Septic tanks: myth busting the things you shouldn’t be doing
The common septic tank myths that do more harm than good, from adding worms to fitting soakaway crates.
Key takeaways
- Adding worms or dead animals won't help a septic tank, and using soakaway crates will make its soakaway fail.
- No credible evidence backs septic tank worms, and smells or poor performance point to a problem with the tank.
- Foul and grey water from your household provides everything a septic tank needs, so dead animals add nothing.
- Soakaway crates are for rainwater only and must not take waste from a septic tank, alone or mixed with rainwater.
- Neither BS 6297 nor Building Regulations Part H lists crates for wastewater, because suspended solids choke them.
- If your tank smells or underperforms, get an inspection to find the cause, such as a crack or a tree root.
Septic tanks. You would think operating one would be a simple, uniform process. Stuff goes in. It naturally separates. The cleaner content soaks away into the surrounding ground. The nasty stuff is periodically vacuumed out and taken away by a friendly tanker driver. Rinse (not literally) and repeat. In a nutshell, that is it.
Until they stop working as they should, of course. Septic tank problems can cause a real headache, and some people will sell you `solutions’ that they say will help to avoid them. But are they really the solution they claim to be? Read on to find out!

A septic tank is a simple system: waste separates, the liquid soaks away, and a tanker clears the rest about once a year. When it stops working, the fix is seldom a bottle of additives, which is where a few popular myths fall apart.
Septic tank worms
Suppliers that advertise septic tank worms normally position them as some sort of additive that will assist in the biological process and remove odours. Ignore for one moment that no credible evidence has been produced to confirm these claims. More importantly, what we, as humans, put into a septic tank is more than sufficient for any processes to occur and bacteria to grow. If there’s an issue with a septic tank or there are noticeable odours, worms just aren’t going to help. You’ll just now have a tank with noticeable odours and a worm colony. Performance issues and unpleasant smells are a sign of a problem with the tank. Get a septic tank inspection sooner rather than later. It could be a crack in the tank or a tree root coming through. Either way, you are far better advised to get to the bottom of the issue as it is not likely to fix itself and will only get worse. Don’t waste your time, money and effort chucking worms in.
Whatever you've heard, a tank still needs emptying about once a year.
Assorted dead animals
A dead chicken, a dead sheep, perhaps a small dead horse, there are plenty of old wives’ tales about what to throw into a septic tank to assist the growth of bacteria. Just like the worms, there is absolutely no need to deposit anything into a septic tank apart from that which us humans produce. Without wishing to be needlessly graphic, foul and grey water from our toilets, dishwashers, washing machines, showers and baths will generate all that is needed for our septic tanks and sewage treatment plants to work. Throwing in dead creatures isn’t going to help and your tanker operator won’t appreciate having to spend his or her time dislodging bones from the suction pipe.
Damaged tank?
Many property owners are unaware that, typically, buildings insurance policies provide cover for damaged off-mains drainage systems.
Find out more →
Soakaway crates
A less grim myth but important all the same. Soakaway crates must only be used for rainwater. They are not to be used as a soakaway for a septic tank nor can they be used for a combined soakaway i.e., receiving both foul and rainwater.
There are two separate documents detailing the test procedures, design parameters and installation options for foul and rainwater soakaways, namely, the BS 6297:2007+A1:2008 British Standard: Code of practice for the design and installation of drainage fields for use in wastewater treatment and the BRE DG 365 (revised 2016) design and construction procedures for rainwater soakaways.
There are several options described for a wastewater soakaway both in the BS6297 and the Building Regulations 2010 Part H document, but you will not find soakaway crates listed in either. Crates cannot handle the suspended solids and other matter that finds its way out of a septic tank or sewage treatment plant, they choke and blind the soakaway, quickly causing it to fail. A crate soakaway system is relatively cheap and easy to install, but it’s a false economy if used as a foul water soakaway as it will fail quickly. A classic example of buy cheap, buy twice. Save yourself the bother and get it done properly by avoiding crate solutions for foul water systems.
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Frequently Asked Questions
Do I need to add special bacteria or septic tank treatments?
No. A working tank gets all the bacteria it needs from normal household waste. Shop-bought additives rarely do anything useful, and some harsh ones kill the good bacteria.
What's the worst thing you can put in a septic tank?
Fats, oils and grease. They set hard in the pipework and soakaway and are a common cause of failure. Close behind are wet wipes, even ‘flushable’ ones, and heavy use of bleach or antibacterial cleaners.
Can a septic tank really go 20 years without being emptied?
No. A tank with no obvious problem can still be quietly overfilling and pushing solids into the soakaway, which fails it for good. Empty it about once a year.
Does a septic tank clean the water before it leaves?
Only partly. A septic tank separates waste; the real treatment happens in the drainage field as the liquid soaks through the soil. That's why a working soakaway matters as much as the tank.
Is it true you should never pump a septic tank completely empty?
No, a full empty is fine. The bacteria re-establish quickly from normal use, so there's no need to leave waste behind to ‘seed’ it.
About the author
Sam Warren
UKDP co-founder
Sam Warren co-founded UKDP with James Warren in 2009 as an off-mains drainage specialist.
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