. The sanitation of cities. ebeen expended on the improvement of the rivers;with the two objects in view of making themvaluable commercial assets, and of reducing theneed of sewage treatment expense to a commercial waterways and deep chan-nels, not only provides for water-borne freight,but also greatly increases the quantity of waterpresent available for sewage dilution, and pro-vides favorable conditions for the mechanicaltreatment of sewage. The danger from floods isalso greatly reduced, and the natural drainage ofextensive tracts of country much improved;swamps and marsh la
. The sanitation of cities. ebeen expended on the improvement of the rivers;with the two objects in view of making themvaluable commercial assets, and of reducing theneed of sewage treatment expense to a commercial waterways and deep chan-nels, not only provides for water-borne freight,but also greatly increases the quantity of waterpresent available for sewage dilution, and pro-vides favorable conditions for the mechanicaltreatment of sewage. The danger from floods isalso greatly reduced, and the natural drainage ofextensive tracts of country much improved;swamps and marsh lands are dried, directly pre-venting malarious conditions, and destroying thebreeding places of mosquitos. The commercialand sanitary conditions are all benefited by suchuse of money—and the saving in cost of sewagetreatment is large. Money used in waterway im-provements gives immense returns on the invest-ment, while that used on sewage treatment plants—notwithstanding their sanitary importance—islocked up forever. 95. DRESDEN, SAXONY Interior view of the screen house, showing general arrangement of the Riensch-Wurl screens (each twenty-six feet three inches diameter), screen bridges, motors and driving mechanism, with elevators for screenings, effluent discharge channel at left 96 CHAPTER X THE PURIFICATION OF SEWAGE From the foregoing considerations we now turnto the need of a more complete treatment of sew-age than the removal of suspended matter willgive. Such treatment may be required if the efflu-
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