Energy Valorization and Resource Recovery from Municipal Sewage Sludge: Evolution, Recent Advances, and Future Prospects

Abstract

Municipal sewage sludge, a by-product of urban wastewater treatment, is increasingly recognized to be a strategic resource rather than a disposal burden. Traditional management practices, such as landfilling, incineration, and land application, are facing growing limitations due to environmental risks, regulatory pressures, and the underuse of the sludge’s energy and nutrient potential. This review examines the evolution of sludge management, focusing on technologies that enable energy recovery and resource valorization. The transition from linear treatment systems toward integrated biorefineries is underway, combining biological, thermal, and chemical processes. Anaerobic digestion remains the most widely used energy-positive method, but it is significantly improved by processes such as thermal hydrolysis, hydrothermal carbonization, and wet oxidation. Among these, hydrothermal carbonization stands out for its scalability, energy efficiency, and phosphorus-rich hydrochar production, although implementation barriers remain. Economic feasibility is highly context-dependent, being shaped by capital costs, energy prices, product markets, and policy incentives. This review identifies key gaps, including the need for standardized treatment models, decentralized processing hubs, and safe residual management. Supportive regulation and economic instruments will be essential to facilitate widespread adoption. In conclusion, sustainable sludge management depends on modular, integrated systems that recover energy and nutrients while meeting environmental standards. A coordinated approach across technology, policy, and economics is vital to unlock the full value of this critical waste stream.

1. Introduction

The escalating generation of municipal sewage sludge (MSS) poses a significative environmental challenge globally, necessitating the development and implementation of sustainable management strategies [1]. Sewage sludge is a by-product generated during the treatment of wastewater in wastewater treatment plants (WWTPs), and it primarily consists of two fractions: primary sludge and secondary or waste-activated sludge. Sewage sludge is generated as a byproduct of wastewater treatment processes, primarily during primary and secondary treatment stages. In primary treatment, settleable and floatable solids are separated to form primary sludge. Secondary treatment, typically involving the activated sludge process or other biological systems, removes dissolved organic matter and produces excess microbial biomass, known as secondary or waste-activated sludge. While tertiary treatment may be applied for further polishing of the effluent, its contribution to overall sludge production is minimal.

MSS, which is the predominant type encountered in WWTPs, generally contains a high proportion of organic matter and water. Its effective management is essential due to its volume, composition, and potential for resource and energy recovery [2,3]. Typically, MSS production is affected by several factors, such as the country and its regulations, the habits and size of the population served, as well as the level of urbanization, wastewater treatment cycle (with particular attention to the biological process and the operating conditions applied), and the sludge-handling methods. In addition, sludge production has been increasing in recent years due to the commissioning of new municipal WWTPs, driven by population growth, urban expansion, and the need to improve sanitation infrastructure, as well as the enforcement of increasingly stringent discharge standards imposed by national and European regulations [4,5].

To better understand the evolution and focus of recent research in this area, Figure 1 provides a visual analysis of literature related to MSS keywords published between 2023 and April 2025.

Currently, comprehensive global estimates of sewage sludge production are lacking, with available data limited to studies published over the past three years. Therefore, we derived our estimate using the most recent national and regional statistics, as summarized in Figure 2, which shows the annual MSS production and per capita generation in selected countries. As illustrated, Europe, East Asia, and North America are the largest contributors to global MSS output, which is estimated to be approximately 56 million tons (Mt) of dry matter per year (which could almost double in 30 years) [6,7]. This rapid growth makes the search for circular, low-carbon sludge valorization solutions an urgent priority.

MSS, a by-product of wastewater treatment operations, is typically generated with a total dry solids (TS) content of approximately 0.5–15%, depending on the degree of dewatering. Most of these solids are organic, with volatile solids (VS) comprising between 60% and 80% of TS. The quantity and characteristics of MSS are highly variable and are largely influenced by the composition of the incoming wastewater and the specific treatment processes applied [9].

Table 1. Typical chemical composition of untreated MSS.

Component	Composition (% of TS)
Volatile solids	60–80
Grease and fats
Ether soluble	6–30
Ether extract	7–35
Protein	20–30
Nitrogen (N)	1.5–4
Phosphorus (P2O5)	0.8–2.8
Potash (K2O)	0–1
Cellulose	8.0–15.0
Iron (not as sulfide)	2.0–4.0
Silica (SiO2)	15.0–20.0

shows a typical composition of untreated MSS [10].

While MSS is often regarded as a waste stream requiring disposal, dewatered MSS also represents a significant reservoir of recoverable resources, including organic matter and essential nutrients. This has led to increasing interest in its valorization through the energy (e.g., pyrolysis, incineration) and material recovery pathways [3]. However, the complex composition of MSS presents substantial challenges for its management and safe utilization. In addition to its high organic content, MSS typically contains a range of potentially hazardous constituents. These include heavy metals (e.g., cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn)) [3], pathogenic microorganisms (which can enter the food chain due to disposal [11]), microplastics (MP) [12,13], pharmaceutical and personal care products (PPCPS) [14], and a variety of persistent organic pollutants and micropollutants [15,16], which can pose serious health and environmental risks. Landfilling can generate leachate rich in these contaminants, and land application may introduce pathogens and heavy metals into soil and crops. Organic contaminants and trace metals are particularly concentrated in primary sludge due to their affinity for particulate matter [3]. The presence of these pollutants complicates the treatment, reuse, and disposal of MSS, particularly in regions lacking the infrastructure and technological capacity for advanced sludge processing. In many developing countries (such as parts of Sub-Saharan Africa, South Asia, and Latin America), the absence of adequate treatment and sustainable management systems remains a critical barrier to safe sludge handling, raising concerns about environmental contamination and public health impacts [6].

In many contexts, the absence of adequate infrastructure for sludge handling significantly increases treatment and management costs, which can account for up to 60% of the total operating costs of municipal WWTPs [9,13]. Furthermore, conventional disposal methods (such as landfilling, incineration, and land application) are often unsustainable, as they contribute to greenhouse gas emissions and fail to recover the resource potential of dewatered MSS. For example, landfilling can generate leachate, a harmful effluent that poses serious risks to soil and groundwater quality. Incineration, on the other hand, is economically impractical due to high cost and energy requirements; however, to date, some European Union (EU) countries are practicing this method, including Italy. The direct land application of MSS has been shown to present a substantial environmental risk to water, soil, and human health, as the released contaminants may enter the food chain. Nevertheless, other methods, such as composting, have shown potential for nutrient recycling and waste reduction. However, if not properly managed, environmental challenges, such as odor emissions, nutrient runoff, and incomplete pathogen inactivation, may arise, requiring appropriate operational controls. As a result, the sustainable management of MSS remains a formidable challenge, necessitating a reconsideration of production, disposal, and resource utilization to enhance its social, economic, and environmental sustainability [6]. In 2022, Italy produced approximately 3.2 million tons of MSS from the treatment of urban wastewater (EWC code 190805). According to the ‘Special Waste Report—2023 Edition’ by ISPRA, the sludge was primarily managed through four main pathways. Around 40% was applied to agricultural land, benefiting from its organic and nutrient content. Approximately 30% underwent incineration, while about 20% was disposed of in landfills. The remaining 10% was exported. Although land application remains a relevant recovery option, the presence of contaminants, such as heavy metals, pathogens, and persistent organic pollutants, poses significant limitations and requires adequate treatment to ensure environmental safety and compliance with health regulations [17].

This article pursues three complementary objectives. Firstly, it will provide facility managers, researchers, and policymakers with an updated and practical overview of the shift from traditional linear sludge management to circular and energy-self-sufficient systems: “Water resource recovery facilities” (WRRFs). Secondly, two years later, we aim to assess whether the forecasts presented in our previous studies [7,8] materialize, identifying confirmations, critical points, and potential delays in technological advancement and industrial implementation. Finally, this study will outline the key trends in sustainable municipal sludge management for the next decade (2025–2035), highlighting emerging opportunities and challenges in order to guide future strategies.

2. Current Methods of Sewage Sludge Treatment and Valorization

2.1. Overview of Treatment Processes

A streamlined overview of the principal technological pathways for municipal sewage-sludge treatment and valorization is illustrated in Figure 3, providing a visual roadmap for the discussion that follows throughout this chapter.

In a conventional WWTP, sludge management begins with thickening, a stage that reduces the volume of excess sludge by 30–80% [18]. Standard technologies include gravity thickeners, dissolved air flotation, rotary drum thickeners, and, increasingly, centrifuges. The selection depends on plant size, layout constraints, and subsequent treatment steps. Typically, 3–5% solids concentrations are achieved, while the practical “solidity threshold” is reached at about 15–20% dry solids. Fresh thickened sludge undergoes spontaneous fermentation and contains pathogenic loads that generate unpleasant odors. To prevent health and logistical issues, the sludge is stabilized. Three main stabilization routes are available:

• Alkaline chemical stabilization: Adding lime (sometimes combined with solid waste materials) raises the pH > 12 for several days, yielding a sterilized, easily transportable solid sludge [19,20,21,22].
• Advanced oxidation processes: Ozone, Peroxone, or other radical-based treatments have proven to be effective for stabilizing sludge and reducing its mass or as a pre-treatment step before biological stabilization [23,24,25,26,27,28,29,30,31].
• Biological stabilization: This operation can be aerobic or anaerobic.

Aerobic stabilization (AS) operates at high sludge retention times with no fresh organic input; endogenous respiration lowers solids and pathogen levels and can be intensified through autothermal thermophilic aerobic digestion (ATAD), where exothermic oxidation spontaneously raises reactor temperatures to 70–75 °C [32,33]. Anaerobic digestion (AD), recommended by the Water Environment Federation (WEF) guidelines for plants > 20 ML d⁻¹ (about 60–100 k population equivalents or PE, where 1 PE represents the average daily organic biodegradable load, measured as biochemical oxygen demand [BOD], produced by one person) [34], produces biogas containing 45–60% CH₄, offsetting part of the energy demand of the entire treatment train [35,36,37,38,39,40,41,42,43,44]. When the C/N ratio is unbalanced, nitrogen-rich co-substrates (manure, urea, food waste) are added. Systematic comparisons show that AD is superior in net energy balance, whereas AS is simpler but more energy-intensive and yields no biogas [45,46].

After stabilization, the sludge must be dewatered to separate the solid fraction from a liquor usually returned to the plant’s headworks. Unlike thickening, which removes only free water and raises the dry-solids content to about 4–6%, dewatering removes a far larger share of water (interstitial, capillary, and some bound water), achieving typical concentrations of 15–25% dry solids. The product is no longer a pumpable suspension, but rather a compact “cake” that will not flow and must be moved with conveyors, mechanical shovels, or site equipment.

Dewatering requires high mechanical forces in filtration or centrifugation systems: multi-cone centrifuges, belt presses, and filter presses are the dominant solutions in medium- and large-scale WWTPs. As low-energy alternatives, water can evaporate naturally in drying lagoons or drain by gravity on sludge-drying beds, but these options demand large footprints and depend heavily on climate [47,48,49]. In either case, 75–85% of the remaining water (intracellular, interstitial, and surface moisture) remains within the matrix even after mechanical dewatering.

In large plants, overall efficiency improves when thickened sludge undergoes thermal hydrolysis (TH) before anaerobic digestion: treatment at about 6 bar and 400–440 K for about 30 min disrupts flocs, releases readily fermentable substrates, converts interstitial water to free water and, after flashing to atmospheric pressure, increases both biogas production and the dewaterability of the digestate [50].

This well-established chain provides mature, scalable technologies, and, thanks to anaerobic digestion, converts part of the organic load into renewable energy. Nonetheless, several bottlenecks remain: electricity and chemical consumption (polyelectrolytes); odorous and greenhouse gas emissions from aerobic steps; and a cake that, even after dewatering, still retains more than 60% moisture. To address these issues, plants have introduced intensification pre-treatments (ultrasound, hydrothermal heating, cryogenic lysis with solid CO₂) and thermochemical routes (pyrolysis, gasification, hydrothermal carbonization) that concentrate energy and nutrients into separate streams.

Although the conventional thickening–stabilization–dewatering sequence remains the operational backbone, regulatory, energy efficiency, and environmental pressures drive facilities toward hybrid schemes that blend biological intensification techniques with high-energy-density thermochemical conversions.

2.2. Regulatory Landscape

The current regulatory framework governing MSS management is highly heterogeneous at the global level. It reflects, in addition to cultural and socio-economic differences, the specific natural vocations of each territory. The European Union, which has one of the most organic-focused bodies of legislation, addresses the entire chain with a phased approach:

• Directive 91/271/EEC (Urban Waste Water Treatment Directive) governs the collection and treatment of municipal wastewater [51].
• Framework Directive 2008/98/EC sets the waste management hierarchy, traceability, and transport rules [52].
• Directive 2010/75/EU on industrial emissions establishes performance limits and BATs for digestion, drying, and incineration [53].
• Directive 86/278/EEC (Sewage Sludge Directive) regulates land application by imposing heavy-metal limits [5].

Overall, environmental protection is reinforced by the Water Framework Directive 2000/60/EC, its “daughter” directives, and the Nitrates Directive 91/676/EEC [54,55].

Although sludge is classified as non-hazardous waste (EWC code 19 08 05), Brussels, also following the inclusion of phosphorus among critical raw materials, has introduced binding recovery targets that, in Germany, will become effective from 2029 above the threshold of 20 g P kg⁻¹ dry solids; some states, such as the Netherlands and Switzerland, already prohibit agronomic use without prior thermal treatment. The need for frequent analytical monitoring has fueled a lively market for chemical and thermochemical extraction technologies, putting circular nutrient valorization and minimizing environmental impact throughout the sludge life-cycle at the center of innovation.

In the United States, the Clean Water Act and the NPDES permit system set limits on discharge and biosolids use; the EPA updates the list of contaminants to be monitored every two years and focuses on emerging micropollutants (PFAS, MP). With over 14,700 publicly owned treatment works (POTWs), annual production is about 14 Mton DS, but 15% of the plants operate beyond their nominal capacity, highlighting the need for modernization and solutions resilient to climate change [56].

In China, urbanization has led to over 10,000 WWTPs that treat about 95% of urban centers, generating 10 Mt DS yr⁻¹ (destined to reach 18 Mton by 2025). The Five-Year Plans finance the expansion of sewer networks and the “Sponge Cities,” promoting anaerobic digestion, hydrothermal carbonization (HTC), and biochar-oriented pyrolysis in line with the 2060 carbon-neutrality goal [7].

India, with treatment coverage below 25% of wastewater, is investing in the “Clean Ganga” program for new plants and pipelines; the theoretical potential of 100–200 Mton DS yr⁻¹ of sludge could become an energy resource through waste-to-energy (WtE), but the infrastructural gap remains the main challenge [57].

Other areas, Russia, the Middle East and North Africa, sub-Saharan Africa, and Latin America, show treatment rates between 8% and 38%; uncontrolled landfilling and community composting initiatives prevail here, while the Organisation for Economic Co-operation and Development (OECD) reports that high-income countries treat over 70% of wastewater, compared to the global average of 20% [58,59,60,61,62].

These regulatory differences translate into divergent technological choices: Europe favors thermochemical routes with nutrient recovery to meet climate and raw-material security objectives; in the USA, concern over contaminants drives advanced stabilization and filtration; and China and India aim to increase treatment capacity by integrating urban-scale WtE technologies. Understanding these regulatory trajectories is essential for assessing the technical and economic feasibility of emerging sludge valorization technologies discussed in the following sections.

2.3. Drivers, Gaps, and Recent Advances in Sustainable Sludge Management

The drive toward decarbonization, the circular economy, and the achievement of SDGs 11 and 12 has turned sludge from a costly waste into a strategic resource. Global initiatives such as “Waste Wise Cities” are already steering utilities toward reduction, recovery, and reuse policies that have influenced many technology choices [63]. However, the transition is still slowed by technical and scientific gaps. One critical research gap concerns emerging contaminants, particularly MP, which persist in biosolids due to the ineffectiveness of conventional treatment methods. This calls for the development of integrated removal processes capable of producing “MP-free” sludge suitable for agricultural or material reuse [64]. On the process side, traditional biological treatment lines are increasingly limited in scale, efficiency, and adaptability as sludge volumes grow. To address this, integrated biorefinery models that combine biological, thermochemical, and nutrient recovery processes are gaining interest [7], but their widespread application is still constrained by technological maturity and economic feasibility.

Additional key research and development gaps include the following:

• Techno-economic optimization: More detailed site-specific studies are needed to evaluate the performance, scalability, and cost-effectiveness of advanced treatment technologies under varied operational conditions.
• Regulatory and policy alignment: The absence of harmonized standards and economic incentives often delays the adoption of innovative sludge valorization strategies, especially in low- and middle-income regions.
• Digital tools and process monitoring: The application of smart monitoring, automation, and data-driven decision-making in sludge treatment remains limited. Research into digital integration can enhance process efficiency and compliance.

Bridging these gaps is essential to transform regulatory and environmental pressures into opportunities for innovation, investment, and the large-scale implementation of sustainable sludge management systems. Ultimately, all of these challenges converge on a common goal: to convert MSS from a liability into a valuable energy carrier, nutrient source, and a contributor to critical raw material recovery.

A targeted analysis of different papers published between 2023 and 2025, subsequently aggregated into the eight research clusters, is shown in

Table 2. Thematic clusters of 2023–2025 sludge valorization studies, with focus, keywords, and article count.

Cluster Theme	Representative Keywords	N. Articles	Ref.
Anaerobic digestion and co-digestion—methane/H2 production, pretreatment to boost biogas	AD, co-digestion, VFA *, methane yield, organic loading, hydrothermal-AD	16	[65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80]
Thermal processing and energy (pyrolysis/gasification/combustion)—syngas, bio-oil, kinetic synergy, flue gas control	Pyrolysis, gasification, co-combustion, syngas, activation energy	18	[81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98]
Composting, vermicomposting and soil amendment—nutrient recovery, plant growth, soil enzymes	Compost, vermicompost, biochar soil, plant uptake, phytostabilization	25	[99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123]
Adsorbents and pollutant removal—sludge-derived sorbents for metals, PFAS, P, ARG * reduction	Adsorption, biochar sorbent, heavy metals, PFAS, ARG, P recovery	16	[97,118,120,124,125,126,127,128,129,130,131,132,133,134,135,136]
Construction and ceramic materials—bricks, concrete, foamed ceramics, nano-biochar fillers	Brick, concrete, ceramic, compressive strength, sintering	7	[137,138,139,140,141,142,143]
Environmental risk, monitoring and surveys—ARG/microplastics loads, IoT sensing, risk indices	Risk, ARG, microplastics, IoT *, monitoring, survey	13	[12,87,133,144,145,146,147,148,149,150,151,152,153]
Life-cycle assessment, reviews, and policy—comparative LCA, SWOT/FAHP *, national inventories	LCA, review, SWOT, survey, policy, circular economy	12	[72,86,91,154,155,156,157,158,159,160,161,162]
Integrated biorefinery and advanced catalytic routes—multigeneration plants, HTL-AD-MEC *, specialty catalysts	Biorefinery, catalyst, multigeneration, HTL, MWCNT *, biodiesel	11	[82,83,84,124,163,164,165,166,167,168,169]

* ARG: antibiotic resistance genes; IoT: Internet of Things; VFA: volatile fatty acids; MEC: microbial electrolysis cell; SWOT: Strengths–Weaknesses–Opportunities–Threats; FAHP: Fuzzy Analytic Hierarchy Process; MWCNT: multi-walled carbon nanotubes.

. The corpus reveals a convergent trajectory toward hybrid processes that maximize energy and nutrient recovery while suppressing emerging contaminants.

Anaerobic digestion remains the backbone of sludge valorization, yet the focus has shifted from classical mesophilic operation to high-solids, co-digestion, and two-stage thermophilic schemes. Blending MSS with other waste effluent [65,66,67,75,80] consistently boosts methane yields. Pretreatments as mild as solidified CO₂ dosing [69], alkali dissolution [76], and hydrodynamic cavitation [170] are reported to raise soluble substrates and volatile fatty acid levels. In contrast, ultrasound installations already operating in Polish facilities only need energy optimization fine-tuning [71].

Thermochemical routes constitute the second most vigorous front. Co-combustion of sludge with coal-gangue lowers ignition temperatures and captures heavy metals in mineral matrices [98]. At the same time, steam co-gasification with tire char enhances H₂-rich syngas production through catalytic ash components [94]. Hydrothermal pretreatment at 200 °C shifts subsequent pyrolysis toward larger H₂ and CH₄ fractions and produces high-surface-area biochar [95]. Complementary studies deploy red-mud-doped CaO catalysts that lift steam-gasified hydrogen to 54 vol% [83] or use CaO scrubbing to reach 94% H₂ purity in upgraded pyrolysis gas [84]. However, only a handful of papers venture beyond laboratory scale; one life-cycle comparison of Chinese incineration lines still notes 351–1133 kg CO₂-eq tDS⁻¹, even when heat recovery is optimized [86], suggesting that the scale-up economics and real gas clean-up challenges remain largely unsettled.

A third line of inquiry explores composting, vermicomposting, and soil amendment. Response surface optimization identifies optimal parameters for sludge–biowaste blends yielding nutrient-rich compost [123]. Long-term field trials confirm that such amendments enhance soil enzymatic profiles more than a decade after application [122], sustain maize yields rivaling mineral fertilizers [119], and can even facilitate arsenic phytoextraction in contaminated soils [121]. Nevertheless, antibiotic-resistance and heavy-metal-resistance genes linger after aerobic composting [150], and microplastic loads in finished biosolids reach 10⁴–10⁵ particles kg⁻¹ dry matter [12], signaling that agronomic deployment still lacks a universally accepted contaminant safety framework.

Parallel research converts sludge into functional adsorbents. Willow-char and activated carbon remove up to 95% of multi-metal loads from wastewater [120], while ferrite-loaded biochar captures 160 mg Pb²⁺ g⁻¹ [128] and nitrogen-doped char oxidizes 55% NO at ambient temperature [124]. Oxalate-modified zero-valent iron accelerates ozone decomposition to slash ARG abundances by three log units in sludge [135].

Construction-material valorization of sludge ash is gaining momentum. Fly-ash replacement in concrete sustains compressive strengths up to 55 MPa [143], while self-foamed ceramics containing slag achieve 65 MPa with 0.45% water absorption [141]. Lightweight bricks sintered with Brazilian sludge remain within performance norms at ≤6 wt% addition [139].

Risk-monitoring and digitalization threads weave through multiple clusters. An edge-IoT architecture employing Faster Region-based Convolutional Network (Faster R-CNN) replaces conventional PLCs and furnishes real-time sludge-quality data with minimal network load (S1). Image analysis neural networks predict total and volatile solids during composting with ≤3% error [133], and computational fluid dynamics models inform injector placement in 660 MW boilers retrofitted for sludge co-firing [171].

Fewer in number, life-cycle, and techno-economic perspectives are beginning to anchor technology selection. A comparative review concludes that composting is the most greenhouse-intensive route, whereas pyrolysis can approach net-negative emissions when energy and material recovery are optimized [162]. A SWOT–FAHP evaluation ranks pyrolysis over anaerobic digestion, gasification, and incineration for overall feasibility in developed contexts [91]. However, another meta-analysis underscores the four-fold spread in reported biogas yields and the lack of digestate data, calling for harmonized full-scale inventories before LCA outcomes can be trusted [72].

Finally, a vanguard of studies conceptualizes integrated biorefineries. A solar-assisted multigeneration plant promises 7.7 MW of electricity, freshwater, and cooling from sludge [169]. A municipal solid-waste and municipal-liquid-waste hybrid couples hydrothermal liquefaction, anaerobic digestion, and microbial-electrolysis cells to deliver bio-oil, biogas, hydrogen, and reclaimed water in alignment with SDGs 7, 11, and 13 [168]. Co-Mo-catalyzed gas upgrading converts sludge-derived gases into carbon nanotubes and hydrogen [164].

Collectively, these articles paint a landscape where biological and thermochemical approaches are no longer competing, but are increasingly complementary, united by digital monitoring and driven by circular economy imperatives. However, critical hurdles persist, such as comprehensive control of emerging contaminants, rigorous pilot-scale techno-economics, harmonized life-cycle datasets, and policy mechanisms that translate laboratory innovation into deployable infrastructure. Overcoming these barriers will determine whether municipal sewage sludge completes its transition from regulated waste to a scalable source of renewable energy, nutrients, and critical raw materials.

3. Energy Recovery from Sewage Sludge

3.1. Anaerobic Digestion

Anaerobic digestion remains the cornerstone for energy recovery from sewage sludge, converting organic solids into biogas containing 45–65% methane via four sequential microbial stages. Typical yields range from 0.20 to 0.35 m³ CH₄ kg⁻¹ vs. removed, and most plants deploy mesophilic continuously stirred tank reactors (35–38 °C, 15–25 d HRT); thermophilic, two-stage, fixed-bed, or UASB designs are selected only when high organic loads or very dilute feeds justify the extra control requirements [7]. Although capital and operating costs exceed those of aerobic stabilization, the biogas generated can satisfy a large share of a facility’s energy demand. Process performance is highly feed-dependent; co-digesting carbon-rich wastes, such as fruit and vegetable residues or manure, corrects low C/N ratios and can raise methane production several-fold [77,172]. Targeted pretreatments further boost efficiency; solar-assisted, low-temperature hydrothermal heating (100–130 °C) that is applied downstream of digestion enhances the biodegradability of particle-rich sludges [77]; solid-CO₂ dosing at a SCO₂:sludge ratio of 0.3 increases solubilization and lifts the methane yield to 337 cm³ CH₄ g⁻¹ vs. under thermophilic conditions [173]; Fenton oxidation reduces pharmaceutical toxicity and achieves 446 cm³ CH₄ g⁻¹ vs. with an optimized reagent balance [174]; and full-scale ultrasound installations in Poland improve gas yield and cake dewaterability when the energy balance is tuned [71]. Metagenomic profiling links these gains to increased Firmicutes and a shift toward hydrogenotrophic methanogenesis, which is more robust at high organic loads [175]. Finally, coupling digestion with plasma pyrolysis produces a biochar that acts as a microbial carrier, further enhancing biogas quantity and quality [176].

3.2. Thermal Processes

3.2.1. Drying

The most basic thermal treatment in the chain is drying, a unit operation that removes residual water by forced evaporation, lowering the moisture content to 8–10% [177]. Unlike dewatering, which relies solely on mechanical pressure, drying supplies heat in the vapor phase through three configurations: direct dryers, where the sludge is in contact with a stream of hot gas; indirect dryers, in which heat crosses metal surfaces without mixing air and solids; and solar dryers, which use solar radiation in dedicated greenhouses. Plants can run in batch or continuous mode, and the choice depends on throughput, inlet dry-solids content (15–25%), and downstream destination.

The water industry has for years employed drying of stabilized, mechanically dewatered sludge to further shrink volumes, cut logistics costs, and enable use as EQ biosolids or as a pre-treatment for more intensive thermal processes (pyrolysis, gasification) [178,179,180,181,182]. Within a circular economy framework, the ability to package high-calorific-value dried pellets (about 13–15 MJ kg⁻¹) opens co-fuel markets. At the same time, waste heat from combined heat and power (CHP) units or biogas congenators is often recovered to run the dryer, improving the plant’s overall energy balance.

3.2.2. Torrefaction

Torrefaction is a low-temperature thermochemical treatment (200–300 °C) carried out at atmospheric pressure in an inert or low-oxygen atmosphere (flue gas, super-heated steam) with short residence times. As with lignocellulosic biomass, dried sludge undergoes dehydration and partial devolatilization, yielding a carbonaceous, hydrophobic solid easily milled with residual moisture <10% [183]. The process lowers the O/C and H/C ratios, raises the calorific value, and makes the material suitable for co-combustion or gasification. Over two thousand studies published in the past decade attest to growing interest; a “coal-like” solid is the main product. At the same time, condensable vapors and combustible gases are valuable by-products.

For municipal sludges, torrefaction produces a solid fuel of higher quality than dried sludge, facilitates subsequent thermochemical steps (pyrolysis, gasification), and opens applications as a soil amendment or adsorbent material thanks to its greater porosity [184,185,186,187,188,189].

3.2.3. Incineration and Co-Incineration

Incineration is the oldest form of thermal valorization; first developed in the United Kingdom at the end of the nineteenth century [190], it is based on excess-oxygen combustion that oxidizes the organic fraction of waste at temperatures above 850 °C, converting it into three streams: a flue gas that must be cleaned, a mineral solid fraction (ashes), and heat that can be recovered to produce steam and electricity. Large municipal waste plants can handle more than 3000 ton per day⁻¹. However, small mobile units and “co-incineration” configurations exploit existing infrastructure, such as cement kilns, in which the clinker’s calcium oxide incorporates the sludge ashes [191,192].

For MSS, the main limitation is moisture; fluidized-bed studies show that, below 40% water, the sludge burns stably without auxiliary fuel [193]. This aspect makes an efficient dewatering and, if necessary, drying line essential. In Germany, Japan, and other economies lacking natural phosphate deposits; sludge mono-incineration is now the preferred treatment because it couples the destruction of persistent micropollutants (pharmaceuticals, MP, pathogens) with the possibility of extracting phosphorus by valorizing the ash [194,195,196]. Environmental constraints are strict: European regulations require multi-stage abatement systems for particulates, NO_x, SO₂, heavy metals, and dioxins. A critical operational aspect is the risk of self-ignition of sludge cakes (160–186 °C) during storage and transport, which calls for rigorous fire-prevention protocols [197].

Net energy recovery varies with inlet moisture: co-combustion with coal in existing boilers has shown NO_x reductions of about 150 ppm and comparable electrical efficiencies, but introduces challenges in managing the resulting slag [88]. Ash from mono-incineration provides a feedstock concentrated in phosphorus and critical metals that can be recovered via acid leaching, thermo-phosphatic processes, or bio-extraction; compositional variability makes a site-specific approach necessary [198].

Worldwide, sludge incineration remains uncommon in low-income countries, owing to its high cost and operational complexity. Nevertheless, evidence that biological stabilization alone does not eliminate emerging contaminants leads several nations to consider incineration as a long-term solution to health concerns and nutrient-circularity objectives.

3.2.4. Pyrolysis and Gasification

Among the thermochemical routes, pyrolysis and gasification share the goal of converting the sludge’s organic fraction into high-density energy carriers, but they differ in thermal regime and end-products. In the classical sense, pyrolysis is the decomposition of organic matter in the complete absence of oxygen: starting at about 300 °C, biomass undergoes exothermic cleavages that, in lignocellulosic materials, can self-sustain heating up to 500–600 °C [199,200]. The result is an anhydrous carbon-rich solid with a high LHV, accompanied by a mixture of vapors and gases which, if the feed water content is below 35%, can be burned in situ to supply the process energy.

Applying pyrolysis to digested, mechanically dewatered sludge nevertheless requires an additional drying step to avoid forming a viscous bituminous phase that is hard to handle and highly polluting. By contrast, prior passage through HTC yields a hydrochar (HC) with a favorable organic- compounds-to-moisture ratio. During pyrolysis, the combined organic vapors and water are combustible and can fully support the endothermic phase, eliminating any need for condensation [201]. This aspect opens a possible territorial scheme in which WWTPs deliver HC to a centralized pyrolysis plant, minimizing the final char quantity and producing a solid of very high stability and energy value, together with oils rich in phenolic compounds usable as a chemical platform.

Gasification operates under a controlled oxidative regime: above 700 °C, with metered doses of air or steam, the sludge carbon reacts to generate syngas composed of H₂, CO, CO₂, and traces of heavier hydrocarbons. Subsequent water–gas shift (CO + H₂O → CO₂ + H₂) and membrane separation allow for the production of high-purity hydrogen [202]. For sludges, the reaction rate is lower than for coal, and the hydrocarbon fraction in the raw gas is higher: a catalytic reforming stage and a particulate/tar cleanup system are therefore required, raising the plant’s minimum economic scale. Pilot studies indicate that gasification becomes viable only above supply thresholds of >30–50 kton DS yr⁻¹ [202,203,204,205,206], which is why it is often preceded by pretreatments (drying, torrefaction, HTC, or flash pyrolysis) that increase energy density and reduce moisture.

In summary, the sequential combinations HTC–pyrolysis or HTC–gasification can drastically cut solid mass and maximize energy recovery: the first yields high-quality char and liquid fractions for chemical applications, and the second yields a syngas that can be steered toward power-to-gas, hydrogen production, or chemical looping. The choice between the two paths will depend on territorial scale, the availability of recovery heat, and the target product (solid vs. gas).

3.2.5. Hydrothermal Carbonization

Hydrothermal carbonization is one of the most promising routes for valorizing sludge with extremely high water content today, without resorting to costly drying steps. Operating at 180–250 °C under the autogenous pressure of water, with liquid/solid ratios of 5–10%, the process exploits the peculiar chemistry of water in sub-critical conditions; the increase in the ionic product, combined with the reduction in dielectric constant, turns the reaction medium into a quasi-apolar solvent that triggers decarboxylation, dehydration, and polymerization [207,208,209,210]. In less than two hours, thickened sludge is converted into a carbonaceous solid, the HC, a process water rich in organic compounds and nutrients, and a small gaseous stream mainly composed of CO₂. Typically, 55–65% of the dry matter is captured in the HC, while the liquid holds the soluble share of COD and nitrogen.

The operational impact is considerable, especially for small- and medium-sized wastewater-treatment plants that lack anaerobic digestion: installing an HTC reactor directly on-site can cut the volume of sludge to be disposed of to about one-fifth of the original. The hydrochar (hydrophobic, sterilized, and with an LHV close to 22 MJ kg⁻¹) can be further dried with recovered warm air and used as a fuel, soil amendment, or precursor for carbon-based adsorbents; at the same time, the process water can be recycled upstream or sent to digestion to recover additional energy.

Moving to harsher operating conditions, hydrothermal liquefaction (300–360 °C; 10–25 MPa) converts up to 50% of the dry matter into a refinable crude, whereas supercritical-water gasification (SCWG) (>374 °C; >22 MPa) transforms the organic fraction into a syngas rich in H₂ and CH₄, leaving an inorganic residue extremely concentrated in phosphorus [211,212,213,214,215,216,217,218,219,220,221,222]. The HTC-SCWG pairing, besides avoiding incineration, thus offers a potential route that couples energy recovery with the recovery of critical nutrients.

While pilot plants and first full-scale applications (Europe, China), often organized as “carbon hubs” that receive sludge from several WWTPs, are multiplying, several barriers remain: the lack of explicit regulatory recognition in sludge directives, the need for dedicated handling of process liquor in stand-alone installations, and corrosion or fouling problems in supercritical reactors that still await mature engineering solutions [223].

Since 2017, the international community interested in HTC technology has been meeting every two years at the International Symposia on HTC—key events for sharing research, experiences, and technological advancements. After the first edition held in London, the symposia series continued in Berlin (2019) and, despite the break in 2021 due to the pandemic, resumed in 2023 in Seoul, with the participation of OECD representatives. The next event is already scheduled for 2027 in Sardinia, Cagliari. These symposia represent an important platform for promoting the adoption of HTC, especially in MSS, where the technology has shown particular effectiveness and promising application prospects [224].

3.3. Technological Advances and Process Integration

The latest frontier for WRRFs is no longer limited to tailor-made catalysts, reactors in series, or CO₂-capture systems to curb emissions from pyrolysis and gasification [160,225]. Attention has shifted to layouts that network the various unit operations, turning the plant into a biorefinery able to extract energy, nutrients, and critical raw materials from sludge simultaneously. In large-scale activated sludge processes (ASPs) + AD lines, for instance, cogeneration heat feeds dryers, thermal-hydrolysis units, and, further upstream, advanced oxidative treatments for micropollutant removal. The Danish VARGA project at Avedøre illustrates the “waste-as-feedstock” logic: ozonation, ammonia stripping, and biopolymer production from the process water operate in cascade, using the residual stream of one unit as the raw material for the next.

Hydrothermal carbonization completes this circular model, integrating with the biological and thermochemical processes already in place. When the high-COD aqueous stream leaving the HTC reactor is sent to anaerobic digestion, the organic load falls sharply, and the biogas produced allows for an overall energy recovery of 50–90%, with a self-sustaining heat balance [226,227,228,229]. If the same liquid instead undergoes catalytic aqueous-phase reforming at temperatures above 250 °C, additional methane- and hydrogen-rich fractions are generated, and the recovered energy can approach 95%. At the same time, the residual COD is markedly reduced [230].

The HC produced by HTC, in turn, is a secondary source of strategic elements. Coupling it with a hydrometallurgical module solubilizes phosphorus, often present at concentrations equal to or higher than those of natural phosphate rock (11–15% P), thus offering a competitive secondary supply source [231,232]. HTC efficiency improves further when homogeneous catalysts (organic acids or salts such as ZnCl₂ and FeCl₃) are used, shortening reaction times and enhancing HC properties (co-HTC) [233].

Critical issues remain, however. Managing the liquid stream, laden with PAHs and heavy metals, requires targeted treatments (adsorption, wet-air oxidation, selective stripping); valorization of the light gases is not yet widespread, and the initial investment remains high [234].

3.4. Tecnology Comparison

To guide technology selection, a concise comparison of the principal sludge-to-energy routes is presented in

Table 3. Qualitative comparison of the main energy valorization technologies for MSS.

Technology	Main Advantages	Main Drawbacks
AD	Mature, positive energy balance, adaptable to co-digestion	Requires strict process control; digestate handling
Thermal hydrolysis + AD	+20–40% methane; better dewaterability	High-pressure steam demand
HTC	Treats wet sludge; produces high-energy hydrochar; captures phosphorus	Process-water treatment; comparatively high CAPEX
Drying	Reduces transport costs; enables pellet fuel	Significant heat demand; odor/emission control
Torrefaction	Upgrades fuel quality; eases downstream pyrolysis/gasification	Needs prior drying; condensate management
Incineration/co-incineration	Complete pathogen and micropollutant destruction; phosphorus-rich ash	High capital/operating cost; stringent flue-gas limits
Pyrolysis	Generates bio-oil and biochar for energy/material use	Requires low-moisture feed; tar clean-up
Gasification	Produces syngas/hydrogen; large volume reduction	Tar reforming CAPEX; economical only at large scale

. The table highlights the key operational advantages and drawbacks of each process in order to see at a glance where a technology excels and where its main constraints lie.

As Table 3 shows, anaerobic digestion remains the most established option, while hydrothermal carbonization is emerging as a versatile solution for high-moisture sludges. Thermal routes, such as pyrolysis, gasification and incineration, become attractive when complete contaminant destruction or high-density energy carriers are required. Building on the qualitative analysis in Table 3, the next section translates those operational advantages and drawbacks into numbers using recent techno-economic data.

While the land application of stabilized sludge can still deliver nitrogen, phosphorus, and organic carbon to soils at relatively low cost, its viability is steadily eroding as contaminant limits for heavy metals, microplastics, and PFAS tighten and public acceptance wanes in densely populated regions. Against this backdrop, energy-oriented routes provide a resilient alternative whenever pollutant levels exceed agronomic thresholds. Crucially, energy and nutrient recovery are not mutually exclusive: hydrochar and incinerator ash retain up to 80% of the incoming phosphorus, which can subsequently be extracted via acid leaching or thermochemical processes. Hence, technology choice hinges on (i) current and forthcoming contaminant regulations, (ii) the relative value of displaced energy versus fertilizer, and (iii) the availability of infrastructure for downstream phosphorus extraction. Given the trajectory toward ever-stricter standards, high-quality biosolids will continue to go to agriculture, but an increasing share of sludge is likely to be routed first through energy valorization pathways, with phosphorus recovered later from the residual solids.

4. Economic, Environmental, and Future Considerations

4.1. Techno-Economic Analysis

Economic analysis of sludge-to-energy valorization yields markedly different results depending on the technology and the incentive landscape. A recent Aspen Plus study of four AD configurations showed that coupling the digester with 100% downstream wet oxidation (WO) is both the cheapest option (lowest unit treatment cost) and the most energy-efficient, thanks to solids reduction and better dewaterability (cake at 51% DS) [235]. Pre-treatment, such as TH or partial wet oxidation (20% partial WO), raises biogas yield, but increases OPEX; TH, in particular, tops the scenarios for operating cost, while the two WO layouts require the highest CAPEX. Sensitivity analysis highlights plant capacity and operating cost as the main bankability drivers, followed by CO₂ management: the optimal WO scenario generates up to 2.1 ton d⁻¹ of CO₂, making a capture or reuse strategy essential to stay climate-competitive.

In Denmark, HTL of sludge + fat, oil, and grease (FOG) + food waste mixtures reports 62–76% energy recovery and payback < 7 years [236]; co-HTC without drying yields fertilizer HC with internal rate of return (IRR) > 12% for plants < 100,000 PE [237]. A recent techno-economic comparison of four AD + HTC/combustion scenarios revealed further points: replacing a thermal dryer with HTC in an AD + combustion line cuts heat demand, but, because of the high-temperature-steam requirement, can penalize power output and offers only a modest investment margin when considered alone [238]. Economics become attractive when HTC is paired only with AD (no combustion section) and the process water is recycled straight to the digester; in that case, the extra biogas, disposal-cost savings, and HC sales offset the high CAPEX, provided that the HC price and avoided gate fee stay above critical thresholds (key sensitivity drivers). Figure 4 shows the average coal selling price trend and MSS HC selling price.

Covering HTC’s heat demand with wet oxidationWO is not competitive: it costs more, lowers power export, and increases CO₂ generation. The economics of HTC + combustion improve if the plant accepts external sludges for a fee to co-combust with HC.

Growing attention is being paid to sludge co-combustion in municipal WtE plants, which is seen as a win–win in cutting disposal costs and raising boiler thermal load. Experimental and modeling studies have confirmed technical feasibility (lower NO and HCl, controlled dioxins), but lacked a complete economic picture. A recent study [239] simulated the heat balance and performed the first comparative techno-economic analysis on four sludge-drying schemes before mixing with municipal solid waste (MSW): flue gas drying, steam drying, in-plant electric heating, and in situ electric heating at the WWTP. With sludges at 30–40% moisture and a dry-sludge/MSW ratio of 2–3%, boiler efficiency drops by only 0.6–1.1% and net power by 0.004–0.38 MW, depending on the scheme. Flue gas drying, which uses the incinerator’s hot exhaust directly, shows the highest NPV (>EUR 124,000.00) and the lowest sensitivity coefficient, and thus the least risk even under energy–price or subsidy changes. In-plant electric drying, by contrast, fails to repay capital because of high OPEX. The most influential profitability factors are the dry-sludge/MSW ratio (SC of about 1.12–1.17) and the unit sludge-startup subsidy (SC of about 1.23). Stand-alone plants require CAPEX two to four times higher than co-incineration [240].

A further study of four configurations (anaerobic digestion (AD), dedicated mono-incineration, co-incineration in coal power plants (CINP), and co-incineration in cement kilns (CINC)) showed that organic content strongly affects efficiency and economics. As the organic/mineral ratio falls from 70% to 40%, environmental emissions change by only 1.6–7%, but energy efficiency drops by 43–66% and project NPV by up to 317% (100 ton DS d⁻¹) [241]. AD retains the best energy and economic performance, but carries the most significant environmental burden due to heavy-metal land spreading; cement-kiln co-incineration (CINC) has the lowest environmental impact and energy/economic figures similar to AD, emerging as the preferred option for low-organic sludges where AD loses competitiveness. The results point to differentiated strategies: AD or CINC for sludges rich in organics, and CINC (or CINP) for matrices with low biomethane potential.

The SCWG + fast-pyrolysis pairing for woody residues produces 23 mol H₂ kg⁻¹ biomass, 684 kW of electric power, and 3.4 MW of excess heat, with 61% energy efficiency and 45% exergy efficiency. The minimum hydrogen selling price (EUR 2.67 kg⁻¹) is lower than for stand-alone SCWG but higher than for pyrolysis alone, indicating that feasibility hinges on the H₂ market and heat valorization. The integration is technically feasible and can treat wet sludges and dry biomass simultaneously, but it needs high CAPEX and access to green hydrogen incentives [242].

These techno-economic evaluations demonstrate that while many sludge valorization strategies are technically feasible, their economic viability often depends on external factors. CAPEX remains a significant barrier for advanced systems. Incentive schemes, such as feed-in tariffs for renewable energy, gate fees for accepting external sludge, or subsidies for green hydrogen and biochar, can significantly improve project bankability. In contrast, regions without stable regulatory frameworks or financial incentives may struggle to attract investment in these technologies. Furthermore, the market value of outputs (e.g., hydrochar, recovered nutrients, hydrogen) and avoided disposal costs play a crucial role in determining the IRR and payback time. As such, policy-driven financial mechanisms are essential to accelerate the deployment of sludge valorization systems, especially in contexts with limited economic margins

4.2. Life-Cycle Assessment (LCA)

Life-Cycle Assessment (LCA), complementary to techno-economic analysis, makes it possible to quantify environmental impacts along the entire treatment chain, from pre-thickening to final residue management. A comparative study assessed nine different options (land spreading, composting, anaerobic digestion, HTC, pyrolysis, gasification, incineration with energy recovery, WO, and landfill), applying mid-point CML-IA indicators and expanding the system boundary (system expansion) to account for credits from energy export and from replacing fertilizers or aggregates [243]. A recent global review ranks composting as the most GHG-intensive option, followed by anaerobic digestion, pyrolysis, and incineration, yet shows that targeted energy or fertilizer substitution can even turn some routes into being net-negative for climate impacts [162].

The analysis confirms that the environmental effectiveness of each route depends on operating conditions and the fate of the residues. Incinerators show real benefits only when the energy produced is exported and the phosphorus in the ash is recovered; a case-specific LCA in China reduced the climate impact of incineration by switching from coal-assisted drying to self-sustained combustion and improving heat-recovery design [86]. WO and HTC become competitive if their respective solid outputs are used as aggregates or fertilizers. Land spreading cuts the carbon footprint, but raises soil toxicity concerns. In contrast, anaerobic digestion, pyrolysis, and gasification achieve GWP or volume reductions at the cost of more complex digestate, bio-oil, or syngas management. Co-pyrolyzing sewage sludge with agricultural residues can cut aggregate mid-point impacts while delivering a strongly positive net-energy balance, whereas single pyrolysis remains the least favorable option [244].

HTC, in particular, offers an intermediate trade-off: it consumes less energy than high-temperature pyrolytic processes. However, it requires the treatment of the process water and carefully handling metal-laden char.

Systematic literature reviews reinforce this picture. A meta-analysis of twenty technologies ranks anaerobic digestion, pyrolysis, and supercritical water oxidation among the best compromises between volume reduction, GWP, and toxicity, while warning that digestate, biochar, or HC can re-introduce contaminants if not properly managed [245]. Another review reiterates that no single technology simultaneously eliminates all impact categories: performance remains tied to local context, nutrient- and energy-recovery options, and the possibility of integrating front-end pretreatments such as Advanced Oxidation Processes or TH [246].

Ultimately, the optimal cradle-to-grave choice varies with climate priorities, sludge characteristics, and available infrastructure. Only site-specific LCAs can balance energy benefits, nutrient recovery, and soil health protection, guiding investment toward sustainable technological solutions.

4.3. Future Outlook

A comprehensive 2022 study on the prospects for managing sewage sludge originating from municipal wastewater treatment [7] concluded that the most promising future strategies hinge on a close integration of well-established bio-based processes with thermal treatments that are able to recover both energy and materials.

• Large activated sludge plants: The most advantageous option is an on-site hybrid process that couples the HTC of surplus sludge with anaerobic digestion of the process water and subsequent leaching of the HC. This configuration maximizes biomethane production, minimizes residual solids, and yields a sterilized, phosphorus-rich HC that, after partial removal of heavy metals, can be marketed as a fertilizer or soil improver.
• Medium-to-small plants: For these facilities, the outlook favors practical, sustainable thermal techniques (HTC or mild thermolysis) with the off-site treatment of the dewatered material, which is then directed either to agronomic use or to phosphorus recovery. Because the process water cannot be recycled into the biological loop, it requires polishing via membrane filtration followed by oxidation.

Overall, conventional biological routes, activated sludge processes (ASP) and anaerobic digestion, are expected to remain the backbone of treatment. However, biology alone will no longer suffice; composting and incineration are losing centrality to “mild” thermal technologies that can fully sanitize sludge, destroy micro-plastics and recalcitrant organics, and enable selective phosphorus recovery (a strategic element, owing to its scarcity). Finally, the modernization of sewer networks and treatment plants, transitioning from combined to separated sewers and integrating units for material and energy recovery, is identified as an indispensable infrastructural condition to make these solutions technically and economically attractive in the long term.

Although a 2023 article [8] theoretically confirmed all these perspectives, today’s reality still shows no decisive leap forward. That study stressed that HTC was no longer an emerging technology, but a concrete solution, evidenced by pilot plants operating in several countries and more than 500 patents filed. It concluded, however, that full industrial take-off would require overcoming key hurdles. Foremost among these are the management of process water and off-gases, which remain under-utilized; the liquid phase, often contaminated with polycyclic aromatic hydrocarbons and heavy metals, poses serious treatment and disposal challenges, limiting reuse options and driving up operating costs. Moreover, a stand-alone HTC plant demands substantial upfront investment to achieve a competitive production capacity, and long-term economic viability depends on its flexibility to treat a wide range of feedstocks.

A recent study [247] adds further insights into HTC development toward 2050. It predicts that the sector will handle a broad spectrum of wet feedstocks, including MSS, becoming a competitive, low-cost, and widely recognized option for organic-residue management. In the short-to-medium term, the authors set out several quantitative milestones, summarized in

Table 4. Milestones and operational targets for global industrial rollout of HTC in sewage sludge management (2028–2035).

Target	Timeline	Operational Significance
Cost parity with conventional disposal options	2028	The total HTC treatment cost (CAPEX + OPEX) must equal that of incineration or composting
20% cost reduction versus current state-of-the-art techniques	2033	Achieved through learning-curve effects and plant standardization
80% of products for carbon capture or bio-energy, 20% for high-value materials	2030	Market focus on sludge/MSW-derived biochar and nutrient-recovery fertilizers
50,000 t day−1 of residues treated worldwide	2035	Global rollout of HTC plants in urban (sludge) and agro-industrial settings

of that work.

These targets rest on the fact that the process is now deemed mature for sewage-sludge treatment, having proved capable of handling rising volumes while boosting the energy efficiency of WWTPs. Strategically, permanent carbon detection through the land application of biochar opens a dynamic, scalable market. If this route is recognized within carbon-credit schemes, the rapid construction of new plants could be self-financed with the revenues from those credits. The policy roadmap identifies complementary milestones: the establishment of an HTC industry association for standards, lobbying, and dissemination of best practices; the publication of a BAT reference document and creation of shared databases; and the development of specific legislation on carbon sequestration via bio-/hydrochar and of minimum criteria for HTC-derived fertilizers [247].

The current snapshot of the HTC market is still markedly Euro-centric, with Germany first, followed by Italy, Spain, Switzerland, and the United Kingdom. The study shows that Asia hosts four operators (three in Japan, one in Korea) and the Americas count for only a single player in the United States. On the plant side, 11 companies run at least one full-scale facility for 14 commercial units; half of the known installations are already classified as “full-industrial,” confirming the move from pilot-scale to continuous production [247].

The start-up of new HTC plants (or integrated plants with HTC) will likely coincide with the revamping cycles of existing infrastructure because that is precisely when water utilities and municipalities can swap out obsolete lines for higher-value “plug-in” solutions. This aspect also explains today’s slow up-scaling: across much of Europe, the major wastewater-treatment works date from the 1990s–2000s and will not reach the end of their useful life before the second half of this decade; replacing them sooner would require sizeable CAPEX, which is hard to justify without a targeted incentive framework.

At this stage, environmental-mitigation financing tools, ranging from EU Green Deal grants to carbon credit schemes, become decisive, converting refurbishment outlays into investments that deliver tangible returns: revenues from carbon credits, energy savings, lower sludge-disposal costs, and phosphorus recovery. The geographic distribution of plants confirms the trend: of the 14 commercial sites cataloged in a recent study (Figure 5), over 70% lie within EU member states, indicating that EU funding packages and waste directives are already steering the market.

The future of integrated sludge treatment will probably revolve around “retrofit + HTC” programs backed by environmental incentives; current delays do not reflect technological skepticism, but rather the wait for favorable investment windows. Once those windows open, spurred by regulatory pressure on climate, critical nutrients, and micro-pollutants, the leverage of the environmental benefits will make HTC an increasingly unavoidable option for wastewater treatment operators.

5. Conclusions

Municipal sewage sludge is transitioning from a disposal challenge to a valuable resource, driven by stricter environmental regulations, climate goals, and the need for phosphorus and energy recovery. This review outlines the evolution of sludge management technologies, highlighting a shift from conventional linear treatment toward integrated biorefinery approaches that aim to close energy and nutrient loops. Anaerobic digestion remains the foundational technology due to its positive energy balance and maturity. However, its performance is significantly enhanced when combined with pre- or post-treatment methods such as thermal hydrolysis, hydrothermal carbonization, or wet oxidation, which improve methane yields, reduce sludge volume, and enable phosphorus recovery. Thermochemical routes, particularly hydrothermal carbonization and mono-incineration, offer viable alternatives for sludge with low biodegradability or high contaminant loads. These technologies support both resource recovery and pollutant destruction, but require supportive policies and market development to become widely adopted.

Economic feasibility is highly context-dependent, with CAPEX, energy prices, and revenue from by-products playing a central role. Incentives, gate fees, and regulatory drivers, such as EU phosphorus mandates or emerging contaminant limits, can strongly influence implementation pathways.

Looking forward, research and policy should prioritize standardizing hybrid configurations, enabling smaller plants to access regional treatment hubs and ensuring the safe management of residuals. A new generation of wastewater treatment plants, as integrated, modular, and resource-oriented systems, will be critical to achieving environmental and economic sustainability in sludge management.