Following preliminary treatment, the water moves on to a clarifier or settling tank. This is primary treatment. Here, under very calm conditions, sewage solids settle to the bottom of the tank. Not all of the solids are removed here in the primary clarifier because very small, light particles take too long to settle. These small particles and dissolved organic matter are removed later. The solids that collect at the bottom of the tank are called primary sludge and are pumped from the tank for treatment elsewhere in the plant.

Alt | Win