Top ten things you should never flush into a septic tank

The ten things that damage a septic tank or block its soakaway, and what to do instead.

Sam WarrenUKDP co-founder
Published9 May 2016
Updated26 Oct 2026
Read time5 min

Key takeaways

Never flush grease, antibacterial products, sanitary products, wet wipes, paints, cotton buds, nappies, cat litter or rainwater into a septic tank.
Grease and fat can block the pipe to the tank and the perforated holes in the soakaway pipework.
Antibacterial products kill the good bacteria that break down solids, and paints and varnishes disrupt the treatment process.
Sanitary products, wet wipes, cotton buds and nappies belong in the bin, as they block pipes or jam mechanical parts.
Cat litter is mostly clay particles, which block pipes and can't break down in the tank.
Rainwater from downpipes or gullies overloads the tank and can churn solids out into the soakaway.
Keep rainwater pipes away from the tank and bin anything that isn't toilet or household waste water.

Septic tanks take all the waste water from your property’s toilets, sinks and showers. So we probably don’t need to spell out exactly what you should be flushing into it. Instead, we thought we’d focus on some of the things you should never flush into a septic tank, to avoid any septic tank problems in future (or strange looks from the company that comes to empty it for you).

Naturally, there are hundreds, possibly thousands of things you should never flush into a septic tank. And in our line of work, we have seen some very peculiar things lurking in septic tanks which might put you off your lunch if we showed you a picture.

There are however some things that you could innocently flush into the septic tank which would cause some real issues, and at worst could stop the whole system working. This is our top ten:

Grease and fat: don’t pour cooking oils or fats down your kitchen sink. Firstly it might block the pipe running from your house to the tank, but it could also create real problems with the soakaway system if your septic tank connects to one. This is because it can block the perforated holes in the pipework, preventing the waste water from percolating into the soil.
Antibacterial products: things like antibacterial hand washes and kitchen cleaners can kill off the `good’ bacteria in your septic tank, which can stop it working as well as it should in breaking down the solids that get flushed into it (solids is a much more acceptable word than poo)
Sanitary products: these are best disposed of in a bin.
Wet wipes (including face wipes): some brands will claim to be `flushable’. But they won’t be the ones to come and deal with the problem if the system gets blocked, so don’t take the chance.
Chemicals such as paints and varnishes: wash your paint brushes outside, not in the sink. The chemicals can mess with the treatment process in your septic tank.
Cotton buds: these little sticks of mischief can create havoc for your septic tank. They are not biodegradable and if you have a sewage treatment plant, or any kind of mechanical element to your drainage system, they can get stuck and cause it to fail.
Nappies: bag them and bin them instead of flushing them into your septic tank, they’re not biodegradable and could cause a horrible blockage
Cat litter: we’re not entirely sure who puts cat litter down their toilet anyway, but it seems it’s a fairly common problem. Either way, it’s best avoided as it is made mostly of clay particles which can block pipes and also can’t be broken down in the tank.
Rainwater: in order to avoid overloading the septic tank, it’s best to avoid connecting any rainwater down pipes or gullies to it. Excessive levels of water entering the tank can churn everything up inside it which can result in some of the lumpy stuff being flushed into the soakaway (trying not to be too graphic, but we are talking septic tanks here!)
Puppies: ok so it wasn’t into a septic tank, but you can read this article about what happened when a 4 year old thought that his puppy was in need of a wash…and thought that the toilet would be a good place to do it….don’t try this at home, kids.

Finally, we’ll leave you with this sign that we found, which we think sums it all up nicely. Remember, if you’re having any problems with your septic tank or soakaway, call the team at UKDP on 0800 028 9903.

Frequently Asked Questions

Are ‘flushable’ wipes safe for a septic tank?

No, despite the label. Flushable wipes don't break down like toilet paper and are a common cause of blockages in tanks and soakaways. Bin them instead.

What cleaning products are safe to use with a septic tank?

Use normal amounts of ordinary products and you'll be fine. The thing to avoid is heavy use of bleach and antibacterial cleaners, which kill the bacteria the tank relies on. Septic-safe brands exist if you want to be careful.

Can I put bleach down the drain with a septic tank?

In small amounts, occasional bleach won't hurt. Regular heavy use is the problem, because it kills the bacteria that break down the waste. Spread cleaning out and use it sparingly.

Is it safe to use a dishwasher and washing machine with a septic tank?

Yes. Everyday use is fine; the tank is built for it. Just avoid lots of washes back to back, and keep detergents to normal amounts.

Is there anything I can add to help a septic tank work?

Nothing special. A healthy tank needs no additives; normal household use keeps the bacteria going. The best thing you can ‘add’ is regular emptying and keeping the banned items out.