Sewage treatment. Aerial view of pools used in bacterial aerobic treatment of sewage at a sewage treatment plant. The pools contain organic matter suspended in solution. The water in the pool is agitated to prevent the matter from settling. The water is oxygenated to promote aerobic bacteria which convert the sewage into carbon dioxide and other stable chemicals. The bacteria are then filtered out of the liquid. Inorganic solids are then removed before discharging the water into the sea or a river. Photographed in Germany.