You don't, really. A septic tank is built to float grease into a scum layer and hold it until the tank is pumped. Bacteria digest it slowly at best. The EPA doesn't recommend additives, and solvent degreasers can harm the tank and groundwater. Keep grease out of the drains and pump when the scum layer gets close to the outlet.
What grease actually does inside a septic tank
Grease floats and stays put. The EPA explains that the tank holds wastewater long enough for solids to settle into sludge "while the oil and grease floats to the top as scum." A T-shaped outlet and the tank's compartments keep that sludge and scum from leaving the tank and traveling into the drainfield.
Bacteria in the tank do break down organic matter, but with grease the tank's main job is storage. Every pan of bacon grease adds to the scum, and the scum stays until someone pumps it out.
That's fine as long as the scum stays in the tank. The trouble starts when it doesn't. Maricopa County's septic FAQ warns that skipping routine maintenance "can cause solids or greases and oils in the tank to migrate into the drain field and clog the system." That's the outcome to avoid. University of Arizona Extension says that when solids clog the soil, the soil treatment area often has to be abandoned and a new one built.
Do enzymes, bacteria or degreasers break it down?
Biological additives (bacteria and enzyme products) are the ones marketed for grease. The EPA's 2024 fact sheet says the use of septic additives "is not recommended for domestic wastewater treatment because there is already a significant presence of bacteria, enzymes, yeasts, fungi, and other microorganisms" in the system. The same fact sheet allows that some biological additives can reduce scum and sludge, then adds that the long-term effect on the soil's ability to treat wastewater is unknown. University of Arizona Extension is a bit milder: enzyme and yeast products "will not harm your system," but adding them "will not eliminate the need for regular cleaning."
Degreasers are worse. The EPA fact sheet says these products often contain chlorinated solvents, and that organic solvents "can destroy decomposers and other helpful microorganisms in the treatment system." It notes that products containing them are banned in many states. UA Extension warns that degreasers may contain cancer-causing agents that can reach groundwater. If the label promises to dissolve grease with chemicals, keep it away from a septic system. Our page on whether septic additives work goes through the other product types.
Hot water and dish soap just move the grease along. A county grease flyer from Henry County, Virginia says "Hot water will cool down in pipes causing fats and grease to coagulate," and that detergents claiming to dissolve grease may cause blockages farther down the pipeline. In a septic home, farther down the pipeline means your tank or your drainfield.
The only real fix: pump the tank on time
Pumping is how grease leaves a septic tank. The question is when.
The EPA's rule of thumb is to pump "if the bottom of the scum layer is within six inches of the bottom of the outlet," if the sludge is within 12 inches of the outlet, or if sludge and scum make up more than 25 percent of the liquid depth. University of Arizona Extension sets a tighter scum trigger, pumping when the bottom of the scum layer is less than 3 inches above the lower end of the outlet, or when scum is more than 12 inches thick if the outlet can't be found. Ask your septic service to measure and write down both layers each visit so you can see how fast grease is building.
A household that cooks with a lot of oil, or runs a garbage disposal hard, fills the scum layer faster. The EPA says limiting the disposal "will significantly reduce the amount of fats, grease, and solids that enter your septic tank." It also says homes that use one will need to pump more often. For the general schedule, see how often to pump a septic tank.
Keeping grease out to begin with
- Pour cooled grease into a sealable container, let it harden, and put it in the trash. The EPA's instruction is short: "Never pour cooking oil or grease down the drain."
- Wipe pans with a paper towel before washing. Fat, butter, cheese and heavy cream all count; the EPA lists each of them as grease to keep out of the drain.
- Go easy on the disposal, or skip it for greasy scraps.
- Use a sink strainer to catch food solids.
Our page on what not to flush with a septic system covers the rest of the list.
When grease has already caused a problem
A slow kitchen sink in a septic home is usually a grease clog in the house drain, not the tank. That's a drain cleaning job: a cable or a water jet clears the line, and the grease goes on to the tank where it belongs. HQ Plumbing & Air clears house drain lines and can run a camera to see whether the clog is in the pipe or at the tank inlet.
If drains all over the house are slow, sewage backs up, or the ground over the drainfield is soggy with no rain, the problem may be the tank or the field itself. Those are signs a septic system is failing, and they call for a licensed septic contractor to pump and inspect the system. Maricopa County notes that replacing a failed tank or disposal field requires an alteration permit.