Abstract

Sewer-solids sediment is one of the major sources of pollutants in urban wet-weather flow discharges that include combined sewer overflow, separate sanitary sewer overflow, and storm water runoff. During low-flow, dry-weather periods, sanitary wastewater solids deposited in combined sewers have significant adverse effects on the integrity of the sewerage system and the quality of the receiving water.

In the United States, estimates of dry-weather flow deposition in combined sewers vary from 5 to 30 percent of the daily inputs of solids and pollutants. In Europe, average deposition rates have been measured between 30 and 500 grams per meter per day. Even sewers that are designed to be self cleansing will have transient sediment deposits, and part of the load in transport will move near 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 biochemical oxygen demand, chemical oxygen demand, and ammonia in sewer sediments can be as high as 150,000 milligrams per liter (mg/L), 200,000 mg/L, and 300 mg/L, respectively. During a storm event, resuspended sediments are discharged directly into receiving waters.

This report covers sources of sewer solids, sewer-solids loading, sewer sediment and associated pollutants and their impacts, sewer cleaning, and in-sewer sediment control. For in-sewer sediment control, the report presents a number of in-sewer flushing systems with case studies.

Contact

Joyce Walling

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