URBAN POINT SOURCE DESIGN PROBLEM (Circa 1920s) River Critical concentration,c [M/L3] Waste loading,W[M刀 WTP WWTP Waste removed CITY WATER-QUALITY MODEL: c fW,physics,chemistry,biology) WATER-QUALITY MODEL: URBAN POINT SOURCE DESIGN PROBLEM (Circa 1920s) WTP Waste removed CITY Waste loading, W [M/T] Critical concentration, c [M/L3 ] WWTP River c = f(W, physics, chemistry, biology)