**1. Introduction**

In 1991, the European Union (EU) Directive 91/271/EEC set new benchmarks for the collection, treatment and monitoring of wastewater in urban areas [1]. In 2000, the EU produced over 10 million dry tons of sewage sludge (latest available official EU data [2]). Since then, production has steadily increased, increasing burdens on municipal wastewater treatment systems [3]. In 2017, the global sewage sludge production reached approximately 45 million of dry tons per year [4]. In Italy alone, over 1100 kilotons of dry sludge matter were produced in 2010, with almost 40% going to landfills and less than 30% used in agricultural applications [2]. In 2016, the European Commission reported that the 1991 directive was successful in terms of current improvements seen in EU water quality despite an increasing population growth. However, the European Commission also underlined the need to develop "innovative solutions to increase resource efficiency, such as solutions for energy recovery, nutrient recovery, and processing to marketable products and water re-use" [5]. Beyond the EU, global urbanization and the growth of the middle class, combined with stricter environmental regulations,

have forced municipalities to re-examine their sludge management practices—transitioning away from traditional disposal via incineration, landfilling, or discharge to oceans/waterways—to favor beneficial reuse [6,7].

Sludge is a complex, heterogeneous mixture comprised of organic compounds such as proteins, peptides, polysaccharides, phenols, aliphatic, aromatic and furan compounds, as well as inorganic materials such as nutrients (phosphorous, potassium, nitrogen), silica, and heavy metals [8] and pathogens and other microbiological pollutants [9]. In a typical wastewater treatment process, primary sludge is characterized as the sludge following mechanical processing (screening, grit removal, sedimentation) containing between 93 and 99.5 wt% water, with a high content of suspended and dissolved organics. Secondary sludge (also known as Waste Activated Sludge, WAS) follows from biological treatment and contains a high amount of microbial cells with a total solids concentration ranging between 0.8 and 1.2 wt% depending on the type of process used [10].

Anaerobic Digestion (AD) is the most widely used sludge management technique. AD converts the organic solids to biogas (predominantly CH4 and CO2) via hydrolysis, acidogenesis, acetogenesis, and methanogenesis [11,12]. Despite its popularity, a considerable amount of solid remains after AD; as little as 20–30 wt% of the total organic matter is mineralized [9]. While these solids were once thought to be environmentally benign, their use into the soil may well be a potentially large source of greenhouse gas emissions and point pollutant sources for mercury, lead, cadmium, and copper on arable land, contributing to environmental acidification [13] and posing a toxicological risk in terms of pathogens present [9]. The microbiological processes occurring on land-applied sludge (mainly anaerobic decomposition, nitrification, and denitrification) lead to considerable emissions of methane and nitrous oxide, as well as ammonia and nitrate. While land application of sludge offsets the use of industrial nitrogen-containing fertilizers and represents a considerable net reduction of N2O, NH3, and NO3 <sup>−</sup> emissions, CH4 emissions are still estimated to be 6.3 kgton−<sup>1</sup> of applied sludge [13]. As such, direct land application of secondary sludge may not be the optimal nutrient recovery pathway and may well represent a waste of a renewable energy source.

Hydrothermal Carbonization (HTC) is a process to concentrate the carbon in a given biomass, occurring in water at elevated temperatures in the range of 160–280 ◦C [14], above saturated vapor pressure, where water's dielectric constant decreases so drastically that it catalyzes the carbonization of biomass while acting as an organic solvent [15,16]. HTC proceeds via a series of mechanisms, including hydrolysis, dehydration, decarboxylation, decarbonylation, and demethanation. HTC is performed in 80–95 vol% water [17–20], making it an ideal processing pathway for wet biomasses [21,22] such as sewage sludge. As well recognized in the literature, HTC not only leads to reduced volume and energy densification of the solid residue, considerably improving its dewaterability [23,24], but also significantly increases its solid fuel properties while stabilizing and disinfecting the sludge [25,26]. Interestingly, HTC of sludge enhances NO + NH3 reactions during solid combustion, significantly reducing NOx emissions across combustion modes [27].

While multiple studies focused on nutrients recovery such as phosphorous [28–30] or probed the impact of processing conditions on the solid and energy yields of hydrochars produced from the HTC of sludge from one withdrawal point in the wastewater treatment (WWT) process [23,31–39], the impact of upstream WWT processes on hydrochars is often not part of the experimental design. Thus, a primary goal of the present work was to determine the most viable point in the WWT process to employ HTC for sludge treatment from a solid fuel production and potential nutrient recovery standpoint. There are two sets of variables of interest in the present work: (1) feedstock withdrawal point from the WWT process; (2) hydrothermal reaction conditions on the composition and oxidation properties of resulting hydrochars. While some research has been done to quantify the environmental and economic benefits of using HTC to treat sewage sludge, understanding the impact of both feedstock and processing conditions on resulting hydrochars is critical to enable better optimization of this waste-to-energy conversion pathway [40].
