Troubleshooting: Excessive Corrosion Around the Outlet Baffle

Troubleshooting: Excessive Corrosion Around the Outlet Baffle
Jim Anderson

Interested in Sewer/Drain Cleaning?

Get Sewer/Drain Cleaning articles, news and videos right in your inbox! Sign up now.

Sewer/Drain Cleaning + Get Alerts

Suppose you arrive at the site, open the septic tank and find that the outlet baffle in the concrete tank is severely eroded and crumbling with exposed rebar? 

A septic tank is meant to vent back through the house sewer vent, so the gases produced inside from anaerobic digestion can dissipate to the atmosphere. One of the gases formed during the digestion process is hydrogen sulfide. Because hydrogen sulfide is heavier than air, it has a tendency to settle to the lowest part in the tank over the top of the effluent. Here it combines with water vapor to form sulfuric acid. Over time, the highly corrosive acid will eat away at concrete at the surface of the liquid in the tank, breaking down the concrete and the baffle. 

Proper ventilation

If the tank is properly vented, the gases are removed and the acid does not form. If the tank is not venting there is some type of blockage or interruption. The first place to check is the piping into the tank at the inlet baffle. 

One common condition is that the inlet piping was shoved too far into the tank so it has come in contact with the wall of the baffle. This space often becomes blocked with solids, which does not allow for proper ventilation and can cause wastewater to backup into the house. Often where there has been a blockage at the inlet, the service provider will also see damage to the outlet baffle. 

Piping angle at installation

Another common condition is when the pipe entering the tank is inspected, it appears to be cocked at an angle into the baffle. With the pipe pointing slightly upward, venting is not totally interrupted, but not all of the gas is able to escape. The cause of this condition is also related to the installation. The piping into the tank was not properly bedded or set on firm earth. Differential settling takes place around the tank and the piping is not supported in the area between the tank and the sidewall of the excavation. 

To fix the venting problem, the piping into the tank needs to be replaced and repositioned so it enters the tank at the proper angle and to the proper distance. If the outlet baffle is corroded to the point of not functioning or will likely reach that condition soon, the baffle should be replaced. This is typically done by removing the concrete baffle and replacing it with a sanitary tee and PVC piping. 

Corrosion concerns

As a final note, there has been a lot of discussion recently about corrosion in concrete tanks when the venting appears to be working properly. There have been a number of theories and ideas expressed by experienced service providers. Some of the suggested causes include poor concrete mix at the plant, tank construction, incorrect mix and additives in the concrete, water softeners, or the quality of the well water. So if venting does not seem to be the cause, the service provider will likely have to do some more detective work. 

Often the end result is replacing the tank. Hopefully, for the homeowner, this is not the case, but if the tank is no longer structurally sound due to corrosion, replacement is the best option. 

Next up, do homeowners have complaints about odor? 

For other troubleshooting articles, visit

About the Author
Jim Anderson is connected with the University of Minnesota onsite wastewater treatment education program, is an emeritus professor in the university’s Department of Soil Water and Climate, and education coordinator for the National Association of Wastewater Technicians. Post a comment below or send him questions about septic system maintenance and operation by email to


Comments on this site are submitted by users and are not endorsed by nor do they reflect the views or opinions of COLE Publishing, Inc. Comments are moderated before being posted.