what is sludge explain

Dear student,After the wastewater passes through the grit and sand removal tank, it is allowed to settle down in a large tank, with a slope toward the middle. Solid materials like faeces settle at the bottom and are removed with the help of a scraper. The deposited waste material thus collected is called sludge. The contaminants that float over the surface of the water are removed by a skimmer. The water that is treated in this process is called clarified water and the process is called clarification. Sludge in the tank is allowed to decompose with the help of anaerobic bacteria (bacteria which can survive in the absence of oxygen) to produce biogas, that can be used for the generation of electricity.Regards