The agricultural case for sludge is genuinely strong when the material is clean. Field studies cited in the review show that sludge application can raise crop yields by as much as 60 percent, rivaling the effect of fertilizers made from non-renewable resources. Two years of sludge amendment increased not only soil organic matter but also the stability of soil aggregates, improving soil structure in ways that conventional mineral fertilizers cannot. Biosolids even boosted the phenolic content and antioxidant activity of sweet basil, hinting at quality benefits beyond sheer yield. In a world facing dwindling phosphorus reserves and a nitrogen fertilizer crisis, the logic of recycling nutrients from human waste back into farmland is compelling. Circular-economy advocates argue that every tonne of sludge diverted to agriculture is a tonne of mined phosphate rock that does not need to be extracted.

The problem is what else rides along with those nutrients. Heavy metals, including cadmium, zinc, copper, chromium, nickel, arsenic, silver, and lead, make up roughly 0.5 to 2.0 percent of sludge on a dry-weight basis, and they originate from both industrial discharges and everyday household plumbing. Once these metals enter soil they are not biodegradable and can persist in the environment for decades, migrating through soil profiles into groundwater and accumulating in the edible tissues of crops. One influential study found that while sludge boosted yields of mung bean by up to 60 percent, it simultaneously raised the accumulation of lead, nickel, and cadmium in the grain, undermining its suitability as food. Under European rules for organic-mineral fertilizers, permissible limits are 100 milligrams of chromium, 5 milligrams of cadmium, 60 milligrams of nickel, 140 milligrams of lead, and 2 milligrams of mercury per kilogram, but the actual metal content of a given sludge depends heavily on its origin, and the same production technology can deliver strikingly different results depending on whether the process runs at laboratory, pilot, or full industrial scale.

Pathogens constitute a second, more insidious barrier. Municipal sludge is a biological concentrate of fecal bacteria such as Escherichia coli and Salmonella, enteric viruses including rotavirus, norovirus, and hepatitis A, protozoan parasites like Giardia intestinalis, and fungal genera such as Aspergillus and Penicillium. Farmers handling sludge-based fertilizers report skin allergies, digestive ailments, and respiratory infections, and communities living near fields where sludge is sprayed face elevated risks of pharyngitis, coughing, and shortness of breath from airborne transmission. Contaminated soil can also shed pathogens into surface water and groundwater during rainfall, threatening drinking supplies and aquatic biodiversity. The review stresses a subtle analytical problem: pathogen counts measured with culture-based methods versus molecular assays such as quantitative PCR are not directly comparable, and detecting viral genetic material does not necessarily mean the virus is still infectious. Comparisons across studies are further muddied by differences in sludge stabilization history, storage conditions, and seasonal variation in the incoming wastewater.

Organic micropollutants form perhaps the most chemically diverse contaminant group. The most frequently detected compounds include linear alkylbenzenesulfonates from detergents, the plasticizer di-ethylhexylphthalate, nonylphenols, polychlorinated biphenyls, dioxins and furans, and polycyclic aromatic hydrocarbons, which have been measured at concentrations ranging from 0.7 to 75.2 milligrams per kilogram in sludge. Persistent compounds such as DEHP and PAHs are of particular concern because they have been linked to cancer, reproductive damage, and metabolic disease. Pharmaceuticals add another layer: global antibiotic consumption rose 65 percent between 2000 and 2015, and most antibiotics pass through the human body unmetabolized, ending up in sludge at concentrations from micrograms to milligrams per kilogram. When antibiotic-laden sludge is applied to soil, it exerts selective pressure that fosters antibiotic-resistant bacteria, suppresses microbial diversity, and can inhibit carbon mineralization, nitrogen cycling, and enzyme activity. Natural and synthetic steroid hormones, active at vanishingly low concentrations, raise endocrine-disruption worries, while persistent pesticides accumulate in sludge solids because of their hydrophobic chemistry.

Microplastics have emerged as the contaminant that most sharply exposes the limits of current treatment and regulation. An estimated 3.2 million tonnes of primary microplastics enter the environment annually, and between 60 and 99.9 percent of the microplastics entering wastewater treatment plants are retained in the resulting biosolids. Studies report anywhere from hundreds of thousands to several millions of microplastic particles per kilogram of sludge, depending on the analytical method and particle-size range considered. Once sludge is spread on fields, these particles, which can adsorb dyes, heavy metals, and pharmaceuticals onto their surfaces, effectively seed agricultural soils with a persistent plastic burden and a mobile carrier for co-transported pollutants. The United Nations listed microplastic pollution among the top ten environmental issues requiring immediate attention in 2017, yet quality standards for biosolids and soils, many written three decades ago, still test only for heavy metals and a handful of persistent organic pollutants. Microplastics, pharmaceuticals, hormones, and antibiotic resistance determinants fall almost entirely outside the regulatory net.

The review catalogues a battery of treatment technologies that can shrink these risks, while candidly admitting that none is a universal solution. For heavy metals, options include alkaline modification with geopolymers that immobilize metals in monolithic matrices, chemical extraction using chelating agents such as EDTA and NTA followed by sulfide precipitation, microbiological bioleaching with organisms like Acidithiobacillus ferrooxidans that achieved up to 39 percent lead removal in one study, and electrokinetic treatment that drives metal ions toward electrodes under a direct-current field. Each approach carries trade-offs: stabilization does not remove total metal mass and can reverse if soil pH or redox conditions shift, extraction generates secondary metal-rich effluents, bioleaching is slow and finicky, and electroreclamation is energy-hungry and electrode-sensitive. For pathogens, composting under controlled conditions eliminated 99 percent of Salmonella and 96 percent of fecal coliforms within a month, while thermophilic anaerobic digestion at 50 to 60 degrees Celsius denatures bacterial enzymes and outperforms mesophilic digestion. Quicklime treatment pushes pH above 12 and heats sludge to around 70 degrees, destroying organisms including Clostridium perfringens, Listeria, and Enterococcus, and hybrid schemes combining 160-degree thermal hydrolysis with anaerobic digestion came close to complete pathogen destruction, sparing only Clostridium perfringens. Irradiation with cobalt-60 or cesium-137 can shred pathogen DNA but remains rare at full scale because of cost and safety barriers.

For organic micropollutants, well-managed composting reduced hormones below detection limits in field-scale post-treatment, whereas uncontrolled open-air storage produced variable and often disappointing removal. Anaerobic digestion shows compound-specific performance, achieving over 80 percent removal for selected anti-inflammatory drugs under laboratory conditions but low or even negative removal for other pharmaceutical classes at full scale. Thermal routes are the heavy artillery: combustion at 850 to 1000 degrees destroys most organic micropollutants but transfers a fraction of them to flue-gas residues, and pyrolysis at 600 degrees reduced polychlorinated biphenyls, PAHs, and pharmaceutical and personal-care compounds by at least 99.9 percent in one comparative study, although contaminants can partition into condensates and oils that then require controlled disposal. Against microplastics, incineration removes roughly 99.9 percent of particles and pyrolysis achieves 91 to 97 percent removal, but both funnel nutrients out of closed-loop recycling into construction materials or energy, while emerging alternatives such as hydrothermal liquefaction and hyperthermophilic composting show promise yet rarely exceed 50 percent removal efficiency and remain poorly characterized in terms of degradation by-products.

The review’s most sobering findings concern the law rather than the lab. Council Directive 86/278/EEC protects soils by setting heavy-metal thresholds for agricultural sludge, but the EU fertilizer framework, Regulation 2019/1009, excludes sludge from its list of approved fertilizer ingredients precisely because of toxicity concerns, leaving farmers who wish to use it dependent on temporary national permits. Because member states write their own rules on application rates, permissible microorganism levels, microplastics, antibiotics, hormones, and pH, soil protection is inconsistent across the bloc, and internationally traded food faces uneven safety standards. The authors call for a harmonized directive specifying contaminant limits across all pollutant classes, mandatory annual soil testing for heavy metals, microplastics, hormones, and antibiotics, adoption of best available techniques, and on-site sludge processing to cut transport emissions. Public skepticism, fueled by odors and documented health complaints near land-application sites, adds a social dimension that educational campaigns and community engagement must address. The bottom line is pragmatic: sludge-derived fertilizers can genuinely anchor circular nutrient management in a phosphorus-scarce world, but only under science-based regulation, standardized analytical methods, and treatment strategies that account for contaminant fate, secondary waste streams, and real-world field conditions rather than laboratory promises alone.

Subject of Research: Contaminant risks and mitigation strategies in sewage sludge-derived fertilizers

Article Title: Assessing and Mitigating Risks of Emerging Contaminants in Fertilizer Production from Sewage Sludge: Challenges and Opportunities

Article References: Skrzypczak, D., Mikula, K., Izydorczyk, G., Samarina, T., Gil, F., Wijatkowska, A., Szyszka, D., & Chojnacka, K. (2026). Assessing and Mitigating Risks of Emerging Contaminants in Fertilizer Production from Sewage Sludge: Challenges and Opportunities. Waste and Biomass Valorization. https://doi.org/10.1007/s12649-026-03780-z

Image Credits: AI Generated

DOI: 10.1007/s12649-026-03780-z

Keywords: sewage sludge, heavy metals, pathogens, microplastics, pharmaceuticals, composting, anaerobic digestion, pyrolysis, fertilizer production, circular economy, wastewater treatment, regulatory gaps