What Is Scum in a Septic Tank?
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Quick answer
Scum is the floating layer of grease, oils, soap residue and other light material that rises to the top of the liquid in a septic tank. A normal amount is part of how a tank works, but a thick scum layer reduces usable space and can reach the outlet, so it is removed when the tank is pumped.
Open a septic tank and you would find three layers: scum on top, clear-ish liquid in the middle and sludge at the bottom. Scum is the top one, and it tells a technician a lot about what goes down your drains.
What is septic scum made of?
Anything lighter than water rises. In a household tank that means fats, cooking oil, grease from dishes, soap film, hair-care product residue and bits of food and other light solids. Bacteria in the tank slowly break some of it down, but the grease-heavy part breaks down slowly and builds up.
Is scum in a septic tank normal?
Yes. A thin layer is part of normal operation, because the tank is designed to separate floating material from liquid. The trouble starts when the layer gets thick, hard or crusty. Then it takes up room the tank needs, shortens the time wastewater has to settle and can push closer to the outlet.
How does the tank keep scum from leaving?
The outlet draws liquid from the middle of the tank, below the scum, and a baffle at the outlet holds the floating layer back. If a baffle is cracked, missing or rotted away, scum can escape toward the effluent filter, the pump or the drain field. Grease reaching a drain field can coat the soil and stop it absorbing water.
What causes too much scum?
- Pouring cooking grease or oil down the kitchen sink.
- A garbage disposal sending lots of food waste into the tank.
- Going too long between pumpings.
- Heavy use of harsh cleaners or antibacterial products that slow the bacteria that break waste down.
- A damaged baffle or an overloaded tank.
Kitchens that handle a lot of cooking grease, such as restaurants, usually add a grease trap upstream to catch it before it reaches the tank.
How do you manage scum?
Prevention is mostly habits: scrape plates into the trash, wipe greasy pans with a paper towel and keep cooking oil out of the drains. Pumping does the rest, because it removes scum and sludge together. How often depends on your household and system, so follow your technician's advice. Our guide to how often to pump your septic tank explains what affects the timing.
Do not try to skim or check scum yourself. A septic tank holds dangerous gases, and lids should only be opened by a trained technician. For a scheduled pumping, see our septic pumping service or call 832-789-5357. Se habla español.
Related septic terms
Quick questions and answers
- What is the difference between scum and sludge?
Scum floats on top of the liquid and is made of grease and light solids. Sludge sinks to the bottom and is made of heavier solids. Both build up over time and are removed when the tank is pumped.
- Do septic additives remove scum?
Not reliably. No additive replaces pumping, and some can stir up solids that then travel to the drain field. Ask your technician before adding anything to your system.
- Can too much scum cause a backup?
Yes. A thick scum layer can reach the outlet and clog the filter or line, which can lead to slow drains and backups. Pumping and keeping grease out of the drains reduce the risk.
Related guides and services
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