Municipal Sludge Resource Recovery: Technologies, Challenges, and Future Directions

Abstract

Municipal sludge generation has increased rapidly with urbanization, creating significant challenges for sustainable waste management. This review proposes a system-oriented framework for sludge resource utilization by linking sludge characteristics, conversion technologies, environmental risks, and product applications. Major treatment pathways, including anaerobic digestion, pyrolysis, ozonation, and hydrothermal carbonization, are critically compared, with emphasis on their inherent trade-offs between resource recovery, energy consumption, and contaminant control. Particular attention is given to emerging contaminants, such as microplastics, per- and polyfluoroalkyl substances (PFAS), and antibiotic resistance genes, where the distinction between pollutant removal and actual risk reduction remains insufficiently addressed. The review highlights that no single technology can achieve optimal performance under all conditions, and integrated treatment trains are generally required for sustainable sludge management. Among these pathways, pyrolysis shows considerable potential for applications requiring enhanced contaminant control and value-added biochar production due to its ability to promote organic contaminant degradation, heavy metal immobilization, and carbon storage. However, the feasibility of pyrolysis and other technologies depends strongly on site-specific factors, including sludge properties, energy availability, economic conditions, and regulatory requirements. Future research should focus on integrated process optimization, comprehensive pollutant fate assessment, and standardized evaluation frameworks to advance sludge management toward a circular economy.

1. Introduction

Rapid urbanization, population growth, and the continuous expansion of municipal wastewater treatment have resulted in a substantial increase in sewage sludge production worldwide. According to industry reports, global sludge production is estimated at approximately 80–90 million metric tons on a dry solids basis in 2025, with the market size reaching approximately $6.8 billion [1]. China has become the world’s largest sludge producer, generating more than 60 million tons of municipal sludge (80% moisture content) annually, while the European Union and the United States produce approximately 10 million and 4.0–6.26 million dry tons per year, respectively [2]. With further improvements in wastewater treatment coverage and increasingly stringent discharge standards, global sludge production is expected to continue rising over the coming decades. The enormous quantity of sludge generated each year poses significant challenges to environmental protection, public health, resource management, and greenhouse gas mitigation, making sustainable sludge treatment an increasingly important global priority [3].

Municipal sewage sludge is a highly heterogeneous material composed of organic matter, nutrients, microorganisms, inorganic minerals, and various contaminants. It typically contains substantial amounts of organic matter (380–730 g·kg⁻¹), nitrogen (13.3–78 g·kg⁻¹), phosphorus (approximately 11.1 g·kg⁻¹), and potassium (approximately 7.9 g·kg⁻¹), making it a potentially valuable secondary resource for energy and nutrient recovery [4]. However, sludge also contains pathogens, heavy metals, and emerging contaminants, including microplastics, per- and polyfluoroalkyl substances (PFAS), pharmaceutical residues, and antibiotic resistance genes (ARGs) [5]. Inadequate treatment or improper disposal may lead to secondary environmental pollution through contaminant release, greenhouse gas emissions, and long-term ecological risks [6]. Consequently, municipal sludge exhibits a dual role as both an environmental liability requiring safe management and a valuable resource that can contribute to a circular economy and carbon-neutral development through appropriate valorization technologies [7].

Several comprehensive reviews have been published on various aspects of sludge management. Gusiatin et al. [8] reviewed municipal sewage sludge as a resource in the circular economy. Di Giacomo and Romano examined sludge management toward integrated biorefineries [9]. Jellali et al. [10] reviewed sludge-derived biochar synthesis conditions and pollutant removal efficiency. Recent advances in sludge pyrolysis have been systematically reviewed, highlighting co-pyrolysis as a key strategy for enhancing biochar pores and stabilizing heavy metals [11]. On phosphorus recovery, comprehensive summaries of extraction technologies have been published, with increasing attention to vivianite crystallization [12,13]. Recent advances in machine learning have opened new frontiers for optimizing sludge treatment [14,15].

While these existing reviews have substantially advanced the field, several critical gaps persist. First, most reviews focus on individual technologies in isolation (e.g., pyrolysis, anaerobic digestion, or ozonation) without systematically comparing their system-level trade-offs in terms of energy consumption, contaminant removal, and economic viability under comparable boundary conditions. Second, the emerging challenges posed by co-occurring micropollutants (microplastics, PFAS, and antibiotic resistance genes) are typically treated separately, whereas in real sludge matrices they may interact synergistically and fundamentally alter treatment performance and product safety. Third, the rapid proliferation of machine learning applications in sludge management, while promising, has not been critically assessed for practical scalability and data requirements. Fourth, the transition from laboratory-scale proof-of-concept to field-scale economic feasibility remains poorly characterized, particularly for advanced oxidation and thermochemical conversion routes.

The overall architecture of this review is structured around three interconnected pillars: feedstock characterization, conversion technologies, and end-use valorization, as conceptually illustrated in Figure 1. Unlike previous approaches that mainly evaluate individual treatment technologies or isolated unit operations, this framework emphasizes the interdependencies among sludge properties, treatment train selection, contaminant fate, energy balance, and final product quality. This integrated perspective highlights that sustainable sludge management requires coordinated optimization across the entire resource recovery chain rather than maximization of a single performance indicator. In this context, pyrolysis-based biorefinery platforms represent a promising pathway for applications requiring enhanced contaminant control, heavy metal immobilization, and carbon storage potential, while anaerobic digestion and ozonation may serve as complementary processes depending on specific treatment objectives and operational conditions. Furthermore, this review identifies three major challenges currently limiting practical implementation: (i) the over-generalization of laboratory-scale findings to diverse real-world sludge matrices; (ii) insufficient consideration of system-level trade-offs among resource recovery, environmental safety, and economic feasibility; and (iii) the persistent gap between pollutant removal from solid phases and actual environmental risk reduction.

Recent reviews have provided valuable insights into individual sludge treatment technologies, including anaerobic digestion, pyrolysis, ozonation, and hydrothermal carbonization, as well as specific resource recovery pathways. However, a comprehensive framework integrating sludge characteristics, conversion processes, environmental risks, and end-use applications remains insufficiently developed. To address this gap, this review was developed based on a comprehensive literature search of the Web of Science Core Collection and Scopus databases, covering publications from January 2015 to the latest available studies. The search strategy combined keywords related to sludge management (“sewage sludge”, “municipal sludge”, and “waste activated sludge”) with major conversion technologies (“anaerobic digestion”, “pyrolysis”, “ozonation”, and “hydrothermal carbonization”) and resource recovery pathways (“phosphorus recovery”, “biochar”, and “energy recovery”). Peer-reviewed journal articles were primarily considered, supplemented by selected technical reports and conference papers from recognized sources. Studies involving pilot- or full-scale demonstrations, comprehensive pollutant mass balances, and techno-economic assessments were particularly emphasized to improve practical relevance. Additional studies were identified through backward and forward citation tracking of key review articles. Through this integrated systems perspective, this review aims to establish a decision-oriented framework for evaluating and optimizing sludge management strategies under different operational, economic, and regulatory conditions.

2. Physicochemical Characteristics and Environmental Risks

2.1. Physical Properties

Municipal sludge possesses several unique physicochemical characteristics that fundamentally distinguish it from other solid wastes. One of its most prominent features is the exceptionally high moisture content. Primary sludge generally contains 95–97% water, whereas waste activated sludge may contain 98–99.5% [16]. Even after conventional mechanical dewatering, the moisture content typically remains between 70% and 85%, substantially increasing transportation, storage, and disposal costs. For thermochemical conversion processes such as pyrolysis, additional drying to below 20% moisture is generally required, depending on reactor configuration and product specifications [17]. Typical physical properties of municipal sludge are summarized in Table 1.

The poor dewaterability of sludge originates from its complex floc microstructure. As illustrated in Figure 2, sludge flocs are composed of microbial aggregates, extracellular polymeric substances (EPS), and inorganic particles. EPS forms a three-dimensional network that effectively retains bound water through capillary and physicochemical interactions while simultaneously providing binding sites for heavy metals and other contaminants. Consequently, sludge exhibits complex non-Newtonian rheological behavior, with activated sludge generally displaying considerably higher apparent viscosity than primary sludge owing to its higher EPS content and finer particle size. These physicochemical characteristics strongly influence sludge dewatering performance, biological degradation, thermal conversion efficiency, and contaminant migration, thereby playing a fundamental role in determining the effectiveness of subsequent sludge treatment and resource recovery technologies.

Table 1. Typical physical properties of municipal sludge [16,17].

Parameter	Primary Sludge	Waste Activated Sludge	Digested Sludge	Dewatered Sludge (80% Moisture)
Moisture content (%)	95–97	98–99.5	94–96	70–85
Particle size (μm)	50–200	10–50	10–100	—
Density (g/cm3)	1.01–1.03	1.02–1.05	1.03–1.06	1.2–1.6
Apparent viscosity (mPa·s)	10–100	50–1000	20–200	—

Table 1. Typical physical properties of municipal sludge [16,17].

Parameter	Primary Sludge	Waste Activated Sludge	Digested Sludge	Dewatered Sludge (80% Moisture)
Moisture content (%)	95–97	98–99.5	94–96	70–85
Particle size (μm)	50–200	10–50	10–100	—
Density (g/cm3)	1.01–1.03	1.02–1.05	1.03–1.06	1.2–1.6
Apparent viscosity (mPa·s)	10–100	50–1000	20–200	—

2.2. Chemical Composition

The organic matter content ranges from 380 to 730 g·kg⁻¹, comprising primarily proteins (20–35%), lipids (5–20%), carbohydrates (10–20%), and humic substances (15–30%) [5,18]. The elemental composition includes carbon (25–45%), hydrogen (3–8%), oxygen (15–30%), nitrogen (2–7%), and sulfur (0.5–2.5%). The C/N ratio of sludge generally ranges from 5 to 15, necessitating co-digestion with carbon-rich substrates for optimal anaerobic digestion [19]. The dry-basis calorific value ranges from 8 to 15 MJ/kg.

Figure 2. Morphological characteristics of sludge flocs.

Figure 2. Morphological characteristics of sludge flocs.

The inorganic fraction contains SiO₂ (15–40%), CaO (5–25%), Al₂O₃ (3–15%), MgO (1–5%), and Fe₂O₃ (3–10%) [20]. A nationwide review of heavy metals in Chinese municipal sludge revealed the descending order Zn > Mn > Cu > Cr > Pb > Ni > As > Cd > Hg [21,22]. Substantial geographic variability in sludge composition is revealed in

Table 2. Chemical composition characteristics of municipal sludge from different regions (dry basis) [19,23].

Region	Organic Matter (g·kg−1)	Total N (g·kg−1)	Total P (g·kg−1)	Total K (g·kg−1)	SiO2 (%)	CaO (%)
China (national average)	350–550	15–45	8–18	5–15	25–35	10–20
Europe (average)	450–650	25–60	10–20	4–8	20–40	12–22
USA (average)	400–600	20–50	8–16	3–6	22–38	10–18
Poland	380–730	13.3–78	11.1	7.9	—	—

. Notably, organic matter in European sludge (450–650 g·kg⁻¹) exceeds that in Chinese sludge (350–550 g·kg⁻¹), while total phosphorus in Polish sludge reaches 11.1 g·kg⁻¹, values that directly affect the economic viability of phosphorus recovery (discussed in Section 4.3). More critically, the C/N ratios calculated from Table 2 (typically 5–15) fall below the optimal range (20–30) for anaerobic digestion, mandating co-digestion with carbon-rich wastes such as food waste or agricultural residues, a strategy whose effectiveness we discuss further in Section 3.2. The high SiO₂ and CaO contents (25–35% and 10–20%, respectively) also imply that sludge ash has inherent pozzolanic activity, making it suitable for cement replacement, but these same inorganic constituents accelerate fouling and slagging during pyrolysis and incineration, imposing stringent temperature control as elaborated in Section 3.3 [23].

2.3. Emerging Contaminants

Microplastics have emerged as a critical concern. Sewage sludge has been identified as a significant sink for microplastics (MPs), with wastewater treatment plants retaining substantial quantities during treatment. Reported microplastic concentrations in sewage sludge vary considerably worldwide. A 2025 study from Turkey reported MP concentrations of 6078 ± 695 MP/g (dry weight) in sludge from urban WWTPs, 4726 ± 2939 MP/g (dry weight) from industrial WWTPs, and 2133 ± 1690 MP/g (dry weight) from drinking water treatment plants [24]. For composted sewage sludge, the microplastic fiber concentration averaged 278 MP/g of dry compost, with polyester being the dominant polymer type [25]. A systematic review found that reported microplastic concentrations varied considerably from 0.193 to 1.69 × 10⁵ microplastics/g. This exceptionally wide range (spanning six orders of magnitude) is largely attributed to methodological disparities, including differences in sampling protocols, sample pretreatment (e.g., digestion methods), and analytical techniques (e.g., visual identification, FTIR, or Raman spectroscopy). The lack of standardized analytical methods for microplastics in complex organic matrices like sludge remains a significant barrier to data comparison and risk assessment. Consequently, the current concentration data are unsuitable for regulatory standard-setting or quantitative risk assessment, a critical limitation that is rarely acknowledged in studies reporting single-point measurements. The presence of synthetic polymers inhibits key enzymatic activities in anaerobic digestion, disrupts microbial communities, and reduces methane yields [26]. Anaerobic digestion does not consistently remove microplastic particles; MPs frequently persist and, at elevated or sensitive loadings, have been shown to affect methane production, microbial communities and sludge quality [27].

Recent studies have revealed polymer-specific effects of MPs on anaerobic digestion performance. A systematic evaluation was conducted to assess the dose-dependent impacts of PP and PE MPs on sludge anaerobic digestion. Specifically, high-dose PE-MPs (300 particles/g TSS) suppressed methane yield by 24.0% and reduced VFA consumption by 3.8%. In contrast, equivalent concentrations of PP-MPs enhanced methane production by 10.8% and accelerated VFA consumption by 25.5%. These contrasting outcomes were attributed to the proliferation of key functional microbial groups, particularly acetogens and methanogens, under PP-MPs exposure, whereas these populations were inhibited in PE-MPs treatments. Moreover, PE-MPs induced a pronounced and dose-dependent enrichment of antibiotic resistance genes (ARGs), with the abundance of the class 1 integron integrase gene (increasing by up to 1990.8%). Compared to PP-MPs, PE-MPs expanded the host range of ARGs by 50.0% and significantly amplified the abundance of human pathogenic bacteria, such as Clostridium, by an average of 428.0%. Consequently, the presence of PE-MPs presents a considerably higher risk profile than PP-MPs, primarily through its marked promotion of horizontal gene transfer and substantial enrichment of pathogenic bacteria [28].

Per- and polyfluoroalkyl substances (PFAS) represent another class of emerging contaminants. Sewage sludge is a major reservoir and secondary pollution source of PFASs [28]. PFAS have been detected in sludge and biosolids across all inhabited continents [29,30]. Global monitoring reveals stark regional disparities: U.S. sludge shows ∑PFAS up to 3390 ng/g (dry weight) with dominant long-chain PFOS/PFOA, while Nordic countries exhibit lower levels (PFOS < 10 ng/g) owing to early regulations [31,32]. PFOS is the most widely detected compound in biosolids, whereas the concentration of 6:2 FTCA reached up to 22,000 ng/g (dry weight) in an industrial wastewater sludge [33]. USEPA risk assessment reveals that land application of sewage sludge containing 1 ppb PFOA or PFOS presents human health risks beyond acceptable thresholds [30]. Pyrolysis emerged generally as the most efficient removal method for PFAS, with removal efficiencies of ≥96.9% [30]. Pyrolysis at 700 °C achieved 99% PFAS removal for anaerobically digested biosolids [30]. Hydrothermal carbonization (HTC) of sewage sludge achieved over 94% removal of C5–C10 perfluorocarboxylates [34,35]. Antibiotic resistance genes (ARGs) in sewage sludge pose significant public health challenges. Hyperthermophilic composting achieves efficient removal of antibiotics (100%) and ARGs (98.8%) [36]. Biochar addition during sludge composting promotes the removal of both intracellular and extracellular ARGs by reshaping microbial communities [37].

The co-occurrence of emerging contaminants, microplastics, PFAS, ARGs, and trace organics in sludge matrices creates a risk profile fundamentally different from that of any individual contaminant. The six-order-of-magnitude variation in reported microplastic concentrations (0.193 to 1.69 × 10⁵ MP/g) renders current data unsuitable for regulatory standard setting. Thermochemical processes (pyrolysis >500 °C) degrade most organic pollutants, but the transformation products, particularly volatile PFAS intermediates, remain largely uncharacterized. A critical gap exists between reported “removal efficiencies” (disappearance from the solid phase) and true risk reduction, which requires knowledge of degradation pathways and product toxicity.

2.4. Pathogenic Microorganisms

Municipal sludge harbors a diverse range of pathogenic microorganisms, including bacteria, viruses, protozoa, and helminths, which pose significant risks to public health and environmental safety if inadequately treated. Raw sludge typically contains fecal coliform bacteria at densities of 10⁶–10⁸ CFU g⁻¹ together with viable helminth eggs at concentrations of 10²–10⁴ eggs kg⁻¹ [38]. Consequently, effective pathogen inactivation is a prerequisite for sludge recycling, particularly for agricultural land application. Biological stabilization, alkaline treatment, thermal drying, anaerobic digestion, and thermochemical conversion have all been demonstrated to substantially reduce pathogen viability [39].

3. Current State of Research on Sludge Treatment Technologies

3.1. Classification and Overview

Municipal sludge can be managed through biological (e.g., anaerobic digestion), thermochemical (e.g., pyrolysis, hydrothermal carbonization), and chemical (e.g., advanced oxidation processes) routes, with landfill disposal serving as the primary destination for non-recoverable residues. Among these, three routes have emerged as the most promising for sustainable management: anaerobic digestion, pyrolysis-carbonization with thermal activation, and ozonation-based advanced oxidation [9]. The annual generation of sewage sludge in China has surged substantially, exceeding 60 million tons (at 80% moisture, equivalent to 12 million tons on a dry solids basis) in 2023, according to the Ministry of Housing and Urban-Rural Development, and is projected to exceed 100 million tons (at 80% moisture, i.e., >20 million tons dry solids) by 2025, highlighting the urgent need for efficient treatment technologies. The following sections critically examine these three routes with emphasis on their performance, limitations, and application contexts.

3.2. Anaerobic Digestion

Mesophilic AD (35–37 °C) offers greater stability, while thermophilic AD (55 °C) provides higher reaction rates and superior pathogen reduction. AD performance is fundamentally constrained by the slow hydrolysis of particulate organic matter, which has motivated extensive research into pretreatment strategies. Pretreatment technologies: Various pretreatments have been categorized into physical (ultrasonic, microwave, and thermal hydrolysis), chemical (acid, alkali, Fenton, calcium peroxide, and ozone), and biological groups [40,41,42]. Thermal hydrolysis pretreatment (THP) at 160–180 °C improves biogas yields by 30–50%. Recent research has explored radio frequency technology as an emerging pretreatment method for waste activated sludge [43,44].

Conductive materials for enhanced AD: Conductive materials such as biochar, magnetite, and activated carbon facilitate direct interspecies electron transfer (DIET) between syntrophic bacteria and methanogens [45,46,47]. A novel hybrid conductive material, namely nitrogen-doped biochar supported magnetite (Fe₃O₄@N-BC), was synthesized and applied in continuously operated up-flow anaerobic sludge blanket reactors. The addition of Fe₃O₄@N-BC increased the methane production rate by 25%, higher than that of Fe₃O₄ (19%) and biochar (13%). Chryseobacterium and Methanothrix were enriched as key bacteria and archaea responsible for hydrolysis and DIET in methanogenesis, respectively. Fe₃O₄@N-BC enhanced hydrolase activities and the construction of e-pili networks for electron transfer. Metagenomic data further revealed that electrons were efficiently transferred for CO₂-to-CH₄ conversion [43,48].

Sludge-derived biochar prepared at 500 °C (BC500) demonstrated excellent capacitance (0.31 mF/g) and electrical conductivity (3.1 × 10⁻⁴ S/m), promoting the enrichment of acetotrophic methanogens, Methanosaeta, from 24.0% to 32.3–34.0%. The maximum CH₄ production for the control, BC300, BC500, and BC700 groups was 326.8 mL/g VS, 401.8 mL/g VS, 423.8 mL/g VS, and 420 mL/g VS, respectively. BC500 promoted 29.7% higher maximum CH₄ production than the control group. These findings suggest that DIET was preferentially stimulated to promote methanogenesis during WAS AD, replacing the thermodynamically unfavorable hydrogen-mediated interspecies electron transfer, providing a theoretical foundation for in situ enhanced AD via sludge-derived biochar and proposing a closed-loop “treating waste with waste” strategy [49]. Magnetic biochar (MBC), as a composite conductive material, is capable of enhancing methane yield and production rate because of its favorable characteristics. Interestingly, recent research has highlighted the potential of sand as an alternative to conductive materials for biomethane production during AD, suggesting that the conductive paradigm may be broader than previously thought [50,51].

Machine learning optimization: Recent machine learning applications have demonstrated promising predictive performance for methane yield and biogas production under laboratory conditions, with reported R² values generally exceeding 0.85 [52,53,54,55]. However, critical scrutiny reveals that nearly all studies use single-facility datasets with strong autocorrelation; cross-facility validation, the true test of generalizability, is almost never performed. The fundamental barrier is data-related: most full-scale WWTPs lack the validated, continuous, multi-parameter monitoring required to train transferable models. Furthermore, the “black-box” nature of ensemble methods obscures mechanistic understanding that engineers require for confident decision-making. Hybrid approaches that embed domain knowledge to constrain predictions within physically plausible bounds offer a more promising direction than pure data-driven models [56,57]. Until open-access, multi-facility datasets become available, machine learning will remain primarily a laboratory tool rather than an engineering solution.

3.3. Pyrolysis-Carbonization and Thermal Activation Technology

Biochar can be used as a stable carbon sink, soil amendment, and functional adsorbent, while syngas can be recycled as a fuel to supply process heat, thereby improving the overall energy efficiency of the pyrolysis system. Although bio-oil possesses a relatively high heating value (20–35 MJ kg⁻¹), its high oxygen and moisture contents generally require further upgrading before practical utilization [11]. The three-phase product distribution of sludge pyrolysis is schematically illustrated in Figure 3.

The distribution and properties of pyrolysis products are strongly governed by operating conditions, particularly temperature, heating rate, and residence time. Among these parameters, temperature exerts the greatest influence on product yield and quality. Lower temperatures (approximately 400 °C) generally favor biochar production but produce materials with relatively low specific surface area and limited pore development. In contrast, higher temperatures (>600 °C) promote secondary cracking reactions, increasing syngas production and enhancing biochar porosity while reducing solid yield. Therefore, optimization of the pyrolysis process requires balancing carbon sequestration, energy recovery, and biochar performance according to the intended application [58,59].

Figure 3. Schematic diagram of the pyrolysis process and three-phase product fractions.

Figure 3. Schematic diagram of the pyrolysis process and three-phase product fractions.

3.3.1. Thermal Activation for Sludge-Based Activated Carbon Production

Beyond conventional biochar production, pyrolyzed sludge can be further upgraded through activation to produce sludge-derived activated carbon (SAC) with enhanced ad-sorption performance. The production of SAC generally involves three sequential steps: drying, carbonization, and activation. During physical activation, steam or carbon dioxide at elevated temperatures (typically around 800 °C) reacts with the carbonized sludge, en-larging existing pores and generating abundant micropores and mesopores. Consequently, the activated carbon exhibits significantly improved surface area and adsorption capacity, with iodine values typically ranging from 320 to 420 mg g⁻¹ [60,61].

SAC can be prepared through both physical and chemical activation. Chemical activation using alkaline reagents (e.g., NaOH and KOH) or salt-based activating agents (e.g., ZnCl₂), followed by pyrolysis at 500–700 °C, has been widely reported to further enhance pore development and adsorption performance [62,63]. More recently, microwave-assisted one-step conversion has emerged as a promising approach for simultaneously achieving carbonization and activation, providing a simpler and more energy-efficient route for producing multifunctional magnetic activated carbon from sewage sludge [64]. Representative sludge-based activated carbon production projects are summarized in

Table 3. Sludge-based activated carbon production performance from different projects [60,61].

Project Location	Feedstock	Processing Capacity	Iodine Value (mg/g)	Product Application
Wuxi, China	Municipal sludge (80% moisture)	35 t/d	−320	Flue gas adsorption, soil/water treatment
Wuxi, China (Blue algae)	Blue algae (97% moisture)	50 t/d	−800	Adsorption materials
Xuancheng, China	Municipal sludge (65% moisture)	70 t/d	−500	Adsorption materials
Yidu, China	Municipal sludge + food waste	43,000 t/a	320–420	Multi-purpose adsorption

.

3.3.2. Effect of Pyrolysis Parameters on Biochar Properties

The physicochemical properties of sludge-derived biochar depend critically on pyrolysis parameters, with temperature and residence time identified as the most critical factors affecting biochar quality. Systematic investigation revealed that higher pyrolysis temperatures (>500 °C) increased biochar pH to alkaline values (>10), ash content, and nutrient concentrations. Temperatures above 600 °C significantly increased biochar surface area and yielded H/C ratios below 0.57, improving suitability for soil remediation. When the pyrolysis temperature was 600 °C and the pyrolysis time was 60 min, an excellent pore structure was obtained for CO₂ adsorption applications [57,65]. The opposing trends of biochar yield and surface area with increasing temperature are quantitatively depicted in Figure 4. As the temperature rises from 300 °C to 900 °C, the biochar yield plummets from 60% to 22%, a loss of nearly two-thirds, while the BET surface area soars from 5 to 115 m²/g. This inverse relationship forces a critical engineering compromise: maximizing adsorption capacity demands high-temperature processing, but the accompanying mass loss drastically reduces the product output per ton of dry sludge. However, this trade-off is justified when considering heavy metal immobilization. As detailed in

Table 4. Performance comparison of sludge pyrolysis under various conditions from the literature.

Feedstock	Temp. (°C)	Residence Time	Biochar Yield (%)	BET Surface Area (m2/g)	Heavy Metal Immobilization	Reference
Municipal sludge	300	60 min	−58	−5	Cu residual: −15%; PERI: >1000	[23]
Municipal sludge	500	N/A	−42	−25	Cd removal: −70%; Pb residual: −45%	[56]
Municipal sludge	600	60 min	−32	−68	Cu residual: −75%; PERI: <50	[60]
Municipal sludge	700	N/A	−26	−95	Cd removal: >90%; Pb residual: −55%	[66]
Municipal sludge	900	60 min	−22	−115	Cu residual: −95%; PERI: <50	[23]
Sewage sludge	300	N/A	−55	−5	F1+F2 fraction: >40%	[63]
Sewage sludge	500	N/A	−40	−50	F1+F2 fraction: <15%	[69]
Sewage sludge	700	N/A	−25	−100	F1+F2 fraction: <5%	[63]
Municipal sludge + wheat straw	600	30 min	−38	−95	Zn/Cu/Pb residual: >75%	[65]
Municipal sludge + sawdust	600	30 min	−36	−100	Zn/Cu/Pb residual: >78%	[65]
Municipal sludge + oak bark	600	60 min	−35	−80	Metal leaching reduced >80%	[67]
Municipal sludge + hemp hurd	600	60 min	−33	−85	Metal leaching reduced >75%	[67]
Municipal sludge + corn stover	600	60 min	−30	−90	Zn/Cu/Pb residual >80%	[70]
Municipal sludge (biosolids)	600	N/A	−30	−70	PFAS removal >99%; organic pollutants eliminated	[71]
Municipal sludge + sugarcane residue	600	N/A	−32	−88	Heavy metal risk significantly reduced	[72]
Sewage sludge (molten salt)	600	60 min	−30	−70	RI: 33.6 (vs. >1000 for raw)	[67]
Paper mill sludge (microwave)	500	N/A	−45	−180	Magnetic AC; pharmaceuticals removal	[58]

Notes: F1 = exchangeable fraction; F2 = reducible fraction; PERI = potential ecological risk index; RI = risk index. N/A = not available in the cited reference. Biochar yield and BET values are approximate ranges derived from the cited literature; exact values vary with sludge composition and experimental conditions. Co-pyrolysis with biomass generally improves biochar porosity and heavy metal stabilization compared to sludge-only pyrolysis.

, the residual fraction of copper increases from a mere 15% at 300 °C to 95% at 900 °C. Parallel trends for Zn, Pb, and other metals (though not tabulated here) lead to an overall potential ecological risk index (PERI) plummeting from 1158 (extremely high risk) in raw sludge to below 50 (low risk) in biochar produced above 600 °C. In other words, the data in Figure 4 and Table 4 collectively demonstrate that the “cost” of lower yield at higher temperatures is repaid with a “benefit” of safer and more adsorptive biochar, a finding that directly supports the recommendation in Section 5.2 to prioritize co-pyrolysis and thermal activation for value-added products rather than relying on low-temperature char for bulk land application. Most heavy metals were retained in the residual fraction and became further immobilized during pyrolysis, thereby lowering bioavailability, although Pb and Ni showed increased mobility at 600–700 °C [58,66]. Critically, the temperature dependence revealed in Figure 4 and Table 4 defines a narrow Pareto-optimal window: below 500 °C, immobilization is insufficient (Cu residual fraction <60%, PERI > 500); above 700 °C, biochar yield plummets to <25% with only marginal immobilization gain. This forces an engineering compromise where 600–650 °C emerges as the practical optimum for most municipal sludges, achieving >75% metal stabilization while retaining 28–32% solid yield [67]. However, this generalization is feedstock-dependent; sludges with high chloride content may volatilize Pb and Ni at 600–700 °C, requiring pre-washing or co-pyrolysis with Ca/Mg-rich biomass [56,68].

Molten salt pyrolysis: Recent research has explored pyrolysis of sewage sludge in molten salt environments to enhance conversion efficiency and biochar quality. Both molten carbonate and molten chloride systems enhanced heavy metal immobilization [67]. The MC system facilitated the transformation of all heavy metals into more stable speciation (F3, F4), benefiting from features such as an alkaline environment, enhanced biochar pores, and silicate formation. Overall, molten salt pyrolysis reduced the potential ecological risk of heavy metals in SS biochar, with risk index (RI) values for MC-derived biochars dropping to 33.6 and 52.6.

Heavy metal immobilization: Metal analysis confirmed the immobilization of heavy metals in biochar, significantly reducing the environmental risk from high (PERI = 1158) in the sewage sludge feedstock to low (PERI < 50) in biochar obtained at temperatures above 600 °C. Pyrolysis also facilitated volatilization of heavy metals, particularly As and Cd, which were reduced to safe levels, with Cd removal exceeding 90% at 700 °C [23]. Biochar produced through slow pyrolysis at 300 °C and 500 °C from sewage sludge does not significantly increase the bioavailability of potentially harmful trace elements, such as Cd, Cr, Ni, and Pb, in soil over a five-year field-testing period [71]. Sludge pyrolysis could be an effective treatment technique by producing valuable and safe-to-apply products (biochar); specifically, biochar leached up to 15 times fewer heavy metals than the unpyrolyzed sludge samples [63,69].

Both incineration and pyrolysis effectively stabilized heavy metals in sewage sludge, with heavy metals in the by-products remaining stable in the long term after weathering [73]. During pyrolysis above 500 °C, heavy metals are progressively converted to stable forms [23] through: (i) incorporation into the aromatic carbon matrix; (ii) formation of insoluble metal phosphates; (iii) formation of metal silicates; (iv) reduction followed by encapsulation in the carbon matrix. The sequential evolution of heavy metal stabilization with increasing pyrolysis temperature is mechanistically conceptualized in Figure 5. At 300–400 °C, metals remain largely in exchangeable or carbonate-bound fractions; between 500 and 600 °C, decomposition of organic ligands releases metals, which then react with phosphate to form insoluble metal phosphates; above 700 °C, molten silicates encapsulate metal particles, while simultaneous carbothermal reduction converts mobile metal oxides to zero-valent species that become physically trapped within the aromatic carbon sheets. This mechanism-based visualization (Figure 5) explains why the PERI reduction is not a linear function of temperature but exhibits a sharp inflection point around 600 °C, corresponding to the onset of silicate vitrification. Crucially, this mechanistic insight informs our later recommendation that co-pyrolysis with Ca- or Mg-rich biomass can lower the required threshold temperature by providing exogenous silicates, thereby achieving the same immobilization effect at 500 °C and saving significant energy [74,75].

3.3.3. Co-Pyrolysis with Biomass

Co-pyrolysis has emerged as an effective strategy for improving the performance of sludge pyrolysis by utilizing complementary feedstocks such as lignocellulosic biomass, gypsum, and agricultural residues. Compared with sludge pyrolysis alone, co-pyrolysis not only improves biochar quality but also reduces the environmental risks associated with heavy metals [70].

The underlying mechanisms are illustrated in Figure 6. Biomass decomposition releases volatile compounds that generate reducing gases (e.g., H₂ and CO), which promote secondary cracking reactions and facilitate pore development in the carbon matrix, thereby enhancing the adsorption performance of sludge-derived biochar (Figure 6a). In addition, the higher volatile matter content of biomass supplies supplementary combustible gases, improving the energy self-sufficiency of the pyrolysis process and reducing external heat demand. Furthermore, interactions between sludge minerals and biomass ash promote the transformation of heavy metals into more stable residual fractions, thereby decreasing their mobility and bioavailability (Figure 6b). Overall, co-pyrolysis simultaneously improves product quality, energy efficiency, and environmental safety, making it a promising strategy for sustainable sludge valorization. However, the reported benefits are non-linear with blend ratio and feedstock type; an optimal ratio exists (typically 20–30% biomass), beyond which antagonistic effects may degrade performance, yet consensus on this optimum remains elusive due to wide variation in experimental conditions.

3.3.4. Pyrolysis for Emerging Contaminant Removal

Pyrolysis has emerged as the most efficient removal method for emerging contaminants in sewage sludge. Pyrolysis resulted in a ≥96.9% removal of PFAS, with residual PFAS (0.1–3.4 ng·g⁻¹) detected in biochars obtained at temperatures up to 750 °C, dominated by long-chain PFAS [30]. Whether this represents true mineralization (conversion to F⁻) or merely partitioning to syngas and bio-oil phases remains unclear, as fluorine mass balances across all product fractions are rarely reported. Biosolids pyrolysis incorporating autothermal drying and pyrolysis with after-combustion of all pyrolysis gas achieves over 95% PFAS removal, with PFAS in the biochar reduced by 99.9%, leaving no detectable residues in the resulting biochar or flue gas. Pharmaceuticals are also eliminated at rates exceeding 97%, alongside LAS, nonylphenols, DEHP, and microplastics [75,76]. Overall, the most recalcitrant pollutants in sludge were PFAS, certain antibiotics, and estrogens [77]. Amendment with 1% sludge biochar reduced the level of leaching of perfluorooctanesulfonate (PFOS) from contaminated soil by up to 92–99%, with notably better effectiveness for long-chain PFAS (92–99%) than for short-chain C4-C5 PFAS (40–70%) [78].

To provide a comprehensive quantitative overview of pyrolysis performance under diverse conditions, Table 4 summarizes key literature data on biochar yield, BET surface area, and heavy metal immobilization compiled from 12 independent studies across a range of pyrolysis temperatures, feedstocks, and residence times. The data consistently demonstrate that temperatures above 600 °C are generally required to achieve both high specific surface area (>60 m²/g) and effective heavy metal stabilization (PERI < 50), while co-pyrolysis with biomass further enhances biochar porosity and reduces metal leaching. These literature findings strongly support the earlier recommendation that 600–650 °C represents a practical Pareto-optimal temperature window for most municipal sludges, balancing product quality, contaminant safety, and process economics.

3.4. Ozonation for Sludge Reduction and Dewatering Enhancement

The destructive action of ozone on activated sludge follows a distinct three-stage mechanism, quantitatively delineated in

Table 5. Three-stage mechanism of ozone action on activated sludge [79,80].

Stage	Ozone Dosage (mg O3/g MLSS)	Primary Effects	Key Observations
Low dosage	<11	EPS disruption, floc disintegration	Limited cell destruction
Medium dosage	11–90	Cell lysis, organic matter solubilization	Most effective for sludge reduction
High dosage	>90	Mineralization of released organics	Diminishing returns, over-oxidation

and conceptually illustrated in Figure 7. At low dosages (<11 mg O₃/g MLSS), ozone preferentially attacks extracellular polymeric substances (EPS), causing floc disintegration and release of bound water; this explains the initial improvement in dewaterability observed in some studies. At medium dosages (11–90 mg O₃/g MLSS), the ozone penetrates the cell membranes, inducing lysis and solubilization of intracellular organic matter, which is the primary contributor to sludge mass reduction (up to 90%). Beyond 90 mg O₃/g MLSS, further ozonation merely mineralizes the released organics to CO₂ and H₂O, yielding diminishing returns in mass reduction while potentially impairing dewaterability due to over-oxidation that generates colloidal fine particles. The data in Table 5 provide a critical operational guideline: the optimal economic dosage window is constrained to the medium range (11–90 mg), because exceeding this threshold incurs additional energy costs without proportional reduction benefits, a trade-off that we revisit in the techno-economic analysis.

3.4.1. Microbubble and Ultrafine Bubble Ozonation

Conventional ozonation suffers from relatively low ozone transfer efficiency. Microbubble (MB) and ultrafine bubble (UFB) technologies have been developed to enhance ozone utilization [80]. The ability of ultrafine bubble/microbubble ozonation to enhance the disintegration and solubilization of sewage sludge for methane fermentation has been investigated. Waste activated sludge (WAS) and digested sludge were ozonated and subsequently used as feed sludges to promote sludge reduction and biogas production. During the methane fermentation of WAS, the rate of sludge reduction based on the total chemical oxygen demand increased to 75% from 40% in untreated sludge, and biogas production of 0.558 NL per gram of volatile solids (VS) added was achieved [81].

Beyond the enhanced mass transfer effects, the collapse of microbubbles generates localized hot spots with transient temperatures reaching several thousand Kelvin and pressures exceeding 100 atm [80]. These extreme conditions, albeit short-lived (microseconds), contribute to the thermal degradation of organic pollutants and enhance the disruption of microbial cell walls. The combination of localized thermal effects and oxidative radical attack makes microbubble ozonation particularly effective for recalcitrant pollutant removal. However, the energy dissipated as heat rather than chemical oxidation represents an efficiency trade-off; the optimal bubble size must balance mass transfer enhancement against energy losses to thermal effects.

Microbubble ozonation (O₃/MB) has been applied to activated sludge flocs to assess its effects on particle structure and removal efficiency. The strong oxidative action and flotation from O₃/MB disrupted the sludge floc matrix and aided in separating floc particles from the liquid phase. After 60 min of treatment, the mixed liquor suspended solids (MLSS) removal efficiency reached 95.1% [82]. The median particle size (D50) decreased from 150 µm to 17 µm, and the sludge particle density rose by a factor of 6.66, indicating near-complete floc breakdown. Surface examinations revealed porous etching and disintegration, accompanied by reduced intensities of functional groups related to polysaccharides, proteins, and aromatic structures. Simultaneously, the sludge zeta potential increased from −31.78 mV to −27.58 mV, reflecting a diminished negative surface charge, and the sludge moisture content also declined. During O₃/MB treatment, organic compounds such as extracellular polymeric substances (EPS) were released into the supernatant and subsequently degraded. As a result, concentrations of soluble chemical oxygen demand (SCOD), proteins (PN), and polysaccharides (PS) in the supernatant first rose and then diminished with time, while the oxidative degradation of organic nitrogen elevated the ammonia nitrogen concentration. The substantial size reduction in sludge flocs under microbubble ozonation is visually evident in Figure 8, where the median particle diameter (D50) shrinks from 150 μm to 17 μm after 60 min of treatment, a nearly 90% reduction. This fragmentation is not merely a physical phenomenon; it has substantial implications for ozone utilization efficiency. As compared in

Table 6. Ozone dosage requirements for different bubble generation methods [70,75,80].

Bubble Generation Method	Bubble Diameter	Ozone Dosage for 80% Bacterial Death (mg O3/g MLSS)	Relative Ozone Requirement
Conventional diffuser	−2.0 mm	45	100% (baseline)
Microbubble generator (jet)	−35 μm	25	56%
Ultrafine bubble generator	−120 nm	15	33%

, achieving 80% bacterial inactivation requires 45 mg O₃/g MLSS with conventional coarse bubbles, but only 25 mg with microbubbles and a mere 15 mg with ultrafine bubbles (UFB). This 67% reduction in ozone demand (from 45 to 15 mg/g) is directly attributable to the mechanisms captured in Figure 8: smaller bubbles rise more slowly, have higher internal pressure, and generate locally high shear forces upon collapse, which not only enhance mass transfer but also physically fragment flocs, exposing encapsulated bacteria to direct oxidant attack. Based on the reduced ozone dosage requirements (Table 6), UFB ozonation has been estimated to substantially reduce the specific energy consumption compared to conventional ozonation, with reported values in the range of 0.4–0.8 kWh per kg of solids reduced versus approximately 1.2 kWh/kg for conventional diffuser systems [80,81]. However, these estimates must be interpreted cautiously, as bubble generation itself consumes 0.5–1.0 kWh/m³, which must be included in the net energy balance. The actual energy savings are, therefore, process- and scale-dependent.

Hashimoto et al. [75] further confirmed that UFB ozonation effectively kills bacteria inside flocs, with floc fragmentation by shear forces making a significant contribution. Ultrafine bubble/microbubble ozonation also reduced sludge viscosity, although dewaterability deteriorated at high ozone dosages [81].

3.4.2. Synergistic Ozone-Based Processes

Ozone with Calcium Peroxide (CaO₂/O₃): Under optimal conditions (pH = 9, CaO₂ = 1 g/L, O₃ = 2 g/h, 60 min), sludge water content decreased from 85.67% to 46.54%, representing a 39.13 percentage-point reduction, and specific resistance to filtration was reduced by approximately 40% [83]. While ozone alone is effective, its combination with catalysts or co-oxidants yields markedly superior dewatering outcomes, as systematically summarized in Table 8 and mechanistically rationalized in Figure 9 (showing the CaO₂/O₃ synergistic pathway). The O₃/peroxymonosulfate (PMS) system achieves a 62% reduction in capillary suction time (from 70.5 s to 26.7 s) and lowers the final cake moisture from 81.93% to 65.65%, improvements that are unattainable by either oxidant individually. This synergy arises from the simultaneous generation of hydroxyl radicals and sulfate radicals, which attack both polysaccharides and proteins in EPS with different regioselectivities, leading to near-complete disruption of the hydrophilic gel network. Even more remarkable is the CaO₂/O₃ system: under optimal conditions (pH = 9, 1 g/L CaO₂, 2 g/h O₃, 60 min), it reduces the sludge moisture content from 85.67% to a substantial 46.54% (Table 8). The catalytic mechanism in Figure 9 demonstrates that CaO₂ releases H₂O₂ in a controlled manner, which then reacts with O₃ to generate highly reactive ·OH, while the alkaline environment simultaneously neutralizes the negative charges on sludge particles, promoting floc re-agglomeration after oxidation, a dual action that explains why this particular combination outperforms all others in dewatering efficiency. This substantial dewatering enhancement directly translates to downstream savings, as every 10% reduction in moisture content cuts thermal drying energy by approximately 15–20%.

Ozone with Chitosan: A combination of 60 mg/g TS ozone pre-oxidation and 20 mg/g TS chitosan reflocculation reduced sludge cake water content to below 60%, with calorific value increasing from 9366 kJ/kg to 10,670 kJ/kg. Similarly, the substantial 67% reduction in ozone requirement reported for ultrafine bubble ozonation comes with high energy consumption for bubble generation (0.5–1.0 kWh/m³), which must be included in the net energy balance. Chemical costs for synergistic systems (O₃/PMS or CaO₂/O₃) may offset disposal savings unless local tipping fees exceed $80/t. The economic viability of ozone-based processes, therefore, depends critically on local disposal costs, a factor frequently omitted in laboratory-focused studies.

While ozonation is effective for sludge reduction, it can generate oxidation by-products of toxicological concern, notably bromate (BrO₃⁻) and N-nitrosodimethylamine (NDMA), both probable carcinogens with stringent regulatory limits (10 μg/L and 10 ng/L, respectively, in some jurisdictions). Bromate forms from bromide oxidation, while NDMA arises from nitrogen-containing precursors. Both are dose-dependent and matrix-sensitive; at moderate ozone doses typical for sludge treatment (0.5–0.8 g O₃/g DS), they generally remain below critical thresholds, but elevated concentrations have been reported in bromide-rich wastewaters. Mitigation strategies, including H₂O₂ addition, catalytic ozonation, and sequential dosing, exist but introduce trade-offs: O₃/H₂O₂ suppresses bromate but is less effective for NDMA control. Critically, most available data derive from drinking water or effluent polishing contexts, where the matrix is substantially different from return activated sludge with high suspended solids and radical scavenging capacity. Whether bromate and NDMA formation in sludge ozonation poses a comparable risk is poorly characterized, representing a knowledge gap that deserves attention, particularly where ozonated sludge liquor is recycled to the mainstream WWTP or discharged to sensitive water bodies [84,85,86,87].

3.4.3. Full-Scale Applications and Economics

The feasibility of ozonation for sludge reduction has been demonstrated at full scale. Chiavola et al. applied ozone oxidation to 30% of return activated sludge in a full-scale WWTP [80]: sludge sent to final disposal decreased by 50% at 0.55 kg O₃/h and 75% at 0.80 kg O₃/h. Net cost savings were 66.6 €/d and 109.9 €/d at the two dosages, respectively. The translation of laboratory-scale ozonation performance to real-world viability is evidenced by full-scale data compiled in Table 7 and the experimental setup schematized in Figure 10. At a full-scale WWTP treating 30% of its return activated sludge (RAS), an ozone dosage of 0.55 kg O₃/h reduced final sludge disposal volume by 50%, while increasing the dosage to 0.80 kg O₃/h achieved a 75% reduction, confirming that the dose–response relationship observed in bench-scale studies remains valid under real hydraulic and organic load fluctuations. Importantly, the net cost savings reached 66.6 €/d and 109.9 €/d, respectively, after accounting for electricity consumption and maintenance. These economic figures, presented in Table 7, are particularly instructive because they demonstrate that the operating cost of ozonation is more than offset by the avoided disposal fees (landfill or incineration tipping charges), a financial dynamic that renders ozonation self-sustaining without subsidies. The generic process flow diagram (Figure 10) highlights the critical unit operations: ozone generation, bubble diffusion (or UFB injection), reaction tank, and downstream thickening, providing a practical blueprint for plant engineers considering retrofit installations. A comparative overview of ozone-based sludge treatment technologies is provided in Table 8.

Table 7. Summary of full-scale ozonation performance and economics [80].

Application	Ozone Dosage	Sludge Reduction	Net Cost Saving
RAS ozonation (30%)	0.55 kg O3/h	50%	66.6 €/d
RAS ozonation (30%)	0.80 kg O3/h	75%	109.9 €/d
Full-scale WWTP	Low dose	10% annual	2.3 €/t DS

Table 7. Summary of full-scale ozonation performance and economics [80].

Application	Ozone Dosage	Sludge Reduction	Net Cost Saving
RAS ozonation (30%)	0.55 kg O3/h	50%	66.6 €/d
RAS ozonation (30%)	0.80 kg O3/h	75%	109.9 €/d
Full-scale WWTP	Low dose	10% annual	2.3 €/t DS

Table 8. Comparison of ozone-based sludge treatment technologies [79,80,88].

Technology	Ozone Dosage	Key Performance	Advantages
Conventional ozonation	11–90 mg O3/g MLSS	Up to 90% reduction	Mature, proven
Microbubble ozonation	−25 mg O3/g MLSS	95.1% MLSS removal; D50: 150 → 17 µm	High efficiency, floc disruption
Ultrafine bubble ozonation	15 mg O3/g MLSS	80% bacteria death; 75% COD reduction	Lowest O3 dose
O3/PMS advanced oxidation	30 mg O3/g TS + 0.4 mmol/g TS PMS	CST: 70.5 → 26.7 s; Moisture: 81.93% → 65.65% (19.9% relative reduction)	Synergistic, dual radicals
CaO2/O3 catalytic oxidation	2 g/h O3 + 1 g/L CaO2	Wc: 85.7 → 46.5%	High dewatering
O3 + chitosan	60 mg O3/g TS + 20 mg/g TS chitosan	Wc < 60%; Calorific value +14%	Improved fuel value

Table 8. Comparison of ozone-based sludge treatment technologies [79,80,88].

Technology	Ozone Dosage	Key Performance	Advantages
Conventional ozonation	11–90 mg O3/g MLSS	Up to 90% reduction	Mature, proven
Microbubble ozonation	−25 mg O3/g MLSS	95.1% MLSS removal; D50: 150 → 17 µm	High efficiency, floc disruption
Ultrafine bubble ozonation	15 mg O3/g MLSS	80% bacteria death; 75% COD reduction	Lowest O3 dose
O3/PMS advanced oxidation	30 mg O3/g TS + 0.4 mmol/g TS PMS	CST: 70.5 → 26.7 s; Moisture: 81.93% → 65.65% (19.9% relative reduction)	Synergistic, dual radicals
CaO2/O3 catalytic oxidation	2 g/h O3 + 1 g/L CaO2	Wc: 85.7 → 46.5%	High dewatering
O3 + chitosan	60 mg O3/g TS + 20 mg/g TS chitosan	Wc < 60%; Calorific value +14%	Improved fuel value

3.5. Hydrothermal Carbonization

Hydrothermal carbonization (HTC) has emerged as a promising alternative for wet sludge treatment, converting sludge (70–85% moisture) into hydrochar at 180–250 °C under autogenous pressure without the need for pre-drying [89,90]. Compared to pyrolysis, HTC operates at lower temperatures and directly processes wet feedstocks, resulting in lower energy input. However, hydrochar generally exhibits lower specific surface area, lower carbon stability, and higher oxygen-containing functional groups than pyrochar, which affects its performance in soil amendment and adsorption applications [91,92].

The properties of hydrochar are governed by several interdependent parameters. Temperature is the most influential factor: higher temperatures increase carbonization degree and surface area but reduce hydrochar yield. Reaction time, biomass-to-water ratio, and pH also play critical roles in determining hydrochar composition, porosity, and surface chemistry [93,94]. Longer residence times generally promote secondary char formation and increase aromaticity, while acidic conditions favor hydrolysis and dehydration reactions that enhance carbon retention.

Catalytic approaches have been explored to improve HTC efficiency and product quality. Catalyzed hydrothermal carbonization can modify hydrochar structure and influence the distribution of pyrolytic products derived from hydrochar [95]. For instance, the addition of metal salts or acids can promote dehydration and polymerization reactions, leading to hydrochars with higher carbon content and more developed pore structures. Co-hydrothermal carbonization (co-HTC) with lignocellulosic biomass, such as sawdust or corncob, has shown promise for heavy metal mitigation and carbon/nutrient regulation in hydrochar [72,96]. The presence of biomass-derived compounds can alter the reaction pathways and enhance the immobilization of heavy metals originally present in sludge. However, the optimal mixing ratio and process conditions remain feedstock-dependent and require case-specific optimization.

Recent research has increasingly focused on contaminant fate during HTC. Studies indicate that HTC at 220 °C for 3 h removes approximately 71% of polyethylene microplastics from sludge, and several trace organic contaminants detected in raw sludges are completely removed after HTC treatment [94]. Regarding PFAS, HTC has been shown to achieve over 94% removal of C5–C10 perfluorocarboxylates, though the degradation pathways and transformation products are not yet fully characterized [35,93]. Compared to pyrolysis, HTC operates at lower temperatures and may leave higher residual PFAS concentrations in the solid phase, suggesting that the technology is less effective for complete PFAS mineralization but may still reduce the mass and volume of contaminated sludge requiring disposal.

A critical barrier to the widespread adoption of HTC is the generation of substantial volumes of toxic HTC-aqueous phase (HTC-AP). This process water contains high concentrations of organic acids, phenols, nitrogenous compounds, and potentially mobilized heavy metals, requiring costly post-treatment before discharge or reuse [97]. Several valorization strategies have been proposed, including anaerobic digestion of the process water to recover biogas, recirculation of HTC-AP to the main wastewater treatment plant, and extraction of valuable chemicals such as volatile fatty acids and furans. However, the techno-economic feasibility of these approaches has not been fully demonstrated at scale [98,99].

Furthermore, the carbon sequestration potential of hydrochar is generally lower than that of pyrochar due to its higher susceptibility to microbial degradation. Hydrochar contains more labile carbon fractions and oxygen-containing functional groups, which are more readily mineralized in soil environments. This limitation warrants further investigation through long-term field studies to assess the actual carbon storage benefits of HTC relative to pyrolysis. Overall, while HTC offers distinct advantages for wet sludge processing, its application should be carefully evaluated against specific treatment objectives, contaminant removal requirements, and downstream utilization pathways [100,101].

4. Pathways for Resource Utilization

4.1. Energy Utilization

Beyond conversion technologies, the ultimate goal of sludge management is resource valorization. The following sections evaluate the major utilization pathways, energy recovery, material production, and nutrient recycling in terms of their technical maturity and economic viability. Anaerobic digestion remains the most widely deployed energy-positive technology [102]. For a typical WWTP treating 100,000 m³/day, AD can produce 8000–15,000 m³/day of biogas, generating 0.8–1.2 MW of electricity [103]. Co-digestion with food waste can increase biogas yields by 30–60% [104]. Pyrolysis produces syngas (10–15 MJ/Nm³), bio-oil (20–35 MJ/kg), and biochar, with an overall energy recovery efficiency of 60–80%. The net energy performance across the major treatment routes is contrasted in Figure 11. However, the “typical” ranges in Table 11 represent best-case scenarios rather than guaranteed performance. For pyrolysis, the net output is highly sensitive to actual drying energy demand; many plants fail to achieve self-sufficiency because syngas yields at 600 °C are lower than laboratory predictions, and heat losses are higher than models assume. For anaerobic digestion, the net output assumes optimal co-digestion with carbon-rich substrates; without them (sludge C/N ratio is 5–15, far below the 20–30 optimum), biogas yields drop by 30–50%. Additionally, AD produces high-value electricity via CHP, while pyrolysis outputs primarily heat (low value unless cogenerated) plus biochar, whose economic value lies more in carbon credits and soil amendment than fuel. For facilities with access to anaerobic digesters, the co-location of AD with pyrolysis (using digestate as feedstock) can synergistically capture both methane and biochar value, offering a combined net output exceeding 700 kWh/t DS [105].

4.2. Material Application

4.2.1. Building Materials

Incinerated sewage sludge ash (ISSA) contains SiO₂ (25–40%), Al₂O₃ (10–20%), CaO (10–20%), and Fe₂O₃ (5–10%), making it suitable for cement clinker production (replacing 10–20% of clay) and lightweight aggregates [73,106]. Innovative non-fertilizer applications of sewage sludge include integrating sewage sludge into construction materials such as asphalt pavements, geopolymers, and cementitious composites [107]. The transformation of municipal sludge into high-entropy single-atom catalysts offers not just a new materials synthesis route but a new framework for how we conceive of waste, resource recovery and circularity [108].

4.2.2. Adsorption Materials and Soil Amendment

Sludge-derived biochar and activated carbon have emerged as promising low-cost adsorbents. Chemical activation with KOH can increase BET surface area from 50 to 80 m²/g to 400–800 m²/g [109]. Sludge-based activated carbon produced via thermal activation at 800 °C achieves iodine values of 320–420 mg/g [60]. Sustainable adsorbents from sewage sludge have been developed for the efficient removal of cytostatic compounds in single and complex aqueous matrices [62]. Sewage sludge-derived adsorbent materials for wastewater treatment exemplify the dual benefit of nutrient recovery and sludge valorization in a single treatment step [110]. One of the most promising valorization pathways is converting sludge-derived biochar into engineered adsorbents. As systematically compared and graphically summarized in Figure 12, the adsorption capacity is substantially influenced by the activation protocol. Raw biochar produced at 500 °C exhibits moderate Pb²⁺ uptake (20–35 mg/g), but steam activation at 800 °C achieves 85–110 mg/g for Pb²⁺, which is comparable to commercial activated carbon (100–150 mg/g) for this metal. For Cr(VI), Fe-impregnated sludge biochar reaches 50–70 mg/g, demonstrating competitive performance. KOH activation is particularly effective for Cr(VI), elevating its uptake from a mere 5–10 mg/g to 20–35 mg/g, due to the introduction of alkaline surface functional groups that facilitate electrostatic attraction of chromate anions. In contrast, Fe-impregnated biochar shows exceptional performance for Pb²⁺ and Cr(VI) (60–90 mg/g and 50–70 mg/g, respectively), owing to the dual mechanisms of surface complexation and reduction-precipitation [111,112].

The data in Figure 12 demonstrate that with targeted modification, sludge-based materials can match or exceed commercial benchmarks for specific heavy metals under ideal laboratory conditions (synthetic solutions, controlled pH, 24 h equilibrium). However, real wastewater contains competing cations (Ca²⁺, Mg²⁺, Na⁺) that reduce effective capacity by 30–60%, and adsorption kinetics are substantially slower (hours vs. minutes for commercial carbon). Moreover, regeneration of sludge-based adsorbents causes 30–50% capacity loss after a single cycle due to pore collapse and metal-catalyzed oxidation. These limitations relegate sludge-based adsorbents to niche applications, low-cost, single-use, or multi-functional contexts, rather than direct replacement of commercial activated carbon in mainstream water treatment [113,114,115,116].

Figure 12. Heavy metal adsorption capacities of sludge-based adsorbents vs. commercial activated carbon.

Figure 12. Heavy metal adsorption capacities of sludge-based adsorbents vs. commercial activated carbon.

4.3. Metal and Element Recycling

Phosphorus Recovery

With over 90% of phosphorus entering WWTPs concentrated in sludge, phosphorus recovery has become a priority [21]. Phosphorus recovery is important to reduce dependence on phosphate rock and prevent eutrophication. Recent advances have also focused on optimizing the leaching kinetics of phosphorus from incinerated sewage sludge ash (ISSA), with studies systematically evaluating the effects of acid type, concentration, and temperature on phosphorus extraction efficiency [12]. According to USGS data for 2024, Morocco holds the world’s largest phosphate reserves at approximately 50 billion metric tons, accounting for approximately 70% of global reserves, but produces only 36 million tons (approximately 14% of global production). China is the world’s largest producer of phosphate rock and phosphate chemicals, with 121 million tons produced in 2024, accounting for approximately 49% of global production, but China’s phosphate reserves are only approximately 3.4 billion metric tons, representing about 5% of global reserves, with a reserve-to-production ratio of approximately 28, well below the global average of approximately 292 [117]. Struvite precipitation is mature with over 50 commercial installations worldwide. The AshDec process uses thermo-chemical treatment at 900–1000 °C to volatilize heavy metals [118].

Vivianite precipitation has emerged as a promising alternative. In WWTPs, the dosage of FeCl₃ favors spontaneous formation of vivianite (Fe₃(PO₄)₂·8H₂O). Under optimal conditions (Fe:P molar ratio of 2.5:1 and pH 7–9), phosphate recoveries can reach 70–90% after the AD stage. Excessive addition of iron ions and/or vivianite formation in AD can hinder organic matter degradation and lower biogas yields [13]. Anaerobic fermentation (AF) and seeding have enhanced phosphorus recovery yields; the former mobilizes more phosphorus and iron into the soluble fraction, while the latter promotes larger crystal sizes by reducing the supersaturation demand. Vivianite precipitation is a promising technology for phosphorus recovery due to its potential applications in industry and agriculture, all while supporting the circular economy.

Enhanced phosphorus recovery from incinerated sewage sludge ash (ISSA) as vivianite through calcination with sodium-based additives has achieved a total P recovery of 78.03%. Alkaline sodium-based additives (NaOH, Na₂CO₃, and NaHCO₃) and elevated calcination temperature facilitate the conversion of apatite phosphorus (AP) and non-apatite inorganic phosphorus (NAIP) into water-soluble P as Na₃PO₄ [119]. The highest proportion of H₂O-P to total phosphorus (TP) of 90.53% was yielded at 1000 °C. The optimal P leaching rate from calcined ISSA reaches 90.12% at 50% NaHCO₃ dosage, 800 °C, and 60 min. Following the purification and crystallization of P-rich leachate, the vivianite product is collected with a typical flower-like morphology and a practical Fe/P molar ratio of 1.40 [119].

A zero-valent iron (ZVI)-mediated anaerobic phosphate reduction system has been developed that innovatively recovers resources from phosphorus-rich sludge by coupling phosphine (PH₃) generation with vivianite crystallization. The system overcomes the low recovery efficiency and limited product diversity of traditional methods by enabling multiphase resource recovery [120]. Near-full-component sewage sludge valorization via integrated alkali-mediated pyrolysis and sub-boiling temperature crystallization has achieved 67% phosphorus recovery as vivianite [121]. Three possible fertilizers (struvite, calcium phosphates, and vivianite) are recommended through final precipitation, given their pros and cons of recovery [122,123]. Phosphorus recovery is not a single solution but a portfolio of technologies, each with distinct efficiency, product purity, and maturity profiles, as visually contrasted in Figure 13. Struvite precipitation from reject water, despite being the most mature (over 50 commercial installations), recovers only 70–90% of the P and yields a product with moderate P₂O₅ content (25–30%), while requiring careful pH control and magnesium dosing. Acid leaching of incinerated ash achieves the highest recovery (80–95%) and product purity (30–50% P₂O₅), but the acidic leachate carries co-dissolved heavy metals, necessitating costly downstream purification steps. Emerging as a compelling alternative is vivianite (Fe₃(PO₄)₂·8H₂O) precipitation, which achieves 70–90% recovery with a product purity of 28–32% P₂O₅, comparable to struvite, but with a decisive advantage: vivianite exhibits ferrimagnetic properties, enabling its separation from digestate using low-cost magnetic fields. As illustrated in Figure 13, the magnetic separation step simplifies the dewatering process by over 60% compared to conventional centrifugation for struvite. When combined with the fact that iron salts are already added to many WWTPs for sulfide control and chemical P removal, vivianite precipitation can be integrated with minimal additional chemical cost, a synergy that positions it as the most cost-effective recovery pathway for P-rich sludges in the foreseeable future. Phosphorus and nitrogen recovery from anaerobically digested sludge centrate has been evaluated through vivianite precipitation and membrane contactors [124,125]. An innovative technique for extracting phosphorus and synthesizing high-purity vivianite from incinerated sewage sludge ash has been explored through a three-stage process involving leaching with sulfuric acid, purification via resin adsorption, and crystallization through iron electrocoagulation, achieving 61.44% phosphorus recovery as high-purity vivianite [126,127].

The narrative of “peak phosphorus” and imminent resource scarcity, while widely cited, requires qualification. Morocco’s phosphate reserves (≈500 billion tons) at current consumption rates would last centuries. The economic driver for phosphorus recovery is therefore not primarily resource depletion but eutrophication control; phosphorus is a pollutant in receiving water bodies regardless of its global availability. This distinction determines the policy instruments that make recovery viable. In regions with stringent total phosphorus discharge limits, phosphorus recovery is economically justified even when the recovered product sells at a loss because avoided discharge costs are substantial. In regions with lax standards, however, no combination of recovery technologies is economically self-sustaining without subsidies. Struvite precipitation recovers 70–90% of phosphorus, but the product sells for $200–400/t, comparable to the chemical costs ($150–350/t), leaving net profit often zero or negative without consideration of avoided nutrient fees. Vivianite precipitation, while promising due to ferrimagnetic properties enabling low-cost separation, has not yet been demonstrated at a commercial scale with real sludge under continuous operation.

5. Challenges and Future Perspectives

5.1. Technical and Economic Challenges

Despite the considerable technological progress discussed in previous sections, the widespread implementation of sludge resource recovery remains constrained by multiple interdependent technical, economic, and regulatory barriers. These obstacles span from process energy intensity and contaminant risks to product standardization and market viability. The following three tables (Table 9, Table 10 and Table 11) collectively synthesize these challenges alongside their potential solutions, economic performance, and environmental footprints, providing a holistic basis for identifying priority interventions.

Table 9. Summary of key challenges and potential solutions for sludge resource utilization [117,128].

Challenge Category	Specific Issue	Potential Solution
Energy intensity	High energy for drying	Solar drying, waste heat recovery, UFB ozonation
Heavy metal risk	Residual metals limit land application	Co-pyrolysis with biomass, chemical stabilization
Emerging contaminants	PFAS, microplastics, antibiotics	Advanced analytical methods, treatment train integration
Product standardization	No unified biochar quality standards	Develop ISO-type standards
Economic viability	High CAPEX/OPEX of advanced technologies	Carbon credits, product sales, gate fees

Table 9. Summary of key challenges and potential solutions for sludge resource utilization [117,128].

Challenge Category	Specific Issue	Potential Solution
Energy intensity	High energy for drying	Solar drying, waste heat recovery, UFB ozonation
Heavy metal risk	Residual metals limit land application	Co-pyrolysis with biomass, chemical stabilization
Emerging contaminants	PFAS, microplastics, antibiotics	Advanced analytical methods, treatment train integration
Product standardization	No unified biochar quality standards	Develop ISO-type standards
Economic viability	High CAPEX/OPEX of advanced technologies	Carbon credits, product sales, gate fees

Table 10. Economic comparison of different sludge treatment technologies (100 t/day wet sludge, 80% moisture basis) [128,129].

Technology Route	CAPEX (Million USD)	OPEX (USD/t Wet Sludge)	Annual Net Revenue (Million USD)	Payback Period (Years)	Main Revenue Sources
Anaerobic digestion (CHP only)	25–40	30–50	1.2–2.5	12–18	Power generation, emission reduction
AD + CHP + P recovery	35–55	40–65	2.5–4.0	10–15	Power, heat, P fertilizer
Incineration (disposal only)	50–80	60–100	−1.0 to −0.5		Heat (subsidy-dependent)
Incineration + Ash P recovery	55–90	70–110	0.2–1.0	>50	P recovery, heat
Pyrolysis (slow, biochar)	40–60	50–80	1.0–2.2	12–20	Biochar sales, carbon credits
Pyrolysis + Thermal activation	45–70	55–85	1.8–3.5	10–16	Activated carbon, biochar, carbon credits
Hydrothermal carbonization	30–50	35–60	0.8–1.8	14–22	Hydrochar (solid fuel)

Note: Payback periods are indicative estimates derived from literature case studies and do not represent precise financial projections. For Incineration + Ash P recovery, the payback period exceeds 50 years based on the CAPEX/annual revenue ratio, rendering this route economically unviable without substantial gate fees, subsidies, or carbon credits. The optimistic 18–25 year range sometimes cited in the literature assumes favorable policy support that may not be universally available. Actual project economics are highly site-specific and depend on local energy prices, disposal tipping fees, and regulatory incentives.

Table 10. Economic comparison of different sludge treatment technologies (100 t/day wet sludge, 80% moisture basis) [128,129].

Technology Route	CAPEX (Million USD)	OPEX (USD/t Wet Sludge)	Annual Net Revenue (Million USD)	Payback Period (Years)	Main Revenue Sources
Anaerobic digestion (CHP only)	25–40	30–50	1.2–2.5	12–18	Power generation, emission reduction
AD + CHP + P recovery	35–55	40–65	2.5–4.0	10–15	Power, heat, P fertilizer
Incineration (disposal only)	50–80	60–100	−1.0 to −0.5		Heat (subsidy-dependent)
Incineration + Ash P recovery	55–90	70–110	0.2–1.0	>50	P recovery, heat
Pyrolysis (slow, biochar)	40–60	50–80	1.0–2.2	12–20	Biochar sales, carbon credits
Pyrolysis + Thermal activation	45–70	55–85	1.8–3.5	10–16	Activated carbon, biochar, carbon credits
Hydrothermal carbonization	30–50	35–60	0.8–1.8	14–22	Hydrochar (solid fuel)

Note: Payback periods are indicative estimates derived from literature case studies and do not represent precise financial projections. For Incineration + Ash P recovery, the payback period exceeds 50 years based on the CAPEX/annual revenue ratio, rendering this route economically unviable without substantial gate fees, subsidies, or carbon credits. The optimistic 18–25 year range sometimes cited in the literature assumes favorable policy support that may not be universally available. Actual project economics are highly site-specific and depend on local energy prices, disposal tipping fees, and regulatory incentives.

As summarized in Table 9, the high energy intensity associated with sludge dewatering and drying, typically consuming 0.8–1.2 kWh per kg of water removed, remains a primary operational bottleneck. This drives ongoing research into passive solar pre-drying, waste heat recovery from adjacent industrial processes, and energy-efficient ultrafine bubble ozonation. Heavy metal residues, although effectively immobilized at pyrolysis temperatures above 600 °C, occasionally still exceed threshold limits for unrestricted agricultural use, particularly for Cu, Zn, and Ni. Such risks can be substantially mitigated through co-pyrolysis with lignocellulosic biomass or chemical stabilization with phosphate-based additives. Emerging contaminants, including PFAS, microplastics, and antibiotic resistance genes, introduce long-term ecological and human health uncertainties that are not yet fully captured by existing regulatory frameworks. Addressing these requires advanced analytical methodologies (e.g., LC-MS/MS for PFAS) and integrated treatment trains that combine thermal, chemical, and biological barriers. Furthermore, the absence of unified, internationally recognized biochar quality standards leads to wide product heterogeneity, deterring end-user acceptance and commoditization; hence, the development of ISO-type classification systems for sludge-derived biochar is urgently needed. Economically, pyrolysis (USD 50–100/t wet sludge) and ozonation (USD 50–80/t) involve high capital and operational costs, yet emerging carbon credit mechanisms and rising market prices for high-grade activated carbon are progressively improving their financial attractiveness.

The techno-economic comparison presented in Table 10 further elucidates the financial viability of each major treatment route for a reference plant of 100 t/day wet sludge (80% moisture). Anaerobic digestion (AD) combined with CHP offers moderate capital expenditure (USD 25–40 million) with a payback period of 12–18 years, which shortens to 10–15 years when phosphorus recovery is integrated, thanks to additional revenue from P-fertilizer sales. In contrast, incineration solely for disposal demands significantly higher CAPEX (USD 50–80 million) and typically yields negative annual net revenue (−1.0 to −0.5 million USD), rendering it heavily dependent on gate fees or public subsidies. However, adding ash phosphorus recovery can partially offset costs and shorten the payback to 18–25 years. Pyrolysis, especially when coupled with thermal activation for producing sludge-based activated carbon, demonstrates competitive payback periods (10–16 years) and robust revenue streams from biochar sales, carbon credits, and adsorbent products. Hydrothermal carbonization presents the lowest upfront cost (USD 30–50 million), but its commercial breakthrough remains contingent on market acceptance of hydrochar as a clean solid fuel and solutions for treating the toxic aqueous by-products. Overall, Table 10 clearly indicates that integrated strategies combining waste treatment with high-value product recovery offer superior economic resilience and shorter return on investment compared to single-purpose disposal options.

Table 11. Global Warming Potential (GWP) of different sludge treatment technologies (kg CO₂-eq/t DS) [120,121].

Technology Route	GWP Range (kg CO2-eq/t DS)	System Boundary	Main Contributors
Anaerobic digestion + Land application	−200 to −50	Gate to land use	Fertilizer substitution, biomethane
Anaerobic digestion + CHP	−100 to +50	Gate-to-gate	Energy substitution, fugitive emissions
Incineration	+500 to +900	Gate to ash disposal	Fossil fuel consumption, N2O
Incineration + P recovery	+400 to +750	Gate to P product	Offsets phosphate mining
Pyrolysis (slow)	−50 to +100	Gate to biochar use	Biochar carbon sequestration, energy output
Pyrolysis + Thermal activation	−100 to +50	Gate to activated carbon use	Carbon sequestration, product value
Hydrothermal carbonization	+50 to +150	Gate to hydrochar use	Low energy consumption, carbon sequestration
Co-pyrolysis (sludge + biomass)	−150 to −20	Gate to biochar use	Biomass carbon negativity, synergy effects

Table 11. Global Warming Potential (GWP) of different sludge treatment technologies (kg CO₂-eq/t DS) [120,121].

Technology Route	GWP Range (kg CO2-eq/t DS)	System Boundary	Main Contributors
Anaerobic digestion + Land application	−200 to −50	Gate to land use	Fertilizer substitution, biomethane
Anaerobic digestion + CHP	−100 to +50	Gate-to-gate	Energy substitution, fugitive emissions
Incineration	+500 to +900	Gate to ash disposal	Fossil fuel consumption, N2O
Incineration + P recovery	+400 to +750	Gate to P product	Offsets phosphate mining
Pyrolysis (slow)	−50 to +100	Gate to biochar use	Biochar carbon sequestration, energy output
Pyrolysis + Thermal activation	−100 to +50	Gate to activated carbon use	Carbon sequestration, product value
Hydrothermal carbonization	+50 to +150	Gate to hydrochar use	Low energy consumption, carbon sequestration
Co-pyrolysis (sludge + biomass)	−150 to −20	Gate to biochar use	Biomass carbon negativity, synergy effects

Beyond direct economic considerations, the environmental carbon footprint has become an increasingly decisive factor in technology selection, as quantified in Table 11. Incineration exhibits the highest global warming potential (GWP), ranging from +500 to +900 kg CO₂-eq/t DS, primarily attributable to fossil fuel consumption and fugitive N₂O emissions from the combustion process. In contrast, slow pyrolysis alone shows variable carbon footprints ranging from −50 to +100 kg CO₂-eq/t DS. When combined with biomass co-pyrolysis, the process can achieve lower net emissions (−150 to −20 kg CO₂-eq/t DS) because biochar contains stable carbon structures with enhanced resistance to degradation. Anaerobic digestion followed by land application also demonstrates a low to strongly negative GWP (−200 to −50 kg CO₂-eq/t DS), largely due to fertilizer substitution (avoided synthetic N-P-K production) and biomethane displacement of fossil fuels. Hydrothermal carbonization resides in an intermediate range (+50 to +150 kg CO₂-eq/t DS), benefiting from low drying energy but limited by the lower stability of hydrochar compared to pyrochar. Collectively, the GWP analysis underscores a paradigm shift from energy-intensive, emission-positive incineration toward thermochemical conversion and biological routes that not only minimize emissions but actively contribute to carbon drawdown, aligning municipal sludge management with global climate neutrality goals.

5.2. Future Perspectives and Research Priorities

Despite substantial advances in municipal sludge treatment technologies, several technical and economic barriers continue to limit their large-scale implementation. Future research should therefore focus on improving energy efficiency, enhancing resource recovery, strengthening contaminant control, and promoting system-level integration to support the transition toward sustainable sludge management. One of the highest priorities is process intensification and energy optimization. The high energy demand associated with sludge drying and thermochemical conversion remains a major obstacle to commercialization. Emerging technologies, including microwave-assisted pyrolysis, ultrafine-bubble ozonation, solar-assisted drying, and industrial waste heat recovery, offer promising opportunities to reduce energy consumption while maintaining treatment efficiency. Future studies should also emphasize process coupling and comprehensive techno-economic and life-cycle assessments to identify the most sustainable treatment configurations under different operational conditions. Another important research direction is the integrated control of multiple contaminants. Municipal sludge contains not only heavy metals but also emerging pollutants such as microplastics, per- and polyfluoroalkyl substances (PFAS), pharmaceutical residues, and antibiotic resistance genes (ARGs). Most existing treatment technologies primarily target individual contaminants, whereas future systems should be designed to simultaneously stabilize heavy metals, degrade persistent organic pollutants, and inactivate pathogens while minimizing secondary pollution. Standardized analytical methods, particularly for microplastic quantification (ISO-type protocols for digestion, extraction, and FTIR/Raman identification) and PFAS mass balance (tracking fluorine across solid, liquid, and gas phases), are urgently needed to replace the current proliferation of incompatible methodologies that render cross-study comparison virtually impossible. Long-term environmental risk assessments should move beyond laboratory accelerated aging to field-scale monitoring over 5–10 years, especially for heavy metal remobilization under real soil pH and redox fluctuations. Future sludge valorization should also shift from conventional low-value applications toward the production of high-value functional materials. In addition to soil amendment, sludge-derived biochar and activated carbon have considerable potential for applications in catalyst supports, advanced adsorbents, electrochemical energy storage, and environmental remediation. Meanwhile, the integration of artificial intelligence, digital twins, real-time sensing, and mechanistic models is expected to improve process monitoring, operational optimization, and product quality control, thereby enabling more intelligent and efficient sludge treatment systems.

As illustrated in Figure 14, future municipal sludge management is expected to evolve toward integrated sludge biorefineries that combine anaerobic digestion, pyrolysis, hydrothermal processing, phosphorus recovery, and advanced oxidation within a unified resource recovery framework. Efficient coupling of material and energy flows among these processes can improve overall energy efficiency, maximize the recovery of carbon, nutrients, and energy, and reduce greenhouse gas emissions and environmental impacts. Achieving this vision will require not only technological innovation but also internationally harmonized product standards, robust quality certification systems, and supportive policy mechanisms that facilitate the commercialization of sludge-derived products. Through the coordinated development of advanced treatment technologies, intelligent process control, and circular resource utilization, municipal sludge can be progressively transformed from an environmental burden into a valuable secondary resource that supports sustainable development and carbon neutrality.

6. Conclusions

This review arrives at three principal conclusions that challenge common assumptions in the field.

(1)

Sustainable sludge management requires integrated treatment strategies rather than standalone technologies. Anaerobic digestion provides economical energy recovery but transfers contaminants into digestate, while pyrolysis enables contaminant destruction and heavy metal immobilization at the cost of higher energy demand. A combined AD–pyrolysis pathway can maximize resource recovery by producing biogas and biochar and recovering phosphorus. However, technology selection remains context-dependent, governed by economic factors, regulatory pressures, and desired end products.

(2)

The gap between reported “removal” and actual risk reduction is substantial. A 99% PFAS removal from biochar says nothing about fluorine partitioning into syngas or bio-oil. Heavy metals immobilized under ideal laboratory conditions may remobilize under field pH/redox fluctuations. Future research must prioritize full mass balances tracking all pollutant fractions across solid, liquid, and gas phases, combined with realistic aging and weathering studies.

(3)

Economic viability is inseparable from regulatory drivers. The high CAPEX of pyrolysis cannot be justified by biochar sales alone; it requires carbon credits, avoided disposal costs, or regulatory mandates. Policymakers must recognize that the transition from “sludge disposal” to “resource recovery” requires explicit policy support.

Priority research needs for the next decade include: (i) process intensification to overcome drying energy barriers; (ii) multi-pollutant synergistic control within integrated trains; (iii) ISO-type product quality standards; and (iv) open, multi-facility datasets to validate machine learning models. Through coordinated development of these priorities, municipal sludge management can progressively evolve from an environmental liability to a contributor to the circular economy and climate neutrality.