Why is there pink or black residue in my bathroom/kitchen?

The appearance of pink or dark gray film in your bathroom or kitchen is generally not a problem with water quality. The likely culprit is airborne bacteria that rely on moisture to survive. The most commonly identified species of bacteria is Serratia marcescent, which produces a pink hue, but other bacteria species are often found. These bacteria are usually reported in or around construction areas when dirt and dust are stirred up and most often at times of the year when windows and doors are left open.

Once the bacteria enter the home, they prefer to grow along the waterline in toilet bowls, tubs, or sinks and on faucets, showerheads, tiles and drains; all locations that stay moist but are not exposed to water for long periods. The water entering your home is disinfected with chlorine which kills any bacteria after moderate exposure time. The chlorine residual in your water reduces over time, and activated carbon filters remove chlorine from your water, so both filtered and stagnant water (toilets with limited use) are less resistant to bacterial growth.

To limit the growth of the bacteria, make sure to dry susceptible areas and disinfect them regularly. Using a disinfectant cake in the toilet reservoir and making sure the lid is down may also help. The most effective way to remove bacterial growth is through regular cleaning with a chlorine solution such as bleach. Avoid scrubbing with a brush or abrasive cleaner, which can cause small scratches, making it harder to remove the bacteria in the future.