What type of treatment primarily focuses on removing smaller particles that may not settle by gravity alone?

The focus of direct filtration treatment is on the removal of smaller particles from water that are too fine to settle by gravity alone. This method typically employs a series of physical filters, often using sand, anthracite coal, or other filtration media, to capture these tiny particulates. Unlike processes that rely solely on sedimentation, direct filtration combines the removal of these small particles with other treatment processes to produce high-quality water.

Direct filtration is particularly advantageous for treating water that is relatively low in turbidity, as it can efficiently reduce small suspended solids and some microorganisms. The success of this method hinges on effective filtration media and proper operational techniques to maintain filter efficiency.

Other options such as coagulation mainly focus on aggregating smaller particles into larger flocs to promote their settling or subsequent removal, while reverse osmosis primarily separates water from dissolved solids and contaminants at a molecular level. Advanced oxidation is a method that uses powerful oxidants to degrade organic pollutants but is not primarily about particle removal. Thus, direct filtration stands out as the most appropriate answer for the specific focus on removing smaller particles that may not settle by gravity alone.