Bloom often fields questions that are rooted in confusion about biosolids. It’s understandable, because there are many different types of biosolids products, intended for different applications. Every wastewater treatment plant has different influent (meaning the sources of wastewater) and different treatment methods, therefore the end products will vary too, while also having similarities, like all biosolids generally being rich in organic matter and nitrogen. It’s important to understand the distinctions when assessing a biosolids product.
What’s in a Name? Biosolids versus Sludge
According to the US Environmental Protection Agency (EPA), the terms “biosolids” and “sewage sludge” are often used interchangeably by the public. However, the EPA typically uses the term biosolids to mean wastewater that has been treated to meet the requirements defined in its regulation 40 CFR. Part 503, Standards for the Use or Disposal of Sewage Sludge, and are intended to be used as a soil conditioner or fertilizer.
The EPA’s reference page states, “Biosolids can be used on agricultural land, forests, rangelands, disturbed land in need of reclamation, or nonagricultural lands like parks, golf courses, and home lawns and gardens. Land application of sewage sludge can have environmental benefits including improved soil health, carbon sequestration, and reduced demand on non-renewable resources like phosphorus. Land application also generates reduced emissions of greenhouse gases compared to other management practices.”
The term “sludge” is also used to refer to biosolids, but can encompass a much boarder range of materials which includes municipal, industrial and commercial residuals.
Class A, Class B, and Class A Exceptional Quality
Biosolids are frequently referred to as a Class A or Class B product. These classes are set by the EPA and refer to the degree to which a biosolids product is treated for potential contaminants.
Class A and Class B biosolids are both treated sewage sludge approved for land application, but they differ in pathogen levels and vector attraction reduction (measures taken to minimize the attraction of disease-carrying organisms), leading to different use and application restrictions.
Class A biosolids, including Exceptional Quality (EQ) biosolids, undergo more stringent treatment and are considered safe for unrestricted use, including residential applications and edible crops, while Class B biosolids require specific management practices to mitigate potential risk, and are not available to the public.
Class A Exceptional Quality: Sometimes referred to as “EQ,” Class A Exceptional Quality biosolids meet the EPA’s most stringent pollutant, pathogen, and vector attraction reduction limits. Once biosolids are treated to be Class A EQ there are no additional requirements for land application. Thus, Class A EQ biosolids are often sold directly to the public for use in home gardens and on lawns in stores or directly from the wastewater treatment plant.
According to a 2018 national biosolids survey, only 11% of biosolids are Class A EQ products marketed and distributed to the public, including DC Water’s Bloom and Milorganite. The country’s oldest commercially available product (more than 100 years), Milorganite is created from Milwaukee’s wastewater and distributed through big box retailers nationwide.
DC Water’s product began earning the Class A EQ distinction 10 years ago, when it was the first in North America to install a thermal hydrolysis system called CAMBI. This Norwegian technology works like a pressure cooker, heating the wastewater solids beyond pasteurization and pressurizing them at seven times the atmospheric pressure – a process pathogens cannot survive. Across the board, Bloom generally far exceeds the EPA’s most stringent criteria for pathogens and metals, putting it on par with other organic soil amendments or fertilizers on the market.
Class A: Class A biosolids must meet specific pathogen reduction requirements, eliminating detectable levels of pathogens like fecal coliform and salmonella, and meet specific pollutant concentration limits, particularly for heavy metals. Class A biosolids must be treated to reduce their attractiveness to vectors such as rodents or insects that could transmit disease. Class A biosolids are commonly produced through processes like composting, heat-drying or anaerobic digestion.
Class B: Class B biosolids have detectable levels of pathogens but undergo a process to reduce them to levels that are often below those found in animal manures. They require specific management practices, such as site restrictions, buffer zones, and incorporation into the soil, to minimize potential risks. Class B biosolids are commonly used for agriculture, forestry and land reclamation applications. States generally require site permits for use of Class B biosolids.
Only an estimated 11% of biosolids nationwide are Class A Exceptional Quality products distributed to the public, like Bloom.
Restrictions on Edible Crops Grown with Biosolids
There are no restrictions for growing or harvesting crops grown with Class A EQ biosolids. If Class B treated biosolids are used for food crops, the EPA has issued a number of restrictions, largely around how soon the crops can be harvested after application, in some cases restricting harvest for as long as 38 months after application.
In addition to the EPA’s biosolids regulations, specific states or localities may have additional requirements, permitting restrictions or other criteria, such as the Maryland Department of the Environment’s 2024 regulations issued on PFAS limits for biosolids. For more information about classes of biosolids and land application in your area, please contact your regional EPA office and/or state department of environment.
Learn more at the EPA’s web page, including information about the different types of biosolids land applications, including agriculture, reclamation sites, and forestry.
Industrial Impacted vs. Residential Biosolids
In addition to differences in treatment processes, the inputs into the wastewater system also determine the nature of a biosolids product. This distinction is important when considering a potential contaminant like PFAS, as there are significant differences between biosolids derived from residential and industry wastewater sources.
Municipal wastewater, mostly from residential properties, has low concentrations of PFAS chemicals from the products we use in our homes, but at much lower concentrations than wastewater solids from industrial dischargers (thousands of times higher). Soils amended with municipal biosolids show PFOS and PFOA levels comparable to soils where no biosolids have been applied. In contrast, farms with high PFAS concentrations in their soils were often found to be treated with industrial solids known to have highly concentrated PFAS directly attributable to the industry. Learn more.
