Does Tallow Soap Harm a Septic Tank? Let’s Clear Up the Confusion

During one of my recent soapmaking workshops, a lovely participant shared that her plumber advised never to let fat go down the sink because it’s on a septic tank.

A fair concern — and one I hear often. But here’s the good news:

Tallow soap is completely safe for septic systems.

Let’s walk through this in a simple, homestead-friendly way.

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🌿 Soapmaking 101: What Actually Happens to the Oils?

When we make soap, the oils and lye go through a natural chemical reaction called saponification. During this process, the majority of the oils — including tallow — are transformed into:

• Soap

• Glycerin

If we made a 0% superfat soap, then yes, every bit of oil would be converted.

But in handmade soap, we intentionally superfat (often around 5–8%). This simply means a small, controlled portion of the oils remain unsaponified so the final bar is more moisturising and gentle on the skin.

This leftover oil doesn’t act like liquid fat being poured down a drain. It’s held within a solid bar, released slowly during use, and most of it is absorbed onto:

• your skin

• your washcloth

• your towel

• your soap dish

Only a tiny, diluted amount reaches your plumbing — far less than what enters a septic tank from normal household living.

🌼 What About Cleaning Up After Making Soap?

This is the part many people worry about, but it’s actually the easiest to explain.

✔️ I always recommend waiting 24–48 hours before washing up

By then, every drop of leftover batter has fully saponified. You’re no longer washing bowls coated in raw oils — you’re washing bowls that simply have soap stuck to them.

A bit of hot water and a sprinkle of laundry powder, and everything rinses clean.

This makes the “fat down the drain” concern irrelevant. There is no fat going down — it has already turned into soap.

✔️ The amount is tiny

Even if you didn’t wait, it’s still only a light film of batter — nothing like pouring oil into a sink.

✔️ Septic tanks already manage far more challenging things

Realistically, you get more oil in your drains from washing:

• a mechanic’s overalls

• a chef’s apron

• a roasting dish

• hands and face moisturised with oils or skincare products

• body lotions and conditioners

…than from cleaning up after a soapmaking session.

And the septic tank copes with all of this without trouble.

🧮 A Quick Example With Real Numbers (For the Soap Nerds)

Here’s the updated 500 g recipe example:

• 150 g Tallow

• 110 g Olive Oil

• 110 g Sunflower Oil

• 130 g Coconut Oil

Total oils = 500 g

At 7% superfat, about 35 g of oils remain unsaponified in the entire batch — these are the “conditioning oils” that make handmade soap so lovely.

Breakdown:

• 10.5 g tallow

• 7.7 g olive oil

• 7.7 g sunflower oil

• 9.1 g coconut oil

• 
  

If you make five bars, each 100 g bar contains roughly:

• 2.1 g tallow

• 1.5 g olive

• 1.5 g sunflower

• 1.8 g coconut

• 
  

Around 7 g of extra oils per bar, slowly released over weeks — and mostly absorbed into your skin or onto your washcloth. None of it hits your drain as “liquid fat.”

🌱 When DO Fats Cause Septic Troubles?

Plumbers are referring to situations like:

• pouring a cup of hot fat into the sink

• draining deep fryer oil

• emptying a pan full of cooled fat

This kind of oil cools, hardens, and builds up in pipes.

Soap isn’t oil. Superfat isn’t poured. Soapmaking dishes aren’t coated in liquid fats if you wait for saponification.

🕰️ A Quick Look Back: Tallow Soap Has Always Been Septic-Safe

Long before modern sewer pipes existed, most homes relied on septic tanks or simple soak pits — and the only soap people used was traditional tallow or lard soap. All household greywater, including bath water and laundry water, flowed straight into those early septic systems without any problems. If saponified tallow were harmful, generations of homeowners would have had to deal with constant plumbing failures. History itself shows that tallow soap is not only safe but perfectly compatible with septic systems.

🧼 Using Tallow in Soap Is Like Washing a Pan That Once Had Butter on It

Nothing dramatic. Nothing harmful. In fact, it's even gentler because most leftover batter has become soap by the time it hits the water.

🍃 The Homestead View: Natural, Waste-Free, Traditional

Tallow is:

• a sustainable ingredient

• 100% biodegradable

• perfect for creating long-lasting bars

• gentle on the skin

• nose-to-tail and waste-free

• safe for septic tanks

  
  

Sustainable living is all about honouring resources, using traditional skills, and reducing waste — and tallow soap fits beautifully within that way of life.

🌻 Final Thoughts

If you’re on a septic tank, you can make and use tallow soaps confidently.

The key idea:

Once tallow becomes soap, it is no longer fat — and even the superfat is minimal, slow-releasing, and mostly absorbed by your skin.

Waiting 1–2 days before washing your soapmaking tools ensures you’re rinsing off soap, not oils. And the amount of oil from a handmade bar is far less than what enters drains from normal household living.

Your septic tank is absolutely safe with tallow soap.