A Comprehensive Review on Sewage Sludge Biochar: Characterization Methods and Practical Applications

Abstract

Sewage sludge (SS) management and wastewater (WW) treatment remain among the most critical environmental challenges. The pyrolysis of sewage sludge to produce biochar (BC) represents a sustainable and circular strategy for waste valorization and pollution mitigation. This scoping review provides a comprehensive overview of BC derived from SS (BCxSS), with particular emphasis on how pyrolysis conditions affect key physicochemical characteristics such as yield, ash content, pH, surface area, and functional groups. Although substantial research has focused on the removal of heavy metals and organic pollutants using BCxSS, far less attention has been directed toward its potential for pathogen adsorption and inactivation, revealing a notable research gap. Recent studies highlight BCxSS as a versatile material with considerable promise in adsorption and catalysis. However, its application in pathogen removal remains insufficiently investigated, underscoring the need for further investigation into sorption mechanisms and biochar–microbe interactions.

1. Introduction

The rise in global population has substantially increased wastewater (WW) generation worldwide. At the same time, water scarcity has emerged as one of the most critical challenges of the twenty-first century [1]. The limited, and often deteriorating, condition of freshwater resources, coupled with rising water demand and the impacts of climate change, has accelerated the global shift toward water reuse strategies. These strategies promote the reuse of treated wastewater for a wide range of applications, including agriculture, industrial processes, and even potable reuse, as already implemented in countries such as Australia, Israel, Namibia, Singapore, and the United States [1,2,3].

Treated water and sludge are the two primary outcomes of WW treatment processes and, in the context of reuse, can be viewed either as a “Trojan horse”—carrying hidden risks—or as a “beautiful Helen”—offering valuable benefits [4]. Inevitably, as wastewater generation increases, the volume of sludge produced also rises. In Europe, sludge production exceeds 13 million tons (dry weight) [5], which corresponds to an average of approximately 9.49 kg per capita. According to Eurostat [6], Germany, France, and Poland are the largest producers of SS in Europe, generating more than 1100–1700 thousand tons annually, followed by the Netherlands, Turkey, and Hungary (Figure 1). Albania, Ireland, Slovakia, Lithuania, Bulgaria, Croatia, Slovenia, Estonia, Latvia, Serbia, Luxemburg, Malta and Cyprus produce less than 150 thousand tons annually, according to the original data. This uneven distribution reflects both population size and WW treatment coverage across member states. In Greece, sludge generation was estimated at 99.06 thousand tons of dry weight for the year of 2022 [6]. However, the abovementioned data for Greece may be uncertain, since no official sludge production data were reported for the country, which is listed among those with missing values [7]. This highlights a broader and persistent challenge in the field of WW treatment and management: the lack of a standardized and systematic framework for data recording across all treatment facilities nationwide, as well as the delayed reporting of such data [8]. These gaps hinder both operational optimization and scientific research aimed at developing new methods and techniques to improve the quality of final byproducts, ensuring they are free from pathogenic microorganisms and hazardous contaminants, including heavy metals, polycyclic aromatic hydrocarbons (PAHs), micro- and nanoplastics [9], per- and polyfluoroalkyl substances (PFAs) [10], dioxins, as well as micropollutants such as pharmaceuticals (PhACs), personal care products (PPCPs) and persistent organic pollutants (POPs) [11].

Besides the treated WW, sludge management represents equally a global environmental challenge due to its complex and variable composition. Seasonal fluctuations influence its characteristics, and while it contains valuable macronutrients such as nitrogen, phosphorus, and potassium, along with other trace elements, it also harbors the abovementioned broad spectrum of contaminants [12]. The management of sewage sludge is closely linked to each country’s broader waste treatment practices (Figure 2). Northern and Western European countries with high sludge generation place strong emphasis on recycling, backfilling, and energy recovery, thereby reducing reliance on landfilling. In contrast, Southern and Eastern European countries remain heavily dependent on landfilling, with rates approaching 90%, which poses environmental risks and constrains recovery potential.

In Greece, over 50% of sewage treatment sludge is disposed of in landfills, with limited reuse reported [6]. The sustainable management and utilization of sewage sludge (SS) as a resource has become a key priority within the frameworks of the circular economy (CE) and sustainable development agendas in Europe, the United States, and globally [12,13]. These initiatives aim to reduce pollutant loads and to convert sludge into a valuable product. Commonly implemented strategies include composting for agricultural and non-agricultural uses [14], biogas production, and thermal processes [15]. The production of biochar (BC) from SS through pyrolysis aligns with the principles of sustainable resource recovery and promotes the CE deriving from WW. Thermochemical conversion of SS into BC simultaneously addresses two major key challenges: reducing sludge disposal costs and generating a value-added material capable of removing toxic contaminants from water and WW [16].

BC is a porous, carbon-rich material formed through the thermochemical decomposition of biomass under limited or no oxygen conditions. The biomass feedstock may consist of a range of organic waste materials, such as crop and forest residues, food-industry wastes, wood chips, algae, sewage sludge, animal manures, and the organic fraction of municipal solid waste [17,18,19]. Its properties can be adjusted for specific environmental applications by chemical modification using acids, alkalis, or metal ions [20]. BC offers a list of significant advantages, including high carbon content and cation exchange capacity, large specific surface area, and structural stability. These properties fulfill the requirements of a versatile, effective, low-cost, and accessible material due to the abundance and affordability of its raw feedstock. BC has already been widely applied in pollutant and microbial agents’ sorption and as a soil conditioner, enhancing soil fertility and crop yields [21,22]. Furthermore, studies have proved that BC application to soil [23] or WW can enhance the sorption of pathogenic microorganisms [24].

This study aims to provide a comprehensive scoping review of sewage-sludge-derived biochar (BCxSS), examining how pyrolysis conditions affect its physicochemical properties, evaluating its potential use as an adsorptive and catalytic material for the removal of organic micropollutants, dyes, heavy metals and pathogens in water matrices and highlighting the role of BCxSS in advancing circular-economy strategies and sustainable WW management. Despite increasing interest in BC as a multifunctional sorbent for WW treatment, applications of BCxSS have not focused sufficiently on the removal of pathogens. This research gap underscores the need for further research into the interaction mechanisms between BCxSS and pathogens. The timeliness and necessity of this review are supported by the increasing production of SS worldwide, the challenges in its management, the growing importance of circular-economy approaches as well as the concern about emerging contaminants that remain after conventional WW treatment.

2. Methodology

A structured literature search was conducted between March and May 2025 in accordance with the PRISMA 2020 (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines to identify studies related to BCxSS. The search was carried out in the Scopus, ScienceDirect, PubMed databases and the MDPI publisher platform and was limited to English-language publications from 2015 to 2025, including both research articles and review papers. To address the study’s main focus on the characteristics of BCxSS and its role as an adsorptive and catalytic material, a set of relevant keywords was employed, including “biochar”, “sewage sludge”, “adsorption”, “heavy metals”, “pathogens” and “catalysis”. Additionally, a few references were manually added.

The initial database search identified 1264 records. After removing 574 duplicate entries, 690 unique studies remained for screening. During the first-step screening, 334 papers were excluded based on their titles and abstracts. Full-text papers assessed for eligibility were 326. Based on exclusion criteria, 171 of these records were finally excluded. Following the screening and eligibility assessment, 155 studies were included in the final review. Studies were excluded if they did not focus on BCxSS or if they primarily examined the use of BC for soil improvement, energy recovery, nutrient recycling, or sludge composting rather than for pollutants’ adsorption or catalysis.

The selection process adhered to the PRISMA 2020 framework, and the corresponding PRISMA flow diagram is presented in Figure 3. The PRISMA-ScR checklist is provided as Supplementary Material. Following the PRISMA-based selection of 155 eligible studies, a bibliometric co-occurrence analysis of author keywords was conducted using VOSviewer 1.6.20 to visualize research trends and relationships within the retrieved literature (Figure 4). The resulting network revealed seven major thematic clusters. Colors refer to different clusters, and the size of each circle refers to how often the keyword is used. The most frequently used keywords in the network are “biochar,” “sewage sludge,”, “adsorption” and “pyrolysis”. These terms occupy central positions in the map, indicating their dominant role and interconnection within the research landscape. Importantly, the VOSviewer results were consistent with the PRISMA-based search strategy, as the keywords initially used in the systematic search also emerged as the most influential and interconnected terms in the bibliometric map, thereby reinforcing the consistency and focus of the review.

3. The Potential Role of Biochar in Closing the Loop

The concept of CE emphasizes the transition from a linear “take-make-use-dispose” model to one that promotes the continual use and regeneration of resources [25]. In this context, waste streams are viewed not as liabilities, but as valuable inputs for new production cycles. Thus, biochar which has historically been regarded as a waste requiring significant cost, energy, and substantial space for disposal purposes [26] can now be valorized into a resource with environmental and economic advantages, as presented in Table 1. The production and application of BCxSS represent a promising strategy for closing resource loops while supporting sustainable goals, whereby nutrients, carbon, and materials embedded in waste are reintroduced into productive use [27,28]. Pyrolysis of SS not only stabilizes organic carbon but also transforms nutrients into bioavailable forms, making BCxSS suitable for agricultural and environmental applications. Consequently, BCxSS acts both as a sink for pollutants and a carrier of beneficial elements, aligning with CE strategies that seek to retain resource value while minimizing harm.

One of the key contributions of BCxSS to the circular economy lies in its capacity for carbon sequestration. Through pyrolysis, a significant fraction of the sludge’s organic carbon is transformed into a stable, recalcitrant form that resists microbial decomposition for centuries. From soil health enhancement [29] to water quality protection and energy recovery, the transformation of SS into BC represents a paradigm shift in how wastewater byproducts should be managed to turn a costly waste into a valuable resource. Nevertheless, its safe application requires compliance to regulatory limits on contaminants, particularly heavy metals, which can become concentrated in BCxSS [30,31].

Analyzing the abovementioned pathways, as well as the SS—due to its nutrient content—BCxSS offers a way of reducing the need for synthetic fertilizers and the environmental costs associated with their production. Its porous structure enhances soil aeration, water retention, and cation exchange capacity, which improves nutrient availability to plants [32]. When used as a soil amendment, BCxSS has been shown to improve crop yields, enhance microbial activity, and buffer against soil activity [33]. Beyond soil applications, this matrix can function as an adsorbent material in water, stormwater filtration, and WW treatment systems. Its high specific surface area and functionalized surfaces—especially when chemically modified—enable the sorption of a wide spectrum of pollutants [34,35,36]. By capturing contaminants before they enter water bodies or are applied to soil, BCxSS contributes to closing water quality loops, reducing downstream treatment costs, and protecting ecosystems [37]. In this way, BC application exemplifies a dual strategy, waste valorization and pollution mitigation [38], that lies at the heart of advanced CE infrastructures. Lastly, this integration offers opportunities for energy recovery and operational synergy. Pyrolysis generates not only BC but also bio-oil and syngas, which can be utilized on-site for heat and electricity generation. This energy can offset plant energy demands, reducing reliance on fossil fuels and lowering greenhouse gas emissions [39]. Each plant has an energy footprint of 0.4–0.9 kWh/m³ of treated WW, depending on technology and scale [40]. Such energy-material recovery systems are a hallmark of advanced CE infrastructure, where outputs from one process serve as inputs for another.

Table 1. Advantages and disadvantages of using BCxSS.

Category	Advantages	Disadvantages
Environmental	- Reduces the volume of municipal sludge waste [18] - Carbon sequestration [29] - Soil health enhancement [30] - Lower CO2 emissions compared to other disposal methods [30] - Pathogens and hazardous compounds removed; metals stabilized [33]	- Risk of heavy metal or toxic compound (micropollutants, pathogens) release [41] - Requires careful monitoring before soil application [41]
Economic	- Produces adsorbent materials or soil amendment [26] - Potentially reduces waste management costs [26]	- Biochar production costs (drying, pyrolysis) [42] - Requires investment in equipment and technical know-how [43]
Social	- Supports circular use of resources [25] - Promotes sustainable practices [25]	- Community perception and acceptance - Health concerns - Lack of information and education [44]

Table 1. Advantages and disadvantages of using BCxSS.

Category	Advantages	Disadvantages
Environmental	- Reduces the volume of municipal sludge waste [18] - Carbon sequestration [29] - Soil health enhancement [30] - Lower CO2 emissions compared to other disposal methods [30] - Pathogens and hazardous compounds removed; metals stabilized [33]	- Risk of heavy metal or toxic compound (micropollutants, pathogens) release [41] - Requires careful monitoring before soil application [41]
Economic	- Produces adsorbent materials or soil amendment [26] - Potentially reduces waste management costs [26]	- Biochar production costs (drying, pyrolysis) [42] - Requires investment in equipment and technical know-how [43]
Social	- Supports circular use of resources [25] - Promotes sustainable practices [25]	- Community perception and acceptance - Health concerns - Lack of information and education [44]

4. Biochar Deriving from Sewage Sludge (BCxSS)

Biochar derived from sewage sludge (BCxSS) represents a promising pathway for the valorization of WW treatment residues; however, its composition is strongly influenced by the nature of the sludge feedstock and the conditions of pyrolysis. A recurring concern in the literature is the presence of both inorganic and organic pollutants, many of which are persistent, bioaccumulative, and potentially toxic. Overall, the literature highlights that the origins of pollutants in BCxSS are diverse, ranging from household activities to industrial and medical sectors and from diffuse urban runoff to agricultural practices, as summarized in

Table 2. Potential pollutants in sewage sludge biochar and their origins.

Heavy Metals	Domestic wastewater (plumbing corrosion, detergents, cosmetics), Industrial discharges (electroplating, tanning, smelting, electronics), Hospitals (incineration residues), Urban runoff (vehicle brake/tire wear, paints, road dust).	Cd, Pb, Cr, Cu, Ni, Zn, Hg, As	[41,45]
PAHs	Vehicle exhaust & fuel combustion (urban runoff), Industrial (coking, asphalt, petrochemicals), Domestic oily discharges formed during low-temp pyrolysis.	Naphthalene, Anthracene, Benzo[a]pyrene	[46,47]
PCDDs and PCDFs	Industrial (chlorinated chemical/pulp production), Waste incineration (plastics, solvents), Pyrolysis of chlorine-rich sludge.	TCDD (dioxin), TCDF (furan)	[48]
PCBs	Industrial discharges (transformers, lubricants), Urban runoff (old paints, sealants), Improper disposal of PCB equipment.	PCB-28, PCB-118, PCB-180.	[49,50]
VOCs	Domestic wastewater (solvents, paints, fuels, cleaning products), Hospitals (disinfectants, lab solvents), Industrial discharges (chemical/textile/paint), Urban runoff (fuel spills, leaking tanks).	Benzene, Toluene, Trichloroethylene.	[51]
PhACs and Hormones	Domestic wastewater (excretion of antibiotics, analgesics, contraceptives), Hospitals (cytostatics, antibiotics, radiocontrast agents), Agriculture (veterinary drugs, hormones).	Diclofenac, Ibuprofen, Trimethoprim, Sulfamethoxazole.	[52,53]
Pesticides and Herbicides	Agricultural runoff (pesticides washed into sewers), Urban green areas (parks, golf courses, gardens), Industrial manufacturing.	Atrazine, Glyphosate, DDT.	[54]

.

In order to overcome these barriers, a variety of techniques have been proposed. For removing heavy metals from SS, techniques such as electrokinetic remediation, supercritical fluid extraction, chemical treatments, plant-derived washing agents, ion-exchange processes, advanced oxidation methods and bioleaching have been used. Most of these approaches have been tested only at the laboratory scale, with the exception of electrokinetic remediation, which has also been demonstrated at the pilot scale [55].

Another process to address the challenge of heavy metals and toxic pollutants presence in SS is pyrolysis. Zhang et al. [56] who performed the Toxicity Characteristic Leaching Procedure (TCLP), demonstrated that pyrolysis significantly lowers heavy metal leaching. As temperature increases greater leaching reduction is observed, mainly due to the alkaline pH, which entraps the metals in the porous structure of the BC. The main mechanisms of metal immobilization during pyrolysis include the formation of new stable crystalline phases within the mineral components. These phases bind heavy metals through substitution or vacancy occupation. In addition, heavy metals can be incorporated onto the microporous surface of BC, which further reduces their mobility and toxicity. Increasing the pyrolysis temperature enhances these processes, as the proportion of micropores increases and the metals are covered or entrapped in the BC matrix. Soils enriched with BCxSS showed lower heavy metal availability and risk of leaching compared to the ones enriched only with SS [41,57] (metal leaching reduced up to 15 times) [58]. Pyrolysis further enhances sludge detoxification by effectively degrading persistent organic pollutants, including pharmaceuticals, endocrine-disrupting compounds, PCBs and PFAS. Their concentration at temperatures > 400–600 °C is decreased below the detection limit [59,60].

Co-pyrolysis and composting of SS can reduce the concentration of heavy metals, mainly due to dilution effects. During composting, the addition of bulking agents, such as sawdust, which typically contains much lower concentrations of heavy metals, results in a reduced overall metal concentration in the final compost [61]. Similarly, in co-pyrolysis, mixing the sludge with biomass that contains minimal heavy metal dilutes the metal-rich feedstock, resulting in lower concentrations in the produced BC [62].

The combined application of heavy metal removal techniques prior to SS pyrolysis enhances BC safety for application. Additionally, a comprehensive characterization and the development of a legislative framework is needed before reuse in soils, water treatment, or other purposes. Without such safeguards, the circular use of SS may inadvertently transfer environmental risks rather than mitigate them.

5. Pyrolysis Methods

The main techniques for BC production include pyrolysis (conventional and microwave-assisted [63,64], gasification, hydrothermal carbonization (HTC), laser ablation (LA), electron beam irradiation (EBR), and torrefaction [65]. In this study, emphasis is placed on pyrolysis, as it is the most widely applied and effective method for producing BC from sewage sludge. Several pyrolysis approaches have been developed. The yields and properties of the resulting products (bio-oil, BC, and syngas) vary considerably depending on operating conditions such as temperature, reactor type, and the presence of co-combustible materials. The main pyrolysis routes are slow pyrolysis, fast pyrolysis and flash pyrolysis, in which BC may serve as either the primary or a co-product. Regardless of the route, the process involves thermal decomposition of biomass under oxygen-free or oxygen-limited conditions, producing three fractions: (a) vapors that condense into bio-oil, (b) non-condensable gases, and (c) a solid residue, the char [66]. The routes are further explained below and illustrated in Figure 5.

Slow pyrolysis is a thermal process in which the feedstock is heated gradually at temperatures ranging from 350 to 600 °C, with low heating rates (5–7 °C/s) and a reaction time of more than 450 s. The main product is BC (35–45%), followed by bio-oil (25–35%) and syngas (20–30%) [66].

Fast pyrolysis is conducted at higher temperatures between 800 and 1300 °C, with heating rates of 100–200 °C/min and very short reaction times [66]. This method primarily targets the production of bio-oil, BC and biogas, with yields of 50–70% w/w, ~20% w/w, and 20% w/w, respectively [66,67].

Flash pyrolysis, a variant of fast pyrolysis, is performed at temperatures between 400 and 600 °C, with very low residence times (0.1–0.5 s) and high heating rate [67]. It focuses on bio-oil production, achieving yields of 60–80% bio-oil (w/w), 10–20% biogas (w/w) and 15–25% BC (w/w) [68].

Across these methods, typical pyrolysis conditions generally span 300–900 °C, with heating rates of 1–10 °C per minute and residence times of 2–6 h [16,69,70]. Additional approaches include conical spouted bed pyrolysis of SS at 500 °C [71].

5.1. Key Parameters in BC Characterization

• Yield

The yield of BC is influenced by several factors, including feedstock properties, environmental conditions, and pyrolysis temperature. Studies consistently show that higher temperatures reduce BC yields. Mbasabire et al. [72] observed that increasing the temperature from 300 °C to 500 °C reduced yields due to intensive thermal decomposition, volatilization, and enhanced charring. Similarly, Altikat et al. [73] reported a decline in yield between 400 °C and 600 °C, while Khanmohammadi et al. [74] found a decrease from 72.5% at 300 °C to 52.9% at 700 °C, accompanied by increased gas production. Overall, elevated pyrolysis temperatures accelerate the breakdown of organic matter, increasing vapor and gas release at the expense of solid residue.

ii.

Ash Content

Pyrolysis temperature strongly affects the ash content of BCxSS. Zhang et al. [75] recorded that BC produced at 900 °C had the highest ash content, primarily due to volatilization of organic matter and the accumulation of inorganic oxides such as Si, Al, and Fe. XRF analysis confirmed that concentrations of Si, Al, Ca and Fe increased with temperature, indicating the enrichment of inorganic constituents. Similarly, Wang et al. [76] observed that raising the pyrolysis temperature from 350 °C to 550 °C led to higher ash content in BCxSS. Consistent results were also obtained by de Souza et al. [77], further supporting the positive correlation between temperature and ash accumulation in BCxSS.

iii.

pH and Electric Conductivity (EC)

Pyrolysis temperature strongly influences the mineral composition, pH, and electrical conductivity (EC) of BC. Increasing temperature generally leads to higher concentrations of K, Na, Ca and Mg, resulting in higher pH and EC values [78,79]. This effect is attributed to the formation of oxygen-containing functional groups, and alkali/alkaline metal salts, which enhance alkalinity and conductivity. Hossain et al. [45] observed that EC increased progressively up to 500 °C but declined by half at higher temperatures. As pyrolysis temperature rises from 300 °C to 700 °C, the acidity of the sludge decreases and the resulting BC becomes increasingly alkaline, mainly due to the decomposition of organic acids and carbonates [80]. Xu et al. [81] and Figueiredo et al. [82] reported similar, observing that pH increased with temperature. The effect was attributed to the accumulation of alkaline elements, such as Ca and Mg oxides, and their associated alkaline reactions.

iv.

Surface and Porosity

According to Vali et al. [83] who performed Brunauer–Emmett–Teller (BET) surface area analysis, increasing the pyrolysis temperature, particularly in the range of 700 °C and 900 °C, significantly enhances the specific surface area (SSA) and pore volume of BC, primarily due to the formation of nanopores. In contrast, BC produced at 500 °C exhibited limited pore development and the lowest adsorption capacity, likely resulting from incomplete decomposition of organic matter, lower ash, inorganic content, and reduced aromatic structure formation. Similar results were reported by Aktar et al. [84] where BET analysis demonstrated an increase in the specific surface area of BC. Under an N₂ atmosphere, the area increased from 7.6 m²/g at 400 °C to 32.0 m²/g at 600 °C, while under CO₂ atmosphere the increase was more pronounced, ranging from 11.4 m²/g at 400 °C to 45.5 m²/g at 600 °C. This enhancement is attributed to the volatilization of organic components at higher temperatures, which leads to the development of additional mesopores and micropores. In both cases, increasing the pyrolysis temperature also led to a gradual reduction in pore size. Similarly, Ghorbani et al. [85] concluded that increasing pyrolysis temperatures significantly enhance the SSA and total pore volume of BC, owing to progressive devolatilization and structural reorganization of carbon matrices. In another study, Gopinath et al. [16] reported an increase in BET surface area from 69.7 m²/g at 500 °C to 89.2 m²/g at 700 °C, while Zhang et al. [86] observed a lower value of 12.7 m²/g at 600 °C. This discrepancy between the results is most likely due to variations in the feedstock characteristics of sewage sludge.

v.

Functional groups via Fourier transform infrared spectroscopy (FTIR)

FTIR spectroscopy is a widely used technique for identifying functional groups in the matrices under analysis, providing valuable insights into the degree of carbonization, aromaticity, and the presence of oxygen-containing moieties. The main characteristic absorption bands typically observed in sewage-sludge-derived biochar are summarized in

Table 3. Main FTIR absorption bands identified in BCxSS, corresponding functional group, and qualitative changes in peak intensity with increasing pyrolysis temperature.

Wavenumber/Band (cm−1)	Functional Group	Effect of Pyrolysis Temperature [8,68,87]
3600–3100	O–H, N–H (water, alcohols, carboxylic acids, amines)	peak often disappears ≥ 500 °C
3000–2825	Aliphatic C–H stretching	Τ ↑: gradual intensity decrease
1650 (1634–1653)	Adsorbed water deformation or amide-related vibrations	Τ ↑: weaker intensity
1644	C=O (amide or carbonyl), bound water	Τ ↑: intensity decrease
1622	C=O stretching of carboxylic groups	Τ ↑: peak absent
1545	Nitro-aromatic compounds	Τ ↑: peak often disappears
1440	Carboxyl groups	Τ ↑: intensity decrease
1396	Methyl (CH3) groups	Τ ↑: gradual intensity decrease
1030–1021	C–O stretching (alcohols)	Τ ↑: weaker intensity
1028	Si–O–Si stretch	Τ ↑: relatively stable
<1000	–CHO, C=O, C–N–C (various oxygen-/nitrogen-containing groups)	more pronounced at lower temperature

Τ ↑: Increase Pyrolysis Temperature.

, together with their associated functional groups and the corresponding changes in peak intensity as the pyrolysis temperature increases.

It should be noted that the assignment of FTIR peaks to specific functional groups may vary depending on the initial SS composition and pyrolysis conditions.

vi.

Total Carbon and Total Organic Carbon

BC contains a high proportion of stable carbon, making pyrolysis an effective method for carbon sequestration. The total carbon content, which is the sum of organic and inorganic carbon, is closely linked to vital properties such as stability, aromaticity, and nitrogen immobilization, deriving from the ratio of O:C, H:C and C:N, accordingly. Total carbon content is influenced by both the feedstock type and the pyrolysis temperature; generally, higher temperatures enhance stability but decrease the total organic carbon (TOC). Notably, the overall carbon content in BC is typically lower than that of the raw SS and decreases further with rising the pyrolysis temperature [88]. However, the addition of organic materials to the feedstock prior to pyrolysis has been reported to improve the TOC content of the resulting BC [89].

vii.

Elemental Ratios (H:C, O:C, N:C)

The molar ratios H/C, O/C, and N/C decline as BC is produced at higher temperatures, indicating increased aromatization and enhanced structural stability [47]. The C:N ratio is an important indicator of the ability of BC, when applied as a soil conditioner, to either immobilize or release nitrogen. The H:C ratio is commonly used to evaluate aromaticity and stability. The International Biochar Initiative [90] sets 0.7 as the upper limit, while values above this indicate incomplete pyrolysis or the presence of non-pyrolytic material. Similarly, the O:C ratio serves as a measure of stability in soils, with the European Biochar Certificate [91] defining an upper limit of O:TOC ratio < 0.4. A reduction in the O:C ratio with increasing temperature indicates enhanced BC stability and resistance to microbial degradation. As for nutrients, the two most important elemental nutrients in BC are P, usually present as phosphate ions, and N, found mainly as ammonium and nitrate. Nitrogen content is strongly influenced by sludge treatment, with aerobic digestion maintaining higher levels. Nonetheless, the N content generally decreases as pyrolysis temperature increases [88,92,93].

5.2. BCxSS as Adsorbent Material

BC effectively removes a wide range of contaminants from water, including heavy metals, organic pollutants, and other substances, through different adsorption mechanisms (Figure 6). Furthermore, chemical and physical modifications of BC have been shown to significantly enhance its adsorption capacity, thereby improving the removal of both organic and inorganic contaminants [94,95].

• Organic micropollutants

Organic micropollutants represent one of the most critical categories of emerging contaminants due to their resistance to conventional water and WW treatment processes. Even when partially degraded, they may be transformed into intermediates that pose risks to both the environment and human health. Among the most prominent are endocrine-disrupting compounds and pharmaceutical and personal care products, a broad group that includes antibiotics, hypnotics, steroids, cosmetics and fungicides [96].

Several studies have highlighted the efficiency of BCxSS in removing such organic micropollutants. Regkouzas et al. [97] produced BC from sewage sludge at three different temperatures and employed it as a sorbent for the removal of organic micropollutants from both drinking water and treated wastewater. The adsorption efficiency ranged between 67 and 99% in table water and 35–97% in treated WW, with the lower performance attributed to the presence of organic and inorganic constituents competing for available adsorption sites. In addition, adsorption in the actual WW required significantly longer contact times (i.e., up to 70 h), compared to the controlled laboratory conditions (i.e., 12–24 h). The study also underscored the influence of solution pH, noting that sorption capacity decreased under neutral pH conditions.

Other studies have demonstrated further enhancement of adsorption through modification of BCxSS. For example, Fan et al. [98] focused on the adsorption of three common antibiotics—tetracycline (TC), sulfamethoxazole (SMX) and amoxicillin (AMC)—using BCxSS modified with Fe, Mn and Al ions. The Fe-amended BC exhibited the highest sorption capacities, reaching 123.35, 99.01 and 109.89 mg/g, for TC, SMX and AMC, respectively. The dominant adsorption mechanisms were identified as pore filling, Van der Waals forces, and hydrogen bonds. Consistently, Gao et al. [99] reported high TC removal efficiencies, with BCxSS activated using KOH achieving an adsorption rate of 86.35%, further confirming the potential of modified BC as cost-effective and sustainable adsorbents for antibiotic removal.

Beyond SS, BC produced from other industrial sludge sources has also been shown to remove organic micropollutants. Kalderis et al. [100] investigated the adsorption of 2,4-dichlorophenol (2,4-DCP), a common byproduct detected in municipal WWTPs effluents, using BC derived from pulp mill sludge and wheat husks as adsorbents. The Response Surface Methodology revealed that pH is the most decisive factor influencing the adsorption efficiency. The findings revealed that pH-dependent electrostatic interactions, along with non-covalent π-donor-electron acceptor mechanisms, were the predominant mechanisms governing the sorption of 2,4-DCP onto the tested BC. Ferreira et al. [101] investigated the sorption of three anesthetics widely applied in intensive aquaculture to control fish stress—tricaine methanesulfonate, benzocaine, and 2-phenoxyethanol—by BC derived from paper mill sludge, alongside commercial activated carbon for comparison. Although activated carbon exhibited higher adsorption capacities (approximately four to eightfold greater), the authors emphasized that sludge-derived BC represents a promising, cost-effective, and environmentally sustainable adsorbent, particularly given its potential to valorize large quantities of industrial sludge, contributing to pollution mitigation.

Table 4. Sludge-derived biochars used as adsorbent materials for the removal of emerging pollutants.

Type of Sludge Biochar	Pyrolysis Conditions	Chemical Modification	Target Pollutants	Modified Biochar	Key Mechanisms	Adsorption Capacity (mg/g)	Ref.
Sewage Sludge Biochar/Leaf	mass ratio 1:3, 200 °C, 1 h	HCl 9%	Diclofenac	Pore widening: 6.7 to 11.7 nm, SSA: 3.30 to 4.17 m2/g	chemical adsorption	877	[102]
Sewage Sludge Biochar/Bamboo waste	mass ratio 4:1, 700 °C, 30 min, 10 °C/min,	-	Ciprofloxacin	-	π–π interaction, H-bond, ion exchange, Fe-complexation	62.48	[103]
Sewage Sludge Biochar/Lignin	mass ratio 50:50, 800 °C	-	Methylene Blue	-	H-bond, electrostatic interactions	38.10	[104]
Pharmaceutical Sludge Biochar	800 °C, 90 min	KOH, ZnCl2 and CO2	Levofloxacin	SSA: 9.46 to 534.91 m2/g total pore volume: 0.014 to 0.335 cm3/g	π–π interaction, H-bond, surface complexation, pore filling, electrostatic interaction	59.26 (ZnCl2 modification)	[105]
	600 °C	HCl 1 mol/L	Tetracycline	SSA: 214.97 to 319.80 m2/g mesopore volume increased	physical and chemical adsorption	157.38	[106]
Sewage Sludge Biochar	400–900 °C, 1 h, 5 °C/min	-	Sulfadiazine	-	chemical adsorption	1.79	[107]
	500 °C, 2 h, 10 °C/min	Fe/Zn + H3PO4	Ciprofloxacin, Norfloxacin, Ofloxacin	SSA: 14.0 to 39.1 m2/g	pore filling, H-bond, π–π interaction, electrostatic interaction, functional groups complexation	83.7, 39.3, 25.4	[108]
	800 °C, 2 h, 5 °C/min	NaOH, HNO3	Tetracycline	SSA: 67.387 to 202.519 m2/g, increased oxygen-containing functional groups	π–π stacking interactions, pore-filling	286.913	[109]
	600 °C, 20 min	-	Malachite green and Crystal violet	-	H-bond, π–π interaction, electrostatic interactions, ion exchanges	69.5, 49	[110]
Industrial Sludge Biochar	750 °C, 2 h, 5 °C/min	ΚOH	Cationic dye from aqueous solution	SSA: 2 to 157 m2/g, total pore volume: 0.019 to 0.119 to cm3/g	π–π interactions and complexation with hydroxyl functional groups	65.9	[111]
Tannery Sludge Biochar	550 °C	Μelamine and KOH/Fe3O4 nanoparticles	Cationic blue, Reactive red, Direct yellow and Acid blue	SSA: 21.48 to 47.67 m2/g	Chemical adsorption	5.13, 84.10, 154.80, 120.92 (Fe3O4 nanoparticles modification)	[112]
Dyeing Sludge	400 °C	-	Ofloxacin	-	Electron donor–acceptor interactions, π–π interactions, and H-bond	21.6	[113]

-: Any Modification.

summarizes recent studies on BCxSS used as adsorbent materials for the removal of emerging pollutants. Feedstock origin, pyrolysis temperature, residence time and heating rate, subsequent modification techniques, and target pollutant removal, are presented to emphasize the versatility of BCxSS.

ii.

Dyes

Wastes from textile industries represent a major source of pollution for both surface and groundwater [112]. Removal of dyes from WW is particularly challenging due to their complex molecular structures, which make them resistant to degradation and conventional treatment processes [16]. The adsorption mechanism varies depending on the type and nature of dye molecules and may involve physical adsorption (H-bonds), ion exchange, electrostatic interactions, catalytic degradation, and surface complexation [65].

Ravindiran et al. [114] evaluated the sorption behavior of the azo dyes Acidic Blue 210 (AB210) and Acidic Blue 7 (AB7) using BCxSS, achieving removal efficiencies of 99.32% for AB210 and 94.28% for AB7. Al-Mahbashi et al. [115] investigated the use of BC from SS produced at 700 °C for 60 min in continuous flow experiments using a fixed-bed column under varying flow rates to remove toxic dyes. The maximum adsorption capacity was 42.3 mg/g. Sun et al. [116] explored the removal of Reactive Black 5 by using BCxSS activated with persulfate (PDS). A removal efficiency of 99.52% was achieved for 100 mg/L Reactive Black 5 wastewater after 180 min of reaction.

In addition to dyes and pigments, textile industries also produce sludge, commonly referred to as dyeing sludge. Qian et al. [117] studied the use of BC derived from dyeing sludge and modified with ΖnCl₂ for the adsorption of Malachite Green. The modified BC exhibited a maximum adsorption capacity of 224.0962 mg/g and a removal efficiency of 99.3% under the optimal conditions. Even after five cycles of reuse, the removal efficiency remained around 45%. In the last five years, more than 2861 studies related to BCxSS and dye adsorption have been published (Scopus search, keywords: “sewage sludge biochar” AND “dye adsorption”, accessed October 2025). This finding highlights the growing research interest and the pressing need to address wastewater treatment challenges in the textile industry. The use of BCxSS appears to be a viable model for waste reuse, both for the removal of pigments and for resource recovery.

iii.

Heavy metals

The presence of heavy metals such as Cd (II) and Pb (II) in wastewater constitutes a serious environmental risk [118]. The removal efficiency of BC depends on the pH, metal type, surface chemistry, and functional groups of the material. The main mechanisms include physical sorption (Van der Waals, H-bonds) and chemical adsorption which includes (a) ion exchange, where dissolved metals replace cations and protons on the surface of BC depending on the functional groups of both the metal and the BC; (b) surface complexation, where oxygen-containing functional groups such as carbonyl (-C=O), carboxyl (-COOH), and hydroxyl (-OH) effectively remove heavy metals such as Cr (VI), Cd (II), Ni (II), Hg (II), Pb (II), and nitrates; and (c) precipitation, where the metals are precipitated and deposited in the BC [119].

Chemically modified BC demonstrates enhanced efficiency due to its increased surface area and the enrichment of diverse functional groups, highlighting its potential for environmental remediation of contaminated water and soil. Specifically, acidic treatments remove mineral components and soluble impurities, thereby creating additional micropores, while alkaline treatments can partially dissolve organic and inorganic matter, enlarging existing pores and generating new porosity [120]. Khalil et al. [121] investigated the use of BCxSS and H₃PO₄-modified BCxSS in water polluted with Pb. The addition of H₃PO₄ increased the BET surface area and enhanced the biochar’s ability to adsorb Pb (87.36%). Similarly, Zhang et al. [122] studied the adsorption of Pb²⁺ using BCxSS activated with KOH, which improved the surface area and increased sorption capacity, reaching 57.48 mg/g. For Cd²⁺ removal, Yin et al. [123] used BCxSS modified with melamine and reached maximum sorption of 126.75 mg/g. Moreover, Zeng et al. [124] studied the adsorption of Cu²⁺ and Cd²⁺ by co-pyrolyzing SS and phosphorus tailings at a ratio of 0.4:1. These studies, along with many others published over the last decade [125,126,127,128], highlight sewage sludge as a potential feedstock for producing effective adsorbents for heavy metal removal, with the additional benefit of waste sludge reduction.

iv.

Phenolic Compounds

Phenolic compounds (PC) constitute one of the most common organic pollutants in WW because of their use in industry. PC adsorption depends on the properties of the BC (surface area, pore volume, and oxygen-containing functional groups), on the properties of PC (molecular size, pKa, functional groups, and polarity), and on the environmental conditions (pH, temperature, ionic composition, and concentration). The major adsorptive mechanisms include donor–acceptor complexes where carbonyl groups (C=O) on BC act as electron donors, while aromatic rings of PC act as acceptors, and π–π dispersive interactions, where the adsorption occurs via π–π interactions between the aromatic rings of PC and the graphene layers of BC. H-bonds, ion exchange, covalent bonding, and Van der Waals also contribute to adsorption [129]. Sierra et al. [130] used both BCxSS and BCxSS chemically (NaOH or K₂CO₃) and physically (CO₂) activated for the adsorption of phenol and methylene blue. The activation led to lower activation temperatures and an increase in the adsorption of phenol up to 152%, especially on the chemically activated BC at 600 °C. The phenol’s adsorption is due to the generation of new functionalities containing nitrogen and/or oxygen and the adsorption of phenol by surface polymerization. Chemical activation impacts magnetic properties of BC and further enhances its capacity for heavy metal removal. Zhao et al. [131] showed that despite the adsorption on the BCxSS the PC can also regenerate and recycle. More specifically, they used BCxSS pyrolyzed at 900 °C, activated with KHCO₃, achieving a maximum sorption capacity of 305.83 mg/g.

v.

Microbes and antibiotic-resistant genes

Nowadays, agriculture and livestock farming have a significant environmental footprint, as they generate large amounts of waste, which include pathogenic microorganisms, many of which end up polluting aquatic ecosystems. This has significant implications for human and ecosystem health, as well as environmental security [131]. Gastrointestinal pathogens present in contaminated water are linked to bacterial, viral, protozoan and other parasitic diseases in humans, which can cause serious illness or even death, while also adversely impacting agricultural production. More than 100 human intestinal viruses are known to cause diseases such as gastroenteritis and hepatitis, with noroviruses, rotaviruses and hepatitis A viruses being the most prevalent [132]. BC has been widely studied for its beneficial effects on soils and plants, including the enhancement of plant resistance to diseases [133,134,135]. Recently, Silva et al. [136] demonstrated that the combined application of BCxSS and Trichoderma afroharzianum significantly inhibited Agroathelia rolfsii both in vitro and under greenhouse conditions.

The removal of pathogens from WW through adsorption processes has been extensively investigated using various sorbent materials, such as sand [137,138], clay [139], and activated carbon [140]. In recent years, investigations into pathogen adsorption or inactivation by BC of plant origin have emerged [141], although research examining the use of BCxSS remains limited. Chen et al. [142] investigated the removal of Escherichia coli and bacteriophage MS2 using columns that contained iron filings, calcined magnesite, natural ore limestone or corn stalk BC. BC showed a removal capacity of 62.59% for E. coli and 69.82% for MS2. Zhou et al. [143] evaluated BC and ZVI-amended sand filter columns, including BC, Ag-BC, and ZVI individually, as well as their combinations (BC + ZVI and Ag-BC + ZVI) and reported >90% removal of E. coli and contaminants of emerging concern from wastewater effluent. Abit et al. [144] compared how BC produced from two feedstocks and pyrolyzed at two different temperatures affects the transport of E. coli through a water-saturated and partially saturated fine sand at different rates. A more pronounced effect of BC application on E. coli transport was recorded in the unsaturated soils while differences were also observed between the E. coli isolates, indicating that bacterial surface properties play a role in how BC affects E. coli transport [144]. On the other hand, Graham et al. [145] investigated fecal pollution and human viruses in urban stormwater in the San Francisco Bay Area and evaluated the effectiveness of BC-amended biofilters and vegetated biofilters in viruses’ removal. BC-amended biofilters remove human viruses (adenovirus, enterovirus) and viral indicators (MS2) from urban runoff with low to moderate efficiency. Similar results are found in the study of Chen et al. [146], in which they observed lower removal capacity of viruses in soil for BC. Despite the lack of studies directly addressing pathogenic microorganisms in BCxSS, the aforementioned studies indicate that BCxSS exhibits strong adsorption capacity for organic and inorganic pollutants, suggesting that the surface and porous properties of the material are also promising for microbial removal applications—a fact that requires targeted experimental studies to evaluate the adsorption/inactivation of pathogens by BCxSS.

Pathogen removal by BCxSS is expected to result from the combined effects of physical entrapment, electrostatic attraction, chemical adsorption, and physicochemical inactivation [33,147,148]. Higher SSA and pore volume have been associated with enhanced physical retention of bacteria but studies on other types of microbes (i.e., viruses and protozoa) are lacking [147]. Pore tuning may indeed support an effective design strategy to optimize BCxSS for pathogen removal applications [149,150]. Surface charge also plays a central role in controlling interactions between BC and microorganisms. BCs with positively charged surfaces or enhanced ion-exchange capacity tend to exhibit higher affinity toward microbial cells and viral particles. Interactions of BC surface functional groups with biological entities are particularly relevant for microbial inactivation, as strong binding may induce structural damage to bacterial membranes or viral capsids.

However, limitations also have to be mentioned. Large macropores may provide protective niches for microorganisms, while changes in pH or ionic strength can lead to desorption or re-release of retained pathogens. Thus, the importance of carefully controlling BC physicochemical properties when designing pathogen removal systems should be highlighted. According to Shahraki & Mao [147], BC amendment to on-site wastewater treatment systems may improve pathogen removal performance. Recently, Fadi et al. [148] showed that production conditions alone can modulate the ability of biochar to sequester bacteria in a species-dependent manner, and Active Pharmaceutical Ingredients (APIs) as a function of physicochemical properties [148].

Wu et al. [151] studied the effects of BCxSS and commercial BC on the reduction in intracellular and extracellular sulfonamide antibiotic resistance genes (ARGs) during sludge composting. They showed that the “waste-to-waste” strategy to convert sludge into BC for composting achieves comparable ARGs removal efficiency to commercial BC. Although BCxSS may increase the abundance of certain ARGs-hosting bacteria, it was found to effectively limit ARGs dissemination by regulating community assembly and physicochemical interactions [151]. Selected ARB genes and mobile genetic elements present on the free-floating extracellular DNA fraction and on the total environmental DNA were removed at 85% and 97% by sewage-sludge BC, in the study of Calderón-Franco et al. [152] Interestingly, Huang et al. [153], developed an advanced oxidation process for simultaneously removing ARGs and antibiotic-resistant bacteria (ARB) by two types of iron and nitrogen-doped BC derived from rice straw (FeN-RBC) and sludge (FeN-SBC) [153]. Thus, BCxSS seems to be a promising strategy to deal with a potential public health threat, i.e., ARGs and ARB, in both water and sludge.

5.3. Biochar as a Catalyst Precursor

The development of BCxSS-based catalysts represents a recent and promising application of BC in wastewater treatment, providing a sustainable pathway for pollutant degradation and sludge volume reduction. Given the inherent complexity of SS, BCxSS exhibits a heterogeneous composition rich in both organic and inorganic moieties. This mineral-rich carbonaceous matrix typically contains Si, Al, iron species (Fe₃O₄, Fe₂O₃, Fe⁰, and Fe₃C) originating from the feedstock, along with metallic phase structures dispersed on its surface formed during the pyrolysis and carbonization of SS [36]. Such a composition, together with a large SSA and an intricate pore network, offers an ideal medium for the dispersion and deposition of catalytically active particles [154]. In addition, its remarkable electrical conductivity reduces electron-hole recombination during the photocatalytic process, thereby enhancing the oxidation efficiency toward target compounds [155]. These unique features make BCxSS an emerging platform for catalytic and photocatalytic applications in wastewater treatment [156].

Ji et al. [157] described the catalytic capacity of BCxSS, emphasizing their structural features and potential catalytic pathways. Specifically, BCxSS can act as both an electron donor and acceptor due to the presence of graphitized and (semi-)quinone structures. Moreover, they can catalyze the degradation of organic pollutants through both free-radical and non-free-radical pathways, driven by oxygen-containing functional groups, redox-active metals that activate oxidants, heteroatoms that enhance oxidant affinity, and carbon/oxygen defects that facilitate electron transfer. The non-free radical mechanism primarily involves surface activation, electron transfer and the generation of singlet 1O₂ [158].

To improve the catalytic and photocatalytic efficiency of BCxSS, recent studies have employed various modification strategies. A common strategy involves doping with inorganic transition metals (Fe, Mn, Co, Cu, etc.), which enhances the generation of reactive oxygen species and persistent free radicals, especially when the original feedstock lacks sufficient intrinsic active sites. These transition metals act as active centers, facilitating electron transfer and accelerating oxidation reactions. Among these metals, Fe impregnation not only renders BCxSS magnetic properties, facilitating easy separation, but also enables its application in Fenton-like and persulfate-based advanced oxidation processes [159]. For instance, Bao et al. [160] reported that Fe-loaded BCxSS in a Fenton process achieved 90.7% degradation of tetracycline. Similarly, Shi et al. [161] reported that BCxSS-Co₃O₄ achieved 93.2 % tetracycline removal due to the enhanced activation of peroxymonosulfate from the well-dispersed Co₃O₄ encapsulated in the BC. Moreover, metal loading improves catalyst stability and allows for multiple reuse cycles while minimizing metal leaching into solution [159,161]. In addition to transition metals, heteroatom doping (e.g., N, S) offers further opportunities to enhance catalytic activity, with nitrogen being particularly effective due to the activity of pyridinic and graphitic N species [159].

Beyond metalation, incorporating semiconductor materials such as TiO₂, ZnO, and SiO₂ onto the BCxSS surface improves light absorption and suppresses electron–hole recombination, thereby making these composites highly efficient in photocatalytic pollutant degradation and hydrogen evolution reactions. TiO₂, in particular, has notable advantages: it is chemically stable, exhibits high photocatalytic activity and strong oxidative potential, is non-toxic, and has low cost [155,162]. A TiO₂/Fe/Fe₃C-BCxSS composite was synthesized by Mian and Liu [163] for methylene blue degradation. The heterogeneous catalyst, via photoreaction and H₂O₂ activation, exhibited a removal capacity of 376.9 mg L⁻¹. Additionally, the nanotexture of BCxSS was used to construct a BC/TiO₂ composite with metallic elements, which exhibited efficient photocatalytic hydrogen production via water splitting [164]. Furthermore, a novel Fe–Mn bimetallic oxide supported on SiO₂-composited BCxSS demonstrated excellent stability, low metal leaching, and high tetracycline degradation. Its superior performance was attributed to enhanced electron transfer and persulfate activation, driven by metal redox reactions and silica doping [165].

Despite the promising catalytic and photocatalytic performance of BCxSS, several limitations should be acknowledged. The heterogeneous composition of sewage sludge leads to variability in the resulting BC properties, which can affect reproducibility and catalytic efficiency. Scale-up of BCxSS-based catalysts for industrial wastewater treatment remains challenging due to processing complexities and potential leaching of heavy metals [165]. Nevertheless, the performance of these systems is generally influenced by factors such as solution pH, catalyst dosage, reaction temperature, and the presence of coexisting anions [159]. In photocatalytic applications, the intrinsic structure of BCxSS may limit light absorption and charge separation efficiency, necessitating additional modification steps to achieve optimal performance. Moreover, long-term stability under operational conditions and the potential environmental impacts of residual metals or radicals must be carefully evaluated.

6. Regeneration of BCxSS

Τhe regeneration of BC, particularly when derived from SS, plays a crucial role in enhancing its sustainability and cost-effectiveness as an adsorbent for environmental applications. A competitive adsorbent should exhibit high reusability and recyclability for industrial applications, thereby significantly reducing the cost of BC sorbents through repeated sorption–desorption cycles. Regeneration can be achieved either through desorption or decomposition of BC, as shown in Figure 7, depending on the characteristics of the adsorbent and the nature of the adsorbed pollutants.

Decomposition-based regeneration methods include chemical oxidation using strong oxidants such as ozone or hydrogen peroxide, as well as advanced oxidation processes (AOPs) involving ultraviolet irradation (UV), Fenton reactions, or reactive oxygen species, which enhance the contaminant degradation. Electrochemical regeneration achieves selective decomposition by applying an electric field, while ultrasound treatment induces cavitation and generates radicals that oxidize the adsorbed pollutants. In some cases, biodegradation is applied, where microorganisms convert pollutants into simpler and less toxic products [166].

On the other hand, desorption-based methods involve thermal or no-thermal approaches. Thermal regeneration, one of the most effective techniques, promotes further carbonization and pollutant decomposition at elevated temperatures, although it may reduce the original pore size of BC. Non-thermal desorption includes the use of surfactants, supercritical fluids, or chemical solvents and reagents in order to remove both organic and inorganic pollutants adsorbed onto the BC surface [167].

Several studies demonstrate the efficacy of BC regeneration. Shah et al. [168] studied the adsorption capacity of BCxSS on heavy metals and its regeneration by using 1M HCl, with solution pH adjustments, across cycles to enhance performance. Although removal efficiency declined after the first cycle, optimal results were restored at neutral pH (Cd²⁺: 99.5%, Zn²⁺: 84.5%), demonstrating that BC can be effectively regenerated and reused under favorable pH conditions. Similarly, Madhau et al. [169] examined the reuse of BCxSS through re-pyrolysis following tetracycline adsorption. After five regeneration cycles, all samples exhibited only a slight decrease in removal capacity, with a maximum performance loss of approximately 10%. BCxSS produced at 900 °C maintained over 90% efficiency, attributed to its superior thermal stability. Overall, efficient regeneration of BC not only reduces operational costs but also enhances sustainability, positioning BCxSS as a promising candidate for large-scale environmental remediation.

The selection of an appropriate regeneration method depends on various factors, including pollutant type and BC properties. While thermal regeneration is generally effective in restoring adsorption capacity, it may reduce pore size; chemical methods offer selectivity but can generate secondary waste. Factors such as pH, contact time, pollutant characteristics, and BC properties strongly influence regeneration efficiency [170].

7. Cost Analysis of BCxSS

The economic viability of BCxSS is a critical factor for its large-scale application and market acceptance. Literature-based cost analyses indicate that the feedstock cost for BCxSS production is relatively low, ranging from USD 6.71 to 110 per ton. However, the total production cost, including capital investment, energy consumption, labor, storage, and operational expenses, varies considerably between USD 51 and 5668 per ton depending on process configuration and scale [42]. By comparison, conventional disposal of sewage sludge, involving transportation, treatment, and landfilling, can amount to approximately 195,000 USD per year, representing a substantial economic burden. Although production costs exceed feedstock costs, the estimated market price of BCxSS is around 246 USD per ton, which is nearly six times lower than the reported cost of commercial activated carbon (~1500 USD per ton). Moreover, BCxSS production offers significant indirect economic benefits by reducing sludge disposal costs and mitigating environmental impacts, thereby enhancing its attractiveness within a circular-economy framework [93,171].

8. Conclusions

Circular economy promotes the continuous use and regeneration of resources, aiming to minimizing waste. Ιn this context, BCxSS provides a tangible example of CE in practice, as wastewater treatment byproducts are transformed into a valuable resource that reduces waste, recovers nutrients, and mitigates pollution. The transformation of SS into BC through pyrolysis represents a paradigm shift in how WW byproducts are managed, turning a costly waste into a valuable resource, and it is also an effective and environmentally important solution, given the enormous quantities of sludge produced annually. This process leads to the creation of a new carbon-rich and highly porous material, which can be used both as an adsorbent and as a catalyst. The pyrolysis temperature appears to play a crucial role in the physicochemical properties of the produced BC. Currently, there is no standardized protocol for the production of BCxSS, resulting in considerable variation in the properties of the produced materials. There is therefore a need to develop a standardized production and characterization framework to enable reliable comparison and evaluation of their properties.

As demonstrated by the studies reviewed, BCxSS has proven particularly effective in environmental applications, serving as an efficient sorbent for organic micropollutants, pathogens, dyes, and heavy metals. Its modification with acids or bases further enhances this capacity. The synergistic interplay of multiple mechanisms—shaped by sludge origin, pyrolysis conditions, surface modifications, and chemical composition—underpins the high efficiency of BCxSS in contaminant removal, thereby establishing its key role in pollution mitigation and water purification. Similarly, several studies have highlighted the potential of BC for the sorption and inactivation of microbial indicators and pathogens, as well as its role in reducing ARGs and ARB across diverse environmental matrices. Nevertheless, to fully understand and safely assess the potential of BC for large-scale applications, further research remains essential.

BC stands out as a promising catalyst precursor due to its high surface area, abundant active sites, and ability to facilitate diverse catalytic reactions. BC-derived catalysts are applied across biorefineries, waste management, renewable energy, and environmental remediation. Moreover, BC offers distinct advantages over conventional catalysts, being low-cost, sustainable, and highly tunable through pyrolysis or chemical modification. However, while BCxSS presents clear circular-economy benefits, several challenges must be addressed before its large-scale adoption. These include the contaminant risk management by pollutants, which may be persistent and should be monitored to ensure environmental safety, standardization of guidelines for characterization and production conditions, and economic viability. The latest is a very important parameter towards the argument: “operational costs of pyrolysis units are higher than the value of recovered products and avoided disposal costs”. Social acceptance is equally important, and public awareness about the use of wastewater-derived products in water treatment must be raised. Addressing these issues through research and within a well-regulated and economically viable framework, BCxSS can serve as a cornerstone technology in the transition to a CE, fostering sustainable resource management at the intersection of urban sanitation, agriculture, and environmental protection.