The U.S. EPA recently ordered the city of Ridgecrest, Calif., to investigate and address the discharge of metals to its sewer system. The order was issued after the city’s wastewater treatment facility applied sewage sludge with excessive molybdenum concentrations to city-owned fields to be planted with animal fodder.
“The city of Ridgecrest needs to develop an adequate source control program to prohibit excessive discharges of pollutants such as molybdenum to its municipal wastewater system,” said Alexis Strauss, director for the EPA Pacific Southwest region’s Water Division.
Molybdenum is discharged as a waste product from cooling towers and other industrial sources. Ridgecrest’s wastewater plant serves the China Lake Naval Weapons Center as well as the town of Ridgecrest. The metal is used in the manufacture of high-strength alloys and in high-temperature steels.
City officials reported in their annual sewage sludge report molybdenum levels of 92 ppm. The EPA’s limit for the metal in sewage sludge is 75 ppm. Molybdenum levels in sewage sludge in most areas of California are between 5 and 20 ppm.
Cows that have consumed plants grown in soils with high molybdenum levels can suffer from molybdenosis, where the animal is unable to absorb sufficient copper from their food. This copper deficiency can cause growth and reproductive problems.
When treated and processed, sewage sludge becomes biosolids, which can be safely recycled and applied as fertilizer to improve and maintain productive soils and stimulate plant growth. Only biosolids that meet stringent federal and state standards can be approved for use as a fertilizer.