"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.
A 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: 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 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.
Low-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 systemwide
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.
Includes tables, figures.
| Edition : | Vol. - No. |
| File Size : | 1
file
, 590 KB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 19 |
| Published : | 06/16/2002 |
