# Combining process severity and response surface methodology: a comprehensive approach to phosphorus speciation in sewage sludge hydrothermal treatment

Antonello Tangredi, Cristian Barca, Jean-Henry Ferrasse, Olivier Boutin

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Antonello Tangredi, Cristian Barca, Jean-Henry Ferrasse, Olivier Boutin. Combining process severity and response surface methodology: a comprehensive approach to phosphorus speciation in sewage sludge hydrothermal treatment. Journal of Environmental Management, 2025, 381, pp.125239. 10.1016/j.jenvman.2025.125239. hal-05039217

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# Combining process severity and response surface methodology: a comprehensive approach to phosphorus speciation in sewage sludge hydrothermal treatment

Antonello Tangredi, Cristian Barca*, Jean-Henry Ferrasse, Olivier Boutin

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

## ARTICLE INFO

### Keywords:

- Sewage sludge

- Phosphorus recovery

- Hydrothermal treatment

- Process severity

## ABSTRACT

Phosphorus is essential for global food production, but intensive agriculture disrupts its natural cycle, increasing reliance on non-renewable sources. A sustainable alternative is recovering phosphorus from waste streams using thermochemical processes. This work investigates the hydrothermal treatment of sewage sludge digestate to explore phosphorus conversion and speciation, aiming to optimize solid by-product as fertilizer. Through a novel integration of response surface methodology and process severity modeling, the study examines the influence of process parameters (temperature, time) on phosphorus behavior. A sewage sludge digestate, sampled from a wastewater treatment plant, was treated in a batch reactor at temperatures from 250 to 350 °C for 5-45 min. Products were centrifuged into a solid pellet and process water, followed by characterization of their physico-chemical properties. Results show that temperature and time significantly impact by-product characteristics and phosphorus speciation. Treatments at 250-300 °C promote organic phosphorus mineralization and increase soluble phosphate concentration, while treatments at 350 °C lead to greater phosphorus recovery in the solid pellet, mostly as calcium phosphates. This research provides a framework for sustainable phosphorus recovery, suggesting hydrothermally treated pellets as potential slow-release fertilizers. Future work should comprehensively investigate the effect of calcium addition on phosphorus precipitation as calcium phosphates.

## 1. Introduction

Water scarcity and quality degradation is an escalating global issue, including in Europe (Bixio et al., 2006), and wastewater treatment is a key point for a sustainable circular approach. Modern approaches emphasize wastewater as a resource rather than waste (Kehrein et al., 2020). Moreover, wastewater treatment plants (WWTPs) are increasingly viewed as important hubs for the recovery of different resources beyond water, such as energy, nutrients, heavy metals, cellulose, volatile fatty acids, and CO₂ (Kehrein et al., 2020). Most of these resources are concentered in sewage sludge (SS) and SS digestate, which is the by-product of SS anaerobic digestion for bio-gas production.

SS digestates contain important amounts of nitrogen(N), phosphorus(P) and potassium(K), the three primary macronutrients central for modern agriculture and food production. However, nowadays N, K and P fertilizers production is not sustainable (Cordell et al., 2009; Penuelas et al., 2023; Walling and Vaneckhaute, 2022). P fertilizers production relies on extraction from natural apatite rocks (Zhu et al.,

2022), but global reserves are expected to be depleted in the coming decades. Consequently, it is crucial to maximize and implement large-scale P recovery from P rich waste streams such as SS digestate, to develop a more sustainable anthropogenic P cycle and to avoid nutrients run-off in the environment, which leads to eutrophication (Liu et al., 2021b). Legislation has already moved in this direction by restricting sludge disposal and imposing P/N discharge limits (Directive 91/271/EEC).

Numerous technologies are available to recover nutrients, namely adsorption, ion exchange, ammonia stripping and the precipitation of N, P, K in the form of various minerals species, such as struvite (NH₄MgPO₄·6H₂O), K-struvite (KMgPO₄·6H₂O), hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂), and, more generally, calcium phosphate (Di Cos;tanzo et al., 2021).

Hydrothermal (HT) processes (carbonization, liquefaction, and gasification) have proven effective in converting various biomasses, including SS digestates, into valuable by-products such as hydrochar, biofuel, syngas, and other chemicals (Huang and Yuan, 2016). These

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: cristian.barca@univ-amu.fr (C. Barca).</footnote>
treatments involve subjecting wet biomass to high temperatures and pressures, eliminating the need for pre-drying and thereby reducing the energy costs typically associated with traditional thermal treatments (Biller and Ross, 2012). During HT treatments, the altered properties of sub-critical water, including a reduced dielectric constant, ionic product, and lowered viscosity, lead to a mass transfer enhancement and help catalyze acid/base reactions.

When SS is subjected to HT, nutrients and specifically P, are recovered in the solid by-products (hydrochars and ashes) (Zhu et al., 2022), while simultaneously pathogens and oxidable pollutants are destroyed (Schnell et al., 2020). Hydrochars and ashes can be repurposed as fertilizers, soil amendments, or subjected to chemical leaching, depending on the macronutrients relative content and pollutants such as heavy metals.

Previous studies have investigated macronutrients pathways and recovery during wet biomass HT treatments (Aragón-Briceño et al., 2021; Huang et al., 2017; Liu et al., 2021b). Possible N reaction pathways have been identified and elucidate, suggesting that most of N is recovered in the liquid by-product, but underlining the presence of N-compound in the solid by-products resulting from precipitation and incorporation in heterocycles (Aragón-Briceño et al., 2021). In the same way, there is a general understanding of P conversion mechanisms and pathways during HT treatments of biomass but systematic reaction schemes remain incomplete (Huang et al., 2017) and the interaction between the organic matter and the mineral phase remains unclear. Currently, existing applied technologies for N and P recovery from HT products involve gas stripping or struvite precipitation for the liquid by-product, and wet-chemical acidic/alkaline extraction from the solid chars (Aragón-Briceño et al., 2021; Liu et al., 2021b). Less studies are available on direct recycling of HT products through soil application. More research is needed to increase the more thermodynamically stable P-complexes speciation and thus to increase P fertilizer bio-availability and efficiency (Huang et al., 2017). Moreover, the HT products need to be comprehensively characterized in terms of environmental compatibility (heavy metals content and other contaminants). This would allow to integrate nutrients reclamation and energetic valorization, leading to novel chains for digestate utilization. This is particularly pertinent given the increasing significance of digestate utilization and proper disposal, which has become a critical bottleneck in anaerobic digestion and biogas production (Brebbia and Itoh, 2016).

Most of the studies that have investigated P conversion and speciation during the HT treatment of SS focused on lower temperatures, mainly within the HTC range of 180–280 °C (Tangredi et al., 2023). More recent studies demonstrated that HTL of SS realized in a temperature range of 270–350 °C concentrates P into hydrochar, with over 85 % of P migrating from SS(Zheng et al., 2024) and reaching up to three times the original content (Neha et al., 2024). P can then be effectively recovered using acid extraction and struvite precipitation (Abeyratne et al., 2023; Neha et al., 2024), highlighting HTL as a promising approach for P recovery.

In this study, SS digestate is subjected to an HT treatment to characterize the resulting products and investigate P conversion pathways and speciation. The selected temperature range (250–350 °C) is typically associated with HTL but also includes particularly severe HTC conditions, allowing to observe the transition between these two processes and their effects on P behavior. Reaction times of 5, 15, and 45 min reflect the short residence times typical of HTL(Tangredi et al., 2023). Unlike previous studies, which mainly used autogenic conditions (Tangredi et al., 2023), this study decouples pressure from temperature effects by adjusting initial N₂ pressure for each operative temperature. This approach helps to clarify the individual impacts on P conversion and HT product characteristics and allows investigation of P speciation at pressures exceeding water's autogenic pressure. This work primary objective is to recover P in the solid product to evaluate its availability and the potential for using hydrochar as a fertilizer or soil amendment. The by-products are comprehensively characterized through a novel

integration of process severity analysis and response surface modeling, to evaluate not only the individual effects of temperature and time but also their combined impact on P conversion and speciation. Their environmental compatibility is assessed with a critical analysis of the current European legislation on the subject, incorporating regulatory requirements and safety considerations and determining whether hydrochar can be used as a fertilizer or soil amendment.

## 2. Materials and methods

### 2.1. Sewage sludge digestate collection and preparation

Fig. 1 shows a schematization of the experimental protocol. The SS digestate used in this study was collected from the Aix-La Pioline WWTP, treating domestic wastewater from a large size community in the south of France (175000 population equivalent). The collected SS digestate contains a low total solids (TS) content (2.5 %). It was concentrated by centrifugation at 4000 rpm for 20 min (Sigma 2-16P centrifuge), producing a thickened sludge with a higher TS content (6.7 %) that was used as feed for the HT treatment, and a digestate leachate. Both products have been stored in sealed plastic containers and refrigerated at 4 °C.

### 2.2. Hydrothermal treatment experiments

The HT treatment experiments have been realized by introducing 110 g of thickened SS digestate in a 0.2 L austenitic chromium-nickel-stainless steel batch reactor. The reactor is heated with a resistance wire system in the reactor jacket and cooled with the injection of cooled pressurized air on the reactor external shell. Temperature regulation is automated thanks to a proportional integral derivative (PID) controller. The reactor is operated between 250 and 350 °C for a time of 5–45 min. Pressurization and inerting of the reaction medium are managed with N₂ gas injection inside the reactor. All experiences have been realized with a N₂ pressure of 18 MPa. Mixing is provided with a Rushton turbine at 700 rpm. At the end of treatment, the HT product is collected and centrifugated at 4000 rpm for 20 min (Sigma 2-16P centrifuge), separating it into a solid pellet and a process water, both stored in sealed plastic containers and refrigerated at 4 °C.

### 2.3. Characterization of digestate feed and solid pellet by-product

The TS contents of the thickened digestate feed and the solid pellet by-product were characterized by drying 3 g of wet samples at 105 °C until constant weight mass (EN 12880, 2000).

Total Solid Recovery Yield (TSRY%) is defined by equation (1), where mS,dry, mL,dry and mD,dry are the dry masses (g) of respectively the solid pellet, the process water and the digestate fed to the reactor.

$$TSRY\% = \frac{m_{S,dry} + m_{L,dry}}{m_{D,dry}} \qquad (1)$$

Elemental CNHS analysis (Thermo Scientific FlashEA 1112 Elemental Analyzer) of the dried digestate and solid pellets allowed to determine carbon (C), hydrogen (H), nitrogen (N), and sulfur (S) content. Ash content is determined by exposing the dried sample at 900 °C in a muffle furnace for 1 h and measuring the residual mass. Oxygen (O) is calculated by difference. The dried solid digestate and pellet samples (0.5 g of dried sample) have been mineralized with 8 mL of aqua regia according to EN 13346 (2008). Then, total P, calcium (Ca), iron (Fe), aluminum (Al), magnesium (Mg), potassium (K), copper (Cu), and zinc (Zn) concentrations of the aqua regia digestion extracts were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Jobin Yvon Horiba Ultima-C). Moreover, heavy metals contents of aqua regia extracts, such as mercury (Hg), lead (Pb), vanadium (V), chromium (Cr), nickel (Ni), arsenic (As), silver (Ag), and cadmium (Cd), were measured by inductively coupled plasma mass spectrometry (ICP-MS)
<figure><img src="image_2.png" /><figcaption>Fig. 1. Left: Schematization of the experimental protocol; right: process parameters employed in the experimental plan, with the corresponding severity value determined with equation (2).</figcaption></figure>

(PerkinElmer® NexIon 300X). The Standard Measurement and Testing (STM) sequential extraction method (Ruban et al., 2001) has been applied to wet digestate and pellet by-products to quantify different P fractions. SMT method procedure is described in supplementary material. According to this approach, total P (TP) encompasses both organic P (OP), P present in organic compounds like phospholipids, DNA, and ATP, and inorganic P (IP). The latter can be split into non-apatite inorganic P (NAIP), which is IP not bonded to Ca compounds, more specifically IP bound to Al and/or Fe compounds that are soluble at high pH or readily available phosphates that can be desorbed by OH-exchange, and apatite inorganic P (AIP), consisting of IP associated with various Ca compounds generally soluble at low pH.

## 2.4. Characterization of digestate leachate and process water

The TS content of the digestate leachate and the HT treatment process water were characterized by drying the samples at 105 °C (EN 12880, 2000). Both liquid phases were then filtered (0.45 µm filter) before quantifying pH, alkalinity, density, total P, phosphate (PO₄-P), total nitrogen (TN), ammonia nitrogen (NH₄-N), chemical oxygen demand (COD), Ca, Fe, Al, Mg, and K contents. pH values were measured using a multiparameter analyzer (Consort C5020), whereas density measures were performed by a density meter (Anton Paar DMA 5000 M). The alkalinity of liquid samples was determined according to standard methods (EN 9963-1, 1998). PO₄-P concentrations were determined according to the ammonium molybdate-ascorbic acid method (EN 6878, 2004), whereas TN, NH₄-N, and COD concentrations were determined by kit cell tests WTW (Weilheim in Oberbayern, Germany) using ultra-violet-visible (UV-VIS) spectrophotometry (Photolab 7100 UV-vis). Total P, Ca, Fe, Al, Mg and K concentrations in liquid samples were determined by inductively coupled plasma atomic emission spectrophotometry (ICP-AES) (Jobin Yvon Horiba Ultima-C).

## 2.5. Hydrothermal severity factor

The parameter known as hydrothermal severity factor, which is dependent on both time and temperature, was employed to better understand the synergistic impacts of temperature and time (Tangredi et al., 2023). The Ruyter model (equation (2)) stands out in existing literature as the predominant function utilized for determining the HT

severity factor f (T, t), with t denoting reaction time (s) and T representing temperature (K) (Ruyter, 1982).

$$f(T, t) = 50t^{0.2} \exp(-3500/T) \quad (2)$$

## 2.6. Experimental design and statistical analysis

To investigate the role of temperature and time on HT by-products characteristics and P speciation, the experimental design methodology has been employed, using a complete symmetric factorial design with two factors and three levels (250, 300, and 350 °C for temperature; 5, 15, and 45 min for time), resulting in $3^2 = 9$ experiments. Fig. 1 shows the process parameters employed in the nine experiments, with the corresponding severity value determined with equation (2). Response surface approach has been employed to describe the relationship between response and the two factors in a second order polynomial regression model. The statistical significance of the obtained models is evaluated with the ANOVA (ANalysis Of VAriance) test while the statistical significance of the model's coefficients has been evaluated with Student t-test.

Each experiment has been conducted in duplicate, resulting in a total of 18 experiments. The duplicate weighted average value is presented for each experimental result. Errors reported in this work correspond to the combination of propagated uncertainty and standard deviation, for the measurements realized multiple times. For heavy metals content, error represents the propagated uncertainty combined with the relative standard deviation associated with the spectrometric intensity measurement (n = 3). For other results, reported errors correspond solely to propagated uncertainty in measurement. For each set of analyses, limit of detection and limit of quantification were determined by analysis of blanks. All chemicals used were of analytical grade.

# 3. Results and discussion

## 3.1. Sewage sludge digestate characterization

The concentered digestate and the relative leachate main characteristics are presented in the first column of Table S1.

The digestate liquid fraction (digestate leachate) obtained as by-product of the raw digestate centrifugation, appears as a clear opaque and colloidal liquid. The digestate leachate is characterized by a pH of
8.12 ± 0.04, aligning with the pH range reported in previous works (Liu et al., 2021a). Total alkalinity is influenced by both upstream processes and biomass origin, but it is also connected to the quality and inherent characteristics of water entering the WWTP. Since alkalinity represents a solution's ability to neutralize acids to the carbonate or bicarbonate equivalence point (pH 4.5), the relatively high total alkalinity of the digestate in this study, measured at 3.71 ± 0.05 g CaCO₃ L⁻¹, may be correlated with the observed basic pH. TN concentration in the digestate liquid fraction is 1.10 ± 0.07 g L⁻¹, which is also in line with previously observed values (Akhiar et al., 2017). Most of this N (>90 %) is found as NH₄-N, confirming the observation that NH₄-N constitute the main source of N in liquid digestate (Akhiar et al., 2017; Krzemińska et al., 2019), due to N-organic species degradation during the anaerobic digestion process (Kayhanian, 1999). Leachate contains 339 ± 16 mg L⁻¹ of soluble P, almost completely in the form of PO₄-P (Table S1). K, Ca, Mg and Fe are present in a solubilized form in digestate leachate respectively with 294 ± 25 mg L⁻¹, 42 ± 2 mg L⁻¹, 3.7 ± 0.2 mg L⁻¹ and 1.47 ± 0.04 mg L⁻¹. N/P, Mg/P and K/P molar ratios in the digestate leachate are respectively 7.2 ± 0.7, 0.014 ± 0.002 and 0.69 ± 0.09. Therefore, this effluent is not a favorable candidate for nutrient recovery through struvite (NH₄MgPO₄ • 6 H₂O) precipitation. The process would require the addition of Mg to fully recover P, but it would hinder the retrieval of a significant portion of N. Alternatively, the K-struvite (KMgPO₄ • 6 H₂O) option would demand the integration of K to complete P recovery.

The thickened digestate appears as a relatively homogeneous and viscous black sludge. It still retains an important amount of water, with a total solids content of 6.7 %, which is consistent with the values typically observed for SS digestate, ranging from 2 % to 30 % (Tangredi et al., 2023). The digestate ash content is equal to 22.7 % ± 0.2 % of dry matter. Ultimate analysis results quantify the digestate C, O, N, H and S amounts, respectively 37.5 % ± 0.7 %, 26 % ± 1 %, 6.4 % ± 0.3 %, 5.8 % ± 0.3 % and 1.1 % ± 0.3 % of dry matter. The thickened digestate contains 32 ± 2 mg g<sub>dry</sub><sup>-1</sup> of P. Among the most common metals that precipitate to form metal-P compounds, Ca and Fe are prevalent, with respectively 29 ± 2 mg g<sub>dry</sub><sup>-1</sup> and 23 ± 2 mg g<sub>dry</sub><sup>-1</sup>, whereas K, Al, and Mg are on the order of few mg g<sub>dry</sub><sup>-1</sup> (Table S1). P, Ca, Fe, Al, Mg and K contents are substantially comparable to the average values found in literature (Tangredi et al., 2023). Ca/P, Fe/P, and Al/P molar ratios are respectively 0.70 ± 0.08, 0.40 ± 0.03 and 0.082 ± 0.007, lower than the values of the most common P phosphates that can precipitate in water solutions (FePO₄, CaHPO₄, Ca₅(PO₄)₃(OH), AlPO₄). Ratios below 1 indicate that P is present as part of several compounds, and it cannot be solely complexed with Ca, Fe or Al. The digestate also contains 0.44 ± 0.02 mg Cu g<sub>dry</sub><sup>-1</sup>, 0.57 ± 0.03 mg Zn g<sub>dry</sub><sup>-1</sup>, 26 ± 1 mg Pb kg<sub>dry</sub><sup>-1</sup>, 25 ± 1 mg Cr kg<sub>dry</sub><sup>-1</sup>, 19.2 ± 0.9 mg V kg<sub>dry</sub><sup>-1</sup>, 17.7 ± 0.9 mg Ni kg<sub>dry</sub><sup>-1</sup>, 6.6 ± 0.3 mg Ag kg<sub>dry</sub><sup>-1</sup>, 3.4 ± 0.2 mg As kg<sub>dry</sub><sup>-1</sup>, 0.96 ± 0.05 mg Cd kg<sub>dry</sub><sup>-1</sup>, and 0.15 ± 0.02 mg Hg kg<sub>dry</sub><sup>-1</sup>. These values are consistent with the ranges reported in the literature (Liu et al., 2021a). P in digestate is mostly present as NAIP, representing 61 % ± 7 % of Total Phosphorus (TP), followed by 25 % ± 3 % of AIP, and 14 % ± 9 % of OP. This speciation is consistent with the distributions typically observed in SS, for which the NAIP fraction is usually the prevalent one (Tangredi et al., 2023).

## 3.2. Total solid content in by-products

The recovered product constitutes between 93 % and 97 % of the mass fed to the reactor (Fig. S1). The not recovered mass is associated with the gas produced, a small part of the water and volatile products that evaporate at the reactor opening, and with the small amounts of non-recovered products.

Fig. 2a and b presents respectively the TS content in pellet and in process water, showing a clear trend with the process severity. A more severe process leads to a reduction of the total solids in the process water, from 2.31 % ± 0.03 % down to 0.45 % ± 0.05 %, and to an increase of total solids in pellet, from 25.5 % ± 0.2 % up to 41.9 % ± 0.7 %.

<figure><img src="image_3.png" /><figcaption>Fig. 2. (a) Total solids in pellet, (b) total solids in process water and (c) Total Solid Recovery Yield (%) (equation (2)) as a function of process severity (equation (2)). Dashed lines included to help data interpretation. Data reported in Table S1.</figcaption></figure>

%. Therefore, a process severity increase leads to a better by-products separation and could be linked to colloids degradation and dissolved compounds precipitation in the solid phase after the treatment (Ekpo et al., 2016).

Fig. 2c shows a clear TSRY% reduction at increasing process severity, dropping from 77.4 % ± 0.6 % down to 40.7 % ± 3.1 % at the most severe conditions. The linear decrease of the TSRY% with severity has been comprehensively described in previous works, confirming that a higher f (T,t) (>0.5) is generally associated with a low Solid Recovery Yield (<55 %) (Tangredi et al., 2023).

Increasing temperature leads to lower TSRY%, as high temperatures promote the dissolution and degradation of organic matter through the solubilization of inorganic material and biopolymers (Ekpo et al., 2016; Zheng et al., 2022) resulting from dehydration and decarboxylation reactions (He et al., 2015; Pérez et al., 2021; Qi et al., 2022; Smith et al.,
2016; Wang et al., 2019). TSRY% also decreases with longer time (Huang and Tang, 2016; Wang et al., 2019, 2021). The response surface of TSRY% as a function of process temperature and time (Fig. S2 and Table S2) is statistically significant ($p < 0.01$ %, $R^2 = 0.97$), and confirms the correlation between increasing temperature and time and lower TSRY%. However, the linear coefficient for temperature is more than four times higher than the coefficient for the linear effect of time, supporting previous observations (Funke and Ziegler, 2010) showing that time influences TSRY%, but to a lesser extent than temperature.

### 3.3. Process water characterization

The filtered process water density has an almost constant value of $1008.2 \pm 0.5 \text{ g L}^{-1}$ and does not seem to depend on the process severity. Fig. 3 shows the process water pH and total alkalinity evolutions as a function of process severity. An increase in process severity leads to an alkalinity increase, which corresponds to a higher content of dissolved $HCO_3$ resulting from the mineralization of organic carbon and $CO_2$ dissolution. Both temperature and time influence the increase in total alkalinity, with temperature having the predominant effect. After the least severe HT treatment (250 °C, 5 min), process water shifts towards a more neutral pH $7.52 \pm 0.02$ compared to the digestate feed liquid fraction pH $8.12 \pm 0.04$ (Fig. 3). This could be explained by considering the release of organic acids following carbohydrates breakdown (McIntosh et al., 2022). However, for longer times, process water pH increases ($7.82 \pm 0.03$ for 250 °C, 45 min), and for higher temperature treatments, it levels off at approximately 8.0.

Fig. 4 shows the evolution of the concentration of $NH_4-N$ in process water and its relative proportion over TN. TN in process water exhibits no discernible trend, consistently maintaining an approximate value of $4.5 \text{ g L}^{-1}$ irrespective of temperature and reaction time. In contrast, the concentration of $NH_4-N$ in process water rises with increasing temperature and time. The relative fraction of TN in the form of $NH_4-N$ increases, indicating that a more severe HT treatment enhances organic N mineralization into ammonium. Previous works have already observed that a temperature increase leads to the recovery of soluble N mainly in process water, primarily in the $NH_4-N$ form (Aragón-Briceño et al., 2021). A temperature increase has been correlated with protein and pyridine-N compounds hydrolysis and deamination, leading to the formation of inorganic N ($NH_4-N$) (Wang et al., 2018). The fact that total soluble N remains relatively constant above 250 °C suggests that the process of breaking down solid protein compounds into soluble N is complete at temperatures below 250 °C. Then, at temperatures above

<figure><img src="image_4.png" /><figcaption>Fig. 4. $NH_4-N$ in process water expressed in g L$^{-1}$ (♦) and as fraction of TN (●) as a function of process severity (equation (2)). Dashed lines included to help data interpretation. Data reported in Table S1.</figcaption></figure>

$250$ °C, further degradation of soluble $Org-N$ occurs, leading to a decrease in its concentration and an increase in $NH_4-N$ concentration. The evolution of the process water pH suggests that for the $250$ °C, 5 min treatment, the hydrolysis of carbohydrates and the release of organic acids into solution are prominent, leading to the reaction medium acidification (Fig. 3). Meanwhile, for longer time and higher temperature, the hydrolysis of proteins and mineralization of $org-N$ leads to higher amounts of $NH_4-N$ in solution which provides a buffer effect that counteract the process water acidification, resulting in an increase in pH. pH, TN and $NH_4-N$ contents observed for process water are in line with the ranges associated with digestate HT aqueous products already available in literature (Liu et al., 2021a).

Process water shows a high concentration of COD ($18.5–30.5 \text{ g L}^{-1}$) compared to the digestate leachate ($0.39 \text{ g L}^{-1}$) (Fig. S3). Indeed, COD in process water has been observed to have a concentration of two orders of magnitude higher than a typical WWTP influent (Silva Thomsen et al., 2020). COD concentration decreased with increasing temperature and time: a more severe treatment allows to reduce the COD content (40 % reduction at 350 °C, 45 min compared to COD content at 250 °C, 5 min), suggesting that part of water soluble organic compounds could move into other phases (e.g. adsorbed in the solid by-product) or are converted into other species (e.g. gas) (Su et al., 2019).

As shown in Fig. 5, the HT treatment leads to a strong reduction of soluble Ca and an increase of soluble Fe, Mg, P, and K concentrations compared to the digestate leachate. This suggests that Ca cations precipitate in the solid pellet while Fe and Mg cations are released in the process water, at least at less severe conditions. Then, soluble Fe and Ca concentrations decrease steeply with increasing process severity. At $250$ °C, Mg concentration increases from $16 \pm 1$ to $35 \pm 2 \text{ mg L}^{-1}$ with increasing time from 5 to 45 min. For higher temperatures, Mg concentration drops again and decreases with increasing reaction time (Fig. 5a). K is mostly found in process water, and its concentration in process water decreases for most severe treatments (350 °C).

### 3.4. Solid pellet by-product characterization

Almost 100 % of Ca, Fe, Al and Mg are recovered in the solid pellet. After the HT treatment Ca, Fe and Mg contents in the solid pellet increase with increasing process severity, and they double compared to the digestate feed (Fig. S4a). Al contents of solid pellets from the most severe treatments are more than three times higher than those of the feed digestate. K content in the solid pellet is generally lower than the feed digestate (Fig. S4b), suggesting K solubilization from solid

<figure><img src="image_5.png" /><figcaption>Fig. 3. Process water pH (♦) and total alkalinity (●) evolutions as a function of process severity (equation (2)). Dashed lines included to help data interpretation. Data reported in Table S1.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 5.** Ca, Fe, Mg (a) P and K (b) concentration in process water as a function of process severity (equation (2)). Dashed lines included to help data interpretation. Data reported in Table S1.</figcaption></figure>

compounds during HT treatment. Indeed, it has been observed that most of K is recovered in the process water after the HT treatment (Alhnidi et al., 2020). For the most severe conditions (T = 350 °C), K content in the solid pellet increases, suggesting a partial (re-) precipitation of solubilized K. Heavy metals content in solid pellet (Fig. S5) increases with increasing process severity, except for Hg which could be partially lost to volatilization. The content of metals and heavy metals in the solid pellet are of the same order of magnitude as the values observed in the digestate feed, but generally higher, indicating metals' accumulation in the solid pellet. Their values are also in line with the ranges observed in literature (Liu et al., 2021a; Tangredi et al., 2023).

### 3.5. Phosphorus fate and speciation

The HT treatment at 250 and 300 °C leads to an increase of soluble P concentration in process water compared to the digestate leachate (Fig. 5b). For the most severe treatments (350 °C), soluble P concentration of process water is lower than that of the digestate leachate. An increase in temperature has been correlated to a decrease of soluble P concentration (Fig. 5b). Soluble P concentration slightly increases from 0.54 ± 0.01 up to 0.58 ± 0.01 mg L⁻¹ with increasing time at 250 °C, from 5 to 45 min. However, at 300 and 350 °C this trend is reversed. The response surface of the soluble P concentration in process water as a function of process temperature and time (Fig. S6 and Table S3) is statistically significant ($p < 0.01$ %, $R^2 = 0.98$). The model's linear coefficient for temperature is more than 7 times higher than the coefficient for the linear effect of time, confirming the predominant influence of temperature on soluble P concentration of process water. Besides, the model highlights the presence of an interaction between temperature and time: time influence is not significant at 250 °C but becomes more important for temperatures higher than 300 °C.

P content in the solid pellet is higher than that of the feed digestate, and it increases with increasing process severity (Fig. S4a). This result has been comprehensively described in previous works (Tangredi et al., 2023), showing that at more severe conditions, P tends to persist within the solid fraction despite the enhanced solubilization of solid organic matter and TSRY reduction. The temperature effect on HT solid pellet P content seems to be prevalent over time (Ovsyannikova et al., 2019; Wang et al., 2019).

Fig. 6 shows P relative repartition in process water and pellet as a fraction of the total P in the feed. The fraction of P recovered in the pellets increases from 79 % to 94 % by increasing process severity from 0.19 (250 °C, 5 min) to 0.88 (350 °C, 45 min). A temperature increase is

<figure><img src="image_7.png" /><figcaption>**Fig. 6.** P repartition in HT by-products: process water and pellet, expressed as a function of process severity (equation (2)). Error bars represent propagated uncertainty.</figcaption></figure>

associated with a higher P recovery in the pellet. The time effect is less straightforward: at higher temperatures (300 and 350 °C) P recovery yield in process water decreases with time, while at 250 °C a longer treatment leads to slightly more P recovered in the process water. At relatively low temperatures, in fact, the mineralization kinetics of OP, following an Arrhenius kinetic law (Feng et al., 2020), is relatively slow, requiring several minutes to fully mineralize solid OP to PO₄-P (Wang et al., 2021). At higher temperatures the mineralization kinetics is much faster (Amrullah and Matsumura, 2019), and the precipitation of PO₄-P with metal cations becomes the dominant phenomenon that determines the P precipitation and accumulation in the solid pellet (Shi et al., 2019; Xu et al., 2018).

The response surface of P recovery yield in solid pellet as a function of process temperature and time (Fig. S7 and Table S4) is statistically significant ($p < 0.01$ %, $R^2 = 0.98$). The obtained response surface confirms once more the predominant influence of temperature over time
with the effect of time becoming more significant at higher temperatures. P contents and P concentration in the solid and liquid by-products are in line with the ranges available in literature (Liu et al., 2021a; Tangredi et al., 2023).

Fig. 7 shows P speciation in the solid pellets. AIP is the prevalent P fraction in solid pellets, in contrast to the digestate feed, where the predominant fraction is NAIP. The most noticeable result is the increase in AIP and the decrease in NAIP as the temperature increases, in particular from 300 to 350 °C. This phenomenon has been extensively observed in other works (Tangredi et al., 2023). High temperatures lead to the precipitation of PO₄³⁻ with Ca²⁺ cations into the form of Ca-phosphates, with a concomitant dissolution of NAIP complexes (such as Fe- and Al-phosphates) into Fe³⁺, Al³⁺, and PO₄³⁻, which could be then reprecipitated as stable Fe- and Al-(hydro)-oxides (Barca et al., 2019). In contrast to the digestate feed, for which the OP has been estimated to be around 10 % of TP, no statistically significant residual OP fraction has been reliably observed in the solid pellets. Time does not seem to have a particular effect on P speciation for HT realized at 250 and 300 °C, but at 350 °C longer time lead to higher AIP fractions.

The response surface of AIP fraction as a function of process temperature and time (Fig. S8 and Table S5) is statistically significant ($p < 1\%$, $R^2 = 0.98$). Moreover, the temperature coefficients are statistically significant. The model indicates a particularly notable increase in AIP fraction for the highest temperature (350 °C), while temperature effect is statistically less relevant between 250 and 300 °C. No significance has been observed for time.

These results allow to support the mechanistic model of P conversion during HT of SS illustrated in a previous work (Tangredi et al., 2023). According to the model, P speciation is controlled by the relative significance of three different groups of reactions:

(I) Organic P mineralization into PO₄³⁻, which reduces solid organic P concentration and increases soluble P concentration.

(II) Inorganic P dissolution and/or desorption to PO₄³⁻, which reduces solid inorganic P concentration and increases soluble P concentration.

(III) PO₄³⁻ (re-)precipitation into metal phosphates, highly dependent on the presence and stoichiometry of Ca²⁺, Fe³⁺, Al³⁺, Mg²⁺ and Mn²⁺ cations in the reaction medium.

<figure><img src="image_8.png" /><figcaption>Fig. 7. Total P speciation in solid pellet: Non-Apatite Inorganic P (NAIP), Apatite Inorganic P (AIP) and Organic P (OP) relative fractions expressed as a function of process severity (equation (2)). Error bars correspond to the combination of propagated uncertainty and standard deviation ($n = 3$).</figcaption></figure>

After HT treatment at 250 °C, process water has a higher soluble P concentration compared to the digestate feed (from 320 to almost 600 mg L⁻¹), indicating that HT at relatively low process severities favors OP mineralization (I) and IP dissolution and/or desorption (II) to PO₄-P. However, the solid pellet, despite being obtained at relatively low temperatures (250 °C), predominantly consists of AIP, contrasting with the P speciation of the feed digestate, where the prevailing P-form is NAIP. This implies that there has been a conversion from NAIP to AIP, and thus a partial re-precipitation of phosphates with Ca cations (III). This result is in line with the evolutions of Ca and Fe concentrations in process water as a function of process severity (Fig. 5a), which can be explained by concomitant precipitation of Ca-P compounds (AIP), and dissolution of Fe-P complexes (NAIP). At 300 °C the free PO₄³⁻ concentration starts decreasing, showing that the phosphates precipitation with metallic cations is becoming prevalent. Finally, at 350 °C, it is clear that reaction (III), specifically precipitation with Ca²⁺ cations, is the predominant one, considering the low PO₄³⁻ concentration in process water and the strong increase in the AIP relative fraction (up to 80 % of TP).

It is important to note that in this study complete P precipitation was not achieved even under the most severe process conditions. For instance, after HT treatment at 350 °C for 45 min, the residual P concentration in the process water was 150 mg L⁻¹, corresponding to 6 % of feed P. For all process water by-products, Ca/P, Fe/P and Mg/P molar ratios are always lower than 1, indicating that the available Ca²⁺, Fe³⁺, and Mg²⁺ concentrations in solution are insufficient for complete PO₄-P precipitation. Ca/P molar ratio in the digestate and in all solid pellet by-products is roughly 0.7, meaning that not more than 70 % of total P can precipitate with the available Ca, assuming the formation of dicalcium phosphate (CaHPO₄). Under the most severe process conditions, it was indeed observed that AIP corresponds to about 70 % of the TP. To achieve complete P precipitation and higher amounts of AIP, it is therefore necessary to introduce additional Ca into the feed, to increase the molar ratio of Ca/P to at least 1. This means that the stoichiometry of the key metals involved in P speciation is a crucial parameter in defining the conversion pathway of P during HT treatments.

### 3.6. Digestate and by-products valorization and disposal

The digestate leachate and HT treatment process water are not compatible with discharge into the environment due to their high COD concentration, exceeding the limit prescribed by the European Directive 91/271/EEC. Furthermore, the concentration of soluble P and N is likely incompatible with direct discharge into the environment, as it could lead to eutrophication phenomena (Silva Thomsen et al., 2022). To reduce COD, NH₄-N and P levels, potential treatments include wet oxidation (Silva Thomsen et al., 2022) or precipitation as calcium phosphate, struvite (Munir et al., 2017), or K-struvite (Yesigat et al., 2022), though Mg or K supplementation may be required (Tangredi et al., 2023). Alternatively, process water could be repurposed as a medium for microalgae cultivation (Koçer et al., 2023), promoting nutrient recovery.

As shown in Table 1, the thickened digestate could be exploited as a solid organic fertilizer in a dried form, as it conforms to the guidelines for minimum macronutrients and organic carbon values and to all the limits for heavy metals contents, except for Cu. The dried HT pellet could be exploited as a compound inorganic macronutrient fertilizer (Table 1). The sum of primary and secondary macronutrients is higher than 18 % and the contaminants limits (Cd, Hg, Ni, Pb, As, Zn) are respected except for Cu (900–1050 mg kg⁻¹) against a maximum value of 600 mg kg⁻¹).

For both the dried digestate and HT pellet, exceeding Cu content could be reduced by an upstream approach: Cu contamination in wastewater often comes from industries such as mining, electroplating, and chemical manufacturing (Li et al., 2024). Additionally, Cu-based bactericides and fungicides used in agriculture may contribute to elevated Cu levels (Ochoa-Herrera et al., 2011). Addressing this requires targeted treatment at the source, including optimized metal recovery or
<caption>**Table 1**</caption>

Dried sewage sludge digestate and dried HT pellet compatibility as EU fertilizing and amending products as indicated in the European Regulation 2019/1009. P₂O₅ = P/0.436; K₂O = K/0.830; MgO = Mg/0.603; CaO = Ca/0.715; SO₃ = S/0.4. *1A(I): SOLID ORGANIC FERTILIZER; ** 1(C)(I)(a)(ii): COMPOUND SOLID INORGANIC MACRONUTRIENT FERTILISER. ***expressed as mg kg⁻¹ P₂O₅.

<table><thead><tr><th rowspan="2">Product</th><th rowspan="2"></th><th>Composition</th><th rowspan="2">Guidelines</th><th rowspan="2">Compatibility</th></tr><tr><th>%</th></tr></thead><tbody><tr><td rowspan="10">Dried sewage sludge digestate</td><td>Macronutrients</td><td>14.6</td><td>4</td><td>✓</td></tr><tr><td>Organic carbon</td><td>43.3</td><td>15</td><td>✓</td></tr><tr><td>Contaminants</td><td>mg kg<sup>-1</sup></td><td>limit mg kg<sup>-1</sup>*</td><td></td></tr><tr><td></td><td>Cd</td><td>Cd</td><td>✓</td></tr><tr><td></td><td>Hg</td><td>Hg</td><td>✓</td></tr><tr><td></td><td>Ni</td><td>Ni</td><td>✓</td></tr><tr><td></td><td>Pb</td><td>Pb</td><td>✓</td></tr><tr><td></td><td>As</td><td>As</td><td>✓</td></tr><tr><td></td><td>Cu</td><td>Cu</td><td>X</td></tr><tr><td></td><td>Zn</td><td>Zn</td><td>✓</td></tr><tr><td rowspan="10">Dried HT pellet</td><td>Macronutrients</td><td>%</td><td>minimum value %**</td><td></td></tr><tr><td>Contaminants</td><td>>22.2</td><td>18</td><td>✓</td></tr><tr><td></td><td>limit mg kg<sup>-1</sup>**</td><td></td><td></td></tr><tr><td></td><td>Cd</td><td>Cd</td><td>✓</td></tr><tr><td></td><td>Hg</td><td>Hg</td><td>✓</td></tr><tr><td></td><td>Ni</td><td>Ni</td><td>✓</td></tr><tr><td></td><td>Pb</td><td>Pb</td><td>✓</td></tr><tr><td></td><td>As</td><td>As</td><td>✓</td></tr><tr><td></td><td>Cu</td><td>Cu</td><td>X</td></tr><tr><td></td><td>Zn</td><td>Zn</td><td>✓</td></tr></tbody></table>

using alternative compounds in pesticides. Besides, further investigations must be conducted to ensure that these two products are compatible for fertilizer use, specifically to assess biuret (C₂H₅N₃O₂), perchlorate (ClO₄), hexavalent chromium (CrVI) and polycyclic aromatic hydrocarbon (PAH) contents.

Future studies should focus on optimizing process conditions to enhance P recovery while minimizing heavy metal contamination in the solid residues. Techniques such as acidic extraction, sequential extraction, and precipitation (e.g., with Ca or hydroxyapatite) should be further explored for efficient P recovery (Liu et al., 2021b, 2023). Besides, further characterization of P species via XRD, XANES, and XPS would improve the assessment of the solid product as a fertilizer. A similar HT study on pig manure suggested amorphous calcium phosphate precipitation due to short reaction times and rapid cooling (Ochoa-Martinez et al., 2025), which would enhance P fertilizing efficiency by improving its bioavailability.

The optimization of the SS macronutrients recovery would allow to obtain products with higher added value. Integrating P valorization with energy production (bio-oil from HT liquefaction) would allow to reduce the costs associated with proper SS treatment and disposal, and to develop a sustainable circular approach to wastewater management and reclamation. More severe treatment conditions can increase P concentration in the solid phase as AIP, but bio-oil yield follows a non-linear trend with temperature, peaking before decreasing at higher temperatures (Liu et al., 2021a). Finding an optimal balance between treatment conditions is essential to maximize both P recovery and bio-oil yield. Further cost-effectiveness or Life Cycle analyses could explore the addition of Ca for complete P precipitation and its impact on bio-oil yield and overall cost.

## 4. Conclusion

SS digestate has been subjected to HT treatment to assess the resulting by-products characteristics and specifically investigate P fate and conversion. Results indicate that the increase in process severity leads to higher alkalinity and NH₄-N concentration in process water, which could be linked to proteins and other N-containing species mineralization. Soluble P slightly increases with time at 250 °C, due to solid OP mineralization and solid IP dissolution, but at higher temperatures it decreases steeply, suggesting P precipitation. The pellet P content increases with increasing severity. The prevalent P form in the

pellet is AIP, in contrast to the prevailing form of the feed digestate that is NAIP. This suggests a re-precipitation of phosphates produced by OP mineralization and Fe-P and Al-P compounds dissolution with Ca²⁺ cations to form more stable Ca-P complexes.

Complete P precipitation was not achieved in these experiments; under most severe process conditions, 6 % of feed P remained in solution. A Ca/P ratio of 0.7 in the digestate allowed to precipitate up to 70 % of P as AIP. Adding Ca to achieve a Ca/P ratio of 1 is necessary for full P precipitation as AIP, showing the critical role of metal stoichiometry in P speciation during HT treatments.

Both the feed digestate and HT pellet could be compatible as fertilizing and amending products according to EU parliament guidelines, but more investigation is required to completely assess all variables. Following a Ca addition to complete P precipitation, the solid pellet would be compatible with agricultural use as an inorganic fertilizer rich in Ca and with slow-release P, particularly suitable for acidic soils or soils sensitive to eutrophication or Fe/Al bioaccumulation.

Future studies should focus on optimizing conditions for P recovery while minimizing heavy metals contamination. Exploring extraction and precipitation methods, along with P characterization via XRD, XANES, and XPS, will improve fertilizer assessment.

Optimizing SS nutrient recovery enhances value and promotes sustainable wastewater management. Integrating P valorization with bio-oil production can reduce treatment costs, but finding the right balance of process conditions is crucial. Further analysis should evaluate the impact of Ca addition on P recovery and overall cost-effectiveness.

## CRediT authorship contribution statement

**Antonello Tangredi:** Writing – original draft, Visualization, Investigation, Formal analysis, Data curation. **Cristian Barca:** Writing – review & editing, Validation, Supervision, Project administration, Funding acquisition, Conceptualization. **Jean-Henry Ferrasse:** Writing – review & editing, Supervision, Conceptualization. **Olivier Boutin:** 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.

## Acknowledgments

This work has been supported by the French National Research Agency, France (ANR) for the Project D2LIFE under the reference number ANR-21-CE43-0013-01. We thank Grégory Excoffier (Spectro-pole, Aix-Marseille Université) for elemental CNHS analysis, Hélène Miche and Bernard Angeletti (Plateforme LA-ICP-MS, CEREGE, Aix-Marseille Université) for ICP-AES and ICP-MS analyses, and Cyrielle Girard (M2P2, Aix-Marseille Université) for technical support.

## Data availability

Data will be made available on request.

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