This article, written by the American Ground Water Trust was originally published in THE AMERICAN WELL OWNER, 2002, Number 1]
SOLUTIONS TO MANGANESE PROBLEMS
Manganese (Mn) is very common in soils and sediment. It is commonly found with iron as mineral oxide coatings on the surface of soil and rock grains. When ground water contacts these coatings, the oxides are dissolved and may be transported to your well in the ground water. Manganese is commonly found in water as manganese ions (Mn++) or as manganese bicarbonate (Mn[HCO3]-). Dissolved manganese is colorless. Manganese coatings are commonly black.
Manganese is an essential trace element that is necessary for good health. The US Environmental Protection Agency (EPA) recommends that the level of manganese in drinking water be less than 0.05 milligrams per liter (mg/L) to avoid aesthetic staining problems. This limit is also enforced by the Food and Drug Administration (FDA) on bottled water products and is considered satisfactory to protect human health by the Centers for Disease Control (CDC) in Atlanta, Georgia. The CDC reported that an Estimated Safe and Adequate Daily Dietary Intake (ESADDI) for manganese for adults is 2 to 5 mg/day. The average adult diet includes 1 to 10 mg/day of manganese. Infants may require only 0.3 to 0.6 mg/day.
If dissolved manganese levels are above 0.05 mg/L, black or gray staining and a bitter metallic taste may result from oxidation of the water. Oxidation and precipitation of manganese will occur when well water mixes with oxygen in the air. This process may begin as ground water containing dissolved manganese enters a well and continues within the building’s plumbing. Adding oxidizing cleaning products such as laundry bleach or scouring powders to the water may intensify or speed up the oxidation process. Using these products may cause staining during clothes washing or other situations where the manganese is oxidized, precipitates as a solid and is left behind on a surface.
There are several technologies for reducing the level of manganese in water. Choosing the most efficient treatment will depend on the specific chemistry of the water being treated. It is recommended that the total iron, manganese, hardness, alkalinity and pH of the water be tested by a state-certified laboratory before purchasing equipment.
Ion Exchange Treatment
Ion exchange using a sodium chloride salt (NaCl) brine solution can remove low concentrations of dissolved manganese when the pH is greater than 6.8. The system should have a forceful backwash (100% bed expansion), preferably in a down flow direction to help remove the relatively heavy manganese oxide particles. The backwash should be scheduled to occur before the softening capacity of the system is exhausted.
Oxidation and Filtration
Oxidation of dissolved manganese creates particles of manganese dioxide (MnO2). Oxidation treatment is more effective for removing higher concentrations of dissolved manganese than ion exchange. Dissolved manganese generally oxidizes more slowly than iron which means it is relatively slower to form scale (precipitate), but is also harder to remove efficiently (quickly and cost effectively) from a water source by a treatment system.
Oxidation is commonly accomplished with chlorine although some newer home treatment systems are now using ozone (O3) as the oxidant. Oxidation treatment systems add an oxidant to the raw water in a mixing tank so that enough contact time is created to allow for maximum oxidation. The particles of MnO2 are then removed from the water by a mechanical filter. This may be followed by an activated carbon filter to remove any excess chlorine. Oxidation with chlorine is most effective when the water pH is greater than 9.5.
Potassium permanganate is a stronger oxidant than chlorine. Treatment with potassium permanganate commonly uses a “greensand” resin bed in conjunction with a regenerative backwash of concentrated potassium permanganate similar to the mechanical operation of an ion exchange water softener system
(the potassium permanganate solution substitutes for the salt brine. Note that the permanganate system will not remove calcium carbonate “hardness” from the water). The greensand is a resin coated with manganese oxide. As the raw water passes through the greensand the manganese oxide reacts with the dissolved manganese to form particles that become trapped in the resin bed. The particles must be flushed out during the backwash cycle so that the resin bed does not become clogged and fouled. Greensand systems do not require high dissolved oxygen contents but work best when the water pH is above 7.5.
Some oxidizing filters are self-contained treatments that precipitate and filter dissolved metals (manganese) in one filter canister/ tank. They must meet specific water chemistry requirements to work effectively. Unlike the greensand systems they do not need regeneration although they must be backwashed to remove the filtered particles. Examples include BirmTM and FiloxTM. These oxidizing filters work best when the water pH is above 7.5 and the dissolved oxygen content is greater than 15 percent of the manganese concentration.
Water quality problems are not always straight forward to solve. Be sure to get a written contract with your water treatment installer that specifies how any lingering water quality issues will be addressed, who will by responsible financially and what will be done if a water quality concern cannot be satisfactorily treated.
Refer to the Centers for Disease Control website for health information on manganese: http://www.atsdr.cdc.gov/toxprofiles/tp151.html. Refer to the National Sanitation Foundation (www.nsf.org) and Water quality Association (www.wqa.org) websites for more manganese treatment information.
[© American Ground Water Trust. This article may be reprinted for non-commercial educational purposes provided it is used in its entirety and that reference is made to its source as an article in THE AMERICAN WELL OWNER, 2002, Number 1]
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