Using sewage sludge – one of two end products from any wastewater treatment plant (the other is effluent) – as fertilizer on food crops is a hotly debated food safety issue, but only among a very small group of people.  Most likely, the majority of Americans who are even aware of the issue actually work in the sewage industry.

Despite sludge’s relative obscurity, the newly formed Food Rights Network, founded by John Stauber, former Executive Director of the Center for Media and Democracy and author of the book “Toxic Sludge is Good For You,” is taking on sewage sludge as its flagship issue. Simply put, the group says that it is not safe to grow food in sewage sludge.

Why isn’t it safe?

Sewage sludge regularly tests positive for a host of heavy metals, flame retardants, polycyclic aromatic hydrocarbons, pharmaceuticals, phthalates, dioxins, and a host of other chemicals and organisms.  Of the thousands of contaminants that have been found in sludge, the U.S. government regulates exactly 10 of them (nine heavy metals and fecal coliform) if you want to spread the sludge on farm fields growing food crops.

When industry, hospitals, and households send their waste to wastewater treatment plants, the plants remove as many contaminants as possible from the water and then discharge the water as effluent.  The leftover solids are sludge.

Sewage sludge is typically treated to remove some–but not all–of the contaminants.  In recent decades, the sludge lobby (yes, there is one) has rebranded the treated sludge as “biosolids.”

Sludge that is applied to farmland–or even golf courses, home gardens, and, in the past, the White House lawn–comes in two flavors: Class A Biosolids and Class B Biosolids.  The only regulatory difference between the two is the level of fecal coliform, which is lower in Class A.

Class B Biosolids may be applied only to land where crops fed to animals are grown.  No restrictions apply to Class A Biosolids.   You as a home gardener can even buy these at your local gardening store and grow your own food in them.  Various cities get very creative at “branding” their sludge, so that gardeners can choose between “Milorganite” from Milwaukee, “Hou-Actinite” from Houston, or “GroCo” from Seattle.

Often, but not always, the various contaminants are found in sewage sludge at low levels. What happens to them once the sludge is applied to the soil is anyone’s guess.  Some chemicals bind to the soil; others do not.  Some chemicals leach into groundwater; others are insoluble in water.

Some chemicals are taken up by plants–perhaps into the roots only, or into leaves, or all the way into fruits.  Some chemicals break down into harmless components, others break down into dangerous components, and others don’t break down at all.

Understanding the path that low levels of thousands of chemicals take in the environment is a daunting task.

Once a contaminant makes its way from sewage sludge to soil, and into the human food supply, what happens?

Again, it depends.

Some chemicals are stored in the human body, and others pass through it.  Some break down in our digestive system and others don’t.  And each person is different, with a different body size, stage of development, and metabolism.  The same chemical may wreak devastating effects if a pregnant woman eats it but go unnoticed if eaten by a man.

Consider also the interaction between the many contaminants individuals may be exposed to if they regularly eat food grown in sludge.  While the effects of individual chemicals are often studied, less is known about the interactions between low levels of large numbers of chemicals.

To provide a more concrete example, take the chemical triclosan.  It has been used for several decades in antibacterial products like soaps, deodorants and cosmetics.  It is also nearly universally found in sewage sludge.  A recently published study found that soybeans planted in soil containing triclosan took the triclosan up into their beans.

Triclosan is a suspected endocrine disruptor and recent CDC reports show more than a 40 percent increase in triclosan levels in the urine of Americans over a recent two-year period.  The amount in our bodies isn’t entirely due to sewage sludge; humans can absorb triclosan through their skin and those who use triclosan-containing toothpastes put the chemical directly into their mouths.

Scientists are also finding that triclosan breaks down into dioxins in the environment.

A more extreme example of sewage sludge posing health risks

Andy McElmurray was a Georgia dairy farmer who accepted sewage sludge to fertilize his fields where he grew food for his cows over many years.  As the years went by, he noticed that his land was becoming more and more acidic.  McElmurray applied lime to raise the pH of his soil.  Soon after he did so, his cows became sick.

After many tests, he traced the cows’ illnesses back to the sludge.  The sludge he had applied contained high levels of molybdenum, cadmium, and thallium.  Molybdenum and cadmium are regulated in sewage sludge, whereas thallium is not.  

When McElmurray applied the lime, the contaminants became more bioavailable to the plants, and the cows ate the plants.  His cows suffered from telltale signs of molybdenum poisoning, and their milk was contaminated with thallium (a rat poison toxic to humans in small doses).

By the time the cows’ illnesses were traced to the sludge, it was too late to save McElmurray’s farm.  Worse, both McElmurray and his father became sick themselves from breathing sewage sludge dusts blowing from their fields.  

In this case, the Augusta, GA wastewater treatment plant that provided the sludge fudged its numbers and broke the law, providing McElmurray with sludge that contained higher levels of heavy metals than allowed.  In the end, however, scientists also found that if the wastewater treatment plant had followed the law and limited molybdenum to the legal levels, McElmurray’s cows still would have gotten sick.

Sludge Regulation

Given the complexity of the many chemicals found in sewage sludge–and consider that each wastewater treatment plant’s sludge is different over time and different from another plant’s sludge–how could sewage sludge be regulated in such a way that it is safe to use?

Regulators must consider that humans will be exposed to any contaminant in sludge in several ways.  McElmurray and his father became ill after inhaling sewage sludge dusts.  Gardeners who use sewage sludge as fertilizer touch the soil directly, and small children may even eat soil.  If it could be done, regulating sewage sludge to guarantee safety and then following through with those regulations would be infinitely difficult and expensive.

Currently, sewage sludge is disposed of via landfills and incineration as well as land application. According to the EPA, about half of all sewage sludge is applied to land, but it is only applied to about one percent of the nation’s farmland.  The likely result is that, if dangers do lurk in the sludge applied to land, we rarely find out about them.

Most people’s chances of eating enough tainted food from farms that apply sewage sludge as fertilizer to cause an acute reaction are pretty slim.  The chance that anyone who got sick would be able to correctly trace his or her illness back to the farm and to sewage sludge is even smaller.  However, a lack of easily traceable acute illnesses does not prove that sewage sludge is safe.  Health harm due to exposure to low levels of toxins over a long period of time is no more acceptable than acute problems, even if they are less obvious.

As a consumer, the only sure way to avoid food grown in sewage sludge is to buy organic food (or grow your own).  If you are a gardener and you wish to avoid sewage sludge fertilizers or composts, avoid any product that says it contains “biosolids.”  Last, if you wish to keep sewage sludge from being spread on farm fields near where you live, you can take action locally to make it illegal in your city or county.

Editor’s Note:  Jill Richardson has written about sewage sludge for the Center for Media & Democracy.