Biosolids Return Nutrients to the Soil

Photograph of a large field of grass on a farm with rolling hills in the background against a clear blue sky. There are wind turbines in the distance, and in the center of the field and horizon is a tractor.
Biosolids are another valuable resource recovered through the wastewater treatment process. Rich in nutrients and organic matter, Portland’s biosolids increase the productivity of dryland agriculture in Eastern Oregon by improving the soil while helping to capture and store carbon.

What Are Biosolids?

Biosolids are the result of treating the raw solids extracted from wastewater during the treatment process. Multiple times during the treatment process, solids are separated from the water. These solids are sent to the treatment plant’s digesters. In the digesters, the solids are heated to a minimum of 95 degrees Fahrenheit while anaerobic microorganisms – very tiny bugs who live without oxygen – break down the solids. These processes destroy pathogens and reduce odors.

After about a month in the digesters, biosolids go through a “dewatering” process. This drains any excess liquid from the biosolids making them lighter and more compact for transport.  

Biosolids are regulated by the Environmental Protection Agency and the Oregon Department of Environmental Quality. There are strict requirements for biosolids production and application. To make sure Portland's biosolids meet those requirements, Environmental Services tests its biosolids on a daily schedule and monitors wastewater throughout the treatment process to detect any problems. In addition, Environmental Services works with business and industry to pre-treat their wastewater before discharging it to the sewers to further protect the treatment plant, water quality, and biosolids production.

What Are the Benefits of Biosolids?

When added to soil, biosolids return beneficial nutrients to the land. They contain nitrogen, phosphorus, sulfur, and potassium, which helps make soil better for plants to grow. Biosolids also contain small amounts of other important minerals such as magnesium, manganese, iron, zinc and calcium. Not usually found in commercial soil amendments or fertilizers, these additional minerals help grow nutrient-rich plants.

Biosolids add valuable organic matter to the soil. Organic matter provides a food source for soil microbes, increases the ability of the soil to hold water, and improves soil structure. Field monitoring of land application sites shows that about 20 percent of the organic matter from Portland's biosolids decomposes very slowly, increasing soil fertility for several years.

In addition, biosolids provide climate benefits. They capture and store carbon in the soil, and they reduce the need for commercial fertilizers, which are produced by a fossil fuel-intensive process.

Environmental Services has been working with farmers in Eastern Oregon for more than 30 years to use biosolids to improve the soil of their pastures and grain fields. Applying biosolids to farm and ranch land makes the land more productive and is the most cost effective and sustainable use of biosolids.

Continuously Improving

The Oregon Department of Environmental Quality has praised Portland’s biosolids program for operating in compliance with state requirements and serving as an example of a "well-run program compared to other biosolids programs within Oregon as well as nationally."

Biosolids are another example of how we are recovering resources from Portland’s wastewater. By finding and recovering all that is valuable in our wastewater, we can shift to a more sustainable future – one where our current needs are met without compromising the ability of future generations to meet their own needs.