Sewage sludge is a solid, semisolid, or liquid muddy looking residue that results after plain old sewage (human and other waste from households and industries) is treated at a sewage plant. After being treated, the sewage sludge may be spread on non-organic agricultural land as a fertilizer or dust suppressant. The USDA notes that sewage sludge includes: "Domestic septage; scum or solids removed in primary, secondary, or advanced wastewater treatment processes; and a material derived from sewage sludge."
Ash generated during the firing of sewage sludge and grit and screenings generated during treatment of domestic sewage are not categorized as sewage sludge.
KEY FACT: Agricultural products claiming USDA organic certification cannot be produced using sewage sludge.
Sewage sludge is rich in organic content and plant nutrients, so technically it can be categorized as a useful fertilizer. Many advocate of sewage sludge also point out that sewage sludge is a great solution to the waste problem. That said, sewage sludge for land application is not a safe practice.
The Environmental Protection Agency (EPA) points out that sewage sludge can contain volatiles, disease-causing pathogenic organisms, bacteria, heavy metals, inorganic ions along with toxic chemicals from industrial wastes, household chemicals and pesticides. Sewage sludge may also negatively affect land and water quality and cause erosion and runoff.
Organic farmers should cannot use sewage sludge but they can choose a crop nutrient or soil amendment that is an allowed substance on the National List.