Primary treatment removes 40-50% of the solids. Sanitary sewers carry wastewater from homes and businesses to the treatment plant. Bar screens let water pass, but not trash. The trash is collected and properly disposed. A grit chamber is a large tank that slows down the flow of water. This allows sand, grit, and other heavy solids to settle at the bottom for removal later.
Secondary treatment completes the process, so that 85-95% of the pollutants are removed. A secondary sedimentation tank allows the microorganisms and solid wastes to form clumps and settle. Some of this mixture, called "activated sludge," can be mixed with air again and reused in the aeration tank. A disinfectant, such as chlorine, is added to the wastewater before it leaves the treatment plant. The disinfectant kills disease-causing organisms in the water. After treatment, the water can be returned to nearby waterways. It can also be used on land for agriculture and other purposes.