Biosolids – understanding benefits and risks

by Georgine Yorgey

Biosolids? Yes, that means sewage sludge. Well, sort of.  But before you say YUCK and click off the page, let’s start with what they really are: biosolids are the materials produced from digestion of sewage at city wastewater treatment plants. They are rich in plant nutrients such as organic carbon, nitrogen, and phosphorus, and can be applied to wheat, alfalfa, and timber land for plant fertilization and soil conditioning. When biosolids are applied at rates that meet plant nutrient needs, farmers and researchers are seeing crop yields equal to or greater than those seen with synthetic fertilizer. Applying biosolids as fertilizer also allows them to be recycled for a useful purpose rather than disposed of in landfills or incinerated.

Biosolids being spread on agricultural fields. Photo: A. Bary.
Biosolids being spread on agricultural fields. Photo: A. Bary.

In addition to the benefits to plant growth and the waste stream, biosolids can serve another role.  Applying biosolids to the land can benefit the climate because they sequester carbon in the soil in the form of enhanced organic matter. Given current climate concerns, that could be one small but important piece of a wider mitigation strategy.

While the benefits of biosolids are many, there are also perceived risks.  A new WSU Extension fact sheet: Guide to Biosolids Quality by Shannon Mitchell, Chad Kruger, and me, digs into these risks and discusses major categories of contaminants and explains what is currently known about them. Concern about contaminants arises because municipal facilities treat wastewater from industrial and household sources that may contain small amounts of various contaminants including metals, pathogens, antibiotics, industrial and household chemicals, odorants, and aerosols. Some of these contaminants (often called “emerging contaminants”) may be compounds whose impacts are not well understood.

To date, research indicates that the major classes of contaminants in biosolids pose a minimal risk to human, animal, or environmental health. This is often because contaminants do not appear in sufficiently high concentrations to cause harm or because they are not taken up by crops even when present in soils. To further minimize risk, the application of biosolids is highly regulated by state environmental protection departments and by the EPA.

Notice that I said “to date.” Ongoing research on biosolids continues to investigate contaminants and potential impacts. New research findings are reviewed periodically and risk assessments conducted to reevaluate the effectiveness of existing biosolids land-application regulations.

For all the details, see the extension publication or visit the Washington State University Biosolids Management website.

Thank you to Amy Pendegraft for her contributions to earlier drafts of this post.

This is reprinted, with permission, from the Center for Sustaining Agriculture and Natural Resources’ (CSANR) blog.

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