Sewage sludge can find a second life on farm fields. Here's how it's made


Sewage Sludge: From Wastewater Treatment Plants to Farmland

What you flush down the toilet may eventually end up being utilized on agricultural land throughout the United States. Biosolids, also known as sewage sludge, represent the solid materials produced during the wastewater treatment process. These nutrient-rich byproducts can serve as fertilizer for farm fields or as compost for residential lawns, offering a beneficial reuse for waste materials.

Understanding Biosolids and Their Agricultural Application

Biosolids, essentially treated sewage sludge, are created as a byproduct of cleaning wastewater. They are recognized for their high nutrient content, making them potentially valuable as agricultural fertilizers and soil amendments. This practice of applying biosolids to land recycles nutrients and can improve soil health.

The Biosolids Production Process: A Step-by-Step Overview

The creation of biosolids involves a multi-stage process at wastewater treatment facilities designed to remove contaminants and stabilize the material:

  • Initial Debris Removal: Wastewater enters the plant and undergoes preliminary screening to eliminate large items like trash and paper products.
  • Sedimentation: The wastewater flows into large sedimentation tanks where gravity separates heavier solids, which settle to the bottom. Floating solids are also skimmed off.
  • Aeration: Liquid from sedimentation is moved to aeration tanks. Here, air is introduced to promote the growth of microorganisms that break down organic pollutants.
  • Secondary Sedimentation: The wastewater undergoes another sedimentation phase to remove further solids after aeration.
  • Anaerobic Digestion: Solids collected from various stages are thickened and then placed in anaerobic digestion tanks. In these tanks, microbes continue to decompose organic matter in the absence of oxygen. The tanks are heated to specific temperatures for a set duration to eliminate most pathogens.
  • Final Thickening: After digestion, the biosolids undergo a final thickening step, preparing them for application.

Potential Concerns: PFAS “Forever Chemicals” in Biosolids

While biosolids offer agricultural benefits, the production methods can vary, and undesirable substances can sometimes be present. Recent research from the U.S. Environmental Protection Agency (EPA) has indicated that there may be elevated health risks associated with per- and polyfluoroalkyl substances (PFAS) in areas where sewage sludge is applied to farm fields. These “forever chemicals” can find their way into biosolids depending on the initial water source entering the treatment facility.

Source of PFAS Contamination

Carsten Prasse, a professor at Johns Hopkins University specializing in biosolids research, notes that PFAS are widespread but not universally present in all biosolids. Their concentration can also vary significantly.

Benefits of Biosolids in Agriculture

Despite potential concerns, the utilization of biosolids in agriculture presents significant advantages. According to Prasse, biosolids are valuable in farming due to their substantial concentrations of essential nutrients, including nitrogen and phosphorus. Furthermore, they can enhance soil structure and aid in irrigation efficiency, contributing to sustainable agricultural practices.

Variations in Biosolids Composition

The makeup of biosolids can vary considerably based on the content of the incoming wastewater. Besides human waste, biosolids may contain a wide range of common household chemicals from personal care items to medications, some of which may include forever chemicals. Facilities like the one in Columbus, Ohio, primarily process waste from residential and commercial sources.

Quality Control and Testing

The Columbus wastewater treatment facility routinely tests its final biosolids product monthly for heavy metals and nutrients like phosphorus and nitrogen, even during storage before land application. This rigorous testing ensures the biosolids meet specific quality standards before being used as fertilizer.

Alternative Uses: Composting Biosolids

In Columbus, half of the produced biosolids are sent to a nearby composting facility. There, they are combined with other organic materials such as tree trimmings and leaves. This mixture undergoes an additional heating and aeration composting process to further eliminate pathogens. These composted biosolids face less stringent regulations and can be utilized in various settings like parks and residential gardens.

PFAS Testing Limitations and Future Regulations

Currently, the Columbus facility, like many others, does not routinely test incoming wastewater for PFAS due to the high cost of such analysis. Facility management indicates that future PFAS testing or treatment will likely depend on forthcoming federal regulations regarding these persistent environmental contaminants.


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