What are biosolids?
Biosolids are the mostly organic solids resulting from the treatment of wastewater. They are rich in nutrients such as nitrogen and phosphorus and contain other supplementary nutrients such as potassium, sulfur, magnesium, calcium, copper, and zinc. When used as a fertilizer, biosolids provide extended benefits due to the slow-release nature of their performance. Our customers have been enjoying the benefits of this high quality yet cost-effective fertilizer for years.
What is wastewater?
Wastewater comes from home kitchens, bathrooms, and laundries, as well as from process and wash water from industries and businesses.
What is in wastewater?
Wastewater is mostly water (about 99.5%). Less than 1/2% is suspended and dissolved solids which must be removed so the water can be returned safely to the environment. The solids are either inorganic (sand, grit, nutrients such as nitrogen and phosphorus, salts, and metals) or organic (primarily waste products of animal or vegetable origin, but also a variety of synthetic chemicals). Wastewater also contains living organisms such as bacteria.
Why do we have wastewater treatment?
Wastewater treatment is necessary to protect public health and the environment. Discharging raw sewage into lakes and streams harms those environments and can spread disease. In lakes and streams, native bacteria, algae, and other microscopic organisms (microbes) use the waste as food. These microbes eat and reproduce, using the available oxygen dissolved in the water. If too much waste enters a waterbody, the microbes will use too much of the available oxygen needed by fish and other aquatic life.
How does wastewater treatment work?
Wastewater treatment plants mostly use the same biological and physical processes by which water is cleaned in nature. The steps to clean water include industrial pre-treatment, preliminary treatment, primary treatment, and secondary treatment.
Industrial Pre-treatment
Advance removal of unacceptable levels of chemicals or metals prior to delivery to a community wastewater treatment facility.
Preliminary Treatment
Initial physical screening to remove debris (rags, wood, plastic)
Primary Treatment
Storage of wastewater in a sedimentation tank motionless for several hours to allow solids to settle to the bottom. Grease and oils float to the top and are removed for disposal.
Secondary Treatment
Secondary treatment is a biological process which relies on the same microbes that clean natural waterways. After primary treatment, wastewater is held in another large tank in which microbes feed on the suspended and dissolved solids in the wastewater. Treatment plant operators carefully monitor and control the temperature, pH (acidity), and amount of oxygen in the wastewater to ensure the health of the working microbes. Gradually, older microbes, which have eaten their fill, die and settle to the bottom. They take with them the suspended and dissolved solids that they consumed. The nearly pure water flows out of the top of the secondary treatment tank and is disinfected prior to being released into a river, ocean, or groundwater. The collected “secondary solids” are mixed with the “primary solids” and, if they are going to be recycled, treated and tested. Secondary treatment removes about 85% of the remaining suspended solids and nutrients.
Where do the biosolids come from?
The solids or “sludge” collected from the primary and secondary treatments are mixed together and undergo further treatment. Most often, these combined solids are “dewatered” to make a more manageable, semi-solid material. Dewatering usually involves centrifuges, vacuum filters, drying beds, or presses that remove excess water. The solids are also “stabilized” by some form of digestion (aerobic or anaerobic), and/or composting, or some other treatment. There are stringent U. S. federal guidelines (the federal “Part 503” regulations of the U. S. Clean Water Act) which identify approved treatment processes. In Canada, provincial regulations or guidelines do the same. These stabilization processes reduce pathogens (microscopic organisms that can cause disease in humans) and odors. They also make the solids less attractive to “vectors” (animals that can carry pathogens, such as flies). Only once they have been through these treatment and testing procedures can “sludges” be called “biosolids” and used as fertilizers and soil amendments.
Is using biosolids safe? How do we know?
Yes. When properly managed in accordance with federal, state, and/or provincial regulations and best management practices, the risk to public health and the environment from recycling biosolids are negligible. Federal and state standards and management practices for biosolids recycling were developed from a detailed scientific risk assessment completed by the U.S. Environmental Protection Agency (EPA). Input included research and expertise from the U.S. Department of Agriculture and premier universities including the Universities of Arizona, California, Colorado, Florida, Maine, Michigan, Massachusetts, Minnesota, New Hampshire, Oregon, Vermont, Washington, Colorado State, Ohio State, Penn State, and Cornell Universities.
What about heavy metals?
Biosolids contain trace amounts of “heavy metals,” as do natural soils, manures, and commercial fertilizers. The U.S. EPA and university researchers around the continent have studied the potential risks from these trace metals and U. S. EPA, state, and provincial regulations set maximum levels in biosolids to ensure protection of public health and the environment. Because of required industrial pretreatment and the risk-based standards for biosolids recycling, the risks posed by trace metals in biosolids are minimal.
What about per- and polyfluoroalkyl substances (PFAS)?
PFAS are manufactured compounds that are utilized for their water and grease resistant properties in many common household products that we interact with on a daily basis, including but not limited to non-stick cookware, furniture, clothing, food packaging, dental-floss, makeup, and carpeting. They are also found in industrial products including firefighting foam, herbicides, and pesticides. Dust found within the average American household contains measurable levels of PFAS compounds.1
Biosolids fertilizer is the most regulated soil amendment on the market and must meet strict quality standards set forth by federal and local law. RMI’s biosolids are routinely tested to ensure that these products meet the requirements set forth in these rigorous standards. Similar to some dried manures, liquid fertilizers, and organic yard and waste composts,2 municipal biosolids have been found to contain trace levels of PFAS, the presence of which can be attributed to our daily interactions with many of the household products noted above. These minimal levels are often comparable to ambient background levels of PFAS found in soils. It is important to differentiate between municipal biosolids (produced during the treatment of domestic wastewater) and industrially contaminated biosolids (produced during the treatment of wastewater from an industrial or commercial source utilizing PFAS chemicals) as the latter are not used for beneficial land application in our recycling program.
Decades of careful research on the land application of municipal biosolids has concluded that it is a safe, effective, and beneficial practice that improves soil health, increases crop yields, sequesters carbon, and reduces irrigation demands, amongst many other significant benefits. Two of the most common types of PFAS (PFOA and PFOS) have already been phased out of production in the United States, and this action, along with continued source reduction efforts, will see these compounds continue to decrease in the environment. Furthermore, a 2020 study completed by the University of Arizona found that long-term, land applied municipal biosolids had limited mobility through soil depth, thus posing minimal risk to groundwater.3 RMI is currently voluntarily participating in a larger, nationwide version of this study that will provide further data on the transport of PFAS in soil, and we look forward to sharing the results of these efforts when available.
For more information about the benefits of biosolids fertilizer, and additional technical resources on PFAS, please visit the following links:
Safety of Biosolids for Farmers
PFAS in Biosolids and Residuals
Will the use of biosolids adversely impact nearby surface waters and/or groundwater?
No. Properly treated and properly managed biosolids products do not have a negative impact on surface water or groundwater quality. As with any fertilizer or soil amendment, best management practices must be followed to prevent impacts via surface water runoff or via leaching to groundwater. By law, biosolids recycling programs in the Northeast must follow such best management practices; the same is generally not true for the use of manures and chemical fertilizers.
Is biosolids recycling the right thing to do?
Yes!
- Biosolids recycling returns nutrients and organic matter to soils.
- Biosolids recycling avoids the need for more costly landfill space or incinerators.
- Biosolids products are efficient fertilizers and soil amendments that build healthy soils, restore barren lands, and help keep New England and eastern Canadian landscapes open and growing.
- Biosolids recycling in accordance with regulations and best management practices has been proven safe and beneficial by decades of scientific study and peer review.
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- Strynar M.J., Lindstrom A.B – Perfluorinated compounds in house dust from Ohio and North Carolina, USA. Environmental Science & Technology 42:3751-3756. 2008
- Northeast Biosolids & Residuals Association. (2022, May). PFAS in the environment. https://www.nebiosolids.org/s/May-2022-PFAS-in-the-Environment_NEBRA.pdf
- PFAS in Biosolids: A Southern Arizona Case Study, The University of Arizona, 2020