Sewage sludge is a slurry residue produced from wastewater treatment operations. Typically, sewage sludge comprises approximately 3% solids and 97% liquids (Kumar et al., 2017). On the other hand, biosolids are treated sewage sludge that adheres to specific standards for its utilization or disposal, with an approximate solid content of nearly 90% (USEPA, 2021). Different methods, from composting, acidic oxidation/disinfection, thermal drying, alkaline stabilization, and aerobic and anaerobic digestion, are used for biosolids treatment. These treatments distinguish the physicochemical properties of biosolids from those of sewage sludge (Mohajerani et al., 2017).

The concept of resource recovery from wastes has provided an alternative use for biosolids, sewage sludge and composts because these wastes are rich in organic matter and essential plant nutrients, such as phosphorus, nitrogen, potassium, magnesium, calcium, and sulphur (Dad et al., 2019). The total nitrogen, phosphorus, and potassium concentrations in biosolids can be as high as 3%, 1.5%, and 0.7%, respectively, while the organic content can be from 30% to 40% of organic matter (Kumar et al., 2017). However, the methods used to stabilize sewage sludge greatly influence the contents of organic matter and nutrients and the fertilizer value of the resulting biosolids (Dad et al., 2019). Owing to the chemical composition inherent in these biowaste materials, they find application as fertilizers in agricultural practices, land reclamation and reforestation efforts. This utilization is a viable alternative source of essential macronutrients for soil enrichment (Rigby et al., 2016; Dad et al., 2019). Studies have also shown that biosolids-based soil amendments enhance plant accessibility of nitrates, ammonium ions, and nitrites in the soil as a consequence of the mineralization of organic nitrogen (Sharma et al., 2017; Wang et al., 2017).

Applying sewage sludge/biosolids and composts to agricultural lands has become an increasingly essential aspect of sustainable nutrient management as it contributes positively to nutrient recycling, soil fertility and properties. There are clear circular economy arguments for closing the nutrient and carbon loops so that nutrients extracted from the soils to produce our food are returned to the soils to maintain their fertility and quality. The use of nutrients from biosolids also replaces chemical fertilizers that would otherwise need to be produced, mined and transported with all of the associated environmental impacts.

Although the land application of biosolids is advancing in popularity and is beneficial in terms of improving soil health and resource recovery, there are recent apprehensions concerning environmental and human health hazards associated with their application to agricultural land. Several organic and inorganic pollutants have been found in biosolids that can potentially be transferred from the applied biosolids to crops (Clarke and Cummins, 2015; Gworek et al., 2021; Hušek et al., 2022; Kumar et al., 2022; Thompson et al., 2022; Adu et al., 2023; Marchuk et al., 2023). Recent studies have called into question the agricultural use of biosolids due to the documented reciprocal interaction between pollutants such as trace metals, per- and polyfluoroalkyl substances (PFAS), pharmaceuticals, hormones, polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), pesticides and plasticizers, polycyclic aromatic hydrocarbons (PAHs), flame suppressants, alkyl phenols, as well as other contaminants that could be potentially released in environment (Rathankumar et al., 2020). Many of those contaminants pose a significant risk because they can produce direct biological effects such as genotoxicity, endocrine disruption, neurotoxicity, immunotoxicity or reproductive defects in living organisms (Mohajerani et al., 2017; Rathankumar et al., 2020). Irrespective of how sewage sludge/biosolids are treated, organic and inorganic contaminants are found in biosolids; however, substantial treatment processes will usually significantly abate the volatile components and pathogens (Mohajerani et al., 2017).

PFAS are a class of synthetic aliphatic compounds characterized by a completely fluorinated carbon chain (per-) or partially fluorinated carbon chain (poly-) making up the hydrophobic tail of the compound. This hydrophobic tail is usually connected to at least one hydrophilic head that constitutes the functional group of the compound. PFAS have unique chemical features due to their extremely polar and strong carbon-fluorine linkages, such as excellent thermal and chemical stability and exceptional surface-tension-lowering capabilities (Xiao, 2017). Because of their hydrophobic and hydrophilic characteristics, PFAS have been extensively utilized globally in a variety of applications like firefighting foams, surfactants, industrial emulsifiers, non-stick cooking utensils, paints, stain and water-resistant textiles and carpets, oil-repelling containers, etc. The C-F bond found in PFAS compounds is known as the strongest bond in organic chemistry. This bond confers strong resistance to usual ecological transformation processes such as microbial, heat or chemical degradation (Meegoda et al., 2020; Nguyen et al., 2020; Wackett, 2021; Berhanu et al., 2023).

According to studies dating back nearly 2 decades, the probable route through which PFAS enter sewage sludge is mainly from industrial or residential wastewaters that flow into the wastewater treatment plants (Yu et al., 2009; Ma and Shih, 2010; Coggan et al., 2019; Semerád et al., 2020). PFAS in residential wastewaters results from the use of PFAS-containing products, such as personal care products, food packaging, carpets, and clothes (Schultes et al., 2018; Seltenrich, 2020; Zhu et al., 2021; Kumar et al., 2022; Rodgers et al., 2022). Additionally, extensive discharge of landfill leachate into municipal sewage systems has also been observed to increase PFAS concentrations in WWTPs (Masoner et al., 2020).

In a study conducted in the United States, PFAS concentrations in landfill leachate, influent and effluent samples collected from three landfill WWTPs were contrasted with similar samples from two WWTPs that received no landfill leachate (Thompson et al., 2022). The study concluded that the detection rate for all 73 PFAS analyzed in leachate (92%) was higher than in influent (55%). The total PFAS content in leachate (93,100 ng L) was ten-fold higher than that of influent (6,950 ng L) and effluent (3,730 ng L). The leachate load contributions to PFAS in the WWTPs were found to vary from 0.78 to 31 g d⁻¹ (Thompson et al., 2022).

Investigations reveal that PFAS resists degradation in conventional WWTPs and are retained in both treated wastewater and biosolids; hence, the prevalence at different concentrations was observed (Table 1) from location to location (Meegoda et al., 2020; Kumar et al., 2022; Leung et al., 2022; Starnes et al., 2022; Brunn et al., 2023).

Exposure to PFAS has been associated with several health hazards, including decreased vaccination response in infants (Grandjean et al., 2017), heightened risk of type 2 diabetes in both men and women (Roth and Petriello, 2022), gestational diabetes in women (Birru et al., 2021), disrupted metabolic rate (Schillemans et al., 2021), reduced fertility (Rickard et al., 2022), reduced fetal development and obesity (Sevelsted et al., 2022; Liu et al., 2023), elevated risk of some cancers (Fenton et al., 2021; Ojo et al., 2021) and decreased immune system (Rappazzo et al., 2017).

This article systematically assessed the global generation of sewage sludge/biosolids, the diverse treatment methodologies employed, and the prevalence of contamination of per- and polyfluoroalkyl substances (PFAS) within these waste products globally. We also explored the factors influencing PFAS contamination in sewage sludge/biosolids and compiled the existing PFAS regulations or recommendations for land application.

To better understand the trends of global PFAS release into the environment through biosolid/sewage sludge applications, we reviewed multiple studies that addressed biosolid generation and management worldwide and the PFAS concentrations in biosolids from these regions.

A database search approach was implemented by independently selecting relevant keywords and terms and, in sequence, acquiring the appropriate publications from Web of Science, PubMed, ScienceDirect, Google Scholar, Scopus database, and federal and state records. The keywords were connected by the Boolean operators “AND” and “OR,” for instance, “PFAS in sewage sludge” OR “PFAS in biosolid AND “Countries” “Sewage Sludge AND Agriculture.” Other keywords include: “Biosolid generation in countries,” “PFAS variation in sewage sludge,” etc. Article exploration for this appraisal focused on articles published between 2003 and 2023. This extensive search was vital because it encompasses a substantial number of studies that will assist in validating the likely trends and movement of PFAS in the environment over time regarding PFAS use regulations. From the vast search results, the articles used in this review were selected based on their pertinence, thoroughness and focus on this review. Supplementary Table S1, presents the list of abbreviations used in the review.

The annual global production of biosolids is experiencing a remarkable upswing, primarily due to the continuous expansion and technological progress of existing WWTPs. This upsurge can be directly linked to the escalating demand for water resources (Mohajerani et al., 2017). The global generation of biosolids is estimated at 100 to 125 million tons dry weight and is predicted to increase to 150–200 million tons dry weight by 2025 (Kumar et al., 2022). Countries are constantly looking for eco-friendly and sustainable ways to manage the increasing amounts of biosolids generated.

The global evaluation of biosolids generation and management is presented in Figures 1, 2. The figures revealed that a significant percentage of biosolids end up in agricultural land applications or as landfills. Consequently, PFAS and other pollutants in these products are accessible for plant uptake and may enter our food chain.

Based on the United States Environmental Protection Agency reports in 2021, the United States generated 4.75 million dry metric tons of biosolids in 2019 (USEPA, 2021). Currently, the total biosolids generated in the USA are being managed in a variety of ways: land application (51%), landfill (22%), incineration (16%), additional management techniques (10%) and surface discharge (1%) (USEPA, 2021). Biosolids have been widely utilized in US agriculture since the early 1970s, supported by various regulations to promote their safe and beneficial application on land. Under stringent assessments, biosolids were no longer identified as wastes but as potential resources for nutrients, resulting in greater flexibility for biosolids management through agricultural application.

To evaluate the environmental risks linked to biosolids, the United States Environmental Protection Agency (USEPA), through a literature survey in 2019, assessed different contaminants present in biosolids generated across the United States (Zhen et al., 2017). The study found more than 500 potential pollutants in at least one instance, and variations of contaminants found in biosolids were observed to depend upon inputs to individual wastewater treatment facilities over time (USEPA, 2019).

A survey conducted between 2009 and 2019 on biosolid production from different provinces in China estimated that China generates 7.8 million tons of dry biosolids annually with a population of more than one billion (USEPA, 2019). Anaerobic digestion, thermal hydrolysis, composting and incineration are China’s predominant sludge treatment technologies, with an estimated output of 6,944, 8,342, 11,250, and 27,122 t/dw in 2019, respectively. Zhen et al. (2017) affirmed that the status of sewage sludge management in China is significantly far from satisfactory, as more than 80% of sewage sludge generated is being discarded inappropriately. Under safety regulations for sanitary landfills in China, the threshold of sludge water content is 60% (Liu et al., 2021). Zhen et al. (2017) noted that even though landfill management is the most prevalent outcome for biosolids in China, most sewage sludge is disposed of directly after mechanical dewatering, with the sewage sludge still containing more than 80% moisture content and minimal compressive strength. Generally, 29.3% of the total biosolids generated in China are disposed of through land application, subsequently followed by incineration (26.7%) and sanitary landfills (20.1%), with much smaller quantities reused through their incorporation in building materials (15.9%) and various other methods (8.0%) (Wei et al., 2020; Chen et al., 2022).

Japan generates up to 2.0 million tons of biosolids annually, with reclaim and disposal rates climbing up to approximately 97%. Over 50 percent (52%) of Japan’s biosolids are recycled into construction materials, while 31% is used in landfills, and 12% is digested anaerobically for bioenergy generation (Yosuke Matsumiya, 2020). Biosolid applications for agricultural purposes in Japan are relatively low, as farmers are concerned about the environmental risks of applying biosolids to agricultural land.

Based on a study by Collivignarelli et al. (2019), Europe generates more than 10 million tons of dry sewage sludge annually, with Germany, the United Kingdom, France, Spain, and Poland being the major contributors. Similar to biosolid management in China, land utilization, either through direct application or after composting, as well as incineration, are the primary sludge management process in European countries. About 50% of the total biosolids generated in Europe are spread on agricultural land; 28% are incinerated, while 18% are disposed of via landfills (Eurostat, 2022). For instance, in Ireland, 99% of sewage sludge generated in 2018 from urban areas was reused in agriculture (Eurostat, 2022).

The residual part is disposed of through landfill cover (e.g., Sweden), reuse in forestry (e.g., Slovakia, Latvia, and Ireland), pyrolysis, and storage (e.g., Greece, Italy, and Poland) (Eurostat, 2022). The compost approach is largely utilized in some European agricultural countries such as Luxembourg, Slovakia, Estonia, Hungary, Lithuania, etc. Between 40% and 60% of sewage sludge produced in these countries is managed through composting (Eurostat, 2022).

In Türkiye, Croatia, and Malta, about 42%–100% of the total sewage generated is disposed of in landfills. In contrast, incineration has been the predominant biosolid management practice in countries such as Belgium, Netherlands, and Germany due to the strict regulation on the agricultural utilization of biosolids as a result of possible contamination of the biosolids (Gianico et al., 2021).

Based on a survey conducted by the Canadian Council of Ministers of Environment (CCME) in 2012, Canada generates 660,000 tons of dry biosolids yearly with no data on the management of the biosolids. To encourage the advantageous use of biosolids, the CCME proposed dewatering, drying, and nutrient reclaiming from wastewater as acceptable management methods to enhance the quality of the biosolids generated (Canadian Council of Ministers of the Environment, 2012). In 2015, about 28.8% of biosolids generated were incinerated, while 15% were used in landfill (Tessier, 2017). According to Tessier, disposal of biosolids via incineration is declining, while land application is rising in Canada (Venegas et al., 2021). This trend is related to regulatory influences and voluntary biosolids quality enhancements but has been drastically slowed down by recent media attention on the presence of PFAS in biosolids.

Due to the paucity of information from these regions and the lack of appropriate and sufficient wastewater treatment facilities in India, South America, and Africa, estimating the annual biosolid production and management is challenging. Biosolids legislation, generation, stabilization, and application are underdeveloped in these continents and countries.

In Latin America, the primary strategy for managing biosolids/sewage sludge predominantly involves recycling them on agricultural land. This trend is exemplified by Colombia, where, in 2018, seven cities and four districts collectively generated 250,172 dry metric tons of biosolids (Venegas et al., 2021). Notably, 65% of this quantity was employed in agricultural areas. However, several WWTPs in this country dispose of sewage sludge into the environment without proper stabilization (Venegas et al., 2021).

In some areas of Chile, the percentage of biosolids recycled to land increased to 75% between 2009 and 2017. Between 2007 and 2017, over 285,836 tons of biosolids were dumped on 15,423 ha of farmland in the Brazilian state of Paraná (Bittencourt et al., 2014; Venegas et al., 2021).

Data from the India Central Pollution Control Board sets sewage generated in India at 62,000 million litres per day (MLD); however, only 23,277 MLD are being treated, which represents 37% of total sludge generated (PFAS, 2019). In 2021, the percentage of treated sewage increased to 50%, with more treatment plants being constructed. However, the remaining 50% of untreated sewage sludge is indiscriminately discharged into the environment (CPCB, 2021).

For Africa, it is difficult to estimate the amount of sewage sludge generated because individual households are often for the most part responsible for managing their own sewage (especially in rural and sub-urban settlements). While individual households rely on on-site septic tank systems for safe treatment of their wastewater, industrial wastewater is primarily treated through advanced water treatment methods (Onu et al., 2023).

Based on the Australian and New Zealand Biosolids Partnership Report 2019, Australia generates approximately 371,000 dry tonnes of biosolids yearly. Application to agricultural land is the main disposal route (70%), while 19% is used for landscaping or land restoration, and the residual is used as landfill or discharged to the ocean (The Australian and New Zealand Biosolids Partnership ANZBP, 2019).

There is a substantial correlation between a country’s level of development, industrialization and the prevalence of PFAS pollution in the environment. Basically, the development level in each country and the eventual disposal of more than 50% of biosolids generated globally to land, either through landfill or application to agricultural land, appear to be the primary local contribution to PFAS in soils globally. Potential atmospheric inputs distributed over large areas still need to be better assessed as rain and dry deposition are certainly contributing to low level large scale distribution of PFAS over soils. Albeit it is difficult to determine the actual levels of PFAS in developing countries, given the paucity of relevant data in these areas. This review article revealed a global, ubiquitous presence of PFAS in biosolids and little to no limits on the presence of PFAS in biosolids aimed for land application and introduction into our agri-food production. This does present a potential threat from the release of PFAS into the environment through agricultural land application, landfill or indiscriminate disposal of biosolids that are not usually systematically monitored for their levels of PFAS. The scant data on the occurrence of PFAS in biosolids worldwide reflects a lack of interest and recognition of the potential risks caused by PFAS in biosolids.

This review observed that only Germany, Maine, Michigan State, and the USA have established some form of limits for a few PFAS in biosolids intended for land application. Moreover, in countries with regulatory standards, there appears to be little uniformity in the approach taken to determine the limits for PFAS in biosolids or soils. For instance, while some countries adopt lower limits of ≤10 μg/kg for PFOS or PFOA for soils or biosolid-amended soils (Canada, Netherlands, and Australia), others adopted a rather higher concentration ≥100 μg/kg for the sum of FAS (Germany and the US).

A national database that records PFAS usage and its presence in consumer products, including foods, biosolids, and manufacturing and processes using PFAS can help track and prevent the release of PFAS into the environment. We must also better regulate the use of PFAS and prevent their release into wastewater and biosolids and the potential transfer to our food chain and underground water.

We must also implement better monitoring programs to document the presence and concentration of PFAS in biosolids and their production, management, and end-of-life, considering PFAS and the other emerging contaminants they contain.