Kitchen Gardening

The Sludge Report: Using Recycled Municipal Wastewater Solids as a Soil Amendment

Should you put compost derived from wastewater solids on your kitchen garden? The answers are hard to come by, but the questions are worth asking.

Since the 1920s, the City of Milwaukee Sewerage Commission has been converting biosolids into Milorganite, a heat-treated pelletized fertilizer.
Photo/Illustration: courtesy Milorganite Lawn & Garden Products

by Marc Vassallo
April 1999
from issue #20

The solid stuff left after municipal wastewater has been treated is rich in organic matter but also contains heavy metals and possibly pathogens and other pollutants, all in amounts deemed acceptable or unacceptable depending upon whom you ask. Advocates call the stuff biosolids, detractors call it toxic waste. More and more of it is being made available to the public. Is this greenwashing or responsible recycling? Should you put compost derived from wastewater solids on your kitchen garden? The answers are hard to come by, but the questions are worth asking.

Let’s begin with Jim Pitts, a process analyst at Seattle’s wastewater treatment facility south of the city in Renton. Pitts took me into the bowels of the plant: down concrete stairways, through labyrinthine tunnels lined with thick pipes. I felt like I was traveling through a vast gastrointestinal tract, which in essence I was. The solids even give off gas—methane—which is recovered and sold to the local energy utility. Mud-colored and never, in fact, more solid than a milkshake, the solids became thicker and thicker along the way, and I had to remind myself that the plant’s primary purpose was to clean Seattle’s wastewater and return it to Puget Sound, and that, excepting the largest debris and grit, every­thing that makes the water unfit for the ocean ends up concentrated in the solids.

I next paid a visit to Curley Winebrenner, whose operation, GroCo, takes some of the digested solids from the Renton plant, mixes them with sawdust (3 parts sawdust to 1 part wastewater solids), and composts the mixture, windrowing it for more than a month at temperatures above 131ºF and then curing the mix for a year. The finished compost, called GroCo, is sold in bulk to landscapers, nurseries, and homeowners. Winebrenner has samples lab-tested once a month, and finished GroCo more than meets the requirements for what the U.S. Environmental Protection Agency (EPA) calls a “Class A biosolids” product: it’s essentially free of pathogens, and its heavy metals content falls well below EPA’s allowable limits. GroCo tests low enough, in fact, to earn the “EQ” label, certifying it as an “exceptional quality” product. The low heavy metals content of GroCo can be attributed to Seattle’s robust wastewater pretreatment program and the small army of investigators who help to ensure that the city’s industries and residences keep many pollutants out of the wastewater stream in the first place.

What happens in the soil?
From the GroCo yard, I went to see Washington State University soil scientist Craig Cogger. Cogger believes that people’s objections to biosolids (to use his preferred term for now) are based on perception and cultural bias—the yuk factor, which he doesn’t discount. But the preponderance of science, he insists, suggests that properly managed wastewater solids are sufficiently safe to use as a soil amendment. Cogger’s field tests have found biosolids to be a reliable, slow-release source of nitrogen. And because these solids contain organic matter, they improve soil structure and tilth.

Cogger’s most intriguing proposition  is that biosolids form a matrix in the soil that actually binds up heavy metals, reducing their availability and slowing their uptake by plants.

Cogger described for me two prevailing theories of what happens when you add biosolids to the soil. The time-bomb theory has it that once the organic matter tying up the metals in bio­solids decomposes, as it inevitably will, all the metals will be released en masse, with predictably dire consequences. The plateau theory postulates that if biosolids are applied at agronomic rates, the availability of metals in the amended soil will plane off at well below levels deemed unacceptable by the EPA. Looking at the literature, Cogger says, “I don’t see any evidence that the time-bomb effect is occurring.” As for the plateau effect, he’s not so sure. “There’s a minority of scientists who are still concerned,” Cogger says, “but I don’t see the long-term studies as supporting them at this point.” Cogger himself uses Tagro, Tacoma’s biosolids product, on his own vegetable garden and has no qualms recommending a certified “Class A, EQ” biosolids product to any gardener for any soil condition.

What to do with wastewater solids?
The situation we now face with municipal wastewater solids is a direct result of our decision to use water as a conveyance to remove the combined wastes of our homes and industries via a single sewerage system. One big pipe for everything. Our more recent decision to clean the water we’ve sullied before returning it to nature—a decision given some teeth by the 1972 Clean Water Act (pdf file)—necessitates the removal of solids from the wastewater stream and the production of sludge. As we have expanded our industrial activities while at the same time demanding cleaner water, we have created ever more sludge with an ever growing list of contaminants in it, despite lower levels of some due to stepped up pretreatment efforts: not just heavy metals and pathogens but PCBs, dioxins, petroleum products, pesticides—every organic and inorganic chemical disposed of by industries, businesses, and households.

Faced with twice as much sludge as in 1972, the municipal wastewater treatment industry has three options: burying the sludge in a landfill, incinerating it, or applying it on the land. A fourth option, ocean dumping, has been banned since 1988.

Dumping sludge in a landfill—still a popular option—raises all of the potential problems afflicting landfills generally: pollutant concentration, groundwater contamination, lack of space, high costs, and the tying up of organic matter we might put to better use. As for incineration, even the best efforts still produce some air  pollution and in any event result in an irreducible amount of potentially toxic ash—and where to put that?

Hence, the move to composting. In 1997, there were 262 operating wastewater solids composting facilities in the United States, an increase in composting activity of 327 percent since 1983. The best-known wastewater solids product is surely Milorganite, not a compost but a heat-treated pelletized fertilizer produced and distributed nationally by the city of Milwaukee since 1926.

Should we put our faith in science?
In 1993, the EPA adopted regulations—commonly referred to as Part 503—that establish minimum standards for the quality of municipal wastewater solids destined to be land-applied. The standards are based on a lengthy risk assessment conducted on behalf of the EPA by numerous government and university researchers. Part 503 stipulates the limits of human pathogens and nine heavy metals in wastewater solids, as well as requirements for the production and use of Class A, Class B, and EQ materials.

Part 503 begs two questions: Are the standards sufficient and are they enforced? For one set of answers, I paid a visit to Al Rubin, who heads up the biosolids program at the EPA. Rubin defines pollution as movement of an element, organic chemical, or pathogen that results in a concentration above levels deemed protective of health and the environment. In other words, nothing is a pollutant per se, unless there’s too much of it in the wrong place at a given time. Or as Rubin puts it, “The dose makes the poison.”

Rubin spoke at length and in great detail about the design of the risk assessments that determined the standards, the assumptions made, the conservative thinking and safety margins built in, the worst-case gardening practices presumed, and so on, making his case that Part 503 is sufficiently protective, and that “biosolids,” as he says, “are wastes that have the potential to be a resource when treated and managed properly.” Rubin is convinced—utterly convinced—that it is possible to apply biosolids to the land at rates such that the concentration of pollutants will forever remain below levels that would cause environmental or health problems. And he claims Part 503 stipulates the acceptable levels with an ample margin of safety.

On my second question, about oversight and enforcement, Rubin is honest enough to concede that this is the weak link in the biosolids policy: EPA simply doesn’t have the resources to enforce Part 503 at all locations. There are, nevertheless, municipalities that can document that the wastewater solids leaving their treatment plant meet EPA standards, and there are municipalities that must meet state or local standards, some of which are more stringent than Part 503. My own conclusion is that you should look carefully into your source of supply should you choose to use a wastewater solids compost.

What really gets under Rubin’s skin is that we’re obsessing about sludge without having taken a hard look at animal manures and synthetic fertilizers, which he feels are sorely in need of the same degree of research, risk assessment, and regulation as municipal wastewater solids. Indeed, he suggests that the EPA’s experience with biosolids would make a good template for dealing with manures. In the meantime, the bad sludge press continues to hit the fan, and all the while, as Rubin insists with fists banging the table, the EPA has never come across a documented case of a negative impact on human health from the land application of biosolids.

Is this a case for caution?
In 1997, the Cornell Waste Management Institute (CWMI) issued a working paper titled, “The Case for Caution,” wherein Ellen Harrison and the other authors took Part 503 to task, concluding that while the land application of sludge is not an unreasonable practice, the need to maintain human health and preserve the agricultural productivity of the land is so great and the potential problems of added contaminants so persistent that much greater caution than is provided for by Part 503 is required.

The 44-page report lists no fewer than 14 assumptions and decisions made in the risk assessment leading to Part 503 that the authors believe to be insufficiently conservative and protective. To me, the most alarming “non-protective aspect” brought to light in the CWMI report is that the risk assessment considered less than 10 percent of the pollutants in sludge.

The CWMI report does not, however, recommend the prohibition of the land application of sludge. But it does lay out recommendations for farm and home use of sludge that are far more conservative than those of Part 503. The report advises that the heavy metals limits stipulated in Part 503 could and should be much lower, more like the limits imposed by some European governments. The report also recommends more stringent standards for home use, “reflecting the greater likelihood of child ingestion of sludge and the potential for less careful management.” The CWMI report is a model of clear, accessible writing about a technical issue and should be required reading for anyone contemplating the use of a wastewater solids compost. [Note: you can download a new document, published in 2008: Case for Caution Revisited: Health and Environmental Impacts of Application of Sewage Sludges to Agricultural Land (pdf).]

I called Harrison to get at the essence of CWMI’s concerns, and also at its motivation for publishing the report, which was greeted with consternation by the wastewater treatment industry.

Harrison wrote the report because CWMI, as part of the land-grant system for New York, is responsible both for advising growers and for safeguarding the long-term agricultural productivity of the state. Harrison reasoned that if Cornell were going to make recommendations to farmers and landowners for the application of wastewater solids, it should also provide accessible information about the issues involved to help them make decisions.

The bottom line for Harrison is that until we know more about the long-term impact of municipal wastewater solids on the land and until we do a better job of eliminating contaminants in these solids, caution should be the rule. “It is wise to be cautious,” the CWMI report says, “since many of these added contaminants are persistent and, once applied, will remain in soils and the ecosystem for many years.”

Should you bring some home?
As “The Case for Caution” points out, asking whether the land application of wastewater solids is safe is like asking: Is it safe to drive a car? The real question on land application is: Is the risk acceptable? I can’t supply the answer, because it depends on your values and beliefs, and on your degree of faith or skepticism in technology and the standards.

No matter how you feel about sludge/ biosolids, call it what you will, I encourage you to consider doing a few prudent things. Insist on specific quality information about any sludge-based product you choose. Press for a scientific inquiry into the constituents of animal manures and synthetic fertilizers, and perhaps for regulation, so you can compare soil amendments on a level field. And consider putting pressure on yourself and on your community to improve the quality of wastewater. Cleaner wastewater coming into the treatment plant means cleaner wastewater solids coming out, some­thing I’m sure none of us would mind.

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