Sources of iron and manganese in drinking water
Iron and manganese are the 4th and 13th most common metallic elements found in the Earth’s crust, respectively. Water percolating through soil and rock can dissolve minerals containing iron and manganese and hold them in solution in varying amounts, depending on other water properties, such as pH, oxygen levels, salinity, and the presence or absence of other chemicals in solution. Iron pipe corrosion may be a source of iron in drinking water.
In aquifers, where oxygen content is low, reduced forms of iron and manganese predominate in clear and colorless forms. When water from aquifers containing iron and manganese is exposed to air, these elements are oxidized (combine with oxygen) to less water soluble forms. Upon oxidation, colored forms of iron and manganese become visible in water. In the case of iron, white, then yellow and finally red-brown solid particles form that settle out of the water. Iron oxide particles may not settle out and can impart the water with a red tint. Oxidized forms of manganese usually remain dissolved in water, giving it a black tint. These abrupt changes in the chemical forms of iron and manganese are responsible for the staining properties of aquifer waters containing high concentrations of these elements. Iron will cause reddish-brown staining of laundry, porcelain, dishes, utensils and even glassware. Manganese causes a brownish-black stain. Soaps and detergents do not remove these stains, and use of chlorine bleach intensifies the stains. In rare occasions, the addition of laundry bleach to manganese-rich water may turn the water purple due to the formation of permanganate ions.
Deposits of iron and manganese can build up in pipelines, pressure tanks, water heaters and water softeners. This reduces the available quantity and pressure of the water supply.
Iron and manganese can affect the flavor and color of food and water. They may react with tannins in coffee, tea and some alcoholic beverages to produce a black sludge, which affects both taste and appearance.
A problem that frequently results from iron or manganese in water is iron or manganese bacteria. These non-pathogenic (non-health threatening) bacteria feed on iron and manganese in water, forming red-brown (iron) or black-brown (manganese) slime, often detected in toilet tanks, and can clog water systems. In addition, a “foul” odor can be produced.
Potential health effects of iron and manganese in drinking water
Iron and manganese in drinking water are not considered health hazards. In addition, iron and manganese bacteria are not known to present a health risk.
Testing drinking water for iron and manganese
The quality of water supplied by public water systems is regulated by the U.S. Environmental Protection Agency (EPA.) Iron and manganese are both classified under the Secondary Maximum Contaminant Level standards, which are based on aesthetic factors such as color and staining properties of water rather than health effects. The standard in drinking water is 0.3 milligrams per liter (mg/l) for iron, sometimes expressed as 0.3 parts per million (ppm), and 0.05 mg/l (ppm) for manganese. Secondary standards are guidelines and are not enforced.
Consumers wanting to know the concentration of dissolved iron and/or manganese in a private water supply will need to have the water tested. If foul odor (not a rotten egg smell) and a red or black slime layer are found in places like the toilet bowl or reservoir, then individuals should request to have water tested for iron and manganese bacteria.
Note that if a rotten smell is detected, it might indicate the presence of reduced forms of sulfur, such as hydrogen sulfide, a toxic gas. The rotten egg smell presence in water should be treated with extreme caution. Pretreatment is required to fully oxidize sulfur species to sulfates and to control pipe corrosion.
Tests to determine the presence of iron or manganese, and of iron and manganese bacteria, in drinking water should be done by a state certified laboratory utilizing approved EPA methods for the detection of iron and manganese.
Options for iron and manganese in drinking water
Secondary iron and manganese standards are established as guides to manage taste, odor, and color of water. Drinking water suppliers are not required by federal law to meet these secondary standards. If iron and/or manganese levels in drinking water approach or exceed the standard, some public water suppliers voluntarily reduce or remove iron and manganese from the water.
If excessive iron or manganese is present in your water supply, you might consider an alternative source for drinking water, or water treatment. It may be possible to obtain a satisfactory alternate water supply by drilling a new well in a different location or at a different depth in the same or different aquifer.
Several methods of removing iron and manganese from water are available. The most appropriate method depends on many factors, including the concentration and form of iron/manganese in the water, if iron or manganese bacteria are present, and how much water you need to treat. Point-of-use (POU) devices such as reverse osmosis and distillation can remove dissolved iron and manganese. However, these treatment systems are not generally recommended. Since excess iron and manganese are aesthetic problems that affect all potential uses of the water they are most often removed from all water entering the home using Point-of-entry (POE) treatment devices. The four most commonly applied methods for treating water containing dissolved iron and manganese, are: ion exchange water softeners; oxidizing filters; aeration (pressure type) followed by filtration, chemical oxidation followed by filtration.
The most common approach to control iron and manganese bacteria is shock chlorination. It is almost impossible to kill all the iron and manganese bacteria in a system. In most cases, they will grow back eventually and the shock chlorination procedure will most likely need to be repeated from time to time. If bacteria regrowth is rapid, repeated shock chlorination becomes time consuming. Continuous application of low levels of chlorine may be more effective. Because chlorine changes dissolved iron into oxidized iron that will precipitate, a filter may be needed to remove oxidized iron if continuous chlorination is used to control iron bacteria.