(4) The sewers never overflow, as is frequently the case in the combined system.
(5) The sewers being small, no decomposition takes place therein.
(6) Sewers of small diameter need no special means of ventilation, or main traps on house drains, and can be ventilated through the house pipes.
On the other hand, the disadvantages of the separate system are:
(1) The need of two systems of sewers, for sewage and for rain water, and the expense attached thereto.
(2) The sewers used for sewage proper require some system for periodically flushing them, which, in the combined system, is done by the occasional rains.
(3) Small sewers cannot be as well cleaned or gotten at as larger ones.
The separate system has been used in Memphis and in Keene, N. H., for a number of years with complete satisfaction. Most cities, however, use the combined system.