To reduce the rate of significant noncompliance with National Pollutant Discharge Elimination System (NPDES) permits at lagoon facilities, improve water quality, and protect environmental and public health, the EPA has developed this troubleshooting manual. Specifically, this manual is designed to help operators at small wastewater treatment plant (WWTP) lagoons, also called wastewater stabilization ponds, identify causes of lagoon upset conditions and the corresponding troubleshooting steps.