How Often to Pump a Septic Tank

Why tank size and household size decide it

A septic tank separates what goes down the drain into three layers. Grease and soap float to the top as scum, solids sink to the bottom as sludge, and the liquid in the middle flows out to the drain field. The bacteria in the tank break some of the sludge down, but never all of it, so the layers keep growing.

Every person in the house adds to the sludge at roughly the same rate, so a bigger tank fills more slowly and a fuller house fills it faster. A 1,000-gallon tank serving two people can go five years or more. The same tank with five people in the house might need it every two or three. EPA's rule of thumb is to have the tank checked every few years and pumped when the sludge is within 12 inches of the outlet, or the bottom of the scum is within 6 inches of it.

Signs you are overdue

• Drains slowing down all over the house, not just one sink.
• Gurgling in the pipes when the toilet flushes or the washer drains.
• A sewage smell near the tank or the drain field.
• Wet ground or bright green grass over the field.
• Sewage coming up in the lowest drain in the house, usually a basement floor drain or a downstairs shower.

If you cannot remember the last time it was pumped, that counts too. Most tanks give no warning at all until the solids have already started moving out into the field.

What happens if you wait too long

Once the sludge reaches the outlet, solids flow out with the liquid and settle in the drain field trenches. They seal the soil, the field stops taking water, and no amount of pumping brings it back. A pump-out typically costs $300 to $500. A replacement drain field on Knox County clay typically runs $8,000 to $20,000. Going too long between pump-outs is the most common reason a field fails early.