Sewerage systems may be laid out on any one of three systems: separate, storm, or combined. A separate system of sewers is one in which only sanitary sewage or industrial wastes or both are allowed to flow. Storm sewers carry only surface drainage, exclusive of sanitary sewage. Combined sewers carry both sanitary and storm sewage. The use of a combined or a separate system of sewerage is a question of expediency. Portions of the same system may be either separate, combined, or storm sewers. Some conditions favorable to the adoption of the separate system are where: _a._ The sanitary sewage must be concentrated at one outlet, such as at a treatment plant, and other outlets are available for the storm drainage. _b._ The topography is flat necessitating deep excavation and steeper grades for the larger combined sewers. _c._ The sanitary sewers must be placed materially deeper than the necessary depth for the storm-water drains. _d._ The sewers are to be laid in rock, necessitating more difficult excavation for the larger combined sewers. _e._ An existing sewerage system can be used to convey the dry weather flow, but is not large enough for the storm sewage. _f._ The city finances are such that the greater cost of the combined system cannot be met and sanitary drainage is imperative. _g._ The district to be sewered is an old residential section where property values are not increasing and the assessment must be kept down. Some additional points given in a report by Alvord and Burdick to the city of Billings, Montana, are: The separate system of sewerage should be used, where: 1st. Storm water does not require extensive underground removal, or where it can be concentrated in a few shallow underground channels. 2nd. Drainage areas are short and steep facilitating rapid flow of water over street surfaces to the natural water courses. 3rd. The sanitary sewage must be pumped. 4th. Sewers are being built in advance of the city’s development to encourage its growth. 5th. The existing sewer is laid at grades unsuitable for sanitary sewage, it can be used as a storm sewer. A combined system must be relatively larger than a separate storm sewer as the latter may overflow on exceptional occasions, but the former never. A combined system of sewerage should be used where: 1st. It is evident that storm and sanitary sewerage must be provided soon. 2nd. Both sanitary and storm sewage must be pumped. 3rd. The district is densely built up.