Iron- corrosion in water

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Corrosion is the degradation of metal. In water distribution pipelines, valves, and fixtures, it can cause the degradation of the quality of our drinking water. EPA’s research helps to control corrosion and reduce the release of metals into our drinking water distribution systems. Iron makes up about 5% of the earth’s crust and can exist as soluble ferrous iron or as the relatively insoluble ferric form found in water.

Soluble ferrous iron is found in ground water, in anaerobic reservoirs, in dead-ends in water distribution systems, and in scale within pipes. When soluble ferrous iron is exposed to oxygen or to a disinfectant during water treatment, it oxidizes to the relatively insoluble iron (i.e., suspended colloidal and particulate iron) that is responsible for discolored water.

The primary sources of iron in drinking water are the natural geology, and aging and corroding distribution systems and household pipes. Iron-based materials, such as cast iron and galvanized steel, have been widely used in our water distribution systems and household plumbing.

Unlike lead and copper, iron ingestion is not associated with adverse health effects. Iron is even essential to human health in small concentrations. (Iron deficiency can lead to anemia.) Although iron doesn’t pose a direct health risk, trace impurities and microorganisms that are absorbed by iron solids may pose health concerns.

The effects associated with iron contamination can be grouped into two categories: aesthetic effects and physical effects.
Aesthetic effects are undesirable tastes or odors. Iron in quantities greater than 0.3 milligrams per liter (mg/L) in drinking water can cause an unpleasant metallic taste and rusty color. Taste is a useful indicator of water quality even though taste-free water is not necessarily safe to drink. Taste is also an indicator of the effectiveness of different kinds of treatment, such as water softening or reverse osmosis treatment systems, which effectively remove iron from drinking water. Elevated levels of iron in drinking water can also cause a rusty color that can stain laundry or household fixtures. Discolored water is one of the most frequent consumer complaints about drinking water.

Physical effects are damage to water equipment and reduced effectiveness of treatment for other contaminants. Corrosivity and staining related to corrosion not only affect the aesthetic quality of water, but may also result in distribution system problems. Among other things, corrosion of distribution system pipes can produce sediment or loose deposits that block water flow.

Corrosion and Metal Solubility Control

Control of corrosion and metal solubility is perhaps the single most cost-effective method for preventing iron contamination. Significant benefits include:

Reduction of contaminants at our taps
Cost savings by extending the useful life of water mains and service lines
Energy savings from transporting water more easily through smoother, non-corroded pipes
Reduced water losses resulting from leaking or broken mains or other plumbing

Corrosion control is used to manage the acidity, alkalinity, and other water qualities that affect pipes and equipment used to transport water. By controlling these factors, the chance of iron leaching into our water supply is reduced.

Conventional treatments can also be used to remove secondary contaminants, such as iron, from our drinking water. These treatments include coagulation/flocculation, filtration, aeration, and granular activated carbon. Nonconventional treatments include distillation, reverse osmosis, and electrodialysis. However, these are fairly expensive technologies and may be impractical for smaller systems. Nontreatment options include blending water from the principal source with uncontaminated water from an alternative source.

Regulations

Unlike lead and copper, the ingestion of iron is not associated with adverse health effects, so mandatory iron regulations are not in place. However, based on aesthetic issues, EPA has issued a Secondary Maximum Contaminant Level (SMCL) of 0.3 mg/L for source water iron. (Note: EPA does not enforce SMCLs; rather, they are used as guidelines to assist public water utilities in managing their drinking water for aesthetic considerations.)

Although federal regulations do not require them to do so, state health agencies and public water utilities often decide to monitor and treat their supplies for secondary contaminants. For example, source water iron can be removed by oxidation followed by sedimentation and filtration or by iron sequestration (that is, using any mechanism to prevent visible precipitation).