Sewage treatment is the process of removing contaminants from wastewater, primarily from household sewage.Physical, chemical, and biological processes are used to remove contaminants and produce treated wastewater (or treated efluent) that is safer for the environment. A by-product of sewage treatment is usually a semi-solid waste or slurry, called sewage sludge. the sludge has to undergo further treatment before being suitable for disposal or application to land.