=5. Methods of Treatment.=—Screening and sedimentation are the primary methods for the treatment of sewage. By these methods a portion of the floating and settleable solids are removed, preventing the formation of unsightly scum and putrefying sludge banks. Chemicals are sometimes added to the sewage to form a heavy flocculent precipitate which hastens sedimentation of the solid matters in the sewage. The process in these methods is mechanical and the solid matters removed from the sewage must be disposed of by other methods than dilution with the sewage effluent. More complete methods of treatment are dependent on biologic action. Under these methods of treatment complete stabilization of the effluent is approached, and in the most complete treatment an effluent is produced which is clear, sparkling, non-odorous, non-putrescible, and sterile. Sterilization of sewage, usually with chlorine or some of its compounds, has been used, not to reduce the amount of diluting water necessary, but to reduce the number of pathogenic germs and to minimize the danger of the transmission of disease. =6. Definitions.=—Sewage and sewerage are not synonymous terms although frequently confused. Sewage is the spent water supply of a community containing the waste from domestic, industrial or commercial use, and such surface and ground water as may enter the sewer.[9] Sewerage is the name of the system of conduits and appurtenances designed to carry off the sewage. It is also used to indicate anything pertaining to sewers. A difference is made between sanitary sewage, storm sewage, and industrial wastes. Sanitary sewage, sometimes called domestic sewage, is the liquid wastes discharged from residences or institutions, and contains water closet, laundry and kitchen wastes. Storm sewage is the surface run-off which reaches the sewers during and immediately after a storm. Industrial wastes are the liquid waste products discharged from industrial plants. A sewer is a conduit used for conveying sewage. The names of the conduits through which sewage may flow are: _Soil Stack._—A vertical pipe in a building through which waste water containing fecal matter or urine is allowed to flow. _Waste Pipe._—A vertical pipe in a building through which waste water containing no fecal matter is allowed to flow. _House Drain._—The approximately horizontal portion of a house drainage system which conveys the drainage from the soil stack or waste pipe to the point of discharge from the building. _House Sewer._—The pipe which leads from the outside wall of the building to the sewer in the street. _Lateral Sewer._—The smallest branch in a sewerage system, exclusive of the house sewers. _Sub-main or Branch Sewer._—A sewer from which the sewage from two or more laterals is discharged.[10] _Main or Trunk Sewer._—A sewer into which the sewage from two or more sub-main or branch sewers is discharged.[11]