In the June column I addressed a question I received about low-profile septic tanks. While going through some calculations related to that discussion, I decided it would be good to discuss septic tank baffles and why they are important for system operation. Septic tank baffles are something service providers deal with one way or another every day. This is either because they are operating correctly or, most often, because they weren’t installed properly, they are missing altogether, or they are out of alignment.

Baffles come in different shapes and of different materials. Some baffles extend across the width of rectangular tanks and are fastened to the tank walls. The most common type of baffle and material are plastic sanitary tees. These are durable and relatively easy to install. With requirements for effluent screens at the outlet, many products fit within the 4-inch diameter tee. 

There should be baffles at the inlet and outlet. They each serve a purpose. The inlet baffle directs the raw sewage downward into the middle zone of the septic tank. From there, the larger solids settle to the bottom and form the sludge layer. The outlet baffled allows sewage effluent to flow out from the liquid clear zone while retaining scum in the tank. 

TRAPPING SOLIDS

Elbows cannot be used for either the inlet or outlet baffles because gases generated in the tank through bacterial action cannot escape the tank, leading to problems due to corrosion. Gases from the septic tank are meant to move from the tank up through the house sewer and out the plumbing stack vent. It is important there be space for gases to move under the lid and out the inlet piping.

Baffles need to be of proper length above and below the liquid surface level. This is so the tank can effectively trap and hold settleable solids (sludge) as well as floating solids (scum). There also needs to be a minimum of a 3-inch drop between the inlet above the liquid level, which should be at the invert of the outlet of the tank. This difference is required to provide free flow of raw sewage into the tank to minimize the potential for plugging at the inlet. It is also the reason there needs to be adequate distance between where the inlet pipe enters the tank and the baffle wall. If because the pipe was pushed in too far or some other reason solids can hang up there and plug the piping, sewage can back up into the house.

The inlet baffle directs the raw sewage downward and prevents the floating scum from plugging the inlet pipe. The inlet baffle must be submerged a minimum of 6 inches below the liquid level but must not penetrate any deeper than 20% of the liquid depth. A deeper submergence will cause the solids at the bottom to be disturbed when sewage flows into the tank. This can re-suspend settled solids. Similarly, the baffle needs to extend a minimum of 6 inches above the liquid level up to 20% of the liquid depth.

To give you an idea of the length of the inlet baffles properly installed with an operating depth of 60 inches below the outlet, the minimum length both above and below would be 6 inches while the maximum would be 12 inches above and below.

Submergence of the outlet baffle is critical to the retention of both sludge and scum. Again, these days we are helped in terms of maintenance frequency by effluent screens which will plug if there are excessive solids delivered to the outlet. Regardless, the bottom of the outlet baffle must be submerged to a point equal to 40% of the liquid depth. In a tank with a 60-inch operating depth, the submergence would be 24 inches. 

INSPECTION PORTS

The upper part of the outlet baffle should extend above the liquid level 20% of the liquid depth, or 12 inches for this example. This provides for maximum storage of sludge and scum. Current maintenance recommendations are that when the depth of sludge and scum exceeds 25% the operating depth, the tank should be cleaned. This means when the depth of sludge and scum exceeds 15 inches, the tank should be pumped.

As I indicated, it is important that the tops of both baffles be left open and extend no closer than 1 inch to the inside of the top of the tank. This allows for movement of gases through the tank. Improper venting is one cause of tank corrosion at the outlet baffle. 

Access or the ability to inspect the tank contents and baffles is important to determine maintenance frequency and to ensure the baffles are in place and operating correctly. There should be at least one manhole at least 20 inches in diameter allowing access to remove the solids. Inspection pipes at least 4 inches in diameter should be installed directly above and in line with the inlet and outlet baffles. This allows access to measure sludge levels at both tank locations and removal for cleaning the effluent screen at the outlet. 

Remember that inspection pipes are for inspection and not removal of tank solids. That should be done through the manhole. Even with the best backflush procedures, the tank cannot be adequately cleaned through the inspection pipes!