In which stage of sewage treatment are floating materials, like grease, typically removed?

Floating materials, such as grease, are primarily removed during the primary treatment stage of sewage treatment. This stage is designed to physically separate solids from the liquid waste. It typically involves processes like screening and sedimentation. During screening, large objects like sticks, plastic, and grit are filtered out. After this initial separation, the sewage is allowed to sit in large tanks, where heavier solids settle to the bottom, forming sludge, while lighter materials like grease and oils float to the surface. This floating layer can then be skimmed off, effectively reducing the overall organic load and improving the quality of the wastewater entering subsequent treatment stages.

In contrast, secondary treatment focuses on the biological processes that break down dissolved organic matter, while tertiary treatment involves further purification methods, often using chemicals or advanced filtration methods. Disinfection is a final step that ensures any remaining pathogens are neutralized but does not specifically target floating materials. Each stage has its unique purpose in achieving effective sewage treatment, with primary treatment being crucial for the removal of floating substances.