Human fertilizer refers to any soil amendment or nutrient product derived from human waste, and it has been used in agriculture for thousands of years. Modern versions include treated sewage sludge (commonly called biosolids), urine-derived fertilizer crystals, and composted human excreta. A comprehensive review found that these products are nutrient-rich and can contribute to sustainable, circular agriculture, though they may carry contaminants that need careful management.1PubMed. Opportunities and challenges of using human excreta-derived fertilizers in agriculture: A review of suitability, environmental impact and societal acceptance The science behind turning human waste into crop food is more advanced than most people realize, but the concerns are real and worth understanding in detail.
What Counts as Human Fertilizer
The term covers a surprisingly wide range of products. At one end sits the most common form: biosolids. These are produced at municipal wastewater treatment plants, where sewage goes through biological processing, thickening, and stabilization before being applied to farmland. In the United States alone, roughly half of all biosolids produced end up on agricultural land or in land reclamation projects. At the other end are source-separated products, where urine or feces are collected independently and processed into concentrated fertilizer pellets or crystals. Between these extremes sit composted human excreta from dry toilets, thermally dried sludge, and biochar made by heating sewage sludge in the absence of oxygen.
What unites all these products is their nutrient content. Human waste is rich in nitrogen, phosphorus, and potassium, the same trio that forms the backbone of synthetic fertilizers. A single person’s annual urine output contains enough phosphorus and nitrogen to fertilize a meaningful patch of cropland. The appeal is straightforward: instead of flushing these nutrients into waterways where they cause algal blooms and dead zones, capture them and return them to the soil.
How Pathogens Are Removed
Raw human waste is a serious health hazard. It harbors bacteria like Salmonella and E. coli, parasitic worms, and viruses. The entire point of treatment is to reduce or eliminate these organisms before the material touches food-producing soil. Multiple technologies accomplish this, including composting, anaerobic digestion, aerobic digestion, and microwave irradiation, each with different effectiveness against different types of pathogens.2PubMed Central. Inactivation and risk control of pathogenic microorganisms in municipal sludge treatment: A review
In the U.S., the EPA classifies treated sewage sludge into two tiers. Class A biosolids have been processed until pathogens drop below detectable levels. These can be sold to the public in bags and used without site restrictions. Class B biosolids have undergone less intensive treatment and may still contain detectable pathogens. Their use on land is regulated to prevent human and animal exposure.3PubMed Central. A review on the fate and effects of contaminants in biosolids applied on land: Hazards and government regulatory policies Class B material, for instance, typically cannot be applied near homes or where people might contact it directly, and harvesting of food crops is restricted for a set period after application.
Internationally, the picture is more complicated. A risk assessment study found that WHO guidelines for the reuse of sludge in developing countries may actually be too lenient for parasitic worm eggs, even as the same guidelines appear more stringent than necessary for wastewater reuse in irrigation.4PubMed. Evaluation of the WHO helminth eggs criteria using a QMRA approach for the safe reuse of wastewater and sludge in developing countries This disconnect matters because parasitic worms (helminths) are extremely persistent in soil and pose one of the greatest risks from insufficiently treated human waste in agriculture.
Heavy Metals and Their Persistence
Pathogens can be killed with heat and time. Chemical contaminants are a different problem entirely. Biosolids commonly contain heavy metals including zinc, cadmium, copper, chromium, lead, nickel, arsenic, and mercury. The concentrations depend on the nature of the wastewater and where it originated, with domestic wastewater carrying far less metal contamination than industrial wastewater.5PubMed Central. A review on the fate and effects of contaminants in biosolids applied on land: Hazards and government regulatory policies
Some metals are actually needed by plants in small amounts. Zinc, copper, and manganese serve as micronutrients. Others, like lead, cadmium, mercury, and arsenic, have no beneficial role and accumulate in soil over time because most metals do not break down biologically or chemically. One study found that zinc, nickel, chromium, and copper concentrations in biosolid-amended soil rose sharply over just two years.6PubMed Central. A review on the fate and effects of contaminants in biosolids applied on land: Hazards and government regulatory policies Plants absorb these metals through their roots, which means the contamination can travel up the food chain. The effects on people from long-term low-level exposure range from kidney problems and immune dysfunction to skin lesions and cancer.
That said, the dose matters enormously. Research on lettuce grown in composted biosolids found that while heavy metal concentrations were higher than in control plants, they did not exceed the range typically found in commercially grown lettuce.7Communications in Soil Science and Plant Analysis. Growth and metal uptake of lettuce [Lactuca Sativa L] on soil amended with biosolids and gypsum Regulatory limits exist precisely to keep metal loading within safe bounds, and properly managed biosolids programs stay well below those limits. The risk rises when biosolids are applied repeatedly to the same land for decades, or when the source wastewater contains industrial discharges.
PFAS and “Forever Chemicals”
The contaminant that has drawn the most recent public attention is PFAS, a class of synthetic chemicals used in nonstick coatings, waterproof fabrics, and firefighting foam. These chemicals resist breakdown in the environment and accumulate in living tissue. They enter wastewater from household products and industrial sources, concentrate in sewage sludge during treatment, and persist in biosolids applied to farmland.
Research has confirmed that crops take up PFAS from biosolid-amended soil. Certain compounds, particularly short-chain varieties like PFBA and PFPeA, accumulated in lettuce and tomato at levels well above what was present in the soil when biosolids were heavily contaminated with PFAS from industrial sources.8Environmental Science & Technology. Uptake of Perfluoroalkyl Acids into Edible Crops via Land Applied Biosolids: Field and Greenhouse Studies Longer-chain compounds like PFOS tend to accumulate more in plant stalks and leaves than in the parts people eat, but they have been detected in corn stover and rye.9PubMed Central. Per‐ and polyfluoroalkyl substances in land‐applied biosolids: Accumulation in soils, crop uptake, and potential dietary risk PFAS transfer from soil to plants was also demonstrated for spinach, tomato, and corn, with transfer rates for PFAS running several times higher than for other persistent organic pollutants like brominated flame retardants.10PubMed. Uptake of perfluoroalkyl substances and halogenated flame retardants by crop plants grown in biosolids-amended soils
This is arguably the most difficult contaminant problem facing human fertilizer today. Heavy metals have established regulatory limits and decades of management experience. PFAS regulations are still emerging, and the chemicals are so widespread in the waste stream that keeping them out of biosolids is essentially impossible without addressing their use upstream in consumer products and industry.
Microplastics in Biosolids
Another emerging concern is microplastics. Wastewater treatment plants are effective at removing plastic particles from the water itself, but many of those particles end up concentrated in the sludge. A study measuring microplastic content found an average of roughly 12,000 particles per kilogram of dry biosolids. Long-term biosolid application led to significantly higher soil plastic concentrations compared to unamended soil.11PubMed. Accumulation of microplastics in soil after long-term application of biosolids and atmospheric deposition An Australian study found even starker differences, with biosolid-amended soils averaging over 1,100 microplastic particles per kilogram versus about 36 in reference soils.12PubMed. Transport and Accumulation of Microplastics from Biosolids to Australian Agricultural Soils: Detection of Microplastics Down to 1 μm
The long-term effects of microplastics in agricultural soil are not yet well understood. Soil organisms interact with them, plastic particles can carry other contaminants on their surfaces, and there is concern about them entering groundwater. Like PFAS, microplastics in biosolids remain largely unregulated, and their presence is essentially guaranteed as long as the waste stream contains synthetic materials.
Antibiotic Resistance Genes
A less visible but potentially serious concern is antibiotic resistance. People excrete antibiotics and antibiotic-resistant bacteria, both of which end up in sewage. When biosolids are applied to farmland, they can introduce resistance genes into soil bacterial communities. One study found that biosolid application significantly increased the diversity and abundance of antibiotic resistance genes in soil and that these genes moved into lettuce roots and even into the edible leaves, primarily through bacteria living inside the plant tissue rather than on its surface.13Soil Biology and Biochemistry. Application of biosolids drives the diversity of antibiotic resistance genes in soil and lettuce at harvest
How much the treatment process matters is now becoming clearer. A mesocosm study found that higher biosolid application rates produced a dose-dependent increase in antibiotic-resistant bacteria in soil. Limed or thermally dried biosolids performed better than air-dried or untreated sludge, suggesting that treatment intensity genuinely reduces the risk. Still, some antibiotic resistance was present even in unamended control soils, indicating a background level that exists regardless of biosolid use.14PubMed Central. From biowastes to risks? Impact of biosolids treatment and dose on antibiotic resistance in agricultural soils – A mesocosm study Greenhouse experiments showed that after biosolid amendment, E. coli and enterococci concentrations in soil jumped dramatically within the first week and then declined slowly over time, remaining detectable throughout the study period.15PubMed Central. Impact of biosolids amendment and wastewater effluent irrigation on enteric antibiotic-resistant bacteria – a greenhouse study
Urine-Derived Fertilizers
One way to sidestep many of the contaminant problems associated with mixed sewage is to collect urine separately before it enters the waste stream. Human urine is naturally rich in nitrogen and phosphorus but low in pathogens and heavy metals compared to feces or mixed sewage sludge. A growing body of research focuses on recovering phosphorus from urine as struvite, a crystalline mineral that works as a slow-release fertilizer.
A comparative study found that struvite recovery from human urine achieved phosphate recovery efficiencies above 90%, with struvite yields roughly twice those from cow urine.16Materials Today: Proceedings. A comparative study of phosphorus recovery as struvite from cow and human urine Research on struvite precipitated from source-separated human urine confirmed that heavy metal concentrations fell well below regulatory limits, and about 61% of the total phosphorus content dissolved within 10 days at neutral pH, making it available to plants relatively quickly.17PubMed Central. Struvite precipitation from source-separated human urine: mineralogical characterization, phosphorus release kinetics, and heavy metal safety assessment
Source separation requires different plumbing. Specialized toilets and urinals can divert urine into collection tanks, where microbial activity naturally raises pH and drives struvite crystallization without added chemicals. The approach is gaining traction in parts of northern Europe, where pilot programs have scaled urine collection to neighborhood and district levels. A life-cycle assessment comparing urine-based fertilizer to synthetic fertilizer for wheat production found lower environmental impacts across climate change, eutrophication, fossil resource use, and water consumption categories. The avoided impacts from mineral fertilizer production and from reducing nutrient discharge to rivers were both significant.18Journal of Cleaner Production. Comparative study of environmental impacts related to wheat production with human-urine based fertilizers versus mineral fertilizers
Pyrolysis and High-Temperature Processing
For mixed sewage sludge, one of the most promising advanced treatments is pyrolysis, which heats the material to high temperatures in an oxygen-free environment. This destroys pathogens, reduces volume, and produces biochar, a carbon-rich solid that can be used as a soil amendment.19PubMed Central. Biochar from Co-Pyrolyzed Municipal Sewage Sludge (MSS): Part 1: Evaluating Types of Co-Substrates and Co-Pyrolysis Conditions
Pyrolysis also addresses the organic contaminant problem far more aggressively than composting or digestion. An industrial-scale pilot study found that slow pyrolysis at 500°C reduced PFAS concentrations by 30 to 93%, brominated flame retardants by 97 to 98%, and most endocrine-disrupting compounds by 82 to 96%. Pharmaceuticals and estrone were driven below detection limits.20PubMed. Industrial pilot scale slow pyrolysis reduces the content of organic contaminants in sewage sludge The researchers recommended pyrolysis temperatures above 500°C to ensure efficient removal. Inorganic contaminants like heavy metals, however, are not destroyed by heat. They remain in the biochar and their fate depends on the feedstock. This means pyrolysis is a powerful cleanup tool for organic pollutants but not a silver bullet for all contamination.
What Biosolids Do to Soil and Crops
When contaminant levels are properly managed, the agronomic case for human-derived fertilizers is strong. A review of the evidence found overall positive effects on crop yield, soil nutrient levels, plant-soil-microbe interactions, and even plant pathogen suppression.21PubMed. Opportunities and challenges of using human excreta-derived fertilizers in agriculture: A review of suitability, environmental impact and societal acceptance Biosolids add organic matter to soil, improving its water-holding capacity and structure, which is especially valuable for degraded or sandy soils.
The effect on soil microbes is more nuanced. A short-term flooding study found that while biosolid application increased soil phosphorus, nitrogen, and organic matter content, it did not significantly change the overall makeup of the soil’s bacterial community, despite the biosolids themselves containing far more genetic material and much lower bacterial diversity than the native soil.22PubMed Central. Response of soil bacterial populations to application of biosolids under short-term flooding Long-term studies paint a more complex picture. Research at a site that received sewage sludge for over a century found that moderate zinc loading (roughly 700 to 1,000 mg/kg) appeared optimal for maximum bacterial and fungal diversity, but that microbial community structure shifted in response to accumulated trace metals, especially zinc, cadmium, and copper.23PubMed. The response of soil microbial diversity and abundance to long-term application of biosolids In other words, moderate biosolid use can enhance soil life, but heavy or prolonged application reshapes the microbial community in ways driven by metal accumulation.
The Environmental Tradeoffs
Recycling nutrients from human waste back to farmland reduces the need for synthetic fertilizers, which require fossil fuels to produce. A life-cycle assessment comparing nutrient recycling from human excreta to advanced wastewater treatment found that the recycling scenarios conserved energy and reduced greenhouse gas emissions. However, the urine and blackwater scenarios increased potential eutrophication and acidification compared to the conventional treatment-plus-synthetic-fertilizer approach, primarily because ammonia is released during storage and field spreading of the fertilizers.24Science of the Total Environment. Environmental impact of recycling nutrients in human excreta to agriculture compared with enhanced wastewater treatment
This tradeoff is worth understanding honestly. Human fertilizer is not a zero-cost environmental win. It shifts the environmental burden from one category (fossil energy, climate change) to another (local ammonia emissions, potential nutrient runoff). Whether that shift is net positive depends on how the material is stored, transported, and applied, and on what it replaces. In regions that rely heavily on imported synthetic fertilizers, the calculus tends to favor recycling. In places with tight eutrophication controls on nearby waterways, the ammonia issue may be a dealbreaker unless injection or immediate incorporation into soil reduces emissions.
Composting Toilets and Decentralized Approaches
Not all human fertilizer comes from centralized treatment plants. Composting toilets and dry toilet systems offer decentralized alternatives, particularly for rural areas, off-grid communities, and developing regions where sewage infrastructure is minimal. These systems collect excreta on site, often mixing it with carbon-rich bulking agents like sawdust, and rely on thermophilic composting to reduce pathogens over weeks or months.
A risk analysis of dry toilet contents identified the key hazards: pathogens come primarily from feces; plant diseases can enter through composting materials; pharmaceuticals and chemical additives are potential pollutants; and urine can contribute excess ammonia and salinity. The researchers concluded that these risks can be significantly reduced through proper user education, appropriate waste handling, and modern sanitization processes tested under local regulations.25PubMed Central. Qualitative Risk Analysis for Contents of Dry Toilets Used to Produce Novel Recycling Fertilizers
A comparative evaluation of a farm using both open-air composting and composting toilets found different strengths in each system. Open-air composting more effectively removed fecal pathogens but tended to increase antibiotic resistance in the resulting material. Composting toilets offered faster breakdown and better removal of antibiotic-resistant bacteria, though with slightly less pathogen reduction.26PubMed. Microbiological safety and antibiotic resistance risks at a sustainable farm under large-scale open-air composting and composting toilet systems Neither system was perfect, but both produced usable material when managed attentively.
Worker Safety During Application
One dimension of human fertilizer that rarely makes it into the public conversation is what happens to the people who actually spread it on fields. A NIOSH evaluation of workers land-applying biosolids found that all five employees involved reported at least one episode of gastrointestinal illness shortly after performing the work. The investigation included air sampling for bacteria, endotoxins, volatile organic compounds, and metals in the workers’ breathing zones.27Centers for Disease Control and Prevention. Exposure assessment of employees land-applying biosolids This is a small case report, not a sweeping epidemiological study, but it highlights that even treated biosolids can produce aerosol exposures during spreading that affect the people doing the job. Protective equipment and application methods that minimize dust and aerosol generation are not optional extras.
How People Actually Feel About It
Even if all the safety and environmental questions were resolved tomorrow, human fertilizer would still face a significant psychological barrier. A study on consumer acceptance found that fertilizers derived from human urine and biosolids were equally preferred, and both were actually preferred over synthetic fertilizers. However, urine-derived fertilizer was not preferred as strongly as conventional organic fertilizers like compost from plant or animal waste. Acceptance dropped further when the fertilizer was to be used on crops for human consumption versus ornamental plants. The strongest predictors of acceptance were people’s judgments about risks and benefits, not disgust alone.28Environmental Science & Technology. Consumers’ Acceptance of Agricultural Fertilizers Derived from Diverted and Recycled Human Urine
This finding is encouraging but also reveals the marketing challenge. People are not uniformly repulsed by the idea. Many are open to it, especially when they understand the environmental rationale. But the “ick factor” intensifies the closer the product gets to their dinner plate. Urine-derived fertilizer on a lawn? Mostly fine. Biosolids on strawberries? Much harder sell. Building public trust requires not just safety data but transparency about what is in the product, how it was treated, and what testing was performed, all of which loop back to the regulatory and processing questions the industry is still working through.