Which process involves the gathering of small particles into larger masses during water treatment?

The process that involves the gathering of small particles into larger masses during water treatment is known as coagulation. This process is essential in water treatment as it helps to remove suspended solids and impurities from water sources. During coagulation, chemicals called coagulants, such as aluminum sulfate, are added to the water, which bond with the small particles. This interaction causes the small particles to clump together, forming larger aggregates known as flocs. These flocs can then be more easily removed during subsequent water treatment steps, such as sedimentation or filtration.

The other processes mentioned play different roles in water treatment. Filtration refers to the removal of particles from water as it passes through a medium, sedimentation is the settling of suspended particles under the influence of gravity, and aggregation is a broader term that can refer to the joining of particles but is not the specific term used in water treatment practice to describe the clumping of small particles into larger masses. Coagulation specifically focuses on the chemical processes involved in forming flocs, making it the correct answer in this context.