Sustainable Utilization of Dewatered Sewage Sludge via Hydrothermal Conversion: Focus on Steroid Transformation

Abstract

With the acceleration of urbanization, the disposal of dewatered sewage sludge (DSS) has become an urgent environmental issue worldwide. Hydrothermal conversion (HC) of DSS is an important method for sludge sustainable utilization due to its combination of efficiency and economic and environmental advantages. This study investigates the product distribution and composition of products during the HC of DSS under subcritical and supercritical water conditions (200–450 °C, 5–90 min), with a particular focus on the formation and conversion mechanisms of steroid compounds. The results indicate that increasing temperature and reaction time leads to a rise in gas-phase products (GPs) and a corresponding decline in solid-phase products (SPs), with phenolic compounds identified as the predominant constituents. In the water-soluble products (WSPs), nitrogen-containing cyclic compounds are the major products. Saturated nitrogen heterocycles dominate at lower temperatures (200 °C), while at elevated temperatures (300–350 °C), saturated azapolycyclic compounds emerge, ultimately transitioning into unsaturated aromatic nitrogen heterocycles at 450 °C. Steroids are primarily concentrated in the oil-phase products (OPs). The conversion process involves the initial conversion of lipids in the DSS to long-chain olefins at 200 °C, which are then converted to steroids at 250–350 °C. At higher temperatures (400–450 °C), these steroids might decompose into gaseous products or undergo polymerization to form char. This suggests the potential for steroids to act as precursor compounds in the process of char formation. This work could contribute to a deeper understanding of the HC mechanism of DSS and provide valuable technical insights for improving bio-oil quality.

1. Introduction

Dewatered sewage sludge (DSS), a solid waste with high water content generated during municipal wastewater treatment, has become an important environmental challenge, as it contains a large number of pollutants [1]. With the acceleration of urbanization, DSS production continues to rise, making its sustainable disposal an urgent global environmental concern. Meanwhile, DSS is also a valuable biomass resource that can be used as a source of carbon and energy due to its high organic matter content. Hydrothermal conversion (HC) is an advanced thermochemical process that facilitates the transformation of biomass into energy products, including fuel gas, bio-oil, and biochar, under high-temperature and high-pressure water conditions [2,3,4]. In this process, water acts both as a reactant and a catalyst [4], allowing wet biomass such as DSS to be directly utilized without the need for extensive drying. Therefore, hydrothermal conversion of DSS has become an important direction for sludge sustainable utilization due to its combination of efficiency and economic and environmental advantages [5].

Typically, bio-oil derived from HC contains a diverse range of chemical compounds, such as organic acids, fatty acids, alkanes/alkenes, ketones, imines/amines, phenolic derivatives, and heterocyclic compounds [2,6,7]. In addition to the aforementioned compounds, steroids have been identified in certain bio-oil samples [8,9,10], and their presence significantly influences bio-oil quality. Structurally, steroids are characterized by the cyclopentanophenanthrene framework, consisting of a fused assembly of three cyclohexane rings and one cyclopentane ring. This condensed polycyclic structure theoretically enhances the higher heating value (HHV), increases viscosity [11], and improves thermal stability [12,13]. The notable thermal stability of steroids suggests that they are not readily degradable, making them key components in the hydrothermal conversion of biomass. Therefore, investigating the reaction pathways of steroids is essential for optimizing their formation and degradation during hydrothermal reactions. Despite the importance of steroids in hydrothermal processes, research on their formation under hydrothermal conditions remains limited. Hietala et al. [12] studied the reaction of cholesterol under hydrothermal conditions and found that dehydration and parallel isomerization were the predominant reactions at 350 °C, yielding cholestadienes as the major products. Jocz et al. [14] observed that the addition of H₂ increased the yield of cholestadienes while simultaneously reducing the conversion rate of cholesterol. Rushdi et al. [13] examined the thermal decomposition of cholesterol and reported that cholestenes and cholestenones were the primary products at lower temperatures, with reaction pathways significantly influenced by the presence of catalysts. However, these studies predominantly focused on model compounds; the formation and conversion pathways of steroids in real sludge during HC remain poorly understood. Further research is necessary to elucidate the mechanisms governing steroid transformation in complex sludge and to develop strategies for optimizing bio-oil quality.

This study aims to elucidate the formation and conversion pathways of steroids during the hydrothermal conversion of DSS. Specifically, we investigated the transformation of intermediate products as the reaction temperature increased and proposed fundamental reaction pathways for steroids DSS under subcritical and supercritical water conditions. The findings of this study contribute to a deeper understanding of HC reactions in DSS and provide foundational knowledge and technical insights for optimizing bio-oil quality.

2. Materials and Methods

2.1. Materials and Apparatus

The dewatered sewage sludge (DSS) used in this study was obtained from the Jiangxinzhou Sewage Treatment Plant in Nanjing, Jiangsu Province. Its fundamental properties are presented in

Table 1. Properties of Jiangxinzhou DSS.

Moisture Content (wt%)	pH	Proximate Analysis (wt%) a			Ultimate Analysis (wt%) a					HHV (MJ/kg) c
		VM d	FC d	Ash	C	N	S	H	O b
89.2	7.8	56.91	1.34	41.75	25.56	4.57	0.15	4.42	22.10	11.06
Biochemical (wt %) a		Crude protein			Lignin	Lipids	Hemicellulose		Cellulose
		22.27			18.03	12.22	4.07		0.32

^a: On an air-dried basis. ^b: By difference (O% = 100% − ash% − C% − N% − S% − H%), samples for ultimate analysis were dried at 105 °C. ^c: Higher heating value (HHV) calculated by the Dulong Formula, i.e., HHV (MJ/kg) = 0.3383C + 1.443 (H − O/8) + 0.0927S. ^d: VM: Volatile matter (burned at 550 °C); Ash: burned at 815 °C; FC: Fixed carbon.

. The experimental apparatus has been described in detail in a previous study [15]. The 316L stainless steel (SS316L) reactor has an internal capacity of 100 mL with maximum design parameters of 650 °C and 35 MPa for temperature and pressure, respectively.

2.2. Methods

2.2.1. Preparation and Separation of the Products

The experimental procedures for product preparation and separation were conducted in accordance with the methods described in Ref. [16]. In a typical experiment, 44.9 g of dewatered sewage sludge (DSS), containing 40.0 g of water, was loaded into a batch reactor and subjected to reaction temperatures ranging from 200 to 450 °C, with reaction times varying from 5 to 90 min under autogenous pressures of 2 to 26 MPa. The organic matter present in the sludge, measured on a dry and ash-free (daf) basis, was fractionated into four distinct product categories: gas-phase products (GPs), water-soluble products (WSPs), oil-phase products (OPs), and solid-phase products (SPs). Since CO₂ was the predominant component of the gas-phase products, their mass was calculated based on the standard density of CO₂ (1.977 g/L).

2.2.2. Sludge and Products Analysis

Proximate analysis was conducted according to the following Standards [17,18]. Elemental analysis was carried out using an elemental analyzer (Elementar Vario EL III, Langenselbold, Germany). The composition of DSS was determined using the methods described in our previous work [16].

The GPs were primarily determined using a gas chromatograph (Kejie GC5890, Nanjing, China) to analyze H₂, CO₂, CO, and CH₄, while other gaseous components were not considered. High-purity nitrogen (N₂) was used as the carrier gas. The analysis was performed with a thermal conductivity detector (TCD) and a TDX-01 column (2 m × 3 mm). The column oven was maintained at 90 °C, while the inlet and detector temperatures were set at 120 °C and 150 °C, respectively. The extraction and analysis of WSPs followed a specific procedure: 1 mL of WSPs was freeze-dried for 24 h. After complete water removal, acetone was added to adjust the volume to 5 mL. A 0.2 mL aliquot of the resulting solution was then diluted with chromatographic-grade acetone to a final volume of 1 mL and analyzed using gas chromatography-mass spectrometry (GC-MS, Shimadzu QP2010Plus, Kyoto, Japan). The OPs were extracted using the following method: 1.50 g of solid residue, containing both OPs and SPs, was subjected to multiple (3–4) elution steps with a solvent mixture of benzene and ethanol (V_benzene/V_ethanol = 1:2). The eluate was purified using activated anhydrous sodium sulfate, then concentrated to approximately 1 mL using a rotary evaporator (Yarong RE-2000B, Shanghai, China). The concentrated eluate was subsequently dried under a nitrogen stream and redissolved in 1 mL of chromatographic-grade acetone for GC-MS analysis. The GC-MS system was equipped with a DB-5MS quartz capillary column (60 m × 0.25 mm I.D. × 0.25 μm), with parameter settings based on those described in Ref [19]. The characterization of SPs was conducted using Fourier Transform Infrared Spectroscopy (FTIR, ALPHA Bruker, Bruker, Karlsruhe, Germany).

3. Results and Discussion

3.1. Analysis of General Products

The distributions of products across different phases at temperatures ranging from 200 to 450 °C are illustrated in Figure 1. As temperature and reaction time increased, the SPs progressively transformed into other phases, resulting in a gradual reduction in their mass. At 450 °C for 15 min, the mass of SPs stabilized at approximately 5% of the total organic matter in the sludge, suggesting that SPs primarily consisted of coke at this temperature. The GPs yield increased with rising temperature and reaction time. At temperatures below 300 °C, GPs constituted no more than 15% of the total organic matter. However, beyond 350 °C, their mass percentage increased significantly, reaching a peak of 38.30% at 450 °C after 90 min.

The mass percentage of water-soluble products (WSPs) exhibited an initial increase, followed by a subsequent decrease with increasing temperature and reaction time. At 400–450 °C, WSPs rapidly reached their peak concentration at 3 min, followed by a gradual decline. This trend indicates that sludge initially underwent hydrolysis before transforming into other phases. Similarly, the oil-phase products (OPs) displayed an initial increase followed by a decline. Above 350 °C, OPs began to decrease, with a marked reduction at 450 °C, likely due to intensified free radical reactions occurring in supercritical water.

Across all temperatures, product distribution varied significantly within the first 30 min, indicating a rapid reaction phase. After this period, changes in product distribution became less pronounced, and the influence of reaction time decreased. Below 350 °C, an increase in temperature predominantly led to a rise in OPs, with minimal impact on gas yield. Conversely, above 350 °C, higher temperatures resulted in decreased OPs and increased GPs. Therefore, the temperature range of 350–400 °C was critical for product distribution, aligning with the transition of water into its supercritical state.

3.2. Gas-Phase Products

As illustrated in Figure 2, the gas yield exhibited a positive correlation with both temperature and reaction time, with CO₂ consistently being the predominant component of the GPs across all tested temperature conditions. The gas yield is up to 8.65 mol/kg at 450 °C for 90 min. After 90 min of reaction at 400 °C, CO₂ remained the primary gas-phase product, constituting more than 92.61% of the total gas yield. Significant increases in H₂ production were observed only at temperatures exceeding 450 °C. The generation of H₂ is primarily attributed to the decomposition of small-molecule acids or the direct decarboxylation of amino acids at relatively low temperatures [20]. The decarboxylation of long-chain carboxylic acids was not considered, as these compounds are generally resistant to decarboxylation under the conditions tested [21].

3.3. Water-Soluble Products

Figure 3 shows the GC-MS total ion chromatogram (TIC) spectra of substances in the aqueous phase at different reaction temperatures. A reaction time of 30 min was selected based on the observation that the gas, solid, and liquid phases of the products exhibited minimal changes beyond this duration, as shown in Figure 1. The 15 largest peaks, are listed in

Table 2. Molecular Structures of the Main WSPs during HC of DSS.

Temperature	Molecular Structures of the Main WSPs
	Molecular Structures of the Top Five Peaks					Molecular Structures of Other Main WSPs
	1	2	3	4	5
200 °C	S-W-200(11) Peak area:19.74	S-W-200(7) 18.70	S-W-200(9) 11.99	S-W-200(6) 11.74	S-W-200(3) 11.35	S-W-200(4), S-W-200(1), S-W-200(2), S-W-200(8)
250 °C	S-W-250(7) 28.57	S-W-250(3) 20.76	S-W-250(5) 9.68	S-W-250(11) 6.55	S-W-250(9) 4.76	S-W-250(8), S-W-250(9), S-W-250(12), S-W-250(13), S-W-250(14), S-W-250(15)
300 °C	S-W-300(4) 26.94	S-W-300(2) 13.49	S-W-300(3) 12.33	S-W-300(6) 6.73	S-W-300(11) 5.53	S-W-300(7), S-W-300(8), S-W-300(10)(12)(13), S-W-300(9), S-W-300(14), S-W-300(15)
350 °C	S-W-350(5) 22.44	S-W-350(6) 22.30	S-W-350(3) 10.45	S-W-350(2) 9.35	S-W-350(1) 8.52	S-W-350(7), S-W-350(8), S-W-350(9), S-W-350(10), S-W-350(11)
400 °C	S-W-400(12)(13)(14) 6.20 + 3.80 + 4.39	S-W-400(1) 13.55	S-W-400(7) 10.83	S-W-400(3) 9.14	S-W-400(4) 8.74	S-W-400(8), S-W-400(9), S-W-400(10), S-W-400(11)
450 °C	S-W-450(7) 19.71	S-W-450(2) 19.19	S-W-450(1) 16.72	S-W-450(3) 9.35	S-W-450(8) 7.69	S-W-450(5), S-W-450(6), S-W-450(9), S-W-450(10), S-W-450(11)

(ranked by peak area) and Tables S1–S6 (ranked by retain time). The peak numbers of water-soluble products (WSPs) are denoted as S-W-T(X), where “S” refers to the sludge, “W” indicates the water-phase products, “T” represents the reaction temperature, and “X” corresponds to the serial number of the peaks. The data under each peak number reflect the peak area. It is important to note that a portion of the WSPs adhered to the walls of the centrifugal glass tube following freeze-drying and could not be fully dissolved in acetone. Consequently, this portion of the WSPs was not detected by GC-MS. For example, much of the nitrogen in the sludge is typically converted into ammonium (NH₄⁺) [16], which is not detectable by GC-MS.

As shown in Figure 3a and Table 2, at 200 °C, saturated nitrogen heterocycles and amines were the predominant compounds in the WSPs of the sludge. The other WSPs consisted mainly of small molecule ketones, esters, alcohols and unsaturated nitrogen heterocycles, etc. The five peaks with the largest peak area ratios, in descending order, were S-W-200(11), S-W-200(7), S-W-200(9), S-W-200(6), and S-W-200(3). A substantial number of nitrogen-containing compounds were detected in the aqueous phase, likely resulting from the dehydration and cyclization of amino acids [22]. Ammonium (NH₄⁺-N) was not detected, as it is nearly insoluble in acetone. It is important to note that the peaks S-W-200(11) (similar index: 83) and S-W-200(9) in the product were likely generated from small ketones and ammonia (NH₃) [23], possibly due to the use of acetone as a solvent during the WSP collection process.

The WSPs of the sludge at 250 °C were similar to those at 200 °C (Figure 3b and Table 2), with saturated nitrogen heterocyclic compounds remaining the dominant products. At 250 °C, additional nitrogen heterocyclic peaks, such as S-W-250(8), S-W-250(9), S-W-250(12), S-W-250(13), S-W-250(14), and S-W-250(15), emerged in comparison to those observed at 200 °C. Furthermore, the heights of many peaks increased, indicating a higher concentration of substances in the liquid phase. Notably, the appearance of saturated azapolycyclic compounds at 250 °C suggests that condensation reactions occurred within the nitrogen heterocycles.

As shown in Figure 3c and Table 2, saturated nitrogen heterocyclic compounds remained the predominant components in the aqueous products at 300 °C. Some small-molecule substances, such as glycerol, which were present at 200 °C and 250 °C, no longer appeared in the top 15 peaks at this temperature. New saturated nitrogen heterocyclic compounds, such as S-W-300(8), emerged at 300 °C. Additionally, nitrogen-containing fused rings, including S-W-300(10)(12)(13) (likely three structural isomers), as well as S-W-300(14) and S-W-300(15), were present in significantly higher concentrations compared to 250 °C. At this temperature, the formation of saturated nitrogen-cyclic compounds such as S-W-300(11)   and S-W-300(10)  may result from the intermolecular dehydration of amino acids [22].

Saturated nitrogen heterocyclic compounds remained predominant at 350 °C (Figure 3d and Table 2). Compared to 300 °C, the quantity of various nitrogen compounds and the height of their respective peaks increased. The peak S-W-350(7) may have been produced through intramolecular dehydration of amino acids [24]. Additionally, an unsaturated nitrogen-aromatic heterocyclic compound, S-W-350(8), appeared; however, its content was relatively low, with a peak area ratio of 2.58%.

Figure 3e and Table 2 show that at 400 °C, the predominant WSPs were saturated nitrogen heterocyclic compounds, with saturated azapolycyclic compounds constituting a significant proportion. Aromatic compounds, such as S-W-400(7), appeared among the five largest peaks. Notably, the total concentration of each compound in the aqueous phase decreased at 400 °C, indicating the conversion of WSPs into other products. Nitrogen compounds in the aqueous phase may convert to NH₄⁺-N through direct deamination or ring-opening deamination [16], or they may transition into the OPs (see Figure 3f and

Table 3. Molecular Structures of the Main OPs during HC of DSS.

Temperature	Molecular Structures of the Main OPs
	Molecular Structures of the Top Five Peaks					Molecular Structures of Other Main OPs
	1	2	3	4	5
Raw sludge	S-O-(2) Peak area:42.46	S-O-(10) 12.55	S-O-(13) 7.98	S-O-(14) 5.92	S-O-(6) 4.64	S-O-(1), S-O-(3), S-O-(4), S-O-(7), S-O-(11), S-O-(14)
200 °C	S-O-200(13) 34.12	S-O-200(14) 9.50	S-O-200(7) 8.77	S-O-200(8) 6.14	S-O-200(10) 5.97	S-O-200(2), S-O-200(3), S-O-200(11), S-O-200(12)
250 °C	S-O-250(4) 37.20	S-O-250(6) 12.74	S-O-250(12) 12.24	S-O-250(7) 8.80	S-O-250(5) 6.34	S-O-250(8), S-O-250(13), S-O-250(14), S-O-250(15)
300 °C	S-O-300(6) 34.93	S-O-300(13) 10.90	S-O-300(9) 9.99	S-O-300(10) 9.53	S-O-300(8) 6.47	S-O-300(3), S-O-300(4), S-O-300(5), S-O-300(7)
350 °C	S-O-350(5) 32.92	S-O-350(12) (13) (14) 8.34 + 6.17 + 6.13	S-O-350(11) 9.05	S-O-350(15) 6.97	S-O-350(10) 6.60	S-O-350(1), S-O-350(2), S-O-400(9)
400 °C	S-O-400(9) 27.78	S-O-400(12)(13)(14) 9.97 + 6.88 + 4.33	S-O-400(10) 12.23	S-O-400(15) 6.56	S-O-400(11) 6.46	S-O-400(1), S-O-400(2), S-O-400(3), S-O-400(5), S-O-400(6), S-O-400(7)
450 °C	S-O-450(9) 26.25	S-O-450(12)(13)(14) 6.56 + 7.41 + 5.99	S-O-450(10) 14.12	S-O-450(15) 10.19	S-O-450(5) 5.50	S-O-450(1), S-O-450(2), S-O-450(3), S-O-450(4), S-O-450(7)

). Similarly, deamination of soluble proteins could lead to an increase in NH₄⁺-N, as suggested by [25].

As shown in Figure 3f and Table 2, nitrogen heterocyclic compounds remained the predominant products in the aqueous phase at 450 °C. However, saturated nitrogen heterocycles began to convert into unsaturated nitrogen aromatic heterocycles. The peak S-W-450(3), representing an unsaturated nitrogen aromatic heterocycle, appeared among the top five peaks, with a peak area ratio of 9.35%. Other nitrogen aromatic heterocycles, including S-W-450(5), S-W-450(6), S-W-450(9), S-W-450(10), and S-W-450(11), were also detected.

In general, as the temperature increased, both the variety and concentration of water-soluble products (WSPs) increased, reaching a maximum between 300 °C and 350 °C, before gradually decreasing. At 200 °C, several types of WSPs were detected, predominantly within the first 15 min, with saturated nitrogen heterocycles as the main components. At 300–350 °C, the concentration of saturated azapolycyclic compounds in the aqueous phase increased significantly. At 450 °C, unsaturated nitrogen-containing aromatic heterocycles gradually became the predominant products.

3.4. Oil-Phase Products

The lipids extracted from raw sludge and the OPs at different reaction temperatures were qualitatively analyzed by GC-MS, as shown in Figure 4. The peak numbers of substances in the OPs are labeled as S-O-T(X), where “S” represents sludge, “O” denotes OPs, “T” indicates the reaction temperature, and “X” is the serial number of the peaks. The 15 largest peaks, are listed in Table 3 (ranked by peak area) and Tables S7–S13 (ranked by retain time).

As shown in Figure 4a and Table 3, the largest peak area in raw sludge was observed for peak S-O-(2), which accounted for 42.46% of the total area of the first 15 peaks. The steroid compound peak S-O-(10) represented a significant proportion of the fatty substances in the sludge, with its peak area constituting 12.55% of the total area of the first 15 peaks. This could be attributed to the maturation of the sludge during storage. Other major components in the OPs included alcohols, long-chain esters, alkenes, and amines.

As shown in Figure 4b and Table 3, long-chain hydrocarbons and olefins, such as S-O-(14), which corresponds to peak S-O-200(13) at 200 °C, became the dominant components of the oil-phase products (OPs), indicating that the lipids in the original sludge were hydrolyzed. At 250 °C (Figure 4c and Table 3), the area of peak S-O-250(13), which corresponds to peak S-O-200(13), decreased significantly, while the steroid content increased. This suggests that cyclization of long-chain compounds commenced at this temperature. At 300 °C, as shown in Figure 4d and Table 3, steroids became the predominant products. Notably, nitrogen heterocyclic compounds, such as S-O-300(3), S-O-300(4), S-O-300(5), and S-O-300(7), appeared in the OPs, which is consistent with the increase in saturated nitrogen heterocyclic compounds in the WSPs. Therefore, the nitrogen heterocyclic compounds in the OPs may originate from the WSPs. At 350 °C (Figure 4e and Table 3), new steroids, including S-O-350(12)(13)(14) and S-O-350(15), were formed. The nitrogen heterocycles in the OPs decreased sharply, while phenolic compounds, such as S-O-350(1) and S-O-350(2), emerged. Two primary mechanisms could explain the formation of phenolic substances: one involves the dehydrogenation of saturated alicyclic hydrocarbons from WSPs to aromatic compounds, which then enter the OPs; the other involves the formation of aromatic substances through an addition reaction between small molecules.

As shown in Figure 4f,g, and Table 3, the peak heights at 400 °C and 450 °C decreased, which is consistent with the reduction in the yield of the OPs observed in Figure 1. Steroid compounds began to decompose, and their absolute contents declined. However, the percentage content of steroids remained relatively high, indicating that they were more resistant to decomposition compared to other products. Furthermore, the relative content of phenolic and nitrogen heterocyclic substances in the OPs increased. Based on Figure 1 and Figure 2, there was a significant increase in gas products, particularly in H₂ yields. Therefore, the increase in phenolic substances may be attributed to the further dehydrogenation of small-molecule saturated alicyclic hydrocarbons in the WSPs.

In summary, the hydrothermal conversion (HC) of DSS to form the oil-phase products (OPs) primarily occurs as follows: Initially, chain hydrocarbons are formed through hydrolysis at 200 °C. Steroid compounds are subsequently formed around 250 °C. From 300 °C onward, nitrogen in the sludge is incorporated into the OPs as amines and nitrogen heterocycles. As the temperature increases, nitrogen may re-enter the aqueous phase due to deamination, with only a small amount of nitrogen remaining in the oil phase, primarily as nitrogen heterocycles. Around 400 °C, steroid compounds degrade, while aromatic rings increase significantly, likely due to the dehydrogenation of cyclic compounds.

The GC-MS results (Table 3) reveal that the oil-phase products are rich in steroids, nitrogen heterocycles, and phenolics. The increased presence of cyclic compounds in the bio-oil indicates a higher degree of polymerization, which is associated with a higher viscosity of the bio-oil [11]. This higher viscosity is further evidenced by the substantial amount of bio-oil adhering to the walls of the reactors during the experiments, as previously reported in our study [26]. Additionally, the presence of heteroatoms, such as nitrogen and oxygen, in the oil phase reduces both the calorific value and stability of the bio-oil. Therefore, further up-grading of the bio-oil is needed [4].

To regulate or control the compounds in OPs, catalysts are commonly employed. Catalysts play a critical role in enhancing the selectivity of desired bio-oil compounds during biomass conversion processes. Different catalysts are utilized to catalyze the formation of various compounds. For example, CuSO₄ promotes the formation of esters [27], Fe³⁺ contributes to the generation of furfural derivatives [28,29], HBeta zeolite facilitates the formation of aromatic hydrocarbons, and Pt/C aids in the decarboxylation of fatty acids [30]. In the case of steroids, acid catalysts enhance bond fracture (cracking), while sediments and montmorillonite contribute to aromatization [13].

3.5. Solid-Phase Products

Figure 5 presents the FTIR spectra of eluted solid residues, which include both the generated products (GPs) and inorganic components, at 200 °C. The main peak near 1000 cm⁻¹, along with additional peaks around 768 cm⁻¹, exhibit an increasing intensity with rising temperature. This trend suggests the continuous formation of phenolic compounds during the process. The main precursor of char is humus [26], which contains a large number of aromatic rings. Some of these rings are likely not ring-opened during the hydrothermal process, leading to the accumulation of phenolics in the char. Furthermore, the increase in the broad peaks around 3270 cm⁻¹, which are likely associated with O-H, C-H, and N-H stretching vibrations, coupled with the enhanced intensity of the C=O stretching vibration at 1629 cm⁻¹, indicates the presence of amide groups in the solid residues. This observation suggests that, at 200 °C, the sludge retains certain protein-like substances, as previously discussed in our earlier studies [16]. Moreover, it implies that the hydrolysis reaction has not been fully completed at this temperature. The FTIR spectra of eluted solid residues at various temperatures have been further elaborated in our previous works [16]. In summary, as the reaction time and temperature increase, the solid products predominantly undergo conversion into char, which is enriched with phenolic compounds.

3.6. Formation and Conversion of Steroids

Steroid compounds are typically synthesized within organisms. Figure 6 illustrates the biosynthetic pathway for the conversion of squalene to lanosterol in biological systems [31]. However, steroid compounds can also be synthesized outside of living organisms [32] or under high-temperature conditions [33]. Between 250 °C and 350 °C, the formation of steroid compounds continues to increase, indicating that long-chain olefins are capable of forming steroid compounds under subcritical water conditions. The steroid compounds generated during this process exhibited a relatively high degree of stability and resistance to further decomposition.

The possible reaction pathways for the formation and conversion of steroid compounds are proposed in Figure 7. Steroid compounds primarily occur in the oil-phase products, with a significant decrease in their concentration observed between 400 °C and 450 °C, indicating that steroids undergo degradation at these temperatures. In microbial degradation of steroid compounds, side-chain cleavage typically occurs first, followed by the cleavage of core rings into smaller molecular compounds, such as acids [34,35]. However, few intermediate products resulting from side-chain cleavage, ring opening, or small molecular acids were detected in the oil-phase products. This observation, coupled with the increased yields of gas-phase products (as shown in Figure 1) at higher temperatures, suggests that these intermediate products are highly unstable and decompose rapidly, generating gaseous compounds. Another possible decomposition pathway for steroids is the direct degradation of ketones at elevated temperatures [36,37]. Furthermore, steroid compounds can undergo rearrangement and dehydrogenation to form aromatic steroid hydrocarbons, which can subsequently lead to the formation of polycyclic aromatic hydrocarbons (PAHs) through the breakdown of the backbone [13,38]. These PAHs can further polymerize into char or tar [39]. However, the limited detection of PAHs suggests that they may rapidly polymerize into char, implying that steroid compounds may serve as precursors to char formation.

4. Conclusions and Prospects

In this manuscript, we explored in detail the compounds in bio-oil from DSS, especially steroids and their formation and conversion, which could help provide better technical support for bio-oil upgrading. This work contributes to the recovery of energy from sewage sludge and thus to the sustainable development of urbanization. Future studies should focus on the impact of steroids on the properties of bio-oils, investigate the catalytic effects on steroid conversion, and further analyze the solid products to develop a more comprehensive understanding of the underlying reaction mechanisms. The main conclusion is as follows.

During the hydrothermal conversion of DSS, an increase in temperature and reaction time led to further decomposition of the sludge, with a corresponding reduction in solid products, where phenolics were identified as the predominant components. Gas-phase products increased with both temperature and reaction time, with CO₂ being the primary constituent. Above 450 °C, a significant increase in H₂ concentration in the gas phase was observed. The water-soluble products primarily consisted of nitrogen-containing compounds. As temperature increased, saturated nitrogen heterocycles gradually converted into saturated azapolycyclic compounds, which subsequently transformed into unsaturated nitrogen aromatic heterocycles. In the oil-phase products, lipids initially underwent hydrolysis at 200 °C, forming long-chain hydrocarbons. Between 250 °C and 350 °C, these long-chain hydrocarbons gradually converted into steroid compounds, while nitrogen heterocycles also entered the oil phase. Between 400 °C and 450 °C, steroids degraded into smaller molecules and gases, or polymerized into char. Additionally, unsaturated aromatics, such as phenols, increased in concentration at this temperature range, likely due to the dehydrogenation of cyclic compounds. Steroids in the bio-oil appear to be primarily derived from long-chain olefins and may act as precursor compounds to char formation. For the reduction of steroids in oil-phase products, temperatures exceeding 400 °C are optimal.