**1. Introduction**

Biowaste can be defined as a mixture of vegetal and animal biomasses that are residues of human activities. It contains large carbon-based macromolecules that can be used as a renewable material for energy production, carbon sequestration, and soil conditioner and fertilizer. Nevertheless, biowaste management requires a specific attention in order to increase the overall sustainability of the treatment and to define guidelines to increase its effective recovery. In Europe, more than 118 millions of tons of biodegradable waste are produced each year; only 25% of this amount is collected and sent to the given recovery option [1]. The recovery treatments are mainly based on biological processes: the most used is the composting (low capital and operating cost, low value of products) followed by the integrated anaerobic/aerobic digestion (medium–high capital cost, low operating cost,

medium value of products, depending of country incentives program). Although the composting is widely used, its sustainability is not always guaranteed because of long process time, large areas needed for storage and processing, environmental impact due to annoying odors released by diffuse and fugitive emissions other than a not favorable ratio between the value of the product (compost), and the cost of the process. The recourse to an anaerobic digestion as preliminary stage allows to improve the overall process by permitting the production of biogas in addition to the compost. The main limitation of biological process is the low economic value of the compost obtained from biowaste coming from separate collection of municipal waste. This important source of biodegradable matter is often contaminated by other waste with a fraction between 10 and 25%, depending on the waste collection system adopted for the separate collection [2–4]. The presence of this fraction, generally represented by plastics and metals, can further decrease the economic value of the compost that is sold at a price between 0 and 3 €/Mg [5].

An alternative to compost production is the transformation of the biowaste, including digestate, into different products, either solid, liquid, or gaseous obtained by means of thermochemical treatments. Depending on the specific production process and feedstock, the obtained products are different: thermal decomposition of wood, peat, or some related natural organic materials produces charcoal [6, 7]; the torrefaction produces biocoal [8]; if the charred organic matter is applied to soil with the intent to improve soil properties, it is called biochar [9]; moreover, the product of hydrothermal pyrolysis, is called hydrochar [10].

The hydrothermal pyrolysis (HTC) converts all substrates containing carbohydrates and molecules, including biowaste, into hydrochar, gas, and leachate by means of extraction of nitrogen and oxygen in a subcritical water environment [11]. The HTC stage can be applied to the fresh biowaste or to the digestate produced by the anaerobic digestion plants. In this latter case, the integration is able to avoid the aerobic treatment that is time- and space-consuming and obtain a high-added value product, in a limited footprint. The other waste stream that needs to be exploited is the not biodegradable waste; the fate of this waste is the landfilling or the energy recovery by combustion [12], and the gate fee for its disposal strongly increased in the last years. The thermochemical processes applicable to this kind of waste are pyrolysis and gasification: the latter is preferable since it is energetically self-sustainable and allows to produce both heat and electricity [13–15]. The main advantage of gasification is the limited size of the plant and the possibility to install it with capacities starting from few hundreds of kilograms in an hour.

The present chapter aimed to demonstrate the feasibility of integration between small-scale thermochemical processes and the biowaste biological treatment facility with the target to reduce the waste production, increase the energy recovery, and, more in general, increase the sustainability of the plant.
