Pathogens which adversely affect human
health, including bacteria, protozoa, helminths, and viruses, exist within
sewage. Sludge formed during wastewater
treatment contains concentrated levels of these pathogens. Treated sludge applied as a soil additive
provides beneficial nutrients to fields, which can greatly enhance the growth
rate of the vegetation. If not treated,
the pathogens within sludge can pose serious health threats to humans and
animals. Pathways of exposure (as
listed in “Control of Pathogens and Vector Attraction in Sewage Sludge (1999
Edition)”) include:
Direct Contact:
· Touching the sewage sludge.
· Walking through an area- such as a field,
forest, or reclamation area- shortly after sewage sludge application.
· Handling soil from fields where sewage
sludge has been applied.
· Inhaling microbes that become airborne
(via aerosols, dust, etc.) during sewage sludge spreading or by strong winds,
plowing, or cultivating the soil after application.
Indirect Contact:
· Consumption of pathogen-contaminated crops
grown on sewage sludge- amended soil or of other food products that have been
contaminated by contact with these crops or field workers, etc.
· Consumption of pathogen-contaminated milk
or other food products from animals contaminated by grazing in pastures or fed
crops grown on sewage sludge-amended fields.
· Ingestion of drinking water or
recreational waters contaminated by runoff from nearby land application sites
or by organisms from sewage sludge migrating into ground-water aquifers.
· Consumption of inadequately cooked or
uncooked pathogen-contaminated fish from water contaminated by runoff from a nearby
sewage sludge application site.
· Contact with sewage sludge or pathogens
transported away from the land application or surface disposal site by rodents,
insects, or other vectors, including grazing animals or pets.
The threat of exposure to pathogens by
direct or indirect contact with sewage sludge can be significantly reduced
through treatment. Reducing pathogen
levels while maintaining the beneficial nutrient value of the sludge is the
goal of sludge treatment. Some
treatment technologies include (as listed in “Control of Pathogens and Vector
Attraction in Sewage Sludge (1999 Edition)”):
· Application of high temperatures
(temperatures may be generated by chemical, biological, or physical processes
· Application of radiation
· Application of disinfectants
· Reduction of the sewage sludge’s volatile
organic content (the microbial food source)
· Removal of moisture from the sludge
References and Related Material:
Much of the information on this page was taken
from a guidance document on the topic of pathogens in sewage sludge prepared by
Eliot Epstein and Nerissa Wu of E&A Environmental Consultants (pdf format):
Control of Pathogens
and Vector Attraction in Sewage Sludge (1999 Edition)
Click
here if you do not have Adobe Acrobat Reader
The company that Eliot Epstein co-founded
and currently is the president of: E&A
Environmental Consultants, Inc.
A very good summary of how sludge is
produced, how sludge is applied to land, and some potential problems with sludge
application: Land
Application of Sludge
(Page
created by David
Senus. Last revised: February 11,
2001)