FREQUENTLY ASKED QUESTIONS

 
 
 

 Solano County, CA

 

What are biosolids?

Biosolids are the carbon- and nutrient-rich alternative to synthetic fertilizers. They are produced during the wastewater treatment process by recovering the solids and nutrients from wastewater that would otherwise be discarded, and processing them into an effective, soil-enhancing product.

When incorporated into the soil, biosolids boost crop yields, improve soil quality, and help fight and adapt to climate change by capturing carbon in the soil and reducing the need for irrigation water. Biosolids are a renewable, safe, reliable resource with multiple benefits — good for people and the planet.  

 

How are they made?

Biosolids are produced during the wastewater treatment process. Homes and businesses generate wastewater that goes through sewers to a treatment plant. The treatment plant cleans the water and captures the solids. The solids are typically heat-treated by anaerobic digestion, where microorganisms destroy pathogens and break down pollutants. The end product is a nutrient- and organic-matter-rich, soil-like product called biosolids. Biosolids are filled with essential nutrients that farmers can use as a 100% renewable alternative to synthetic fertilizer. 


 

Wastewater travels to the treatment plant from businesses and homes. Solids are captured from the liquids through a process called settling. Solids are treated through an engineered process called digestion. Dewatering removes excess water to create a soil-like product. The finished product, biosolids, are transported to customers who typically use the material instead of synthetic fertilizer. 

 

How do they help farmers?

As a natural alternative to synthetic fertilizer, biosolids enrich and condition the soil, producing high-quality crops and increased yields. They contain all 16 elements essential for plant growth, including nitrogen, phosphorus, and a variety of essential micronutrients. Biosolids are also a low-cost alternative to synthetic fertilizers, providing a significant financial savings for farmers.

 

Why are biosolids a good alternative to chemical fertilizers?

Biosolids

Recycled
Low-energy or energy positive production
Environmentally friendly
Slow, steady nutrient release
Low-cost or free
Macro and micronutrients
Contain organic matter

Synthetic fertilizers

Mined or manufactured
Energy-intensive production
Environmentally damaging
Immediate nutrient release
Expensive
Typically only 1 or 2 macronutrients
No organic matter

 

How do biosolids help fight climate change?

By helping plants grow, biosolids play a supporting role in removing carbon dioxide from the atmosphere and returning it to the soil. Biosolids contain significant amounts of carbon-rich organic matter, and some of that carbon gets stored in the soil for a very long time. What’s more, using biosolids offsets the carbon emissions associated with synthetic fertilizer. By confining more carbon underground, versus warming the planet in the atmosphere, biosolids can play a critical role in helping to fight climate change. 

Fertilizing with carbon-rich biosolids allows us to 1) reduce the use of synthetic fertilizers, which require a lot of energy to produce; 2) grow larger plants, which removes carbon dioxide (CO2) from the air and returns carbon to the soil; 3) stores carbon in the soil (sequestration).

 
 

How do we know biosolids are safe?

The safety of biosolids has been studied for more than 50 years and is backed by abundant research performed by the Environmental Protection Agency (EPA), United States Department of Agriculture (USDA), and top universities across the United States and Canada.

Biosolids must meet exceedingly protective EPA and California environmental requirements. In fact, they are regulated more than other commonly used synthetic fertilizers, manures, and composts.

 

Do biosolids contain toxic substances?

There are some misconceptions about the safety of biosolids. Here are some facts that set the record straight on many common questions. Do biosolids contain….

Metals?

Thanks to mandatory pre-treatment programs, which prevent industries and manufacturing facilities from discharging metals into sewers, the metals content of biosolids is typically very low. Compare biosolids to natural soil, manure, or compost, and you will likely find that biosolids contain similar concentrations. Moreover, biosolids typically are at least an order of magnitude lower in concentration than the EPA’s rigorous safety standards for heavy metals.

Pathogens?

Biosolids must go through an approved process to destroy pathogens. Depending on the process employed, biosolids are classified as either Class A or Class B, and these classifications determine where and how the product can be used. In Class B biosolids, 95-99% of pathogens are eliminated in the treatment process, and the rest quickly die after application from exposure to oxygen, ultraviolet light, changing temperatures, and competing soil microorganisms. In California, Class B biosolids can only be used in places such as pasturelands. 

In Class A biosolids, the treatment process is hotter, and 100% of the pathogens are eliminated. Class A biosolids can be used in home gardens and to fertilize food crops. Class A biosolids have been used this way across the United States for almost a century. Class A biosolids can be purchased as fertilizers and composts at hardware stores across the country. Many cities with Class A biosolids use their products to support community gardens, parks, and other public lands.

Pharmaceuticals?

Extremely low amounts of pharmaceuticals can be measured in biosolids. The majority of pharmaceuticals are broken down during the wastewater treatment and biosolids production process, and the remaining are quickly broken down by soil microorganisms. Research and risk analysis demonstrate that it would take many lifetimes of working with biosolids to equal everyday exposure to many common products. For example, a person would have to farm or garden with biosolids for over 20,000 years to take in the equivalent of a single tablet of ibuprofen.

Per- and polyfluoroalkyl substances (PFAS)?

PFAS are found everywhere in the environment, and biosolids are no exception. The amount of PFAS found in Bay Area biosolids is extremely low, similar to the amount measured in human blood serum and garden soil. Generally, PFAS enter the wastewater stream through household and office building sinks and drains. In some parts of the United States, a few wastewater treatment facilities have been found to have higher levels of PFAS due to manufacturing facilities producing or using materials containing PFAS in those communities. Those are a very rare exception. There are no PFAS manufacturing facilities in our region. 

It’s also important to note that groundwater contamination has not been observed in areas with background PFAS soil concentrations.  

Human exposure to PFAS is mostly from everyday household products like non-stick cookware and waterproof and stain-resistant textiles, not biosolids. Below is a graphic depicting relative amounts of the two most common PFAS chemicals, Perfluorooctanesulfonic acid (PFOS) and Perfluorooctanoic acid (PFOA), measured in various substances, including Bay Area biosolids (SFPUC Biosolids).  

 

Data Sources: Forest Soil: Vedagiri et al., 2018; Garden soil: MN Dept. of Health; 2005, Backyard and Greenwaste Composts: Choi et al., 2019; SFPUC biosolids: SFPUC monitoring data; Human blood: CDC 2019, Dust in U.S. daycare centers: Strynar and Lindstrom, 2008.

 
 

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