What is the purpose of a septic tank baffle?

This is a question that homeowners often ask, usually in connection with a conversation after a recent inspection shows the baffles need to be repaired or replaced. So it is a legitimate question as your customers realize they are going to have to spend some money.

Before directly answering the question, a brief explanation about what the inside of their tank looks like is in order. The tank should have three separate layers: a floating scum layer on top, relatively clear water in the middle and a sludge layer in the bottom. If these three layers are not present when evaluating the tank, the service provider is on a mission to determine the cause. One of the most likely causes is related to our question above: One or both of the septic tank baffles are missing or damaged.

A septic tank should have baffles at both the inlet and outlet. The purpose of the inlet baffle is twofold: to direct flow from the house sewer downward into the tank to create a longer detention time for the sewage to allow settling of solids, and to keep the floating scum layer from plugging the inlet pipe. The outlet baffle also has two functions: to prevent floating scum or debris from passing to the drainfield, and to ensure the effluent moving to the next part of the system is from the clear effluent zone. These days we enhance the first function through the use of effluent screens to keep large floating solids or debris from passing downstream.

TWO TYPES OF BAFFLES

There are two basic types of baffles: plastic sanitary tees and wall baffles. Wall baffles are built-in and often create more space for the solids carried through the house sewer to move into the tank. Having said this, sanitary tees, by nature of their construction, are less likely to have plugging problems. Either type of baffle will work if they are properly installed. But without proper tank installation, baffles can rapidly deteriorate so they do not operate the way they should. And in older tanks with wall baffles, it is difficult to add an effluent screen to the system. When wall-attached baffles need replacement, it is usually accomplished by retrofitting a sanitary tee.

Sanitary tees are different from a standard tee as they are curved to avoid a straight lip that can be a solids catcher, particularly at the inlet. These days this hardly bears mentioning because professionals readily recognize the difference. Back when I started in the industry there were a lot of standard tees installed as repairs that led to their own problems. Now many prefabricated septic tanks come with a sanitary tee already installed.

Baffles must be properly connected. A wall baffle should be connected in a way that will not corrode. All baffles must be attached so they remain in place over the life of the tank, and they must be accessible for inspection and replacement. Baffles made of PVC sanitary tees must be properly glued and affixed to the inlet and outlet piping. The holes the pipe passes through must be properly sealed to keep the tank watertight and to prevent root entry.

This is where many repair jobs fall down – a wall baffle is replaced with a sanitary tee but the job of patching around the hole is slipshod, allowing space for roots or surface water to enter the tank. If you are inspecting the tank, verify nothing is plugging the baffles. If there is an effluent screen, it should be evaluated for cleaning.

CHECK FREE FLOW

During a routine inspection, verify there is enough free space between the inlet pipe and the wall baffle to allow free flow of water and solids to the tank. There should be 2 to 4 inches of space. This is a common problem due to installation where the pipe was shoved past the inside wall of the tank, reducing the space for solids to pass through. This creates a place for toilet paper to clog the pipe, which can cause backups into the house. The bottom of the inlet baffle should extend down at least 6 inches so the flow into the tank is directed downward.

One other item to evaluate at the inlet is the type of pipe used for the house sewer line. In older tanks, and even still in some states newer construction, this piping can be cast iron. This type of pipe can react with soap products causing corrosion, plugging the pipe and causing flow problems. Of course, this may lead to another conversation with the homeowner; but the piping should be replaced to avoid problems in the future.

Similarly, the outlet baffle should be evaluated for adequate space. This is less critical since the outlet baffle should extend to a depth equivalent to 40 percent of the operating depth of the tank, drawing clear liquid out of the tank.

A FINAL THOUGHT

The piping into and out of the tank should be looked at to determine if it runs straight in and out. If the pipe is “cocked” at an angle due to settling after installation, it needs to be repaired. This condition can lead to pipe blockages and backups, and contribute to venting and corrosion problems.