What is the black slime in my sink drain or faucet?

Biofilm forms anywhere water sits or splashes and then lingers. Bacteria that live in tiny amounts in air, water, and on our bodies settle on a damp surface and start to multiply. As they grow, they release a slimy protective coating that glues them in place and shields them. That coating is why the slime feels slippery and clings to the stopper, the drain walls, and the inside of the faucet aerator even when you rinse them.

The color comes from two things working together. First, the biofilm catches the organic material that goes down a bathroom sink every day: soap, skin oils, shampoo, toothpaste, and shed hair. As that material breaks down it darkens. Second, many water supplies carry trace manganese, a metal that picks up a black or brownish-black tint once it is exposed to oxygen. When manganese-rich water flows over a biofilm, the slime can take on that signature inky black look.

This is different from mold, even though people often call it "black mold." True mold is a fungus, while this slime is mostly bacteria and trapped debris. The fix for both, though, is similar: clean the surface, dry it out, and keep it clean.

For a healthy person, the biofilm you find in a sink drain or on a faucet is not a health emergency. The bacteria involved are common in the environment, and the manganese that darkens the slime is regulated as an aesthetic concern rather than a toxic one. The EPA places manganese under its secondary drinking water standards, which cover substances that affect taste, color, or staining instead of safety. The manganese secondary standard, called a Secondary Maximum Contaminant Level (SMCL), is set at 0.05 mg/L.

The EPA explains the purpose of these limits plainly. As the agency states, secondary standards are "non-enforceable guidelines regulating contaminants that may cause cosmetic effects (such as skin or tooth discoloration) or aesthetic effects (such as taste, odor, or color) in drinking water." In other words, manganese at typical levels is a nuisance that stains fixtures and may affect taste, not a contaminant the EPA treats as a direct safety threat in most drinking water.

That said, a few sensible cautions apply. People with weakened immune systems should avoid letting heavy slime build up and should keep fixtures clean, because some waterborne bacteria can be a concern for vulnerable groups. And while trace manganese is normal, unusually high manganese in a water supply can be a separate issue worth testing for, especially for infants. The slime itself, scrubbed away during regular cleaning, is not the part to worry about.

The black slime tends to appear in the same handful of spots because those are the places that stay wet and collect the most organic material.

• The drain opening and stopper. This is ground zero. A pop-up stopper sits in standing water, catches hair and toothpaste, and rarely gets cleaned, which makes it an ideal home for biofilm. Pull the stopper and you will often find the heaviest black coating clinging to its underside and the rod it rides on.
• The faucet aerator. The little mesh screen at the tip of the faucet stays damp and traps mineral bits and debris from the water. Black flecks or a dark ring here usually point to a mix of biofilm and oxidized manganese.
• The overflow hole. Bathroom sinks have a hidden overflow channel that almost never dries out and never gets scrubbed, so slime quietly accumulates inside it and can give off a musty smell.
• Toilet tanks and bowl rims. Manganese and biofilm can leave black or gray streaks here too, since the water sits still for long stretches.

The common thread is moisture plus food. Bathroom sinks beat kitchen sinks for this kind of slime because of all the soap, skin, and toothpaste, and because the water often sits longer between uses.

Cleaning black slime is straightforward and uses ordinary household supplies. Work through these steps and the dark film will lift away.

1. 1Remove and scrub the stopper. Lift out the pop-up stopper (most twist or unclip, or you can loosen a nut under the sink). Scrub it with an old toothbrush and dish soap to break up the slime, then rinse.
2. 2Clean the aerator. Unscrew the aerator from the faucet tip. Soak it in plain white vinegar for 30 to 60 minutes to dissolve mineral buildup, then brush off the residue and rinse before reinstalling.
3. 3Scrub the drain itself. Push a bottle brush or a stiff drain brush down into the opening and scrub the walls. Reach into the overflow hole with a small brush as well.
4. 4Soak and flush. Pour white vinegar around the drain and let it sit, then follow with very hot water to rinse loosened slime down the line. Baking soda followed by vinegar can help lift buildup too.
5. 5Clean regularly. A quick weekly wipe of the stopper and drain, plus drying the sink after use, keeps biofilm from ever getting a foothold.

Avoid harsh chemical drain cleaners for routine slime. They are unnecessary here and can damage pipes. Regular mechanical cleaning does the job better and is safer for your plumbing.

To prevent the slime from coming back, reduce what feeds it and how long water sits. Run hot water down the drain for a few seconds after brushing teeth or washing, wipe the stopper down often, and let the sink dry out between uses. The less standing moisture and food, the slower biofilm grows.

Most of the time the black slime is a surface issue you can scrub off, and the cleaning above solves it for good. The key question is where the black comes from: the surface or the water.

If the slime sits on the stopper, drain, and aerator and disappears after you scrub, it is biofilm. That is the normal, harmless case. But if you see black flecks coming out of the water itself, staining clothes in the wash, coloring ice cubes, or leaving black grit in glasses even from a clean faucet, the source may be the water supply rather than the fixture. That points to manganese in your water.

In that case, the next step is to test your water for manganese. A certified lab test tells you whether levels are above the EPA aesthetic guideline of 0.05 mg/L. Hard water, which is common in many regions, can carry these minerals and also speeds up scale and staining on fixtures, so a water test often answers more than one question at once. If manganese is high, a water treatment system such as a filter or softener sized for your supply can remove it at the source so the black staining stops returning.

For everyday black slime on the stopper and aerator, though, you do not need a test or a treatment system. A toothbrush, some vinegar, and a regular cleaning habit handle it. If you suspect the problem is in your water and not just the drain, a water test is the smart move before deciding on any treatment. For related slime questions, see our pages on pink slime in the bathroom (Serratia) and the signs you need water treatment.