Tips for Taps Blog

Black gunk on faucest and fixtures - Explanation

What’s The Black Gunk on My Fixtures?!


Sludge...slime...gunk—whatever you call it, you probably don’t want it near your drinking water. It’s sticky, it often smells, and it can leave you puzzled.

Black residue on faucets and fixtures is a very common problem and we’re here to answer:

  • What causes black slime buildup?
  • Is black slime on fixtures dangerous?
  • How can you get rid of the black gunk?

What Causes Black Slime on a Faucet?

Before we get into the exact cause of the gross gunk, let’s take a step back and look at what flows from the tap. Along with H2O, tap water often contains dissolved minerals and metals. Two of these metals are iron and manganese.

Iron and manganese are both naturally occurring, non-hazardous elements found throughout the earth’s crust. As water travels through the ground, it can dissolve soil and rocks containing these elements and then hold them in solution. Most drinking water contains traces of dissolved iron and manganese. 

Iron and manganese in drinking water are not known to have any health impacts. However, elevated concentrations of either can be a nuisance in water supplies–producing an unpleasant taste and off-putting odor. [1] 

Because iron and manganese are chemically similar, they often create similar aesthetic problems. These include black film, gunk, or sludge. The sticky, slimy, stinky residue can make itself at home nearly anywhere water flows in your home.

Whether it accumulates in the faucet aerator, around the tub drain, inside the toilet tank, or even inside your tea kettle, black slime is usually due to bacteria that feeds on oxidized iron and manganese in your water supply. [2]

Is Black Slime on Fixtures Dangerous?

There are no federal primary drinking water standards set for either manganese or iron because their presence in drinking water is not associated with health effects.[3] However, there are regulations regarding secondary standards for both.These standards are set to fight nuisance problems (e.g. black slime) and aesthetic issues (i.e. taste, odor, color).

Manganese and Manganese-related Bacteria:

The U.S. EPA recommends maintaining a manganese concentration at or below 0.05 parts per million (PPM) in drinking water.[4] Neither manganese nor manganese-related bacteria are considered dangerous at the levels that typically occur in drinking water.[5] Manganese exposure from water and food (our largest source of exposure) are not known to have a negative health effect.

In fact, manganese is an essential nutrient and is required by the human body in small amounts. Similarly, manganese bacteria is categorized as non-pathogenic.[6] Some evidence does suggest that if manganese is inhaled in high concentrations over time, it can lead to neurological issues—but this is rare and not caused by drinking water. [7]

Iron and Iron-related Bacteria:

Like manganese, iron (and related bacteria) are not dangerous to human health.[8] Drinking water standards for iron are set based on potential nuisance and/or aesthetic issues. The EPA recommends a secondary maximum contaminant level (SMCL) of iron in tap water at >0.3 PPM.

While iron-related bacteria often produce reddish-brown slime, when they react with naturally occurring tannins (organic matter from vegetation) it frequently forms black, sludgy residue. This is why many people spot black slime inside their tea pots–as tea contains a high concentration of tannins.

How Do I Know if There Is Iron or Manganese in My Water?

The only sure-fire way to know what is in your water is to test it. Choose a test according to your water source. All Core kits include iron and manganese:

While most contaminants are best tested in a lab, there are some things that are well-suited to be tested for at home... such as iron-related bacteria. We have an at-home iron bacteria test as an available add-on to any of our testing packages.

How to Get Rid of Black Slime on Your Fixtures

Although black slime may have a few other causes  (i.e. oxidizing pipes or dissolving rubber seals in your water heater), iron- and manganese-related bacteria are the most common culprits (especially in homes supplied by a private well). 

Once you’ve confidently identified the presence of iron- or manganese-related bacteria through testing, it’s time to solve the problem once and for all. Unfortunately, online research often points you toward temporary treatment measures, rather than lasting fixes. The temporary “fixes,” such as replacing pipes and regularly cleaning the affected area, will cost you time and money–but will not solve the problem. Because the root of the problem stems from the water supply itself, you must focus your attention there (i.e. the cause) rather than the slime (i.e the effect).

How to Deal With Black Slime if You Are on a Private or Shared Well:

Shock chlorination might be the answer. Take a look at our comprehensive, step-by-step shock chlorination guide.

Just remember: It is nearly impossible to kill all of the iron- and manganese-related bacteria in your well water system. The bacteria will eventually re-grow, so you may want to repeat the treatment from time to time.  

Dealing With Black Slime if You Are Served by a Public Water Utility:

If the source of your bacterial problem is your city water system, then using at-home chlorination will not have the desired effect. The best treatment method depends on a variety of factors—such as the concentration and form of iron and manganese in the water, whether or not iron- or manganese-related bacteria are present, and how much water you need to treat. Each of these factors will help determine the most efficient/cost effective treatment method for you. [9]

Treatment options include:

    • Ion exchange water softener
    • Sequestering
    • Oxidizing filters

However, before you spend unnecessary money on treatment options that may not be suited for your specific water quality issues, it is best to know what problems you are dealing with at the tap. That’s why testing your water is so important. There is not a one-size fits all treatment for water quality—it’s crucial to know what you need to target.

There you have it. The black sludge at the bottom of your drain is no longer a mystery. While certainly alarming to find, it shouldn’t leave you scared. It’s not as spooky as it seems—just a little stinky and a little slimy.

If you have black slime on your fixtures and want to know if you should test your water, send us some photos of the issue and our team of chemists, engineers, and water treatment experts can help!

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What's the Takeaway?

  1. Common Causes of Black Slime: Black residue on faucets and fixtures is often caused by bacteria that feed on oxidized iron and manganese in the water supply.
  2. Safety of Black Slime: Black slime on fixtures is not associated with health effects, but it can be a nuisance due to taste, odor, and appearance.
  3. Testing for Iron and Manganese: To determine the presence of iron or manganese in your water, laboratory testing is recommended.
  4. Removing Black Slime: Temporary fixes like cleaning and replacing pipes are not effective. Treatment methods such as shock chlorination for private wells or specific water treatment options for public water utilities can help address the root cause.
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About The Author


Serving as the CCO at SimpleLab, Jorgen has been an integral part of Tap Score since its inception. With a passion for combating misinformation in the water industry, he is dedicated to empowering individuals with a clear understanding of the fundamentals of their drinking water. In his spare time, Jorgen enjoys creating immersive social experiences for Virtual Reality with his wife.
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