You are here:
THE EFFECT OF CHLORIDE AND ORTHOPHOSPHATE ON THE RELEASE OF IRON FROM A DRINKING WATER DISTRIBUTION SYSTEM CAST IRON PIPE
Citation:
Lytle*, D A. AND V. L. Snoeyink. THE EFFECT OF CHLORIDE AND ORTHOPHOSPHATE ON THE RELEASE OF IRON FROM A DRINKING WATER DISTRIBUTION SYSTEM CAST IRON PIPE. Presented at 2003 AWWA WQTC, Philadelphia, PA, November 02 - 06, 2003.
Impact/Purpose:
To inform to public
Description:
"Colored water" describes the appearance of drinking water that contains suspended particulate iron although the actual suspension color may be light yellow to red depending on water chemistry and particle properties. The release of iron from distribution system materials such as cast iron water mains is a complex process involving mass transfer and many interactions including corrosion and oxidation-reduction, precipitation and dissolution, solid phase reactions, microbiological activity, and is most greatly influenced by water chemistry.
Objective
The objective of this research was to evaluate the effect of two water quality parameters, chloride and orthophosphate concentration, on the release of iron from an old cast iron pipe section removed from a drinking water distribution system and on the properties of the resulting suspensions.
Experimental
Experiments were conducted using a section of cast iron pipe removed from a local drinking water distribution system. Local tap water was used to carefully fill the pipe section daily. Water was held in contact with the pipe for 23 hours (72 hours over the weekend), after which samples were drawn and the pipe section was refilled. The process was repeated daily. Sodium chloride and sodium phosphate were chemicals added if applicable to adjust water chemistry. Fill water and water sampled from the pipe following stagnation were tested for a number of parameters including pH, reduction-oxidation (REDOX) potential, dissolved oxygen, orthophosphate, free chlorine, ferrous and total iron, and a number of cations and anions. Filtered color and total iron measurements were also made using 0.45 m syringe filters.
Results
Increasing the chloride concentration by 100 mg/L significantly increased the concentration of iron released from the pipe section while the presence of orthophosphate at 3 mg/L decreased iron release. Chloride and orthophosphate also affected the water color and turbidity that were caused by the suspended iron particles, but the impact was not linear with respect to the concentration of iron released. Chloride and orthophosphate affected suspension appearance in two ways. For example, orthophosphate reduced the total amount of iron released from the pipe, and the color and turbidity (per mg of iron) of the resulting iron particle suspensions. Therefore, orthophosphate affected both the nature of the iron corrosion scale and the properties of the resulting particles. Water chemistry measurements showed that phosphate, as well as calcium, aluminum, and silicate were lost to the iron pipe during stagnation. Phosphate was also incorporated into the suspended iron particles. The study findings improve our understanding of the factors that affect colored water resulting from iron released from drinking water distribution system materials.