How do I look after a septic tank?
How the system actually works, the general binding rules you have to meet, and what destroys the bacteria it depends on.
- Difficulty
- intermediate
- Time
- 1 hr
- Read
- 4 min
- Safety
- warning
Short answer
It works by settling solids and letting bacteria break them down, so the sludge has to be emptied — typically annually — and the bacteria kept alive. Since 2020, a septic tank discharging directly to a watercourse in England must have been replaced or upgraded. Never enter a tank.
A septic tank is a biological system rather than a container, and it fails in slow expensive ways when that is misunderstood. There is also a legal position most owners are unaware of.
Safety
Step by step
- Understand how it works.Waste enters the tank, solids settle as sludge and fats float as scum, and bacteria digest what they can. The liquid in between flows out to a drainage field where soil organisms complete the treatment.
- Find out what you actually have.A septic tank, a cesspit — a sealed container needing regular emptying with no treatment at all — or a package treatment plant with a powered aerator. They have very different requirements and owners often do not know which.
- Check your legal position.Under the general binding rules in England, a septic tank discharging directly to a watercourse should have been replaced or upgraded by January 2020. Discharging to ground via a drainage field remains permitted subject to conditions. Scotland, Wales and Northern Ireland have their own rules.
- Locate the tank and drainage field.Mark them on a plan. You need to know where they are for emptying access, and to avoid driving or building over them.
- Have it emptied regularly.Typically annually, or as the manufacturer or your usage indicates. Sludge building up past the design level carries solids into the drainage field, which blocks it — and replacing a drainage field is a very large expense.
- Use a registered waste carrier.Emptying must be done by a licensed operator and you should keep the receipts, which are the evidence of maintenance if it is ever questioned, including on sale.
- Protect the bacteria.Bleach, disinfectant, antibacterial cleaners, paint, solvents and large quantities of detergent all kill the biological process. Moderate normal use is fine; pouring bleach down regularly is not.
- Flush only the three things.Pee, paper and poo. Wet wipes, sanitary products, cotton buds, nappies, condoms and kitchen roll do not break down and are the leading cause of blockages.
- Keep fat and oil out.Cooking fat solidifies in the tank and thickens the scum layer, reducing capacity and eventually reaching the outlet. Wipe pans and bin the residue.
- Watch the water volume.A leaking loo, a dripping tap or several loads of washing in one day can flush the tank through faster than it can settle, carrying solids out. Spread washing across the week and fix leaks promptly.
- Keep rainwater out of it.Roof drainage connected into a foul system overwhelms a septic tank. Surface water should go to a soakaway or a separate drain.
- Look after the drainage field.No driving, parking, building or heavy landscaping over it, and no trees near it — roots find and block the pipes. Grass over it is ideal.
- Be sceptical about additives.A healthy tank generates its own bacteria and most additives are unnecessary. They do not remove the need for emptying, whatever the marketing says.
- Learn the warning signs.Slow drains throughout the house, gurgling, smells, wet or unusually lush ground over the drainage field, or effluent surfacing. Any of these means get it looked at promptly.
- Sort out the paperwork before selling.Buyers' solicitors ask about compliance with the general binding rules and about maintenance records. Discovering a non-compliant system during a sale is expensive and delays it.
Tips
- Keep every emptying receipt. It is the evidence of maintenance and it is asked for when you sell.
- Grass over the drainage field and nothing else. No parking, no building, no trees — root damage and compaction are what kill them.
- Spread washing loads across the week. A day of back-to-back loads pushes solids through before they can settle.
Common mistakes
- Pouring bleach and disinfectant down regularly — It kills the bacteria the whole system depends on, so solids stop being broken down and the tank fills far faster.
- Flushing wet wipes — They do not break down, so they accumulate and block the system. They are the leading cause of septic tank blockages even when labelled flushable.
If it doesn't work
Slow drains throughout the house
Cause: Tank full, or an outlet or drainage field blockage — Fix: Have it emptied and inspected. Slow drains across the whole house point at the system rather than one appliance.
Smell around the tank or drainage field
Cause: Overdue emptying, a blocked vent, or a failing drainage field — Fix: Empty it first. A persistent smell after emptying needs the system inspected.
Wet or very green patch over the drainage field
Cause: Field failing or saturated, often from solids carried over — Fix: Get it assessed promptly. Continuing to use a failed field makes replacement more likely and it is a large cost.
Gurgling from drains
Cause: Venting problem or a partial blockage — Fix: Check the vent pipe is clear and have the system inspected.
Filling much faster than it used to
Cause: More water entering, or bacteria killed off — Fix: Check for leaking loos and dripping taps, keep rainwater out, and reduce disinfectant use.
Questions people ask
How often should a septic tank be emptied?
Typically once a year, though it depends on tank size and household use. Letting sludge build up carries solids into the drainage field and blocking that is a very expensive repair.
What should you not put in a septic tank?
Wet wipes, sanitary products, cotton buds and nappies, cooking fat, and large amounts of bleach, disinfectant or antibacterial cleaner — which kill the bacteria the system depends on.