Fertilizers have revolutionized agriculture. But they come with a high environmental cost. They require a huge amount of energy during manufacture, leading to a significant carbon footprint. They also pollute our waterways due to run-off. Added to that, the wholesale cost of fertilizer has remained stubbornly high following a massive spike during the pandemic. So it’s hardly surprising that farmers are looking to find alternatives to use for their crops and fields.
One attractive option is to use so-called sewage sludge (sometimes more euphemistically called bio-solids). This is a highly processed, sterilized byproduct of sewage treatment. In effect it’s human-origin manure. But there’s a problem. While the sludge is treated to remove all pathogens, some chemicals survive and are in fact concentrated in the sludge. Now, evidence is growing that suggests sewage sludge may be a major source of PFAS pollution.
What are the downsides of conventional fertilizers?
Conventional fertilizers are generally a mix of nitrogen, phosphorus, and potassium, or NPK. These fertilizers are often viewed as being harmful to the environment in several ways.
High carbon footprint
Fertilizers have a significant carbon footprint (up to 1.5% of global emissions). The nitrogen component is made from ammonia which is almost always produced from natural gas. Phosphorus is produced from treating phosphate rich rock with sulfuric acid. Potassium fertilizers come from mining potash, a natural mineral. These fertilizers have high carbon footprints: nitrogen fertilizers emit 2.6 tons of carbon dioxide for each ton of fertilizer, phosphorus fertilizers emit 1.7 tons of CO2 per ton, and potassium fertilizers emit 0.6 tons. Additionally, there’s a significant carbon footprint from the fertilizers biodegrading once they are applied to the soil.
Soil runoff
Intensive farming often applies high levels of fertilizer to the soil. Rainfall causes chemicals in the soil to wash off into watercourses, a process known as soil runoff. Fertilizers are a key component of soil runoff and are quite damaging for watercourses. Nitrogen and phosphorous fertilizers cause algal blooms in water which in turn causes many problems. Firstly, some forms of algae produce poisonous toxins that can affect both aquatic and human health. Secondly, the algae affect the levels of oxygen in the water, and can lead to fish and other aquatic life forms dying. Thirdly, the algae affect the clarity of the water, blocking sunlight and causing aquatic plants to die, which in turn can further impact the health of the watercourse.
High costs
Manmade fertilizers are relatively expensive for farmers to buy. Prior to the 2020 pandemic, the global price index for fertilizers was about 75. This rose to a peak of 293 in 2022 before dropping back to around 130 in 2025. While it’s hard to convert these into exact dollar amounts, farmers today are paying anything between $330 and $740 per ton for fertilizer, depending on the type. That makes fertilizer a major cost that in turn can impact food prices for consumers.
What is sewage sludge?
Sewage sludge is a byproduct of wastewater treatment that can be used as a fertilizer. Sewage sludge is treated to remove pathogens and other harmful substances. However, some chemicals, such as PFAS, can persist in the sludge and may pose a risk to human health and the environment. The Environmental Protection Agency (EPA) has established standards for the use and disposal of sewage sludge.
Why is it so important for farmers?
Sewage sludge is often used as a fertilizer because it is cheaper than conventional fertilizers. However, there are concerns about its potential environmental impacts. For example, sewage sludge can contain heavy metals, which can contaminate soil and water. It is also a source of PFAS chemicals which seem to get concentrated in the sludge during the treatment process.
How widespread is its use?
According to the EPA, around 1.24 million dried metric tons of sewage sludge was used as an agricultural fertilizer in 2023. To put that into perspective, the US uses around 20 million tons of conventional fertilizer each year. So sewage sludge still only makes up around 5% of all fertilizer. However, it is likely to become increasingly popular as the price of fertilizers goes up.
Why is it a major source of PFAS pollution?
The process used to create sewage sludge tends to concentrate certain pollutants such as heavy metals, PFAS, and other organic compounds. This is increasingly true as water treatment plants start to filter out PFAS compounds to meet new regulations. A 2024 study identified that sewage sludge contains up to 8.2 µg/g of PFOS (perfluorooctane sulfonic acid). That’s a worryingly high level, especially since PFOS was supposedly eliminated from manufacturing processes over a decade ago.
How does this affect us?
PFAS are increasingly linked to a whole host of serious health conditions including cancers, liver and kidney disease, and premature birth. Earlier this year, an EPA draft report identified that people living near farms where sewage sludge has been spread have increased risks for cancer and other diseases. This risk also applied to people who consumed meat or dairy products produced on those farms.
There seems to be three routes of exposure. Firstly, through drinking contaminated water that has been affected by soil runoff from these farms. Some estimates suggest around 1 in 3 people are already exposed to PFAS in their drinking water. Secondly, livestock grazing on those fields are exposed to PFAS. Not only does this then accumulate in the animal, it also leads to high mortality rates among livestock. Thirdly, any crops grown on fields treated with sewage sludge may introduce PFAS into other parts of the food chain.
What can you do if you’re worried?
PFAS is hard to avoid in our modern world. But more and more research is looking at ways to reduce the levels in our water and the environment. Meanwhile, you can take steps to reduce your exposure.
- Test your water for PFAS. If you have high levels, install water filters to remove PFAS from your drinking water.
- Get tested to check your blood PFAS levels, and then re-test periodically to monitor this
- Reduce other common forms of PFAS exposure, such as food packaging, such as microwave popcorn bags and fast food wrappers, and non-stick cookware,
Ultimately, you can’t remove PFAS exposure completely. But you can reduce it to levels that the National Academies deem to be safe for humans.
