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:
· 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.
· 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)
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)