prior to be used with real substrates. In Chapter 2 the reactor and the preliminary tests done are described. In this work, the HTC process applied to three different substrates have been studied: grape marc, the EWC 19.05.03 residue and the EWC 19.12.12 residue. In Chapter 3 the three raw substrates are described. Grape marc is produced by the winery industries or by distilleries. This feedstock is composed by woody seeds and holocellulosic skins and it presents an average humidity content of about 60%. At present, it is used for the production of animal food or it is landfilled. In this case, the application of HTC can be an interesting alternative to these end uses because, through this process, grape marc can be recovered, for example, for energy production. The hydrochar produced from this feedstock could be even used as a soil conditioner. In Chapter 4 several analyses on the hydrochar, on the process water and on the gaseous phase obtained during the carbonization tests are presented. The EWC 19.05.03 residue is a by-product of the composting treatment applied to the organic fraction of municipal solid waste (MSW). In collaboration with Contarina S.p.A., a company that collects and treats MSW in the province of Treviso, in the North-East of Italy, this by-product was carbonized and tested both as a soil conditioner and for energy production. Results of the analyses on the solid, liquid and gaseous phases produced by the HTC process are reported in Chapter 5. The EWC 19.12.12 residue is a by-product of the refuse derived fuel (RDF) production, from the residual fraction of the MSW. This substrate was provided by Contarina S.p.A. and preliminary tests on the exploitability of the hydrochar for energy production are reported in Chapter 6, together with analyses on both the liquid and gaseous phases. A rigorous energy balance has been proposed in Chapter 8, based on the experimental data obtained for grape seeds. In this chapter, all the hypotheses and the assumptions taken to evaluate the enthalpy of the HTC reaction at different process conditions (namely, three different temperatures and three residence times) are described. In Chapter 8 a kinetic model is proposed, based on a two-step reaction mechanism. The activation energy and pre-exponential factor of the various degradation reactions were determined by means of least square optimization versus the experimental data of grape marc. A thermo fluid model is even proposed in this chapter. The model integrates mass, momentum and heat equations within the reactor domain by means of the finite volumes method (f.v.m.) approach. Convective and radiative exchange between the reactor and the fluid within the reactor have been implemented in the f.v.m. model. Under two strong assumptions (mono-component and mono-phase fluid, which fulfils the reactor), it was possible to estimate the behaviour of an equivalent fluid (eq_fluid), in terms of thermal properties of the fluid (thermal capacity, thermal conductivity and thermal diffusivity). Moreover, a simplified dynamic analytic model is also presented – based on lumped capacitance method – in order to simulate the thermal behaviour of the system, using the actual temperature profile imposed by the reactor external heater. A resistance-capacitance network was used to describe the system. Finally, the Henry’s law has been applied to assess the amount of gas really produced during the HTC process. In Chapter 9, the main conclusions of this work are reported.