Sewage treatment in most large cities involves two main steps, primary and secondary. Some cities require an additional step called tertiary treatment. Primary treatment removes the heaviest solid material from the sewage. At the treatment plant, sewage first passes through a screen that traps the largest pieces of matter. It then flows into a large sedimentation tank where many of the suspended solids sink to the bottom of this tank forming sludge.
Grease floats to the surface, where it can be removed by skimming.
Secondary treatment removes from 85 to 90 per cent of the solids and oxygen-consuming wastes remaining in sewage after it has undergone primary treatment. The most common methods of secondary treatment are the activated sludge process and the trickling filtration process. Tertiary treatment may include chemical treatment, microscopic screening and radiation treatment to make effluent safer to discharge into waterways or the sea.