It might surprise you to know that we all drink and bathe in recycled water. The water we flush down the toilet is turned into potable water, and it is the job of sewage treatment plants to make this water fit for human consumption or release into rivers and oceans. Wastewater from sinks, baths, washing machines, toilets and other appliances has to go somewhere. After traversing miles of sewer network, it ends up in sewage treatment plants whose job is to treat and discharge it. Sewage treatment plants collect, treat, and discharge wastewater, providing a service essential to environmental and public health. Without adequate treatment, sewage will leach into the environment and contaminate ecosystems. For example, sewage contains bacteria and chemicals that break down using oxygen in the water. In doing so, they use oxygen that fish and aquatic life needs to survive, so it needs treatment to preserve the ecosystem. Returning sewage/wastewater to a specified quality for safe discharge is the most crucial role of the sewage treatment plant. Sewage treatment plants run wastewater through multiple treatment stages. After preliminary filtration, there are three main stages of wastewater treatment (primary, secondary, and tertiary), with the third stage reserved for polishing.