## 1 Introduction  
The adverse effects of climate change have prompted discussions about the sustainability of current production systems. In this context, it is well-known that the transition from the current linear economic model to a circular one is necessary to achieve sustainable development. The circular economy also involves transforming waste into products, which can be understood as waste valorization approaches. Thus, research concerning waste valorization has been the subject of several studies about thermochemical processes, such as pyrolysis, gasification, and hydrothermal carbonization (HTC) [1; 2; 3]. Concerning HTC, its objective is to convert wastes into a solid, carbon-rich material named hydrochar. HTC has received great interest given the number of publications and established industries over the last decade [4]. In this process, waste is mixed with water at temperatures ranging from 180 to 350 \({}^{\circ}\)C under autogenous pressure conditions in sealed reactors [5]. The water between 100 and 374 \({}^{\circ}\)C becomes subcritical, acting not only as a solvent but also as a catalyst [6]. Advantageously, HTC uses water as the reaction medium, which is non-toxic, environmentally safe, and an economically cheap reagent [7]. Thus, HTC is convenient for wastes with high moisture content (e.g., sewage sludges), as it is carried out in an aqueous environment and does not require a pre-drying stage [8]. Furthermore, the temperatures used in HTC are considerably lower than those used in pyrolysis and gasification processes. When more than one waste is employed in HTC, the process is named Co-HTC, which is an interesting approach for combining wet and dry wastes [9]. Thus, Co-HTC is advantageous in circumventing some drawbacks of single-feedstock HTC. It can avoid dependence on only one feedstock, provide lower demands for water when wet wastes are used, and mitigate the inconvenience of an undesired waste characteristic, such as a high inorganic content [3]. For example, by combining inorganic-rich (e.g., sewage sludge) and inorganic-poor (e.g., sawdust) wastes, it is possible to improve the hydrochar characteristics compared with a hydrochar obtained only from the waste with a high content of inorganics [10; 11; 12]. The application of hydrochar depends on its characteristics. Hydrochar has the potential to be used, for example, for agriculture and crop improvement, catalyst support, carbon sequestration, electrochemical devices, and pollutant adsorption [13]. Regarding pollutant adsorption, hydrochar has been proposed extensively to remove heavy metals, nutrients, emerging contaminants, and dyes from aquatic environments [3]. Dye removal is a pertinent issue from wastewater from the textile industries. Given the limited adsorption capacity of fabrics, 85% of the dye mixture is discharged from the dyeing process. It makes textile industries the primary source of dye effluents in the environment because of the high usage of dyes in the coloring process [14]. If not properly managed, this could have adverse environmental consequences due to the dye toxicity [15]. It is therefore essential to implement effective dye removal methods to prevent the contamination of water sources by dye effluents. Among the proposed solutions, adsorption has been suggested as the best alternative because of its advantages, including high efficiency and short reaction times required [14; 16]. Thus, sustainable and cost-effective adsorbents, such as hydrochar, can be a viable alternative for removing dyes from wastewater. In addition to hydrochar, HTC or Co-HTC produces a liquid fraction named process water (PW) that generally presents an acidic pH and high values of chemical oxygen demand (COD) [17]. According to the waste used in the HTC or Co-HTC, PW can present potentially toxic compounds, such as furfural, 5-hydroxymethylfurfural, phenol, cresol, catechol, resorcinol, and polycyclic aromatic hydrocarbons [18; 19]. In the case of Co-HTC, PW can be more toxic due to the combination of different wastes.