4. Water Flows Into the Secondary Clarifier

  • Similar to the grit chamber tank, the secondary clarifier utilizes settling velocity to treat effluent. This process removes any remaining solids larger than 10 micrometers. Scrapers rotate along the bottom of the tank skimming the sludge from the bottom and directing it to the middle of the tank where it is collected. A percentage of the collected sludge is reintroduced to the aeration basin as return activated sludge while a portion called waste activated sludge is sent to biosolids treatment, stabilized and used as soil or is sent to the landfill.
    • At this stage in the treatment process, roughly 85% to 95% of biological matter in the water should already be removed. The secondary clarifier is the final treatment process to remove solids and biological matter from the water.
    •  The top layer of water in the tank flows over a weir wall towards the final treatment process – disinfection.