← All guides · Reviewed 2026-07-22
Common septic tank problems (and what to do)
Most septic tank problems come down to one of three things: the tank is overdue for emptying, the drainage field has failed, or something’s gone down the drains that shouldn’t have. Here’s how to recognise and deal with the common ones.
Full tank / overdue emptying
Cause: the tank hasn’t been emptied on schedule, so solids build up until there’s no capacity left to settle new waste properly. What to do: book an emptying. Most family tanks need it about once a year — see how often to empty and signs your tank needs emptying. This is the cheapest problem on this list to fix, and the root cause of several of the others.
Blocked or failing drainage field
Cause: usually a knock-on effect of an overdue tank — solids carry over and clog the soil — but can also be age, poor original sizing, or a high water table. What to do: a failing field shows up as boggy or lush patches, smells, or slow drains. Once clogged, a field usually needs replacing rather than repairing. See our drainage field guide for how it works and what replacement involves.
Blocked pipes
Cause: most often wipes, fats or other non-flushable items caught in the pipe between the house and the tank, or in the outlet pipe to the drainage field. What to do: a blockage close to the house is often a straightforward clear. A blockage at the outlet can be a sign the tank itself is over-full — worth checking before assuming it’s “just” a blockage.
Smells
Cause: an overdue tank, a blocked vent, a failed seal, or (less often) a failing drainage field. What to do: a well-maintained system shouldn’t smell. Persistent odour is worth investigating rather than ignoring — it’s often an early warning of one of the problems above, before it becomes a bigger one.
Sewage backing up or surfacing — treat as urgent
Cause: the tank is full and has nowhere for new wastewater to go, or the drainage field has stopped accepting outflow. What to do: stop adding water to the system straight away (fewer flushes, no washing machine or dishwasher), keep people and pets away from any effluent on the surface, and get it looked at urgently. This is the one problem on this list that shouldn’t wait.
When to call a professional
The milder signs — a smell, a slightly slow drain, “it’s probably due” — are worth booking in normally. Backing up, surfacing effluent, or a treatment plant alarm should be treated as urgent. Either way, book an emptying or repair or see repairs & troubleshooting — the cheapest likely cause is checked first, with groundworks only recommended when they’re genuinely needed.
Check your system
Not sure whether what you’re seeing is a maintenance issue or a compliance one? Our free compliance checker is a quick way to understand your setup.
Sources: GOV.UK — General binding rules: small sewage discharge to the ground
Common questions
Why is my septic tank backing up?
Almost always because the tank is full and overdue for emptying, or the drainage field has failed and can no longer take the outflow. Stop adding water to the system and get it looked at as soon as possible.
Why does my septic tank smell?
A well-running system shouldn't smell. Persistent odour usually points to an overdue tank, a blocked vent, a damaged seal, or a drainage field that's no longer soaking away properly.
What causes a septic tank to fail?
Most failures trace back to an overdue emptying schedule letting solids carry over into the drainage field, misuse (wipes, fats, chemicals), or age and wear on an old system. A failed drainage field is the most expensive of these to put right.
My septic tank is overflowing — what should I do?
Treat it as urgent. Stop using water in the house as much as possible, keep people and pets away from any surfacing effluent, and get it emptied and inspected straight away.
Need this sorting in Cornwall?
Kernow Septic covers Cornwall's towns and rural parishes. Call or run the free tools.