AWWA ACE56363

"Red water" is a corrosion-induced problem common in many distribution systems. Themechanism 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 detailedinvestigation of chronic red water problems in distribution systems, which consisted of using in-situpipe for pilot testing and multiple bench scale tests. This study was initiated after years ofred water complaints in Oklahoma City, and the growing costs of an aggressive program toreplace all unlined cast iron pipes with new cement-lined piping. The results of the study shedlight on the site-specific factors at work in Oklahoma City and on the red water phenomenon ingeneral. The cause of the troublesome red water (verified through sampling and bench-scaletesting) was the oxidation of ferrous ions precipitating to ferric hydroxide. Furthermore, thesource of the iron was found to be ferrous ions releasing from the existing scale in the unlinedpipes.A 2,700-foot section of existing 6-inch unlined cast iron (CI) pipe was isolated from thedistribution system and used for pilot-scale testing. The line was isolated with a backflowpreventer to isolate experimental chemicals from mixing with the bulk water supply. Using thein-situ pilot pipe provided an excellent means for conducting a variety of tests under conditionsaccurately matching the actual distribution system. The two key objectives to the pilot studywere: investigate the relationship between rate of flow (particularly low flow or stagnation)and red water formation; and, investigate the impact of chemical addition on red waterformation. Several chemicals were trialed including: orthophosphates, polyphosphates, sodiumsilicate, and stannous chloride. Each product was tested on a bench scale, prior to testing with thepilot pipe. In all, twelve experimental field tests were performed with the in-situ pilot pipe.Low-flow testing verified that iron released from the scale was most closely associated withstagnation, and that alternating periods of stagnation and flow provided the needed conditions forthe separate steps of iron release and oxidation. Orthophosphates were the most effective inpreventing color formation. Full-scale testing is underway to assess the benefits on a systemwidebasis. A cost analysis was conducted on the various alternative approaches to eliminatingthe red water complaints, including pipe replacement, pipe cleaning and lining, automatedflushing, supplemental pumping through low-demand areas, and chemical treatment usingpolyphosphates. The analysis showed that orthophosphate treatment, assuming it proves to beeffective under all conditions, is the least-cost approach for preventing red water complaints inOklahoma City. Includes tables, figures.

Product Details

Edition:

Vol. - No.

Published:

06/16/2002

Number of Pages:

19

File Size:

1 file , 590 KB