top of page

What Is Sewage Sludge?

What is Sewage Sludge? Is it a safe feedstock for a pyrolysis/biochar plant?

A backgrounder on Saratoga Biochar, Feb. 2024


Sewage sludge is the semi solid residue from after raw sewage is treated at a
wastewater treatment plant, also known as a sewage treatment plant.
Sewage sludge has been re-branded as biosolids by the trade association of
wastewater treatment plants. Biosolids sounds less threatening than sewage sludge.
Regardless of what interested parties may claim, they are the same exact material.
A whole book has been written about this re-branding of sewage sludge. It’s called
“Toxic Sludge is Good for You: Lies, Damn Lies, and the Public Relations
Industry,” (2002).


Sewage sludge contains many pollutants. It contains the remains of human
excrement which is broken down by bacteria as part of the treatment process. But
human wastes are only one of the components of sewage sludge.
Many factories and other industrial facilities release chemical-laden wastewater
into sewer pipes. This includes accidental releases as well. Also numerous
wastewater treatment plants accept enormous quantities of landfill leachate, which
contains high concentrations of hazardous pollutants, including PFAS “forever


The title of a 2018 report by the US EPA Office of the Inspector General highlights
our worries. It’s entitled “EPA Unable to Assess the Impact of Hundreds of
Unregulated Pollutants in Land-Applied Biosolids on Human Health and the
Environment.” To clarify, sewage sludge biosolids that are land-applied are no
different than other sewage sludge.


Wastewater treatment plants are not equipped to remove or detoxify harmful
chemicals. That’s just not what they were designed to do. So whatever pollutants
come into a wastewater treatment plant either end up in the sewage sludge or in the
effluent which is discharged into a water body, like the Hudson River.


Currently sewage sludge in the U.S. is either trucked to landfills, burned in an
incinerator, or spread as a fertilizer on farmland or other types of land.
Landspreading sewage sludge disperses toxicity, often resulting in contaminated

wells. It can also threatens our food supply and the health of farm families and
farm workers.


Two states have banned the use of sewage sludge as fertilizer. The Maine
legislature banned this practice in 2022 and Washington state banned it in 2024. In
Maine, farms on which sewage sludge was spread 30 or 40 years ago were found
to have extremely high levels of toxic PFAS “forever chemicals” in the soil, water,
crops, and livestock, leading to broad support for such a ban. In 2022, Maine also
established a $60 million farmer compensation fund to assist those whose farms
have been impacted by this practice, which often predated the current farm family.
EPA has long had extremely weak standards for chemicals in sewage sludge. The
federal government requires that wastewater treatment plants test their sewage
sludge for 9 heavy metals and E. coli. Some states, including NY, now require
testing for a few PFAS “forever chemicals” in sewage sludge before it is used as a


PFAS chemicals have been linked to cancers, thyroid disorders, ulcerative colitis,
and reproductive and developmental problems in humans. There are around 15,000
PFAS compounds, but NYS DEC only has limits for 2 of them and new state limits
in sewage sludge allow very high levels of PFAS that are not health protective for
sludge to be “recycled” as a fertilizer. DEC only requires testing for 2 of the 15,000
PFAS compounds in existence.

bottom of page