Story at-a-glance

• Sewage sludge, a waste product that comes from wastewater treatment facilities, is widely applied to agricultural fields as a fertilizer
• The EPA identified 352 pollutants in biosolids including metals, pharmaceuticals and flame retardants
• In a report from the U.S. Office of Inspector General, it’s concluded that the EPA is unable to assess the impact of the hundreds of unregulated pollutants applied to land via biosolids on human health and the environment
• When researchers evaluated 31 fields that had applications of sewage sludge, microplastics were found in the samples, with levels increasing on fields with higher rates of sludge applications
• While the EPA claims sewage sludge is safe, the long-term effects of sewage sludge on human health and the environment are unknown

If you've never heard of sewage sludge — a term often used interchangeably with biosolids — you're in for a surprise, as this waste product, which is every bit as unappealing as it sounds, is applied to farmland, gardens, schoolyards, lawns and more across the U.S. The food you eat may very well have come from land treated with sewage sludge, which could have implications for human health and the environment.

What exactly is sewage sludge? When wastewater and stormwater enter wastewater treatment facilities, the solid and liquid waste are separated. The solids are "digested" using bacteria, treated, dried and then sent to landfills or used for agricultural purposes as "fertilizer."

It may sound shocking, but this practice is allowed and endorsed by the U.S. EPA. It's not only legal but routine to grow food on sewage sludge-treated land, even though the sludge, by definition, can contain any number of toxic chemicals that may not be removed via treatment. The Center for Food Safety explained:1

"These separated processed solids — sewage sludge — contain numerous known and unknown hazardous materials.

This includes everything that is flushed into the sewer system, including: household, medical, chemical, and industrial waste; chemicals and metals that leach from the sewer pipes themselves; and novel materials that are created in the wastewater treatment plant as a result of the combination of chemicals and organic compounds present."

Applying Sewage Waste to Soil 'Defies Common Sense'

In the U.S., 54% of sewage sludge biosolids are used for so-called "beneficial" purposes. Most often this means they're applied to agricultural sites, although small amounts are also applied to forestry sites and reclamation sites, including Superfund and Brownfield lands and urban areas, including park land.

Broken down, it's estimated that 36% of biosolids are used for agricultural purposes while 28% end up in landfills and 15% are incinerated.2

The fact is, humans produce a healthy amount of waste — an estimated 300 million pounds of feces are produced daily by Americans alone, for instance. How to dispose of this biosolid sludge is a vexing problem worldwide, so theoretically, turning the waste product into a beneficial product like fertilizer makes sense, assuming it could be thoroughly purified.

Therein lies the problem, however. The Guardian quoted former EPA scientist David Lewis, who opposed the use of sewage sludge on cropland. Lewis noted, "Spending billions of dollars to remove hazardous chemicals and biological wastes from water, only to spread them on soil everywhere we live, work and play defies common sense."3

While it's true that sewage sludge contains similar ingredients to synthetic fertilizer, such as nitrogen and phosphorus, it also contains countless other pollutants that are byproducts of modern-day life. As noted by The Guardian:4

" … [T]he excrement from which sludge derives has mixed with any number of 80,000 manmade chemicals that are discharged from industry's pipes or otherwise pumped into the sewer system.

By the time the mix lands in treatment plants, it can teem with pharmaceuticals, hormones, pathogens, bacteria, viruses, protozoa and parasitic worms, as well as heavy metals like lead, cadmium, arsenic or mercury. It often includes PCBs, PFAS, dioxins, BPAs and dozens of other harmful substances ranging from flame retardants to hospital waste."

It's worth noting that while sewage sludge used to be disposed of primarily by burning it or releasing it into the ocean, this practice was banned over concerns that it would pollute the air and water. But spreading it onto soil has somehow received a safety approval from regulatory agencies,5 including the EPA, which describes them as purely beneficial:6

"They [biosolids] are nutrient-rich organic materials resulting from the treatment of domestic sewage in a treatment facility. When treated and processed, these residuals can be recycled and applied as fertilizer to improve and maintain productive soils and stimulate plant growth."

EPA Identified 352 Pollutants in Sewage Sludge

As part of the Clean Water Act, the EPA must review biosolids standards every two years. Technically speaking, the EPA refers to sewage sludge that has gone through treatment and meets EPA standards for land application as "biosolids."

Part of the review includes identifying pollutants that are present. Based on these biennial reviews and three national sewage sludge surveys, the EPA identified 352 pollutants in biosolids,7 including the following:8

Metals

Dioxins and dioxin-like compounds

Inorganic ions

Certain organics (i.e., polycyclic aromatic hydrocarbons, semivolatiles)

Polybrominated diphenyl ethers (flame retardants)

Pharmaceuticals

Steroids and hormones

Per- and polyfluoroalkyl substances

In a report from the U.S. Office of Inspector General (OIG) released November 2018, it's concluded that the EPA is unable to assess the impact of the hundreds of unregulated pollutants applied to land via biosolids on human health and the environment.9

The report was the result of an audit conducted by OIG to determine whether the EPA has controls over the application of biosolids to land in order to protect human and environmental health.

OIG concluded that the EPA "lacked the data or risk assessment tools needed to make a determination on the safety of 352 pollutants found in biosolids" and noted that 61 of the identified pollutants are "acutely hazardous, hazardous or priority pollutants in other programs."10

EPA Biosolids Program Not Protecting Public Health

Further, while the EPA could conduct full risk assessments to gauge biosolids risks, it is not required to do so. Overall, OIG found that the EPA biosolids program was likely not protecting public health and the environment:11

"The EPA has reduced staff and resources in the biosolids program over time, creating barriers to addressing control weaknesses identified in the program.

Past reviews showed that the EPA needed more information to fully examine the health effects and ecological impacts of land-applied biosolids. Although the EPA could obtain additional data to complete biosolids risk assessments, it is not required to do so.

Without such data, the agency cannot determine whether biosolids pollutants with incomplete risk assessments are safe. The EPA's website, public documents and biosolids labels do not explain the full spectrum of pollutants in biosolids and the uncertainty regarding their safety.

Consequently, the biosolids program is at risk of not achieving its goal to protect public health and the environment."

Research from the U.S. Geological Survey (USGS) has also shown household chemicals and drugs are found in biosolids originating from wastewater treatment plants.12 The researchers purchased or obtained nine different biosolids and analyzed them for 87 organic chemicals, finding 55 were detected in measurable amounts and as many as 45 were found in a single sample.

Plastics in Sewage Sludge

Researchers have also looked into how polyester microfibers may be affecting microorganisms in the soil, especially since sewage sludge is loaded with microfibers.13 They found that the microplastics did, indeed, lead to changes in the soil, including altering the bulk density, water-holding capacity and microbial activity.

Writing in the journal Environmental Science & Technology, researchers noted that wastewater treatment plants act as receptors for the "cumulative loading of microplastics." The solids and liquids are separated using a settlement process, which results in the majority of microplastics (MP ending up in sewage sludge.

Different methods of treatment affected the end number of particles found in the sludge, but the study found microplastic amounts ranging from 4,196 to 15,385 particles kg–1 (dry weight) in sludge samples.14

The researchers noted, "This study highlights the potential for sewage sludge treatment processes to affect the risk of MP pollution prior to land spreading and may have implications for legislation governing the application of biosolids to agricultural land."

Microplastics may act like sponges for contaminants including heavy metals, persistent organic pollutants, polychlorinated biphenyls (PCBs) or pathogens, for instance, and may cause harm on a cellular or subcellular level,15 raising serious questions about the risks of exposing soil to them.

Indeed, wastewater treatment plants are efficient at removing microplastics from sewage, but they become trapped in the sludge. This helps keep them out of waterways, unless they're applied to agricultural soils (which may run off into waterways).

When researchers evaluated 31 fields that had applications of sewage sludge, microplastics were found in the samples at levels ranging from 18 to 41 particles g−1, with a median of 34 particles g−1.16 What's more, the microplastic levels increased on fields with higher rates of sludge applications.

"Our results indicate that microplastic counts increase over time where successive sludge applications are performed," the researchers noted, adding, "Sludge is proposed as a primal driver of soil microplastic pollution."17

Sewage Sludge Contaminating Farms

Sewage sludge is passed off as a cost-effective fertilizer for farmers, but some have lost their livelihoods after the toxic waste contaminated their farms. One such farmer is Fred Stone in Maine, who applied biosolids to his hayfields intended to feed his dairy cattle for decades, not knowing it could be contaminated with PFAS, chemicals associated with cancer, liver damage, low birth weight and hypothyroidism.

Milk from Stone's cows later tested positive for PFAS, forcing him to dump hundreds of gallons of milk a day.18 In March 2019, the Maine Department of Environmental Protection added a requirement to test sewage sludge for PFAS before it's applied to land.19

This is just the tip of the iceberg, as long-term application of sewage sludge also increases the abundance and diversity of antibiotic resistance genes in soil.20 In a study from the University of York in the United Kingdom, data even revealed plants suffer when biosolids are applied to the soil.21

Even with low-level exposure, the drugs studied interfered with plant hormones that support defense against predators and diseases. The drugs also damaged the plants' ability to make energy from sunlight, and at higher concentrations the research team saw a drop in the leaves' levels of chlorophyll. At high concentrations, the plants experienced stunted roots and burnt edges on the leaves.

How to Avoid Biosolids

Foods grown on biosolid-treated soil are not labeled as such, so your best bet for avoiding them is to support sustainable agriculture movements in your area. Make it a point to only buy food from a source you know and trust — one using safe, nontoxic organic or biodynamic farming methods.

If you grow your own food, also be aware that companies do not have to disclose when biosolids are used, so there's really no way of knowing what's in your bag of potting soil or compost. Composted products can have the USDA organic label on them and still be loaded with toxic biosolids.

If you see "milogranite" on the label, it contains biosolids from the City of Milwaukee — a national distributor. Your best bet is to buy organic potting soil and/or compost from a local nursery you know and trust, that can guarantee no biosolids have been added.