The belief has been expressed by John H. Gregory, consulting engineer, that as a general proposition the cost of building a combined system is less than that of constructing a separate system, especially where the territory to be served is more or less closely built up and streets paved. In suburban territory, not closely built up and where storm water is easily and quickly diverted into natural water courses, he believes the separate system will in general cost less, for then only sanitary sewers need to be built first, the storm water sewers being deferred for years or only such drains constructed as are immediately required. When there are steep grades and relatively high velocity all authorities agree with Gregory that it is advisable to build combined sewers, even though the development of the territory may hardly be such as to require the removal of the storm water.
Discussing the merits of the two systems so far as they affect the cost of disposal Clark P. Collins, sanitary engineer, concludes that generally speaking “it is unwise to dilute sewage with storm water and to befoul storm water with sewage in the attempt to remove both by the same underground channel.” Gregory has expressed the opinion that if sewage is to be discharged into a body without treatment the combined system will offer the simplest and cheapest solution of the problem.
Among the principal objections to the combined system when the sewage is treated are the increase it causes in the volume of liquid which necessarily requires a larger plant and expenditure, the changes it causes in the character of the sewage which complicates operation of the plant, and the frequency with which it causes the flow of sewage to exceed the maximum of the plant, thereby making it necessary to discharge untreated sewage into the stream. With a combined system all kinds of trade wastes must be run through the disposal plant, whether they are offensive or not; automatic devices, which should be avoided whenever possible, are necessary between the combined and intercepting sewers to limit the amount of flow; a greater amount of grit is deposited at the disposal works unless in the separate system the first wash of the street is intercepted. The New York State Board of Health advocates the separate system.