In the management of water-supply wells, iron bacteria are bacteria that derive the energy they need to live and multiply by oxidizing dissolved ferrous iron. The resulting ferric oxide is insoluble, and appears as brown gelatinous slime that will stain plumbing fixtures, and clothing or utensils washed with the water carrying it. They are known to grow and proliferate in waters containing as low as 0.1 mg/l of iron. However, at least 0.3 ppm of dissolved oxygen is needed to carry out oxidation.
Iron bacteria colonize the transition zone where de-oxygenated water from an anaerobic environment flows into an aerobic environment. Groundwater containing dissolved organic material may be de-oxygenated by microorganisms feeding on that dissolved organic material. Where concentrations of organic material exceed the concentration of dissolved oxygen required for complete oxidation, microbial populations with specialized enzymes can reduce insoluble ferric oxide in aquifer soils to soluble ferrous hydroxide and use the oxygen released by that change to oxidize some of the remaining organic material:
- H2O + Fe2O3 → 2Fe(OH)2 + O2
- (water) + (Iron[III] oxide) → (Iron[II] hydroxide) + (oxygen)
When the de-oxygenated water reaches a source of oxygen, iron bacteria use that oxygen to convert the soluble ferrous iron back into an insoluble reddish precipitate of ferric iron:
- 2Fe(OH)2 + O2 → H2O + Fe2O3
- (Iron[II] hydroxide) + (oxygen) → (water) + (Iron[III] oxide)
Since the latter reaction is the normal equilibrium in our oxygen atmosphere while the first requires biological coupling with a simultaneous oxidation of carbon, organic material dissolved in water is often the underlying cause of an iron bacteria population. Groundwater may be naturally de-oxygenated by decaying vegetation in swamps; and useful mineral deposits of bog iron ore have formed where that groundwater has historically emerged to be exposed to atmospheric oxygen. Anthropogenic sources like landfill leachate, septic drain fields, or leakage of light petroleum fuels like gasoline are other possible sources of organic materials allowing soil microbes to de-oxygenate groundwater.
A similar reversible reaction may form black deposits of manganese dioxide from dissolved manganese, but is less common because of the relative abundance of iron (5.4 percent) in comparison to manganese (0.1 percent) in average soils. Other conditions associated with iron bacteria result from the anaerobic aqueous environment rather than the iron bacteria visibly colonizing that habitat. Corrosion of pipes is another source of soluble iron for the first reaction above and the sulfurous smell of rot or decay results from enzymatic conversion of soil sulfates to volatile hydrogen sulfide as an alternative source of oxygen in anaerobic environments.
Clues which indicate that iron bacteria may be present in well water:
- Iron bacteria often produce unpleasant tastes and odors commonly reported as
- oily or petroleum
- rotten vegetation
- The taste or odor may be more noticeable after the water has not been used for some time.
- Iron bacteria will usually cause yellow, orange, red, or brown stains and colored water
- It is sometimes possible to see a rainbow colored, oil-like sheen on the water.
- Iron bacteria produce a sticky slime which is typically rusty in color, but may be yellow, brown, or grey.
- A feathery or filamentous growth may also be seen, particularly in standing water such as a toilet tank.
The dramatic effects of iron bacteria are seen in surface waters as brown slimy masses on stream bottoms and lakeshores or as an oily sheen upon the water. More serious problems occur when bacteria build up in well systems. Iron bacteria in wells do not cause health problems, but they can reduce well yields by clogging screens and pipes.
Iron bacteria can be introduced into a well or water system during drilling, repair, or service. Elimination of iron bacteria once a well is heavily infested can be extremely difficult. Normal treatment techniques may be only partly effective. Good housekeeping practices can prevent iron bacteria from entering a well:
- Water placed in a well for drilling, repair, or priming of pumps should be disinfected, and should never be taken from a lake or pond.
- The well casing should be watertight, properly capped, and extend a foot or more above ground.
- When pumps, well pipes, and well equipment are repaired, they should not be placed on the ground where they could pick up iron bacteria.
- The well, pump, and plumbing should be disinfected when repaired.
Treatment techniques which may be successful in removing or reducing iron bacteria include physical removal, pasteurization, and chemical treatment. Treatment of heavily infected wells may be difficult, expensive, and only partially successful.
Physical removal is typically done as a first step in heavily infected wells. The pumping equipment in the well must be removed and cleaned, which is usually a job for a well contractor or pump installer. The well casing is then scrubbed by use of brushes or other tools. Physical removal is usually followed by chemical treatment. Pasteurization has been successfully used to control iron bacteria. Pasteurization involves a process of injecting steam or hot water into the well and maintaining a water temperature in the well of 60 °C (140 degrees Fahrenheit) for 30 minutes. Pasteurization can be effective, however the process may be expensive.
Chemical treatment is the most commonly used iron bacteria treatment technique. The three groups of chemicals typically used include: surfactants; acids (and bases); and disinfectants, biocides, and oxidizing agents.
Surfactants are detergent-like chemicals such as phosphates. Surfactants are generally used in conjunction with other chemical treatment. It is important to use chlorine or another disinfectant if phosphates are used, since bacteria may use phosphates as a food source.
Acids have been used to treat iron bacteria because of their ability to dissolve iron deposits, destroy bacteria, and loosen bacterial slime. Acids are typically part of a series of treatments involving chlorine, and at times, bases. Extreme caution is required to use and properly dispose of these chemicals. Acid and chlorine should never be mixed together. Acid treatment should only be done by trained professionals.
Disinfectants are the most commonly used chemicals for treatment of iron bacteria, and the most common disinfectant is household laundry bleach, which contains chlorine. Chlorine is relatively inexpensive and easy to use, but may have limited effectiveness and may require repeated treatments. Effective treatment requires sufficient chlorine strength and time in contact with the bacteria, and is often improved with agitation. Continuous chlorine injection into the well has been used, but is not normally recommended because of concerns that the chlorine will conceal other bacterial contamination and cause corrosion and maintenance problems.
Iron filters have been used to treat iron bacteria. Iron filters are similar in appearance and size to conventional water softeners but contain beds of media which have mild oxidizing power. As the iron-bearing water is passed through the bed, any soluble ferrous iron is converted to the insoluble ferric state and then filtered from the water. Any previously precipitated iron is removed by simple mechanical filtration. Several different filter media may be used in these iron filters, including manganese greensand, Birm, MTM, multi-media, sand, and other synthetic materials. In most cases, the higher oxides of manganese produce the desired oxidizing action. Iron filters do have limitations. Since the oxidizing action is relatively mild, it will not work well when organic matter, either combined with the iron or completely separate, is present in the water and iron bacteria will not be killed. Extremely high iron concentrations may require inconvenient frequent backwashing and/or regeneration. Finally, iron filter media requires high flow rates for proper backwashing and such water flows are not always available.
"Shock" chlorination is the process of introducing a strong chlorine solution into the well, at a concentration of 200 parts per million. If a concentration of 1000 ppm chlorine is used the pH may be raised significantly and the biocidal effect may be reduced to less than 2 percent for water beginning with an original pH of approximately 8 or higher. Ideally, the well should be pumped until clear, or physically cleaned before introducing chlorine. It is recommended to prepare the chlorine concentration to the proper concentration and with enough volume to not only displace the volume of water in the well but also spread out into the aquifer as well. The chlorinated water should be drawn into the household plumbing and remain overnight, from 24 hours to 48 hours. Heavy infestations of iron bacteria may require repeated treatments. Shock chlorination may only control, not eliminate, iron bacteria.
Before attempting to chlorinate, or doing any maintenance on a well, it is important to disconnect the electricity and understand how the well and water system works. It is usually advisable to hire a licensed pump installer or well contractor.
High concentrations of chlorine may affect water conditioning equipment, appliances such as dishwashers, and septic systems. You may want to check with the manufacturer of the appliances before chlorinating. The equipment can be bypassed, however, iron bacteria or other organisms may remain in the units and spread through the water system. It may be possible to disinfect the well with higher chlorine concentrations; and if the water storage and treatment units are not heavily infected, disinfect the treatment unit and piping with lower concentrations circulated through the water system.
After the chlorine has been in the well and plumbing overnight or for 24 hours, the water should be pumped out. Do not over pump the well during this purging as it can damage the aquifer and increase general well problems. Water with high chlorine concentrations should not be disposed of in the septic system. It may be possible to discharge the water to a gravel area, run the water into a tank or barrel until the chlorine dissipates, or contract with a hauler to properly dispose of the water. Water from the well should not be consumed until the chlorine has been removed.
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-  Iron Bacteria in Well Water