Why does my septic system smell bad?

It’s a fact, sewage smells bad.  If you’ve walked through the steam of a city manhole or stood down wind of septic pumping truck, you know the odor.  We all try to avoid the smell, but some septic systems owners have a hard time evading it.

Sewer gasses are produced throughout your septic system.  the gas is created in the tank, piping, distribution box and in the trenches.  In a properly vented septic system, the gas produced in the septic system will move “upstream”  through the piping, septic tank and back through the house gravity sewer where it will then rise up through the house vent (called a vent stack) and disperses into the air.  These process works because low pressure in the pipe draws the gasses up and out of the vent.  Generally speaking, the taller your roof vent, the greater pressure difference and the more effective the venting action.  Folks sometimes find that simply extending  their vent stack on the roof eliminates the problem of unwanted stink.

Obstructions such as an unvented elbow in the distribution box (on the inlet pipe) or a  plug in the sewer line can trap gasses.  Unable to escape through the pipes and vent stack, the trapped gasses are now forced to leak out of the ground and cause embarrassing smells.  “Frank, I think your yard just passed gas!”

A failing absorption field can cause water to back-up in the septic tank and completely cover the inlet and outlet pipes.  This will also block the path of the gasses and cause your yard to smell like septage.

Therefore, if you are smelling sewer gasses for the very first time, it might mean that your absorption field is beginning to fail.

Sometimes, the venting system works properly but a down draft, caused by winds blowing across the roof a nearby row of trees can bring the odors down into your yard, window or air-conditioning unit.  If this is happening to you, you may want to try installing a charcoal vent filter.
These filters have become popular with people that cannot find ways to otherwise fix their stinky vent issues.

For more on venting Read This.