What is primary treatment in wastewater treatment?

Primary treatment is the physical treatment stage that removes settleable and floating solids from wastewater. It includes screening to remove large objects, grit chambers to settle sand and gravel, and primary sedimentation tanks where suspended solids settle as sludge while oils and grease float and are skimmed off. Primary treatment typically removes 50-70% of suspended solids and 25-40% of BOD.