It’s Time to Vent About Hydrogen Sulfide Corrosion

Onsite experts and tank manufacturers continue to seek solutions for concrete deterioration. Sulfate-resistant formulas and secondary venting can have an impact.

It’s Time to Vent About Hydrogen Sulfide Corrosion

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It has been three or four years since I’ve had a specific question about septic system venting and the impact it has on reduction of gas concentrations at septic tank outlets. Every service provider has seen corrosion in concrete tanks. Most commonly, excessive deterioration is taking place around the outlet baffle in a septic tank. The question is whether anything can be done to stop it. The reader in this case was seeing problems in an aerobic system in the pump tank.

When I started in the industry, it was recognized that the sewer line, septic tank and, by extension, the drainfield vented back to the indoor plumbing and through the roof vent. Most systems being evaluated at the time ran septic tank to distribution box to drainfield. As we are aware, there are often a number of additional pieces in our systems that make venting all parts back through the roof vent problematic, resulting in the need for additional venting.

We won’t go into tremendous detail because a lot has been written about the causes and stages of concrete corrosion. Interior concrete walls are also attacked when sulfuric acid forms in the airspace above the waterline. The hydrogen sulfide gas is at first dissolved in the sewage, but then exsolves out into the airspace when it reaches a certain concentration. In the presence of oxygen from incoming water or airspace in the septic tank or pump tank, exsolved hydrogen sulfide gas is converted to sulfuric acid, or H2SO4.


When hydrogen sulfide gas comes out of solution from sewage, it collects at the lowest point because it is heavier than air. The corrosion is most often noticed first in the area of the outlet baffle where the gases will collect. Note, however, corrosion will also occur above the waterline in the tank and on the bottom of the lid. In this moist environment reacting with water and microbes, the sulfuric acid in combination with microbes begins to attack the concrete.

The corrosive action reduces the structural strength and durability of the concrete. It also increases the permeability of the concrete itself, which further leads to cracking and a condition called “crown rot.” This can cause concrete baffles, the lid and the tank itself to slough and crumble. Since bacteria and other microbes are essential for the process to occur, the entire process is called microbiologically induced corrosion.

Since I am a soil scientist by training, I recognize that in areas where the soils are high in sulfates, the corresponding groundwater also has more sulfates, and problems with concrete corrosion are more prevalent. So at least in some areas, soils and water quality have a role to play in terms of the severity of the problem.

Now to the question of what can be done. As with most of the problems we see, there is no single solution that will take care of all of the issues. This is due to the wide diversity of systems and each one is unique in some respect.

For a long time, I said either inadequate ventilation of the septic tank or use of a poor concrete mix during manufacture made the tank and baffles more susceptible to corrosion. The solution was either investigate and make sure the tank vented back through the roof vent, or replace the tank with one that used a sulfate-resistant mix. While these are still options, both the occurrence and solution are more complex.


The National Precast Concrete Association has standards for tank manufacture including sulfateresistant mixes and has continued to do research on the efficacy of different adhesive coatings that can be applied above the waterline and other areas susceptible to corrosion. In areas such as I mentioned above with groundwater high in sulfates, these should be used at a minimum. The NPCA continues its efforts to find solutions to this issue.

Service providers have also observed the problem does not seem to be only from within the septic or other sewage tank, but from components downstream in the system such as the drainfield. This was evidenced by corrosion directly above the outlet baffle in the tank and in distribution or drop boxes serving the soil treatment area.

Within the last 4-5 years there has been some research on whether adding additional or differential venting to a system helps improve air flow through parts or all of the system. The answer is yes. Adding additional vents and constructing each part of the system from septic tank, pump tank, distribution boxes and either above- or inground soil treatment areas with venting in mind can improve the situation. There are two rules to differential venting: The additional vent needs to be at a lower level than the roof vent and slightly above the elevation of the soil treatment area.

Additional vents may improve air flow, but it does not mean simply adding them will solve the problem. Since they need to work in tandem with the roof vent, the same issues that plague venting sewage odors will interfere with the supplemental venting. These include back drafts due to topography, roof steepness, winds and tall trees altering air flow around the system. Unfortunately, as indicated earlier, it makes working on each system a mystery to be solved on its own with a unique set of circumstances.

One last comment for now: Some modifications in pump tanks have helped in some situations. Since pump tanks are receiving effluent from the septic tank, directing flow at the inlet to near the bottom of the tank and placing the weep hole in the outlet pipe where it will be under water most of the time can cut down on turbulence within the tank. This reduces the opportunity for effluent to mix with oxygen in the tank, resulting in the formation of the gases and sulfuric acid as described. This may also be helpful in other aerobic tank situations.


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