Biosolids is a term coined in the United States that is typically used to describe several forms of treated sewage sludge that is intended for agricultural use as a soil conditioner. Although sewage sludge has long been used in agriculture, concerns about offensive odors and disease risks from pathogens and toxic chemicals may reduce public acceptance of the practice. Modern use of the term biosolids may be subject to government regulations, although informal use describes a broader range of semi-solid organic products separated from sewage.
Description of biosolids in conformance with local regulations may reduce confusion; but some use an expanded definition including any solids, slime solids or liquid slurry residue generated during the treatment of domestic sewage including scum and solids removed during primary, secondary or advanced treatment processes. Use of alternative terms like solids or wastewater solids may be preferable for non-conforming biosolids.
Biosolids may be defined as organic wastewater solids that can be reused after suitable sewage sludge treatment processes leading to sludge stabilization such as anaerobic digestion and composting.
Alternatively, the biosolids definition may be restricted by local regulations to wastewater solids only after those solids have completed a specified treatment sequence and/or have concentrations of pathogens and toxic chemicals below specified levels.
The United States Environmental Protection Agency (USEPA) defines the two terms - sewage sludge and biosolids - in the Code of Federal Regulations (CFR), Title 40, Part 503 as follows: sewage sludge refers to the solids separated during the treatment of municipal wastewater (including domestic septage), while biosolids refers to treated sewage sludge that meets the USEPA pollutant and pathogen requirements for land application and surface disposal. A similar definition has been used internationally.
Approximately 7,100,000 dry tons of biosolids were generated in 2004 at approximately 16,500 municipal wastewater treatment facilities in the United States.
In the United States, as of 2013 about 55% of sewage solids are turned into fertilizer, despite demand from farmers who wish to buy more. Challenges to increased levels of recycling include capital needed to build digesters, the complexity of complying with health regulations, and avoiding neighbors who object to unpleasant smells. There are also new forms of contaminants in urban sewage systems which make the process of producing high quality biosolids more complex. These have led some municipalities to ban biosolids on farms and even in forests.
Encouraging agricultural use of biosolids is intended to prevent filling landfills with nutrient-rich organic materials from the treatment of domestic sewage that might be recycled and applied as fertilizer to improve and maintain productive soils and stimulate plant growth. Biosolids may contain macronutrients nitrogen, phosphorus, potassium and sulphur with micronutrients copper, zinc, calcium, magnesium, iron, boron, molybdenum and manganese.
Industrial and man-made contaminants
The United States Environmental Protection Agency (USEPA) and others have shown that biosolids can contain measurable levels of synthetic organic compounds, radionuclides and heavy metals. USEPA has set numeric limits for arsenic, cadmium, copper, lead, mercury, molybdenum, nickel, selenium, and zinc but has not regulated dioxin levels.
So-called "contaminates of emerging concern" may also be present in biosolids. The United States Geological Survey analyzed in 2014 nine different consumer products containing biosolids as a main ingredient for 87 organic chemicals found in cleaners, personal care products, pharmaceuticals, and other products. These analysis detected 55 of the 87 organic chemicals measured in at least one of the nine biosolid samples, with as many as 45 chemicals found in a single sample.
In the United States the USEPA mandates certain treatment processes designed to significantly decrease levels of certain so-called indicator organisms, in biosolids. These include, "...operational standards for fecal coliforms, Salmonella sp. bacteria, enteric viruses, and viable helminth ova."
USEPA has also classified other pathogens that can appear in biosolids such as various protozoa, bacteria, viruses, and prions as "pathogens of emerging concern". EPA regulations allow only biosolids with no detectable pathogens to be widely applied; those with remaining pathogens are restricted in use.
In the United States Code of Federal Regulations (CFR), Title 40, Part 503 governs the management of biosolids. Within that federal regulation biosolids are generally classified differently depending upon the quantity of pollutants they contain and the level of treatment they have been subjected to (the latter of which determines both the level of vector attraction reduction and the level of pathogen reduction). These factors also affect how they may be disseminated (bulk or bagged) and the level of monitoring oversight which, in turn determines where and in what quantity they may be applied.
As public concern arose about disposal in the United States of increasing volumes of solids being removed from sewage during sewage treatment mandated by the Clean Water Act, the Water Environment Federation (WEF) sought a new name to distinguish the clean, agriculturally viable product generated by modern wastewater treatment from earlier forms of sewage sludge widely remembered for causing offensive or dangerous conditions. Of three-hundred suggestions, biosolids was attributed to Dr. Bruce Logan of the University of Arizona, and recognized by WEF in 1991.
- Milorganite is the trademark of a biosolids fertilizer produced by the Milwaukee Metropolitan Sewerage District. The recycled organic nitrogen fertilizer from the Jones Island Water Reclamation Facility in Milwaukee, Wisconsin, is sold throughout North America, reduces the need for manufactured nutrients.
- Loop is the trademark of a biosolids soil amendment produced by the King County Wastewater Treatment Division. Loop has been blended into GroCo, a commercially available compost product, since 1976. Several local farms and forests also use Loop directly.
- TAGRO is short for "Tacoma Grow" and is produced by the City of Tacoma, Washington since 1991.
- Dillo Dirt has been produced by the City of Austin, Texas since 1989.
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