# Influences of temperature and reaction time on nutrient conversion and metal interactions during hydrothermal treatment of pig manure

Carolina Ochoa-Martinez, Cristian Barca, Olivier Boutin, Jean-Henry Ferrasse

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Carolina Ochoa-Martinez, Cristian Barca, Olivier Boutin, Jean-Henry Ferrasse. Influences of temperature and reaction time on nutrient conversion and metal interactions during hydrothermal treatment of pig manure. Science of the Total Environment, 2025, 958, pp.177853. 10.1016/j.scitotenv.2024.177853. hal-04967893

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# Influences of temperature and reaction time on nutrient conversion and metal interactions during hydrothermal treatment of pig manure

Carolina Ochoa-Martinez, Cristian Barca*, Olivier Boutin, Jean-Henry Ferrasse

Aix-Marseille Univ., CNRS, Centrale Med, M2P2 UMR 7340, Marseille, France

## HIGHLIGHTS

* Study of nutrient conversion pathways during hydrothermal treatment of pig manure

* P recovery yield in the solid product >90 % in the temperature range of 107–200 °C

* Increasing temperature and reaction time favors P conversion to apatite inorganic P

* Increasing temperature promotes extraction and dissolution of organic N compounds

* Manure Ca content plays a central role in P speciation in liquid and solid products

## GRAPHICAL ABSTRACT

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

Editor: Huu Hao Ngo

Keywords:

Hydrothermal treatment

Pig manure

Nutrient conversion

Phosphorus recovery

Nutrient management

## ABSTRACT

Pig manure is a renewable source of nutrients, such as phosphorus and nitrogen, that can be used to produce fertilizers. Hydrothermal treatment experiments using real pig manure were conducted to investigate the effect of temperature (107–200 °C) and reaction time (25–95 min) on nutrient conversion and distribution into hydrochar and process water. Sequential extractions were also performed to determine the phosphorus speciation in raw pig manure and hydrochars. The results showed that phosphorus and nitrogen recovery in the hydrochar was consistently above 94 % and 56 %, respectively, for all the experiments. Phosphorus content in the hydrochar increased from 15.4 to 24.6 mg P/g by increasing temperature and/or reaction time. Sequential extractions showed the increase in temperature led to a decrease in non-apatite inorganic phosphorus and organic phosphorus fractions in the hydrochars, while the apatite inorganic phosphorus fraction (Ca phosphates) increased. The concentration of total dissolved nitrogen in process water increased to values higher than 6000 mg N/L, while the ammonium concentration decreased to values lower than 2000 mg N/L by increasing temperature and/or reaction time. These results appear to indicate that the increase in hydrothermal temperature from 107 to 200 °C promoted the extraction and dissolution of organic nitrogen compounds from solid matrices to process water. This study provides valuable insights into P and N transformation during hydrothermal treatment of pig manure, which represent crucial information for developing sustainable treatment processes that aim to minimize waste disposal while enhancing the closure of anthropogenic P and N cycles.

<footnote>* Corresponding author.
E-mail address: cristian.barca@univ-amu.fr (C. Barca).</footnote>
# 1. Introduction

With the growth of the world's population, the increased demand for food has promoted the development of the agricultural sector resulting in an overproduction of animal manure. France is the third largest pig producer in Europe with 12.2 million heads in 2022 (Agreste, 2023), which positively impacts the economic income of the agriculture sector, but can also be a potential vector of environmental impacts due to the composition and amount of manure generated. Sustainable and adequate management is required to mitigate the risks associated with environmental impacts of pig manure (PM) such as, carbon footprint, eutrophication, dissemination of pathogens, antibiotics and heavy metals in natural environments (Ferrentino et al., 2023). Currently, composting or spreading are one of the most widely used methods of PM treatment and management. However, its main disadvantages are related to extensive land requirements, poor odor management, volatilization of nitrogen and production of harmful leachates to the soil, affecting a big part of the total environment (Akdeniz, 2019; Antoniou et al., 2019; Loyon, 2018). In terms of residual biomass, PM has potential application value as nutrient source for agricultural use due to its content in organic carbon, nitrogen (N), phosphorus (P), and mineral elements (Loyon, 2018). Therefore, the development of a technology that aims at exploiting fertilizer potential of PM, with a focus on maximizing resource recovery and minimizing waste disposal while improving closure of the anthropogenic P and N cycles, represents a topical issue for researchers and professionals in the field of resource management for sustainable development.

Hydrothermal treatment (HTT) is a thermochemical process that operates at relatively low temperatures between 120 and 280 °C with residence times <100 min (Ekpo et al., 2016a; Zhang et al., 2020; Aragón-Briceño et al., 2021). It involves the conversion of organic matter by the catalytic properties of water which dissociates into hydronium ions ($H_3O^+$) and hydroxide ions ($OH^-$) with increasing temperature, reaching its maximum value under subcritical conditions (150–280 °C) (Bandura and Lvov, 2006). The products obtained from HTT generally consist in an aqueous phase (known as the process water), a carbon-rich solid phase (known as hydrochar), and a small volume of gas (mainly $CO_2$). The characteristics and distribution of these products are significantly influenced by the feedstock and specific process conditions (Dai et al., 2017; Heilmann et al., 2014). Furthermore, HTT offers a distinct advantage over other thermochemical treatments by avoiding the drying stage, thereby impacting the economic costs (Das et al., 2021; Li et al., 2021; Tekin et al., 2014).

Several studies have reported the applicability of HTT in the nutrients recovery from organic waste (Aragón-Briceño et al., 2021; Ekpo et al., 2016b; He et al., 2022; Heilmann et al., 2014; Marin-Batista et al., 2020). Organic wastes contain P and N in various organic and inorganic forms which could be recovered by HTT process. However, to a better understanding of the HTT nutrients conversion is essential to perform specific analyses for the identification and quantification of the different nutrient species. For instance, organic P exists as a mixture of soluble and insoluble compounds, including phospholipids, nucleotides (e.g. DNA, RNA, ATP), and simple phosphate monoesters. Similarly, inorganic P could be found as insoluble apatite inorganic P (AIP), which refers to P linked to calcium (Ca), or non-apatite inorganic P (NAIP), which mainly represents inorganic P associated with aluminum (Al), iron (Fe), and manganese (Mn) (González Medeiros et al., 2005; Hedley et al., 1982; Heilmann et al., 2014). Ultimately, dissolved inorganic P are basically composed of orthophosphate's species ($H_2PO_4^-$, $HPO_4^{2-}$, and $PO_4^{3-}$). N is present in several complex molecules in solid matrices or dissolved in aqueous solutions. The organic N-compounds include amino acids, nucleic acids, proteins, and coenzymes (Xiao et al., 2019), whereas inorganic N-compounds are mainly attributed to inorganic ions such as ammonium ($NH_4^+$), nitrate ($NO_3^-$), and nitrite ($NO_2$), also called mineral N (Zhang et al., 2023).

Adjusting the HTT process' conditions could have an effect on the

dynamics of nutrients migration among solid or aqueous phases in various types of feedstocks, including algae, sewage sludge, and animal manure (Ekpo et al., 2016a; Xiao et al., 2019; Zhang et al., 2020b; Atienza-Martínez et al., 2014). For instance, Cao et al. (2011) reported that the P content in hydrochar increases by increasing temperature, reaching a value of 47.7 mg P/g (dry mass) after HTT of swine manure at 250 °C. Likewise, Heilmann et al. (2014) observed that >90 % of P could be recovered in the hydrochar after HTT of animal manure (poultry, swine and dairy manure). However, their findings indicated that the used temperature and/or reaction time ranges (200–250 °C and 1–3 h) were higher than those required to achieve >90 % P recovery in the hydrochar. Indeed, it appears that conversion reactions involving nutrient transformation occur at lower temperatures (Song et al., 2019). This was also observed by Ekpo et al. (2016b), who reported that >63 % of P was recovered in the hydrochar at 120 °C, and almost 98 % at 200 °C after 1 h of HTT of swine manure. It should be noted that the effect of HTT operating conditions on the chemical speciation of nutrients in hydrothermal products has been poorly reported. Currently some studies on HTT of sewage sludge have indicated that temperature close to 180 °C promotes the mineralization of organic P to inorganic phosphates in the hydrochar (Huang et al., 2017; Huang and Tang, 2015), and other studies have observed that the initial metal composition of the feedstocks can also affect the speciation of inorganic P in the HTT products (Ekpo et al., 2016b; Heilmann et al., 2014; Xiong et al., 2021). For N, some studies in the literature found that temperature above 150 °C may promote solubilization of organic N compounds from solid matrices to dissolved organic N in process water, then a further increase in temperature over than 180 °C can favor conversion of dissolved organic N to $NH_4^+$ (Inoue et al., 1997; Huang et al., 2016). Despite these insights, only a limited number of studies has explored the combined effect of low-temperature and reaction time on the conversion pathways of both P and N during HTT of pig manure, as well as their interaction with the metals present in the initial manure composition.

This study aims to enhance the understanding of the effect of temperature and reaction time on nutrient conversion and metal interactions during HTT of real pig manure (PM). Thus, (1) a systematic approach was conducted through a series of comparative experiments at different temperatures (107–200 °C) and reaction times (25–95 min). Then, (2) a series of chemical analyses and sequential extractions was carried out to characterize in-depth the P species and metals content in PM and HTT products. Finally, (3) a mechanistic model of nutrients conversion and metals interactions was proposed. By unravelling these mechanisms, valuable insights can be gained to optimize HTT strategies and increase the agricultural value of pig manure-derived products.

# 2. Material and methods

## 2.1. Pig manure collection and characterization

The raw PM used in this study was collected from an organic pig farm located in Rouset (France) one day after it was generated. Then, it was stored in a refrigerator at 4 °C before physicochemical analysis and HTT experiments. Physicochemical characterization of raw PM, such as contents of total solids, volatile solids, ash, fixed carbon, elemental analysis, determination of nutrients, metals and heavy metals, were developed according to the same experimental and analytical methods used to characterize the hydrothermal products, which are described in the following sections.

## 2.2. Hydrothermal experiments' set up

The HTT of PM was performed in a stainless-steel batch reactor (Top Industrie, France) with internal working volume of 0.2 L, maximum pressure of 30 MPa and maximum temperature of 350 °C, equipped with a Rushton type mixer. The temperature in the reactor was regulated by an external electric power heater. Raw PM was first mixed, and 100 g
(total solid content 20.2 %) were loaded into the HTT batch reactor. After being loaded, the reactor was sealed. The air in the reactor was purged with nitrogen gas to generate an inert atmosphere. The reactor was heated from ambient temperature to the desired temperature (107–200 °C) and during several reaction times (25–95 min). The heating speed was between 5 and 10 °C/min. At the end of the chosen operational conditions, the reactor was cooled down to room temperature by pressurized air circulation in a double jacket (about −4.5 ± 1.0 °C/min). The hydrochar and the process water were separated by centrifugation system (Sigma 2-16P, Germany) at 2715g for 20 min and stored in polyethylene recipients in a refrigerator at 4 °C for further analysis. The labels used in this study define the products as HTT-temperature (°C)-reaction time (min).

### 2.3. Proximate analysis of raw PM and hydrochar

Proximate analysis of raw PM and hydrochar were performed according to international standards, including moisture (ASTM D3173), ashes (ASTM 3174), volatile solid content (ASTM D3175) and fixed carbon (ASTM D3172). The analysis was carried out in an electric furnace and samples were placed in ceramic crucibles. For each raw PM and hydrochar samples, total solids content (TS) was determined by weight loss at 105 °C until constant weight, volatile solids content (VS) was determined by weight loss at 550 °C with fitting cover against the admission of air into the crucible, the ash content was calculated based on the residual weight after heating at 900 °C, and fixed carbon content (FC) was determined by weight loss from 550 to 900 °C. The variation of solid mass content before and after hydrothermal treatment was described by the hydrochar yield (HY). The HY (%) was calculated by Eq. (1), where mHC(dry) is the hydrochar dry mass (g), and mPM(dry) is the raw PM dry mass (g).

$$HY (\%) = \frac{mHC, dry}{mPM, dry} * 100 \quad (1)$$

The total carbon, hydrogen, nitrogen, and sulfur (CHNS) elemental composition of raw PM and hydrochars (% of dry mass) were performed using an Instrument Flash EA 1112 series elemental analyzer (Thermo Fisher, USA). The elemental composition was used to calculate total N content in raw PM and hydrochars. Then, the N recovery in the hydrochar (%) was calculated by Eq. (2), where C<sub>N</sub>,HC are the total N content in the hydrochar and C<sub>N</sub>,PM are the total N content in the raw PM expressed as mg N/g of dried mass.

$$N \text{ recovery in the hydrochar (\%)} = \frac{C_N \cdot HC^*HY}{C_N \cdot PM} * 100 \quad (2)$$

### 2.4. Chemical extractions and P speciation

Aqua regia extractions were performed according to standard methods (EN 13346, 2000; EN 13657, 2003) to investigate the total P and metals content of raw PM and hydrochar samples. Firstly, a mixture of 0.5 g of raw PM and hydrochar (dried mass equivalent) and 8 mL of aqua regia (6 mL HCl 37 % and 2 mL HNO<sub>3</sub> 69 %) were placed in a closed digestion-tube and heated to 120 °C for 60 min. Secondly, the aqua regia extractions were filtered (0.45 µm filters) before total P and metal analyses (Ca, K, Mg, Fe, Al, Cu, Zn, Pb, Ni, As, Cr, and Cd). Then, the total P content was used to calculate the P recovery in the hydrochar (%) by Eq. (3), where C<sub>P</sub>,HC represent the total P content in the hydrochar and C<sub>P</sub>, PM is the total P content in the raw PM expressed as mg P/g of dried mass.

$$P \text{ recovery in the hydrochar (\%)} = \frac{C_P \cdot HC^*HY}{C_P \cdot PM} * 100 \quad (3)$$

The modified sequential extraction method of two-step based on the Standards, Measurements and Testing protocol (SMT) (González Medeiros et al., 2005) and from the Hedley fractionation protocol

(Hedley et al., 1982; Tiessen and Moir, 1993), was followed to quantify the different forms of P in the raw PM and hydrochar samples. The principal forms of inorganic P fractions were sequentially extracted from 0.2 g of raw PM or hydrochar (dried mass equivalent) using the following method:

i. Step 1: Non-apatite inorganic-P (NAIP) was extracted in 20 mL of 1 M NaOH. This fraction is primarily related to dissolution of Al and Fe bound P compounds that are soluble at high pH.

ii. Step 2: Apatite inorganic-P (AIP) was extracted in 20 mL of 1 M HCl. This fraction is defined as Ca bound P. The P extraction in this step is mainly due to dissolution of Ca–P complexes at low pH.

The fraction (i) and (ii) defined above were determined after 16 h of rotary agitation at 5 rpm (rotary agitator STR4, Stuart, UK) at room temperature. After each extraction, supernatants were separated by centrifugation (8000 g for 20 min) and the solid residues were washed with 20 mL of 1 M KCl to recover P re-adsorbed on the residue surface. Then, the KCl washes were added to the supernatant solutions. The supernatants were filtered (0.45 µm filter) before P analyses. At the end of the experiment, organic P (OrgP) was calculated by the difference between the total P (determined by aqua regia extraction) and the total inorganic P contents (as sum of AIP and NAIP). Also, supplementary X-ray diffraction (XRD) analyses of hydrochar samples (from HTT of PM at 200 °C and 60 min) were performed to detect the presence of P crystals. All aqua regia and chemical extractions were performed in duplicate, and the results are reported as average value.

### 2.5. Chemical analysis of PM leachate and process water

Raw PM were centrifugated and the supernatant was filtered at 0.45 µm to obtain the PM leachate. Samples of PM leachate and process water were analyzed to determine the pH, total alkalinity, nutrients, and metals content. Total P, total N, phosphate (PO<sub>4</sub>-P), NH<sub>4</sub>-N, and metal (K, Ca, Mg, and Fe) concentrations (mg/L) were determined after sample filtration (0.45 µm). For each sample, dissolved organic P and N were measured by the difference between total P and PO<sub>4</sub>-P, and by the difference between total N and NH<sub>4</sub>-N, respectively, based on the hypothesis that nitrate concentration was always negligible (confirmed by punctual analysis of nitrates). The P and N recovery in the process water (%) were calculated by Eqs. (4) and (5), respectively, where PW is the volume of process water (L), mPM(dry) is the raw PM dry mass (g), soluble TP and TN (STP and STN) are the total P and total N concentrations measured in the process water (mg/L), and C<sub>P</sub>,PM and C<sub>N</sub>,PM are the total P and total N contents in the raw PM, respectively (mg/g of dry mass).

$$P \text{ recovery in the process water (\%)} = \frac{STP^*PW}{C_P \cdot PM^*mPM(dry)} * 100 \quad (4)$$

$$N \text{ recovery in the process water (\%)} = \frac{STN^*PW}{C_N \cdot PM^*mPM(dry)} * 100 \quad (5)$$

### 2.6. Design of experiments and statistical analysis

A circumscribed Central Composite Design (CDD) with the Statistica software (StatSoft v.10.0, Inc., USA) was employed to obtain a sequence of designed experiments (further description of the CDD is provided in Supplementary Information). In this study two operation variables, temperature (107, 120, 150, 180 and 200 °C) and reaction time (25, 30, 60, 80 and 95 min) were investigated to evaluate the influence on nutrients conversion and metals interaction. These ranges of operational conditions were selected based on previous research on the hydrothermal treatment of livestock manure (Ekpo et al., 2016b; Song et al., 2020; Xie et al., 2022; Zhang et al., 2020). The total number of 10 different pairs of temperature and reaction time were investigated and 5
experiments at the conditions of 120, 150, 150, 180 and 200 °C and 30, 25, 60, 80 and 60 min, respectively, were performed in duplicate to quantify the reproducibility.

## 2.7. Process severity model

To evaluate the combined effects of process conditions, this study will use the HT severity factor $f(T,t)$, as already used by other authors (Wang et al., 2019; Yahav Spitzer et al., 2023), which is a function of time and temperature. According to the literature, the Ruyter coali fi- cation model (Ruyter, 1982) is the mostly used severity function to calculate the severity factor $f(T,t)$ of HT processes (Eq. (6)), where t represent the reaction time (s) and T is the temperature (K).

$$f(T, t) = 50t^{0.2}e^{\frac{3500}{T}} \qquad (6)$$

This function has been developed by modeling experimental kinetic data from HT carbonization of various biowastes between 120 and 390 °C, and reaction time from 1 min to 6 months. This function is based on the assumption of time–temperature equivalence, according to which the same products’ characteristics can be obtained with a shorter reac- tion time and higher temperature, or with a longer reaction time and lower temperature.

## 2.8. Chemical equilibrium modeling

Chemical precipitation-dissolution equilibrium modeling was per- formed using PHREEQC (Version 3.6.2.15100, Parkhurst and Appelo, 2020). The minteq.v4.dat database was selected due to its extensive inclusion of environmental minerals and phases, especially calcium- phosphate and magnesium-containing minerals. Since struvite was not included in the minteq.v4 database, it was manually added. Thermo- dynamic parameters for struvite, including the solubility constant (Ksp –13.26, 25 °C) was adopted (Ali and Schneider, 2006; Bhuiyan et al., 2008; Soltani et al., 2023; Xu et al., 2015; Zhang et al., 2014). The initial solution composition was defined based on the hydrothermal feedstock concentrations used in this study, specifically total Ca, Mg, Fe, P and N contents of raw pig manure. The Newton-Raphson algorithm in PHREEQC was used to solve equilibrium, allowing accurate predictions of ion interactions, speciation, and mineral saturation indices (SI) across high ionic strengths typical of hydrothermal systems. The saturation index (SI) for each mineral phase was calculated to assess its potential for precipitation under the different process conditions (i.e. process temperature and pH of process water). The SI for each mineral was computed by Eq. (7), where IAP is the ion activity product for the dissolution reaction, and Ksp is the solubility product constant. SI > 0 indicates supersaturation, suggesting possible precipitation, while SI < 0 indicates undersaturation.

$$SI = \log\left(\frac{IAP}{K_{sp}}\right) \qquad (7)$$

## 2.9. Analytical methods

The pH values were measured by using a pH meter Consort C5020 (Turnhout, Belgium). Total alkalinity was determined by titration using 0.1 M HCl (EN ISO 9963,1996). Total N and NH₄-N analyses of aqueous samples were performed by kit cell tests WTW (Weilheim in Oberbayern, Germany). PO₄-P concentrations were determined by the ammonium molybdate spectrometric method (EN ISO 6878, 2004) through a spec- trophotometer WTW PhotoLab 6600 UV-VIS (Weilheim, Germany). Total P, Ca, Mg, K, Al, Fe, Cu and Zn concentrations were analyzed using an inductively coupled plasma-atomic emission spectroscopy (ICP-AES) spectrometer Jobin Yvon Horiba Ultima-C (Horiba, Japan). Heavy metals Cr, Ni, As, Cd, Hg and Pb concentrations were analyzed using an inductively coupled plasma-mass spectrometry (ICP-MS) instrument

Nexlon 300× (Perkin Elmer, USA). The XRD analysis was developed in a diffractometer (X'Pert PRO MPD, Panalytical), with data recording int the 2 to 70 θ°. For each set of analyses, the limit of detection and limit of quantification were determined by analysis of blanks. All chemicals used were of analytical grade. All measurements were performed in dupli- cate, and the results are reported as average value.

# 3. Results and discussion

## 3.1. Effect of process parameters on physicochemical properties of hydrochar

The effect of the process parameters on the main physicochemical characteristics of hydrochar are presented in Table 1. The hydrochar yield (HY) appears to decrease as the process severity increases. In particular, HY decreases from 90.1 % to 60.5 % with an increase of temperature from 107 °C to 200 °C. No evident effect was observed by the reaction time before the experiments at 180 °C, with the exception in the experiments 120 and 150 °C at 25 and 30 min, which showed a decreasing in HY compared to the other reaction times. These different behaviors could be probably due to the fast kinetics in the hydrolysis of organic molecules in short reaction time, leading to a reduction of HY (Heilmann et al., 2014). The decrease in HY with increasing tempera- tures has been already reported in previous studies with animal manure. Wu et al. (2018) found that increasing the temperature from 150 to 270 °C for a 6-h HTT of cattle manure resulted in a decrease in HY from 74.4 % to 52.7 %. Wu et al. (2017) reported that increasing the tem- perature from 200 to 280 °C for a 4-h HTT of dairy manure resulted in a decrease in HY from 58.3 % to 43.5 %. Similarly, Ekpo et al. (2016b) observed the HY of pig manure decreased from 64.0 % to 55.0 % after 60 min of HTT when the temperature was increased from 170 to 250 °C. The decrease in hydrochar yield with increasing reaction temperature can be attributed to the decomposition of manure’s structural com- pounds at higher temperatures by various chemical reactions. This likely leads to the formation of numerous small molecular compounds through secondary decomposition, thereby reducing the HY (Tito et al., 2023; Wang et al., 2018).

Table 1 shows that the volatile solids (VS) content in the hydrochar appears to decrease from a maximum of 69.0 % to a minimum of 63.2 % with increasing process severity. By contrast, the fixed carbon (FC) and the ash content in hydrochar appears to increase from 15.1 to 16.9 % and 15.9 to 19.9 % of TS, respectively, as the process severity increases. Previous studies have reported that HTT of PM could promote decom- position of volatile content while favoring the formation of fixed carbon (Song et al., 2020; Sun et al., 2024). The ash content of hydrochar re- mains relatively constant between 107 and 150 °C, with values ranging from 15.9 % to 16.8 %. Then, it increases with increasing temperature and time above 180 °C, reaching 19.9 % at 200 °C. According to Smith et al. (2020), the ash content in hydrochar after HTT of PM at low temperatures (120 °C) decreased slightly from 12.2 % to 10.7 % and as the operating temperature approached 180 °C, the ash content increased significantly reaching 21.2 % at 250 °C. This increase in ash content was mainly attributed to the reduction of the organic content and the relative increase in the mineral content of the hydrochar (Xiong et al., 2019).

The elemental analysis of raw PM and hydrochars suggests an in- crease in carbon (C) and a decrease in oxygen (O) content in hydrochars as the process severity rises, whereas hydrogen (H) content did not show any significant change (uncertainty of CHNS analysis 0.3 %) (Table 1). Specifically, the C content increased from 40.6 % to 46.7 %, while the O content decreased from 35.1 % to 24.4 %, respectively, when increasing temperature from 107 to 200 °C. These increases in C and decreases in O contents may result from the dehydration and decarboxylation reactions that occur during the HTT process, especially when temperature in- creases above 180 °C (Ekpo et al., 2016a; Gascó et al., 2018; Leng et al., 2020; Lin et al., 2017). In addition, condensation and polymerization reactions may have further increased the relative carbon content (Huang
<caption>**Table 1**</caption>

Main physicochemical properties of PM and hydrochars (TS: total solids, HY: hydrochar yield, VS: volatile solids, FC: fixed carbon).

<table><thead><tr><th rowspan="2">Samples</th><th rowspan="2">Severity<br>f(T,t)</th><th rowspan="2">TS<br>(%)</th><th rowspan="2">HY<br>(%)</th><th colspan="3">Proximate analysis<br>(% of total solids)</th><th colspan="5">Ultimate analysis<br>(% dried mass)</th></tr><tr><th>VS</th><th>FC</th><th>Ash</th><th>C</th><th>H</th><th>N</th><th>S</th><th>O<sup>b</sup></th></tr></thead><tbody><tr><td>PM</td><td>-</td><td>20.2</td><td>-</td><td>69.6</td><td>14.5</td><td>15.8</td><td>40.9</td><td>5.9</td><td>2.4</td><td>0.4</td><td>34.6</td></tr><tr><td>HTT-107-60<sup>a</sup></td><td>0.02</td><td>33.6</td><td>90.1</td><td>69.0</td><td>15.1</td><td>15.9</td><td>40.6</td><td>5.8</td><td>2.2</td><td>0.3</td><td>35.1</td></tr><tr><td>HTT-120-30<sup>a</sup></td><td>0.03</td><td>30.7</td><td>86.0</td><td>68.2</td><td>15.4</td><td>16.4</td><td>40.7</td><td>5.8</td><td>2.2</td><td>0.3</td><td>34.5</td></tr><tr><td>HTT-120-80<sup>a</sup></td><td>0.04</td><td>26.1</td><td>87.4</td><td>68.6</td><td>15.5</td><td>15.9</td><td>40.9</td><td>5.8</td><td>2.2</td><td>0.3</td><td>34.8</td></tr><tr><td>HTT-150-25<sup>a</sup></td><td>0.05</td><td>24.0</td><td>81.6</td><td>67.7</td><td>15.6</td><td>16.7</td><td>40.6</td><td>5.8</td><td>2.1</td><td>0.3</td><td>34.6</td></tr><tr><td>HTT-150-60<sup>a</sup></td><td>0.06</td><td>29.2</td><td>89.3</td><td>67.6</td><td>16.2</td><td>16.2</td><td>41.3</td><td>5.8</td><td>2.1</td><td>0.3</td><td>34.3</td></tr><tr><td>HTT-150-95<sup>a</sup></td><td>0.07</td><td>29.6</td><td>86.7</td><td>67.4</td><td>16.3</td><td>16.2</td><td>41.5</td><td>5.8</td><td>2.6</td><td>0.3</td><td>33.0</td></tr><tr><td>HTT-180-30<sup>a</sup></td><td>0.10</td><td>31.8</td><td>76.6</td><td>64.6</td><td>16.5</td><td>18.9</td><td>43.0</td><td>5.7</td><td>3.0</td><td>0.3</td><td>29.0</td></tr><tr><td>HTT-180-80<sup>a</sup></td><td>0.12</td><td>25.8</td><td>68.3</td><td>63.7</td><td>16.6</td><td>19.7</td><td>45.0</td><td>5.7</td><td>3.1</td><td>0.4</td><td>26.2</td></tr><tr><td>HTT-200-60<sup>a</sup></td><td>0.16</td><td>25.3</td><td>60.5</td><td>63.2</td><td>16.9</td><td>19.9</td><td>46.7</td><td>5.7</td><td>3.1</td><td>0.3</td><td>24.4</td></tr></tbody></table>

et al., 2017). The sulfur (S) content in raw PM and hydrochars was below 0.5 %, without showing a clear trend with temperature and reaction time in the range studied. N content varied 2.1 and 2.6 % during HTT at temperatures between 107 and 150 °C, and then, when the temperature exceeded 150 °C, increased above 3.0 %. This was in good agreement with the findings of Leng et al. (2021), who reported that biomass such as manure, microalgae and food waste produces hydrochar with N content between 1 % and 6 %. According to Ipiales et al. (2024), the rise in N content at higher temperatures can be mainly attributed to the formation of N-pyrimidine, N-pyrrole, and N-quaternary compounds.

The Van Krevelen diagram, which plots the molar ratio of hydrogen to carbon (H/C) as a function of the molar ratio of oxygen to carbon (O/C), is commonly used to assess the degree of char carbonization (Gascó et al., 2018). As shown in Fig. 1, the evolution of H/C and O/C molar ratios appears to indicate that dehydration was the main reaction affecting the quality of hydrochars during HTT of PM (Xiong et al., 2019) for severity above 0.072. The molar ratio H/C decreased from 1.72 to 1.46, which represents a decrease of 0.6 to 15.6 % compared to raw PM. Similarly, the molar ratio O/C decreased from 0.65 to 0.39, representing a decrease of 3.10 to 38.1 % compared to the initial O/C ratio in raw PM.

## 3.2. Effect of process parameters on pH and alkalinity of process water

The pH of PM leachate (6.9) was higher than that of process water

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Van Krevelen diagram for raw PM and the hydrochars as a function of the severity factor f(T,t).</figcaption></figure>

(Table 2). This is in good agreement with the results of previous studies which mainly attributed the higher alkalinity to the presence of basic amino groups in raw PM (Lang et al., 2018). Besides, as process severity increased the pH in the process water decreased from 6.4 to 5.0. According to previous studies, the decline in process water pH during HTT is probably caused by the decomposition and degradation of biomolecules, such as cellulose, hemicellulose and simple carbohydrates, as well to the hydrolysis of triacyglycerides which may lead to the formation of fatty acids or carboxylic acids (Heilmann et al., 2014; Yuan et al., 2018). The alkalinity at temperatures ranging from 120 to 150 °C appears to fluctuate around a value of 10,300 mg CaCO$_3$/L. As the temperature increased above 180 °C, alkalinity decreased from 7050 to 5680 mg CaCO$_3$/L. This decrease in alkalinity at temperature higher than 180 °C appears to be in good correlation with the decrease in pH, suggesting that a part of the alkalinity is consumed to neutralize the accumulation of organic acids during HTT.

## 3.3. Content of nutrients and metals in process water

As shown in Table 2, TP in process water decreased with increasing process severity from a maximum of 443 mg P/L (process severity f(T,t) of 0.02) to a minimum of 56.6 mg P/L (process severity f(T,t) of 0.16). The difference between TP and PO$_4$-P was primarily attributed to dissolved organic P in process water, as all the process water samples were filtered (0.45 µm filters) before element analyses. It is interesting to observe that PO$_4$-P concentration increased from 136 to 203 mg P/L with increasing temperature from 107 to 150 °C, this suggesting mineralization process of organic P to inorganic phosphate. Then, PO$_4$-P concentration appears to decrease with further increasing temperature over 150 °C, this suggesting PO$_4$-P precipitation in the hydrochar. This trend is consistent with the results of Zhang et al. (2020), who reported the highest PO$_4$-P concentration in process water after 30 min of HTT of PM at 180 °C, and then PO$_4$-P concentration decreased with further increasing reaction time. Moreover, these results are in good agreement to the findings of Heilmann et al. (2014), who indicated that prolonged reaction time may promote PO$_4$-P precipitation in the hydrochar.

Total nitrogen (TN) in process water appears to remain around 5000 to 7000 mg N/L by increasing process severity, while NH$_4$-N appears to oscillate around a value of 2500 mg N/L, reaching the maximum concentration of 2880 mg N/L at 150 °C and 25 min, and then decreasing with a further increase in temperature over than 180 °C (Table 2). According to previous studies, at lower temperatures (150–180 °C) hydrolysis of crude protein plays a significant role in the HTT process, keeping almost stable proportions of NH$_4$-N and dissolved organic N (Wu et al., 2018). This may explain the relatively stable amounts of dissolved organic N and NH$_4$-N in process water. The extraction of dissolved organic N from solid matrices during HTT of PM was already reported by Ekpo et al. (2016a). No significant effect of reaction time

<footnote>a Hydrochar after HTT-temperature(°C)-time(min).</footnote>

<footnote>b Calculated: % O = 100 % - (C % - H % - N % - S % - ash %).</footnote>
<caption>**Table 2**</caption>

Physicochemical properties, nutrients and soluble metal cations of PM leachate and process water.

<table><thead><tr><th rowspan="2">Samples</th><th rowspan="2">Severity<br>f(T,t)</th><th rowspan="2">pH</th><th rowspan="2">Alkalinity<br>(mg CaCO<sub>3</sub>/L)</th><th colspan="4">Nutrients<br>(mg P or N/L)</th><th colspan="4">Metal cations<br>(mg element/L)</th></tr><tr><th>TP</th><th>PO<sub>4</sub>-P</th><th>TN</th><th>NH<sub>4</sub>-N</th><th>Ca</th><th>K</th><th>Mg</th><th>Fe</th></tr></thead><tbody><tr><td>PM leachate</td><td>–</td><td>6.9</td><td>10,310</td><td>234</td><td>101</td><td>4680</td><td>2510</td><td>1410</td><td>2680</td><td>598</td><td>13.4</td></tr><tr><td>HTT-107-60</td><td>0.02</td><td>6.4</td><td>10,260</td><td>443</td><td>136</td><td>5290</td><td>2730</td><td>1020</td><td>2800</td><td>681</td><td>12.3</td></tr><tr><td>HTT-120-30</td><td>0.03</td><td>6.3</td><td>10,300</td><td>398</td><td>142</td><td>5230</td><td>2800</td><td>1120</td><td>2860</td><td>730</td><td>11.9</td></tr><tr><td>HTT-120-80</td><td>0.04</td><td>6.0</td><td>10,380</td><td>384</td><td>146</td><td>6610</td><td>2640</td><td>1200</td><td>2750</td><td>717</td><td>14.1</td></tr><tr><td>HTT-150-25</td><td>0.05</td><td>5.9</td><td>10,120</td><td>360</td><td>203</td><td>6410</td><td>2880</td><td>1500</td><td>2870</td><td>759</td><td>17.3</td></tr><tr><td>HTT-150-60</td><td>0.06</td><td>5.6</td><td>10,930</td><td>265</td><td>146</td><td>6970</td><td>2730</td><td>1770</td><td>3110</td><td>784</td><td>25.6</td></tr><tr><td>HTT-150-95</td><td>0.07</td><td>5.7</td><td>9530</td><td>243</td><td>137</td><td>6630</td><td>2720</td><td>1770</td><td>2650</td><td>745</td><td>21.7</td></tr><tr><td>HTT-180-30</td><td>0.10</td><td>5.2</td><td>7050</td><td>180</td><td>116</td><td>7070</td><td>2050</td><td>2400</td><td>3120</td><td>880</td><td>41.0</td></tr><tr><td>HTT-180-80</td><td>0.12</td><td>5.1</td><td>6600</td><td>85.4</td><td>30.8</td><td>6230</td><td>1380</td><td>3360</td><td>3360</td><td>920</td><td>46.0</td></tr><tr><td>HTT-200-60</td><td>0.16</td><td>5.0</td><td>5680</td><td>56.6</td><td>17.5</td><td>6360</td><td>1300</td><td>4220</td><td>3460</td><td>995</td><td>48.6</td></tr></tbody></table>

was observed, suggesting that temperature was the main variable affecting the N conversion. No nitrite or nitrate concentrations were detected.

As shown in Table 2, potassium (K) was the major metal cation in the PM leachate and process water, followed by Ca and magnesium (Mg). K and Mg concentrations of process water were always higher than those of PM leachate. Whereas Ca and iron (Fe) concentrations of process water were higher than those of PM leachate only when the HTT temperature was higher than 150 °C. The K concentration remained around a value of 3000 mg K/L with no significant correlation to changes in process severity, whereas the Mg concentration appears to increase with increasing process severity. The Al concentration was below the detection limits (<0.027 µg Al/L) in both PM leachate and process water. Although the Fe concentration showed an increasing trend with increasing process severity, its concentration was 2 orders of magnitude lower than those of Ca, Mg, and K in process water. The low Fe and Al

levels in the process water are likely due to the initial composition of raw PM, as these metals are not commonly predominant in animal manures (Heilmann et al., 2014).

### 3.4. Content of nutrients and metals in hydrochar

As shown in Table 3, the total P content in raw PM was 14.5 mg P/g which is slightly lower than the value reported by Smith et al. (2020) (15.2 mg P/g), but higher than that reported by Song et al. (2020) (10.1 mg P/g). The total P content in hydrochar increased from 15.4 to 24.6 mg P/g with increasing process severity. These results are consistent with previous studies. Xu et al. (2018) reported an increase of 31 % in total P content of hydrochar after 60 min of HTT of sewage sludge at 200 °C, while Song et al. (2020) found an increase of about 48 % of total P content of hydrochar after 60 min of HTT of PM at 200 °C. This increase in P content of hydrochar by process severity was attributed to

<caption>**Table 3**</caption>

Main results from studies that have investigated hydrothermal treatments (HTT) of different types of residual biomass (PM, inlet pig manure; SS, inlet sewage sludge; CM, inlet cattle manure), (SLR, solid-liquid ratio).

<table><thead><tr><th rowspan="2">Source</th><th colspan="4">Process parameters</th><th rowspan="2">Severity<br>f(T,t)</th><th colspan="5">Elements' content (mg/g dried mass)</th></tr><tr><th>Sample</th><th>SLR<br>(g/mL)</th><th>T (°C)</th><th>time<br>(min)</th><th>P</th><th>Ca</th><th>K</th><th>Mg</th><th>Fe</th><th>Al</th></tr></thead><tbody><tr><td rowspan="10">This study</td><td>PM</td><td rowspan="10">0.20</td><td></td><td></td><td></td><td>14.5</td><td>48.3</td><td>15.1</td><td>6.23</td><td>0.80</td><td>0.12</td></tr><tr><td>HTT-107-60<sup>a</sup></td><td>107</td><td>60</td><td></td><td>15.4</td><td>56.5</td><td>10.7</td><td>5.23</td><td>2.24</td><td>0.13</td></tr><tr><td>HTT-120-30<sup>a</sup></td><td>120</td><td>30</td><td></td><td>15.3</td><td>55.6</td><td>10.0</td><td>5.45</td><td>1.03</td><td>0.15</td></tr><tr><td>HTT-120-80<sup>a</sup></td><td>120</td><td>80</td><td></td><td>16.2</td><td>58.9</td><td>13.1</td><td>5.94</td><td>1.63</td><td>0.12</td></tr><tr><td>HTT-150-25<sup>a</sup></td><td>150</td><td>30</td><td></td><td>15.3</td><td>53.3</td><td>11.1</td><td>5.66</td><td>1.30</td><td>0.11</td></tr><tr><td>HTT-150-60<sup>a</sup></td><td>150</td><td>60</td><td></td><td>16.2</td><td>55.1</td><td>10.8</td><td>5.65</td><td>1.36</td><td>0.12</td></tr><tr><td>HTT-150-95<sup>a</sup></td><td>150</td><td>80</td><td></td><td>16.3</td><td>55.7</td><td>11.6</td><td>5.71</td><td>1.48</td><td>0.14</td></tr><tr><td>HTT-180-30<sup>a</sup></td><td>180</td><td>30</td><td></td><td>19.2</td><td>63.8</td><td>10.5</td><td>5.69</td><td>1.51</td><td>0.22</td></tr><tr><td>HTT-180-80<sup>a</sup></td><td>180</td><td>80</td><td></td><td>21.6</td><td>69.5</td><td>12.8</td><td>7.12</td><td>1.46</td><td>0.17</td></tr><tr><td>HTT-200-60<sup>a</sup></td><td>200</td><td>60</td><td></td><td>24.6</td><td>86.7</td><td>16.3</td><td>8.84</td><td>2.97</td><td>0.26</td></tr><tr><td rowspan="2">Ekpo et al., 2016a</td><td>PM</td><td rowspan="2">0.11</td><td></td><td></td><td></td><td>15.6</td><td>36.8</td><td>8.06</td><td>9.60</td><td>1.04</td><td>0.64</td></tr><tr><td>HTT-170-60<sup>a</sup></td><td>170</td><td>60</td><td></td><td>15.5</td><td>39.9</td><td>0.22</td><td>5.89</td><td>1.58</td><td>1.35</td></tr><tr><td rowspan="2">Xiong et al., 2019</td><td>PM</td><td rowspan="2">0.10</td><td></td><td></td><td></td><td>29.6</td><td>55.9</td><td>0.19</td><td>9.82</td><td>2.22</td><td>1.66</td></tr><tr><td>HTT-200-30<sup>a</sup></td><td>200</td><td>30</td><td></td><td>25.6</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Xu et al., 2018</td><td>SS</td><td rowspan="2">0.16</td><td></td><td></td><td></td><td>20.7</td><td></td><td></td><td>12.0</td><td>13.0</td><td>7.0</td></tr><tr><td>HTT-200-60<sup>a</sup></td><td>200</td><td>60</td><td></td><td>27.0</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="4">Wu et al., 2018</td><td>CM</td><td rowspan="4">0.10</td><td></td><td></td><td></td><td>10.2</td><td>3.79</td><td>14.3</td><td>1.25</td><td>5.52</td><td></td></tr><tr><td>HTT-150-360<sup>a</sup></td><td>150</td><td>360</td><td></td><td>6.70</td><td>4.24</td><td>6.13</td><td>1.22</td><td>5.37</td><td></td></tr><tr><td>HTT-180-360<sup>a</sup></td><td>180</td><td>360</td><td></td><td>0.16</td><td>7.73</td><td>4.98</td><td>6.27</td><td>1.15</td><td>6.28</td></tr><tr><td>HTT-210-360<sup>a</sup></td><td>210</td><td>360</td><td></td><td>0.26</td><td>10.2</td><td>5.47</td><td>5.31</td><td>1.04</td><td>6.32</td></tr><tr><td rowspan="4">Smith et al., 2020</td><td>PM</td><td rowspan="4">0.11</td><td></td><td></td><td></td><td>15.2</td><td>27.0</td><td>12.1</td><td>0.94</td><td>1.09</td><td>0.43</td></tr><tr><td>HTT-120-60<sup>a</sup></td><td>120</td><td>60</td><td></td><td>13.1</td><td>31.6</td><td>1.49</td><td>6.23</td><td>1.46</td><td>0.54</td></tr><tr><td>HTT-170-60<sup>a</sup></td><td>170</td><td>60</td><td></td><td>21.8</td><td>41.8</td><td>1.04</td><td>5.39</td><td>1.63</td><td>0.67</td></tr><tr><td>HTT-200-60<sup>a</sup></td><td>200</td><td>60</td><td></td><td>0.16</td><td>25.2</td><td>38.0</td><td>0.41</td><td>6.54</td><td>1.22</td></tr><tr><td rowspan="4">Song et al., 2020</td><td>PM</td><td rowspan="4">0.26</td><td></td><td></td><td></td><td>10.1</td><td>55.8</td><td>26.4</td><td>13.1</td><td></td><td></td></tr><tr><td>HTT-160-60<sup>a</sup></td><td>160</td><td>60</td><td></td><td>10.1</td><td>55.8</td><td>25.0</td><td>14.5</td><td></td><td></td></tr><tr><td>HTT-180-60<sup>a</sup></td><td>180</td><td>60</td><td></td><td>0.11</td><td>13.0</td><td>59.0</td><td>15.5</td><td>10.0</td><td></td></tr><tr><td>HTT-200-60<sup>a</sup></td><td>200</td><td>60</td><td></td><td>0.16</td><td>15.0</td><td>65.0</td><td>10.0</td><td>10.0</td><td></td></tr><tr><td></td><td>HTT-220-60<sup>a</sup></td><td></td><td>220</td><td>60</td><td></td><td>0.21</td><td>15.7</td><td>70.0</td><td>10.5</td><td>10.0</td><td></td></tr></tbody></table>

<footnote><sup>a</sup> Hydrochar after HTT-temperature(°C)-time(min).</footnote>
various reactions that can occur during HTT, including P precipitation, polymerization, and/or condensation (Ekpo et al., 2016a; Heilmann et al., 2014; Ischia et al., 2022). However, other studies at similar temperature conditions but at shorter reaction time (30 min) reported a significant amount of TP migrating into the process water rather than in the hydrochar (Xiong et al., 2019). Other operative parameters such as the solid-liquid ratio (SLR) may intensify this effect, particularly at lower SLR, where organic and inorganic P complexes are more susceptible to hydrolysis and solubilization (Deng et al., 2020; Xiong et al., 2019).

The most abundant metal in the hydrochar dry mass composition in this study was Ca (53.3–86.7 mg Ca/g), followed by K (10.0–16.3 mg K/g) and Mg (5.23–8.84 mg Mg/g). Ca shows a similar trend to that of P, as its content in hydrochar appears to increase with increasing process severity, reaching the highest value of 86.7 mg Ca/g for the HTT experiment at 200 °C and 60 min (Table 3). Fe and Al were found in relatively low amounts, ranging from 1.03 to 2.97 mg Fe/g and 0.11 to 0.26 mg Al/g, respectively. This finding aligns with other studies indicating that these elements are usually present in minor quantities in animal manure, while they are usually more abundant in other organic wastes such as sewage sludge (Xu et al., 2018).

The contents of heavy metals in raw PM and hydrochar are shown in Table 4. Eight metals (Zn, Cu, Pb, Ni, As, Cr, Cd, and Hg) were investigated to study the influence of process parameters and the correlation between their concentration and the limit values of the European Regulation for use as organic fertilizer (EU EC 2019/1009). The results suggest that the increase in process severity increases the accumulation of heavy metals in the resulting hydrochar. Previous studies have also reported these findings (Lang et al., 2018, 2019; Sun et al., 2024). The most abundant heavy metals in hydrochar are Zn and Cu, with contents ranging from 616 to 1020 mg Zn/kg and 94.4 to 106 mg Cu/kg, respectively. This is mainly due to the fact that these elements are part of the nutritional supplements usually provided to animals for their health and growth (Loyon, 2018). The raw PM and hydrochar contained only trace amounts of Pb, Ni, As, Cr, Cd, while the concentration of Hg was below the detection limit in all samples (<0.01 mg Hg/kg). The results indicate that the concentration of metals in hydrochar meets the criteria of the European regulation 2019/1009, with the only exception of Zn for the experiments carried out at temperatures higher than 180 °C. However, it should be noted that the Zn content in hydrochars can be further reduced by proper management and improvement of pig feeding. Also, Cr content of some hydrochar samples appears to exceed the limit criteria (> 2 mg/kg). However, inlet-outlet Cr mass balance appears to suggest that the Cr content of hydrochar was affected by Cr release from the Cr—Ni stainless steel reactor used for the HTT experiments.

### 3.5. Yields of nutrients recovery in hydrochar and in process water

The results in Fig. 2a indicate that the P recovered in hydrochar ranged from values around 90.0 % to about 100 %, while the P recovered in process water decreased from values around 5.0 % to values below 1.0 % with increasing process severity. The increase in temperature appears to be more influential than the increase in reaction time on P recovery in hydrochar, at least within the ranges of values examined in this study. These results appear to be in good agreement with the

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 2. (a) P recovery in hydrochar and process water, and (b) N recovery in hydrochar and process water (PW) as functions of the severity factor $f(T,t)$ (experimental data, the dotted lines are there to facilitate interpretation).</figcaption></figure>

<table><caption>**Table 4**

Metals and heavy metals contents in raw PM and hydrochars.</caption><thead><tr><th rowspan="2">Samples</th><th rowspan="2">Severity<br>f(T,t)</th><th colspan="6">Metals and heavy metals (mg/kg dried mass)</th></tr><tr><th>Cu</th><th>Zn</th><th>Pb</th><th>Ni</th><th>As</th><th>Cr<sup>b</sup></th><th>Cd</th></tr></thead><tbody><tr><td>PM</td><td>-</td><td>87.0</td><td>555</td><td>0.50</td><td>2.30</td><td>0.30</td><td>1.10</td><td>0.60</td></tr><tr><td>HTT-107-60<sup>a</sup></td><td>0.02</td><td>94.4</td><td>616</td><td>0.50</td><td>10.7</td><td>2.90</td><td>3.20</td><td>0.30</td></tr><tr><td>HTT-120-30<sup>a</sup></td><td>0.03</td><td>92.1</td><td>592</td><td>0.40</td><td>3.90</td><td>0.40</td><td>2.00</td><td>0.30</td></tr><tr><td>HTT-120-80<sup>a</sup></td><td>0.04</td><td>97.5</td><td>627</td><td>0.70</td><td>5.60</td><td>0.80</td><td>1.90</td><td>0.30</td></tr><tr><td>HTT-150-25<sup>a</sup></td><td>0.05</td><td>90.5</td><td>604</td><td>0.50</td><td>5.40</td><td>1.00</td><td>2.30</td><td>0.30</td></tr><tr><td>HTT-150-60<sup>a</sup></td><td>0.06</td><td>98.4</td><td>643</td><td>0.70</td><td>9.50</td><td>0.70</td><td>3.10</td><td>0.30</td></tr><tr><td>HTT-150-95<sup>a</sup></td><td>0.07</td><td>97.7</td><td>643</td><td>0.40</td><td>7.10</td><td>0.60</td><td>4.20</td><td>0.30</td></tr><tr><td>HTT-180-30<sup>a</sup></td><td>0.10</td><td>113</td><td>769</td><td>0.50</td><td>6.80</td><td>0.50</td><td>5.10</td><td>0.30</td></tr><tr><td>HTT-180-80<sup>a</sup></td><td>0.12</td><td>114</td><td>801</td><td>0.90</td><td>35.4</td><td>0.40</td><td>47.5</td><td>0.40</td></tr><tr><td>HTT-200-60<sup>a</sup></td><td>0.16</td><td>106</td><td>1020</td><td>0.90</td><td>10.3</td><td>1.50</td><td>11.3</td><td>0.50</td></tr><tr><td>Regulation</td><td></td><td>300</td><td>800</td><td>120</td><td>50.0</td><td>40.0</td><td>2.00<sup>c</sup></td><td>1.50</td></tr><tr><td>EU-EC-2019/1009</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

<footnote><sup>a</sup> Hydrochar after HTT-temperature(°C)-time(min).</footnote>

<footnote><sup>b</sup> Total Cr content.</footnote>

<footnote><sup>c</sup> Cr<sup>VI</sup> content.</footnote>
findings of previous studies (Aragón-Briceño et al., 2021; Li et al., 2021; Tangredi et al., 2023; Xu et al., 2018; V. Qaramaleki et al., 2020).

Fig. 2b illustrates that N recovered in the hydrochar ranged from 56.7 % to 76.1 %, which is less than the recovery rate observed for P, suggesting less favorable conditions for N retention. No clear trend between process severity and N transformation during HTT was observed. However, at 180 °C and 30 min, over 40 % of N was present in the process water, demonstrating conditions that facilitate N solubilization. The results of mass balances between the input contents of P and N in the raw PM and the output contents of P and N in the hydrochar and process water indicated balances ranging from 94.2 % to 99.3 % for P, and from 56.2 % to 76.1 % for N (Tables S1, S2 and S3 Supplementary material).

### 3.6. Nutrients' speciation in process water

As shown in Fig. 3a, organic P in process water was the predominant P specie during HTT at lower temperatures (<120 °C), then it is mineralized to soluble inorganic P (PO₄-P) by increasing process severity. The transformation of insoluble organic P into soluble organic P, improved by dehydration reactions, has already been reported in previous studies (Deng et al., 2020). Organic P concentration in process water decreased from values around 300 mg P/L to 30 mg P/L with increasing process severity. At lower HTT temperatures (<150 °C), dissolved organic P was the primary species in the process water but an increase in PO₄-P was observed with increasing temperature. These observations suggest that higher temperatures may promote the transformation and hydrolysis of organic P into inorganic phosphate, leading to an increase in PO₄-P concentration in the process water. However,

<figure><img src="image_7.png" /><figcaption>Fig. 3. (a) P speciation in process water: dissolved organic P (OrgP) and PO₄-P, and (b) N speciation in process water: dissolved organic N (OrgN) and NH₄-N as functions of the severity factor f(T,t) (experimental data, the dotted lines are there to facilitate interpretation).</figcaption></figure>

when the temperature increased from 150 to 200 °C, the PO₄-P concentration decreased, possibly due to its migration to the hydrochar phase. Previous studies on HTT of cattle manure and PM have observed that increased temperatures promote the precipitation of P into hydrochar (Wu et al., 2018; Deng et al., 2020). Hence, given the composition of raw PM in this study, it is highly probable that the decrease in the PO₄-P concentration of process water is related to the precipitation of Ca phosphates due to the high prevalence of Ca among the various metal cations in raw PM.

Fig. 3b suggests a trend of increasing organic N concentration in process water with increasing temperature, which appears to confirm organic N extraction from solid matrices of raw PM and its accumulation in the process water. Ekpo et al. (2016a) also found an increase in dissolved organic N between 170 and 200 °C, followed by a decrease as the temperature rose to 250 °C during the HTT of swine manure. Similarly, Zhuang et al. (2017) reported an increase of organic N in process water during HTT of sewage sludge (150–210 °C). As is reported by Aragón-Briceño et al. (2021), dissolved organic N can be converted to inorganic N through hydrolysis and deamination. However, this process is generally slow at temperatures below 230 °C, requiring extended reaction times for effective N solubilization in low-temperature processes. Zhuang et al. (2017) found that temperatures higher than 240 °C was enough to completely dissolve or hydrolyze N from solid compounds to the form of NH₄-N or NO₃-N ions. In addition, it was observed a slight trend of decreasing NH₄-N concentration with increasing temperature, especially at temperature higher than 180 °C. This decrease in NH₄-N at high temperature conditions can be partially explained by NH₄-N volatilization from the water to the gaseous phase. Additionally NH₄-N may be converted into N-pyrimidine, N-pyrrole, and N-quaternary compounds, which represent the basis of the secondary hydrochar structure, as have been already proposed by Ipiales et al. (2024).

### 3.7. P speciation in hydrochar

Fig. 4 shows the P speciation of raw PM and hydrochar. The initial fractions of P in raw PM, before HTT, were organic P (50.7 % of total P), AIP (42.1 % of total P), and NAIP (7.25 % of total P). The temperature appears to be the most influential parameter in P speciation rather than reaction time and therefore severity factor f(T,t). During the HTT, the content of organic P and NAIP decreased significantly. The HTT at 107 to 120 °C had a weak impact on organic P. Moreover, the impact of temperature on the decrease of organic P appears to be more noticeable at temperatures higher than 150 °C. This probably resulted from mineralization process of organic P to inorganic P (Huang et al., 2017; Yu et al., 2022). The temperature equal or higher than 180 °C gave a

<figure><img src="image_8.png" /><figcaption>Fig. 4. P speciation in hydrochar (% of total P): Organic P (OrgP), Non-Apatite Inorganic-P (NAIP) and Apatite Inorganic-P (AIP) as functions of the severity factor f(T,t) (experimental data, the dotted lines are there to facilitate interpretation).</figcaption></figure>
significant decrease in organic P along with an increase in AIP fractions. It is worth mentioning that the impact of temperature on AIP was more significant. After 60 min of HTT at 200 °C, AIP became the main fraction of P in the hydrochar (85.3 % of total P), followed by organic P (14.3 % of total P) and NAIP (0.39 % of total P). According to the main chemical composition shown in Table 3, the molar ratio of Ca/P in raw PM (2.57 mol Ca/mol P) is much higher compared to those of K/P, Mg/P, Fe/P, and Al/P (0.82 mol K/mol P, 0.55 mol Mg/mol P, 0.1 mol Fe/mol P, and 0.01 mol Al/mol P, respectively). Metal to P molar ratios of most common P precipitates are 1.5 Ca/P for Ca₃(PO₄)₂, 1.67 Ca/P for Ca₁₀(PO₄)₆(OH)₂, 1.5 Mg/P for Mg₃(PO₄)₂, 1 Fe/P for FePO₄, and 1 Al/P for AlPO₄ (Heilmann et al., 2014). Indeed, molar ratios Ca/P of raw PM and hydrochars were always higher than 2.5 (Table 3), suggesting that the increase in AIP fractions could be attributed to the precipitation of Ca phosphates. The Mg, Fe, and Al contents of PM were too low for efficient P precipitation. Previous studies have emphasized the influence of Ca promoting the AIP precipitation within hydrochar during HTT of animal manure (Dai et al., 2015; Yu et al., 2022). The XRD spectra from analysis of hydrochar (HTT at 200 °C and 60 min) indicated a slope in the region of the characteristic peak for hydroxyapatite crystals (HAP), suggesting the presence of Ca phosphate precipitates in the amorphous phase (Fig. S1). This could be attributed to insufficient energy for crystal growth due to the short reaction time and the fast air cooling of the reactor, as reported by Wu et al. (2022). Hydrochar with a lower phosphate crystalline phase may enhance P fertilizing efficiency by improving its bioavailability. The presence of amorphous P precipitates in hydrochars has been reported in previous studies (Deng et al., 2020; Huang et al., 2017).

According to the results of chemical equilibrium modeling by

<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Saturation index (SI) for various P-containing mineral compounds: (a) SI as function of pH at 25 °C; (b) SI as function of process severity (i.e. process temperature and pH of process water) (experimental data, the dotted lines are there to facilitate interpretation).</figcaption></figure>

PHREEQC (Fig. 5) five phases of Ca phosphate were determined in saturation, CaHPO₄ (monetite), CaHPO₄·2H₂O (brushite), Ca₃(PO₄)₂ (tricalcium phosphate), Ca₄H(PO₄)₃·3H₂O (octacalcium phosphate) and Ca₅(PO₄)₃OH (hydroxyapatite), while Fe₃(PO₄)₂·8H₂O (vivianite) and MgNH₄PO₄·6H₂O (struvite) remained undersaturated.

Fig. 5a shows the modelled SI for the various mineral compounds as a function of pH at 25 °C. The results indicate that SI values for all Ca phosphate phases increase by increasing pH from 5.0 to 7.0, confirming that higher pH values favor the precipitation of these compounds. Among these, hydroxyapatite exhibits the highest SI values across the pH range, with a marked increase from 16.5 (pH 5.0) to 27.4 (pH 7.0), indicating it is the dominant mineral likely to precipitate under these conditions. Both monetite and brushite show moderate increases in SI with rising pH, while tricalcium phosphate and octacalcium phosphate display more pronounced increases, indicating a higher sensitivity to pH changes. In contrast, struvite and vivianite remain undersaturated across the entire pH range, with negative SI values that suggest they are unlikely to precipitate under these experimental conditions. Although the SI values for struvite and vivianite become slightly less negative with increasing pH, they do not reach levels sufficient for precipitation, probably due to limited Mg and Fe concentrations, as already suggested by the relatively low Mg/P and Fe/P molar ratios of pig manure (0.55 mol Mg/mol P and 0.1 mol Fe/mol P).

Fig. 5b shows the modelled SI for the studied mineral compounds as a function of the severity factor f(T,t) (i.e. process temperature and pH of process water). The results indicate that all Ca phosphate minerals, particularly hydroxyapatite, maintain high SI values across all the severity levels. This is consistent with the high Ca/P molar ratio of pig manure (2.57 mol Ca/mol P), which favors the precipitation of Ca phosphate. These results suggest that Ca/P molar ratio of pig manure, rather than process severity, is the primary factor driving hydroxyapatite precipitation. In contrast, the low Mg/P and Fe/P molar ratios of pig manure indicate limited potential for struvite and vivianite formation, as these minerals remain undersaturated across all conditions. Therefore, it appears that the operational conditions of HTT and initial metal cation content in the PM, with temperatures increasing from 107 to 200 °C, favor reactions that facilitate the retention of P in the hydrochar via the precipitation of Ca phosphate.

### 3.8. Proposed mechanisms of nutrient conversion

The results of this study suggest that P conversion and speciation during HTT of PM is governed by the following mechanism, as proposed in Fig. 6. First, (i) hydrolysis reactions, which occur at around 150 °C, lead to the decomposition and mineralization of polyphosphates and organic P (e.g. polyphosphates, phosphate monoesters, phosphate esters, phospholipids, nucleotides) to dissolved organic P and orthophosphates (PO₄-P) in the process water (Huang et al., 2017; Deng et al., 2020; Turner and Leytem, 2004). Then, (ii) dissolved organic P is transformed into PO₄-P with increasing temperature after HTT at 150 °C. Finally, (iii) PO₄-P is available for metal-P precipitation, and it is recovered by migration in the hydrochar. The content and molar ratios of metal cations to P in process water, such Ca/P, Fe/P and Al/P, are the main parameter controlling P precipitation and speciation in the hydrochar. For N conversion mechanism, the results indicate that temperature more than reaction time affected N transformation during HTT of PM. Initially, (i) the temperature comprised between 120 and 180 °C can promote extraction and solubilization of organic N in the raw PM, leading to the increase in N recovery yield in process water. Then, the further increase in temperature over 180 °C and reaction time longer than 30 min can (ii) promote the mineralization of dissolved organic N to inorganic NH₄-N, (iii) the volatilization of a fraction NH₄-N to the gaseous phase, and (vi) the conversion of NH₄-N to N-pyrimidine, N-pyrrole, and N-quaternary compounds, which represent the basis of the secondary hydrochar structure, leading to the decrease in NH₄-N concentration of process water and to the increase in N recovery in
<figure><img src="image_10.png" /><figcaption>Fig. 6. Proposed mechanism of P conversion during HTT of PM.</figcaption></figure>

hydrochar.

## 4. Conclusions

This study offers insights into the dynamics of nutrient conversion and distribution during HTT of PM. >94 % of P and 56 % of N were retained in the hydrochar under all conditions studied. P recovery in hydrochar increased by increasing process severity. Also, the increase in temperature from 120 to 180 °C promoted extraction and dissolution of organic N in the process water, while a further increase in temperature over 180 °C had the effect of improving N recovery in the hydrochar. Thus, temperature was the main parameter influencing N conversion. In P speciation, AIP fraction in the hydrochar increased, while NAIP and organic P fractions decreased, by increasing process severity, with temperature the most influential process parameter affecting P conversion and speciation. Chemical speciation and metals analyses of process water and hydrochar leads to the conclusion that Ca was the main metal cation controlling P precipitation during HTT, due to its high prevalence and chemical ratio in raw PM. Overall, these findings underscore the potential of HTT as a promising technology for efficient P recovery and nutrient management in pig farming systems. Future research will also focus on evaluating the most suitable ways for valorizing process water, in particular its N and K contents, to complete the nutrient recovery cycle. In addition, a complete screening of the potential toxicity of HTT by-products, as well as an economic assessment of the process, are needed before full-scale applications of the technology.

### CRediT authorship contribution statement

**Carolina Ochoa-Martinez:** Writing – original draft, Investigation, Funding acquisition, Data curation. **Cristian Barca:** Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization. **Olivier Boutin:** Writing – review & editing, Validation, Supervision. **Jean-Henry Ferrasse:** Writing – review & editing, Validation, Supervision, Project administration, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

This work received support from the French government under the France 2030 investment plan, as part of the Initiative d'Excellence d'Aix-Marseille Université - A*MIDEX - AMX-19-IET-010. Also, to the ECOS-Nord program for French-Colombian Scientific Cooperation (ECOSNORD, Minciencias-225-2021), as well as Minciencias project 860. We thank Hélène Miche and Bertrand Angeletti (CEREGE, France) for ICP analyses, and Grégory Excoffier (Spectropole Aix-Marseille University, France) for CHNS analyses.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.scitotenv.2024.177853.

### Data availability

Data will be made available on request.

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