ESTIMATING URBAN WET-WEATHER POLLUTANT LOADING
Abstract:This paper presents procedures for estimating pollutant loads associated with the major sources of solids and solid materials in urban wet-weather flow. Equations are presented for the annual quantity of litter and floatables; the quantity of sand used on highways for traction during snowstorms; loadings of dust-and-dirt accumulated on street surfaces during dry weather; soil erosion within urban areas; and the load associated with the resuspension of sediments and solids deposited in urban sewers. Although dissolved contaminants are not addressed, these equations may be used to estimate the total maximum daily loads for watershed management plans.
During low-flow, dry-weather periods, sanitary wastewater solids deposited in combined sewers have significant adverse impacts on the integrity of sewerage system. In the U.S., estimates of dry weather flow deposition in combined sewers vary from 5 to 30% of the daily inputs of solids and pollutants. In Europe, average deposition rates have been measured at between 30 and 500 g/m/d. Even sewers that are supposedly designed to be ‘self-cleansing’ will have transient sediment deposits and part of the load in transport will move toward the sewer invert.
Deposited organic matter contains high concentrations of sulfates that can be reduced to hydrogen sulfide under anoxic conditions often reached in a sewer. The hydrogen sulfide is then oxidized to sulfuric acid, a highly toxic and corrosive gas, by biochemical transformation. The concentration of BOD5, COD, and NH3-N in sewer sediments can be as high as 150,000 mg/L, 200,000 mg/L, and 300 mg/L, respectively. During a storm event, resuspended sediments are discharged directly into receiving waters. Thus, sewer sedimentation should be controlled.
Document Type: Research Article
Publication date: January 1, 2003
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