Sewer smell in the bathroom

[Lady B]

Please can anyone tell me why and how to stop a foul smell that periodically comes from our ensuite bathroom, even if the toilet hasn't been used. We are not on mains sewers but have a sesspit.
Thank you

[buildersmate]

One of your water traps is being compromised. Water traps are the U-bend or P-trap devices beneath the waste plug. Each loo, washbasin, shower has one. The water is either being removed by evaporation (if the facility is not used for several weeks so flushing does not replenish the water) or it is being syphoned out by the 'negative pressure' of another waste outlet further down the system. The latter answer is more likely.

[buildersmate]

This then allows smells from the cesspit to pass up into the bathroom.

[gasman]

You can sort this problem out and avoid back syphonage by installind Anti-Vac Traps in place of the old traps.
Also if it is a cesspit as you say and not a ceptic tank then check and make sure the fresh air inlet ( approx 12" above ground level ) has not been blocked by leaves or nesting birds etc. Also check the same for the 100mm vent pipe (approx 9ft above the ground level ).