Last update12:12:22 PM

Back You are here: Home Lifestyle Home and Garden Offensive Odours

Offensive Odours

Many property owners find that there is a time when offensive odours are coming from their drainage system but don't know what to do about it.

Not unexpectedly, many of these problems relate to countryside properties on a septic tank but a good proportion still relate to homes connected to mains drainage.

Most problems are similar. "The problem comes and goes"; "It seems to happen when the wind is in a particular direction", and so on. Blockages or broken pipes are more easily localised and resolved. These other problems appear to have no clear cause. Quest Bulgaria has put together a knowledge base on how to deal with these obnoxious odours.


One of the key things to remember is that it is not just a question of sorting out the smell but also recognising that there is a safety issue here which is a matter of where the smell is coming from.

When human waste is broken down it produces sewer gases. This is a nasty little mix of many gases. Interestingly the gas which makes the smell is only a small percentage of the mix. The largest gas by a long way is methane. This makes up 60% of the total gases. Methane is odourless, colourless, lighter than air and is flammable.

Now, it seems to make sense that if smells arise, then methane may be there at the same time.

Whatever drains system you are on (septic tank or mains), if it has been correctly installed and ventilated then the sewer gases will find their way outside, as they should. Many problems arise where the system which has been installed is not properly ventilated.

Whilst the smell can be truly horrid, it is just as well that these unwanted odours draw your attention to a potential problem. If you fix the smell, you'll fix the other problems at the same time.

What are the most common reasons for these odours occurring?
Odours Inside the Property

Answer One - Evaporation has caused the water in one of the trap seals to dry out, which has allowed sewer gases to escape through one of the plug holes in the house. Holiday homes frequently suffer from this problem. The quickest and cheapest answer to this problem is to run water into the plug holes which are least often used and also through appliances which have not been used when you arrive at your holiday home.

Answer Two - Where a water trap seal has been depleted. This generally occurs in homes on mains where the soil stack vent is too small (or maybe your builder did not fit it at all!). A partial vacuum is formed when the water is siphoned and this cannot be stabilised by the vent. You can easily see if you have a soil stack vent. You'll either see a 100mm pipe poking out of the roof or it may be in your loft space.

Answer Three - If your builder has put the soil stack vent in the loft but has not fitted an air admittance valve. This means that the drafts in the loft on windy days will disperse the gases but on those days when there is no owind, the gases find their way into other parts of the home.

Answer 4 - Again, on windy days, if you have a window near this vent, smells can be carried indoors

Answer 5 - With regard to septic tanks, if it is not property vented to the outside, the pressure caused every time an appliance is discharged can force sewer gases out of weak joints. EU regs state that a ventilation pipe must be fitted, so if you can't see one, get one installed. If you have a concrete septic tank you should be aware that hydrogen sulphide is highly corrosive to concrete and if you don't have correct ventilation, this will quickly cause the concrete to crumble.

Answer 6 - Check that your vent pipes are suitably covered with a cover which deflects wind currents.

Odours Outside the Property

After checking that there are no bad joints, no broken covers or pipes, the source of nasty odours outside your home is the septic tank vent pipe. You may be tempted to block this off but don't! This pipe performs an essential job and has to be there.

To eliminate the smell you can fit an activated carbon filter which allows two way ventilation but blocks the gases that are causing the smells. Whatever, you do, don't fit an air admittance valve as this will do just the opposite and prevent the tank from property venting.