Why does my septic system stink at our vacation home?
For this answer to make sense we need to first explain the operation of your system.
Your system is comprised of two tanks. The first tank is the aerobic treatment unit (ATU). The second tank is a pump tank that receives the treated effluent from the ATU, and holds a pump that discharges this treated effluent via pressure out to your dispersal field.
Where the odor comes from in your case is that this process, which is intended to be a ongoing process, stops for long periods of time when you are gone. When it stops, there is no longer any water moving through the ATU, and this creates the odor problem as follows: The water that comes from our homes contains DO because we have aerators on the faucets, shower droplets falling through the air, etc. When the wastewater flow is continuous, there's a continuous supply of DO to the trash tank via the wastewater from the home.
When the process stops, there is no more DO coming in. The bacteria and waste in the trash tank will then quickly consume what DO is present until there is no more. Without oxygen, the portion of the bacteria that like to live in the absence of oxygen then begin to thrive. The byproducts of their activities include hydrogen sulfide gas, which smells like rotten eggs.
This tank vents to the atmosphere through the house venting just like all other sewage odors whether from city sewer, conventional septic systems, or whatever. That's why you normally don't smell sewage at any home is the gases are diluted in the air by the time they reach our noses.
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