Sewer and tank flushing for sediment, corrosion, and pollution control

This paper presents an overview of causes of sewer deterioration together with a discussion of control methods that can prevent or arrest this deterioration. In particular, this paper covers inline and combined sewer overflow storage tank flushing systems for removal of sediments and minimization of hydrogen sulfide production resulting in the reduction of associated pollution and sewerline corrosion. During low-flow dry-weather periods, sanitary wastewater solids deposited in combined sewer systems can generate hydrogen sulfide and methane gases due to anaerobic conditions. Sulfates are reduced to hydrogen sulfide gas that 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 that 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 (FG)] was evaluated by a detailed examination of 18 facilities in Germany, Canada, and United States. As a result, both the tipping flusher and FG technology appear to be the most cost-effective means for flushing solids and debris from combined sewer overflow-storage tanks, while the FG is considered to be the most efficient method for flushing large diameter, flat sewers. In addition, capsulated reviews of several cost-effectiveness analyses are reported demonstrating the comparative benefits of flushing technology.