There’s Nothing Baffling About Tank Inlet and Outlet Features

Keep ease of maintenance in mind during the wastewater flow design of the next septic tank you install

There’s Nothing Baffling About Tank Inlet and Outlet Features

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A colleague of ours sent us a photo of the inlet baffle in a septic tank and commented that the baffle design will lead to plugging and sewage backups into the house. While this was good for his pumping business, it meant he got to deal with unhappy homeowners, which is not a good thing.

Baffles are defined as any device installed in a septic tank to retain solids. Often, these are sanitary tees at the inlet and effluent screens at the outlet. In early days, both inlet and outlet baffles were often tees or — as in this case — a baffle built into the tank.

Traveling around the country, we still find states or areas where septic tanks are not equipped with inlet baffles. Usually the comment is made that “inlet baffles are not needed because the wastewater flows directly into the tank.”

Inlet and outlet

Inlet baffles perform an important function in the operation of a septic tank. They direct wastewater received from the house downward to the level of the clear zone, dissipating the energy of the incoming flow to prevent turbulence and disruption of the segregation of the scum and sludge layers in the tank. Along with the outlet baffle, the inlet baffle prevents inflow from short-circuiting flow through the tank, allowing time for solids to settle and to maintain the clear zone. Finally, baffles prevent accumulated floating scum from plugging the inlet or outlet.

Outlet baffles today consist of a sanitary tee fitted with effluent screens to ensure larger solids from either the sludge or scum layers are not allowed to move from the tank downstream to impact soil treatment units. Excessive solids and BOD can cause soil treatment trenches to grow excessive biomat and, in some cases, physically plug the soil’s ability to accept effluent.

Several other design and operation criteria are important to consider or incorporate when planning a system. Baffles must be resistant to corrosion or decay: They will not function if they are resting in the bottom of the tank.

In the past, a variety of materials were used that did not stand up to the corrosive environment in the tank. Different metal or other materials with metal fasteners did not prove to be durable. There was a strong movement to use cast-in-place baffles in concrete tanks; but due to several factors, they sometimes did not stand up to corrosion. Today what we see most are either cast-in-place or installed sanitary tees.

The baffle distance above or below the sewage surface is critical for proper operation within the tank in terms of holding the scum layer back and for solids to settle as sludge and provide a clear zone for effluent to be delivered to the soil treatment area. The inlet baffle must extend at least 6 inches below the surface, but not more than 20% of the total liquid depth in low-profile tanks. They should also extend at least 6 inches above the operating surface in the tank. This allows the baffle to do its job of directing flow downward into the tank and to keep any developed scum layer away from the inlet.

The outlet baffle must extend a distance equal to 40% of the liquid depth for rectangular tanks or 35% for cylindrical tanks. For a rectangular tank with an operating depth of 60 inches, the baffle should extend 24 inches. This ensures liquid being delivered to the next component is coming from the clear zone. Similar to the inlet, the baffle should extend at least 6 inches above the liquid surface. This is to keep the scum layer from floating over the top of the baffles, causing plugging or potentially being delivered to the soil treatment area.

Maintain proper venting

In the case of cast-in-place inlet baffles as shown in the photo, it is important to have enough space between the inlet pipe and the baffle. The space needed to avoid plugging with toilet paper or other solids is 6 to 12 inches. Anything less and there will be a lot of service visits to unplug the baffle after sewage has backed up into the house.

Sanitary tees operate well at the inlet. They direct flow downward and through the tank and are less subject to plugging with toilet paper or the wipes people aren’t supposed to flush. The bottom line is that sanitary tees reduce plugging problems when compared to the cast-in-place baffles.

A final important note: For proper venting of the tank back through the house vent, there should be at least 1 inch between the tops of the baffles and the underside of the tank cover. If the gases are not vented properly, there will be corrosion in the concrete around the outlet baffle and the underside of the cover, resulting in the baffles deteriorating and the cover being structurally unsound.

The National Precast Concrete Association has tank design and manufacture standards that should be followed. 


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