Should You be Concerned if a Septic System has No Scum Layer?

When there is no scum layer, you should work with the homeowner to remedy this to extend the life of downstream components

Should You be Concerned if a Septic System has No Scum Layer?

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Septic tanks allow gravity to separate solids from wastewater as heavier solids settle and fats, grease and lighter solids float. You might typically think of solids removal primarily happening via settling but separation of suspended solids by flotation is also very important in the septic tank. The process works best when the flow through the tank is very slow and can be compared to settling in reverse. The flotation process is enhanced by the presence of fats, oil and grease in the wastewater, which congeals on small particle surfaces, making them more buoyant and floatable. The accumulation of scum by flotation is a significant factor relative to the efficient removal of fat, grease, oil and floating solids. 

After septic tanks have been used for six months or more, the tank should have developed three layers: a scum layer on top, a clarified effluent in the middle which is free of large solids, and a sludge layer on the bottom. If these three separate layers are not present, then the system is not operating the way it should, and you need to find out why.

A scum layer should be present, although depending on practices within the home or facility it may be a thin zone of an inch or less, or could be thicker. Items to evaluate are discussed below.

1. Effluent baffle missing - The purpose of the effluent baffle is to assure that the scum layer stays in the septic tank and therefore should be replaced as soon as possible if falls off or becomes ineffective. It is recommended to be replaced along with an effluent screen to further reduce solids in the effluent.

2. Inadequate design detention time - The septic tank capacity should provide long enough residence time in the septic tank for proper suspended solids and oil/grease removal by sedimentation and flotation. The actual residence times in septic tanks may vary significantly, because of the accumulation of sludge and scum, and thus the volume of the tank for treatment will vary as well.

3. Turbulence - The primary purpose of the septic tank is to provide relatively calm conditions to allow settleable solids to sink to the bottom and accumulate and floatable solids to rise to the top and accumulate. Sometimes the layers will form but then become mixed due to turbulence in the water, particularly if there are high-flow events, large leaks into the system, or a pump adding sewage into the system.

4. Emulsification - Emulsifiers are mixing agents that will affect stratification. Used in small, normal amounts they should not cause a problem but when overused, additions such as strong phosphate-based cleaners, fabric softeners and degreasers can affect scum formation. Since many degreasers are chlorinated solvents or other prohibitively strong chemicals, these compounds should not be used. Higher water temperatures and higher water flow rates promote mechanical emulsification of oil as well.

5. Microbial community impacts - To perform properly the septic tank needs a healthy microbial community. Chemical additions that impact bacterial activity should be kept to a minimum. This includes any product in a home that will kill bacteria. Typically this problem it can be diagnosed using your nose and testing the pH level. The pH level should be between 6-8 and similar to the pH of the tap water. If the pH level is above or below the tap water, the tank will be struggling to support active bacterial activity. Low pH values are caused by acids such as cleaners or furnace condensate, where high pH values are caused by basic cleaners or other chemicals. Going through the products being used by the owner will be critical.

6. Water softener recharge - Some, but not all, studies have found that when regeneration water from water softeners enters septic tanks there is a lack or reduction of the scum layer. There have been indications that the brine solution can reduce the tank’s ability to release and hold the soap scum in the tank, causing them to flow through the system. Routing the recharge out of the system or replacing old units that use significantly more salt may assist.

Every septic tank should have a scum layer — not too thick, not too thin. When there is no scum layer, you should work with the owner to determine the likely causes and remedy them to extend the life of downstream components. 

About the author: Sara Heger, Ph.D., is a researcher and educator in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota, where she also earned her degrees in agricultural and biosystems engineering and water resource science. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is the president-elect of the National Onsite Wastewater Recycling Association and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system design, installation, maintenance and operation by sending an email to


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