Biosolids: A Different Kind of Fertilizer

By Grubbycup
Published: April 1, 2013 | Last updated: August 4, 2022 07:57:52
Key Takeaways

Although biosolids are banned for agricultural use in many areas of the United States, the subject deserves a closer look.

Biosolid is a public relations term for treated municipal sewage sludge, which is sometimes sold or given to consumers and farms to be used as a nutrient source for gardens and crops. Biosolids contain about 4% nitrogen, and several micronutrients. An estimated 3.5 to 5.4 million dry tons of biosolids are used annually as nutrients for farms and landscapes, and by homeowners.


Biosolids are a product of sewage treatment. Waste products enter the sewage system through toilets, drains and other venues, and are collected at sewage facilities known as wastewater treatment plants. Untreated sewage contains everything that has entered the system, not only human waste products, but anything else that gets dumped into the sewer system.

Untreated sewage is considered to be a biohazard as it can contain human susceptible pathogens or medical waste. It can also be a chemical hazard depending on the amount of drain cleaners, solvents, or other potentially toxic chemicals present.


Sewage sludge is defined by the Environmental Protection Agency (EPA) as a solid, semi-solid or liquid residue generated from the treatment of domestic sewage in a treatment works.

“Sewage sludge includes scum or solids removed in primary, secondary or advanced wastewater treatment processes and any material derived from sewage sludge,” according to the EPA’s publication The Standards for the Use or Disposal of Sewage Sludge–Part 503. It also specifically mentions that material comprised of 99% industrial waste and 1% domestic waste can still be considered “biosolids” after treatment since it contains some domestic wastewater product.

The details of treatment vary, but it generally starts with the removal of large objects that could damage the processing equipment with the use of a bar screen. The waste is then allowed to settle. The portion of the material that sinks to the bottom and is the sewerage sludge, and the portion that floats is called scum.


The scum is sometimes collected and reintroduced to the collected sewage sludge after the water between the layers has been removed. Much of the water is removed and after filtration and treatment is often released into rivers and oceans. The majority of studies involving sewage treatment effectiveness has centered around this wastewater effluent, and not the leftover sewage sludge. After the sewer sludge has been collected, it is further treated before it can be labeled as biosolids (or in some instances the term “compost” is used).

Treatment to convert biosolids into Class A biosolids can be accomplished by aerobic composting, anerobic composting, heat drying, or pasteurization. These methods reduce the amount of live pathogens present in the material. Class B biosolids have less stringent requirements, and use is restricted to areas where public access is limited, such as food crop production and grazing lands.


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Biosolids can be spread on forest sites, or placed on agricultural fields as “surface disposal sites.” Cattle and other animals meant for human consumption are allowed to graze on surface disposal sites as long as there is documentation that they will “ensure protection of public health and the environment from any reasonably anticipated adverse effects of certain pollutants that can be present in biosolids,” the EPA says. Class B biosolids may be applied to food crops or grazing land as long as the edible portions do not touch the surface of the soil, and harvest is at least 30 days after the last application.

If the edible portions touch the soil, the last application must have been over 14 months before harvest. Grazing animals cannot be allowed back onto the pasture for 30 days after application. People are also restricted from the site for 30 days on a farm, or one year for general public access. Class A biosoilds are those that have undergone additional composting or drying treatments.

EQ biosolids show lower levels of heavy metals and pathogens, and are exempt from these restrictions. These biosolids can be sold or given away in bulk, mixed with soil amendments and sold to consumers, or bagged for direct consumer use.

The United States Geological Survey (USGS) purchased nine off-the-shelf samples of biosolids available to consumers for study. The samples were tested for 87 different pollutant chemicals selected for their ability to pass though current treatments essentially intact. Out of the 87 chemicals tested, 55 were present in at least one of the samples.

Every sample had 25 chemicals in common, with one sample having measurable levels of 45. Some of the organic wastewater contaminants (OWCs) they all had in common include an antimicrobial disinfectant, an antihistamine, an antiepileptic drug and steroids. These contaminates were found to comprise up to 1,811 ppm in the biosolids tested.

The study concluded that biosolids have high concentrations of these emerging contaminants compared to treated liquid wastewater effluent.

“What is not known at present is the transport, fate, and potential ecological effects of these contaminants once biosolids are applied to agricultural fields, garden plots, and landscaped plants and shrubs,” states the USGS document Household Chemicals and Drugs Found in Biosolids from Wastewater Treatment Plants. Another study from the Environmental Science and Technology department of Colorado State University of Pueblo found elevated levels of these types of chemicals found in earthworms taken from areas of repeated biosolid use.

One concern of introducing contaminants such as antibiotics into the environment is that they may encourage the development of antibiotic-resistant bacteria. The thought is that the more bacteria is exposed to antibiotics from sewage products, the more likely a random mutation will result in a resistant form.

Although the information on long term effects of the use of biosolids is inconclusive at this time, the USGS Toxic Substances Hydrology Program, a of long term intensive land disposal of treated sewage in Cape Cod, has shown the damage caused to nearby an aquifer will take at least decades to return to pre-contamination conditions.

While the question of how best to cope with accumulated sewage sludge is becoming increasingly more important as overpopulation continues, there are ways to deal with it which may prove safer than to store it in our lawns, gardens, and food sources. Alternative uses include using it as a fuel source, in bioreactors or to help restore already damaged landscapes like those left from mining or as a cover material for landfills.

One possible cause for concern in the use of treated sewage sludge is the intentional misrepresentation of the product to the consumer. Amendment products that contain biosolids may or may not be labeled clearly as such, and the meaning of the organic-sounding word “biosolid” is not commonly understood by the public. Foods grown under conditions where they are exposed to biosolids, even under direct contact, are not required to bear any indication to allow the consumer to make an informed choice in making purchasing selections.


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Written by Grubbycup | Indoor Gardener, Owner & Writer of Grow with Grubbycup

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Grubbycup has been an avid indoor gardener for more than 20 years. His articles were first published in the United Kingdom, and since then his gardening advice has been published in French, Spanish, Italian, Polish, Czechoslovakian and German. Follow his gardening adventures at his website

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