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\DOI{10.5802/crchim.333}
\datereceived{2024-01-18}
\daterevised{2024-06-11}
\dateaccepted{2024-09-03}
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\begin{document}

\begin{noXML}

%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{Materials and Energy Valorization of Biomass and Waste : The Path for Sustainability and Circular Economy Promotion}
%\def\cdr@specialtitle@french{Valorisation \'energ\'etique et mati\`ere de la biomasse et des d\'echets : la voie de la durabilit\'e et de la promotion de l'\'economie circulaire}
%\makeatother

\CDRsetmeta{articletype}{research-article}

\title{Assessment of the applied potential of sludge-derived hydrochar
in terms of process parameters and product properties}

\alttitle{\'{E}valuation du potentiel appliqu\'{e} de l'hydrocharbon
d\'{e}riv\'{e} des boues en fonction des param\`{e}tres du processus et
des propri\'{e}t\'{e}s des produits}

\author{\firstname{Chuang} \lastname{Zhang}\IsEqualContrib}
\address{Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, 
School of Environmental and Biological Engineering, 
Nanjing University of Science and Technology, Nanjing 210094, China}

\author{\firstname{Bingyu} \lastname{Wang}\CDRorcid{0000-0001-5865-4849}\IsEqualContrib}
\addressSameAs{1}{Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, 
School of Environmental and Biological Engineering, 
Nanjing University of Science and Technology, Nanjing 210094, China}

\author{\firstname{Jixiang} \lastname{Wang}}
\addressSameAs{1}{Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, 
School of Environmental and Biological Engineering, 
Nanjing University of Science and Technology, Nanjing 210094, China}
\address{Key Laboratory of Agro-Environment in Downstream of Yangtze Plain, 
National Agricultural Experiment Station for Agricultural Environment, 
Ministry of Agriculture and Rural Affairs, Institute of Agricultural Resources and Environment, 
Jiangsu Academy of Agricultural Sciences, Nanjing 210014, China}

\author{\firstname{Shaopeng} \lastname{Rong}\CDRorcid{0000-0003-0899-871X}}
\addressSameAs{1}{Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, 
School of Environmental and Biological Engineering, 
Nanjing University of Science and Technology, Nanjing 210094, China}

\author{\firstname{Huifang} \lastname{~Xie}\CDRorcid{0000-0001-6082-7112}\IsCorresp}
\addressSameAs{1}{Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, 
School of Environmental and Biological Engineering, 
Nanjing University of Science and Technology, Nanjing 210094, China}
\email[H. Xie]{huifangxie@njust.edu.cn}

\author{\firstname{Hongxia} \lastname{Qu}}
\address{School of Chemical Engineering, Nanjing University of Science and Technology, 
Nanjing 210094, Jiangsu Province, China}

\author{\firstname{Yanfang} \lastname{Feng}\CDRorcid{0000-0001-6251-7660}}
\addressSameAs{2}{Key Laboratory of Agro-Environment in Downstream of Yangtze Plain, 
National Agricultural Experiment Station for Agricultural Environment, 
Ministry of Agriculture and Rural Affairs, Institute of Agricultural Resources and Environment, 
Jiangsu Academy of Agricultural Sciences, Nanjing 210014, China}

\thanks{National Natural Science Foundation of China (Nos. 42107398 and 42277332), 
Natural Science Foundation of Jiangsu Province (BK20210358 and BK20221428), 
China Postdoctoral Science Foundation (2020M68618), 
``333'' High-level Talents Training Project of Jiangsu Province\unskip\break (2022-3-23-083).}

\shortrunauthors

\keywords{\kwd{Sludge}
\kwd{Hydrothermal carbonization}
\kwd{Hydrochar}
\kwd{Hydrothermal intensity}
\kwd{Solid loading}}

\altkeywords{\kwd{Boues}
\kwd{Carbonisation hydrothermale}
\kwd{Hydrochar}
\kwd{Intensit\'{e} hydrothermale}
\kwd{Charge solide}}

\begin{abstract}
Hydrothermal 
carbonization (HTC) is beneficial for the disposal of
high-moisture organic waste such as sludge. However, the relationship
between HTC parameters and hydrochars has not been fully explored, and
most studies have focused on single or dual parameters. This paper
systematically studied how hydrothermal intensity and solid loading
affected the HTC process, hydrochar yield, and composition properties.
The results revealed that the organic matter or ash content in sludge
was a vital factor affecting the fuel properties of hydrochars. The
hydrothermal intensity was a critical process parameter for hydrochars'
structural properties. The solid loading had no apparent effect on
hydrochars but had a considerable effect on energy consumption in the
HTC process. When the moisture content of sludge was 80.3\%, the energy
{recoverable} by hydrochars could be sufficient for the material to be
{heated up to phase transition.} Evaluations of sludge characteristics
in different regions and
treatment processes were conducted, allowing HTC process parameters to
be optimized in correlation with future resource utilization
approaches. If the sludge contains a great deal of organic matter and
is used as a fuel to generate energy, it can be subjected to a milder
hydrothermal intensity like  180--220 \textdegree C for 1--2 h. If the
heavy metal content in the sludge meets land-use requirements, a
more intense hydrothermal intensity may be applied such as 
260 \textdegree C for 1--4 h. This study provides a reference for the
design, optimization, and scaling of HTC for sludge\unskip\break
treatment.
\end{abstract}

\begin{altabstract}
La carbonisation hydrothermale (HTC) est b\'{e}n\'{e}fique pour
l'\'{e}limination des d\'{e}chets organiques \`{a} forte teneur en eau
tels que les boues. Cependant, la relation entre les param\`{e}tres de
la HTC et les hydrochars n'a pas \'{e}t\'{e} enti\`{e}rement
explor\'{e}e, et la plupart des \'{e}tudes se sont concentr\'{e}es sur
des param\`{e}tres simples ou doubles. Cet article a \'{e}tudi\'{e} de
mani\`{e}re syst\'{e}matique la fa\c{c}on dont l'intensit\'{e}
hydrothermique et la charge solide affectaient le processus HTC, le
rendement en hydrochar et sa composition. Les r\'{e}sultats ont
r\'{e}v\'{e}l\'{e} que la mati\`{e}re organique ou la teneur en cendres
des boues \'{e}tait un facteur essentiel affectant les
propri\'{e}t\'{e}s combustibles des hydrochars. L'intensit\'{e}
hydrothermale est un param\`{e}tre critique du processus pour les
propri\'{e}t\'{e}s structurelles des hydrochars. La charge solide
n'avait pas d'effet apparent sur les hydrocars, mais un effet
consid\'{e}rable sur la consommation d'\'{e}nergie dans le processus
HTC. Lorsque la teneur en eau des boues \'{e}tait de 80,3 \%,
l'\'{e}nergie accumul\'{e}e par les hydrochars pouvait \^{e}tre
suffisante pour chauffer le mat\'{e}riau. Des \'{e}valuations des
caract\'{e}ristiques des boues dans diff\'{e}rentes r\'{e}gions et
diff\'{e}rents proc\'{e}d\'{e}s de traitement ont \'{e}t\'{e}
r\'{e}alis\'{e}es, ce qui a permis d'optimiser les param\`{e}tres du
proc\'{e}d\'{e} HTC en corr\'{e}lation avec les futures approches
d'utilisation des ressources. Si les boues contiennent beaucoup de
mati\`{e}re organique et sont utilis\'{e}es comme combustible pour
produire de l'\'{e}nergie, elles peuvent \^{e}tre soumises \`{a} une
intensit\'{e} hydrothermique plus douce, comme  180--220 \textdegree C
et 1--2 h. Si la teneur en m\'{e}taux lourds des boues r\'{e}pond aux
exigences d'utilisation du sol, une intensit\'{e} hydrothermique plus
intense peut \^{e}tre appliqu\'{e}e, comme  260 \textdegree C et
1--4 h. Cette \'{e}tude a fourni une r\'{e}f\'{e}rence pour la
conception, l'optimisation et la mise \`{a} l'\'{e}chelle de la HTC
pour le traitement des boues.
\end{altabstract}

\maketitle

\vspace*{4pt}

\twocolumngrid

\end{noXML}

\section{Introduction\label{mark-1}}
Dealing with excess sludge in a way that is both economically and
ecologically responsible is a growing concern. Hydrothermal
carbonization (HTC) is a thermochemical conversion technology that can
convert biomass into a combination of carbon materials and liquid phase
products under autogenous pressure  with water as the reaction 
medium~\cite{1}. Unlike conventional pyrolysis processes~\cite{2,3},
its reaction conditions are mild and there is no need for pre-drying
biomass waste, leading to considerable energy  conservation~\cite{4}.
Additionally, HTC can dramatically decrease the amount of 
{sewage sludge (SS),}
improve the dewatering capability of sludge considerably, and
convert it into high-value products (hydrochars) in the field of  
fuels~\cite{5,6}, agricultural applications~\cite{7,8,9}, and 
adsorbents~\cite{10,11}. Consequently, hydrochars have a promising
future as a fuel source, in agricultural applications, and for water 
resource restoration.

Altering sludge characteristics (major component and water content) or
HTC parameters (HTC temperature and residence time) will generate
diverse physicochemical characteristics of hydrochars, thus affecting
their potential applications. Previous research revealed that the ash
content of sludge-derived hydrochars from different production areas
was significantly  dissimilar~\cite{12,13}. 
The greater the solid
content of sludge, the more pronounced the output of 
hydrochars~\cite{14,15}. HTC processing requires much energy for
macromolecular organic matter destruction; thus, the HTC temperature is
the primary factor of HTC processing and restrictive 
factors~\cite{16}. Moreover, the residence time is a critical factor in
the HTC process, impacting product distribution, chemical properties,
and deepening carbonization degree~\cite{17,18}. For industrial purposes, HTC
must be conducted with a shorter period of residence to optimize
efficiency and reduce costs. Consequently, moisture content, HTC
temperature, and residence time all influence the HTC process of
sludge, and the characteristics of hydrochars vary accordingly. Most
studies have focused on individual or dual parameters and their effects
on hydrochar properties. Furthermore, the relationship between these parameters
and hydrochars has not been fully explored. To provide a data basis for
engineering applications, this work studied the HTC process of sludge
by regulating solid content, HTC temperature, and residence time.

HTC has been demonstrated to turn sludge into solid fuels with greater
energy density, providing a feasible alternative to combustible fuels
while avoiding contamination of the environment. Previous studies
indicated that the higher heating value (HHV) and atomic ratios of
hydrochars were comparable to  lignite~\cite{5,19}. Hydrochar
production relies on the ash content of the raw material, and the
sludge types have varying ash content. Conversely, hydrochars have
abundant nutrients that are helpful for plant growth, such as N, P, and
K, and their fertilizer and soil amendment utilization has drawn 
attention~\cite{7,20,21}. Hydrochars can be employed as soil
conditioners to release nutrient components, which is beneficial for
long-term utilization. Due to the presence of certain heavy metals in
the sludge feedstock, before applying hydrochars to the soil, it is
essential to consider both the regulations regarding heavy metals in
soil and the impact of the heavy metal content of hydrochars on 
soil  characteristics~\cite{22,23}. However, most of these works only
investigate the sole application of resources in hydrochars without
comprehensive evaluation.

In this study, HTC treatment of sludge was conducted by controlling
solid content, hydrothermal temperature, and residence time. The
properties of hydrochars generated under various conditions were fully
compared and analyzed to optimize operational parameters and evaluate
the energy recovery efficiency of hydrochars with different
hydrothermal strengths. In addition, heavy metal content in hydrochars
under different HTC conditions was studied to assess its potential for
land use. Few consider hydrochar applications in fuel and land use,
with comprehensive consideration given to solid content, hydrothermal
temperature, residence time, and heavy metal content to create a data
basis for engineering applications.

\section{Materials and methods}\label{sec2}

\subsection{Materials}\label{sec2.1}
Activated sludge (AS) and dewatered sludge (DS) in this study were
collected from the secondary sedimentation tank and mechanical
dewatering workshop of a wastewater treatment plant (Nanjing, China).
The sludge was dispensed in sealed bags, refrigerated at  4~\textdegree
C, and used in the HTC process. For the characterization of the
samples, AS and DS were further dried in an oven at  105~\textdegree C
for 12~h, ground to a fine powder, and sealed in a dry glass vial. The
basic properties of sludge are listed in  Table~S1.

\subsection{HTC process}\label{sec2.2}
The HTC system was implemented by deploying a non-stirred 100~mL
stainless steel Teflon-lined reactor. For each experiment, 60~mL of 
AS (wet volume) or 6 g of DS (dry weight) with a 1:10 solid-to-liquid
ratio was loaded into the reactor, and the reactor was then securely
closed and placed in the oven. The HTC reaction was maintained for 1,
2, 3, and 4~h at the set temperature of 180, 220, and  260~\textdegree
C. When the HTC procedure was finalized, the reactor was taken out of
the oven and cooled to room temperature. The treated slurry was settled
and separated by a 0.45~$\rmmu$m filter membrane. The solid phase of
hydrochar was dried, pulverized, and labeled
AS/DS-HC-180/220/260-1/2/3/4 for each sample before being analyzed. The
AS/DS specified the type of sludge,  180/220/260
indicated the temperature, and 1/2/3/4 represented the retention period
in hours.

\subsection{Analytical method}\label{sec2.3}
The elemental content (C, H, N, S) and ash content of hydrochars were
determined by an elemental analyzer (vario EL cube, Elementar,
Germany) and the combustion method. Based on the above data, the O
content was determined by calculating the difference. The bulk atomic
ratios of oxygen/carbon (O/C) and hydrogen/oxygen (H/C) were further
calculated to estimate the coalification process of HTC based on the
van Krevelen  diagram~\cite{24}. The hydrochar yield was calculated as in 
Equation~(\ref{eq1}). A \mbox{thermogravimetric} (TG; TG 5500, TA Instruments, United States) 
analysis of hydrochars was conducted in the air atmosphere at a heating rate of 
10~\textdegree C${\cdot}$min$^{-1}$ from 50 to  800~\textdegree C. The
recalcitrance index $R_{50}$ of hydrochars based on TG data was
calculated as in Equation~(\ref{eq2}). The heavy metal content of hydrochars was
determined by an inductively coupled plasma mass spectrometer (ICP-OES,
PerkinElmer, United States) after being digested according to our previous 
method~\cite{25}.
{\fontsize{9.5}{12}\selectfont
\begin{eqnarray}
\mbox{Yield} (\%)
&=&
(\mbox{mass of dry hydrochar/mass of dry sludge})
\nonumber\\
&&\times\, 100\%\label{eq1}\Seqnsplit
R_{50}
&=&
T_{50,\mathrm{hydrochar}}/
T_{50,\mathrm{graphite}}\label{eq2} 
\end{eqnarray}}\unskip
where $T_{50,\mathrm{hydrochar}}$ is the corrected temperature
corresponding to 50\% oxidation of the hydrochar; 
$T_{50,\mathrm{graphite}}$ is  886~\textdegree C according to
Harvey et~al.~\cite{26}.

\subsection{Data collection and category}\label{sec2.4}
An extensive literature search was conducted in the Web of Science
using keywords such as ``sewage sludge'' and ``hydrothermal
carbonization''. A total of 826 related articles were retrieved. The
included literature
was further screened to meet the following criteria: (1)~the research
was about sludge HTC and without any auxiliary condition; (2)~the HTC
temperature ranged from 140 to  380~\textdegree C and the corresponding
retention time was from 20 to 1440~min; here, it should be noted
that the retention time is influenced by hydrothermal temperature as
previous research has highlighted the more pronounced impact of
temperature on HTC compared to retention  time~\cite{27}; (3)~the
reactor volume utilized for data collection varied between 100~mL and 
1~L; and (4)~the original data of hydrothermal intensity and
physicochemical properties of hydrochars were measured. 
The information pertaining to (1), (2), (3), (4) could be found in the
manuscript or supporting information of the screened articles.
Hydrothermal intensity, a function of hydrothermal
temperature  ($T$, \textdegree C) and retention time ($\tau$, min), was
recommended as a valuable parameter for interpreting results from hydrochar
production performed at different temperatures and reaction times.
It was calculated  using 
Equations~(\ref{eq3}) and~(\ref{eq4})~\cite{27,28,29,30}. From the 10 selected
research articles, the highest frequency of hydrothermal intensity 
$(\ln R_{0})$
is in the range\unskip\break of 12--14.
{\begin{eqnarray}
&\displaystyle
R_{0}=
\tau \times \exp
\{(T-100)/14.75\}
\label{eq3}\Seqnsplit
&\displaystyle
\ln R_{0}=
\ln \tau +(T-100)/14.75
\label{eq4} 
\end{eqnarray}}\unskip
where $T$ (\textdegree C) is the hydrothermal temperature and $\tau$
(min) is the retention time.

\section{Results and discussion\label{mark-3}}

\subsection{HTC operation and hydrochar yields}\label{sec3.1}
To ensure the successful implementation of HTC technology, as shown in 
Figure~S1, efficient carbonization and the possibility of prolonged
production are necessary. According to our data, if the hydrothermal
intensity is insufficient, the raw material cannot be completely
carbonized, leading to difficulties in solid--liquid separation (data
not shown). The study of He et~al.\ reported that the solid--liquid
separation was similarly difficult after sewage treatment with HTC at
200 and  220~\textdegree C~\cite{31}. Therefore, adjusting the
appropriate hydrothermal intensity for continuous production is
essential to ensure complete carbonization and efficient solid--liquid
separation.

The product yield is one of the critical factors that determine the
feasibility of HTC technology application. When the retention time was
1~h, the product yields of activated sludge hydrochars (AS-HCs) 
were 65.2\%, 55.1\%, and 53.6\% at
180, 220, and  260~\textdegree C, respectively  (Figure~\ref{fig1}a).
Meanwhile, when the retention time was 3~h, the product yields of
dewatered sludge hydrochars (DS-HCs) were 66.6\%, 61.7\%, and 49.8\% at 180, 220, and 
260~\textdegree C, respectively  (Figure~\ref{fig1}a). In addition, He
et~al.\ discovered that the product yields of hydrochars declined from 66.6\% to
43.1\% when the temperature rose from 200 to  380~\textdegree C 
$(\ln R_{0}=9.78)$~\cite{31}. In contrast, the product yields
of hydrochars decreased as the complete carbonization process
progressed at a specific temperature with increased retention time. For
example, the product yields of DS-HCs were 68.5\%, 65.0\%, and 56.1\%,
respectively, when the retention time was 1, 2, and 4~h at 
220~\textdegree C. Under these same conditions, the product yields of
hydrochars were reported as 71.9\%, 68.5\%, and 64.9\%, respectively,
by  Ovsyannikova et~al.~\cite{32}. The same trend was also reported by 
Malhotra et~al.~\cite{24} and He et~al.~\cite{13}. It was noticeable that
the hydrochar yields became closer with increasing  $R_{0}$ for the
two kinds of sludge and ranged from 52\% to 56\% at  220~\textdegree C 
$(\ln R_{0}=12.23\ndash13.62)$  (Figure~\ref{fig1}a). The results
of He et~al.\ showed that the hydrochar yield remained at about 60\% when
the retention time was long enough (${\geq}$6~h) at  200~\textdegree C
$(\ln R_{0}\geq12.67)$~\cite{13}. It can be concluded that the 
$R_{0}$ going up by the elevation of reaction temperature or lengthening
reaction time can generate an equivalent yield of hydrochars,
demonstrating that the hydrothermal intensity can direct the
decomposition extent of organics in sludge during HTC. 

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}Effects of hydrothermal intensity
$(\ln R_{0})$ on hydrochars' (a)~yields; (b)~ash content; (c)~C
content; (d)~H content; (e) O~content; (f)~N content; (g)~correlation
analysis. AS-HCs represent the activated
sludge hydrochars and DS-HCs represent the dewatered sludge
hydrochars.}
\end{figure*}

As the hydrothermal intensity is heightened, the yield of hydrochars
diminishes and the hydrochars are more carbonized 
(Figure~\ref{fig2}a). Moreover, it can be seen in  Figure~\ref{fig2}a
that the hydrochar yields fluctuated in the range of  35.0--80.0\% due
to the group data coming from different solid loading and hydrothermal 
intensity~\cite{15}. The study by Arag\'{o}n-Brice\~{n}o et~al.\ found
that hydrochar yields increased from 67.9\% at 2.5 wt\% loading to
75.6\% at 25.0\% wt\% loading for sewage digestate at  250~\textdegree
C, 30~min $(\ln R_{0} =  13.57)$~\cite{15}. In this study, AS
with the moisture of 98.4\% resulted in a lower AS-HC yield (52.1\%)
than the yield (56.1\%) of DS with a moisture of 80.3\% at 
220~\textdegree C, 240~min  $(\ln R_{0} = 13.62)$. Under this
condition, the hydrochar yield decreased from 71.7\% to 64.9\% when the
DS-HC/water (w/w) ratio changed from 0.2 to  0.1~\cite{32}. Higher
moisture levels would enable the hydrolysis of organic matter and the
{decarboxylation reactions}~\cite{31,33}. Simultaneously, it would be
hard to polymerize the small molecules that had decomposed into the
aqueous phase into hydrochars because of the low 
concentration~\cite{34}. Therefore, the solid loading in the HTC
reactor should be a factor when selecting the parameters.

\begin{figure*}
{\vspace*{2pt}}
\includegraphics{fig02}
{\vspace*{2pt}}
\caption{\label{fig2}Investigation on hydrochars' (a)~yields; (b)~ash
content; (c)~C content; (d)~H content; (e)~O~content; (f)~N content.
A:~$\ln R_{0}\leq10.99$; 
B:~$11.00\leq\ln R_{0}\leq12.49$;
C:~$12.50\leq\ln R_{0}\leq13.49$;
D:~$13.50\leq\ln R_{0}\leq14.99$;
E:~$15.00\leq\ln R_{0}\leq16.99$;
F:~$\ln R_{0}\geq17.00$. AS-HCs (pink circles) represent the
activated sludge hydrochars, DS-HCs (blue circles) represent the
dewatered sludge hydrochars, and the data in the literature (yellow
circles) represent the hydrochars reported in the published literature.}
{\vspace*{2pt}}
\end{figure*}

\subsection{Elemental composition of hydrochars}\label{sec3.2}

\subsubsection{Content of ash in hydrochars}\label{sec3.2.1}
The raw sludge had higher ash content (AS 46.88~wt\% and DS 48.19 wt\%)
than all AS-HCs or DS-HCs. AS-HCs had an ash content of between 60.59
and 75.89 wt\%, while the ash content of DS-HCs was between 
48.81 and 68.84 wt\% in  Figure~\ref{fig1}b. During the HTC process, the
organic matter was partially broken down and dissolved into the
aqueous phase, increasing the inert ash component in 
hydrochars~\cite{13}. As the hydrothermal intensity rises, the ash
content increases as organic materials are lost in the raw material and minerals
{present} in hydrochars, which is demonstrated in  Figure~\ref{fig1}b. It is
possible that the increased ash content in AS-HCs is a consequence of
the accelerated hydrolysis of organic matter and an amplified
decarboxylation reaction for treated  AS~\cite{35}. As depicted in 
Figure~\ref{fig1}b, at relatively low hydrothermal temperatures (e.g.,
180 and  220~\textdegree C), the ash content in both AS-HCs and DS-HCs
is augmented with increased retention time. When hydrothermal 
intensity/$\ln R_{0}$ exceeded 12.92  (220~\textdegree C and
120~min), the changes in ash content of DS-HCs were no longer
noticeable, which was in agreement with the data collected by  Ekpo
et~al.~\cite{36}.

The ash content in hydrochars changed drastically at a specific
hydrothermal intensity based on the literature data, as seen in 
Figure~\ref{fig2}b. This is probably due to the significant disparity in
the organic matter between various types and regions of 
sludge~\cite{37}. As an illustration, the ash content of dewatered
anaerobically digested SS from Singapore was just 21.8\%,
and the ash content of hydrochars (prepared at  200--380~\textdegree C
and 20~min) varied from  32.8 to 43.2\%~\cite{31}. For the dewatered
SS with 52.0\% ash content located in Guangzhou, China,
the hydrochars (at  220~\textdegree C and 12~h) achieved 71.8\% ash 
content~\cite{38}. Comparable to this, the dewatered SS from
Changsha, China, with 49.9\% of ash content, yielded a 70.6\% ash
content in hydrochars after  300~\textdegree C and\break  60~min~\cite{39}.

\subsubsection{Content of C in hydrochars}\label{sec3.2.2}
The C content in hydrochars decreased in comparison to raw materials
due to the change of organic carbon-containing compounds (e.g.,
carbohydrates and lipids) into gaseous products and hydrolysate during
the HTC  process~\cite{38}. Comparatively, the drop of C content in
AS-HCs was more marked than in DS-HCs, as  Figure~\ref{fig1}c shows.
The C content in AS was 26.8\%, but in hydrochars, it dropped to 
20.1--14.6\% and kept shrinking as the hydrothermal intensity
increased. Nonetheless, the disparity became imperceptible when
hydrothermal  intensity/$\ln R_{0}$ surpassed 10.2. The C
content in DS was 25.4\%, whereas the C content in DS-HCs varied
between 26.4 and 21.1\%. At  180~\textdegree C, the C content
experienced a slight rise in 2~h and then a notable decrease after 3~h.
When the HTC temperature is  220~\textdegree C, a significant reduction
can be noticed after 2~h. Under vigorous hydrothermal intensity, the
final C content in DS-HCs remained at approximately 21\%. There was a
discrepancy in the literature concerning the impacts of solid loading
on the C content in hydrochars. A rise in moisture content from 85 to
95\% caused the C content in hydrochars to drop from 42.8 to 37.0\%
at  380~\textdegree C and  20~min~\cite{31}, in accordance with our
result that the C content in AS-HCs was lower\unskip\break 
than DS-HCs. In the case of
sewage digestate (United Kingdom), when the solid loading was  2.5--30\% (w/w) at 
250~\textdegree C for  30~min~\cite{15}, the C content was kept within
32--34\%.

The C content in our experiments was moderate in the collected data 
(Figure~\ref{fig2}c), varying from 11.4 to 44.6\%. The C content in
hydrochars showed a pattern similar to ash content variation,
responding to the type and region of sludge. 
The C content in hydrochars produced from the dewatered SS
in Singapore was approximately 32.8\%~\cite{13}, comparable to the value
(33\%) in hydrochars from mixed centrifuged sewage sludge from 
India~\cite{24}. The C contents of hydrochars derived from SS in 
Shanghai~\cite{40}, Beijing~\cite{18}, and Dalian~\cite{41} in China
were high, surpassing 36\%. At the same time,  Changsha~\cite{39},
Nanjing~\cite{12}, and Guangzhou~\cite{38} had much lower C content of 
22--23\%, 16.8\%, and 16.5\%, respectively. Our experiment and
collected data revealed that the C content remained constant when
hydrothermal intensity reached a certain degree for the same sludge. As
an illustration, hydrochars maintained a C content of 44\%, and the HTC
temperature had no notable effect when held at  260--340~\textdegree
C and 20~min for the dewatered anaerobically digested SS 
(Singapore)~\cite{31}. Studies have shown that the primary organic
materials present in sludge, such as proteins and polysaccharides, can
be broken down significantly even at temperatures  below 
180~\textdegree C~\cite{42,43,44}. The study by Wang et~al.~\cite{12}
reported that 42\% of the volatile matter was degraded at 
250~\textdegree C and 300~min  $(\ln R_{0} = 15.9)$, and due to
the interaction of the organic components in the sludge, char formation
was  affected~\cite{12}. Differences in the content and form of organic
material in different kinds and areas of sludge could result in
distinct behaviors during the HTC\break process.

\subsubsection{Content of H in hydrochars}\label{sec3.2.3}
As the temperature increased, the dehydrogenation response became more 
intense~\cite{31}; thus, the H content in hydrochars was lower than in
the raw materials. The H content in AS was 4.43\%, but in hydrochars,
it dropped to  2.30--3.73\%, as shown in  Figure~\ref{fig1}d. The H
content in DS-HCs was in the range of 3.07 to 4.4\%, which was slightly
lower than in DS. The H content in DS-HC-260 was less than that in
DS-HC-180 and DS-HC-220 when held at a specific retention time. As
determined by Peng et~al., the H~\mbox{content} in DS-HCs decreased from 3.3
to 2.3\% when the temperature rose from 200 to  300~\textdegree C in 
60~min~\cite{39}. With the hydrothermal temperatures set at 
180 and  220~\textdegree C, the H content decreased as
the retention time increased. However, when the temperature rose to 
260~\textdegree C, the H content fell steadily with the hydrothermal
intensity. Similarly, Zhang et~al.\ observed that the H content decreased
from 5.1 to 4.9\% when the duration was prolonged from 60 to
720~min at  190~\textdegree C, whereas at  260~\textdegree C, the H
content stayed steady at approximately  5\%~\cite{18}. 

The H content of this study was of a medium level, which is in
accordance with the data found in the literature  (Figure~\ref{fig2}d).
It has been observed that the H content of sludge can be greater than
5\%, such as dewatered anaerobically digested SS 
(Singapore)~\cite{31}, dewatered SS (Changsha, 
China)~\cite{40} and municipal sludge (Beijing,  China)~\cite{18}.
However, the H content can be less than 2\%, such as in SS
(Guangdong,  China)~\cite{45}. There was no noticeable change in the H
content of hydrochars when solid loadings were taken into consideration
for sludge of the same  type~\cite{15,31}.

\subsubsection{Content of O in hydrochars}\label{sec3.2.4}
As a consequence of the decarboxylation and dehydrogenation reaction,
the oxygen content of hydrochars was significantly diminished from that
of the raw material, ranging from 16.81\% in AS to  12.53--5.93\% in
AS-HCs and from 17.28\% in DS to  16.61--5.33\% in DS-HCs. It decreased
with increase in hydrothermal intensity as shown in 
Figure~\ref{fig1}e. By increasing the hydrothermal time at low HTC
temperatures, the oxygen content in hydrochars is reduced. As an
illustration, the oxygen content in DS-HCs decreased from 14.86 to
8.04\% when the retention time was increased from 60 to 240~min at 
220~\textdegree C. It was also found by Malhotra et~al.\  that 
the oxygen content decreased from 17.4 to
14.5\% when the duration of the process was increased from 60 to
300~min at  200~\textdegree C when the mixed centrifuged sludge was
treated with  HTC~\cite{24}. As the temperature increased, the effect
of retention time became diminished. The O content in DS-HCs decreased
from 7.7 to 5.3\% when the time was extended from 60 to 240~min at 
260~\textdegree C. Zhang et~al.\ reported similar results; the O content
in hydrochars only reduced from 4.3 to 4.0\% when handling municipal
sludge with HTC at  260~\textdegree C, extending the time from 60
to  360~min~\cite{18}.

Data analysis of the literature revealed that the O content of
hydrochars varied from 1.6 to 18.5\%, as shown in 
Figure~\ref{fig2}e. Higher levels of O content  (16.9--18.5\%) appeared
in hydrochars prepared from dewatered SS (Singapore) at low
intensity  (200~\textdegree C and  240--720~min)~\cite{13}. The
temperature significantly affected dewatered anaerobically digested
SS from Singapore, leading to a decrease in the O content of
hydrochars from 10.6 to 1.6\% when the temperature increased from 
200 to  360~\textdegree C~\cite{31}. In milder
conditions, the O content of DS-HCs exceeded that of AS-HCs. However,
this divergence nearly disappeared when hydrothermal 
intensity/$\ln R_{0}\geq13$ showed that the solid loading has
no distinct influence on the O content in hydrochars under these
conditions. Aragon-Briceno et~al.\ obtained comparable results, and it
was determined that the O content varied between  10.5 and 8.7\% when solid
loading changed from 2.5 to 30\% at  250~\textdegree C for 30~min 
$(\ln R_{0} = 13.57)$~\cite{15}.

\subsubsection{Content of N in hydrochars}\label{sec3.2.5}
There was a decrease in N content from AS to AS-HCs, from 4.64 to 
2.73--0.94\%, and from DS to DS-HCs, from 4.21 to  4.38--1.82\% 
(Figure~\ref{fig1}f) when compared to raw materials. This could be
attributed to the dissolution, hydrolysis, and disintegration of
nitrogen-containing organic compounds, such as 
proteins~\cite{24,31,46}. As shown in  Figure~\ref{fig1}f, the N
content in hydrochars declined in line with a growing hydrothermal 
intensity/$\ln R_{0}$. For example, the N content decreased
from 2.73\% in AS-HC-180-1 to 0.94\% in AS-HC-260-4 due to intense
amide hydrolysis and decarboxylation at  260~\textdegree C~\cite{39}.
At a constant temperature, the N content decreased with increased
retention time; however, this effect was no longer apparent at 
260~\textdegree C. The results indicated a fluctuation of N content in
DS-HCs at  180~\textdegree C, possibly due to the complexity of
N-containing substances in DS after the mechanical dewatering process.
In addition, at  220~\textdegree C, the N content in DS-HCs decreased
as the duration increased, and a stable level was achieved  (2.22--2.45\%)
after 120~min. Furthermore, He et~al.\ demonstrated that 40\% of the N
content was retained in the hydrochars. The remaining 60\% was
released into the gas and liquid phases at  200~\textdegree C, with no
decrease in nitrogen content when the carbonization period was extended
to  4~h~\cite{13}.

The literature data indicated that the N content in hydrochars was in the
range 1.1--5.2\%  (Figure~\ref{fig2}f). A higher amount of N
content (5.2\%) was found in hydrochars created from dewatered
anaerobically digested SS at low intensity  (200~\textdegree
C and 20~min), and the value decreased to 3.9 and 3.2\% when the
temperatures were raised to  260 and  380~\textdegree C, 
respectively~\cite{31}. It was also observed that the N content of
hydrochars derived from the mixed centrifuged SS at 
200~\textdegree C reduced correspondingly, decreasing from 4.8 to
3.1\% as the time increased from 60 to  480~min~\cite{24}. The N content in
hydrochars ranged from 1.8 to 1.9\%, with slight variance when the
duration of municipal sludge treatment using HTC changed from 60 to
1440~min at  260~\textdegree C~\cite{18}. This study and literature
data demonstrated that if the solid loading were low, the N content of
hydrochars would be less than that found in hydrochars with a higher solid\break
loading~\cite{15,31}.

\subsubsection{Correlation analysis}\label{sec3.2.6}
Figure~\ref{fig1}g shows the correlation between hydrothermal
intensity and the different parameters (such as yield and 
components) of AS-HCs and DS-HCs. It can be seen from 
Figure~\ref{fig1}g that the C, O, H, N, and S present in hydrochars are
all negatively correlated with the hydrothermal 
intensity/$\ln R_{0}$, indicating that with increase in
hydrothermal intensity, the contents of the five elemental compositions
in hydrochars all show a downward trend. Likewise, C, O, H, N, and S
components were significantly negatively correlated with ash content
but positively correlated with the HHV. The comprehensive analysis shows
that the ash content in hydrochars increases with hydrothermal
intensity, and it also has some influence on the content of other
components in hydrochars and HHV.

{\vspace*{-1pt}}

\subsection{Structural characteristics of hydrochars}\label{sec3.3}

{\vspace*{-1pt}}

\subsubsection{Morphology characterization of
hydrochars}\label{sec3.3.1}
The scanning electron microscope (SEM) images demonstrated the 
distinct morphological structure
between raw materials and hydrochars  (Figure~\ref{fig3}). In~the HTC
process, on the one hand, O and H elements left the surface of the raw
materials in the form of  CO$_{2}$ and H$_{2}$O, creating a surface
with more porosity for  hydrochars~\cite{45}. On the other hand, the
large bulk structure in the raw materials was demolished after HTC and
small spheres with a coarse surface were formed and collected, with
abundant pore and tunnel structures inside the\break  hydrochars~\cite{2,47}.

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}SEM images of (a)~AS and typical AS-HCs;  (b)~DS
and typical DS-HCs. AS-HCs represent the activated sludge hydrochars
and DS-HCs represent the dewatered sludge hydrochars.}
\end{figure*}

\subsubsection{Atom ratios and van Krevelen diagrams of
hydrochars}\label{sec3.3.2}
The ratios of H/C, O/C, (O${+}$N)/C, and C/N were typically used to
evaluate the aromatization, hydrophilicity, polarity, and source
discrimination of biochar,  respectively~\cite{48,49}. As  Table~S2
indicates, the ratio of H/C in hydrochars decreased with increased HTC
intensity; the content of H/C in AS-HCs shifted from 2.23 to 1.78, and
the content of H/C in DS-HCs shifted from 2.04 to 1.67, which is
associated with the enhanced aromaticity of 
hydrochars~\cite{24,38,50}. As the hydrothermal intensity increased,
the O/C decreased from 0.47 to 0.29 for AS-HCs and from 0.48 to 0.19
for DS-HCs, demonstrating a decrease in their surface 
hydrophilicity~\cite{51}. The van Krevelen diagram, presented by the
H/C and O/C ratios, as shown in  Figure~\ref{fig4}, can distinguish the
HTC reaction  pathways~\cite{12,52,53,54}. For AS, it can be
preliminarily deduced that the HTC process was governed by
decarboxylation when hydrothermal intensity was low, while dehydration
became intense with  increased hydrothermal intensity. The high
water content in AS could have promoted the hydrolysis of organic
matter and increased decarboxylation  reactions~\cite{31,33}. For DS,
the HTC process mainly followed a dehydration trend, consistent with
the results reported by  Wang et~al.~\cite{12}, He et~al.~\cite{13,38},
and Shi et~al.~\cite{40}. Significant decarboxylation and dehydration
reactions produced hydrochars of a higher fuel  quality~\cite{31},
suggesting the possibility of utilizing hydrochars as fuel.

\begin{figure}
{\vspace*{3pt}}
\includegraphics{fig04}
{\vspace*{4pt}}
\caption{\label{fig4}Van Krevelen diagram of AS-HCs and DS-HCs. AS-HCs
represent the activated sludge hydrochars and DS-HCs represent the
dewatered sludge hydrochars. For AS, the HTC process was mainly governed
by decarboxylation and dehydration. For DS, the HTC process was mainly
governed by dehydration.}
\end{figure}

The C/N ratio is a useful parameter, and it was found that many studies
use the C/N ratio to assess the contribution of anthropogenic or
agricultural sources to aquatic systems. The C/N ratio of hydrochars
was significantly higher than that of the sludge, indicating
considerable N loss during the HTC process. The C/N values of AS-HCs 
(8.6--19.3) were appropriately close to the C/N from anthropogenic
sources (13.7), and they had similar effects on aquatic systems,
suggesting potential for the application of fertilizer to 
soil~\cite{55}. The C/N values of the DS-HCs were in the range of 
8.3--12.5, which is comparable to the C/N of the humic substance
fraction observed in the soil  (9.74)~\cite{56}. In addition, they had similar
effects on water bodies, indicating that the DS-HCs had potential
to be used as a soil amendment. The polarity index (O${+}$N)/C decreased
when the hydrothermal intensity value was higher, revealing that the
number of polar groups in hydrochars was lower. Polar groups usually function
as water-binding sites and contribute to the formation of water 
clusters~\cite{48}. Therefore, it can be concluded that the surface
hydrophobicity of hydrochars grows with the rise in hydrothermal
intensity, which has a consequent effect on the surface adsorption
features.

\subsubsection{Functional group characterization of\newline
hydrochars}\label{sec3.3.3}
To further investigate the change in functional groups of
hydrochars at different hydrothermal\unskip\break 
intensities, 
Fourier-transform infrared spectroscopy (FTIR) analysis was
employed, which is presented in  
Figures~\ref{fig5}a,b. 
The peak at 1540~cm$^{-1}$ (C--N--C bending
vibration and/or C${=}$O in --COOH) in hydrochars declined to a weaker
shoulder or even disappeared depending on the HTC intensity, which
corroborated the hydrolysis of protein and the decarboxylation
reaction. Simultaneously, the aromatic structure was observed; for
instance, the broader band at  1616~cm$^{-1}$ and strengthened bands at
1460--1450~cm$^{-1}$ and 750--780~cm$^{-1}$ in hydrochars with
increasing intensity could be assigned to aromatic C${=}$O, C${=}$C, and
C--H,  respectively~\cite{48}, which verified the aromatization of
intermediates during the HTC process. Moreover, increased intensity caused
the C-aliphatic peaks to become more pronounced, demonstrating the
production of more hydrocarbon compounds. The absorption peaks at 2920
and  2850~cm$^{-1}$ (C--H$_{\mathrm{aliphatic}}$ stretching) became
stronger with increasing hydrothermal intensity, which indicated that
more hydrocarbon compounds formed.

\begin{figure*}
\includegraphics{fig05}
\vspace*{-2pt}
\caption{\label{fig5}The FTIR curves of (a)~AS and typical AS-HCs;
(b)~DS and typical DS-HCs. The XRD curves of (c)~AS and typical AS-HCs;
(d)~DS and typical DS-HCs. TG and DTG curves of (e)~AS and typical
AS-HCs; (f) DS and typical DS-HCs. 1-quartz, SiO$_{2}$ ($2\theta=
20.6\text{\textdegree}$,  26.6\textdegree, and 50.1\textdegree);
2-gismondine,
$\mathrm{CaAl}_{2}\mathrm{SiO}_{8}\cdot4\mathrm{H}_{2}\mathrm{O}$
($2\theta=27.9\text{\textdegree}$). AS-HCs represent the activated
sludge hydrochars and DS-HCs represent the dewatered sludge
hydrochars.}
\end{figure*}

It was noticed that the Si--O stretching band at  1010~cm$^{-1}$ became
much stronger with hydrothermal intensity, which was in line with the
rising ash fraction in hydrochars. The X-ray diffraction (XRD) patterns 
(Figures~\ref{fig5}c,d) also revealed that the
primary crystal structure of hydrochars was  SiO$_{2}$ ($2\theta= 
20.6\text{\textdegree}$,  26.6\textdegree, and 50.1\textdegree). As
seen in SEM images  (Figure~\ref{fig3}), the hydrochars have some
sheet-like  SiO$_{2}$, which is more obvious in AS-HCs than in DS-HCs,
which is in keeping with the higher ash content in AS-HCs. Si is a
fundamental element in soil ecosystems due to its abundance in the
Earth's crust, and it is a vital nutrient for the growth of certain
species of silicon-loving plants. 
It has been indicated that Si in
biochar could interact with and bind to the carbon fractions, which
positively affected carbon fixation and changed the dissolution
patterns of carbon and silicon in  biochar~\cite{57}. The addition of
Si-rich biochar has been found to act as a slow-release source of Si,
resulting in a noticeable rise in the yield of grain and 
straw~\cite{57}. Additionally, the impact of biochar application in
acidic soil had a positive effect as it decreased  CO$_{2}$ and
N$_{2}$O  emissions~\cite{58}. Therefore, the abundant Si-containing
material in hydrochars has become a prominent factor that must be
considered when applying hydrochars in soil.

To further investigate the evolution of the functional groups of
hydrochars at different hydrothermal intensities, 2D-COS FTIR analysis
was employed, which is presented in  Figure~\ref{fig6}. Autopeaks
appeared at  2920~cm$^{-1}$ and 2850~cm$^{-1}$
(C--H$_{\mathrm{aliphatic}}$),  1640~cm$^{-1}$ (C${=}$O), 1540~cm$^{-1}$
(C--N--C),  1010~cm$^{-1}$ (Si--O--Si),  and 750--780~cm$^{-1}$
(C--H$_{\mathrm{aromatic}}$) on the diagonal of the synchronous map.
The autopeaks are all positive, revealing a consistent tendency in the
alterations of their associated functional groups. As hydrothermal
intensity increases, the peak of the functional group at 
2920~cm$^{-1}$ and 2850~cm$^{-1}$ rises, showing that more hydrocarbons
are produced. The peak at  1010~cm$^{-1}$ reveals the most conspicuous
growth in the functional group, signifying that the Si--O bond
stretching vibration becomes significantly stronger, which is
consistent with the enhancement in the ash content of the hydrochars as
hydrothermal intensity\unskip\break 
increases. 

\begin{figure*}
\includegraphics{fig06}
\caption{\label{fig6}2D-COS FTIR analysis of the functional groups of
hydrochars at different hydrothermal intensities $(\ln R_{0})$.
AS-HCs represent the activated sludge hydrochars and DS-HCs represent
the dewatered sludge hydrochars.}
\end{figure*}

The results of the asynchronous map revealed that the crosspeak
signature altered with an increase in hydrothermal intensity,
suggesting that the increase in hydrothermal intensity modified the
sequence of functional groups that occur during the HTC process. The
change in the sign of the crosspeaks in the asynchronous map was
consistent at hydrothermal intensities of  $\ln R_{0}= 10.21$
for  180~\textdegree C, 2~h and $\ln R_{0}= 12.92$ for 
220~\textdegree C, 2~h. The negative value at the crosspeak $\psi$
(1010, Y) suggested that the stretching vibration of Si--O occurred
last. The positive value at the crosspeak $\psi$ (1640, 2920/2850)
demonstrated that the aromatization reaction is ahead of the synthesis
reaction of hydrocarbons. A positive value at the crosspeak $\psi$
(2920/2850, 1540) revealed that the reaction of hydrocarbons had
occurred before the\unskip\break 
hydrolysis of proteins. Accordingly, at low and
medium levels of hydrothermal intensity, the order of functional groups
that shift during the HTC process is C${=}$O  ${>}$
C--H$_{\mathrm{aliphatic}}$\ ${>}$\ C--N--C  ${>}$
C--H$_{\mathrm{aromatic}}$\ ${>}$\ Si--O--Si. However, as the
hydrothermal intensity increased to 12.92  (260~\textdegree C, 4~h), a
marked change in the order of functional groups was observed compared
to low and medium levels of hydrothermal intensity. A positive value at
crosspeak $\psi$ (X, 1010) was observed, signifying that the Si--O
stretching vibration change occurs first. A positive value at
crosspeak $\psi$ (750--780, 1540) demonstrated that the C--H bond
aromatization reaction had occurred before the protein hydrolysis
reaction. The positive value at crosspeak $\psi$ (1540, 1640)
indicated that the protein hydrolysis reaction occurred before the
C${=}$O bond aromatization reaction. The negative value of crosspeak
$\psi$ (1640, 2920/2850) showed that the C${=}$O bond aromatization
occurred before the synthesis of hydrocarbon compounds. Consequently,
as the hydrothermal intensity increased, the order in which the
functional groups transform during HTC is Si--O--Si  ${>}$
C--H$_{\mathrm{aromatic}}$\ ${>}$ C--N--C ${>}$ C${=}$O ${>}$
C--H$_{\mathrm{aliphatic}}$.

\subsubsection{Thermal stability of hydrochars}\label{sec3.3.4}
The TG and derivative thermogravimetric (DTG)
analyses of sludge and hydrochars helped
investigate their thermal stability, as depicted in  
Figures~\ref{fig5}e,f. Under different values of low 
($\ln R_{0}= 10.21$ for \mbox{AS-HC-180-2),} medium 
($\ln R_{0}= 12.92$ for AS-HC-220-2), and high 
($\ln R_{0}= 16.33$ for AS-HC-260-4) hydrothermal intensities, 
hydrochars derived from
AS exhibited similar TG curves with increasing residual amounts, which
was in agreement with the ash content of the hydrochars. The DTG curves
of AS-HCs indicated that the first maximum weight-loss rate appeared at
317--329~\textdegree C, which was higher than the corresponding rate in
AS. The volatiles in AS were removed and carbonaceous components were
generated simultaneously by HTC, leading to a new peak at higher
pyrolytic  temperatures~\cite{45}. At HTC temperatures ranging from 
520 to  409,  398, and 
400~\textdegree C, the second peak gradually widened and weakened,
signifying that the AS-HCs had higher thermal stability and better fuel
quality~\cite{12}. In the DTG curves of DS-HCs, two peaks were observed
in the first region  (180--400~\textdegree C) and moved to a lower
temperature as the hydrothermal intensity increased. For example, the
first peak corresponding to the peak at  280~\textdegree C in DS
shifted to  278, 211, and  217~\textdegree C
with a reduced intensity at low, medium, and high levels of hydrothermal
intensity, respectively. Like AS-HCs, a new peak appeared at higher
pyrolytic temperatures of  312--323~\textdegree C due to the
carbonaceous components formed during HTC. The peak of  517~\textdegree
C in the DS shifted to a lower temperature of  498~\textdegree C for
DS-HC-180-2,  426~\textdegree C for DS-HC-220-2, and  433~\textdegree C
for DS-HC-260-4. The magnitude of migration was not as
pronounced for DS-HCs as it was for AS-HCs, illustrating that the
pyrolysis morphology of DS-HCs was different from that of AS-HCs. It
could be that the organic components in AS underwent a change after
dehydration treatment and impacted their transformation during the 
HTC\unskip\break 
process.

The recalcitrance index $(R_{50})$ was proposed to quantify biochars' 
degradability~\cite{26}. With  increasing hydrothermal intensity,
the $R_{50}$ for AS-HCs gradually increased from 0.38 to 0.43 and
DS-HCs from 0.39 to 0.44. O/C can also be used as a stability index of
biochar in soils. The two groups are Class 1 with a half-life greater
than 1000 years (O/C $<$ 0.2) and Class 2 with a half-life from 100
to 1000 years  (0.2 $<$ O/C $<$  0.6)~\cite{59}. DS-HC-260-4, with an
O/C of 0.19, was designated to Class 1, and the other hydrochars were
designated to Class 2. To conclude, hydrochars under high hydrothermal
intensity conditions may possess greater resistance to deterioration
and a longer life span in the soil system, suggesting their potential use
as agents for soil remediation and carbon\unskip\break 
sequestration.

\subsection{Potential application of hydrochars}\label{sec3.4}

\subsubsection{Energy recovery as fuel}\label{sec3.4.1}
A major potential utilization of hydrochars is as a fuel source for
energy recovery. The HHV and the lower heating value (LHV) are the critical
fuel properties that determine the energy content of charred materials
and can be evaluated as in Equations~(\ref{eq5}) and~(\ref{eq6})~\cite{13,60}.
{\begin{eqnarray}
&&\displaystyle
\mathrm{HHV}\; (\mathrm{MJ}/\mathrm{kg})=
0.349\times \mathrm{C}+1.1783\times 
\mathrm{H}+0.1005\times 
\mathrm{S}\nonumber\\
&&\qquad-\,0.1034\times 
\mathrm{O}-0.0015\times 
\mathrm{N}-0.0211\times 
\mathrm{A}
\label{eq5}\Seqnsplit
&&\displaystyle
\mathrm{LHV}\; (\mathrm{kJ}/\mathrm{kg})=
4.18(94.19\times 
\mathrm{C}-0.5501-52.14\times 
\mathrm{H})\label{eq6} \nonumber\\
\end{eqnarray}}\unskip
where C, H, S, O, N, and A are the mass percentages on a dry basis of C,
H, S, O, N, and ash contents of hydrochars, respectively.

According to  Table~S2, the HHV and LHV values of hydrochars were
usually lower than those of raw materials. For instance, the HHV
decreased from  11.87 (AS) and 11.37~MJ/kg (DS) to
5.86 (AS-HC-260-4) and 8.81~MJ/kg (DS-HC-260-4), respectively, when the hydrothermal 
intensity ($\ln R_{0}$) was 16.3. As a whole, the
energy recovery ratio decreased with  growing hydrothermal
intensity in accordance with the conclusions of  He
et~al.~\cite{13,31}. The HHV had a strong relationship with C and H contents;
thus, the influence of HTC parameters on HHV followed a similar trend to
that of C and H contents  (Figure~\ref{fig1}g). The HHV had a negative
correlation with the ash content, which signified that the rise in 
ash content during HTC would have a detrimental effect on the 
HHV~\cite{24,61}. The HHV/LHV values of DS-HCs were higher than those
of AS-HCs. According to the HHV, DS-HCs kept  66.60--37.70\% of the
energy from the raw material compared to AS-HCs, which only held 
48.60--22.50\%. Regarding fuel utilization, DS was the better option
for constructing hydrochars. Assuming the HTC process to be
energy-demanding, the effect of HTC parameters on the process energy
needs to be considered.

\begin{figure*}
\includegraphics{fig07}
\caption{\label{fig7}Concentrations of different heavy metals in (a)~AS
and typical AS-HCs;  (b)~DS and typical DS-HCs. AS-HCs represent the
activated sludge hydrochars and DS-HCs represent the dewatered sludge
hydrochars.}
\end{figure*}

Considering the above results, the utilization of DS is proposed.
Therefore, this study was conducted using DS with a moisture content of
80.3\%, a density of  $1.13\times10^{3}~\mathrm{kg}/\mathrm{m}^{3}$,
and a volume of approximately  4.64~m$^{3}$ based on 1~t of absolute
dry sludge. At a reactor filling ratio of 70\% in the HTC process, the
required vapor volume is  1.98~m$^{3}$. Using Equation~(\ref{eq7}), the
vapor pressure can be calculated for the corresponding temperature, and
the vapor density and mass can be seen in  Table~S3. The process in the
reactor is simplified without considering the energy transformation
resulting from the molecular decomposition of raw materials. The heat
needed for the temperature elevation and phase transition in the HTC
process can be determined using  Equation~(\ref{eq8}), and the results are
listed in  Table~S3.
{\begin{equation}
P(T)=
2.99\times 10^{10}\exp
(-4666.3/T)
\quad\mbox{for}\ T \geq 410.57
\label{eq7}
\end{equation}}\unskip
where $P(T)$ is the vapor pressure at temperature $T$ (expressed in 
Kelvin)~\cite{62}.
{\begin{equation}
Q=
c_{\mathrm{w}}q_{\mathrm{w}}\rmDelta T_{1}+
q_{\mathrm{s}}r+c_{\mathrm{s}}q_{\mathrm{s}}\rmDelta T_{2} 
\label{eq8}
\end{equation}}\unskip
where $c_{\mathrm{w}}$ is the specific heat capacity of water, 4.2~kJ/(kg${\cdot}$K);
$q_{\mathrm{w}}$ is the mass of water (kg);  $\rmDelta T_{1}$ is the temperature
change of water from room temperature  (25~\textdegree C) to HTC
temperature; $q_{\mathrm{s}}$ is the mass of steam in the reactor (kg); $r$ is
the phase change enthalpy of water at  100~\textdegree C, 2258~kJ/kg;
$c_{\mathrm{s}}$ is the specific heat capacity of vapor, 
$1.9~\mathrm{kJ}/(\mathrm{kg}{\cdot}\mathrm{K})$;  ${\rmDelta} T_{2}$ is
the temperature change of vapor from  100~\textdegree C to HTC
temperature. 

The recoverable energy (calculated by LHV) of hydrochars that were
applied to combustion to retrieve energy is stated in 
Table~S3~\cite{62}. From the data in  Table~S3, it is evident that the
energy available for recovery in hydrochars is sufficient to fulfill
the heat needs for a material temperature increase and phase
transition. Especially when the HTC temperature is low (180 and 
220~\textdegree C), the energy available for recovery is abundant.
DS-HC-260 has less energy conservation; however, its elemental
composition (O/C:  0.20--0.38 and H/C: 0.65--1.65) is comparable to 
lignite~\cite{12}. It can be succinctly stated that as the hydrothermal
intensity heightened, the energy recovery rate decreased; yet, the fuel
quality of hydrochars was  higher~\cite{31}. It was also observed by
researchers that hydrochars from
SS with high ash content cannot
adequately support energy utilization via  combustion~\cite{38}. In
this way, hydrochars produced under high hydrothermal intensity could
be combined with lignite for  combustion~\cite{12}. Furthermore, the
data demonstrate that the majority of energy was used to heat the
water, which suggests that  solid loading affects hydrochar
performance and substantially impacts the energy consumption of the HTC
process.

\subsubsection{Land application}\label{sec3.4.2}
High ash in sludge can impede the coalification process during HTC,
decrease C content in hydrochars, and adversely affect energy 
utilization~\cite{38,45,53}. Thus, it is essential to investigate other
feasible uses for this kind of hydrochar. In China, it is suggested to
use stabilized sludge products for fertilizers or soil amendments to
improve greening, abandoned mine sites, 
saline-alkaline soil, and desertified
lands. The above analysis of $R_{50}$, (O${+}$N)/C, FTIR, and XRD
suggests hydrochars derived from sludge could be used for practical
purposes. The content of heavy metals in products is a significant
factor regarding land  use~\cite{63}. The utilization of sewage and its
derivatives poses a significant risk due to heavy metal contamination. 
Figure~\ref{fig7} and  Table~S4 indicate the content of heavy metals in
the representative samples of this study. The Cd content was below the
detection limit in all samples. The hydrochars possess more Zn, Ni, and
Cr than the raw materials, but the amount of Cu has decreased.
Generally, the hydrochars produced by sludge in this study have levels
far below the heavy metal index  (Cr 0.15~mg/g) outlined in NY/T
3618-2020, suggesting that its land use is viable. This might result
from the sewage treatment plant dealing with municipal sewage and the
residual sludge containing a low level of heavy metals. Therefore, it
is essential for the specified application to be aware of the heavy
metal content in the raw materials. Besides, our previous research
revealed that after the amendment of sludge-derived hydrochars into the
soil, the porosity, O abundance, and ash content of hydrochars
increased with aging time, resulting in improved Cd sorption. 
The ash content was a critical factor in the heterogeneity of Cd
sorption  sites~\cite{64}. Thus, hydrochars have potential as a
soil amendment because it positively affects soil properties but
depends on the soil type. Therefore, before large-scale land use of
hydrochars, it is necessary to comprehensively evaluate all aspects.

\section{Conclusion}\label{sec4}
The discrepancies in sludge properties in various regions lead to 
differences in hydrochars, which should be thoroughly evaluated when
selecting the process and utilization, emphasizing the organic matter
and heavy metal content as it will affect the yield of products and
potential resource application methods. The study showed that the
hydrothermal intensity influenced the yield, ash content, and elemental
composition of hydrochars. An upper limit of hydrothermal intensity was
necessary for treating a particular sludge via HTC. The upper limit
could be used as the criteria for constructing and operating the
widespread implementation of HTC. The findings showed that an
appropriate hydrothermal intensity could be selected, and the reaction time
is crucial since a short period would minimize the reactor volume
necessary for continuous production, thus decreasing the \mbox{capital} costs
of such a process. To optimize energy use in the HTC process, selecting
a higher solid loading as much as possible is essential. Using the
product as fuel requires more heat and time to raise its coal rank, but
it will not retain heat. Incorporating hydrochars as a secondary
fuel or using co-HTC technology to boost the calorific value of products is
part of current strategies. This study allows us to fully understand
the differences in the structure and properties of hydrochars obtained
under different solid contents and hydrothermal intensities. At the same
time, it not only provides specific ideas for the design, parameter
selection, and optimization of the sludge HTC treatment process but
also provides a basis for the resource utilization of
hydrochars.\looseness=-1

{\vspace*{-2pt}}
\section*{Declaration of interests}
{\vspace*{-2pt}}

The authors do not work for, advise, own shares in, or receive funds
from any organization that could\unskip\break 
benefit from this article, and have
declared no affiliations other than their research organizations.

{\vspace*{-2pt}}
\section*{Funding}
{\vspace*{-2pt}}

This work was funded by the National Natural Science Foundation of
China (Nos. 42107398 and 42277332), the Natural Science Foundation of
Jiangsu Province (BK20210358 and BK20221428), and the China
Postdoctoral Science Foundation (2020M68618). 
Y. F. Feng thanks the support of ``333'' High-level Talents Training
Project of Jiangsu Province (2022-3-23-083).

\CDRGrant[NSFC]{42107398}
\CDRGrant[NSFC]{42277332}
\CDRGrant[Natural Science Foundation of Jiangsu Province]{BK20210358}
\CDRGrant[Natural Science Foundation of Jiangsu Province]{BK20221428}
\CDRGrant[CPSF]{2020M68618}
\CDRGrant[``333'' High-level Talents Training Project of Jiangsu Province]{2022-3-23-083}

\def\Ensuremath#1{\ensuremath{#1}}

\back{}

{\vspace*{-2pt}}
\section*{Supplementary data}
{\vspace*{-2pt}}

Supporting information for this article is available on the journal's
website under \printDOI\ or from the author. 

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crchim-333-suppl.pdf}}

{\vspace*{-2pt}}

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