Aeration Tank: Used in secondary treatment for the aerobic biological process to degrade organic matter. 
Aerated Lagoon: Also a secondary treatment method where microorganisms decompose organic matter under aerobic conditions. 
Trickling Filter: Another secondary treatment unit where organic matter is degraded biologically through microorganisms attached to a media over which wastewater flows. 
Imhoff Tank: Not a secondary treatment unit; it is used for primary treatment. The Imhoff tank combines sedimentation and anaerobic digestion within the same tank and is designed to settle out solids from wastewater and digest the organic matter in the settled solids.