Which of the following treatment methods does not rely on particle size?

Precipitation is a chemical treatment method used in wastewater treatment that does not depend on particle size for its effectiveness. In the precipitation process, chemical agents are added to the wastewater to cause dissolved substances to form solid particles, which can then settle out of the water. This method targets specific pollutants through chemical reactions, such as removing heavy metals or phosphates by forming insoluble compounds.

In contrast, filtration, activated sludge, and flotation are processes that rely heavily on the size of particles. Filtration removes solids based on their size by passing the wastewater through a filter medium. Activated sludge involves the biological treatment of wastewater where larger aggregates, or flocs, formed by microorganisms are separated based on size during the aeration and settling processes. Flotation separates particles from wastewater by promoting their rise to the surface, which is influenced by the size and density of the particles.

By focusing on the chemical processes involved, precipitation stands out as a method that operates independently of the size of the particles it targets, which distinguishes it from the other treatment methods listed.