Septic systems are tougher than you think

One of the biggest misconceptions about septic systems is that they’re fragile. Ask around and you’ll hear warnings like, “Don’t use bleach – it’ll kill your system!” or “Stick to eco detergents only.” No wonder many families hesitate to try cloth nappies if they’re not on mains wastewater.

But here’s the reality: septic systems are designed to handle household wastewater, including the products you’d normally use to wash clothes and nappies. With the right information, you’ll feel assured to use cloth nappies even if you have a septic system.

Detergent and chlorine bleach: boogeymen or best mates?

Historically, one of the biggest worries about detergents was phosphates. This fear needs to be relegated to the past as phosphates were phased out of Australian laundry detergents over a decade ago. All detergents sold in Australian supermarkets, and most in New Zealand, are now either phosphorus-free (P) or contain only very small amounts (NP).

Both P and NP detergents are safe to use with septic systems. They’re formulated to break down during the wash process, and your septic system is more than capable of handling them. If the detergents also comply with the biodegradability standard, they’re also suitable for use in greywater or biocycle systems.

What about chlorine bleach? It gets an unfair reputation. Once it’s mixed with water or exposed to air, the chlorination process begins. This converts chlorine bleach into salt, water and oxygen over time. Totally harmless when fed into a 3,000-litre tank.

Even if you dumped an entire packet of Super-Shock 70% pool chlorine directly into your septic system, it would recover within a day and a half. That’s more than ten times the amount of Super-Shock recommended for treating a 10,000 L pool, and infinitely more than you’d ever use in a nappy wash! (Source: Assessment of the Effects of Household Chemicals Upon Individual Septic Tank Performances)

Occasional bleach use in your laundry isn’t just septic safe, it’s one of the most effective ways to sanitise nappies and keep your wash routine simple.

How septic tanks work

A traditional septic tank is a single underground chamber where solids settle and liquids drain into a soakage field. It’s simple and relies heavily on natural bacterial processes. Modern treatment systems, like aerated wastewater treatment systems (AWTS), add extra stages such as aeration, filtration, and disinfection. These processes produce cleaner, more thoroughly treated water by the time it’s discharged.

Both types of systems are built to cope with everyday laundry detergents and the small volumes of bleach used in cloth nappy care, so you can feel confident that your washing routine isn’t putting your septic system or the environment at risk.

When it comes to cloth nappies, it’s important to remember that the water from washing, especially the first wash, is always considered blackwater, because it contains human waste. Unlike greywater, blackwater can’t be reused without treatment, but both traditional and modern systems are designed to handle it safely.

Products to avoid

The only real red flags for septic systems are cleaners and laundry rinses that contain benzalkonium chloride (BAC), such as Dettol or Canesten. These products actively kill the good bacteria your tank relies on. Other products to avoid are oven cleaners, drain cleaners, and ammonia-based cleaners. 

POV: life with a septic tank

We would never recommend any product or advice that we wouldn’t use for ourselves. Admin team member Jes shares her experience using septic tanks.

I’ve been on a septic tank for over five years, and for almost three of those, I’ve used bleach in my first wash. I’ve never had a single issue with my system—no smells, no blockages, no backups.

I can assure you, the volume of bleach that actually makes its way into your system is minute. For extra reassurance, chlorine bleach breaks down very readily, leaving behind nothing more than salt and water. I go through about three litres of bleach a month in my laundry, and my system has never skipped a beat.

On top of that, since I joined the CCN team, I’ve tested dozens of detergents—mainstream powders, liquid detergents, plant-based options, and eco brands. Every single one has gone through my septic tank. And although some of them left a lot to be desired when it came to cleaning, none caused issues for my septic tank. Many of these detergents (especially the eco ones) required my laundry to be sanitised with chlorine bleach after testing. Still no problems, no side effects, no extra maintenance needed.

~ Jes, Clean Cloth Nappies admin team

Rest assured, septic tanks aren’t nearly as delicate as the myths suggest. With proper use, detergents and chlorine bleach are not only safe, they make cloth nappy washing reliable and effective.

The final flush

If you’ve been told that cloth nappies and septic systems don’t mix, rest easy.

• Your septic system is resilient, not fragile.
• Mainstream detergents and bleach are safe.
• The environmental impact is minimal.

Don’t let the myths and ultraconservative recommendations put you off. Septic systems can and do survive real-life laundry. Cloth nappies can and do work beautifully with septic tanks. All without stress, without guilt, and without specialised detergents. ✨