Solids: The Enemy of the Septic Tank

When a tank inspection reveals excessive solids accumulation, sticking with a three- to five-year pumpout schedule is not enough.
Solids: The Enemy of the Septic Tank

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If a tank is operating properly, solids are retained and take up increasingly more volume. At some time they must be removed. (If there is little accumulation of solids, either the household is extremely conservative with water use and waste generation or there is a problem causing solids to pass through the tank.)

When there is little clear zone left, proper solids separation will no longer occur, detention time for settling is further reduced, and solids will wash out of the tank, eventually clog the soil treatment area, and cause system failure.

Research on solids accumulation shows the interval between pumping depends on tank size, number of people in the house, and the nature of the sewage (which in turn depends on household habits and lifestyles). Many publications and maintenance programs recommend a three- to five-year pumpout interval. This interval is probably reasonable, but checking sludge levels at the time of service can provide a better estimate for the necessary pumpout interval.

Solids are the enemy of a septic tank, but unfortunately many tanks are not regularly assessed for accumulation. What do you do if you are monitoring and something makes you scratch your head?

Is the effluent filter clogging regularly? The filter below was clogged after three months, causing the pump to cavitate, as the effluent could not flow quickly enough to the pump. The filter was truly doing its job to prevent solids going out into the soil treatment area, but what is going on in the house?

Let’s say you are out inspecting and servicing a gas station convenience store. The owner insists that they only do minimal prep for tacos, no fried foods, and care for the syrups and milk product waste through a separate source. But when you go out to do a subsequent service visit, you note that fat, oils and grease have caused premature failure and set off the high-level alarm. This was caused by fluid having to rise up over the top of the filter screen to get out to the soil treatment area. 

Another recent example was from a family that has 11 people living in a five-bedroom house. Their problem is that they milk two cows twice a day, collecting about 6 gallons of milk per day. They process the milk a couple times a week, making cheese and butter. The problem is not the processing, or really the volume of milk, but the fact that they collect the milk in 18 1/2-gallon jars, which are each cleaned after the milk is brought in, and the cheese and butter processing bowls, pans, vats are cleaned in the sink, so the residual milk products are flushed down the drain. The accumulation of products, discharged each day, has the tank discharge BOD at 480-plus mg/L going to the soil treatment area. The scum accumulated 12-plus inches in the one year since the tank was installed. Note the condition of the toilet paper in the photo below; not much degradation. The theory is that the bugs are finding ample organics in the milk products that are easier to break down than the toilet paper.

The most reliable method for determining the need to pump is regular inspection of the tank, including measurement of sludge and scum thickness. If we use regular inspection as a method to determine pumpout needs, a good rule of thumb is to pump before the top of the sludge layer reaches a level 9 to 12 inches below the bottom of the outlet baffle, or when the bottom of the scum layer reaches a level 3 inches above the bottom of the outlet baffle. Another way to evaluate the tank is if more than 25 percent of the liquid depth of the tank is being used to store sludge and scum, that tank needs to be cleaned. 

About the Author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association (MOWA) and the National Onsite Wastewater Recycling Association (NOWRA), and serves on the NSF International Committee on Wastewater Treatment Systems. Send her questions about septic system maintenance and operation by email to


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