In-situ Piping for Pilot Study of Distribution System Corrosion

摘要

"Red water" is a corrosion-induced problem common in many distribution systems. The mechanism of red water formation is a complex phenomenon, and one that is poorly understood. The "In-situ Piping for Pilot Study of Distribution System Corrosion" study included a detailed investigation of chronic red water problems in distribution systems, which consisted of using in-situ pipe for pilot testing and multiple bench scale tests. This study was initiated after years of red water complaints in Oklahoma City, and the growing costs of an aggressive program to replace all unlined cast iron pipes with new cement-lined piping. The results of the study shed light on the site-specific factors at work in Oklahoma City and on the red water phenomenon in general. The cause of the troublesome red water (verified through sampling and bench-scale testing) was the oxidation of ferrous ions precipitating to ferric hydroxide. Furthermore, the source of the iron was found to be ferrous ions releasing from the existing scale in the unlined pipes.rnA 2,700-foot section of existing 6-inch unlined cast iron (CI) pipe was isolated from the distribution system and used for pilot-scale testing. The line was isolated with a backflow preventer to isolate experimental chemicals from mixing with the bulk water supply. Using the in-situ pilot pipe provided an excellent means for conducting a variety of tests under conditions accurately matching the actual distribution system. The two key objectives to the pilot study were: (1) investigate the relationship between rate of flow (particularly low flow or stagnation) and red water formation, and (2) investigate the impact of chemical addition on red water formation. Several chemicals were trialed including: orthophosphates, polyphosphates, sodium silicate, and stannous chloride. Each product was tested on a bench scale, prior to testing with the pilot pipe. In all, twelve experimental field tests were performed with the in-situ pilot pipe.rnLow-flow testing verified that iron released from the scale was most closely associated with stagnation, and that alternating periods of stagnation and flow provided the needed conditions for the separate steps of iron release and oxidation. Orthophosphates were the most effective in preventing color formation. Full-scale testing is underway to assess the benefits on a system-wide basis. A cost analysis was conducted on the various alternative approaches to eliminating the red water complaints, including pipe replacement, pipe cleaning and lining, automated flushing, supplemental pumping through low-demand areas, and chemical treatment using polyphosphates. The analysis showed that orthophosphate treatment, assuming it proves to be effective under all conditions, is the least-cost approach for preventing red water complaints in Oklahoma City.