Can you use a septic tank only with black water from the toilet? I am planning to reuse the other water (shower, sinks, kitchen) and only use the septic tank for the black water coming from the toilet.

Will it be enough water for the septic tank to function correctly ?

  • People do this so should work. Local regulations might be another matter if they allow you to do it. The idea is usually called a grey water system if you want to find more information.
    – crip659
    Nov 11, 2022 at 11:03
  • @crip659 yes I know about the grey water and its' regulations etc, but I am mostly interested to know if the septic tank can function properly, as it's an anaerobic process that requires water and might not have enough just from the toilet
    – Michael
    Nov 11, 2022 at 13:04
  • Do note that there are major differences in the amount of water used in a flush, depending on the design of a toilet.
    – Hot Licks
    Nov 11, 2022 at 13:41
  • 1
    Assuming you flush the toilet two or three times everyday then I'm fairly certain an underground tank of water is not evaporating at a rate of several gallons per day.
    – MonkeyZeus
    Nov 11, 2022 at 14:53

1 Answer 1


That is utterly dependent on the rules of the Local Authority Having Jurisdiction as regards greywater and what treatment or uses greywater is subject to.

Many places have a dubious view of the potential fecal coliform bacteria load from showers, laundry, and lavatory sinks and are thus not terribly open to not treating that water. Thus, the kitchen sink may be the only waste stream permitted to be used as greywater, or greywater use may not be permitted at all.

As for the comment regarding the functioning of the septic tank, yes, that will be fine, there's plenty of liquid from the flush.

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