Iron and Sulfur Bacteria: a Slimy Problem

Have you ever lifted the top of your toilet tank and been surprised by the presence of slimy strands or a similarly looking gelatinous substance? It's likely that this slime is caused by iron bacteria or related slime forming bacteria such as sulfur bacteria. Iron bacteria use dissolved iron or manganese in groundwater to grow, producing a bacterial slime along the way. While these bacteria can be off-putting, they are typically considered more of a nuisance than a health hazard.

What Causes Slime Forming Bacteria?

Iron bacteria prefer environments with low dissolved oxygen, fairly mild temperatures, and the presence of dissolved iron or manganese. Even iron concentrations as low as 0.01 mg/L can be enough for these bacteria to grow.

There are two types of slime forming sulfur bacteria, those that oxidize sulfide into sulfate and, more commonly, those that reduce sulfate into sulfide while producing hydrogen sulfide gas. Sulfur bacteria need similar growing conditions as iron bacteria, except that they require a form of sulfur instead of iron or manganese. In Pennsylvania, elevated levels of iron, manganese, and sulfate are generally found in shale and sandstone aquifers or in areas impacted by mining. Consequently, iron and sulfur bacteria often coexist in private water systems because of their similar needs.

Iron bacteria typically exist on top of the ground, either in soil or surface water. While some iron bacteria may occur naturally in groundwater, more often they are introduced into water wells during well construction or maintenance. These bacteria can then be introduced into the well if well components are laid on soil containing iron bacteria, if contaminated surface water is used as drilling water, if drilling equipment is contaminated by bacteria, or if poor well construction allows for contamination from flooding, septic systems, or other sources.

Detecting Slime Forming Bacteria

While regular water testing is recommended for coliform bacteria, water testing is often unneeded to detect slime forming bacteria. In fact, few labs actually test for iron or sulfur bacteria because there are no health implications or drinking water standards. The main concern related to these bacteria is the nuisance that they cause such as odors, tastes, stains, corrosion of plumbing, and clogged well systems. 

Tastes and odors produced by iron bacteria are described as swampy, musty, or like oil, petroleum, cucumbers, sewage, or rotten vegetation. Iron bacteria can also cause reddish, yellow, brown, or gray deposits or orange or opaque slimy strands inside toilet tanks or orange-colored water. Testing well water for iron and manganese can provide information on the presence of these metals, which could help confirm the likelihood of iron bacteria if aesthetic symptoms are also found. Since sulfur-reducing bacteria result in the production of hydrogen sulfide gas, the best indication of a sulfur bacteria problem is the rotten-egg odor; however, these bacteria can also produce a dark-colored slime. With either of the types of slime forming bacteria, the odor may only be detectable when the water hasn't been used for several hours. Keep in mind that a rotten egg odor in the hot water only is likely indicative of hydrogen sulfide being produced as a result of a chemical reaction with the anti-corrosion magnesium rod in the hot water heater. Replacing this rod with one made of either zinc or aluminum is a way to test for the source of the odor since neither of these metals will produce hydrogen sulfide gas, but be aware that replacing the rod might void the manufacturer's warranty.

Preventing Slime Forming Bacteria

Preventing iron and sulfur bacteria from establishing in your private water system is more effective than treating bacteria because it is extremely difficult to eliminate slime-forming bacteria once they have been introduced. Since these bacteria are often introduced during well construction and maintenance, much of the responsibility for prevention rests with the professional well drillers, pump installers, and plumbers who build and service private water wells. Contractors should take care to use sanitary practices during well construction and maintenance and sanitize drilling equipment between job sites. Additionally, wells should be shock chlorinated after construction and following any maintenance that requires opening the well cap.

More information on shock chlorination can be found in the article, Shock Chlorination of Wells and Springs.  Finally, ensuring sanitary well construction practices are followed can also help prevent wells from being contaminated by runoff, flooding, or other surface activities. While Pennsylvania does not have any statewide well construction requirements for private home wells, recommendations for proper well construction can be found in the article, Drilling a New Well. 

Treating Slime Forming Bacteria

Disinfection is the most common approach to treating iron or sulfur bacteria. Since these bacteria often colonize in the well, shock chlorination of the well is frequently the first disinfection method well owners will try to help control the population and the associated aesthetic issues. It can be impossible to entirely remove iron or sulfur bacteria because the slime layers they form act as protection against chlorine. Frequent shock chlorination can be damaging to private water systems; so, if shock chlorination is required more frequently than every six months to keep slime forming bacteria in check, other control options should be explored.  

Persistent severe slime issues can be addressed with down-well chlorination using automated pellet droppers. These units regularly introduce lower levels of chlorine directly to the well, killing the bacteria, and should be followed by a whole house media filter (often installed in the basement near the pressure tank) to remove any gelatinous masses of dead bacteria. Another option for addressing heavily impacted wells is the physical removal of slime colonies. Experienced licensed well professionals can remove and thoroughly clean well components and scrub the inner walls of the well casing to detach slime masses. Following scrubbing, the well should be flushed to remove bacterial debris and then shock chlorinated. 

Other whole-house water treatment units such as cartridge filters, continuous chlorination or oxidizing filters may also treat or remove the slime forming bacteria or remove iron, manganese, and hydrogen sulfide from the water after it reaches the house but will not control populations within the well itself. Activated carbon filtration can be used to remove chlorine taste as well as minor hydrogen sulfide odors that may remain. 

References

"Drinking Water Contaminant – Iron and manganese bacteria." Extension Foundation online. August 23, 2019.

Saha, Uttam, Leticia Sonon, Edward Ayers, David Parks, and Jason Thomas Lessl. "Iron (Manganese) and Sulfur Bacteria in Your Well." Last modified March 2016.

Swistock, Bryan. "Hydrogen Sulfide (Rotten Egg Odor) in Water Wells." Last modified March 2, 2022.

Swistock, Bryan and James Clark. "Common Drinking Water Problems and Solutions." Last modified September 15, 2014.

Swistock, Bryan and William Sharpe. "Iron and Manganese in Private Water Systems.” Last modified October 18, 2019.

Wagenet, Linda, Karen Mancl, and Martin Sailus. Home Water Treatment. Ithaca, N.Y.: Northeast Regional Agricultural Engineering Service, Cooperative Extension. 1995.