PROBLEM: Dark brown, almost gray/black sudsy foam on the aeration tank surface. Mixed liquor color is very dark brown, to almost black. There is the detection of a septic or sour odor in the area of the aeration tank. What is the MOST likely cause? • Anoxic conditions are occurring in the pre-denitrification zone • Septic conditions have occurred in the clarifiers • Anaerobic conditions are occurring in the aeration tank • Aerobic conditions are occurring in the aeration tank