Strategies for PPCP Removal from Sewage Sludge in a Circular Economy Context

Abstract

The transition to a circular economy requires the safe management of sewage sludge through nutrient and energy recovery. However, pharmaceuticals and personal care products (PPCPs) present a significant challenge. These compounds tend to accumulate in sludge via sorption, shifting the environmental burden from the aqueous phase to the sludge. This manuscript provides a comprehensive review of the scientific literature on technical alternatives for valorizing sewage sludge and removing emerging contaminants. The study evaluates the limitations of conventional biological methods, such as anaerobic digestion and composting, which exhibit variable efficacy and are often insufficient to degrade some commonly used pharmaceuticals. On the contrary, thermal treatments (pyrolysis, gasification, and hydrothermal processes) are considered robust alternatives capable of achieving the high removal of chemical compounds. Furthermore, the article emphasizes the innovative potential of utilizing carbon-based byproducts (biochar and hydrochar) as adsorbents, catalysts, or soil amendment to enhance the removal of PPCPs within the treatment infrastructure itself. The integration of advanced thermal technologies is essential to mitigate the risks of contaminant transfer to the food chain and ensure a safe and sustainable nutrient cycle.

1. Introduction

Traditional wastewater treatment plants rely on biological processes to reduce the organic content present in wastewater. However, this approach also generates a sludge byproduct, which requires additional processing. Concerns regarding the management and valorization of sewage sludge have garnered attention from the scientific community, primarily due to the presence of emerging pollutants and microplastics. The complex links between human well-being and environmental protection have placed growing pressure on wastewater treatment and sewage sludge legislation to address environmental preservation, public health, climate change impacts, and socio-economic benefits [1]. The European wastewater treatment directive was updated to align with the Zero Pollution Action Plan, addressing the impacts of microplastics, pharmaceuticals, and cosmetics, as well as the high costs of their removal [2]. Despite the potential of municipal wastewater to irrigate and fertilize crops and generate energy through sludge valorization [3,4], implementing this concept is challenging. This difficulty stems from the need to eliminate micropollutants and the inherent risks posed by certain heavy metals and persistent compounds such as microplastics and pharmaceuticals. Standardizing detection methods and limits is essential for the safe land application of sewage sludge. This requires common terminology, consideration of soil and climate, and a strategic framework for sustainable resource recovery [5,6].

Wastewater treatment plants (WWTPs) receive residual water containing pharmaceuticals and chemical compounds from cosmetics and personal care products, as well as other contaminants from cleaning and common household disinfectant products. Conventional treatment processes, including waste-activated sludge and biofilm-based systems, have been shown to remove some of these pollutants through degradation or by sorption into the sludge matrix. Nevertheless, a significant proportion of complex compounds may not be effectively treated and thus be permitted to remain in the effluent stream. The recently introduced Directive (EU) 2024/3019 [7] provides for the implementation of quaternary treatment in conventional WWTPs to eliminate organic micropollutants. These compounds constitute a substantial proportion of the pollution load entering the environment, for which removal technologies are commercially available. With regard to the 2024/3019 Directive, “the quaternary treatment should be imposed on the basis of the precautionary principle, in combination with a risk-based approach”. This implies that treatment plants serving populations of 150,000 or more inhabitants should implement this type of facility. This statement highlights concerns about the adequacy of current treatment technologies, the additional energy required, operational costs, and the monitoring tasks required for emerging contaminants. The regulation proposes extended producer responsibility to allocate costs, but this may shift the burden to consumers. A national tax could offer a more balanced approach by avoiding price increases for essential pharmaceuticals that lack viable substitutes or greener alternatives.

The increase in the number of installed WWTPs results in sewage sludge as an annoying byproduct. Directive 86/278/EEC (June 1986) regulating sewage sludge land application needs updating and has been recently submitted to evaluation as part of the Circular Economy Action Plan, with results reported in Document 52023SC0157 [8]. This document highlights that removing pollutants from wastewater can increase sludge contamination, posing a risk of spreading these pollutants through sludge-based agricultural practices, and that targeted source control would reduce treatment requirements.

As shown in Table 1, a collection of analytical studies has been assembled that quantify pharmaceuticals, personal care products, and emerging contaminants in wastewater treatment plants, with some studies reporting low removal values or even null removals for certain compounds [9,10,11,12,13]. A concern is the potential presence of these compounds in water resources [14] and the risk of bypassing process removal in conventional water treatment systems, as reported by Charuaud et al. [15] and earlier by Kloepfer et al. [16].

Table 1. List of some studies available in the scientific literature dedicated to the monitoring of emerging contaminants and pharmaceutical and personal care products in WWTPs.

Country	Monitoring Study	Reference
Studies concerning Pharmaceuticals
China (Guangzhou, Foshan and Zhongshan city WWTPs)	Sulfonamides and metabolites detection in 4 WWTPs	[9]
Italy (Rome): Influents/effluents of four WWTPs serving the city of Rome	12 pharmaceuticals	[10]
5 WWTPs in Sweden	12 human antibiotic substances	[13]
Czech Republic: 18 samples from healthcare facilities and WWTPs; influent and effluent samples.	52 pharmaceuticals and personal care products	[17]
Latvia (Riga): Wastewater treatment plant in Riga; raw wastewater samples.	24 pharmaceuticals	[18]
Norwegian (Oslo and Trondheim): Influent wastewater	51 drugs, pharmaceuticals and metabolites	[19]
Sweden: Sewage treatment plant in Umeå.	Antibiotics (fluoroquinolones, sulfonamide and trimethoprim)	[20]
China: Wastewater monitored included wastewater from landfills and incineration plants, influents from municipal and industrial WWTPs, swine farms, and poultry farms. Water from the Yangtze River in Chongqing City in western China was also monitored.	43 antibiotics in wastewater and water sources	[21]
Spain: WWTP Terrassa, Barcelona. The plant treats a mixture of municipal and industrial wastewater (mostly pharmaceutical and textile industry)	31 pharmaceuticals in aqueous and solid phase	[22]
Studies including emerging contaminants
France (Lyon): Influent/effluent wastewater WWTPs	37 pharmaceuticals and 4 pesticides.	[11]
Greece (Athen): Raw, treated wastewater and sludge	Emerging contaminants	[12]
Spain (Almería): Treated wastewater	115 contaminants (pharmaceuticals, their transformation products and pesticides)	[23]
Spain (Castellón) and Italy (Cremona): WWTP samples; monitoring influent and effluent wastewater samples	Pharmaceutical and illicit drugs (107 compounds and 220 fragment ions)	[24]
England (southwest): Samples from wastewater and treated wastewater from a WWTP using trickling filter, and from digested sludge (derived from anaerobically treated waste activated and primary sludge)	90 emerging contaminants (personal care products, pharmaceuticals and illicit drugs)	[25]
Australia: Bolivar WWTP in Adelaide, South Australia; raw wastewater, water, and sludge line samples	Emerging contaminants (benzotriazoles and UV filter compounds)	[26]
France: WWTPs sampled—Seine Aval, Seine Centre, and Seine Gresillons.	Mass balance of 71 priority and emerging organic contaminants in the sewage sludge treatment train in Paris plants.	[27]

Wastewater from antibiotic manufacturing plants may pose an environmental risk due to the organic molecules released and their transformation products. This requires estimating ecological toxicity and antimicrobial resistance risks, as some of these intermediates may be more persistent or toxic than their parent compound [28,29]. However, PPCPs in domestic wastewater are unavoidable due to daily use, and some contaminants will persist even if industrial sources are removed. Effective treatment is therefore needed to eliminate microcontaminants or reduce their toxicity to acceptable levels. Conventional wastewater treatment processes are often inadequate, as they typically result in only partial biodegradation, sorption onto sludge, incomplete mineralization, and in some cases, the formation and release of toxic intermediate byproducts.

Compliance with government regulations may not be enough in fragile or arid regions, or where water reclamation is required. Gholipour [30] highlighted that the Kashaf River in Iran is an example of the strict measures needed to protect arid regions receiving wastewater treatment plant discharges and to prevent natural wetlands from becoming sacrificial sinks for pollutants. Kekana et al. [31] reviewed wastewater reuse in the Republic of South Africa, a region plagued by persistent droughts and unpredictable rainfall, where there is a risk of exposure to high levels of salts, heavy metals, and microplastics when treated wastewater is used for agricultural irrigation. Another challenge is the high cost of detecting trace contaminants in complex environments, along with the expensive treatments required to remove them when regulations are enforced, especially in developing countries [32]. If providing basic sanitation services to the population is already a significant challenge, implementing advanced technologies to remove emerging contaminants is likely to be unfeasible.

Extensive research has examined contaminant removal, but in conventional plants, much of the reduction results from sorption onto sludge rather than biodegradation. This shifts pollutants between phases rather than eliminating them, underscoring the need for effective solutions to remove PPCPs, microplastics, and antibiotic resistance-related materials. Despite the growing body of literature on the fate and treatment of contaminants in wastewater and sewage sludge, comprehensive assessments that integrate technological, environmental, and regulatory perspectives remain scarce. Existing studies often focus on either removal efficiencies or specific treatment technologies, without providing a holistic analysis of their implications for resource recovery and circular economy strategies. This review synthesizes current knowledge on established and emerging treatment technologies for PPCPs and microplastics in the context of a circular economy, highlighting the challenges and opportunities of integrating these contaminants into resource recovery initiatives. The present manuscript focuses on alternatives for removing contaminants from sludge and organic slurries, paying special attention to thermal treatments for their intrinsic advantages in energy recovery, the conversion of sludge into valuable products, and reduced material requiring final disposal. The aim of the manuscript is to cover the assessment of technologies for the removal of PPCPs and microplastics from sludge, analyze challenges associated with land application of treated sludge and propose strategies for future research directions and policy development regarding resource recovery and sustainable sludge management.

2. Materials and Methods

The literature search covered the period between 2010 and 2026, primarily, but also considered previously published manuscripts that were highly relevant to the subject. The primary databases consulted were Google Scholar, Scopus, and Web of Science. The search keywords used to collect relevant information included: “sewage sludge”, “pharmaceuticals and personal care products”, “treatment”, “removal”, “emerging contaminants”, “monitoring”, and combinations of these terms using “and”, “or” as Boolean operators. Initially, the number of documents found was too high; i.e., when using the first two streams, Google Scholar reported 67,800 documents, which was reduced to 18,100 when only manuscripts published after 2010 were considered. Therefore, a secondary search was performed, considering only the most relevant documents on the first four pages of results (excluding reviews) and those obtained by searching for keywords in the title only, reducing the number to 259.

The previous procedure was repeated for Scopus and Web of Science, yielding 66 and 124 documents, respectively. From this point, manuscripts were individually selected from 181 personally revised manuscripts, after excluding those without a direct relation to the subject, duplicates, and those from unreliable sources.

The selected documents included peer-reviewed journal articles and conference manuscripts. Review documents and book chapters were selected in a second round after choosing the main documents related to experimental work, bringing the total to approximately 460 documents. The literature search prioritized studies that quantified contaminants in the sludge matrix and evaluated treatment methods for their removal from sewage sludge, reducing the total to 350. Particular focus was given to biological and thermal treatments. For this reason, additional keywords were added in line with the previous search, including “biological degradation” and “thermal treatments”, with the specific desired technology serving as an internal selection parameter. Recommended manuscripts by the internal journal algorithm were also considered suitable in the present work. The title and abstract sections were manually revised for a preliminary screening. From this point, all selected documents were carefully read and assessed for inclusion in this review, with relevance determined individually, resulting in a final number of 251 references.

3. Sewage Sludge Treatment and the Presence of Contaminants in the Sludge Matrix

The treatment of wastewater involves more than just removing nutrients and organic matter from the liquid phase. WWTPs must also process sewage sludge—a slurry primarily composed of solids from the primary settling stage and biological material that settles out in the waste activated sludge system, when this treatment configuration is applied. While the principles of the circular economy emphasize the reuse and recycling of materials [33], the presence of persistent and hazardous contaminants in wastewater and sewage sludge can render this strategy ineffective, as it poses a significant risk of dispersing toxic and recalcitrant organic compounds into the environment. Sorption of contaminants onto the sludge matrix concentrates these compounds in a separate phase, shifting the removal challenge to the sludge line. Östman et al. [34] demonstrated this issue in their assessment of chlorhexidine, showing that removal rates reached 91% in the water line thanks to its accumulation in the sludge matrix. However, chlorhexidine did not degrade during anaerobic digestion—a concerning finding given its high influent load (approximately 208 kg/year). This is not the only example of no degradation occurring. Stasinakis et al. [12] likewise reported the accumulation of perfluorinated compounds in sludge via sorption processes, with no degradation during anaerobic digestion.

The work of González et al. [35] showed the presence of the organic pollutants nonylphenol and its derivatives (classified as endocrine disruptors) in raw and treated sludge samples, regardless of the treatment method (aerobic or anaerobic). These compounds are recurrent in wastewater due to the use of nonylphenol polyethoxylates in cleaning products. Previous work by Santos et al. [36] reported similar findings when evaluating the presence of nonylphenol mono- and diethoxylates (NPEs), di-(2-ethylhexyl)phthalate (DEHP), and seven polychlorinated biphenyl (PCB) compounds. DEHP and NPEs were detected in all sludge samples (both aerobically and anaerobically stabilized), whereas PCB compounds were only detected in anaerobically stabilized sludge. In a similar manner, Ismail et al. [37] demonstrated strong adsorption of quaternary ammonium compounds onto the sludge matrix, underscoring the need to determine their fate when biosolids are used as organic amendments in agriculture.

Vom Eyser et al. [38] compared the concentrations of various pharmaceuticals reported in sewage sludge in different countries. They found that most of the compounds investigated (diclofenac, ibuprofen, phenazone, carbamazepine, sulfamethoxazole, bezafibrate, fenofibric acid, metoprolol, propanolol, clarithromycin, roxithromycin, and erythromycin) occurred within similar concentration ranges in all of the cases studied. Table 2 lists some of the PPCPs measured in sewage sludge by various authors.

Table 2. Organic compounds measured in sewage sludge by different authors.

Category	Organic Compound	Reference
Commonly used stimulant	Caffeine	[39,40,41,42,43,44]
Antiseptic	Chlorhexidine	[34]
Pharmaceuticals for bacterial infection treatment	Fluconazole, trimethoprim, ciprofloxacin, azithromycin, chloramphenicol, ciprofloxacin, clarithromycin, erythromycin, nalidixic acid, norfloxacin, roxithromycin, sulfadiazine, sulfadimethoxine, sulfadimidine, sulfamethoxazole, sulfamonomethoxine, sulfathiazole, tetracycline, trimethoprim	[34,39,41,42]
Pharmaceuticals for pain relief and hypertension	Ibuprofen, naproxen, diclofenac, ketoprofen, meclofenamic acid (internal standard), triclosan, acetaminophen, antipyrine, isopropylantipyrine, indometacin, mefenamic acid, valsartan, phenazone, diltiazem, cetirizine, clenbuterol, crotamiton	[12,39,40,41,42]
Pharmaceuticals for treating mental disorders and depression symptoms	Amisulpride, amitriptyline, atenolol, bezafibrate, bupropion, carbamazepine, citalopram, clofibric acid, clozapine, cyclophosphamide, dipyridamole, fluoxetine, fluvoxamine, haloperidol, mefenamic acid and N,N-diethyl-m-toluamide, furosemide, metoprolol, mirtazapine, N-des-Sertraline, N-des-Olanzapine, Norfluoxetine, O-des-Venlafaxine, olanzapine, paroxetine, pirenzepine, primidone, propranolol, quetiapine, risperidone, sertraline, sulfapyridine, sulpiride, telmisartan, theophylline, venlafaxine	[39,40,41,42,43]
Anticorrosive compounds	Benzotriazoles, benzothiazoles	[12,26,34]
Disinfectant-surfactants, clothes and house appliances related products	Quaternary ammonium-based molecules, nonylphenol technical mixture (NP), nonylphenol monoethoxylate (NP1EO) and nonylphenol diethoxylate (NP2EO), perfluorinated compounds	[12,34,35,37]
Cosmetics	UV filters, triclocarban, triclosan	[26,39,40]

The adsorption of contaminants onto the sludge matrix depends on parameters such as the octanol-water partition coefficient (log Kow), the compound’s solubility and concentration, and its pKa [44,45]. However, the sorption capacity is not well predicted by the log Kow parameter in sludge. Thus, it is necessary to determine a similar coefficient that accounts for the sludge-water partition coefficient. The solid-water distribution coefficient (Kd) represents the ratio between the concentration found in the sludge and that in the aqueous phase. Log Kd values aid in more accurately estimating the fate of pharmaceuticals in sludge, as the presence of polar functional groups can alter behavior through interactions arising from the ionization state of the molecule [46,47]. Therefore, interactions are not well-predicted by the simplistic assumption of the octanol-water partitioning coefficient.

The land application of biosolids and digestate is one of the most viable strategies for valorizing these byproducts, as it returns nutrients to the agricultural cycle. Moreover, continued use of biosolids offers a practical way to enhance soil carbon storage, with sequestration capacity generally increasing with the application rate [48,49]. Nevertheless, the presence of emerging contaminants may undermine these potential benefits. This risk should be carefully evaluated, given the significant advantages of nutrient recovery and carbon cycle management. There is a legitimate concern not only about the presence of pharmaceuticals and personal care products, but also of microplastics and per- and polyfluoroalkyl substances (PFAS), which can contaminate groundwater and surface water. This creates a risk that plants will take up these substances from soils amended with biosolids, and consequently, contaminate food resources [50]. The work by Steele et al. [51] highlights the importance of not only monitoring contaminants in sludge, but also gathering information from public databases to enable robust risk analysis and determine the best valorization pathways, and if necessary, constrain the agronomic application of sludge.

The study by Prosser and Sibley [52] evaluated the expected risks of using biosolids and wastewater for soil amendment and irrigation on agricultural land. These authors reported that the hazard quotient was below 0.1 for most residues and application practices studied, with hazard quotients exceeding 0.1 observed for carbamazepine, diphenhydramine, salbutamol, triclosan, and sulfamethazine when assessing biosolids or manure amendments. It should be highlighted that the direct application of biosolids to agronomic land where vegetables and fruit are grown for consumption in their raw state is prohibited by Article 7 of Directive 86/278/EEC of 1986 [53]. Spreading on grazing lands or forage crops shortly before grazing or harvesting is also prohibited (requiring a minimum waiting period of three weeks). A minimum waiting period of 10 months is required before applying biosolids to soils used for growing horticultural or fruit crops intended for consumption. Therefore, any risk quotients estimated should take into account that the experimental assessment is based on a methodology specifically designed to evaluate risk, rather than real-world application conditions. The methodology uses doses that are often much higher than those applied under agronomic practices to ensure reproducible results within a short evaluation period.

Microplastics are commonly detected in WWTPs, with the majority being removed during pre-treatment steps. For instance, in grease-trap units, microplastics are separated by flotation and subsequently accumulate in the lipid phase. Alternatively, they settle with sludge particles and then undergo sludge stabilization [54,55]. WWTPs that use membrane bioreactors, dissolved air flotation, or tertiary filtration as treatment options exhibit greater microplastic retention in sludge [56]. Reports evaluating the effects on the performance of microbiological processes indicate no significant impact on wastewater treatment, but some inhibitory effects on biogas production have been reported at excessively high levels of microplastics [57,58,59]. Nevertheless, there is significant concern about the amount of material released with treated wastewater and stabilized biosolids.

The study conducted by Corradini et al. [60] sought to determine the presence of microplastics in 30 agricultural fields, with a control field contributing to a total sample size of 31. These fields were located in Melipilla County, in the Metropolitan Region of Chile, and had a ten-year record of sludge application. The authors reported a cumulative trend over time, with successive application rates. A similar study was conducted by Van den Berg et al. [61], who evaluated 16 fields near Valencia (Spain). These authors also reported the presence of microplastics due to sludge application but found significant levels in soils not subject to this practice. The average content of soils not submitted to sludge spreading was 2030 particles/kg compared to 5190 particles/kg in soils receiving biosolids. Their results highlight the widespread presence of these substances at levels high enough to indicate multiple sources of contamination in agricultural lands beyond direct biosolid application.

The accumulation rate in soils cannot be linearly correlated with the application rate of biosolids due to particle migration into deeper soil zones, fugitive emissions associated with water runoff, material loss through the formation of airborne dust particles, and degradation by soil microflora [62,63,64]. A similar comparison can be made with any other organic pollutant with a low removal rate in a conventional WWTP, the presence of which in sludge is therefore inevitable.

3.1. Biological Methods for Removing Contaminants from Sludge

Biological methods are widely used to stabilize organic matter in sludge and reduce its putrescible potential. Typical technologies for improving sludge quality are extended aeration, anaerobic digestion, and composting. These processes transform labile organic compounds into more complex structures, reducing the proportion of volatile solids, increasing mineralization, and enriching the material with humic and fulvic acids, which benefit soils [65,66,67]. In addition, these biological processes can also degrade some organic contaminants. He et al. [68] reviewed feasible technologies for removing contaminants from sludge and considered composting, as a biological treatment, to be one of them, although anaerobic digestion and aerobic oxidation of sludge can also degrade some pharmaceutical compounds [69,70,71]. Other feasible alternatives for contaminant removal identified included hydrothermal treatments, electrochemical processes, and advanced oxidation processes. Except for biological treatments, these technologies typically require substantial thermal or electrical energy, as chemical conversion indiscriminately targets all organic matter in the stream, thereby leading to excessive oxidant demand.

The sludge matrix consists of a complex mixture of volatiles (such as extracellular polymers, proteins, and polysaccharides) and inorganic solids. To reduce their putrefaction potential for agronomic valorization, this mixture must be stabilized. However, the presence of hazardous chemical compounds that can become trapped within the sludge structure [72] underscores the urgent need for practical, cost-effective solutions that can ensure their complete degradation and prevent their release into the environment. Narumiya et al. [39] measured PPCPs in sludge samples obtained from 4 WWTPs located in the Kansai region of Japan (Kyoto and Osaka prefectures) and reported the presence of 45 out of 48 analytical standards, indicating that ofloxacin, triclosan, and triclocarban were the most prevalent compounds, exceeding 1 mg/kg of dry sludge. Their findings revealed potentially high levels of organic contaminants and underscored the need for robust monitoring and well-informed decision-making in sludge valorization. This should be based on up-to-date technical knowledge to ensure safe agricultural use or the implementation of advanced contaminant removal technologies.

Although several microorganisms have been reported to degrade chemical pollutants such as atrazine, chlorinated compounds, and various aromatic chemicals, many of these degraders are aerobic species—including Pseudomonas sp., Streptomyces sp., and Sphingomonas sp., among others [72,73,74,75]. On the contrary, large scale sludge stabilization processes typically rely on anaerobic conditions, which favor energy recovery from sludge and reduce operational energy demands by lowering mixing requirements and eliminating aeration. Therefore, many organic contaminants will remain as recalcitrant compounds during conventional anaerobic stabilization in large WWTPs, unless an additional stabilization stage, such as composting, is included to increase sludge mineralization. Composting is one of the most effective ways to stabilize sludge organic matter and improve its quality by producing humic and fulvic substances that benefit soil microbiota. In addition, the organic mineralization occurring under composting conditions is greater than that achieved through anaerobic stabilization processes, resulting in a higher degree of aromatization and greater reduction in labile compounds, as shown by spectroscopic and thermal analyses [76]. Numerous studies have demonstrated significant improvements in the structure of organic matter, reporting greater destruction of aliphatic and polysaccharide components and increased aromaticity [77,78,79].

3.1.1. Composting

Composting is a controlled biological process where organic matter in sludge is decomposed by microorganisms in the presence of oxygen. The sludge is mixed with a structural agent (typically wood chips, straw, or sawdust) to improve the porosity of the composting pile and favor aeration. The process requires adjusting the moisture content and balancing the carbon-to-nitrogen ratio. During composting, microorganisms degrade organic material, generating large amounts of heat from their metabolism, which causes a temperature rise in the mixture. This way, an auto-induced thermophilic phase is created, which helps in destroying pathogens, weed seeds, and harmful organisms. The process finalizes with a maturation stage once the organic load is reduced and the temperature is lowered, both of which indicate reduced microbial activity. The material stabilizes into a humus-like product denoted as compost. The final compost is reduced in volume, less odorous, and can be safely reused as a soil conditioner.

The benefits of contaminant degradation in composting experiments were demonstrated by Martín et al. [80]. These authors studied the degradation of eight priority organic pollutants (linear alkylbenzene sulfonates (LASs), nonylphenol ethoxylates (NPEs), and Di(2-ethylhexyl)phthalate (DEHP)) during the composting process. They reported degradation of LASs (59–66%) and DEHP (although volatile loss prevented them from obtaining a confident value), with higher rates observed in piles that reached thermophilic conditions. Nevertheless, in the case of NPE, these authors observed no clear conversion. Contrary to expectations, they observed an increase in concentration over the experimental period. However, Zheng et al. [81] reported an increase in NPE degradation (from 19.7% to 41.6%) in composting piles in a previous experiment when ventilation was increased, and the process temperature was carefully controlled.

The composting process has been considered a feasible option not only for removing contaminants but also for removing antibiotic resistance genes (ARGs). Bao et al. [82] demonstrated this by adding calcium peroxide as a cost-effective way of enhancing radical oxidation and stabilizing mobile heavy metals. López-González et al. [83] reported a similar outcome without adding an inorganic supplement to enhance radical generation. They conclude that the thermophilic oxidative phase reached during composting was enough to significantly reduce antibiotic-resistant bacteria. The presence of contaminants in sludge is a growing concern, as is the risk of ARG regrowth after composting, since these materials are not completely removed during the process [84]. As sewage sludge contains a diverse range of microorganisms and nutrients, it is reasonable to assume that antibiotic-resistant bacteria will proliferate and accumulate in this matrix, facilitating the transfer of ARGs among different biological species [85]. In addition, most chemical and physical pretreatment techniques, such as alkali pretreatment, coagulation, microwave, or thermal pretreatment, have been demonstrated to be ineffective in achieving an acceptable removal rate of ARGs when these stages are considered as prior steps before biological treatments [86,87]. A recent strategy to enhance ARG removal during composting involves supplementing with biochar derived from biosolids. This approach has been shown to contribute to the closure of waste valorization cycles. As demonstrated by Wu et al. [88], a significant decrease in both intracellular and extracellular ARGs is attained when biochar is added, owing to alterations in community structure, physical adsorption of extracellular ARGs, and disruption of bacterial cell membranes.

3.1.2. Anaerobic Digestion

Anaerobic digestion represents another of the most widely implemented technologies for sludge stabilization in conventional WWTPs. Anaerobic digestion is a biological process in which microorganisms break down the organic material in the absence of oxygen. The process involves several stages: hydrolysis, where complex organics are transformed into simpler compounds; acidogenesis, which is the second stage, where volatile fatty acids are produced in a delicate equilibrium, avoiding their build-up and maintaining neutral conditions. The third phase is acetogenesis, which has as main products acetic acid, hydrogen, and carbon dioxide. The final stage is methanogenesis, which gives rise to methane production and releases, as a final output, a gaseous phase known as biogas, with methane and CO₂ as the main components. During anaerobic digestion, the organic content of the sludge is stabilized, reducing its volume and odor. At the same time, biogas can be valorized to produce energy or upgraded to act as a natural gas surrogate. The remaining material, known as digestate, can be used as a soil conditioner. A valorization option that allows carbon and nutrients to be returned to the soil.

The primary benefits of this process are the generation of biogas and its comparatively low energy requirements, making it efficient and economically viable. However, when addressing the degradation of emerging contaminants, anaerobic digestion presents important limitations, particularly due to the difficulty in transforming certain resistant molecular structures. This limitation substantially reduces the removal efficiency of numerous PPCPs, as their complex chemical bonds often remain resistant to degradation.

The degradation capacity of methanogenic microflora is linked to the presence of acetate kinase enzymes [69]. This relationship was observed during testing of the degradation of galaxolide, naproxen, nonylphenol, octylphenol, ibuprofen, diclofenac, bisphenol A, and triclosan under anaerobic conditions [69]. The division of the digestion process into two phases, a feature commonly applied to increase solid removal and thereby process efficiency, is now seen as an alternative for improving PPCP removal. The two-stage process (acidogenic-methanogenic reactor) was tested by Carneiro et al. [70], who reported removal rates greater than 95% for sulfamethoxazole, methylparaben, propylparaben, naproxen, and acetaminophen. Higher removals were also observed for the elimination of persistent compounds in anaerobic environments, such as ibuprofen, carbamazepine, metoprolol, ciprofloxacin, and diclofenac, with removal rates exceeding 60% [70]. The results of this study establish new research directions for the removal of trace contaminants during conventional anaerobic digestion. This finding is especially noteworthy given the limited degradation observed in this environment in previous studies by Yang et al. [89] and the poor removals (17–50%) achieved even after applying thermal pretreatments at 150 °C [90].

Carballa et al. [91] previously assessed the effects of retention time and temperature on the digestion process when testing the degradation of 13 PPCPs, reporting coincident results: no conversion of carbamazepine and removal percentages between 20 and 60% for the remaining compounds. The study revealed no significant effect when increasing either solid retention time or digestion temperature. However, in a subsequent study carried out by Zhou et al. [92], these parameters were found to be significantly associated with the degradation performance of selected compounds. These authors studied the removal of four pharmaceutical active compounds (diclofenac, clofibric acid, carbamazepine, and triclosan) under thermophilic and mesophilic conditions at different retention times. The findings indicated that thermophilic conditions enhanced the removal efficiency of carbamazepine, and triclosan, whereas diclofenac removal was more effective under mesophilic conditions. The minor discrepancy between the experimental conditions and the modest increase in degradation efficiency with increased solid retention time may have led to this phenomenon being overlooked in earlier research.

Tahir et al. [93] showed that the degradation of carbamazepine was enhanced by the presence of acidogenic and sulfate-reducing bacteria, with an astonishing 46% degradation under anaerobic conditions. These results may explain the high removal rates reported by Carneiro et al. [70] in their examination of the two-stage anaerobic configuration. In a similar vein, Yang et al. [94] reported that the presence of this pharmaceutical enhanced the growth of hydrolytic and volatile fatty acid-producing bacteria, and the addition of persulfate facilitated its conversion via hydroxylation and deamidation.

3.1.3. Aerobic Digestion

The last biological process considered in the present review capable of stabilizing sludge and degrading contaminants is extended aeration, also known as aerobic digestion. This process constitutes an additional biological pathway capable of mineralizing organic matter in a short residence time. However, its implementation is generally restricted to small scale WWTPs due to the substantial energy requirements for aeration. The enhanced removal of organic contaminants may offer a noteworthy advantage, warranting its application under specific operational conditions. The study conducted by Vaithyanathan et al. [71] reported removal percentages ranging from 62 to 92% for various trace organics, including eight pharmaceutical compounds and two pesticides, through the integration of enzymatic pretreatment (GTB2X enzyme) and subsequent aerobic digestion with Bacillus subtilis bioaugmentation. In addition, ultrasonication and alkaline pretreatments were examined, but they proved less efficient than enzymatic pretreatment.

Given the diverse degradation pathways required for the breakdown of complex molecules, integrating anaerobic and aerobic stages within a sequential treatment scheme has been shown to be more effective than using a single microbiological condition. The study carried out by Ahmad et al. [95] assessed the degradation of pharmaceuticals, including azithromycin, carbamazepine, diclofenac, mefenamic acid, and ibuprofen, along with one ibuprofen metabolite (1-hydroxy ibuprofen (1-OH IBP)). The study used a sequence of treatments: thermophilic reactors operated under anaerobic conditions in the first stage and aerobic/anoxic conditions in the second stage, with short retention times and on–off aeration cycles. This approach enabled the authors to achieve full treatment within 18 days, with 15 days allocated to the initial anaerobic stage and 3 days to the second stage. Notwithstanding favorable process conditions, mefenamic acid and carbamazepine showed no signs of degradation under any of the tested conditions, despite achieving 50% removal of ibuprofen. Furthermore, the concentrations of some effluent substances exceeded the initially applied values. This phenomenon has been attributed to the release of these compounds from the original sludge.

The three biological processes available for stabilizing sludge present advantages and disadvantages associated with the degree of mineralization attained, energetic considerations, and their ability to degrade complex organics. Table 3 summarizes their main features.

Table 3. Main characteristics of biological process for sludge stabilization.

Process	Technical Characteristics	Advantages	Disadvantages
Composting	Aerobic stabilization of organic materials under dry fermentation conditions. High porosity is required to allow homogeneous distribution of air. A structural agent is needed to attain porosity.	Efficient pathogen reduction and high sludge mineralization. Moderate removal of PPCP contaminants. Thermophilic temperatures aid in removing ARGs [80,81,82,83].	Aeration penalizes energy demand. No energy recovery possible. Emission of volatiles.
Anaerobic digestion	Microorganisms perform anaerobic stabilization in sequential stages. High treatment capacity. Operates under mesophilic and thermophilic regimens. Suitable for large-scale WWTPs.	Low energy requirements. Energy recovery thanks to biogas production. Two-phase configuration shows moderate removal of some contaminants [70,93].	Higher technical requirements and highly skilled operators. Control of reactor organic loading rate to avoid acidification. Limited removal of some PPCPs [90,91].
Aerobic digestion (extended aeration)	Waste activated sludge is aerated in sludge basins. Aerobic biomass enters the endogenous metabolic phase. Suitable for small-scale WWTPs.	High sludge mineralization due to the presence of oxygen and limited substrate. Short retention time. Moderate to high PPCP removal [71].	High energy requirements. No energy recovery possible.

The findings of researchers examining the biodegradation of contaminants in sewage sludge unequivocally demonstrate that in many cases, biological processes alone are inadequate to attain the effective removal of all chemical compounds. The limited bioavailability of these substances is frequently attributable to strong adsorption onto the sludge matrix or their incorporation into complex organic structures and can significantly hinder microbial degradation. Consequently, the application of pretreatment strategies has emerged as a necessary step to enhance solubilization, promote the release of bound organic contaminants, and increase their accessibility to microorganisms. In this context, pretreatments have been shown to play a critical role in improving the overall efficiency of subsequent biological processes. The following section, therefore, examines pretreatment approaches reported in the extant literature and their potential to enhance the biological degradation of contaminants in sewage sludge.

3.1.4. Pretreatments for Improving Anaerobic Degradation of Contaminants

The application of thermal pretreatments prior to biological degradation of sewage sludge can be regarded as a technological alternative to aid in contaminant removal. The degradation of complex molecules under anaerobic conditions is usually limited, but the addition of a hydrolysis stage may release organic intermediates that are susceptible to subsequent anaerobic conversion. Figure 1 presents a schematic overview of the pretreatments examined in the scientific literature for their efficacy in removing contaminants prior to the implementation of a biological process.

Figure 1. Schematic representation of typical pretreatment methodologies used to enhance biological degradation.

In the study conducted by Balasundaram et al. [96], the application of thermal pretreatments was examined within a temperature range of 120 to 180 °C. The pretreated sludges exhibited removal percentages exceeding 80% for trimethoprim, enrofloxacin, ciprofloxacin, and bezafibrate. However, other compounds, including carbamazepine, 17α-ethinylestradiol, and progesterone, showed removal percentages below 50%. This observation was previously made by Carballa et al. [97], who also evaluated the removal of organic contaminants under mesophilic and thermophilic digestion following thermal and alkaline pretreatments. These authors reported that more than 80% of the naproxen and natural estrogens was removed. However, carbamazepine proved to be recalcitrant. Guo et al. [98] also examined the degradation capacity of combining thermal pretreatments and digestion, testing temperatures between 70 and 170 °C with a posterior digestion stage. The contaminants evaluated in this study were the antibiotics sulfadiazine, oxytetracycline, and enrofloxacin during mesophilic pig manure digestion. The thermal pretreatment-digestion sequence enabled the complete removal of oxytetracycline and enrofloxacin. However, residual levels of sulfadiazine were measured at 150 °C (20% of the initial concentration) and at 170 °C (16% of the original content).

Alternative pretreatment methodologies, such as radio frequency and microwave irradiation, have also been proposed to enhance contaminant removal during biological treatment. These latter technologies can be considered variants of thermal pretreatment, as they involve heating the sample. Despite the superior efficiency and reduced energy consumption of radio frequency systems in comparison to microwaves [99], both systems are deficient in their inability to recover energy, a key advantage of conventional thermal pretreatments. This results in traditional thermal methods demonstrating superior overall energy performance. In the study conducted by Kor-Bicakci et al. [100], the efficacy of radio frequency and microwave pretreatments for the removal of ibuprofen, diclofenac, and carbamazepine was investigated. Although these authors reported an improvement in overall removal efficiency, carbamazepine remained highly recalcitrant, with removal rates of only 3% under thermophilic conditions and 6% under mesophilic conditions. Conversely, ibuprofen demonstrated higher removal rates under thermophilic conditions. However, the reported values were approximately 14%, and the degradation time tested was excessively long (60 days under batch conditions). As is well-documented, ibuprofen is notoriously resistant to degradation under anaerobic conditions. However, it has been shown that this substance is susceptible to rapid aerobic degradation [101,102]. The use of combined strategies, such as ultrasound and thermal pretreatments, has been shown to improve degradation by promoting the solubilization and bioavailability of organic compounds [103].

Pretreatments are frequently justified at the laboratory scale by the improvements in biogas reported by various authors [104,105,106]. Nevertheless, these benefits seldom offset the additional investment and energy requirements of the auxiliary equipment when implemented at full scale [107]. Consequently, the main practical benefit of pretreatment lies in the considerable reduction in sludge production rather than the increased biogas yield [108]. The potential to enhance the degradation of pharmaceutical compounds endows pretreatments with an additional advantage, rendering them appealing for industrial applications—provided that the organic removal efficiencies attained are sufficiently high. Otherwise, the adoption of alternative treatment technologies capable of achieving the complete elimination of recalcitrant molecules is the most rational choice to minimize the risks associated with sludge valorization.

Ultrasounds, mechanical, and enzymatic pretreatments were compared by Zhou et al. [109] in testing the removal of clofibric acid, triclosan, carbamazepine, and diclofenac during the digestion of sewage sludge under mesophilic and thermophilic conditions. Once more, the thermophilic regime showed superior performance when coupled with ultrasonic pretreatment, achieving removal percentages of 64–76%. Enzymatic pretreatment has been shown to enhance diclofenac removal to 74% when combined with papain under thermophilic conditions. These results underscore the critical role of solid retention time in enhancing the removal efficiency of various pharmaceutical compounds examined in this study. In a later experiment, the use of cellulase and zero-valent iron was also tested. While some antibiotics showed higher removals, others achieved only 50% [110].

Advanced oxidation processes encompass a range of techniques, including ozone application, the Fenton reaction, electrooxidation, and novel techniques such as photocatalytic degradation [111,112,113,114,115,116,117]. It is evident that implementing these methods invariably results in higher costs. The effectiveness of these methodologies in removing pharmaceutical compounds from sludge and enhancing methane production has been demonstrated through empirical evidence [111,112,113]. Most applications focus on removing pharmaceuticals from wastewater because of their inherent limitations when applied to the sludge matrix [114,115,116]. This is because oxidation occurs indiscriminately across all organic compounds in the stream. This feature also explains the substantial improvements in digestion reported by some authors when applying these costly pretreatment steps to enhance biogas production [118,119]. Table 4 lists the main features of typical pretreatments applied to improve digestion performance. Thermal hydrolysis and mechanical methods, such as ultrasonication and grinding technologies, are commercially available. However, many pretreatment strategies studied at the research level face challenges when scaled up. This is often due to high energy demands, operational costs, or the use of expensive equipment.

Table 4. Main features of typical pretreatment technologies applied to sewage sludge.

Process	Technical Characteristics	Advantages	Disadvantages
Thermal pretreatments	Sludge temperature is increased to favor thermal hydrolysis. Heat exchangers or direct steam injection are used to attain the desired temperature. Novel techniques involve microwave and radio- frequency, still at laboratory research scale.	Commercial technologies are available for thermal hydrolysis. Mature technology. The process attains sludge hygienization and significant reduction in sludge volume. Conventional thermal hydrolysis benefits from heat recovery.	Careful temperature control is required to prevent the formation of inhibitory compounds. High energy demand unless heat recovery units are incorporated. Highly skilled operators required.
Mechanical pretreatments	Mechanical disruption is achieved through direct physical impact on cells or through localized pressure changes.	Simplicity in implementation for ultrasonication. Commercial technologies are available for ultrasonication and size-reduction-based technologies.	High energy demand. Moderate scalability due to modular configuration. Grinders suffer from wear and abrasion. The energy required is, in many cases, higher than the benefits obtained from biogas improvement.
Chemical pretreatment. Advanced oxidation	Addition of chemical substances causing pH changes or oxidation of organic molecules. The adverse environment created disrupts cellular structure, releasing intracellular material. Advanced oxidation processes rely on the formation of highly reactive radical species.	High sludge solubilization. Technology suitable for small and large-scale plants. Chemical dosification is a simpler process. Advanced oxidation processes attain significant improvement in the destruction of organics.	High energy requirements. High operational cost when considering chemical pretreatment. Large-scale implementation and operational feasibility are low. High installation and operational costs when considering advanced oxidation. Chemical residues in sludge compromise valorization options.

The combination of different oxidation methods, such as the photo-Fenton reaction, with the use of catalysts has also been proposed as an effective way to remove recalcitrant chemicals or compounds [120,121,122]. However, as Pauwels and Verstraete [123] have noted, it is more appropriate to treat pharmaceutical wastewater at its point of origin, where pollutant concentrations are highest, rather than discharge it into municipal sewer networks. International regulation of hospital wastewater is characterized by a lack of standardization, with some countries categorizing it as municipal wastewater and permitting direct discharge into domestic sewage systems. This practice poses risks to waterbodies and public health [124]. Wastewater mixing dilutes contaminants, thereby increasing the technical complexity and economic burden of their removal in large scale WWTPs. An interesting process integration is that proposed by Gavrilaș et al. [125]. These authors contemplated the implementation of membrane bioreactors in conjunction with advanced oxidation processes. The first one is experiencing increased market adoption, attributable to its ability to operate at elevated aerobic sludge concentrations. This capability enables augmented treatment capacity without requiring additional constructed surface area. These authors emphasized the superior performance in contaminant removal. The combination of advanced oxidation processes enables more effective separation of water from sludge, eliminating the need for a secondary clarification stage. In addition, reducing suspended solids in clarified wastewater enhances the efficiency of the subsequent advanced treatment stage [126].

Applying pretreatment technologies prior to biological processes has been shown to significantly enhance the removal of pharmaceuticals and personal care products (PPCPs) from wastewater and sludge [97,98]. However, this improvement is observed primarily in organic compounds amenable to biodegradation. Persistent molecules such as carbamazepine continue to exhibit low removal rates, even when a pretreatment step is incorporated [100]. Pretreatments such as thermal hydrolysis, advanced oxidation, sonication, and chemical conditioning can disrupt complex organic structures, increase substrate solubility, and promote the release of PPCPs from solid matrices. These changes improve the bioavailability of contaminants, making them more accessible to microbial degradation during subsequent biological treatment, but if the necessary metabolic pathways for breaking down a particular compound are absent, no improvement in removal can be achieved. Studies have demonstrated that integrating pretreatment steps can lead to higher PPCP removal efficiencies than conventional biological processes alone [96,97,98,100,109,110], particularly for compounds that otherwise show low removal under composting, aerobic, or anaerobic digestion conditions. As a result, pretreatment serves as a promising strategy to address the limitations of biological processes in mitigating the environmental risks posed by emerging contaminants, although the removal values achieved are far below 90%.

The adoption of Directive (EU) 2024/3019 [7], alongside the forthcoming update of the longstanding Sludge Directive 86/278/EEC, is poised to significantly influence the landscape of sludge treatment technologies both within Europe and beyond. These legislative changes introduce stricter standards for monitoring and restricting contaminants, including microplastics and pharmaceuticals, in biosolids destined for land application. As a result, wastewater treatment plants in European countries will likely need to prioritize advanced and more robust technologies—such as thermal hydrolysis, advanced oxidation, or integrated multi-stage treatment systems—that achieve higher removal efficiencies for emerging contaminants and ensure compliance with the new regulatory thresholds. The high costs associated with pretreatments and advanced oxidation technologies would probably deter plant operators from selecting technologies with higher capacity for energy recovery and overall waste mass reduction, such as thermal treatments.

This shift will not only drive innovation and investment in sustainable sludge stabilization and resource recovery technologies, but may also increase the operational costs and technical complexity. For non-European countries, the updated European regulations could serve as a benchmark, influencing international trade, technology transfer, and the harmonization of environmental standards. Ultimately, the evolving regulatory framework will play a critical role in guiding the selection and implementation of sludge treatment technologies, fostering safer and more sustainable agricultural practices while supporting the broader goals of the circular economy. However, in developing countries, implementing rigorous contaminant control in wastewater treatment plants and sludge stabilization is particularly challenging due to the high installation and operational costs of advanced technologies. Limited financial resources and technical capacity often hinder the adoption of state-of-the-art treatment systems, making it difficult to meet stringent regulatory standards and effectively manage emerging pollutants.

3.2. Thermal Methods for Valorizing Sludge and Removing Contaminants

The efficacy of biological treatments for removing recalcitrant contaminants has been shown to be limited, despite achieving high degradation rates for other, more readily biodegradable organics. Nevertheless, the recovery of nutrients and carbon remains a fundamental objective, aligned with the principles of the circular economy, which emphasizes the need for sustainable resource management. Thermal treatment technologies have been identified as a potentially effective solution to this challenge. As illustrated in Figure 2, a range of thermal treatments can be used to transform sewage sludge into valuable products.

Figure 2. Thermal technologies for the valorization of sewage sludge, transforming this material into heat and/or fuels.

Pyrolysis and hydrothermal processes, for example, can yield char whose properties vary considerably with operational conditions. These carbonaceous products (biochar/hydrochar) have potential value as low-cost catalysts, adsorbents, solid fuels, and as precursors for novel fertilizers. However, the use of sewage sludge as a constituent of a fertilizing product is subject to limitations, primarily due to the explicit prohibition stipulated in the EU Fertilizing Regulation 2019/1009 [127]. The amendment of 30/12/2025 [128] permits the use of ashes derived from sewage sludge combustion while maintaining the restriction on biochar. The utilization of biochar, derived from pyrolysis processes involving sewage sludge, mixed municipal waste, or animal byproducts, as stipulated within the provisions of Regulation (EC) No. 1069/2009, is prohibited in the context of the formulation of fertilizing products in accordance with the stipulations outlined in Regulation (EU) 2021/2088 [129]. Notwithstanding this restriction, certain countries have authorized the “end-of-waste” recognition provided that both process conditions and analytical characterization of the char confirm the production of a safe material. In these countries, the use of biochar for agronomic purposes is permitted, subject to biochar certification that requires adherence to the stipulated guidelines (EBC biochar guidelines [130], IBI guidelines [131]), which specify that the treatment temperature must exceed 500 °C.

In the context of sewage sludge treatment, the principal advantage of these technologies lies in their capacity to substantially reduce the mass of material requiring final disposal while enabling the partial recovery of the energy needed for its valorization. Both processes under discussion rely on the thermal conversion of organics, requiring significant energy input to reach the desired operating temperature. In hydrothermal (HT) processes, water is imperative for generating hydroxyl radicals, which facilitate the conversion of organics. Notably, given the absence of a phase change in this scenario, the energy demand is anticipated to be lower than that for pyrolysis [132], provided that the drying energy demand is incorporated into the balance.

HT processes can operate at mild temperatures and pressures (180–260 °C, 20–50 bars) under the name of HT carbonization (HTC), with the main product being hydrochar [133,134]. It is noteworthy that certain authors regard conventional hydrolysis, such as the CAMBI process (165 °C and 30 min), as a form of hydrothermal treatment [135]. Increasing the temperature up to a range of 200–370 °C with pressures between 40 and 220 bars leads to HT liquefaction (HTL). These operating conditions result in the production of a biocrude rich in oxygenated and nitrogen-containing compounds, along with a remaining hydrochar with a higher C/O ratio and thermal stability [136]. The HT gasification (HTG) process is characterized by temperatures near the critical water point, with a maximum of approximately 600 °C. When the applied temperatures exceed the critical point and are significantly elevated, the technology is better described as supercritical water gasification [137,138]. In the latter case, the reduction in organics is catalyzed by the presence of salts already contained in the raw material, which favor water–gas shift reactions [139].

Pyrolysis and gasification require that the raw material be dried prior to processing. This significantly penalizes the energy demand and may even result in energy needs exceeding the amount recoverable from the feedstock itself [140]. However, the energy constraint can be overcome by harnessing the heat released by microbial metabolism. The Biodryer technology is an example of this principle. The technology was developed by Bioforcetech (San Francisco, CA, USA) [141] and has been demonstrated to be capable of drying biosolids from an 80% water content to a solid content as high as 75% to 90% in 48 h by using the heat produced by the metabolisms of aerobic bacteria. This process has been shown to reduce the drying energy demand by up to 70%.

3.2.1. Pyrolysis as an Alternative Treatment for Removing Contaminants

Pyrolysis is a thermochemical process that transforms dry organic material by thermal degradation in the absence of oxygen. The material is heated to reach temperatures typically in the range of 300 to 800 °C. Under the high temperatures applied, the organic material in the sludge decomposes into smaller, energy-rich products. The pyrolysis process offers the important advantage of producing high-energy-density products, such as syngas, bio-oil, and biochar. Syngas is a gas mixture containing H₂, CO, CH₄, and CO₂ along with small amounts of C₂–C₄ gaseous compounds. Bio-oil is the pyrolysis oil phase with a high energetic content. As a result, syngas and bio-oil are considered valuable energy carriers or valuable feedstocks suitable for further refining into high-value compounds. Biochar is a solid product. This solid phase is a carbon-rich material with applications as a soil conditioner, low-rank fuel, or as a substitute for activated carbon. The distribution of these products depends on operational conditions and the characteristics of the raw material. Additionally, pyrolysis—carried out under reducing conditions—retains heavy metals in biochar while decreasing their extractability. This, in turn, limits metal mobility and lowers the likelihood of plant uptake when biochar is applied as a soil amendment [142,143].

In the present context, the key relevance of pyrolysis lies in its high operating temperature, which is necessary to degrade recalcitrant contaminants. As with hydrothermal processes, pyrolysis can be categorized by the operating temperature and heating rate. It has been demonstrated that slow pyrolysis results in high char yields, a consequence of the prolonged residence time in the reactor. The higher the final operating temperature, the greater the release of H and O atoms from the carbonaceous product. Increasing the heating rate has been shown to favor the formation of gases and liquid products with variable proportions depending on the operating temperature and available time for reforming reactions [144,145,146]. Given the relevance of the heating ramp to product yield, it is used as a parameter to classify the process as slow, intermediate, fast, or flash pyrolysis.

Thoma et al. [147] and Hoffman et al. [148] demonstrated the pyrolysis process’s capacity to destroy various organic contaminants. They found that temperatures above 400 °C were enough to remove the estrogenicity potential from biosolids. However, to completely remove perfluoroalkyl and polyfluoroalkyl substances, temperatures closer to 600 °C are required. This type of contaminant is especially concerning because it affects the efficiency of thermal processes. Buss [149] reviewed results from 20 studies on the pyrolytic degradation of over 100 organic pollutants and found that most contaminants were removed almost completely, with at least 95% removal, even for PFAS. The author highlighted that pyrolysis, combined with the later combustion of oil and gas streams produced, is very effective at removing unwanted materials.

3.2.2. HT Technologies as an Alternative Treatment for Removing Contaminants

Onwudili et al. [150] advanced a hypothesis regarding the efficiency of hydrothermal treatments for oxidizing the organic content of sewage sludge at moderate temperatures (350–450 °C). The hypothesis stated that these treatments would have the additional benefit of removing pharmaceutical compounds. However, it should be noted that using lower temperatures does not guarantee the complete elimination of all types of contaminants [151]. Pham et al. [152] demonstrated the efficiency of the process for removing bioactive compounds (florfenicol, ceftiofur, estrone, and bisphenol A) at temperatures above 300 °C and a 30 min residence time. On the contrary, for the removal of antibiotic resistance genes, lower temperatures (250–300 °C) suffice for complete deactivation. Nahar et al. [153] reviewed the effectiveness of HT treatments for eliminating contaminants and reported that PFAS were the most difficult to remove, requiring higher temperatures and longer residence times. However, adding alkali reagents or metal/carbon additives may enhance efficiency [154].

The potential benefits of obtaining biocrude from HTL are often overshadowed by the challenges associated with subsequent upgrading stages and the difficulties in removing oxygen and nitrogen compounds [155,156]. The HTL process has emerged as a promising solution for the valorization of organic streams into biofuels [157]. However, its current reliance on scale and lower maturity compared to thermal homologous technologies (pyrolysis or gasification) are significant limitations. The accumulation of recalcitrant compounds, such as PFAAs, further complicates the process, thereby reducing its overall feasibility.

3.2.3. Supercritical Water Technology (SCW) as an Alternative Treatment for Removing Contaminants

The increase in the severity of thermal treatment can be considered as a progression in technological development concerning the removal of recalcitrant compounds. The application of higher temperatures and pressures above 374.3 °C (and 22.1 MPa) leads to the range of supercritical water gasification, also denoted as hydrothermal gasification, where H₂, CO₂, and C_1-4 molecules are preferentially produced [133]. Alternatively, the process may be classified as supercritical water oxidation when an oxidizer, such as oxygen or air, is added to enhance the destruction of stable bonds.

De Souza et al. [158] reviewed the application of supercritical water technology for the treatment of complex wastewater and sludge. The study highlighted the difficulties encountered in industrial applications, including significant exposure to corrosion, metal stress, salt deposition, and clogging. Therefore, increasing process severity results in higher installation and operating costs [159]. A study evaluating the efficacy of supercritical water oxidation for treating PCB-contaminated sludge was conducted by Crain et al. [160]. Their results indicated that the oxidation process achieved 99.99% PCB removal. Catalytic supercritical gasification has been proposed to enhance carbon conversion and hydrogen yield by using alkali compounds, including KOH, K₂CO₃, NaOH, and Na₂CO₃ [161,162]. However, the supercritical water gasification (SCWG) process generates char and soluble organic compounds in the water phase as byproducts, requiring further treatment and disposal. Char yield and organic concentration in the liquid phase increase with the solid content of the treated sludge. This parameter also affects syngas composition by reducing the CO₂ content in the gas phase [163,164].

Ribeiro et al. [165] evaluated the effectiveness of SCW technology in treating emerging contaminants by conducting experiments with synthetic wastewater containing hormones to simulate pharmaceutical effluents. Their results demonstrated a removal efficiency of approximately 90%. Dias et al. [166] reported amoxicillin degradation in a separate study using a continuous-flow reactor, with removals exceeding 90%. However, intermediates obtained from the antibiotic conversion process remained toxic when tested on brine shrimp (Artemia salina).

Regarding sewage sludge treatment, the SCWG process has also been proposed as a second stage of the HT carbonization technology. This proposal is attributed to the high COD concentration found in the aqueous phase [167,168]. The need for further treatment arises from the presence of inhibitory compounds, which can affect their degradability through biological processes such as anaerobic digestion. Consequently, specific acclimation stages become imperative, especially when the severity of the treatment is increased [169,170,171]. Another salient issue concerns the fate of contaminants that may be released into the aqueous phase, either due to partial degradation or the presence of substances such as heavy metals in the raw material, making their removal essential. This particular subject was previously reviewed by Ender et al. [172] and Mehrez et al. [173], which underscored the importance of removing contaminants prior to the further valorization of the organic matter contained in HTC process water.

SCWG is considered suitable for treating HTC process water, as the harsh conditions facilitate the transformation of toxic intermediates generated in the preceding phase and can recover some process energy by producing syngas with a high H₂ content. Feng et al. [174] proposed the SCWG of HTC sludge process water, reporting a 59% H₂ content in the syngas along with the formation of sulfur compounds in the gas phase. This latter gaseous compound can be removed prior to syngas valorization. In particular, these authors indicated that increasing the gasification temperature would favor the formation of H₂S over SO₂ and CH₃SH. However, not all compounds undergo complete transformation into inert molecules, as evidenced by Dias et al. [166]. One potential solution to this challenge involves augmenting the process’s degradation capacity by incorporating an oxidant agent.

3.2.4. Water Oxidation as an Alternative Treatment for Removing Contaminants

The addition of an oxidizer transforms conventional SCWG into supercritical water oxidation (SCWO) or into wet oxidation under less severe conditions. This transformation enables the more efficient destruction of organic matter, resulting in the release of ammonia and volatile fatty acids as predominant byproducts. These compounds can undergo more straightforward valorization. Oxidation reactions are known to release heat; this heat can be recovered from the process, thereby reducing the thermal energy demand. However, despite this advantage, the process requires excessive electricity for the operation of high-pressure pumps, feed and gas injection systems, and the oxygen generation unit [175]. The process has been regarded as a viable solution for the treatment of sewage sludge and municipal solid waste [176,177]. However, it has encountered challenges in its industrial implementation due to factors such as corrosion and salt deposition caused by the concomitant presence of oxygen and chlorides, high operating and installation costs, and the complexity of the process [178]. In addition to these previous drawbacks, the critical point of oxygen injection and the risk of exceeding localized temperatures must be considered, as this can result in thermal stress, corrosion pitting, and cracking [179]. The corrosive environment created by oxygen and salts has relegated the technology’s deployment to rare cases where the destruction of highly recalcitrant molecules is required. Recently, the process has been considered more suitable for treating oily sludge due to the inherent difficulties encountered when converting this high-moisture-content stream using other methods [180,181].

The environmental and health risks associated with PFAS have driven the resurgence of this technology. Krause et al. [182] and Rosansky et al. [183] conducted experiments on a device known as the PFAS Annihilator. This device was engineered to withstand corrosion problems thanks to its construction from corrosion-resistant materials, specifically a high-nickel alloy (Hastelloy C276). The device also includes a heat exchanger made of a similarly resistant alloy (Alloy 625) that recovers heat from the oxidation reaction. The extreme conditions of the SCWO process ensure near-total destruction of a wide range of PFAS, with a reported destruction rate exceeding 99.9%. While the process was initially evaluated for its efficacy in treating water-dilution containing aqueous film-forming foams, the findings suggest potential for broader applications in more complex matrices. However, the high operational costs of the treatment and the dependence of the process profitability on the volume processed suggest that SCWO technology is more appropriate for highly contaminated streams [183]. Despite a significant reduction in COD effluent concentration [182], many unquantified fluorine compounds were observed in the study by Rosansky et al. [183]. Yang and co-workers [175,184] developed a commercial-scale plant and described safety issues associated with SCWO applied to wastewater sludge, estimating a running cost of 112 US$/t dry sludge.

3.2.5. Gasification and Combustion as Alternative Treatments for Removing Contaminants

Gasification and combustion represent alternative thermal pathways that enable the complete transformation of organic matter in the presence of oxygen. Gasification is a process that involves partial oxidation, enabling the recovery of part of the material’s inherent energy as combustible gases, such as CO, H₂, and CH₄. In contrast, combustion involves the complete oxidation of substances by supplying air at or above stoichiometric levels, yielding heat as the primary output. Because land availability for applying sludge as an organic amendment is limited—and further constrained by the presence of certain pollutants—the industrial sector has increasingly turned to sludge incineration. This approach offers a practical way to reduce waste volume, recover energy, and limit environmental pollution [185].

Several authors have reported on the benefits of the co-incineration of sludge and diverse fuels and waste (coal, natural gas, municipal solid waste, among others) [186,187,188,189,190]. However, when sewage sludge is valorized in situ or treated in small-scale facilities, other thermal technologies become more practical. Given the operational constraints of ash sintering and fouling at high temperatures, gasification is a suitable option for energy recovery while also enabling the destruction of pharmaceutical residues and other undesirable contaminants. Moreover, integrating gasification into conventional WWTPs has the potential to enhance overall energy efficiency [191] and mitigate the risk of releasing emerging contaminants, microplastics, and materials related to antibiotic resistance into the environment. This rationale underlies the experiment conducted by Varjúová et al. [192], who evaluated the removal of 13 pharmaceutical compounds—including azithromycin, carbamazepine, cetirizine, citalopram and its metabolite N-desmethylcitalopram, diclofenac, fexofenadine, sertraline and its metabolite norsertraline, telmisartan, trazodone, valsartan, and verapamil—by subjecting sewage sludge to different thermal conditions. Their results indicate that treatment at 250 °C achieved nearly complete removal of all target compounds, while increasing the temperature to 500 °C resulted in no detectable residues of any of the substances analyzed.

As illustrated in Table 5, numerous experimental studies have assessed the efficacy of thermal sludge processing for removing pharmaceuticals and contaminants. Despite current regulatory restrictions on the use of sludge-derived biochar in agricultural soils, the significant reduction in waste volume and the inherently inert nature of the resulting char support the consideration of thermal technologies as a viable sludge-valorization strategy. A comparison of the environmental and health risks associated with the application of untreated or insufficiently treated sewage sludge to cropland and the use of thermal processes reveals the latter as a more secure and effective alternative.

Table 5. List of experimental studies assessing the removal of contaminants in sludge using thermal methods.

Experimental Description	Results	Reference
	Pyrolysis
Composted biosolids and biochar derived from biosolids at temperatures between 600 and 650 °C were applied to agronomic land for a winter wheat (Triticum aestivum L.) crop test.	Pyrolysis was more effective in removing pollutants and avoiding plant uptake.	[40]
The study compares the effectiveness of pyrolysis and HTC in removing contaminants (polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and PPCPs) from biosolids.	Most of the contaminants were removed during pyrolysis at 450 °C. At 600 °C, all organic pollutants were eliminated from biochar with ≥99.9% efficacy. However, HTC removal efficiencies were much lower, ranging from 48% to 70% for different compounds.	[151]
Two types of sludge were pyrolyzed using slow pyrolysis at 450 and 500 °C (1 h and 1.5 h, respectively). 161 organic contaminants were monitored.	61 out of the analyzed compounds were measured in the sludge, but only 39 were quantified in the char. Perfluoroalkyl substances showed low removal rates (with average decreases of 32% and 23% for the specific compounds measured). Removal rates for the remaining measured organics were higher (with values above 80%). Paracetamol was the exception, explained by the reactor’s limited ability to easily remove the liquid phase from the generated char.	[193]
The uptake of pharmaceuticals in lettuce growth was tested using dried and thermally treated biosolids (torrefaction at 220 and 320 °C, and pyrolysis at 420, 520, and 620 °C).	42 types of pharmaceuticals were detected in the dried sludge. Pharmaceutical uptake by plants was detected in experiments with biosolids and torrefied char application. Carbamazepine, ethenzamide, solifenacin, and tramadol were detected in the control treatment, which lacked biosolid application. Experiments using biochar derived from pyrolysis showed negligible pharmaceutical uptake by plants.	[194]
Sewage sludge. Slow pyrolysis at temperatures between 400 and 800 °C. Measuring the removal of 4 different groups of contaminants: (i) polychlorinated biphenyls (PCBs), (ii) polycyclic aromatic hydrocarbons (PAHs), (iii) pharmaceuticals, and (iv) endocrine-disrupting and hormonal compounds.	Pharmaceuticals were effectively removed at pyrolysis temperatures above 400 °C, whereas temperatures above 600 °C were required to eliminate the other 3 groups of contaminants.	[195]
The thermal degradation of 14 pharmaceutical compounds in biosolids was studied. The temperature reached was 600 °C at different heating rates (10–50 °C/min).	Antibiotics were detected after treatment. Quantification was not provided.	[196]
	Hydrothermal technology
The authors studied 12 representative pharmaceuticals. Temperature: 210 °C, stirring at 500 rpm, reaction time: 4 h.	Phenazone showed no removal. Matrix effects prevented the reporting of diclofenac values. Unlike other similar studies, ibuprofen showed a conversion of 63%. Metoprolol only attained a 39% conversion. The remaining components showed acceptable removal efficiencies, with propranolol achieving 81% conversion; the other compounds ranged from 89% to 98%. Hydrochar measurements indicated the presence of phenazone, metoprolol, and propranolol; the other studied pharmaceuticals were not detected in hydrochar samples.	[38]
Pilot scale study under continuous conditions with a treatment capacity of 39–94 L/h. Temperature: 300, 325, and 350 °C. Residence time approximately 11–21 min (including heating and cooling stages). The authors tested the removal of pharmaceuticals and biocide compounds.	25 pharmaceuticals and 4 transformation products were measured in the sludge. 7 biocides were measured in the sludge. The optimal temperature for maximizing contaminant destruction was ≥325 °C. Maximum biocrude production was obtained at 325 °C (41% of biocrude yield). Process water poses a treatment challenge. Clindamycin was the most difficult to eliminate, but even at the lowest temperature, the minimum removal was 84.3%; it increased to 85% at higher tested temperatures. The other tested compounds showed removals of 98% and above. However, some removal values need to be contrasted with improved analytical clean-up sample methods.	[197]
The following PPCPs were tested: roxithromycin, sulfathiazole, sulfamethoxazole and sulfadiazine, acetaminophen, caffeine, metoprolol, and propranolol. Reaction temperatures: 180, 240, and 300 °C, reaction times: 2, 6, 10, and 15 h.	Caffeine concentration increased in sludge after hydrothermal treatment, even at high temperatures. Acetaminophen exhibited a similar trend, but at temperatures above 240 °C, the concentration decreased. The remaining tested compounds were effectively removed.	[198]
The authors studied the degradation of 12 organic compounds, using sucrose as a surrogate for biomass matrices and testing acidic conditions. Two temperature conditions were tested: Temperature: 200 °C (reaction time: 4 and 16 h), and temperature: 255 °C (reaction time: 116 h).	Hydrothermal treatment was carried out using water, an aqueous sulfuric acid solution, and an acidified sucrose solution with sulfuric acid as the solvent. Most organic contaminants degraded during hydrothermal treatment, with higher conversion under acidic conditions, although partial transformation was observed. Ibuprofen and chlorinated aromatics exhibited recalcitrance.	[199]
	Wet air/oxygen oxidation
31 pharmaceuticals and 4 pesticides were detected in the sludge samples. Application of RSM 1. Temperature: 170–220 °C, reaction time: 118–421 min, sludge content in water solution: 3.3–10.0%.	The main factors affecting overall pharmaceutical removal were temperature and reaction time. The best removal values were achieved when these parameters were set close to their maximum values, reaching removals of about 90%. Amisulpride and amitriptyline exhibited removal values greater than 90%.	[41]
Experiments were conducted between 200 and 350 °C, adding oxygen as the oxidizer at an initial pressure of 30 bars. Treatment time: 0–60 min at high stirring speed. The pharmaceuticals tested were amoxicillin sodium powder (Ibiamox) and metronidazole solution.	Complete destruction of amoxicillin was achieved at temperatures between 300 and 350 °C with a treatment time of 5 min. However, metronidazole is more thermally stable. Small remnants were observed after 60 min of treatment at 300 and 350 °C (removal percentage: 99.4%).	[200]
Assessing the removal of diclofenac from sludge samples. Temperature: 190–210 °C, reaction time: 1 h. Removal was evaluated from a diclofenac-spiked sludge sample and from sludge samples containing the pharmaceutical.	The removal of the spiked samples exceeded 99%. However, a much lower removal (56%) was attained for the sludge samples. More severe conditions are needed to remove the pharmaceutical in the latter case, which is probably due to aging within the matrix, stronger absorption, and complexation, making it less accessible to radical attack.	[201]
Assessing the removal of caffeine and acetaminophen from sludge samples by HT. Hydrochar was applied to wheat growth tests to measure the migration of contaminants from the char to the plant biomass. HT temperatures: 180 and 240 °C, reaction time: 6 and 15 h.	On average, 46% of caffeine was removed across all treatments, except the most severe, which reported higher levels than those found in sludge. Acetaminophen showed no removal at 180 °C, with values higher than those in the sludge. However, an average of 79% conversion was observed at 240 °C. Plant migration tests showed that although PPCPs were detected in hydrochar and in plant shoots and roots, the measured levels did not pose a risk, as they were below the limits established by the Drug and Food Additive Use Standard in China.	[202]

Note(s): ¹ RSM: Response surface methodology.

Several interesting reviews examined PFAS removal and concluded that processes involving high temperatures and severe conditions were the most suitable alternatives [203,204,205,206,207]. However, even when reporting high levels of destruction, failing to quantify thermal degradation products in the outlet stream may mask risks when valorizing thermal byproducts. Even incineration, which operates at much higher temperatures, cannot eliminate all types of contaminants [208]. Therefore, it is necessary either to identify suitable alternatives or to accept the unavoidable constraints linked to the subsequent removal of these essential compounds, which are widely used across numerous products.

Although thermal technologies offer numerous advantages for contaminant destruction, they are inherently associated with high installation costs and operational challenges, including corrosion and elevated energy consumption. The integration of circular economy principles into thermal treatment operations—through the valorization of waste and the recovery of energy, carbon, and nutrients—must contend with several drawbacks. These include stringent requirements for equipment durability in corrosive and abrasive environments, the need to withstand extreme temperatures and pressures, and the added complexity of managing byproducts. Specific issues arise with syngas cleaning and condensate treatment in gasification and pyrolysis, process water in hydrothermal treatments, and ash and volatile emissions in combustion processes. While no current solution eliminates undesirable byproducts entirely, the significant reduction in their volume relative to the original sewage sludge enables the development of approaches that present a much lower risk of environmental pollution.

4. Application of Biochar and Hydrochar for Removing PPCPs and Emerging Contaminants

The pyrolysis process has the added advantage that the resulting biochar can be used to remove contaminants from wastewater. Therefore, this technology could help valorize contaminated sludge and potentially remove undesirable compounds from wastewater by adsorption. Anastasiou et al. [209] tested the removal of caffeine, carbamazepine, and 17α-ethinyl estradiol using sludge biochar and alkaline-treated biochar. They reported better adsorption results for the latter. Kimbell et al. [210] evaluated the performance of adsorbent-filled columns containing biosolids-derived biochar operating under continuous conditions for triclosan removal. These authors reported lower adsorption capacity than activated carbon, but still high enough to consider the strategy technically feasible. Several authors have reported on experiments using biochar as an adsorbent to remove antibiotics and other contaminants [211,212,213,214,215]. Zhu et al. [216] used machine learning to analyze data from the scientific literature on the characteristics of biochar and PPCP adsorption. The authors reported that biochar adsorbents with average pore diameters 1.5 to 2.5 times larger than the maximum diameter of PPCP molecules favored contaminant adsorption.

Adsorption merely serves as an intermediate step within the treatment process; a subsequent elimination phase remains necessary. Notably, when activated carbon is employed, this additional removal stage can substantially elevate the overall treatment costs. Wang et al. [217] proposed using subcritical water conditions with H₂O₂ as an oxidant to regenerate granular activated carbon used to adsorb PPCPs. Although the characteristics and toxicity of the intermediates produced from the adsorbed contaminants were not assessed, this strategy is interesting because it enables multiple regeneration cycles while maintaining high adsorption capacity.

Not only does biochar act as an adsorbent, but it can also enhance the microbiological degradation of contaminants and act as a catalyst in advanced oxidation processes [218,219]. The strategy of adding biochar as a supplement to biological processes has been widely studied, even though the removal rates are not high enough to consider the single biological stage as a treatment alternative. In general, processes achieve higher removal rates than those without biochar addition. Table 6 lists experiments on the removal of pharmaceutical and emerging contaminants using biological processes aided by biochar supplementation.

Table 6. List of experiments reporting on biochar/hydrochar supplementation for enhancing biological process performance.

Process	Organic Compound	Main Results	Reference
Biochar addition
Photobioreactor + biochar derived from agricultural waste. Microalgae Chaetoceros muelleri	Carbamazepine, sulfamethazine and tramadol	Under optimized conditions (biochar addition), the removal of contaminants reached values of 68.9% for carbamazepine, 64.8% for sulfamethazine, and 69.3% for tramadol. In contrast, the reactor without biochar addition only reached values in the range of 33% to 36.5%	[220]
Immobilization of Chlorella pyrenoidosa on chitosan and FeCl3 modified peanut shell biochar	Sulfamethoxazole	Sulfamethoxazole removal: 45.7% (33% higher than the microalgae removal performance)	[221]
Woodchip bioreactor and biochar (B2)	Ibuprofen, naproxen, sitagliptin phosphate, and estrone	Longer HRTs 1 and smaller biochar particles could enhance the removal efficiency. Removal percentages: ibuprofen: 69.5%, naproxen: 73.6%, sitagliptin: 91.1%, estrone: 97.0%	[222]
Activated sludge process + biochar	Ciprofloxacin	The reactors were fed with activated sludge and filled with biochar. The system operated under semi-continuous conditions with a liquid phase retention time of 2 d and a solid retention time of 7 d. The control reactor lacking biochar showed poor removal performance. The system filled up to 40% of its operating volume with biochar achieved an antibiotic removal value of 94.3%.	[223]
Activated sludge process + biochar	Phenol	The addition of biochar and the acclimation of microflora increased the phenol degradation rate from 0.0017 g/g VSS 2 h in the control to 0.041 g/g VSS h in the adapted experimental conditions containing biochar.	[224]
Anaerobic membrane bioreactor + magnetic biochar	Sulfamethoxazole	The sulfamethoxazole removal was greater than 90% for the system containing biochar. The control system only attained 52% removal. Magnetic biochar reduced the concentration of soluble microbial products by 70%, from 22.2 mg/g VSS in the biochar-free unit to 6.3 mg/g VSS in the biochar-testing unit.	[225]
Anaerobic digestion + biochar, KOH-modified biochar (BCKOH), and FeCl3-modified biochar (BCFe)	Tetracycline	Adding biochar surpassed the inhibition caused by tetracycline in anaerobic digestion. The test using BCFe attained nearly complete tetracycline removal.	[226]
Anaerobic sequencing batch reactor + biochar as catalyst (Fe/Zn@biochar catalyst)	Ciprofloxacin	Treatment of wastewater rich in ciprofloxacin. Ciprofloxacin removal: 80.9 ± 8.6% (a 51.1% increase compared to the control system lacking a catalyst). The biogas yield increased from 0.17 ± 0.06 to 0.34 ± 0.02 L/g of COD removed.	[227]
Composting + zeolite, biochar, zeolite + biochar	Oxytetracycline, tetracycline, and chlortetracycline	Composting deer manure. The addition of adsorbents enhanced antibiotic degradation. Adding biochar promoted temperature increments in composting piles.	[228]
Composting + biochar	Oxytetracycline, tetracycline, chlortetracycline, and doxycycline	Composting pig manure. Biochar enhanced antibiotic degradation and decreased the relative abundance of antibiotic resistance genes. Biochar addition promoted temperature increments in composting piles.	[229]
Hydrochar addition
Co-digestion + Fe-Hydrochar	Sulfametoxydiazine, azithromycin, carbamazepine, megestrol acetate, progesterone, ciprofloxacin, oxytetracycline, naproxen, gemfibrozil capsules, and triclosan	Batch experiments. Improved degradation rate for some contaminants and improved total degradation for others. No single conclusion for all compounds tested.	[230]
Digestion + Fe-Hydrochar, Co-Hydrochar	Sulfadimethazine	Batch experiments. Chicken manure was used as the carbon source. Low degradation (<40%)	[231]
Enzymatic degradation: Use of hydrochar/biochar for laccase immobilization	Acetaminophen, sulfamethoxazole and tetracycline	Oxidative degradation was attained by adding syringaldehyde as a redox mediator. Almost complete degradation was attained for both antibiotics and acetaminophen.	[232]
Soil remediation: Hydrochar addition	Oxytetracycline	Contaminated soil. Pot experiments. Assessed oxytetracycline degradation and plant uptake by using Chinese cabbage. The oxytetracycline content in the soil decreased by approximately 9–25% for the different conditions tested, thanks to the proliferation of oxytetracycline-degrading bacteria. Plant uptake decreased by 10–66%.	[233]

Note(s): ¹ HRTs: Hydraulic retention times. ² VSS: Volatile suspended solids.

Adsorption is not the only phenomenon behind the improved removal of contaminants. The addition of biochar has been shown to enhance microbial growth and facilitate direct interspecies electron transfer (DIET) in biological systems, thereby providing a more effective pathway for microbial degradation [224,225,233]. The presence of char not only favors biological conversion, but also provides protective sites for microorganisms, alleviating inhibitory conditions [226]. Adding iron species, as is the case with Fe-doped biochar or hydrochar, favors the redox cycle between Fe²⁺/Fe³⁺, thus improving electron transfer [226,230]. This element has such a great influence on microbial activity that the presence of iron ores has been shown to modify microbial diversity, increasing nitrogen retention during composting [234].

Analogous to the application of biochar—either as an adsorbent for contaminant removal or as a carbon-conductive material to stimulate metabolic activity—hydrochar, a carbonaceous material produced via HTC—has likewise been investigated for its capacity to facilitate the removal of pharmaceuticals [235]. Table 4 also lists experiments using hydrochar as a supplement in biological treatments. The difference with biochar is that the organic material in hydrochar is assimilated by anaerobic microflora. This increases the methane yield by acting as an electron mediator, facilitating metabolism under low-temperature conditions, and providing an additional carbon source [236]. Although hydrochar has a less resistant chemical structure than activated carbon [237], activation via various chemical and physical processes has been studied. This is similar to its homologous biochar, as its use as an adsorbent is widely recognized for its lower processing cost and the intrinsic advantage of valorizing waste material.

The use of biochar/hydrochar derived from waste (specifically digestates and biosolids) and integrating this material back into the digestion process offers real opportunities for a circular economy. This material acts as a carbon supplement, enhancing methane yields and removing contaminants in the liquid and gas phases [238]. Zhang et al. [239] reviewed the application of bio/hydrochar as adsorbents in environmental remediation, indicating that strategies for eliminating contaminants and regenerating adsorbents are needed in addition to assessing the efficient capture of contaminants.

Yan et al. [230] studied the co-digestion of food waste and sewage sludge, as well as the removal of PPCPs, by adding Fe-doped hydrochar derived from digestate. The contaminants studied included sulfametoxydiazine, azithromycin, carbamazepine, megestrol acetate, progesterone, ciprofloxacin, oxytetracycline, naproxen, gemfibrozil capsules, and triclosan. Batch digestion assays reported removal values greater than 97% for ciprofloxacin, oxytetracycline, progesterone, and triclosan. The addition of hydrochar accelerated the process but did not improve the outcome. However, the removal efficiencies of sulfametoxydiazine, carbamazepine, naproxen, gemfibrozil capsules, azithromycin, and megestrol acetate improved with the addition of the hydrochar catalyst. The authors obtained a 73.2% removal for carbamazepine in the control system, which increased to 81.4% with the highest dose of 5.0 g/L of Fe-doped hydrochar. These values seem too high compared with those reported by other authors [91,93,95,97]. Liu et al. [231] also tested the performance of a hydrochar-based catalyst, assessing the effect of using Fe or Co as the modified metal in the hydrochar. In their experiments, they obtained a poor removal of sulfadimethazine, even after adding the catalyst (31.08–37.40%, Co-Hydrochar reported the highest removal). Although the removal remained low, it still represented an improvement over the control experiment, which yielded a substantially lower value (21.81%).

5. Comparative Performance

A comparative synthesis of available processes for removing PPCPs and emerging contaminants reveals distinct trade-offs across key criteria—energy consumption, removal efficiency, carbon recovery, economic feasibility, and life cycle impacts—within the circular economy framework. Conventional biological processes, such as aerobic and anaerobic digestion, are generally energy efficient and economically viable. These processes offer as a main advantage the stabilization of sludge, which can later be used as an organic amendment in agricultural cycles. Therefore, nutrients and carbon are returned into the production scheme. Nevertheless, this main advantage can pose a significant risk, given that only moderate removal rates are achieved for many persistent contaminants. In the case of anaerobic digestion, the recovery of energy through biogas valorization is the major attractive point of the technology. The application of pretreatments can enhance biodegradability and reduce conversion times, but even in this case, moderate removals were still reported by different authors [96,97,98,100,109,110].

Thermal technologies, including pyrolysis, gasification, and hydrothermal treatments, achieve substantially higher removal efficiencies and enable the recovery of energy, carbon, and nutrients, aligning with circular economy objectives. However, these approaches demand significantly greater energy inputs and investment in robust infrastructure to manage harsh operational conditions and byproducts. Pyrolysis, gasification, and combustion require the complete dryness of the input material, thereby significantly increasing the process’s energy demand unless energy recovery options or novel drying methods (to benefit from microbial metabolism heat) are incorporated. High temperatures attained during these thermal processes enabled the significant removal of contaminants, although research gaps persist regarding intermediate products that can be released into the gas and liquid phases.

In the case of water-based methods, such as HTC and SCW technology, the absence of a drying stage is claimed to be a major energetic advantage, leading to better energy balance and positive repercussions in life cycle assessments (LCAs) as long as heating recovery is included. The study by Poranek et al. [240] reported an LCA comparing incineration, gasification, pyrolysis, and HTC of sewage sludge, finding better performance for the latter, with the lowest relative impacts across almost all indicators. However, the strict requirements for materials in building reactors that must withstand high-pressure conditions in corrosive environments are rarely accounted for in analyzing the environmental performance of these processes, nor are the increased operational costs associated with process control and maintenance to reduce corrosion and pitting. Table 7 compares the technologies currently available for treating sludge and removing contaminants.

Table 7. Comparative assessment of the different technologies currently available for sludge stabilization/treatment and contaminant removal.

Features	Biological Processes: Composting, Anaerobic Digestion, and Extended Aeration	Thermal Processes: Pyrolysis, Hydrothermal, Gasification, and Combustion	Thermal Processes: Extreme Conditions. SWG and SWO
Pollutant removal efficiency	Moderate removal for most contaminants: 17–66% [70,71,80,81,90,91]. Some compounds, such as carbamazepine and oxytetracycline, have shown recalcitrance [97,98]. The application of pretreatments may improve the removal percentage to up to 80% and achieve full removal for some pharmaceuticals, but others remain resistant to degradation [97,98].	Temperatures above 300 °C are needed to achieve the complete removal of pharmaceuticals in hydrothermal processes [152] and above 400 °C in pyrolysis. HTL transforms pharmaceuticals into intermediates that lack the parent compound’s original structure, but the toxicity of degradation products is a relevant issue, with many of them not easily identified [241]. Gasification and combustion operate at higher temperatures, achieving full removal.	SWG achieved pharmaceutical and emerging compound removal above 90% [160,165,166]. However, products released with hydrothermal process water evidence high toxicity [166]. SWO achieves greater destruction of chemical compounds due to the highly oxidative environment and severe process conditions (high temperature and pressure). SWO has been applied to treat sludge with highly recalcitrant components, achieving over 99.9% removal [180,181,182,183].
Energy demand	Low energy demand. Aerobic processes require aeration, whereas anaerobic digestion requires heat addition to maintain temperatures at mesophilic or thermophilic levels. Composting needs 1.5–2.0 MJ/kg compost produced [242]. Energy recovery from anaerobic digestion benefits the process. The estimated energy consumption is 496.13 kWh/t DS (dry solid), with this energy obtained from biogas valorization in a CHP engine [243].	Pyrolysis: 4–10% of the energy input in the raw material is needed to self-sustain pyrolysis [145]. Gasification: An equivalence ratio (ER) between 0.15 and 0.4 achieves autothermal operation, increasing cold gas efficiency as ER increases. Sludge drying is the operation that most penalizes the thermal processes (1.7–4.0 MJ/kg of dewatered sludge [140]). HTL: 57% of the energy contained in sludge is needed for running the process. However, energy recovery can reach 42%, expressed as energy contained in sludge [244]. Combustion: Self-sufficiency.	High energy demand. Energy efficiency is only 45% when considering all process inputs for SWG [245]. Process assessment typically involves obtaining H2 from biomass and waste streams. Energy inputs are highly uncertain when estimating electricity consumption due to a lack of operational data from demonstration plants. Many authors disregard electricity for high-pressure pumping equipment. In the case of SWO, energy demand is lower due to the energy released by the oxidation reaction. The process is autothermal for feeds with an enthalpic content of 900 kJ/kg [246].
Technological readiness level	Commercially available. Wide technological deployment.	Combustion: Mature technology commercially available. Gasification: Mature technology for biomass gasification and fossil fuel gasification. Sludge gasification is an emerging technology with several demonstration plants installed worldwide. Pyrolysis: Emerging technology. Several projects at demonstration-scale. HTL: Small-scale projects at a precommercial stage.	Small-scale demonstration projects at a precommercial stage. Excessively high installation costs due to the highly corrosive and abrasive environment requiring corrosion-resistant alloys capable of withstanding high pressure [175]. High-pressure conditions impose the installation of heavy-duty compressors and pumping equipment.
Environmental impacts	The positive net energy balance and energy recovery make this technology the best option in terms of environmental impacts [243]. CO2 and NH3 emissions are higher for composting [247].	Modern incineration technology is superior to pyrolysis and gasification due to the effectiveness of current flue gas cleaning technologies [248]. Syngas cleaning and bio-oil upgrading still face important challenges in gasification and pyrolysis processes.	Preheating of sludge is the major contributor to environmental impacts [249]. Energy recovery is essential to reduce the carbon footprint. In the case of SWO, energy associated with oxygen generation is another main contributor to environmental impacts [249].
Economic aspects	Wang et al. [243] reported a net energy balance of 418.72 kWh/t DS and a cost of 36.29 US$/t DS. In contrast, composting has a negative energy balance due to the requirements for aeration and pile turning.	Gasification is the most promising method when considering electricity production (0.76 kWh/kg DS). Digestion: 0.53 kWh/kg DS, pyrolysis: 0.34 kWh/kg DS, HTL: 0.13 kWh/kg DS. Levelized electricity costs are lowest for digestion (0.02 US$/kWh), followed by gasification (0.11 US$/kWh), pyrolysis (0.14 US$/kWh), and HTL (2.21 US$/kWh) [250].	Higher installation costs than those expected for anaerobic digestion. Ochieng et al. [251] estimated a treatment cost of 200 US$/t of sewage sludge, which is 52% higher than the projected cost for anaerobic digestion, while reporting comparable levels of energy self-sufficiency. However, their analysis included digestate drying, a step that significantly increases both the cost and energy demand of the digestion process. The estimated heat demand for SWG was 100 kWh/kg DS *. The estimated treatment cost for SWO was 112 US$/t of dry sludge [175,184].

Note(s): * value reported was 42.4 GWh with a treatment capacity of 15.4 t DS/d.

Thermal treatment of sewage sludge, while effective at reducing the concentration of many persistent contaminants, can lead to the formation of a variety of poorly characterized intermediate compounds. Intermediate quantification presents significant analytical challenges due to their chemical diversity, transient nature, and low concentrations. In some cases, these transformation products may exhibit greater toxicity or environmental persistence than their parent compounds, raising concerns about unintended risks associated with the valorization of biocrude and pyrolysis oil, as well as the treatment of gaseous emissions. There is a substantial research gap regarding the identification, fate, and potential impacts of these intermediates. The study by Hegdahl et al. [241] reported on the toxicity expected from thermal-process wastewater when treating pharmaceuticals contained in sewage sludge, despite achieving full removal of the original chemical compound. Rosansky et al. [183] also reported the presence of unidentified fluorine compounds during PFAS treatment in SWO. Comprehensive studies are needed to elucidate their formation pathways, assess their ecological and human health risks, and develop strategies to mitigate their presence in sludge byproducts derived from thermal technologies. Figure 3 illustrates various sewage sludge treatment processes and highlights their effectiveness in removing emerging contaminants. It also identifies the main byproducts and indicates the stages at which intermediate compounds are most likely to accumulate.

Figure 3. Illustration representing the different pathways for sludge treatment based on the feasibility of attaining acceptable levels of contaminant removal. The main byproducts and phases where intermediates are likely to accumulate are also indicated.

Physicochemical techniques such as adsorption (using activated carbon) can effectively reduce the contaminant concentrations, but they often generate secondary waste streams and require further processing, increasing the life cycle costs and environmental footprint. The use of biochar/hydrochar as an alternative to costly adsorbents allows closing cycles when the treatment configuration involves pyrolysis or HTC. The benefits may be increased given the proven improvement of biological processes when chars are added as a supplement.

Ultimately, no single approach can fully resolve all challenges. While thermal processes effectively degrade contaminants, they also generate intermediate compounds that redistribute across different product streams—appearing in syngas and condensates during gasification, in bio-oil and condensates in pyrolysis, and in the biocrude and process water phases in hydrothermal treatments. Achieving high levels of contaminant removal and resource recovery therefore requires balancing performance with energy consumption and economic feasibility. To address these trade-offs, it is essential to integrate multiple complementary technologies, guided by life cycle assessment and circular economy principles, to develop sustainable, low-impact solutions for wastewater and sludge management.

Recent studies on sludge treatment and contaminant removal increasingly focus on integrating advanced technologies—such as thermal processes, advanced oxidation, and biochar application—within a circular economy framework. However, there is ongoing debate over the true effectiveness and environmental trade-offs of these processes, particularly regarding their energy demands, cost implications, and the fate of transformation products. Regulatory restrictions, especially in the European Union, are becoming more stringent, thereby challenging the adoption of certain technologies in resource-constrained regions. Notably, significant research gaps persist in the comprehensive assessment of intermediate byproducts, the long-term impacts of treated sludge application, and the scalability of promising treatments under diverse real-world conditions, particularly given the harsh conditions that reactors must withstand to achieve the complete destruction of emerging contaminants.

The effectiveness of contaminant removal in sludge treatment processes is closely intertwined with opportunities for resource recovery, broader environmental impacts, and life cycle considerations. High removal efficiencies, particularly for persistent and emerging contaminants, are essential to minimize ecological risks and ensure the safe reuse of treated sludge, yet these outcomes must be balanced with the ability to recover valuable resources such as energy, carbon, and nutrients. Integrating these factors within a circular economy framework calls for a holistic evaluation of technologies, prioritizing those that not only reduce pollution but also maximize resource recovery and minimize adverse environmental impacts across the production, use, and end-of-life phases. Such an approach supports the transition toward sustainable, closed-loop systems in wastewater and sludge management, aligning operational strategies with environmental and economic objectives.

6. Conclusions

Managing pharmaceuticals and personal care products in a circular economy remains a major challenge for modern wastewater treatment plants. Simply shifting these contaminants from water to sludge does not eliminate environmental risks but instead moves the problem to the sludge stream. Although land application of sludge and biosolids offers benefits, emerging contaminants and microplastics can compromise the safety of these practices. The present review of the scientific literature reveals that conventional biological treatments, such as anaerobic digestion, extended aeration, and composting, exhibit limited capacity to degrade certain organic contaminants, achieving only partial degradation or no removal for recalcitrant compounds. These molecules tend to remain unchanged or accumulate in the sludge due to their high sorption affinity. Despite the application of various pretreatments to enhance biological degradation, the removals achieved are not acceptable in many cases, suggesting that these techniques are insufficient to ensure the safe use of biosolids as an organic amendment when an excessive content of recalcitrant molecules are present, thereby highlighting the need to adopt more effective methodologies.

Given these constraints, thermal treatment technologies—namely, pyrolysis, gasification, and hydrothermal processes—have emerged as the most robust alternatives for ensuring the effective removal of PPCPs. Processes such as hydrothermal technologies and supercritical water treatments face operational challenges, including corrosion and high capital costs. However, in SCWO, the process can nearly completely break down a wide range of complex organics, suggesting a promising avenue for the remediation of highly contaminated matrices. Despite enormous advances in process development, the high operational and installation costs of thermal technologies remain their major drawback, hindering scalability and technology deployment. The research gap regarding the formation of toxic intermediates needs to be addressed to mitigate the risk of contaminant release during the valorization of thermal byproducts.

Finally, integrating carbon-based byproducts, such as biochar and hydrochar, derived from the sludge treatment process itself, emerges as an innovative strategy for closing waste recovery cycles. These materials function as low-cost adsorbents, capturing micropollutants in the liquid phase, and as catalysts, enhancing metabolic activity in biological processes or advanced oxidation systems. For these technologies to be deployed on an industrial scale, it is imperative to continue advancing the standardization of detection methods and the harmonization of regulations governing the use of biosolids. This will ensure that resource recovery is an economically viable practice, and above all, free of risks to public health and the environment.