What does primary treatment in a POTW allow?

Primary treatment in a Publicly Owned Treatment Works (POTW) is designed to remove a significant amount of solid and floating materials from wastewater. This process involves physical separation techniques, primarily sedimentation and flotation. During primary treatment, wastewater flows into a settling tank where heavier solids settle to the bottom, forming sludge, while lighter materials, such as oils and fats, float to the surface.

The correct choice indicates that not only solids settle, but also lighter materials that float are allowed to separate from the liquid phase. This dual action ensures a more effective removal of both types of contaminants before the water moves on to further treatment processes. Understanding the overall function of primary treatment helps grasp its importance in reducing the organic and inorganic load of wastewater, preparing it for secondary treatment stages, where biological processes further purify the water.