Biosolids Safe for Land Application
Study shows absence of pathogens
Arizona Agricultural Experiment Station Research Report
For at least two decades, wastewater treatment plants have produced Class
A and B biosolids, the organic residues that result from specific treatment
Biosolids are frequently used for land application on cropland, pastures
or timberland, where they decompose while furnishing nitrogen, phosphorus
and potash to growing plants. This method
offers a more ecologically sound and practical alternative to domestic
waste disposal than landfills or incineration, where water or air pollution
Over the past two years questions have arisen over whether Staphylococcus
aureus, a human disease pathogen present in raw sewage, remains in treated
biosolids and potentially causes illness following its application to
soil. A medically important pathogen, S. aureus causes a wide variety
of human skin and wound infections, food poisoning, septicemia, toxic
shock syndrome, pneumonia, meningitis, and other infections.
In a recent study, scientists at the University of Arizona have produced
evidence that S. aureus is not present in biosolids. Their report appears
in the journal Environmental Science and technology.
Chemically and biologically different from raw sewage, biosolids must
meet Part 503 of the federal EPA standards regarding pathogen and heavy
metal content, handling and application precautions, and other regulations.
Air samplers at work in a field near
Sixty percent of all biosolids are land-applied in the United States,
but this amount covers less than 0.1 percent of agricultural land,
says Ian Pepper, a professor in the UA Department of Soil and Water Science
and director of the UA National Science Foundation Water Quality Center
(WQC) (see sidebar).
This center has gained national recognition, and WQC studies on land
application of biosolids are being utilized by EPA as a response to a
2002 National Academy Science Report on land application.
In July 2002, after an 18-month study, the National Academy of Sciences
(NAS) issued a report stating there is no documented, scientific
evidence that the part 503 rule has failed to protect public health regarding
land application of biosolids. At the same time, the Academy noted
that additional scientific work is needed to reduce persistent uncertainty
about the potential for adverse health effects from exposure to biosolids.
Since no scientific data were available to document whether biosolids
specifically contain S. aureus, Pepper and colleagues Patricia Rusin,
Sheri Maxwell, John Brooks and Charles Gerba conducted biosolid and bioaerosol
studies on samples from 15 different sites across the United States.
As the saying goes, Absence of evidence isnt evidence
of absence, Pepper says. Our study focused on finding
the scientific evidence regarding the presence or absence of S. aureus
in biosolids and bioaerosols.
An applicator called a Slinger
catapults biosolids into the air as part of study in Mojave, Arizona.
The sampling sites ranged from the East coast to the Southwest, and all
were full-scale treatment plants; no pilot plants were included in the
study. The researchers took samples of raw sewage and untreated primary
sewage sludge in sterile bottles and transported them on ice to their
laboratory. Biosolid samples were collected in sterile containers at the
production site and likewise
transported overnight to the laboratory on ice. Each sample was assayed
for S. aureus the day it was received.
Pepper and the team collected the bioaerosol samples from four different
sites in the Southwestern United States using commercial land applicators.
We evaluated the potential for bioaerosols from biosolids with
a higher solids content using applicators called slingers,
which literally sling biosolids 80 to 100 feet through the air,
For liquid biosolids (lower solids content) the material was sprayed
from a tanker. In either case we had aerosol samplers hooked up to pumps
so that known volumes of air were sucked into a collection fluid, which
is later analyzed using cultural assays.
The scientists analyzed all bioaerosol samples for S. aureus within 24
hours of collection.
In all, the team analyzed three raw untreated sewage samples and two
undigested primary sewage sludge samples, 23 different biosolid samples,
and 27 aerosols obtained during biosolid land application (biosolid aerosols).
We detected S. aureus in samples of raw sewage and undigested
primary sewage sludge, the scientists state in their report. However,
we did not detect S. aureus in Class A or Class B biosolids after aerobic
or anaerobic digestion, lime stabilization, heat-dry pelleting, and/or
composting. These are conventional methods that treatment plants
use to remove disease-causing organisms from raw sewage.
You can find S. aureus in sewage and you should be able to because
one in three people have it in their systems, Pepper says. Yet
it should be noted that none of the biosolid or biosolid aerosol samples
in our study were positive for S. aureus. The most likely explanation
is that wastewater treatment kills S. aureus along with other pathogenic
Pepper notes that allegations regarding the safety of biosolids are often
not based on good science.
Overall we need more scientific studies to resolve potential issues
of concern, Pepper says. Our study was science-based and indicates
that biosolids are an unlikely source of S. aureus.
Spray tanker spreads biosolids on farm
field in Marana, Arizona.
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Published January 2004
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