Why do concrete septic tanks break down?

It is a fact of chemistry and physics that hydrogen sulfide gas will be produced in all septic tanks as organic waste decomposes and breaks down. The big question is why this seems to affect the integrity of some concrete septic tanks and not others.

Hydrogen sulfide is heavier than air, so it generally affects the concrete just above the waterline. If the gas cannot escape the structure, the level will slowly rise and begin to affect the concrete higher above the waterline.

The waste from every residence and business will produce hydrogen sulfide at different rates, depending on the organic volume of the waste, the temperature, and the retention in the structure. Regardless of the rate of hydrogen sulfide production, if the gas can flow out of the structure along the waste stream, then the gas level in the tank should never build up to the point where it damages the concrete.

Hydrogen sulfide is heavier than air, so when it first begins to develop, it will lie right on the surface of the water in the tank. Being heavier than air, it will flow downhill, just as water does. So in a properly installed septic system, it will flow through the outlet pipe to the drainfield and be dispersed into the soil.

Many people believe this gas follows the plumbing system back towards the house and out the roof vents. This is true of other gases in the tank, especially methane gas which is lighter than air. But getting hydrogen sulfide to exit the tank via the roof vents would be like trying to get water to flow 10 to 20 feet uphill – it’s just not going to happen.

The reason hydrogen sulfide gas attacks some tanks and not others is that in the systems being attacked the gas is not able to flow freely to the leaching bed and disperse in the soil. The flow could be stopped by anything that hinders the gas from entering the soil, such as:

• a sag in the outflow pipe
• a pipe pitched in the wrong direction
• a pipe pitched too steeply and inserted too far into the tank
• an overloaded leaching bed
• roots in the outflow line
• overly saturated or compacted soils
• heavy biomat in the leaching bed soil

Source of Information: Onsite Wastewater Association Newsletter Summer 2011 – reprinted from Onsite Installer Magazine, May 2011, p22