After preliminary treatment, the water goes into into primary clarifiers - settling tanks. The majority of solids that were not removed during preliminary treatment are removed during this process. Primary clarifiers allow the solids to settle to the bottom of the tank, and the water continues on for treatment. The solids at the bottom of the tank is called primary sludge, and is pumped to be treated elsewhere.
Small particles and organic material that are not removed during this process will be removed in the next step. It would take too much time for these particles to settle out during primary treatment.