Does septic tank need to have concrete flooring? If so, why? Will the concrete flooring affect the decomposition of waste?


  • It's probably a poured concrete septic tank. That used to be the way they were made. Now they are more often buried plastic tanks.
    – Tyson
    Commented Dec 1, 2016 at 3:13
  • Are you installing, building or fixing? New tanks are usually precast concrete or plastic. Old tanks may be steel, wood, cast-in-place concrete, mortared stone/brick, etc... - the tank should be waterproof to contain the waste while the anaerobic process proceeds - the next stage is where the aerobic process and filtering into the soil take place.
    – Ecnerwal
    Commented Dec 1, 2016 at 3:51
  • We will build a septic tank but we do not know if we should have a concrete flooring or maybe just put rocks. Commented Dec 1, 2016 at 4:42

1 Answer 1


A septic tanks purpose is to allow the anaerobic bacteria to break the effluent down and also allow the aerobic bacteria to die. This takes time so a septic tanked should be sized based on the inflow to allow a slow enough change over (in days) for this action to happen. Then, in the drain field, the anaerobic bacteria dies off when it meets the soil and aerates.

For this to process to happen septic tanks also need to be completely sealed; this is critical otherwise the effluent does not spend enough time in the tank for the bacteria to work or die.

Concrete septic tanks are not simply concrete but concrete with a coating to make them impermeable to water because concrete by itself allows water through it. This means effluent and its aerobic bacteria could immediately enter the soil before it had a chance to die off and continue to possibly thrive.

Further if you had rocks at the bottom you would not have a septic tank you would have a cess pool. These are a health hazard due to the fact the aerobic bacteria is not "dealt" with all and can proliferate through the soil into ground water and such.

Point is a spectic tank should completely contain the effluent; no leakage at all. This is why sealing cracks if found after time in tanks is important as well.

  • thank you for the answer. What will happen to the scum in the scum layer and the sludge in the sludge layer? I mean, they will grow over time, does it mean that if I have 6 feet deep and 2.5 meters by 1.5 meters septic tank, I will have to get the tank pumped every after 2 years or every after few years? Commented Dec 2, 2016 at 19:13
  • The scum layer should not grow beyond a point. The sludge build-up is going to be based on a multitude of different things. I have heard of people needing to pump every few years with a smaller family and others such as a family of 8 who have never pumped in a decade and do not have too much sludge build up yet. I would just do my best to live "septic" friendly, have easy access to the tank cover and personally monitor the buildup over time by checking it every year or so.
    – Damon
    Commented Dec 3, 2016 at 3:03

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