I noticed the bathroom sink was draining slowly. Our 12-year-old likes to get muddy and then we soak her clothes in that sink, so I thought maybe it was just a bit of mud or debris. But no...

I took out the stopper and had a look -- the inside of the drain pipe was coated with a thick, awful black sludge.

I wiped away as much of it as I could, then I got under the sink and undid the curved pvc trap pipe, and it was also coated with the stuff, although not as badly. It also extended up into the exit pipe, and I cleared away as much of that as I could.

Is this normal? Surely not. Do I need to call a plumber to investigate further?

  • my bathroom drain looks like that also ... looks normal to me
    – jsotola
    Commented Apr 18 at 23:13

1 Answer 1


That black sludge is biofilm, and commonly forms in drains. There are various ways to deal with it, including drain cleaners or snaking. No need to call a plumber. For me, I take apart the drain in my bathroom sink once or twice a year to mechanically clean it out.

  • 1
    You can also boil water and pour it down. A couple of pots and then running hot water will clear out 90-95% if pipes are pvc.
    – DMoore
    Commented Apr 18 at 16:21

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