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: