Biosolids are the treated, semisolid byproduct of municipal wastewater treatment. Every time you flush a toilet or run a dishwasher, the organic solids that separate from the water at a treatment plant become raw sewage sludge. That sludge becomes “biosolids” only after undergoing processes designed to reduce pathogens and make the material stable enough for reuse, most commonly as a soil amendment on farmland. Whether those processes make biosolids genuinely safe is a more layered question than any single government stamp of approval can answer, touching on pathogen reduction, heavy metals, and a newer class of chemical contaminants that existing regulations were never designed to address.
From Sewage Sludge to Biosolids
A modern wastewater treatment plant receives a slurry of water, human waste, food scraps, household chemicals, and anything else that goes down a drain. Primary treatment uses gravity to settle out the heaviest solids. Secondary treatment introduces microorganisms that consume dissolved organic matter; those microorganisms eventually form their own mass of biological solids. The combined settled and biological solids are what the industry calls “sludge.” At this point the material is unstable, odorous, and loaded with bacteria, viruses, and parasites. Turning it into something that can be safely spread on land or otherwise reused requires stabilization, which is where the real engineering begins.
Anaerobic Digestion
The most common stabilization method worldwide is anaerobic digestion, in which sludge is held in sealed, oxygen-free tanks where microorganisms break down organic matter and produce methane-rich biogas. This happens at controlled temperatures for a set period. The two main temperature regimes are mesophilic digestion, which operates around 35–38 °C, and thermophilic digestion, which runs hotter at roughly 50–55 °C. Higher temperatures kill pathogens more effectively and can do so in less time. One study found that shifting from mesophilic to thermophilic conditions achieved complete pathogen removal from the digestate in just ten days, while also maximizing methane output for energy recovery.1Renewable Energy. Optimising sewage sludge anaerobic digestion for resource recovery in wastewater treatment plants
Mesophilic digestion, the cheaper and more widely used option, still reduces pathogens substantially but requires longer retention times to reach comparable results. Research on mesophilic digesters showed that extending the sludge retention time from 11 days to 25 days improved removal of E. coli and Salmonella from about a two-log reduction to a three-log reduction or better, meaning the pathogen count drops by a factor of roughly a hundred to a thousand.2Bioresource Technology. Reactor performance and bacterial pathogen removal in response to sludge retention time in a mesophilic anaerobic digester treating sewage sludge That difference matters for regulatory classification, as discussed below.
Lime Stabilization and Other Chemical Approaches
Not every treatment plant has digestion infrastructure. An alternative is alkaline stabilization, which involves mixing quicklime or hydrated lime into the sludge. The lime drives the pH above 12, creating conditions hostile to nearly all pathogens. A study examining quicklime treatment found that it completely inactivated total coliforms, fecal coliforms, and Salmonella to below U.S. EPA Class A limits.3Journal of Hazardous Materials Advances. Alkaline stabilization of sewage sludge using quicklime: A process-based approach for reducing environmental risk and enhancing safe reuse The high pH also drove off ammonia, reducing total nitrogen by up to 61 percent. Lime treatment is relatively fast and simple, but it produces a more alkaline product, which limits where it can be applied without shifting soil chemistry.
Other stabilization methods include composting, which uses aerobic heat generated by microbial activity, and heat drying or pelletization, where sludge is dried at high temperatures into pellets suitable for bagging and commercial sale. Each method reduces pathogens through a different combination of heat, time, and chemical environment.
Class A Versus Class B
In the United States, the EPA’s Part 503 Rule divides biosolids into two classes based on pathogen levels. Class A biosolids have been treated to reduce pathogens to below detectable levels for Salmonella and fecal coliforms. They can be sold or given away in bags for home garden use, with essentially no site restrictions. Class B biosolids have undergone less aggressive treatment. They still contain detectable levels of pathogens, so their land application comes with restrictions on public access, crop harvesting, and animal grazing for set waiting periods.
A survey of biosolids from 14 U.S. states compared Class A products from advanced processes with Class B products from standard mesophilic digestion. Fecal coliform counts and pathogen genome concentrations were significantly lower in Class A samples. The study ranked processes from lowest to highest pathogen load: composting after digestion came first, then temperature-phased anaerobic digestion, then standard mesophilic digestion. However, human adenovirus genomes were still detected in 70 to 100 percent of Class A samples, depending on the process, and in 88 percent of Class B samples.4PubMed Central. Survey of wastewater indicators and human pathogen genomes in biosolids produced by class a and class B stabilization treatments Viral DNA fragments are not the same as infectious virus, but the finding is a reminder that “Class A” does not mean sterile.
Extended storage after digestion can further reduce both pathogens and the odors that attract insects and rodents, a property regulators call “vector attraction reduction.”5Proceedings of the Water Environment Federation. TWO STAGE ANAEROBIC DIGESTION AND LIQUID STORAGE ACHIEVES ENHANCED PATHOGEN AND VECTOR ATTRACTION REDUCTION
Heavy Metals and Traditional Chemical Contaminants
Pathogens are only one part of the safety picture. Biosolids can carry heavy metals like zinc, copper, cadmium, lead, nickel, and arsenic, among others. Concentrations depend heavily on the nature of the local wastewater. A city with a large industrial base is likely to produce biosolids with higher metal levels than a bedroom community.6Heliyon. Fate, environmental hazard, and effects of contaminants in biosolid applied on land – Section: 5.1 Heavy metals
Both the U.S. EPA Part 503 Rule and the European Union’s Sludge Directive require testing of soil and biosolids before land application and set maximum permissible concentrations for metals.7PubMed Central. A review on the fate and effects of contaminants in biosolids applied on land: Hazards and government regulatory policies – Section: Regulations on biosolid land application When biosolids meet the strictest metal limits under Part 503, they earn what the EPA calls “Exceptional Quality” status, which allows unrestricted distribution. In practice, most biosolids from municipalities without heavy industrial inputs comfortably meet these limits. The concern is cumulative: repeated annual applications can raise soil metal levels over time, and long-term monitoring data remain sparse in many regions.
The PFAS Problem
The contaminant category that has most changed the public conversation around biosolids is PFAS, the family of per- and polyfluoroalkyl substances used in nonstick coatings, food packaging, firefighting foam, and countless other products. These chemicals are extraordinarily persistent in the environment and concentrate in sewage sludge during wastewater treatment. A comparative health-risk assessment found PFAS and polycyclic aromatic hydrocarbons among the most persistent contaminants in biosolids-amended soils, with PFAS still measurable after 20 years of application.8Water Research. Health risk assessment of emerging organic contaminants from land-applied biosolids: A comparative study between the U.S. and China
What makes PFAS particularly worrying is that they can move through soil and into groundwater. One study found that even in a soil with high sorption capacity, up to about 13 percent of certain PFAS compounds leached through, and earthworms accumulated PFAS at levels many times higher than concentrations in the surrounding soil, pointing to a potential pathway into the food web.9Journal of Hazardous Materials: Organics. Unexpected mobility and bioavailability of effluent- and biosolids- borne PFAS and pharmaceuticals in a high-sorption soil system Another study specifically examining composted biosolids found that certain short-chain PFAS compounds, particularly perfluorohexanoic acid, leached at significantly higher rates from biosolids-based compost than from other compost treatments.10Journal of Environmental Management. Leaching of select per-/poly-fluoroalkyl substances, pharmaceuticals, and hormones through soils amended with composted biosolids
Current EPA Part 503 regulations, written in 1993, do not set limits for PFAS in biosolids. Several U.S. states, including Maine and Michigan, have moved ahead with their own restrictions or outright bans on land application. The regulatory gap is one of the central tensions in the biosolids world right now: the pathogen and heavy metal rules that define “safe” in the legal sense do not address the chemicals that many scientists and communities consider the most pressing risk.
Thermal Drying and What It Does to PFAS
After stabilization, biosolids still contain a lot of water. Dewatering and thermal drying reduce the volume and produce a pelletized product that is cheaper to transport and easier to store. Thermal drying also provides some additional pathogen kill and, depending on temperature, can modestly reduce PFAS. One study of a thermal drying system found an average overall PFAS reduction of 15 to 31 percent, with higher dryer inlet temperatures associated with greater removal.11Journal of Water Process Engineering. Understanding dynamics of PFAS in biosolids processed through composting, thermal drying and high temperature pyrolysis That is better than nothing, but it leaves the majority of PFAS intact.
Pellet quality is its own concern. A study examining pelletized biosolids from various digestion processes noted that pellet friability, meaning how easily pellets crumble into dust during handling and transport, remains an overlooked operational and safety issue.12Waste Management. Friability of pelletized biosolids thermally dried from various types of anaerobically digested sludges with and without thermal hydrolysis pretreatment Dusty pellets are harder to apply uniformly and create inhalation concerns for workers.
Pyrolysis and the Search for a PFAS Solution
If conventional thermal drying barely dents PFAS, researchers have been investigating much hotter processes. Pyrolysis heats biosolids to 500–700 °C in the absence of oxygen, breaking down organic matter into biochar, bio-oil, and gas. A pilot study found no detectable PFAS in the resulting biochar, with estimated removal efficiencies above 97 percent on average.13PubMed Central. Pyrolysis processing of PFAS-impacted biosolids, a pilot study
A more detailed study pushed further, showing that pyrolysis at 600 °C reduced PFAS in biochar from about 410 ng/g in the original biosolids to 0.31 ng/g, and that PFAS concentrations in the bio-oil and scrubber water were also very low, indicating roughly 99 percent destruction rather than mere transfer to another waste stream.14Journal of Analytical and Applied Pyrolysis. The fate of per- and polyfluoroalkyl substances (PFAS) during pyrolysis and co-pyrolysis of biosolids with alum sludge and wheat straw The catch is cost and scale. Pyrolysis units require significant capital investment, energy input, and emission controls. For large metropolitan plants generating hundreds of tons of biosolids daily, this technology is still in early commercial adoption rather than routine practice.
Benefits of Biosolids on Farmland
For all the concern about contaminants, there are real agronomic reasons farmers use biosolids. The material is rich in organic matter, nitrogen, and phosphorus. A long-term study of dryland wheat rotations found that increasing biosolids application rates improved soil organic carbon, mineralizable nitrogen, and an overall soil health index. Compared with conventional synthetic fertilizer, biosolids-amended soil scored higher on biological health measures.15Science of The Total Environment. Long-term biosolids land application influences soil health
On the food-safety front, a study that tested 141 pharmaceutical and personal care product compounds in vegetables grown on biosolids-amended fields found only eight compounds detected in any crop replicate, at low nanogram-per-gram concentrations, and none consistently above detection limits across all treated plots. The researchers concluded that crop uptake of micropollutants under normal farming conditions is low.16Science of The Total Environment. Uptake of pharmaceuticals, hormones and parabens into vegetables grown in soil fertilized with municipal biosolids That said, a separate study found that certain compounds, particularly the antibacterial agents triclosan and triclocarban, were taken up by soybean roots and translocated into above-ground plant tissue including beans, with biosolids-amended soils producing higher plant concentrations than irrigated controls.17PubMed. Uptake of pharmaceutical and personal care products by soybean plants from soils applied with biosolids and irrigated with contaminated water The picture is not uniform across all chemicals, and compounds with high sorption to soil organic matter behave differently from water-soluble ones.
Nutrient Runoff and Groundwater
One of the practical advantages of biosolids over synthetic fertilizer is that nutrients are released more slowly, because they are bound in organic matter that microbes break down over time. A study comparing runoff and leachate from biosolids-amended Florida soils with soils receiving commercial inorganic fertilizer found that the fertilizer treatment increased runoff phosphorus concentrations nearly 60-fold compared with untreated soil, while most biosolids treatments yielded runoff phosphorus levels similar to untreated controls. Cumulative nutrient losses were far higher from synthetic fertilizer, with about 38 percent of applied phosphorus and 46 percent of applied nitrogen washing away, versus roughly 3 percent and 6 percent for biosolids.18PubMed. Runoff and Leachate Phosphorus and Nitrogen Losses from Grass-Vegetated Soil Boxes Amended with Biosolids and Fertilizer
That does not mean biosolids have zero impact on water quality. A U.S. Geological Survey study in North Carolina found that shallow groundwater wells adjacent to and downhill from biosolids application fields had significantly higher nitrate concentrations, with median values above 12 milligrams per liter, than wells elsewhere in the study area. Surface water downstream of the application fields also had higher nitrate levels, primarily through baseflow contributions from contaminated groundwater.19U.S. Geological Survey Scientific Investigations Report. Effect of land-applied biosolids on surface-water nutrient yields and groundwater quality in Orange County, North Carolina Over-application or application near sensitive waterways remains a real risk, especially when biosolids are used as a cheap disposal option rather than a precision nutrient source.
Microplastics in Biosolids
Wastewater treatment plants are extremely efficient at capturing microplastics from the water stream, but that efficiency means the captured particles end up concentrated in the sludge. A Canadian study found between 8,700 and 14,000 microplastic particles per kilogram of biosolids. All fields that received biosolids had higher soil microplastic concentrations than control fields, and the field with the longest history of biosolids use had the highest background levels, suggesting that some microplastics persist between annual applications. The most striking finding was that more than 99 percent of the microplastics applied in a single year were unaccounted for in the soil afterward, implying substantial export off-site through wind, water, or deeper soil movement.20Science of The Total Environment. Transfer and transport of microplastics from biosolids to agricultural soils and the wider environment
An Australian study confirmed the pattern, finding that biosolids-amended soils averaged about 1,137 microplastic particles per kilogram compared with 36 per kilogram in unamended reference soils, and that accumulation correlated with the volume and frequency of past applications.21PubMed. Transport and Accumulation of Microplastics from Biosolids to Australian Agricultural Soils: Detection of Microplastics Down to 1 μm No current regulation in any major jurisdiction sets a limit on microplastic content in biosolids, largely because standardized measurement methods are still being developed.
Reclaiming Degraded Land
Biosolids are not only used on farms. One of their more compelling applications is in restoring land wrecked by mining. Over several decades, thousands of acres of abandoned mine lands in Appalachia have been reclaimed using biosolids as the primary soil amendment, re-establishing vegetation, improving water quality, and creating wildlife habitat on formerly barren ground.22Proceedings of the Water Environment Federation. Harnessing Biosolids to reclaim mine lands: Case studies from Appalachia In these settings, the risk calculus is different from food-crop agriculture. The baseline is often toxic, bare tailings; adding biosolids improves the situation dramatically even if the biosolids themselves carry some contaminant burden.
A Canadian study of mine tailings found that 17 years after a single biosolids application, treated areas still had higher carbon, nitrogen, phosphorus, and plant biomass than tailings that received conventional fertilizer or no amendment at all.23Canadian Journal of Soil Science. Influence of a one-time biosolids application on elemental and nutrient concentrations on mine tailings The caveat is that improperly high application rates can push metal concentrations in the receiving soil above safe thresholds, so matching the rate to the site remains critical.
Life Cycle Trade-Offs
Deciding what to do with biosolids is ultimately a comparison between imperfect options: land application, landfilling, or incineration. A life cycle assessment modeling eight scenarios for a mid-size town found that reusing biosolids on land was generally the best environmental option, but that long transportation distances could erode the advantage, and that drying biosolids with fossil-fuel energy instead of biogas from the plant’s own digesters significantly worsened the environmental footprint.24PubMed. Environmental comparison of biosolids management systems using life cycle assessment In other words, whether land application “wins” depends on local conditions: distance to fields, available drying energy, and the contaminant profile of the material itself.
Occupational Exposure for Workers
For the people who actually haul and spread biosolids, the safety question is not abstract. Workers described frequent direct contact during hauling, loading, spreading, post-application field work, cleaning, and maintenance, including ingestion of aerosolized material and dermal contact with sludge and dust.25Journal of Exposure Science & Environmental Epidemiology. Mixed-methods characterization of the tasks and factors influencing occupational exposure during biosolids land application
A large-scale study that collected over 300 air samples downwind of biosolids application sites estimated the occupational risk of infection from bioaerosols at roughly 0.8 to 2.1 percent per year under typical conditions. Extraordinary exposure scenarios, like working very close to a spray applicator in windy conditions, pushed estimated annual infection risk as high as 34 percent, with viruses posing the greatest threat. The researchers noted that even these higher-risk scenarios produced estimated risks lower than those reported for workers at wastewater treatment plants.26PubMed. Estimated occupational risk from bioaerosols generated during land application of class B biosolids A separate modeling study estimated infection risks for workers at biosolids sites at around 2 to 3 per 100 per year for bacterial and viral exposure under moderate wind and one hour of contact.27Journal of Environmental Quality. Bioaerosol Transport Modeling and Risk Assessment in Relation to Biosolid Placement The method of application matters a great deal: spray application generates far more aerosol than injection or surface spreading followed by incorporation into the soil.
These estimates focus on pathogens in Class B biosolids, which still carry measurable microbial loads. Workers handling Class A material face lower biological risk, though chemical exposures from dust and volatile compounds remain relevant regardless of pathogen class.