State-of-the-Art Decarbonization in Sludge Thermal Treatments for Electrical Power Generation Considering Sensors and the Application of Artificial Intelligence

Abstract

This study explores innovative strategies for decarbonizing sludge thermal treatments used in electrical power generation, with a focus on integrating sensor technologies and artificial intelligence. Sludge, a carbon-intensive byproduct of wastewater treatment, presents both environmental challenges and opportunities for energy recovery. The paper provides a comprehensive analysis of thermal processes such as pyrolysis, gasification, co-combustion, and emerging methods, including hydrothermal carbonization and supercritical water gasification. It evaluates their carbon mitigation potential, energy efficiency, and economic feasibility, emphasizing the importance of catalyst selection, carbon dioxide capture techniques, and reactor optimization. The role of real-time monitoring via sensors and predictive modeling through artificial intelligence (AI) is highlighted as critical for enhancing process control and sustainability. Case studies and recent advances are discussed to outline future pathways for integrating thermal treatment with circular economy principles. This work contributes to sustainable waste-to-energy practices, supporting global decarbonization efforts and advancing the energy transition.

1. Introduction

The removal of pollutants from carbon oxidation that can occur in electricity generation processes also called decarbonization through the application of technologies such as carbon capture and storage (CCS) has different strands of technological and scientific development being researched at the level of the evaluation of constructive configurations and control of the physical and chemical variables of the reactors in the thermal processes as pyrolysis/gasification with simulations. In the decarbonization process, the carbon dioxide (CO₂) is converted to carbon monoxide (CO), which is a flammable combustion gas to produce another economic input with reforming temperature and steam, which needs to improve in large-scale plants. However, it is still unknown whether hydrogen H₂ decreased a lot in this operation of gasification/pyrolysis. This balance changed from composition mol% of the CO₂ after being captured and recycled [1]. The processes for the thermal treatments that are covered here are presented in Figure 1.

Reaching a realistic energy transition depends on establishing a pathway in which dense power fuels are important for economic patterns in the global civilization as climate change is a priority [2]. Most studies focus separately on environmental impacts or energy performance inside power plants; however, proposing an indicator that aggregates both of these factors from global warming potential unit exergy efficiency loss evaluating life-cycle impacts of CCS technology combinations about different capacities is crucial to establishing the trade-off of each optimal opportunity, diminishing the power-related emission of post-combustion capture systems [3]. Optimization solutions are a way to improve energy systems in this regard [4,5,6]. As presented by Starke et al. [7], the graph neural networks can help the decision-making in the pump sizing process; the way that the connection is made to the grid may improve motor performances [8], and the finite element method can be applied for the optimal design of equipment [9].

Challenges to the adoption of CCS systems are mainly about the storage/distribution of chemical compounds generation with dioxide carbon transformation and heat steam from a turbine that affects the economic time return schemes to retrofit implementation increasing electricity price. This could differ when producing methanol that almost reaches zero cost of some thermal power plant types according to Liu et al. [10].

Buratto et al. [11] underline the growing demand for efficient and sustainable energy generation, resulting in a significant dependence on thermal treatments in power plants. However, ensuring exact control over these treatments remains a difficulty, and complex automation systems and sensors need to be integrated. Automation can boost safety, enhance operations, and increase energy efficiency in heat treatment processes. Real-time data collection is made possible by sensors, which are essential for monitoring and controlling vital parameters including temperature, pressure, and flow rates. This makes it easier to make quick changes to preserve ideal operating conditions and stop system malfunctions.

The search for decarbonization in thermal treatments of sludge for generating electrical energy appears as an attractive solution with significant environmental, economic, and social implications. Sewage sludge has a high treatment and final disposal cost, with a very polluting life cycle in terms of methane and carbon dioxide emissions. Thermal technologies act as a divider in this process, transforming sewage sludge into valuable inputs for the basic sanitation and energy production chain, and they can be converted into biochar, fuel oil, heat, electricity, and hydrogen, among other products, as addressed in this research (see Figure 2).

This approach not only addresses the urgent need to reduce carbon emissions but also highlights the potential to transform a waste stream into a valuable resource. By harnessing latent energy in sludge through advanced thermal treatments, we contribute to the diversification of energy sources and alleviate dependence on traditional fossil fuel-based power generation. This modification supports a more resilient and sustainable energy infrastructure and is consistent with efforts to mitigate climate change [12].

Bio-energy corresponds to an important renewable source for different countries, and the integration of these resources, increasing dioxide carbon emission capture, can benefit environmental aspects and increase minimum fuel sell price from commercial interactions including cooling chemical compounds available only for fossil fuel, reducing this dependence previously [13]. Potentially, this bio-energy concept could be responsible for producing 12.5 million tons of hydrogen with 133 million tons of CO₂ recovery, corresponding with the development goals for energy transition and carbon neutrality [14].

Chu et al. [15] perform research about the feasibility of carbon neutrality in the sludge treatment industry, and their results showed that realistic prediction will reach 30,291.65 kt CO₂e in 2030 and that optimized possibility depends on the drying process improvement to establish 5816.19 kt CO₂e in this same year. Wastewater fields are responsible for up to 7 and 10% of methane and nitrogen oxide arising from human emissions, 60% of this derived from the processes and 30% providing an energy-related indirect contribution. Decarbonization can be reached through optimization routes of chemical usage and transportation [16].

The main obstacles to clean transportation are financial planning to long-term demand, social acceptance, and willingness to pay higher prices for alternative fuel until production is improved due to research and development and regulatory stability as an investment in direct air capture technology inside large-scale production and storage facilities [17]. According to Yang et al. [18], the mitigation potential strategy has energy-saving improvement, operational optimization, and thermal energy recovery as the three main possibilities to reduce greenhouse gas emissions in China wastewater treatment plants. In the last 15 years, electricity intensity has increased, representing 52% of the total emissions inside of this sector. Promoting solutions between these factors improves pollutant removal performance as well as economic costs.

Implementing energy conversion technologies can eliminate external fuels in wastewater plants in Japan, reducing 118% of greenhouse gas emissions in the next 15 years in about 14 large cities of this country, from 2015 until 2030, integrating incineration with waste heat power generation and anaerobic digestion using solid fuel recovery [19]. In this context, researchers evaluated the efficiency obtained using chemical catalysts produced on an industrial scale in conjunction with process control and obtaining data by sensors [20].

A broad study has been conducted dependent on the different solubility rates, the concentration of CO₂, and applied volume of the catalyst and its adsorption in addition to validating the method of transferring or transporting this material and its disposition associated with cost and economic viability. There is a need to select the carbon capture process in which there is a huge range in a commercial context; it is still under development. It is determined by the percentage of CO₂ found in the oxidation gas used or generated inside or outside of the thermal reactor that produces fuel gases later converted into electricity [21]. In

Table 1. CO₂ emissions from different sludge treatment methods.

Treatment Method	Examples of CO2 Emissions (kg/ton)	Reference
Anaerobic Digestion	120	Li and Zhao [22]
Composting	250	Smith and Kumar [23]
Incineration	950	Zhang and Chen [24]
Landfilling	600	Wang and Lee [25]

, examples of different sludge treatment methods for CO₂ are presented.

Although CCS technologies are still presented as a high investment, around 40% of the economic resource of the complete project, within the thermal typologies mentioned above, can vary significantly depending on the scale, whether the technology will be used in nearby plants, and where the technology will be acquired [26]. Thus, scientific research is necessary to minimize costs and losses with CCS, enabling the generation of new marketable products such as bio-polymers and bio-chemicals resulting from the lower environmental impact providing good ecological balance [27].

One of the great challenges for decarbonization is to convince the world population to accept the change in current lifestyle and energy prices. Current CO₂ removal techniques produce power, mainly in the richest countries with intensive resource usage and discharge. However, increasing demands combined with efficient and consistent environmental policies incentivize improvement in the decarbonization technologies. The waste generated in different processes can be an alternative to reach better emission reduction worldwide [28].

A high-level architecture for integrating thermal units into existing wastewater treatment plants (WWTPs) involves pre-treating sludge through dewatering and drying, followed by conversion via pyrolysis, gasification, or hydrothermal methods. These processes generate energy and valuable by-products like biochar, which can be used for carbon sequestration. Integration with carbon capture systems and real-time AI-based controls enhances efficiency and emission reduction. This setup enables WWTPs to recover energy, reduce greenhouse gases, and support a circular economy [29].

Hydrogen production derived from organic waste is a valuable route to energy supply with lower emissions. It provides better decarbonization than conventional methods from fossil fuels since 90% is used as feed-stock originating from pollutant resources with an estimated demand of 8.6 Mt in 2020 globally [30]. In the complete process of the biomass-to-energy trading chain, analyzing this supply by machine learning tools improves the ability to meet sustainable development objectives by reducing uncertainties and collaborating to increase efficiency. Its relationship with the carbon sequestration ecosystem operated within all planting phases, harvesting, and operation of the transformation plant is also examined by regression analysis, determining the capacity to reduce risks of carbon emission in implementation from the initial stage to long-term operation [31].

Since high investments are demanded that could be associated with greater gains in the previous treatment of biomass through biomass refineries so that lower water consumption is achieved, a higher ratio of energy produced per ton, products of greater economic value, higher electrical power produced, and energy sustainability are evaluated according to the possibilities of maximizing controls concerning safety, health, and environmental measures that can be achieved with the developed refinery [32].

The research highlights various technologies for decarbonizing the thermal treatment of sludge to generate electricity, each with its uses, advantages, and disadvantages. Comparing them is crucial to determining the most effective and sustainable approach in different contexts.

Table 2. Comparative overview of technologies, concepts, applications.

	Concept/Application
Pyrolysis	Thermochemical conversion of sludge into bio-oil, pyrolysis syngas, and biochar in an oxygen-free environment. Energy generation; Biofuel production; Soil improvement; Materials production [33]. Production of biochar, a versatile material with potential for carbon sequestration, soil improvement, and application in various industries. Reduction in the volume of sludge. Production of synthesis gas from pyrolysis [34].
Gasification	Thermochemical conversion of sludge into synthesis gas (syngas). Energy generation; Hydrogen and Chemical products production [35].
Co-combustion	Burning sludge together with other fuels, such as coal or biomass. Energy generation; Reduction in the volume of waste [36].
Hydrothermal carbonization	Treatment of sludge with water at high temperature and pressure to produce hydrocarbon [37].
Supercritical water gasification	Sludge gasification using water in a supercritical state to produce hydrogen and other gases. Hydrogen production; Energy generation [38].

provides a comparative overview of the technologies discussed here.

It is worth noting that some technologies, such as pyrolysis and gasification, offer a wider range of applications and products compared to others. The choice of the most suitable technology depends on the specific characteristics of the sludge, local needs, and the costs associated with each process. In addition, integrating different technologies, such as combining gasification with CCS or using biochar from pyrolysis to improve carbon capture, can lead to more efficient and sustainable systems [39].

1.1. Problem Statement

Municipal sewage sludge, a byproduct of wastewater treatment, poses substantial environmental and economic challenges due to its high carbon and moisture content, leading to significant greenhouse gas emissions and elevated costs for drying and disposal. Conventional thermal treatment routes, such as incineration, pyrolysis, gasification, and co-combustion, often require intensive energy inputs for dewatering, exhibit variable product yields, and lack cohesive integration with carbon capture technologies, resulting in suboptimal energy recovery and a persistent carbon footprint. The absence of real-time, sensor-based monitoring and AI-driven process optimization hampers the dynamic control of key operational parameters, underscoring a critical need for integrated decarbonization strategies to enhance efficiency, minimize emissions, and transform sludge into valuable energy and material resources.

1.2. Justification

The drive to decarbonize sludge thermal treatments for power generation is rooted in the urgent need to mitigate greenhouse gas emissions from wastewater management, where conventional disposal methods release significant CO₂ and methane into the atmosphere. By converting sewage sludge, an otherwise costly and polluting by-product, into valuable energy carriers such as bio-oil, syngas, and biochar through processes like pyrolysis, gasification, and co-combustion, we not only reduce the carbon footprint of sludge disposal but also recover energy that offsets fossil fuel use. Moreover, integrating advanced sensor-based monitoring and artificial intelligence for process control enhances operational efficiency and ensures optimal reaction conditions, thereby maximizing decarbonization potential while supporting a circular economy that transforms waste streams into sustainable resources.

1.3. Research Gaps

Despite significant progress in sludge-to-energy technologies, several critical research gaps persist: the influence of feedstock heterogeneity, particularly moisture content and pH, on hydrothermal carbonization yields and mass results in poorly characterized energy balances; supercritical water gasification lacks robust heat-integration strategies for high-moisture sludges and requires corrosion-resistant reactor designs; microwave pyrolysis systems face high maintenance, corrosion, and capital costs that have yet to be economically mitigated; catalyst recovery and lifecycle performance of CCS sorbents in dynamic thermal processes are still technically and financially challenging; the physicochemical properties, standardization, and field-scale validation of sludge-derived biochars and nano-adsorbents for CO₂ capture need comprehensive assessment; and although AI-driven, reduced-order models and real-time sensor networks show promise for optimizing reactor conditions, their reliability and scalability in pilot- and full-scale sludge treatment plants remain unproven.

1.4. Objectives

Given these challenges, this paper has the following objectives:

• Provide a comprehensive overview of decarbonization strategies in sludge thermal treatments for electrical power generation, situating sludge-to-energy within broader climate change mitigation efforts.
• Explore and critically assess innovative thermal technologies, including pyrolysis, gasification, and co-combustion, focusing on their potential to enhance energy recovery while minimizing CO₂ emissions during sludge conversion.
• Evaluate the impact of process parameters, feedstock characteristics, and operational conditions on the efficacy of each decarbonization route, thereby identifying key levers for optimization.
• Examine the integration of advanced sensor networks and artificial intelligence-based control to enable real-time monitoring, dynamic optimization, and enhanced safety in sludge thermal treatment processes.
• Identify existing research gaps and propose future directions, guiding the development of next-generation decarbonization technologies and supportive policy frameworks for sustainable sludge management and power generation.

1.5. Novelty

This paper delivers a pioneering synthesis of decarbonization strategies for sludge thermal treatments aimed at electrical power generation, distinguished by its integration of advanced sensor networks and AI-driven process optimization. Unlike prior reviews that treat environmental impacts, energy recovery, automation, or AI applications in isolation, this work holistically brings them together to

• Evaluate state-of-the-art thermal conversion technologies (pyrolysis, gasification, co-combustion);
• Propose novel performance indicators that jointly consider global warming potential and exergy efficiency losses;
• Outline a roadmap for embedding real-time sensor feedback with machine learning models to dynamically optimize reactor conditions. By explicitly charting research gaps and future directions, particularly the coupling of AI forecasting with adaptive control, this paper forges a clear path toward more sustainable, resilient sludge-to-energy systems.

1.6. Methods for Literature Search

Databases and Timeframe: The research presented in this paper considered Scopus, Web of Science, and Google Scholar for articles published between January 2010 and December 2024.

Search Terms: We used the following keyword combinations, with Boolean operators: (“sludge” OR “sewage sludge”) AND (“thermal treatment” OR “pyrolysis” OR “gasification”) (“decarbonization” OR “carbon capture”) AND (“power generation” OR “sensors” OR “artificial intelligence”).

Inclusion and Exclusion Criteria: Included: Peer-reviewed articles in English addressing sludge thermal processes with decarbonization or energy-recovery focus. Excluded: Conference abstracts, non-English publications, master or PhD theses.

2. Decarbonization Techniques

When it comes to the two most popular technological paths, the yield and products produced are altered by the primary parameters that inherently separate the pyrolysis process into slow and fast: reaction time and heating rate. However, because it is new and complicated by the mechanisms and difficulties of recovering the chemical molecule used as a catalyst, it is uncertain how CO₂ is captured in either of these two pathways with the addition of different catalysts and the impact of these parameters under investigation. It depends on action that varies based on whether this is in situ or ex situ, and data are easier to recover from the latter mode because the catalyst does not mix inside the primary pyrolysis reactor, allowing for higher filtration [40].

The upgrading stage with the reuse of concentrated CO₂ including bio-methane from anaerobic digestion allows the direct injection in the existing natural gas infrastructure providing minimization of the use of fossil gas including emission savings about the extraction benefiting competitiveness inside the rural ecosystem with the value chain of the agricultural sector in the decarbonization perspective [41].

There are some technical routes for capturing carbon, but the most applied worldwide are divided into two processes that are pre-combustion and post-combustion, both reaching 90% efficiency but needing significant investments and an amount of energy with the capacity to produce NOx resulting in undesired emissions. The second option is implemented with a retrofitting process, making it flexible mainly because of the economical possibilities of input. The first option can be combined with bio-energy to create negative emissions [42].

Regarding trends concerning hydrogen production from low-carbon pathways, they include bio-gas pyrolysis, membrane separation, biomass gasification, bio-gas reforming, and electrolysis to reach sustainable targets and fuels derived from fossil resources; they are alternatives that could reduce emissions in the modern perspective. According to Pleshivtseva et al. [43], the current electrolysis projects with carbon capture and storage have higher efficiency compared to the completed past ones.

Current electrolysis projects with carbon capture and storage increased their efficiency by 7.2-fold compared to the completed ones of 1975 [43].

The economic and technical performance of the membrane system applied in a pre-combustion system is better than physical gas–liquid absorption concerning decarbonization techniques. Biomass gasification provides the highest energy conversion efficiency to produce green hydrogen and cost penalties, with concepts up to 300 MW of output equivalent to 100 thousand Nm³/h of hydrogen with 99.95% purity [44].

Co-pyrolysis is a reaction that occurs when two raw materials are added to the reactor. If organic products are added, water forms, which, once the steam is reformed, produces free hydrogen as gaseous fuel, mostly in slow pyrolysis. The biomass is dependent on the reaction time, which influences the yields and characteristics of the products produced during the pyrolysis process, lowering the rate of carbon and oxygen. Furthermore, the performance and interference of the many carbon capture systems under investigation are unknown and have only recently been used in industrial and experimental settings [45].

Small power plants are more likely to use the thermal–chemical processes of gasification and pyrolysis, which enable plant compaction through methods like incineration and installations near the biomass or raw material. Oxidation occurs, and material is produced in a way that allows for the reuse of the heat produced or by-products like biochar in a variety of energy applications and technological goods including carbon sequestered by the soil, gas adsorbents, fuel cells, super-capacitors, and activated carbon [46].

Li et al. [47] analyzed the carbon footprints of three thermal technologies that are HTL, pyrolysis, and incineration with the software OpenLCA and Ecoinvent Database verifying that the first one cited has the lowest greenhouse gas emission of 172.50 kg CO₂ eq/ton of sewage sludge while pyrolysis and incineration processes present 242.02 and 322.23, respectively, indicating that HTL has an advantage mainly because of its energy consumption ratio in relation of the input and recovered energy.

Incorporation of a mixture of different aggregates as biochar of the sawdust pyrolysis process and bottom ash from municipal domestic waste incineration is evaluated for CO₂ capture and sequestration. It is found to improve mechanical properties in the construction industry and contribute to pore structure attributed to the nucleation of biochar particles forming innovative carbon capture artificial aggregates with maximum carbon rate captured around 26, 27 kg per ton of this mixed compound [48].

Wood biochar is a carbonaceous substance studied as an adsorbent of CO₂ with capacity depending on different factors, from modification techniques such as metal doping, activation, and surface functional group to improve the adsorption mechanism of CO₂ stored for industrial applications to cost-effectiveness of durability and production price, neutralizing another hazardous component in waste gas streams [49].

According to Wang et al. [50], biochar in different applications mitigates 2.56 × 10⁹ greenhouse gas emissions per year, contributes as an immobilization agent in the soil with fertilizer, and has the potential to be a green catalyst of bio-refinery. Digestate solid derived from cattle slurry of anaerobic digestion can produce material for direct carbon oxide capture when inserted in a pyrolysis reactor, increasing the concentration of pollutant gas according to increased temperature from 400 °C to 800 °C due to pore volume and surface area [51].

Catalyst recovery in thermal treatment as gasification and pyrolysis is an important step to decarbonization and circular economy; however, the processes are still expensive, complex, and unreliable since cost-effective factors are challenging, including environmental issues for regenerative catalysts [52].

Ionic liquids capture carbon composed of inorganic salts and coordination anions due to large molecular structures having lower adsorption capacity. However, changing the composition makes it easy to increase the interaction by introducing carboxyl or amine groups. Increasing the proportions adsorbed beyond increased pressure contributes to enhancing this capacity even further [53].

The most widely used strategy of CO₂ capture is from solvent absorption divided into physical, chemical, or physical–chemical methods. The physical solution applies propylene carbonate or polyethylene glycol in different conditions including high operating pressure; those are the main solvents used nowadays. However, this is not an obstacle for the chemical methods that allow operation in low pressures despite the high energy consumption due to evaporation loss and oxidative degradation that needs to be reduced with better heat absorption presenting lower efficiency than physical methods. Because of this problem, integration of both solutions is important to combine the advantages of each one. Other technological possibilities such as electrochemical looping combustion and micro-algae had important progress but are still not used for large commercial applications yet [54].

Post-combustion capture technologies from retrofit power plants result in 3–15 vol% to concentrated CO₂ derived according to different types of adsorbents, recovering 82% with temperature swing adsorption until 100% of CO₂ from vacuum swing adsorption using activated carbon [55]. The main considerations concerning adsorbents and absorbents for CO₂ removal are stability, safety, cost, and durability, as combining two or more different types can improve or decrease efficiency. Incorporation of nano-composites and enzyme-assisted solutions is researched to determine the potential of the action in bio-based resources of carbon capture technologies [56].

Regarding carbon capture for the steam methane reforming of the pyrolysis process, the volatile CO₂ has disadvantages compared to solid carbon because it should not be burned but should be either stored or used in a refined process to produce other chemical compounds for circular economic and decarbonization improvement, producing the blue hydrogen category [57].

Reduction in oil demand including bio-fuels such as green diesel and ethanol must be implemented by 2050, with a major part of production attributed to the freight transport sector as the naphtha manufacturing can be reallocated for biomass co-processing. Increase in emissions can thus be avoided with the insertion of steam methane reforming units. It is important to estimate the individuality of refinery assessment aligned with decarbonization objectives, verifying the asset commercialization, integrating the carbon market exporting these costs, and reducing carbon lock risks. Negotiating negative emissions according to the final product profile and investment inputs is also crucial [58].

Sludge drying technology research is vital to decarbonization since it reduces energy consumption and carbon emissions. This process can be responsible for reducing 74.16% of emissions. According to Chu et al. [15], in China, carbon reduction by sludge incineration with the technology for utilizing waste heat reduces carbon emissions, including in dewatering, and its importance in other thermal application can be predicted by integrating sensors for real-time parameterization and monitoring.Challenges to removing CO₂ at lower temperatures with adsorbent are still being investigated. Materials such as lithium display good potential because of the fast kinetics to capture CO₂ at temperatures as high as 823 K, but better performance in low temperatures is needed despite CaO and MgO-based sorbents being most feasible in hydrogen production compared to other ones, including in higher temperatures using chemisorption [59].

Carbon-based materials have desirable regeneration and cost-efficiency from nanotubes, composites, and aerogels researched in the scientific literature about porosity, surface chemistry, and physical architecture. Still, high-cost chemicals are required to obtain functional properties in CO₂ capture, increasing the search for micro-porous and large-scale applications with inexpensive and abundant biomass such as sewage sludge and micro-algae integrated with artificial intelligence applications to upgrade lab models [60]. Indeed, prediction machine learning models are showing promising results compared to classic approaches [61,62,63], especially regarding time series forecasting [64,65,66,67,68,69,70]. According to Klaar et al. [71], the combination of methods can be promising solutions to enhance prediction deep learning approaches.

According to Quan et al. [72], a global solution of biomass-based carbon materials for CO₂ capture, including biochar with additions of other chemical compounds, that would promote adsorption of greenhouse gas emissions and catalytic effect is a necessary future research topic. One suggested solution of regeneration could be the application of waste materials for products as super-capacitors and batteries.

Shi et al. [73] evaluated two different thermal treatment technologies for sludge: incineration and steam gasification with carbon capture from chemical absorption and chemical looping, respectively. Lower costs were reported for gasification concerning incineration, almost half of the instances showing USD 47.31/ton and USD 91.76/ton in these two processes.

For other typical waste types used nowadays with huge production volumes for the energy market, solutions for chemical compounds accelerating the decarbonization route can be proposed. Sugarcane bagasse is a byproduct that can be greatly responsible for producing methanol worldwide by pyrolysis for the chemical industry, with carbon-negative co-production integrating physical activation and chemical loop, presenting, in evaluated scenarios, a payback for 7 years according to Su et al. [74], where three scenarios were evaluated and a prediction was made that feasibility can be improved when decreasing the green hydrogen costs in the future.

Renewable natural gas produced by different biomass as organic fractions of municipal solid waste and corn stover with the gasification process can be competitive with natural gas prices in 2024, estimated to amount to USD 1.4/J according to [75], if a carbon credit minimum of USD 90 US/ton of CO₂ is considered and the avoided cost of land-filling is included, a contribution of decarbonization policies.

Applying artificial intelligence algorithms to forecast properties of different materials with molecular simulation according to the reaction mechanism of sorption could indicate bi-functional materials that perform better with the optimized operation process, increasing the yield of H₂ in power plants and CO₂ adsorption. Decreasing tar and other pollutant emissions is a laborious task according to short- and long-term data obtained in the practical field, benefitting mainly in the large-scale projects [76]. As presented in [77,78,79,80], machine learning can be also applied for time series forecasting in power energy problems.

Shang et al. [81] evaluated 10 polluting industries in China, verifying that robotic technology and artificial intelligence investments could boost carbon neutralization over short-term and long-term periods. The authors investigated annual data from 2000 and 2020, showing that policies to incentivize these mechanisms are important in integrating green loan, credit, and bonds.

In Chinese cities, it was evaluated that robots are important mechanisms for reducing carbon intensity in labor and productivity as reported by Yu et al. [82], who investigated the International Federation of Robotics from 2010 to 2018 verifying that robotic production associated with digitalization contributes to low carbon emissions, which is necessary to achieve decarbonization goals and improve the variety of robots in different industrial fields that are not used nowadays.

The energy source of the CO₂ removal needs to be low-carbon or carbon-free to generate power for the process. One of the possible solutions is to integrate two technologies: direct air capture integrated with bio-energy with carbon capture and storage, providing the thermal and electrical energy requirements mainly using heat recovery during the cooling of syngas in the gasification and pyrolysis processes [83].

Slavin et al. [84] developed a simulation via Aspen Plus v12.1 to integrate hydrogen production from direct air carbon capture using high-temperature steam electrolysis, finding out in the simulated process that both costs decreased with production scale detaching. This improvement could identify general operating trends and depth of heat transfer calculation, aiding in facilitation of energy transfers and reducing more emissions. The estimation reached the best results in USD 124.15/tCO₂ at energy demands of 31.67 kWh/tCO₂. A summary of the key parameters covered by these authors is presented in

Table 3. Key parameters and AI/software applications for decarbonization techniques.

Technique	Process Parameters	Feedstock Characteristics	Operation Conditions	AI/Software
Pyrolysis	Dewater to ≥60% solids. Heat to 400 °C (constant). Air coefficient 2.6–2.8 in incineration chamber.	Municipal sludge, pre-dried ≥ 60% solids.	Use part of produced oil/gas for self-heating.	Machine learning (ML) to predict product yields, simulate thermogravimetry, optimize routes.
Gasification	—	Sewage sludge dried in solar facility.	CaO bed; steam agent. Syngas: H2 70–73 vol%; CO 2–3 vol%; CO2 8 vol%.	Aspen Plus simulation (syngas mass-balances).
Co-combustion	—	Sludge blended in coal boilers.	Co-incineration in retrofit coal plants. Low-NOx burners; post-combustion amine capture.	—
Hydrothermal Carbonization	—	Dehydrated sludge (15% humidity). Acid catalyst (HCl) to lower Ni/Cr; ↑P in hydrochar.	Wet medium; pressure–temperature as per [85].	—

.

3. Thermal Treatment of Sludge

The energy use of municipal sewage sludge, which is a material with high moisture content, is expensive, and a pre-treatment analysis is usually evaluated to optimize its fuel potential according to the associated costs and possible energy gains in a thermal route. Aiming at the parameterization of the optimized sludge dewatering in relation mainly to the factors of temperature, granularity, and volume that can be performed experimentally or computationally, the application of computational mechanisms is considered of high interest since experimental process costs usually have higher comparative costs, making it possible to verify intervening factors in advance in reactor kinetics and thermodynamics by establishing different heating rates by numerical simulations and machine learning algorithms that can predict the percentage of products generated, simulate laboratory analyzes such as thermogravimetric analysis, chemical distribution rates of elements such as hydrogen, and anticipate optimization routes [86].

For applying the carbonization process with municipal sewage sludge, it is recommended to dehydrate at least 60% of it before inputting it into the system to avoid external energy inputs and establish constant temperature until 400 °C in pyrolysis. Part of the oil and gas produced with an air coefficient between 2.6 and 2.8 is used in the incineration chamber to provide heat to reduce this moisture and ensure energy self-balance [87].

Plasma technology has the adaptability to retrieve phosphorous nutrients from the ash of sewage sludge to develop hydrogen simultaneously with the water contained in this waste through high temperatures in the range of 1100 to 1300 °C. Thus, there is capacity to produce 0.5 m³/kg of hydrogen gas with this wastewater treated used as fuel in an average value of carbon conversion reaching 95% and heat content up to 10 MJ/Nm³ of the flammable gas cited previously [88].

Co-incineration in coal power plants and pyrolysis of municipal sludge are the two best technologies in China as stated by Huang et al. [89] compared to anaerobic digestion and mono incineration in regard to environmental and economic performance. This results mainly from the usage of chemical and electricity consumption, as the organic content and sludge reception fee are the two usual factors that increase the difference in competitiveness of the four compared systems.

Sorption-enhanced gasification of sewage sludge dried in a solar facility using CaO as a bed material and steam as a gasifying agent results in high concentrations of hydrogen content as described by Moles et al. [90] in their experiments obtaining a syngas with 70–73% vol. of H₂ and low CO and CO₂ contents with 2–3 % vol and 8% vol, respectively.

During the process of carbonizing corn stalks together with sewage sludge, the residual heat energy, according to Zhou et al. [91], reduces greenhouse gas emissions by 126.74 kg/ton, obtaining a cost reduction of USD 23.12 compared to sludge incineration, generating 541 kWh of electricity, and allowing absolute dryness of 1.2 tons of sludge for every 1.5 tons of corn stalk. Kostowski et al. [85] evaluated hydrothermal carbonization of dehydrated sewage sludge (15% of humidity), and after gasification of hydro-char to generate syngas promoting hydrogen separation, 10 kg H₂/ton of sludge was shown. Compared to plasma gasification, which allows generating 5.5 kg H₂/ton, the advantage of this technique is non-production of ashes, which, however, reduces yield.

Membrane technology and advanced catalysis in biomass gasification are used to produce methanol, another valuable fuel, similar to hydrogen cited before. It can reduce greenhouse gas emissions and the challenge of tar-rich effluents generated in this process. The implementation of micro-channels and creation of different designs and technologies of modular reactor plug-and-play is being developed in some research [92].

For hydrothermal carbonization of sewage sludge, applying an acid catalyst benefits the produced ash properties by decreasing heavy metal contents such as nickel and chromium in the composition and increasing phosphorous proportion two-fold in comparison to the case without a catalyst. This improves soil fertility and reduces environmental impact [93]. Plasma gasification with carbon capture for municipal sludge was evaluated in simulation from Aspen Plus by Zhang et al. [94] that reached 50% of electrical efficiency operating in suitable conditions and temperatures nearest to 910 °C in a capture rate system with a capacity rate of 97%.

Pilot and demonstration-scale sludge thermal treatment plants vary widely in capacity and performance, reflecting both technological maturity and the diversity of feedstocks [95]. For instance, sorption-enhanced gasification of solar-dried sludge using CaO has produced syngas with 70–73% H₂, CO at 2–3%, and CO₂ around 8% by volume. Another reported setup achieved 2.54 kW/kg thermal and 0.81 kW/kg electrical output using dried solids, reducing CO₂ emissions by 0.59 kg per kg of sludge when compared to natural gas combustion. Pyrolysis systems typically require sludge with at least 60% solids, operating at 400 °C, and can generate significant energy from the reuse of pyrolysis gases and oils. Hydrogen yields from various technologies range from 5.5 to 10 kg/ton of sludge.

Sludge heterogeneity, particularly variability in moisture content, organic composition, and the presence of heavy metals, affects energy yields and reactor stability. Seasonal variability further exacerbates these issues by altering feedstock consistency, often requiring real-time adjustments. Fouling and corrosion are persistent issues, especially in high-temperature systems like supercritical gasification, where materials must withstand both thermal stress and chemically aggressive conditions. These challenges underscore the need for robust process monitoring, corrosion-resistant materials, and adaptive control systems often guided by artificial intelligence to ensure reliable, efficient operation [96].

Sewage sludge gasification with combined heat and power generation can reduce CO₂ emissions compared to natural gas combustion according to Carotenuto et al. [97] applied in Aspen plus simulation software producing until 2.54 kW/kg with dry solids, corresponding to the biggest percentage of thermal power and 0.81 kW of electrical power using air preheating temperature situated in 150 °C and an equivalence ratio of 0.2 reducing 0.59 kg CO₂/kg sewage sludge compared to the natural gas combustion to produce thermal and power energy.

The process of combustion with biomass can reach the highest index of CO₂ capture by applying chemical looping approximately up to 98% as reported by Fleiß et al. [98], including costs between 2 and EUR 40/ton of CO₂ using natural and synthetic oxygen carriers composed of manganese–iron–copper, leveling the costs of heat and electricity.

A novel dual circulating fluidized bed combustion reactor was evaluated by Peltola et al. [99] to perform the thermal treatment of municipal sewage sludge and verified that the process is feasible, mechanically reducing the moisture of this 20% feed-stock until 25% total solid for fertilizer recycling and self-sufficiency in terms of energy. Integrating post-combustion carbon capture technology in sewage sludge gasification makes it possible to reach negative emission in large-scale power plants at more than 50% of load conditions in the flue gas produced in the report of Subramanian and Madejski [100]. The authors applies amine in the absorption and desorption separating CO₂ and vapor collaborating with the heat and power in combined cycle gas power plants. Despite reduced power generation and the attributed costs of amine production and regeneration that need to be improved, this method evaluated and helped policy strategies in mitigating greenhouse gas emissions.

Biochar derived from sewage sludge has different potential uses since the adsorption of micro-pollutants presents a removal rate beginning at 49% and up to 99% in table water and up to 92% in wastewater. Therefore, it is considered a promising adsorbent material to effectively remove organic pollutants, pathogens, and heavy metals. Acting to improve air quality for the purpose of carbon sequestration, it also reaches a capacity of 48 mg/g and a maximum capacity of 182 mg/g of CO₂ with KOH-modified sewage sludge-derived biochar. It is important to establish a standard to ensure consistent quality and efficacy for this environmental application. The government can help the industrial production with incentives such as regulations associated with tax credits and grants [101].

Hydrogen yield can be increased with biochar in the amended dark fermentation process with ranges of contribution from 20% to 328% according to Roychowdhury and Ghosh [102] because of better electron transfer, regulation of pH, and a combination of inorganic nanoparticles. These factors contribute to a biological system producing more bacteria favoring the hydro-genesis step.

In the circular economy of pyrolysis products such as char with potential and a suitable soil enhancer, they perform the function of carbon sequesters. This thermal technology could close the material loop for polymers in their constituent monomers; oil and gas can be reused in the production cycle of industries. The main issues of pyrolysis are high energy and power consumption in developed countries. There are facilities to carry out this technology on an industrial scale; however, legislation needs to recognize the products as beneficial and establish a solid marketplace [103].

If sludge-derived biochar could be applied in brownfield soils in Europe, on degraded post-industrial lands with areas equal to or above 45 thousand km² with 0.8 tons of carbon per hectare, the highest soil carbon sequestration of this material, by 2055, it would be possible to capture between 23 and 84 million metric tons of CO₂ derived from char of pyrolysis from the sewage sludge, according to Sajdak et al. [104].

When plastics are combined with sewage sludge to produce biochar in the pyrolysis process, it is possible to enhance a carbon structure mainly with polypropylene, which benefits the surface properties and pore structure. Mixing polyvinyl–chloride reduces potential ecological risks because the release of hydrogen sulfide, sulfur dioxide, and methyl chloride, which are harmful gases, is inhibited [105].

Wood sawdust and sewage sludge co-pyrolysis is an important step to high-energy recovery and economic feasibility, improving from 141% to 191% the reductionin eco-toxicity and with a net recovery reaching 17,097 kWh/ton of dried matter [106]; it should therefore be investigated. Adding chemical components such as as chitosan, polyacrylamide, and K₂FeO₄ has the potential to increase the CO₂ capture absorption in biochar and improve the re-usability as these compontent and mixes contribute to chemisorption despite reduced specific surface area showing that the chemical capacity is more determining than physical property in this case. This factor also aided in reducing the time required for dring the sludge [107].

Sewage sludge pyrolysis can imply some advantages and disadvantages because seasonality contributes to the high content of water, and a great concentration of carbon benefits the pathogenic microorganisms as elevated nitrogen and phosphorous contents generate harmful substances. In parameters controlling automation, it is important to establish more aggregated solutions with thermal treatments that have outweighing benefits in relation to disadvantages of set-point controllers [108].

Hazardous sludge derived from the oil can be treated with co-pyrolysis by mixing fly ashes to obtain hydrogen, reducing environmental risks, especially because the heavy metals contained are immobilized in stable fractions as related by Yu et al. [109]. The authors concluded that with the addition of 50% of the weight of fly ashes mixed with oily sludge, the hydrogen yield is improved, verifying 21.02 L/kg of hydrogen without this mix and 60.95 L/kg of hydrogen after the addition cited before.

Sewage sludge-derived biochar is also a promising technology for phosphorus recovery. It can be considerably accelerated using CaCO3. The addition of 10% of this chemical compound has the potential to increase the conversion of non-apatite inorganic phosphorous into apatite phosphorous during the pyrolysis process. It may present characteristics to improve acidic soils beyond saving energy because moisture is decreased when the material is used in the sludge dewatering process [110].

Dewatering conditioners such as polymeric aluminum chloride (PAC) and cationic polyacrylamide (CPAM) prepared with biochar can increase the adsorption capacities of CO₂. This was reported by Liu et al. [111] who showed in their results that CPAM had the best efficiency on the reaction with 48.54 mg CO₂/g in the biochar mixture while PAC with biochar obtained 31.86 mg CO₂/g and untreated sludge biochar presented only 28.36 mg CO₂/g. Improved physicochemical properties of sewage sludge integrating post- and pre-treatments contribute to pollutant removal selectivity. Implementation on a large scale depends on adsorption capacity and reduction in application limitations to enable compounds to reduce carbon emissions in connection to actuation of water pollution control in wastewater plants [112].

Control of sludge moisture not only contributed to hydrogen generation but also reduced CO₂ emissions as reported by Lin et al. [113] in the microwave pyrolysis due to the energy consumption that decreased by 35%. This corresponds to 45.60% reduction in global warming achieved with this efficiency, mixing pyrolysis residue with biochar and a catalyst. Calcium-based additives in a looping system of co-pyrolysis of sewage sludge increased the consumption of CO₂ because of the mass and pH of biochar, which improved the dissolved organic matter from carbon, and the component content enhancing the surface area as the total pore volume is important for carbon capture materials [114].

Hydrogen generation promotion can be increased because of higher surface area. The presence of mesopores in the biochar contributes to this purpose according to Farooq et al. [115]. In sewage sludge gasification, the addition of nickel–cobalt and nickel–iron results in more tar cracking. Beyond surface area and porosity, the basicity and functional surface groups are important parameters for capturing CO₂ with adsorption inside the biochar [116].

Carbon neutrality can be reached with value-added energy and products derived from sewage sludge to upgrade the treatment process by applying techniques such as advanced oxidation, reducing the production of the activated sludge process and promoting green deep dewatering technology, especially verifying alternatives to the ash in the thermochemical operations as incineration and gasification that includes concrete-related, ceramic-related, and road pavements when it comes to materials other than generated biochar or bio-oil from pyrolysis evaluating the collaboration of multiple technologies [117]. Torrefaction of biomass is a possible route to substitute coal for biomass in power generation, reducing CO₂ emissions because of implemented advantages. However, it is not a mature commercial technology yet, and it research and development is needed, especially for application in green chemicals [118].

Biochar derived from agriculture as biogas and sawdust mixed with a cold-bonded artificial lightweight coarse aggregate originating from municipal solid household waste incineration bottom ash has good carbon sequestration as investigated by Liu et al. [119]. The authors verified that these compounds, mixed, reach from 30 to 33 kg CO₂/ton, providing a cost-efficient solution.

Martínez-Alvarenga et al. [120] evaluated the potassium hydroxide generation with sewage sludge using an active agent by acid purification and produced activated carbon (AC) that costs less than 8 euros/kg with humidity reduced to 80%, and its transformation yields the equivalent of 0.627 kg of AC/kg of sewage sludge.

The adsorbent technology to promote rapid cost reduction depends on meticulous modeling and in-depth economic analysis, evaluating cost attribution to assure stability in the commercial standardizing and the ranking factors to reach the scale up at the industrial level, mainly verifying the influence of the energy requirements and operational costs [121].

Energy loss caused by CO₂ removal devices by around 20 to 30% is a problem for bio-energy production. However, the best option nowadays is to recover high-purity CO₂ reaching 99 % to obtain an economic market and provide a circular economy. Some researchers are trying to reach this using nano-bio-materials such as carbon nanotubes, micro-algae, sludge, and wood waste [122].

In a closed-loop system, inserting sludge and micro-algae from pyrolysis can result in a value above 90% of CO₂ and chemical oxygen demand (COD) converting the hazardous solid waste into biochar and carbonaceous combustible gas. This application produces char as a photo-catalyst to remove antibiotics from the wastewater treatment plants, decreasing the costs from around USD 471/ton to USD 110/ton [123].

A sustainability model in a refinery that has municipal sewage sludge as feed-stock in a pyrolysis process can presume the capture and storage of carbon through management and decision support by advanced mechanisms such as meta-heuristic components in which techniques for minimizing emissions and maximizing yields in the generation of fuels and fertilizers are applied. Artificial intelligence algorithms such as neural networks are applied in the prediction of these coefficients, and sludge with higher organic content suggests the adoption of a symbiosis of pyrolysis technologies with anaerobic digestion, emitting lower levels of CO₂ and sulfur by reducing the need for drying, which can consume energy, natural gas, or other fuel because some models of pyrolysis reactors that can be damaged with excess water [124].

Muniz et al. [125] focused on the analysis of tools to measure energy sustainability. This can contribute to evaluating the impact and efficiency of thermal technologies used in sludge treatment. The use of these tools can provide valuable information for decision-making regarding the implementation of thermal technologies for sludge treatment, allowing the selection of more sustainable and efficient options.

Beik et al. [126] discussed the economic feasibility and scalability of the small-scale pyrolysis system, emphasizing its potential for decentralized waste treatment solutions. The results contribute valuable knowledge to the field of sludge management, offering a promising alternative for the sustainable treatment of sanitary sludge at smaller scales. This research marks a significant step towards advancing technologies for responsible waste disposal and resource recovery in the context of wastewater treatment.

Increasing the production capacity of the products in a pyrolysis reactor operating with sewage sludge as a biomass aids in the prediction of the char yield and the nitrogen fixation rate on a dry basis. The effect of humidity is discarded and reduces the uncertainties of the machine learning algorithm, helping in determining parameters that can be correlated to the ash produced. Currently, the thermal degradation of the constituents lignin and cellulose structure can also be evaluated, which benefits the predictability of biochemical models promoting greater efficiency in the aggregate thermal process with the obtaining of estimates of fixed carbon concerning maximum and minimum loads of biomass inserted in the reactor. This adds better adsorption conditions to the biochar produced and leads to possible better commercialization [127].

Optimal dispatch is considered essential for operation in thermal power plants, including carbon capture systems as in waste incineration, adjusting output power to load and tariff time, and adopting ladder-type carbon trading mechanisms, reducing operating costs that can be evaluated for other models of generation of sludge and another materials [128]. Table 4 presents studies on the main thermal treatments applied for sewage sludge.

One of the preponderant factors in obtaining an optimal forecast and in raising the optimization capacity in the production of compounds with greater economic added value through any biomass including municipal sewage sludge is the quality of the data, because the greater the homogeneity of the characteristics in the equipment used and more similar the applied methodology, the better this qualitative balance. Thus, the analytical protocol becomes indispensable due to the model of a future new refinery with biomass to deal with changes in biomass compositions and maintain the generation rate of biomass-derived products, minimizing the production of waste incorporated into Industry 5.0. This requires an extensive database from the advancement in the implementation of Industry 4.0 in this sector from the previous data characterization of biomass, pre-treatment methods, and the expanded knowledge of the constituents generated in the biochemical and thermochemical routes of the pyrolysis process [129].

Table 4. Main thermal treatments applied for sewage sludge.

Authors	Thermal Treatment/Challenge
Sikarwar et al. [88]	Thermal plasma with carbon capture, utilization, and storage/It needs considerable energy for plasma torch and a better pathway in innovations to reduce greenhouse gas emissions and phosphorous recovery.
Kossińska et al. [130]	Hydrothermal carbonization/Limited research about degradation, effects of moisture contents and changes in pH of sewage sludge, and its influence in the yield as improvements of mass-energy balance.
Hu et al. [131]	Supercritical water gasification/It needs a supply heat to reach the reaction temperature, especially with municipal sewage sludge, without drying with high water content. Increase in the volume concentration of sewage sludge in this process and collaboration with hydrogen production is required.
Luo et al. [132]	Microwave pyrolysis/High maintenance and operation costs, easy corrosion, and elevated costs of investments for the reactor.
Jadlovec et al. [133]	Co-Incineration/It has the greatest impact on terrestrial ecotoxicity, climate change, and human toxicity, as the challenge evaluated is to find the right blend ratio to maximize cost savings with power plant performance and emission limits.
Salimbeni et al. [134]	Slow pyrolysis and post-chemical leaching/Maximization of the phosphorous recovery and extraction of inorganic compounds separating magnesium, silicon, and aluminum are needed. Chemical extraction of silica requires high equipment and operational costs.
Zou et al. [135]	Co-pyrolysis of sewage sludge with corn stalks/Hard to integrate in the same industrial plant. However, carbon and nitrogen content is increased in biochars, the heavy metal contents present in sewage sludge are diluted, and it can be used to promote corn growth, improving the pore structure and germination rates as the potential to sequester carbon.

Table 4. Main thermal treatments applied for sewage sludge.

Authors	Thermal Treatment/Challenge
Sikarwar et al. [88]	Thermal plasma with carbon capture, utilization, and storage/It needs considerable energy for plasma torch and a better pathway in innovations to reduce greenhouse gas emissions and phosphorous recovery.
Kossińska et al. [130]	Hydrothermal carbonization/Limited research about degradation, effects of moisture contents and changes in pH of sewage sludge, and its influence in the yield as improvements of mass-energy balance.
Hu et al. [131]	Supercritical water gasification/It needs a supply heat to reach the reaction temperature, especially with municipal sewage sludge, without drying with high water content. Increase in the volume concentration of sewage sludge in this process and collaboration with hydrogen production is required.
Luo et al. [132]	Microwave pyrolysis/High maintenance and operation costs, easy corrosion, and elevated costs of investments for the reactor.
Jadlovec et al. [133]	Co-Incineration/It has the greatest impact on terrestrial ecotoxicity, climate change, and human toxicity, as the challenge evaluated is to find the right blend ratio to maximize cost savings with power plant performance and emission limits.
Salimbeni et al. [134]	Slow pyrolysis and post-chemical leaching/Maximization of the phosphorous recovery and extraction of inorganic compounds separating magnesium, silicon, and aluminum are needed. Chemical extraction of silica requires high equipment and operational costs.
Zou et al. [135]	Co-pyrolysis of sewage sludge with corn stalks/Hard to integrate in the same industrial plant. However, carbon and nitrogen content is increased in biochars, the heavy metal contents present in sewage sludge are diluted, and it can be used to promote corn growth, improving the pore structure and germination rates as the potential to sequester carbon.

Among priorities in a processing plant is to reduce energy consumption constantly and raise production and economic gain. Following this objective in thermal treatment, it is important to maximize the use of heat transfer and apply chemical reagents that act as catalysts and can maximize the activated carbon in the biochar of slow pyrolysis or bio-oil in the fast pyrolysis equipment, improving the energy balance processing with other biomass such as micro-algae. This also increases the yield of bio-fuel that can be evaluated by computational tools and machine learning algorithms, verifying the technical and environmental performance of this combination of raw materials in the reactor before the experimental test [136].

Biochar and bio-oil are the main products derived from sewage sludge slow and fast pyrolysis, respectively. The primary product yield and market value consist of the two main factors responsible for applying activated carbon pre-treatment and post-treatment. Thermal drying is crucial to reach economical feasibility. Using less acid as an activating reagent positively impacts financial and environmental sides [137]. Beyond these works, many other applications have been highlighted for sustainable development in various fields [138,139,140]. There are some advantages and disadvantages of each of sludge thermal treatment methods. In

Table 5. Advantages and disadvantages of sludge thermal treatment methods.

Method	Advantages	Disadvantages
Pyrolysis	Produces bio-oil usable as fuel or for chemical refining; generates biochar for carbon sequestration and soil amendment; reduces sludge volume and yields synthesis gas.	High energy demand (especially for sludge drying); variable bio-oil quality; requires tar cleanup from syngas; needs market development for bio-products.
Gasification	Produces versatile syngas for heat, power, or hydrogen; can integrate carbon capture to lower emissions.	Tar formation can foul equipment; requires high temperature and pressure, raising capital and operating costs.
Co-combustion	Utilizes existing combustion infrastructure; lower investment compared to new plants; reduces reliance on fossil fuels.	Still emits significant GHGs without capture; potential NOx/SO2 pollution; ash handling issues.
Hydrothermal Carbonization	Efficient for high-moisture sludge (no drying needed); produces high-energy-density hydrochar.	Emerging technology with high operating costs; process optimization and product quality still under research.
Supercritical Water Gasification	Very high hydrogen yield; low atmospheric pollutant emissions.	Requires corrosion-resistant materials for extreme temperature and pressure; still at development scale with high capital cost.

, a comparison of them is presented. In the next section, the main factors for implementing sustainability in thermal power generation processes are discussed.

Thermal treatment of sewage sludge, including incineration, pyrolysis, gasification, and hydrothermal processes, achieves the highest solids reduction (up to 90%) and ensures near-complete pathogen destruction, with options for energy recovery, but it is capital- and energy-intensive and raises air-emission concerns [141]. In contrast, biological and land-based strategies such as anaerobic digestion, composting, and land application have lower operational costs and greenhouse gas footprints, enabling nutrient recovery in the form of biogas or soil amendments and requiring less auxiliary energy, but they achieve only moderate volume reduction (20–60%) and may leave residual pathogens or contaminants if not carefully managed.

Emerging hydrothermal methods, wet oxidation, thermal hydrolysis, and supercritical water oxidation, bridge these approaches by combining effective pathogen kill, enhanced dewaterability, and energy/resource recovery, though they remain less widely adopted and carry higher technical complexity and capital costs. Life-cycle assessments generally show that while incineration has higher direct GHG emissions, co-processing in existing waste-to-energy facilities and integrating energy recovery systems can improve its net carbon footprint; meanwhile, anaerobic digestion often achieves net negative emissions through biogas utilization, making it a leading low-carbon option [142].

AI Applications

A recent thermally assisted biodrying study presented by Zhang et al. [143] applied support vector regression and random forest models to predict moisture ratio and composting temperature, achieving a coefficient of determination > 0.92 and enabling 12% cycle-time reduction under optimized heating profiles. Municipal sludge drying has also been modeled via genetic algorithm-optimized back-propagation neural networks, with sensitivity analysis showing temperature as the dominant factor and reducing prediction error by 18% compared to pure backpropagation models.

Shao et al. [144] developed a comprehensive comparative study of nine ML algorithms for predicting sludge output in wastewater treatment plants. Using real operational data, including influent volume, temperature, and wastewater quality, they found ensemble methods like extreme gradient boosting and random forest achieved the best accuracy (coefficient of determination up to 0.82), and sensitivity analysis highlighted daily inflow and ambient temperature as key drivers of sludge production.

Li et al. [145] proposed a self-organizing neural network to model the dewatering performance of waste-activated sludge. Their artificial neural network integrates multiple physicochemical sludge properties and conditioning parameters, enabling non-experimental prediction of filter cake moisture and identification of core influential factors. The model demonstrated high predictive accuracy, guiding process adjustments for improved dewaterability under various conditioning schemes.

In a review presented by Sun et al. [146] of ML applications to municipal sludge thermochemical recycling, researchers highlighted how supervised models, particularly gradient boosting and support vector machines, can elucidate reaction mechanisms, optimize process parameters, and reduce energy consumption in pyrolysis and gasification routes. This work provides practical guidance for implementing ML-driven decision support in sludge-to-resource conversion systems.

Rutland et al. [147] conducted a systematic review on ML solutions in anaerobic digestion, showing that neural networks, random forests, and gradient boosting are most prevalent for biogas yield prediction, stability monitoring, and process control. They noted challenges in scaling lab-scale models to full-scale operations due to data heterogeneity and advocated for standardized datasets and explainable AI techniques to improve model transferability.

Wu et al. [148] combined the analytic hierarchy process with an artificial neural network to evaluate competing sludge treatment scenarios, such as chemical conditioning, thermal hydrolysis, and anaerobic digestion, against criteria of energy consumption, cost, water recovery, and residual biosolid quality. Their integrated method produced a transparent ranking of options that aligned with life-cycle assessment results.

4. Important Factors for Sustainable Implementation

Maintenance costs and economic viability are vital factors in the development of a short- to long-term project for the implementation of circular economy. It can be made possible with new technical contextualization of the use of by-products via new technological routes together with the understanding of the energy model structures that can contribute with potential and significant savings of expenses. Such expenses can be reversed in profits or minimized throughout the process as the total loads of phosphorus, nitrogen, chemical oxygen demand, and energy tariffs on blowers and pumps in a Sewage Treatment Plant can aid in the recovery of nutrients such as potassium in addition to phosphorus and nitrogen from marketable heavy metals like iron and copper if they are found within the composition of the sludge [149].

An example of current sewage treatment is shown in Figure 3. In the conventional process, the water is disinfected and sent to a receiving water body. The sewage sludge, resulting from the separation and treatment of water, proceeds to equipment called a densifier, which promotes the concentration of solids and reduces the water in the composition. The thickened sludge undergoes a drying and dehydration process, which has high maintenance costs and high electricity consumption. And then it is sent for final disposal in landfills, with high logistics costs. Once disposed of in the landfill, it is expensive to maintain the buried sludge, reducing the useful life of the landfill with large emissions of greenhouse gases.

In the verification of environmental performance, it is necessary to determine the application assigned to the product generated in a thermal process that can use naphtha or liquefied petroleum gas. With the same assignment and final composition derived from fossil fuel, the gain in sustainability within the final result is little. Due to this problem, the science of the materials produced is vital in the reformulation of economic and environmental improvements. For example, the use of biochar as a substitute material in cement kilns contributes to the reduction in global warming and eco-toxicity in water [150]. When sensitivity analysis is performed, the results call for crucial decisions. Reevaluation and reanalysis is suggested so that different trends are seen, mainly in the insertion of reagents or catalysts in the process that can increase the range of products available and evaluate the gain ratio per inserted concentration of these added compounds [151].

Wastewater treatment plants can offer bio-economy as reported in [152]. According to the authors, the rising population and the ensuing shortage of protein have spurred innovation in the production of protein-rich feeds. The conversion of nitrogen-rich effluents into microbial protein/single-cell protein stands out as a promising solution. This study aims to utilize life-cycle assessment to pinpoint the most eco-friendly approach to re-purpose nitrogen and carbon flows from wastewater treatment plants. Various methods were assessed for production facilities, considering different carbon sources and pretreatment methods for the rejected water. The findings revealed that electrochemical and bio-electrochemical nitrogen recovery not only effectively extracted nitrogen from rejected water but also offered a promising solution for microbial protein production from wastewater treatment plant effluents.

As a way to achieve carbon neutrality, that is, the ability to effectively consume the same amount of carbon produced at every stage of the process, it is a a global goal to develop biorefineries with autonomous control by advanced models from the parameterization of embedded dynamic systems and with data servers that store, receive, and transmit operation data. These biorefineries can also apply minimization techniques to generate acceptable emission levels and maximization techniques in the quality and quantity of fuel, fertilizers, and chemical compounds generated. Advances in training machine learning algorithms that predict thermodynamic behaviors, evaluate immediate biomass analysis predicting volatile, ash, and fixed carbon content, and automatically re-evaluate temperatures, pressure, and heating rates with residence time would help ensure that commodities are commercialized appropriately. These metrics, which were previously calculated by machine learning models, would enable the economic modeling of the costs and financial profits of the biorefineries. They can also currently be estimated using various software programs, but they are still difficult to adapt to and are being developed on the actual scale of biorefineries these days [153].

Because they are smaller programmed systems that can be combined with chemical and environmental modeling from other software to create a complex software environment, reduced-order models equipped with machine learning like long short-term memory networks (LSTMs) are options within the first scope of acquiring autonomous plants. Since reduced-order models with machine learning are smaller programmed systems that can be combined with chemical and environmental modeling from other software to create a complex software environment, they are alternatives within the initial scope of obtaining autonomous plants. The homogeneity of the raw material when it comes to municipal sewage sludge that has its particularities, even though it is complex biomass due to humidity and organic chemical composition, contributes to the analysis of energy consumption in multi-dynamic systems that combine the use of heat and electric power in addition to enabling the prediction of performance improvements. That includes greater energy efficiency from an extensive database of correlations between the parameters of compliance with local environmental standards, climatic data, and the electrical power consumed by aeration devices and in the drying of sewage sludge in sewage treatment plants [154].

As presented by Yamasaki et al. [79], the use of machine learning can be applied to very short-term load forecasting. Some researchers are applying deep learning models [155] such as the LSTM [156,157,158], prophet [159,160,161], neural hierarchical interpolation time series [162], hybrid methods [163,164,165], temporal fusion transformer [166,167,168], or even models that require lower computational effort like the group method of data handling [169] or ensemble learning methods [170] for power system analysis [171,172,173]. This trend, which in some cases helps in sustainability development [174], is becoming popular since these models can deal with non-linearities [175]. Branco et al. [176] applied the wavelet transform with the LSTM network, and they proved that a denoising technique employed for noise reduction is a good strategy when signals with high frequencies are considered. In [177], the wavelet was combined with the neuro-fuzzy. The model is an approach that requires less computational effort and has acceptable error results compared to other structures for the same problem.

Conversion of combined heat and power from sewage sludge can be optimized with computational models [178]. According to Bąk et al. [179], the CO₂ captured locally has huge emissivity reduction incorporated in gasification compared to models globally reaching 460 kgCO₂/MWh and 296 kgCO₂/MWh, respectively, combining heat and power production around 5 MW. More electrical power is generated and the power losses are higher, mainly because of the steam extraction mass flow to reboiled demand and fuel compressor necessary power.

This correlation between internal energy consumed in the sewage treatment plant and potential energy recovery can be observed through computational modeling evaluating the removal of pollutants and determining whether there are improvements in the insertion and conjugation of new technological routes [180]. One of the main challenges to producing hydrogen from sewage sludge is still the lower yield and production rate, as in biological processes like thermal technologies with possibilities to increase both techniques with nano-particle dosage research this aspect must be evaluated, including the participation and each material influence applied in a catalyst route for microorganisms or chemical participation as concluded by Khan et al. [181].

The bio-oil produced from the pyrolysis technological route presents a tremendous potential to be refined in various industrial applications of carbonaceous and binder materials such as bio-asphalt tar, which is the liquid compound of asphalt formation and has a main advantage of the reduction in human emissions compared to the transformation of this product when derived from fossil fuel such as petroleum as well as in its conversion into bio-polymers generated by the poly-condensation technique [182].

This pyrolysis oil can also be converted into high-energy fuels such as bio-hydrogen by processes such as steam reforming, which can also be a future premise in replacing natural gas by reducing CO₂ production. It represents one of the primary ecological raw materials studied in hydrogen production today, in which the main challenge is to find a catalyst(s) that allow reducing the production cost and that achieve the maximum possible yield by compounds derived from biological waste. They can compete with the fossil route in which machine learning can act on the optimization reaction conditions to formulate nano-catalysts, with their economic and environmental evaluation minimizing experimental costs [183].

Technologies like pyrolysis and gasification, though capital-intensive, present promising long-term returns when assessed with comprehensive life-cycle costs that account for reduced landfill dependency, energy recovery, and byproduct valorization such as biochar and syngas [184]. Payback periods for these systems vary significantly based on plant scale and integration with carbon capture technologies, ranging from 5 to 10 years in optimized configurations, with co-benefits from products like methanol and hydrogen improving financial viability. Carbon credit scenarios, especially under pricing frameworks such as EUR 90/ton CO₂ in the European Union (EU) or comparable schemes in China and Brazil, further enhance profitability by offsetting emissions and supporting the economics of negative-emission outputs.

The EU has adopted strong decarbonization incentives via the Emissions Trading System, and it supports waste-to-energy conversion through the Green Deal and Renewable Energy Directive. Brazil, meanwhile, incentivizes bioenergy via RENOVA-BIO, which encourages carbon-intensity reduction in fuel production. China is actively integrating sludge-to-energy within its Five-Year Plans, deploying AI and robotic automation for enhanced emission monitoring and operational control while piloting large-scale CCS projects with favorable loan and green bond mechanisms. These policy ecosystems directly influence technology adoption and economic feasibility, underscoring the need for localized economic modeling that incorporates environmental externalities and market-based carbon instruments [185].

5. Wastewater Treatment Plant Necessary Changes for Reducing CO₂

In the carbon balance estimation, the consumption of chemicals can become one of the main preponderant factors in the emission of greenhouse gases (GHGs), especially when polymers such as polyacrylamide are applied in the processes of flotation and sludge dewatering and glucose in denitrification. This generated pollutant load is even higher when dealing with industrial processes compared to sewage sludge coming from cities, making energy consumption more intensive [186].

The removal of nitrogen and carbon aids in the decrease in GHGs, while advanced modes of control compared to the Proportional–Integral–Derivative (PID) control allow for established parameters of operational efficiency from reinforced learning in evaluating the correlation between the achievement of energy savings in different climatic conditions with distant ranges of occurrence by environmental fines associated with the achievement of optimal standards of electrical consumption of the blowers responsible for aerobic treatment and concentrate ions by chemicals [187].

Evaluation by life-cycle assessment contributes to verifying the relation between CO₂ emission and other environmental impacts of wastewater management such as freshwater ecotoxicity, eutrophication, water depletion, human toxicity, and marine animal toxicity, as identified in [188]. The authors checked over 75% combining categories and found that they are influenced when increasing greenhouse gas emissions, showing that only evaluating financial costs is not enough and that environmental costs must be measured as a broader system perspective needs to be adopted using meta-analysis with different geographic spatial factors when emission is detected in the wastewater treatment plant.

Aa joint interface with reinforcement learning programming languages has to be implemented to Supervisory Control and Data Acquisition (SCADA) systems, as well as thermal, chemical, and electrical kinetics simulator software. However, previously, the implementation and maintenance costs of storing this database was linked to the reinforced learning model, and the recurring costs of subscribing to cloud services must be observed, verifying that all these disbursements are lower than the energy expenses saved as a result of the application of this artificial intelligence mechanism in the Sewage Treatment Plant [189].

Future directions attributed to energy storage and CO₂ capture relations of biochar from sewage sludge pyrolysis and influence by Van Der Waals electrostatic force, surface area, and functional chemical groups depend on low moisture sensitivity and high thermal stability adsorbents produced with activated carbon that can be produced with sewage sludge derived from thermal processes and transformed with chemical reagents in carbon of one dimension (carbon nanotubes), two dimensions (graphene), and three dimensions (activated carbon). Their application, cited before, indicates the possibility to absorb contaminants inside of water. Because of these multiple purposes and applications, we need to evaluate which of these products have higher prices with feature properties in the operation and lower costs to produce with specific thermal technologies [190].

Different models are already being studied at the computer simulation level according to classes by time difference, policy gradient, and Monte Carlo or actor and critic algorithms, but they still need to be further applied in operating sewage plants to predict their efficiency at pilot scales and obtain better adjustments over time [191]. As presented in [192,193,194], artificial intelligence has also been applied for electrical inspections in the power grid, which is a promising solution in energy systems. The inspection of the power grid is an important task performed by the electricity utility to ensure power stability and quality for the user [195]. As presented by Souza et al. [196], the broken insulators may lead to flashovers and lack of energy. Recovered sludge-derived carbon sources can be an auxiliary resource to enhance denitrification according to Wang et al. [197], who estimated an 8.05% reduction target in greenhouse gas emissions.

A comparative analysis of key thermal treatment technologies for sewage sludge is presented in

Table 6. Comparison of Sludge Thermal Treatment Technologies.

Technology	Maturity	Emissions	Energy Efficiency	Cost (USD/ton)
Pyrolysis	Emerging	Medium to Low, depends on reactor design [35]	Medium to High (bio-oil, char, gas outputs)	Medium to High
Gasification	Emerging	Lower emissions with steam/CCS [40]	High H2 yield (70–73%)	Medium
Co-combustion	Mature	High GHGs unless post-treatment used [36]	Moderate, uses fossil fuel co-firing	Low
Hydrothermal Carbonization	Emerging	Low, works with wet sludge [37]	Moderate (hydrochar production)	High
Supercritical Water Gasification	Developmental	Very Low, minimal air pollutants [40]	Very High, efficient hydrogen production	High
Plasma Gasification	Developmental	Very Low with up to 97% CO2 capture [91]	High (∼50% efficiency)	Very High

. The table summarizes each technology’s level of maturity, typical greenhouse gas emissions, energy efficiency, and cost performance. These parameters are crucial for assessing the environmental and economic viability of decarbonization strategies in sludge-to-energy systems. By evaluating these technologies side by side, stakeholders can make informed decisions aligned with sustainability and climate mitigation goals.

In this context, there are alternative treatments, as shown in Figure 4. In comparison to conventional treatment, illustrated in Figure 3, the dense sludge does not undergo the stages of drying/dehydration, transportation, and final disposal in landfills, which are processes that are costly to operate and maintain and which emit greenhouse gases. When subjected to heat treatment, which can occur within the sewage treatment plant, the sludge is converted into added products of economic and energetic value, promoting decarbonization and carbon sequestration and adding new revenues to the conventional system with the commercialization of the resulting products, thereby increasing social gains and environmental benefits.

Carbon emissions during the operation of the sewage plant can be avoided by recycling this waste because mechanical processes are the main contributor compared to electricity and chemical consumption as reported by Zhang et al. [198]. The authors verified water reclamation as the main potential carbon emission reduction for operational sewage sludge plants installed for anaerobic–anoxic–oxic processes. Reducing CO₂ is a constant challenge that various authors have explored [199,200,201], applying different techniques to mitigate these emissions [202].

Table 7. Reducing CO₂ studies.

Authors	Method/Application
Han et al. [203]	A dendrite network-integrated adaptive mean square gradient method for optimizing energy efficiency in buildings.
Han et al. [204]	Enhancing electroactive sites within a three-dimensional covalent organic framework.
Ringe et al. [205]	CO2 adsorption rate in electrochemical processes is constrained by double layer charging.
Hu et al. [206]	Sub-nanometric copper cluster synthesis through double confinement facilitates selective characteristics.
Banerjee et al. [207]	Waste sludge decreases greenhouse gas emissions in a pilot-scale industrial wastewater treatment facility.
Liu et al. [208]	Electrocatalytic carbon applied to fuels on heterogeneous catalysts.
Prabhu et al. [209]	Catalysts with heterostructures for both electrocatalytic and photocatalytic applications.
Zhu et al. [210]	Pavilion of reversible design crafted from recycled materials.
Shi et al. [211]	Sustainable management utilizing a life-cycle assessment.

shows some examples of applications of methods and evaluations of the possibilities for reducing CO₂ emissions.

6. Final Remarks and Conclusions

The pursuit of decarbonization for sludge thermal treatments in electrical power generation emerges as a compelling solution with profound environmental, economic, and social implications. This transformative approach not only addresses the urgent need to reduce carbon emissions but also underscores the potential for turning a waste stream into a valuable resource. By harnessing the energy latent in sludge through advanced thermal treatments, we not only contribute to the diversification of energy sources but also alleviate the burden on traditional fossil fuel-based power generation. This shift aligns with global efforts to combat climate change and fosters a more sustainable and resilient energy infrastructure.

Technological advancements in sludge-to-energy conversion processes play a pivotal role in achieving efficient and environmentally friendly outcomes. Continued research and innovation are essential to optimize these methods, minimizing environmental impact, enhancing energy recovery, and ensuring economic viability. Moreover, the success of decarbonization in sludge thermal treatments hinges on collaborative efforts between governments, industry stakeholders, and the public. Policy frameworks, regulations, and incentives should be aligned to encourage the widespread adoption of cleaner technologies, fostering a supportive environment for sustainable practices.

As we navigate the complexities of energy transition, integrating sludge thermal treatments into the broader landscape of renewable energy sources becomes imperative. This holistic approach not only contributes to a reduction in greenhouse gas emissions but also promotes a diversified and resilient energy portfolio. In essence, decarbonization for sludge thermal treatments represents a proactive and strategic step toward a more sustainable future. By reimagining waste as a valuable asset and embracing innovative technologies, we can pave the way for a cleaner, greener, and more energy-secure world.

Building on the comprehensive overview presented, it is clear that advancing sludge thermal treatments requires not only the continued refinement of core technologies, such as pyrolysis, gasification, and co-combustion, but also the seamless integration of real-time monitoring and data-driven control. The deployment of robust sensor networks enables the continuous tracking of critical parameters (temperature, pressure, flow rates), which, when coupled with AI-driven analytics, can dynamically optimize reactor conditions to maximize energy recovery and minimize carbon emissions. Moving forward, research should prioritize the development of adaptive control strategies, leveraging machine learning models capable of both short-term forecasting and long-term process optimization, to ensure that decarbonization technologies remain efficient and economically viable under variable feedstock compositions and operational demands.

Equally important is the establishment of supportive policy frameworks and market incentives that recognize the multifaceted value of sludge-to-energy processes. Life-cycle assessments (LCAs) have underscored the broader environmental benefits of on-site thermal conversion, avoiding the high emissions associated with sludge transport and landfilling, yet comprehensive LCAs must become standard practice to guide investment decisions and regulatory policies. Collaboration across governments, utilities, technology providers, and the scientific community are essential to align energy markets, carbon pricing mechanisms, and funding programs, thereby catalyzing the large-scale adoption of cleaner thermal treatments and transforming sewage sludge from a disposal liability into a strategic energy and resource asset.

Future research on the decarbonization of sludge thermal treatments should prioritize the development of an integrated, data-driven, and scalable framework that enhances both environmental sustainability and energy efficiency. Critical attention must be given to the optimization of sludge pre-treatment processes, particularly with regard to reducing energy consumption in dewatering and understanding the influence of feedstock heterogeneity, such as moisture content and pH, on process yields. Innovations in reactor design, including the incorporation of corrosion-resistant materials and heat-integrated systems, are essential for the advancement of supercritical water gasification and other high-performance thermal technologies. Moreover, the integration of cost-effective carbon capture systems, especially those utilizing biochar, nano-adsorbents, or ionic liquids, requires further investigation at pilot and industrial scales.

Real-time process monitoring through advanced sensor networks, coupled with AI for predictive modeling and adaptive control, presents a promising avenue to improve operational efficiency, reduce emissions, and facilitate dynamic optimization in sludge-to-energy systems. Additionally, research should focus on the valorization of by-products such as biochar, syngas, and phosphorus-rich ash, fostering a circular economy through the development of standardized, market-ready applications. Finally, comprehensive life-cycle assessments and techno-economic analyses are needed to validate the long-term sustainability and financial viability of these technologies, while policy-oriented studies should explore regulatory incentives and carbon credit mechanisms that support their widespread adoption. Collectively, these directions offer a coherent research roadmap for transforming sludge thermal treatment into a cornerstone of sustainable energy and waste management infrastructure.