How often should a septic tank be pumped?

The honest answer is "when the solids say so," but that's not much help when the tank is buried. So most homeowners use a schedule, then adjust it from what the pumper actually finds. Here's how to build that schedule for a Tennessee house.

Step 1: figure out the tank size

Tennessee's septic rule sets the minimum liquid capacity of a house's tank by the number of bedrooms. If you don't have the original permit, this is a decent guess at what's in the ground for a house built under the current rule:

The rule adds 250 gallons for each bedroom past four. Older homes, and homes where bedrooms were added later, may have a tank smaller than what the bedroom count suggests. Hamilton County's own form warns that adding a bedroom can mean the existing system has to be modified to meet field-line requirements, which is a clue that a lot of houses here have grown faster than their septic systems. The pumper can tell you the real size once the lids are open.

Step 2: count the people, not the bedrooms

The tank is sized by bedrooms, but it fills by people. The EPA puts indoor water use in a typical single-family home at up to 70 gallons per person per day. A couple in a three-bedroom house is lighter on the system than a family of six in the same house. Things that push the schedule shorter:

• A garbage disposal. The EPA says food waste adds solids and means more frequent pumping.
• A running toilet. One leaky toilet can add up to 200 gallons a day, which stirs up the tank and pushes water through faster.
• Lots of laundry on one day instead of spread out over the week.
• Guests or renters. A short-term rental can see more people than the bedroom count ever planned for.

Step 3: pick a starting interval

Two credible sources give you the range. The EPA says household tanks are typically pumped every three to five years, with an inspection at least every three. The University of Tennessee's soil science department recommends pumping every five years to keep solids from reaching the drain field. In practice:

Step 4: let the measurements adjust it

The EPA lists three triggers a service pro can measure. Pump when the bottom of the scum layer is within six inches of the bottom of the outlet, when the top of the sludge is within twelve inches of the outlet, or when sludge and scum together take up more than a quarter of the tank's liquid depth. If your tank is well under those marks at the three-year check, stretch the interval. If it's past them, shorten it.

Write it down

Keep the pumping receipt with the house papers: date, gallons, the company, and what they noted about the baffles. A buyer's inspector will ask for it, and it's the best guide for the next pumping. If you're not sure when it was last done, the safest move is to have it pumped and inspected now and start the clock fresh. The pumping page covers what that visit should look like.

Good questions

Is there a law in Tennessee that says how often I have to pump?

Tennessee's rule makes the property owner responsible for keeping the system in a safe and sanitary condition, but it does not set a pumping interval for a conventional household system. The 3 to 5 year range comes from EPA guidance.

My tank was pumped a year ago and drains are slow again. Is it full?

Probably not. Slow drains soon after a pumping more often point to a clogged line, a blocked outlet filter or baffle, or a drain field that is struggling. That is a repair call, not a pumping call.