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What impacts do antibacterial soaps have on septic systems?

I know that septic systems rely heavily on bacteria (especially regarding the breakdown of wastes by bacteria), so it seems as though antibacterial agents surely could do some harm to septic systems. Is that the case?

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One of my students in my Environmental Science class asked me this question, and it seemed very much worthwhile to post the Q&A to Bio.SE!

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    $\begingroup$ Fortunately, this is not a real problem, as you do not need antibacterial soap anyway. It has many other downsides, and regular soap washes away bacteria just as well. $\endgroup$
    – Cody Gray - on strike
    Mar 27, 2020 at 19:18
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    $\begingroup$ @cody, very true. Antibacterial soaps are bad for long-term human health, and no evidence suggests that antibacterial agents improve soap efficacy. The US FDA has a quick article explaining why you should skip antibacterial soap. An article from Harvard in 2017 further explains the negatives, why the FDA has banned common antibacterial agents, and how/why normal soap works just fine! $\endgroup$
    – theforestecologist ♦
    Mar 27, 2020 at 19:23
  • $\begingroup$ Wouldn't the antibacterial soap be diluted beyond the point of effectiveness, and then biodegraded? $\endgroup$
    – jamesqf
    Mar 28, 2020 at 3:52
  • $\begingroup$ I have dry skin and eczema, and my UK doctor prescribes an antimicrobial lotion which is also a soap substitute to be used in washing, called Dermol 500. It contains benzalkonium chloride and chlorhexidine dihydrochloride. I am advised to use it instead of soap and as a moisturiser, especially after bathing or showering. $\endgroup$
    – Michael Harvey
    Mar 28, 2020 at 9:17
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    $\begingroup$ You can do a simple test using a bottle and a serynge... Divide your sceptic volume by 1000 or something, use a similar bottle, and take yoru septic mix from your drain... Fill the bottle with pond slurry, potatos, organic matter that you want to measure decomposition of, let the bottle kick off bacterially for a week at room temperature, and then add 1/4 of the bottle little by little in the relative amount soap that you use... My suspicion is that antibacterial soap will de-activate in about 10% of the tank, leaving 90% of it working normally. In some setups, it will have little effect. $\endgroup$
    – bandybabboon
    Mar 27, 2021 at 10:38

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The quick answer is: Yes, it can cause harm.

Think about it...The septic system (both the tank and your drain field) rely on bacteria, and antibacterial soap is not designed to kill only specific species of bacteria. In other words, antibacterial soap can kill a whole range of bacteria, and that certainly includes the bacteria needed in your septic system.

According to University of Arizona Cooperative Extension (Farrell-Poe, 2018):

The use of “antibacterial,” “disinfectant,”or “sanitizing” products in the home can and do destroy both good and bad bacteria in septic treatment systems. “Normal usage” (according to directions) of these products will destroy some beneficial bacteria. Fortunately, the normal bacteria population within the septic system is sufficient and adequate to quickly recover. Significant treatment problems, with conservative use, should not occur. Excessive use of these products in the home can cause significant and even total destruction of the bacteria population. Normally, the use of any single product or single application will not cause major problems.

However, the accumulative affect of using too many such products and excessive application may cause serious problems and damage to the septic system.

It appears that some alternative systems may be more affected by “antibacterial” products than other systems. Additional and more conclusive research is needed.

You can find additional info about antibacterial agents in septic tanks as well as discussion about the effects of the common antibacterial agent, triclosan, on septic tanks here: Svenningsen et al. (2011) and Kirjanova et al. (2014).

Works cited:

Farrell-Poe, K., 2018. Antibacterial Products in Septic Systems. The University of Arizona Cooperative Extension fact sheet

Svenningsen, H., Henriksen, T., Priemé, A. and Johnsen, A.R., 2011. Triclosan affects the microbial community in simulated sewage-drain-field soil and slows down xenobiotic degradation. Environmental pollution, 159(6), pp.1599-1605.

Kirjanova, A., Rimeika, M., Vollertsen, J. and Nielsen, A.H., 2014. Retainment of the antimicrobial agent triclosan in a septic tank. Water science and technology, 70(4), pp.586-592.

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    $\begingroup$ This is interesting. Is it really significantly different to the effect any detergent would have on bacteria? I would guess all soaps kill most bacteria simply by disrupting cell membranes. Is that not the case? $\endgroup$
    – terdon
    Mar 27, 2020 at 10:07
  • $\begingroup$ @terdon I would expect it's not significantly different. However, based on the limited sources I could find on the topic (and their own consistent reminder that this topic is under - researched), it looks like there is a measurable difference due to antimicrobial agents. I've just reported what evidence I could find. To be honest, I didn't explore the literature for studies looking at detergent impacts, and perhaps doing so could further inform and improve my answer. If you have any suggested starting points, please feel free to pass on the suggestion or update my response accordingly. $\endgroup$
    – theforestecologist ♦
    Mar 27, 2020 at 15:41
  • $\begingroup$ I don't have the faintest idea! Your answer just made me wonder, is all. I didn't mean my comment as criticism. $\endgroup$
    – terdon
    Mar 27, 2020 at 16:50
  • $\begingroup$ @terdon No worries - I didn't think you meant it as criticism. I was just hoping not to have to start at ground 0 for exploring a topic for which I have limited existing knowledge! :p. I recognize that my post is a superficial scraping of existing literature, so the post can definitely be improved with additional resources and info. And I hope someone can hep with that. This was -- in my mind -- quite an interesting question my student asked me, and I quite honestly wish I had more time to dive deeper into it! $\endgroup$
    – theforestecologist ♦
    Mar 27, 2020 at 18:56
  • $\begingroup$ @terdon triclosan et al have more effects than soaps have; but I'd agree that it's true that it's not significantly different to the effect of any detergent... however it's worth noting that this simply means for a bacteria-based single home isolated septic system like in that picture you should also be selective with what detergents you use. Some forms of septic systems are "dead" and do not care about bacteria, but if you choose to rely on bacteria (which allows for designs with smaller tanks and less draining), then you need to properly select all chemicals that go into your wastewater. $\endgroup$
    – Peteris
    Mar 28, 2020 at 22:26

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