Quality characteristics of urban storm water in Sydney, Australia

Storm water samples collected from three urban watersheds in Sydney, Australia, show that in separately sewered areas the surface runoff has a greater concentration and carries a greater total load of pollutants than effluent from secondary sewage treatment plants. Pollution and nutrient indicators are highly concentrated in the ‘first flush’ at the start of urban storm runoff, but these concentrations fall rapidly to quite low levels during the passage of each flood. An important exception to this trend is phosphate. The phosphate concentration remains approximately constant during each flood. Whenever the discharge increases sharply, the concentrations of suspended solids and phosphate increase slightly. However, the concentrations of BOD and ammonia do not usually increase after the first flush unless a later increase in discharge is extremely rapid. Fecal coliform concentrations in flood flows are usually about 2 orders of magnitude less than those for raw sewage.