A Matter of Inches

Inlet and outlet baffles must be carefully placed to ensure the cleanest effluent and the fewest issues over the life of the septic tank

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In my 10 years of experience, the scum layer in the septic tank is thickest at the inlet end and tapers off, perhaps by 50 percent, at the outlet end. Also, the inlet pipe extends about one-third of the way vertically into the tank, whereas the outlet pipe extends halfway into the tank. This means an excessive scum layer (greater than 12 inches) usually reduces sewage flow into the tank — if not totally capping it off — long before the outlet pipe caps off.

I always measure the scum layer from the inlet end and note this for the customer. Also, I’ve found the bottom sludge layer is pretty uniformly distributed. Am I missing something?



It is a good practice to measure the thickness of the scum layer and inform customers. The critical place, however, is at the outlet baffle so that scum or sludge is not carried out into the soil treatment unit.

From your comments it would seem your area has different specifications for baffle submergence than we use in Minnesota. First of all, I will explain the dimensions Minnesota uses for septic tank baffle submergence and baffle extension above the liquid level.

Research on septic tank performance and dimensions was made quite a few years ago. We have used those results in septic tank specifications for Minnesota. Those dimensions are shown in the graphic.

The length of the septic tank should be two to three times greater than the width. The purpose is to provide a settling distance and prevent short-circuiting, which means some of the sewage inflow to the tank flows out at the same time.

The septic tank liquid depth, D, is used as the basis for other dimensions in the tank. To provide for floating scum storage, the inlet and outlet baffles must extend 0.2D above the liquid level. The top of these baffles must be no closer than 1 inch to the tank cover. These dimensions are to provide for adequate scum storage and movement of gases through the septic tank.

The inlet baffle must extend at least 1 inch above the top of the inlet sewer pipe. The inlet baffles must extend at least 6 inches into the liquid level, but no more than 0.2D. The invert (bottom) of the house sewer must be at least 3 inches above the liquid level of the septic tank. This is to provide a downward velocity to the incoming sewage so the scum is carried down and past the bottom of the inlet baffle.



The outlet baffle should extend a depth of 0.4D into the liquid of the septic tank. Septic tank research studied the location of the bottom of the outlet baffle to determine the depth providing the cleanest effluent to be discharged. That dimension may not be as critical now since the advent of outlet filters.

The septic tank should be cleaned when the bottom of the scum layer is measured to be 3 inches or closer to the bottom of the outlet baffle. Or the tank should be cleaned if the sludge layer is 12 inches or less below the bottom of the outlet baffle.

To explain the various dimensions, I will use a septic tank liquid depth of 60 inches. The inlet baffle should extend at least 6 inches, but no more than 12 inches into the liquid level of the tank. The inlet baffle should extend 12 inches above the liquid level of the tank. This is a total baffle length of 18 to 24 inches.

The outlet baffle should extend 24 inches into the liquid depth and 12 inches above the liquid level, which is the elevation of the invert of the outlet pipe. This is a total outlet baffle length of 36 inches.

You indicated the scum layer was the thickest at the inlet end of the septic tanks you observed. Those tanks may not have the specified 3-inch drop from the inlet sewer to the liquid level in the septic tank. Also, your inlet baffle extends farther into the liquid depth than suggested by the research. This is also true of your outlet baffle.

Scum buildup near the inlet of the septic tank would not be of concern as far as effluent quality. It would be of concern if the scum tended to prevent the flow of sewage into the tank.

Scum buildup near the bottom of the outlet baffle is of concern because of particles being discharged with the effluent. In a similar way, if the sludge is too high, particles will be carried along with the flowing effluent. As we all know, effluent quality is a top concern in the successful operation of an onsite sewage treatment system.



Another column I wrote addressed a question about deteriorating concrete septic tanks. Carl S. Buchman, P.E., executive director of the Precast Concrete Association of New York, responded. He noted the National Precast Concrete Association has a booklet on concrete septic tank design, manufacture, and installation, Best Practices Manual — Precast Concrete On-Site Wastewater Tanks, available at the website, www.precast.org.

Buchman reported that PCANY published a series of Tech Notes on various aspects of septic tanks, including testing for water tightness, proper installation, warranties, etc.

“PCANY instituted a Certification Program for septic tanks, intended to assure the public and officials that tanks are properly designed, fabricated and tested for water tightness,’’ Buchman explained. “The NPCA has a similar program (patterned after ours). I don’t care under which program tanks are certified, as long as they deliver the same quality.’’


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