Biosolids: Organic Materials Management

Biosolids are the nutrient-rich by-product of wastewater treatment, generated by treatment of domestic sewage at nearly 250 facilities located throughout California. At the treatment plants, sewage goes through physical, chemical and biological processes which clean the wastewater and remove the solids. These processes remove physical contaminants and control pathogens. Biosolids may be applied directly to land if federal and state rules are followed. Additional outcomes for biosolids include:

  • Land applied to reclaim fire ravaged land, open pit mines, and deforested areas
  • Processed into compost at composting facilities
  • Used as alternative daily cover or final cover at landfills
  • Disposed at landfills
  • Surface disposed
  • Combusted to produce energy at a Waste-to-Energy facility (i.e., incineration)
  • Alternative uses (e.g., alternative fuel to replace coal in industrial processes such as the cement industry or deep well injection)


Biosolids can be used as a soil amendment/fertilizer, but may be disposed when land application uses are not available or are too costly. When land applied, biosolids are generally used in four forms: as a soil amendment, a dried pellet, a liquid, or after composting. There are three categories of biosolids: Class B biosolids, Class A biosolids, and Exceptional Quality (EQ) biosolids.

  • Class B biosolids may have low levels of pathogens which rapidly die-off when applied to soils, essentially becoming pathogen-free within a short period following application in accordance with “Part 503” Rule requirements. “Part 503” refers to the section in Title 40 of the Code of Federal Regulations, where various standards related to pathogens and metals in biosolids are codified.
  • Class A biosolids are essentially free of pathogens prior to land application. The metal contents requirements under the Part 503 Rule are the same for Class A and Class B biosolids. (Regulation Example: Riverside County, Chapter 13.24). Class A biosolids products include lime pasteurized biosolids and fertilizer pellets.Exceptional Quality (EQ) biosolids have lower metals concentration requirements than either Class A or Class B biosolids and have the same pathogen levels as Class A biosolids.

Land Application

Land application is the primary use for biosolids in California. Approximately 56% of the biosolids generated in California are land applied. This consists of Class A biosolids (36% of total biosolids generated) and Class B biosolids (20% of total generated). Land application of biosolids recycles organic matter and nutrients, improving soil physical, chemical, and biological properties.

Concerns about potential odors and public health impacts resulting from land application of biosolids have been expressed by members of the general public. Best Management Practices (BMPs) should be implemented in order to ensure odors are minimized and human health and the environment are protected. BMPs include applying biosolids at agronomic loading rates, incorporating into soil, limiting soil pH, restricting public access, and other site restrictions.

USEPA and others continue to conduct research and develop methods to better provide more scientifically defensible assessments relevant to USEPA’s regulation of biosolids. For example, research is being conducted on the potential presence and impact of pharmaceuticals and other organic chemicals in biosolids, which have been found in wastewater discharged from treatment plants, causing concern that they may be present in biosolids. The United States Geological Survey (USGS), in collaboration with others, is involved in several projects to develop analytical methods for characterizing potential emerging contaminants in biosolids-derived composts and other products. This includes sampling biosolids to characterize occurrence of emerging contaminants, assessing the ability of a range of wastewater treatment technologies to remove selected pharmaceuticals and other emerging contaminants from municipal sewage, and extending work to characterize biosolids in an investigation to determine the persistence and vertical transport in soil of emerging contaminants derived from biosolids applied to land.

Note that under the United States Department of Agriculture’s National Organic Program, biosolids and compost produced from biosolids are prohibited for use in organic agriculture.


Composting is the second largest use of biosolids. Approximately 16 percent of compost facilities process biosolids for use in agriculture, horticulture, and land reclamation. Biosolids can be composted using a bulking agent such as wood chips or co-composted with green materials. Biosolids composting requires a full solid waste permit. Therefore, biosolids producers who wish to compost either must contract with an existing permitted facility that has the capacity to accept additional material, or put together the significant capital investment and operational outlay to fund the permitting, construction, and operation of a new facility.

Alternative Daily Cover

Approximately 19 percent of the biosolids generated in California are used as alternative daily cover (ADC) or final cover at some landfills. ADC is material used to cover and contain landfilled materials at the end of each day and is a critical part of vector control at landfills. California regulation allows certain materials to be used as ADC because of their physical characteristics and manageability. Regionally, there are areas in California where there are no landfills that accept biosolids for use as ADC.


Approximately 13 percent of the biosolids generated in California are disposed of at landfills. Biosolids can only be disposed of at permitted landfills. Of the 128 permitted landfills located in California, 55 are permitted to accept biosolids for disposal. Some landfills permitted for the disposal of biosolids do not accept biosolids on a routine basis. As with ADC, there are regions in California where there are no landfills that accept biosolids for disposal. 

Surface Disposal

Surface disposal methods account for 3 percent of the biosolids produced in California. Surface disposal methods require large amounts of vacant land which is lined with an impermeable material prior to the implementation of disposal operations. These operations are individually permitted and monitored by California’s Regional Water Quality Control Boards. Surface disposal is used on a limited basis by several wastewater treatment agencies and is not used on a widespread basis due to the dedicated land-area requirements. 


Approximately 2.5 percent of the biosolids generated in California are incinerated. Incineration involves the high temperature burning of biosolids using a fuel supply such as natural gas or diesel fuel. The resultant ash is significantly lower in volume than the feedstock (biosolids) and thus higher in metals concentrations. The ash is typically landfilled. Incinerators require significant capital investment and have high operating costs. There are three operating facilities statewide, each with a very limited capacity relative to the total amount of biosolids produced statewide. Due to air quality regulations, permitting of additional facilities is not considered likely.

A small percentage of the biosolids generated in California are stored temporarily in onsite facilities, such as lagoons. The biosolids are dried and further processed while in storage prior to the final deposition of the material using one or a combination of the management options described above. 

Biosolids Usage

According to the California Association of Sanitation Agencies (CASA), California generated 723,000 dry metric tons of biosolids in 2013, most of which are Class A biosolids. Chart 1 shows how biosolids were managed in California in 2013.

Pie Chart of Biosolids Produced in California

Management of biosolids produced in California in 2013: alternative use (alternative fuel, deep well injection, etc.), incineration 3%, surface disposal 3%, other 4%, landfill disposal 13%, alternative daily or final cover 19%, class B soil amendment/fertilizer 20%, class A soil amendment/fertilizer 36%. Total of 723,000 dry metric tons.

Regulatory Resources

Biosolids reuse and disposal practices are regulated by multiple federal, state, and local agencies. For additional information see April 2004 Agenda Item 4, including Attachments.

In July 2004, the State Water Resources Control Board adopted Water Quality Order No. 2004-12 DWQ, which incorporates the minimum standards established by the Part 503 Rule and expands upon them to conform with  the California Water Code.

For more information contact: Organic Materials,