The short answer
Used occasionally at the dose on the label, borax is unlikely to harm a septic tank, because a small amount is heavily diluted in the tank. The catch is the boron. It doesn't break down, so it passes into the drainage field and soil, and high levels of boron harm plants. Small and occasional is the sensible way to use it.
There is also a good chance that what you have in the cupboard is not borax at all.
Borax or borax substitute?
Genuine borax is sodium tetraborate. It is classified in the UK and EU as toxic to reproduction, which restricts its sale to the general public. Most products on UK shop shelves are therefore labelled borax substitute. These are usually sodium sesquicarbonate, a mix of washing soda and bicarbonate of soda, which does similar cleaning jobs and contains no boron.
Check the label. If yours says borax substitute, the boron question below doesn't apply, and in normal cleaning amounts it is no more of a concern for a septic tank than washing soda.
What borax does in a septic tank
Borax raises the pH of the water slightly and has a mild antimicrobial effect. Diluted in the volume of a domestic tank, a normal cleaning dose has very little effect on the bacteria that break down sludge. Large amounts poured away at once are a different matter, as they would be for any cleaning chemical.
There is no published study that sets a safe amount of borax for a septic system. That is one reason to keep doses modest and occasional rather than making it a daily habit.
Where the boron goes
This is the part most articles miss. A septic tank does not remove boron. It passes out with the effluent into the drainage field and then into the soil and groundwater. Boron is a trace nutrient that plants need in tiny amounts, but it becomes toxic to many plants at higher levels, and it doesn't break down over time.
For a household using borax now and then, the amount reaching the soil is small. Regular heavy use, for example in every wash load, adds up year after year in the same patch of ground.
Borax for blocked drains or toilets
Borax is sometimes recommended for clearing drains or freshening toilets. It can help with light build-up, but it will not clear a real blockage. A plunger or drain rod works better and puts nothing into the tank. If several drains are slow at once on a septic system, the problem is more likely to be the tank or drainage field than the pipes. See how to tell if your septic tank is full.
Using borax sensibly on a septic system
- Follow the label dose. More doesn't clean better.
- Use it for occasional jobs rather than every day or every wash.
- Do not tip leftover solution down the drain in bulk.
- Keep it, and every cleaning product, away from children and pets.
The same moderation applies to bleach and other cleaners, which we cover in is bleach bad for septic tanks.
Treatment plants
The same advice applies to a sewage treatment plant, with a little extra care, since its bacteria do more of the work. Boron also passes straight through a treatment plant to wherever it discharges, which may be a stream.
If your tank is smelling or draining slowly and you are not sure why, a septic tank inspection will find the cause. For the everyday habits that keep a system healthy, see our septic tank maintenance dos and don'ts.
