All settleable solids in sewage may be considered as suspended solids, but not all suspended solids are necessarily settleable solids. Is this statement true or false?

The statement is true because it accurately reflects the relationship between settleable solids and suspended solids in wastewater. Settling refers to the process where certain solids in a liquid can settle at the bottom due to gravity, typically occurring over time in a clarifier or settling tank.

In wastewater treatment, settleable solids are a subset of the broader category of suspended solids. While all settleable solids can indeed be classified as suspended solids (since they are still in the liquid phase until they settle), not all suspended solids have the ability to settle out of the liquid within a reasonable time frame. This can be due to several factors, such as particle size, density, or the presence of colloids that remain suspended in the water.

Understanding this distinction is crucial for efficient wastewater treatment as it influences processes like sedimentation, where the goal is to remove solid particles from the liquid phase. Recognizing that some suspended solids may remain in suspension and not easily settle informs operators about the design and performance of treatment systems. Hence, the statement is correctly assessed as true.