Design & DIY

Smelly septic holding tank

What causes septic holding tank smells to reach the cottage? What can be done to prevent it?

The odour is almost certainly seeping through the lid that caps the holding tank. Some lids are shaped like a bath plug – slightly greater in diameter at the top – and simply drop flush into the top of the tank. Those lids are not typically designed to be sealed tight, and by exposing them to air, you run the chance of wafting sewage smells clearing your cottage patio faster than a swarm of hornets. It doesn’t matter if the tank is empty or full – dirt needs to cover the cap and it needs to have at least a foot of soil on top. (Don’t forget to mark where the opening is, or you might force an irate septic-tank pumper to dig a trench from where the pipe exits the cottage to find the tank.) Another more common lid has a pipe-like collar that usually sticks five to eight centimetres out of the ground. If it doesn’t have a tight-fitting seal, as it should, smelly gases will escape.

A third, though less likely, possibility, is that there is an unrelated plumbing problem. Plumbing stacks provide an escape route for gases in indoor pipes and usually vent about a metre above the cottage roof. If the vent is too low – or poorly positioned – then you could be outdoors breathing in the not-so-great indoors.