# Biochar for Wastewater Treatment--Conversion Technologies and Applications
## 2 Conventional Thermal Methods for the Conversion of Feedstock into Carbonaceous Materials
### Hydrothermal Carbonization  
HTC is one of the most attractive thermochemical conversion methods used for high energy density materials production. This method is well adapted to wet feedstock without need of an energy intensive drying step; it allows high conversion efficiency of biomass to a carbonaceous material with relatively high yield at low operation temperature [47]. During HTC, the biomass in the presence of a liquid is subjected to temperatures ranging from 180 \({}^{\circ}\)C to 300 \({}^{\circ}\)C at a pressure between 2 MPa to 10 MPa for several hours [48]. Different biomasses were submitted to HTC such as two-phase olive mill waste [49], beet root chips [50], starch and rice grains [51], maize silage [48], and even polyethylene and rubber wastes [52]. Other studies have investigated the HTC of fecal biomass [53], sewage sludge [54] and agricultural residues [55]. During HTC of biomass, a decrease in the pH is typically observed; this is reported to be due to the formation of a variety of organic acids such as acetic, levulinic, formic, and lactic acids, which promotes the acid-catalyzed reaction of organic compounds without the addition of acid [56; 57]. The presence of liquid during HTC allows the acceleration of the carbonization process; since water behaves as a solvent and reaction medium that enhances hydrolysis of lignocellulosic biomass [45]. The pretreatment of biomass by HTC follows different mechanistic pathways, resulting in three phases: hydrochar (solid fraction), aqueous phase (mixture of bio-oil and water), and a small volume of gas (mainly CO\({}_{2}\)). The distribution of HTC byproducts and the properties of the hydrochar is also governed by factors such as process temperature, residence time, pressure and water/biomass ratio [58; 59]. The non-soluble carbonaceous hydrochar has generally a structure with spherical particles due to the broken fibrous lignocellulosic chain at some places [60]. Particles contain a hydrophobic part (aromatic ring) and a hydrophilic part, which envelops the first part (Figure 1). The first part consists of stable oxygen atoms which are in the form of ether, quinone, etc. While the second part contains oxygenated functional groups (hydroxyl, phenolic, carbonyl, carboxylic, ester, etc.) [61]. Depending upon the process conditions used, the major part of carbon contained in the starting feedstock is remained in the final hydrochar, while a considerable amount of inorganics is removed from the hydrochar and dissolved in the liquid phase [45; 56]. HTC is considered \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Process**} & \multirow{2}{*}{**Temperature**} & \multirow{2}{*}{**Heating Rate**} & \multirow{2}{*}{**Residence Time**} & \multicolumn{3}{c}{**By-Products [\%]**} \\ \cline{4-7}  & & & & **Biochar** & **Bio-Oil** & **Syngas** \\ \hline Fast pyrolysis & 400–1000 \({}^{\circ}\)C & Very fast (\(\sim\)1000 \({}^{\circ}\)C/s) & \(<\)2 s & 12 & 75 & 13 \\ Slow pyrolysis & 350–980 \({}^{\circ}\)C & Slow (\(<\)10 \({}^{\circ}\)C/min) & 1 h & 27–37 & 19–29 & 25–41 \\ HTC & 180–300 \({}^{\circ}\)C & Slow & 1–16 h & 50–80 & 5–20 & 2–5 \\ Gasification & 700–150 \({}^{\circ}\)C & Moderate-very fast & 10–20 s & 10 & 5 & 85 \\ Torrefraction & 200–300 \({}^{\circ}\)C & Slow, (\(<\)10 \({}^{\circ}\)C/min) & \(\sim\)10–60 min & 80 & 0 & 20 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Thermal conversion processes and products distribution [33; 43; 44; 45]. to be a promising method to convert biomass into materials with abundant oxygen-containing functional groups and porous structure, which is beneficial for the adsorption of contaminants from wastewater [45; 56]. Various hydrochar materials, such as switchgrass hydrochar [62], poultry litter and swine solids hydrochars [63], cassava slag hydrochar [64], banana peels hydrochar [65], and fructose and phloroglucinol hydrochars [66] have been reported as excellent adsorbents for the removal of different inorganic and organic contaminants. In the study by [PERSON] et al.