Biosolids recycling is regulated according to 40 CFR (Code of Federal Regulations) Part 503, Standards for the Use or Disposal of Sewage Sludge. These standards, commonly known as the "503" regulations, are promulgated by the US Environmental Protection Agency (US EPA). The regulations recognize that biosolids, by nature of their origin, have the potential to contain appreciable concentrations of contaminants that may adversely affect human health and/or the environment. The three categories of potential contaminants identified by the regulations are (1) pathogens, or disease organisms, (2) organic compounds, and (3) trace metal content. Federal, state, and local agencies regulate the production, application and marketing of biosolids. There are two classifications of biosolids based on pathogen content: Class A biosolids have been treated to reduce pathogens to a level where access to application sites does not need to be limited, and Class B biosolids have been treated to reduce pathogens to a level that is safe for application on land with an initial period of limited public access. Treatments to produce Class A biosolids do not affect the metals or organic chemicals in the biosolids; odor may or may not be affected. Federal regulations also set maximum limits on trace metal content in biosolids. The state and county health departments may impose stricter standards. We treat our biosolids to meet the Class B standards. The federal regulations on biosolids processing and use and the corresponding state regulations mandate that biosolids be applied at agronomic rates to balance uptake of nitrogen by crops with the potential for nitrate leaching to ground water. The maximum rate and the cumulative amount of biosolids that can be applied to a particular parcel of land are intended to limit the concentrations of contaminants in soil, crops, and receiving waters. These regulations limit the accumulation of contaminants in biosolids-amended soil to levels that are not harmful to the health of humans and other biota. When biosolids are applied in compliance with federal, state and local regulations and permitting requirements, there are no probable significant adverse environmental impacts associated with this practice.