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TOXIC POLLUTANTS IN URBAN WET-WEATHER FLOWS: AN OVERVIEW OF THE MULTI-MEDIA TRANSPORT, IMPACTS, AND CONTROL MEASURES (PRESENTATION)
Citation:
Fan*, C, R Field*, D Sullivan*, AND F Lai*. TOXIC POLLUTANTS IN URBAN WET-WEATHER FLOWS: AN OVERVIEW OF THE MULTI-MEDIA TRANSPORT, IMPACTS, AND CONTROL MEASURES (PRESENTATION). Presented at ASCE World Water & Env Resources Congress, Orlando Fl, 5/20-24/2001.
Description:
This paper presents an oveview of the transport of toxic pollutants through multiple media in the urban environment. Discussions include the sources of particulate-associated toxic substances and the relationship of these toxics to atmospheric deposition, overland accumulation and washoff, contaminated sediments and erosion in urban drainage systems, and their impacts on both surface water and groundwater quality. In addition, remedial measures for controlling toxic pollution in each medium will be addressed. During dry-weather periods, incompletely combusted hydrocarbons from automobiles combine with atmospheric particulates from a host of other sources and deposit on urban roadways, parking lots, and other urban surfaces. During a storm evern, those solids are washed off by surface runoff and drained into either separated storm wewer or combined sewer systems. In levl urban areas where the combined sewer system has relatively low slopes, solids (sewer sediments) tend to accumulate in the sewer during low-flow periods; these solids contain a mix of toxic air pollutants from fallout and solids from sanitary wastewater and have high concentrations of pollutants (150,000 mg/L of BOD(5); 200,000 mg/L of COD; and 300 mg/L of NH3-N). Stormw cause high flow in the sewers that resuspend the solids. When discharged directly into receiving waers, these cause degradation of water quality. In addition, these solids contain organic matter that generate toxic, corrosive, and explosive gases, e.g., hydrogen sulfide and methane, under anoxic conditions. Sulfate, a natural sanitary wastewater constituent, is reduced to hydrogen sulfide and then is oxidized to sulfuric acid; that causes destruction of the sewer structure and subsequent groundwater contamination. Thus, control of sewer sediment not only protects urban receiving-water quality but also presents hazardous conditions in sewers and prevents deterioration of sewer structure. Methods for abating the multi-media transport of toxics in the urban environments are presented, including best management practices for control of stormwater pollution; steeper sewer slopeage and improved pipe bottom shapes to maintain high velocity during low-flow periods; upstream sediment trapping to reduce clogging of sewers; and period sewer flushing.