Troubleshooting Septic Tanks

Troubleshooting Septic Tanks

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When troubleshooting onsite system problems or when an inspection of a septic tank is required, the tank should be opened so the contents can be inspected and the condition of both the inlet and outlet baffles can be assessed. A lot can be learned about how the system is operating as a whole simply by inspecting the tank contents and baffles. This means that the manhole must be opened, and depending on the type of tank, a part of the lid or entire lid will need to be removed to provide access.

The contents of the tank should be evaluated to determine whether the contents exhibit the three distinct layers that should be present: the sludge or settled solids layer, clear water in the middle and a floating scum layer on top. If these layers are not present, the tank is not operating the way it should. Identifying the reason that layers are not forming will likely lead to the cause of the system problems.

If there is a lot of floating material that doesn’t belong in the tank or a lot of undigested food, this should be discussed with the homeowner in terms of how to take care of their system and not introduce these materials into the wastewater. Examples of items that should not be in the tank are feminine hygiene products, baby or other wipes, condoms and cigarette butts. If there is a lot of undigested food, it could indicate that one of the members of the household has an eating disorder or point to excessive use of a garbage disposal; any discussion with the owner needs to be done carefully. The bottom line is they have control over what they introduce to the waste stream.

The scum layer should not be excessively thick and should not extend above or below the outlet baffle. Check the bottom of the tank lid above the outlet for evidence the scum layer has extended over the baffle. If the scum is thick, it may mean that the tank needs to be cleaned; but it can also indicate that there is a backup due to a plugged outlet or plugging in the drainfield that is backing up into the tank. A thick scum layer may also mean that excessive amounts of soap or grease are being delivered to the tank. Here the users of the system may be able to change their habits or have the tank cleaned more often.

The sludge layer should also be evaluated. If the top of the sludge layer is closer than 12 inches to the bottom of the outlet baffle, the tank should be cleaned. If the sludge is not settling into a distinct layer, it may mean that the tank is experiencing turbulent conditions, perhaps due to a pump in the basement delivering sewage to the tank, or that the overall flow from the house is so high the material never has a chance to properly settle. This condition can lead to solids being delivered to the soil treatment area, causing plugging of the effluent screen. A different pump could be a solution if the overall flow is consistent with the tank size. Otherwise, installing a larger tank or additional tank in series may be necessary to solve the higher flow problem.

About the author: Jim Anderson is connected with the University of Minnesota onsite wastewater treatment program and is an emeritus professor in the university’s Department of Soil, Water and Climate. Send him questions about septic system maintenance and operation by sending an email to

This article is part of a series on troubleshooting septic tanks:


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