Sewage treatment is the process of removing contaminants from wastewater, primarily from food industry, milk industry & household sewage. It includes physical, chemical, and biological processes to remove these contaminants and produce environmentally safe treated wastewater (or treated effluent). A by-product of sewage treatment is usually a semi-solid waste or slurry, called sewage sludge, that has to undergo further treatment before being suitable for disposal or land application. Sewage treatment may also be referred to as wastewater treatment, although the latter is a broader term which can also be applied to purely industrial wastewater. Surface runoff and effluents from small-scale industries or pre-treated industrial wastewaters are sometimes routed through municipal sewage treatment plants when the environmental advantages of treatment outweigh the disadvantages of reduced treatment efficiency. Dilution of sewage by stormwater runoff or industrial wastewater with low biochemical oxygen demand (BOD) decreases the efficiency of secondary treatment; because secondary treatment ecosystems require a minimum concentration of biologically decomposable waste to sustain the ecosystem population.