Sewer and Tank Flushing for Corrosion and Pollution Control

This paper presents an overview of causes of sewer deterioration concurrently with a discussion of control methods that can prevent or arrest this deterioration. In particular, discussed are inline- and combined sewer overflow- (CSO) storage-tank-flushing systems for removal of sediments and minimizing hydrogen sulfide production and associated corrosion. During low-flow dry-weather periods, sanitary wastewater organics and solids deposited in combined sewer (CS) systems can generate hydrogen sulfide and methane gases due to anaerobic conditions. Sulfates are reduced to hydrogen sulfide gas which can then be oxidized to sulfuric acid on pipes and structure walls by further biochemical transformation. Furthermore, these solids deposits or sediments are discharged to the urban streams during storm events which can cause degradation of receiving-water quality. Thus, dry-weather sewer sedimentation not only creates hazardous conditions and sewer degradation but contributes significant pollutant loads to the urban receiving waters during wet-weather high-flow periods. Performance of two technologies, i.e., the tipping flusher and the flushing gate was evaluated by a detailed examination of 18 facilities in Germany, Canada, and United States. As a result, both the tipping flusher and flushing gate technology appear to be the most costeffective means for flushing solids and debris from CSO-storage tanks, while the flushing gate is considered to be the most efficient method for flushing large diameter, flat sewers. In addition, cost-effectiveness analysis was conducted for an actual project under construction, comparing flushing gate technology versus conventional large pipe cleaning operations using bucketing methods.