More than $2 billion is spent annually in the United States managing more than 7 million dry tons of biosolids from the nations municipal wastewater treatment facilities.(See Biosolids Fact Sheets.)The manner in which these treated materials is disposed of or recycled affects everyone.As biosolids programs gain in popularity, public interest in such programs is growing.
The most common concerns related to biosolids have to do with odors, nutrients, pathogens and toxic pollutants.The goal of the vast majority of organizations and individuals associated with the subject of biosolids, from regulators to users to private citizens, is to foster a discussion of objective, supportable information and sound scientific research.
As organic materials decompose into a nutrient-rich soil conditioner, odors are released naturally.Depending upon the treatment process used, odors will range from an objectionable ammonia scent to an earthy smell similar to that of fresh potting soil.Most objectionable odors are caused by compounds that contain sulfur and ammonia, both of which are plant nutrients.The siting, design and operation of biosolids facilities based upon decades of study and experience now routinely provide the necessary safeguards against the release of odorous compounds.
In the production of fertilizer products from biosolids, advanced technology that provides for thermal destruction of odorous compounds in a fully enclosed facility is a proven solution for controlling odors.In composting, managers successfully control odors by maintaining aerobic conditions and directing process air through scrubbers and/or biofilters in an enclosed process.During transport between these controlled facilities, odors are managed through the use of enclosed trucks.
As with all organic residuals, biosolids contain nitrogen, a potential pollutant in any fertilizer.According to EPA, the maximum concentrations of organic pollutants allowable in biosolids are well below the amounts determined to be harmful.As a further safeguard, regulations limit the application of biosolids on agricultural land to strict agronomic loading rates, the calculated nitrogen rates needed by plants to grow.In contrast to the uptake of nutrients in chemical fertilizers, the natural release of organic nitrogen associated with biosolids coincides with a plants natural uptake of nitrogen. This eliminates the leaching of excess nitrates (the mobile form of nitrogen) to groundwater or surface runoff, assuring protection of public waterways.
To meet the strict 40 CFR Part 503 Pathogen and Vector Attraction Reduction Requirements, a combination of digesters, stabilization and high temperatures is used in the treatment and production of biosolids.These disinfecting processes pasteurize and sterilize biosolids to a degree that pathogens are reduced to very low levels and the risk of infection or disease is rendered negligible.The natural environment also adds some assurances of its own.The organic nature of biosolids forces pathogens to adhere to amended soil, effectively immobilizing them.This condition prevents pathogens from entering the groundwater while keeping them near the surface of the soil.Other soil conditionsexposure to sunlight, the relative lack of moisture and naturally occurring enemy microbesdestroy the remaining pathogens.
The risk of adverse health effects is negligible once treated biosolids are applied to land. Decades of experience have identified potential risk areas and, where present, appropriate safeguards have been developed, including prudent material handling practices. These include the use of personal protection equipment and healthful hygienic practices, including such fundamental behavior as regular hand-washing.An estimated 100,000 wastewater treatment plant employees working in more than 16,000 wastewater treatment facilities have, to date, demonstrated no adverse health effects attributable to their work environment.These workers are in closest proximity to both untreated and treated wastewater plant residuals.
Metals (including cadmium, lead, copper and zinc) can enter wastewater from industrial drains and metal pipes in homes and businesses.As municipal wastewater treatment and systems have improved over the last 20 years, metal in biosolids have decreased dramatically and typically are well below federal and state standards.Some of these trace metals, known as micronutrients, are necessary in small amounts for plant survival.Biosolids are routinely tested for metal concentrations to ensure compliance with state and federal regulations.
Organic compounds (including pesticides, solvents and polychlorinated biphenyls (PCBs) found in biosolids are present in concentrations near the lowest detectable limits.Studies have found risks to be negligible.As part of an ongoing dioxin reassessment, EPA currently is reassessing standards for dioxin and dioxin-like compounds in land-applied biosolids.On June 12, 2002, EPA published a Federal Register notice of data availability for the Round 2 Part 503 sewage sludge regulations. This document summarizes the new sewage sludge data and risk assessment for dioxin.Based on this revised risk assessment, EPA estimates that the risk of cancer to highly exposed populations is very low.
In 1988, EPA conducted a national survey of biosolids.Sludge and biosolids from municipal wastewater treatment plants nationwide was tested for some 400 different pollutants, most of which were found at very low levels.EPA used the survey and national research data for its comprehensive risk assessment of pollutants in land-applied biosolids.
This assessment, used in developing the 40 CFR Part 503 Biosolids Rule, evaluated human health and environmental risks.EPA compared the relevant toxins exposure data (obtained from a cross-section of representative studies) to the appropriate oral reference dose and human cancer potency values representing the allowable dose of each pollutant.After evaluating 14 possible exposure pathways, environmental and human health risks were found to be very low.