Chemicals in sewage-based fertilizer may persist in farmland soil for years

Swati Mestri

Scientific Editor

Robert Egan

Senior Editor

UNSW engineers have identified a number of chemicals that may merit closer scrutiny to ensure water utility companies and regulators can make informed decisions about the management of biosolids.

Biosolids are treated sewage sludge—that is, wastewater from household kitchens, laundry rooms and bathrooms that has been processed to create a product suitable for beneficial use.

About 80% of biosolids produced in Australia are reused on farmland because they provide valuable nutrients and organic matter that can improve soil quality and support plant growth.

However, biosolids can also contain traces of chemicals entering the system from everyday household activities such as washing, cleaning and taking medicines.

A new study, led by UNSW researchers, has examined what happens to some of those chemicals when biosolids are applied to farmland, including how long they can remain in the environment and whether their concentrations could affect plants and animals.

Their findings, published in the Journal of Hazardous Materials, provide a framework for identifying contaminants that may need closer attention to ensure regulators can continue to support the beneficial use of biosolids.

Denis O'Carroll, a professor at the Water Research Laboratory in UNSW's School of Civil and Environmental Engineering, said, "It is important that biosolids can continue to be applied to the land to act as a fertilizer and get nutrients into the soil.

"What this research has shown is that there are some potential contaminants of concern because of their concentration or how they persist in soil.

"And we hope this work can be the first step in a multistage plan to mitigate the problem."

Environmental screening assessment

Importantly, the study is an environmental screening assessment.

It does not demonstrate that the chemicals examined are causing harm on farms, that food grown using biosolids is unsafe or that people are being exposed to harmful levels of contaminants.

However, the researchers did identify several chemicals that could persist in soil for months or even years. These include some per- and polyfluoroalkyl substances (PFAS), carbamazepine (a medicine used to treat conditions such as epilepsy), triclocarban and triclosan (antimicrobial chemicals in personal care products) and galaxolide and tonalide (fragrance chemicals).

"Whenever people flush their toilet or their water goes down the sink, all of that ultimately ends up at a wastewater treatment plant," says Dr. Calvin He, author of the paper.

"That wastewater can be treated and the resulting biosolid can be applied to farmland as a fertilizer. But if there are still contaminants present, then they can stay in the soil.

"Even though biosolids have a really amazing benefit of being full of carbon, nitrogen and phosphorus, there is also a downside in that you're potentially applying these contaminants."

For their study, the UNSW team examined 47 chemicals, including pharmaceuticals, personal care products, industrial chemicals, hormones and other substances that have previously been detected in biosolids.

These included commonly used medicines, antimicrobial ingredients found in some personal care products and chemicals used in industrial and consumer applications.

The 47 substances represent a small selection of the hundreds of chemicals known to have been detected in biosolids. The researchers focused on compounds that could be candidates for further investigation or regulation, rather than attempting to assess every possible contaminant.

Rather than conducting new experiments on farmland, the team brought together findings from existing scientific studies to assess the behavior of the chemicals.

Three key questions

They investigated three main questions: where a chemical is likely to go after biosolids are applied to land; how long it is likely to remain in the environment; and whether the concentrations reported in biosolids could be of concern in relation to plants and animals.

Some chemicals are more likely to remain in soil, while others may dissolve in water and move into groundwater or nearby waterways. Their behavior also depends on factors such as temperature, moisture and the presence or absence of oxygen.

Understanding these differences is important because it helps determine where monitoring should take place and which environmental pathways may need attention.

"The first question was what happens to these chemicals once they are applied to the land in the form of biosolids. Do they go into the groundwater? Do they evaporate into the atmosphere, or do they just stay in the soil?

"The second was how long does each chemical stay in the environment?"

The study showed considerable variation between chemicals.

Some break down relatively quickly, while others can remain in soil or water for months or, in some cases, considerably longer. That is important to know because chemicals that persist may have more opportunity to accumulate if biosolids are applied repeatedly.

The researchers compared reported concentrations in biosolids with existing scientific benchmarks for effects on organisms, such as algae and small aquatic animals. These benchmarks indicate concentrations below which no measurable adverse effect was observed in particular tests.

The comparisons are intended as a conservative screening tool to identify chemicals that may warrant closer investigation. They are not measurements of human exposure or assessments of whether eating food grown on biosolids-treated land is safe.

The team found that concentrations varied considerably between wastewater treatment plants, reinforcing the need for site-specific monitoring rather than assuming all biosolids have the same chemical profile.

The researchers say further work, including testing actual biosolids and soils under local conditions, will be important to better understand which chemicals present a meaningful environmental concern.

Supporting the continued use of biosolids

The research is intended to help identify where further investigation and management may be most useful, rather than discourage the use of biosolids in agriculture.

O'Carroll says there are potential ways to manage contaminants if particular chemicals are found to pose a concern, including changing wastewater treatment processes, changing how biosolids are applied to land and reducing the use of certain chemicals at their source.

"There are management options that can potentially be applied. The design of wastewater treatment plants can be amended, or their operation can be changed slightly to optimize the removal of specific chemicals if required," he said.

"If there are a few select contaminants of concern, those are the ones that we should figure out how to manage. There are a number of options, including minimizing use of those chemicals in products in the first place, but also options further downstream."

Publication details

Calvin He et al., Fate and persistence of emerging chemical contaminants after land application of biosolids: A screening-level risk assessment, Journal of Hazardous Materials (2026). DOI: 10.1016/j.jhazmat.2026.143937

Journal information:
Journal of Hazardous Materials

Provided by
University of New South Wales

Citation: Chemicals in sewage-based fertilizer may persist in farmland soil for years (2026, November 5) retrieved 5 November 2026 from https://phys.org/news/2026-11-chemicals-sewage-based-fertilizer-persist.html