Basic Information about Sewage Sludge and Biosolids
Basics of Sewage Sludge and Biosolids
When domestic sewage is transported and conveyed to a wastewater treatment plant (WWTP), it is treated to separate liquids from the solids, which produces a semi-solid, nutrient-rich product known as sewage sludge. The terms “biosolids” and “sewage sludge” are often used interchangeably by the public; however, the U.S. Environmental Protection Agency typically uses the term “biosolids” to mean sewage sludge that has been treated to meet the requirements in the EPA’s regulation entitled, “Standards for the Use or Disposal of Sewage Sludge,” promulgated at 40 C.F.R. Part 503, and intended to be applied to land as a soil conditioner or fertilizer.
In the U.S., there are generally three options for use or disposal of sewage sludge: land application, landfilling, and incineration. Land application is the spraying, spreading, incorporation, or injection of sewage sludge into or onto the land to either condition the soil or fertilize crops or vegetation grown in the soil. Landfilling can occur in a sewage sludge-only monofill (i.e., surface disposal) or a municipal solid waste (MSW) landfill. Sewage sludge can also be incinerated in a sewage sludge incinerator (SSI).
Sewage Sludge Management Practices
Land Application
Biosolids can be used on agricultural land, forests, rangelands, disturbed land in need of reclamation, or nonagricultural lands like parks, golf courses, and home lawns and gardens. Biosolids that are to be beneficially used must meet federal, state, Tribal and local requirements.
Landfilling
Biosolids landfilling options include surface disposal (e.g., disposal in a sewage sludge-only landfill called a surface disposal site or monofill) or in a co-disposal landfill (e.g., MSW landfill).
Incineration
Incineration is the combustion of organic matter and inorganic matter in sewage sludge by high temperatures in an enclosed device.
Use and Disposal Statistics
Each year some facilities are required to summarize their sewage sludge management practices and provide pollutant monitoring data in Biosolids Annual Reports (BARs). A BAR is required from a publicly owned treatment works (POTW) that land applies, incinerates, or disposes of its sewage sludge via surface disposal and falls into any one of the following categories:
- It serves 10,000 people or more;
- It is a Major POTWs (POTWs with a design flow rate greater than or equal to one million gallons per day);
- It is a Class 1 management facilities (POTWs with an approved pretreatment program or facilities that have been classified as such by the EPA or state); or
- It is otherwise required to report by the EPA or permitting authority.
The EPA collects annual biosolids reports from roughly 2,350 facilities in the U.S. that are located in the 41 states, the District of Columbia and the Tribes and territories where the EPA is the permitting authority for the Biosolids Program. Based on the reports submitted in 2024 from the facilities that meet these applicability requirements, the EPA estimates:
- Approximately four million dry metric tons (dmt) of sewage sludge was generated.
- Approximately 2.39 million dmt of sewage sludge was land applied.
- Approximately 558,000 dmt of sewage sludge was incinerated in a sewage sludge incinerator.
- Approximately 982,000 dmt of sewage sludge was landfilled, about 880,000 dmt in a municipal solid waste landfill (MSW) and 102,000 dmt was surfaced disposed of in a sewage sludge-only monofill.
- Approximately 81,500 dmt of sewage sludge was used or disposed of using another management practice like deep well injection, use as an auxiliary fuel, or storage.
When facilities submit Biosolids Annual Reports they may provide a more detailed description of the end use for the sewage sludge that is land applied. Facilities can additionally indicate the end uses as “agricultural”, “reclamation”, “distribution and marketing”, or “other”.
- Agricultural. Uses in this category could include application to lands for the purposes of growing food crops, feed crops, fiber crops, energy crops (i.e., grown for ethanol production), or land for animal grazing.
- Reclamation. Uses in this category could include reestablishment of vegetation at lands that have been significantly disturbed like mines and construction sites. Sites can also be reclaimed for agricultural uses.
- Distribution and marketing. This category encompasses sewage sludge that is sold or given away, including sewage sludge available to the public for lawn and home garden use. Note that this sewage sludge may also be used as a soil amendment on agricultural or other lands.
- Other. This category includes all other types of land application, such as use in forestry or where a detailed end use was not provided.
While facilities may indicate an end use in the BAR, sometimes more than one end use category is applicable. For example, sewage sludge that is labeled “distribution and marketing” can be applied to agricultural lands. Similarly, reclaimed lands can be used for agricultural purposes like pasture for animal grazing. The following chart summarizes end uses as reported in the Biosolids Annual Reports.
There are nine states (Arizona, Idaho, Michigan, Ohio, Oklahoma, South Dakota, Texas, Utah, and Wisconsin) that are authorized through the National Pollutant Discharge Elimination System (NPDES) Program to be the permitting authority for the Biosolids Program. Sewage sludge managed in these states are not represented in the EPA’s statistics. The EPA will transition to electronic reporting for the remaining authorized states as part of Phase 2 implementation of the NPDES eRule by December 2025.
The EPA does not receive data from smaller facilities or those that use or dispose of their sewage sludge using alternate management practices (like landfilling in an MSW) unless that information is voluntarily submitted by the POTW. Because the EPA does not currently receive data from some states that are authorized to implement the biosolids program or from smaller facilities, there is no definitive source that reports the amount of biosolids produced annually in the United States. The EPA tries to ensure the information presented here is as accurate as possible; however, the data in biosolids annual reports may contain errors as submitted by the POTWs.