I installed a ( 4’x 4’x 32’) gravel bed gray-water pit in order to relieve the load on my 20 yr old septic tank. I also placed a (4” x 3’) septic-tank effluent bristle brush in the feed pipe going down into the gravel. When I pulled out the bristle brush it had a lot of black mold ( looks like thick black gravy ). I also can see the same black mold in the septic tank clean-out line going out to the septic tank. QUESTION: What are you guys thoughts on using bleach to kill the mold. I’m afraid the mold will grow and eventually clog up the gravel. This kind of mold already clogged up the 2”HVAC condensate line. So would bleach be ok to kill the mold since it can’t be pumped out like the septic tank, thank you Oily Tex

  • are you certain that it is mold?
    – jsotola
    Jan 10 '20 at 0:44
  • No, only guessing. I was suggesting it was mold because when I gather up a sample ( 2” by 2” ) & about 3/8” thick & place it on a flat board for a few days it will shrink down to a very thin 1/16” layer about the thickness of a thin potato chip & the colors changes from black to a light brown.
    – Oily Tex
    Jan 10 '20 at 3:15
  • The only 2 places that the water comes from is the ( clothes washer & the HVAC system )
    – Oily Tex
    Jan 10 '20 at 3:19

Don’t chemically mess with a gray water drain! It is kind of like a septic system and if you kill things eating the little bits of solids it will not last as long! Soap scum is a solid in this case if you kill the bacteria it will plug up faster. I could take the other side and say you should dump a caustic every now and then to turn the scum into less than scum. But I would not use bleach.

  • Ed, Thank you for your reply. I will not use bleach to kill the the mold / soap scum. Question: when you say to use a caustic solution, are you talking like ( ammonia or baking soda ), something with a high PH.?
    – Oily Tex
    Jan 11 '20 at 3:37
  • When I say caustic I am referring to lye or sodium hydroxide that’s what many drain cleaners are made of. It will reduce the soap scum.
    – Ed Beal
    Jan 11 '20 at 16:02

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