A septic tank is "full" when the excess solids build up over time and need to be removed before they start entering and plugging the soil in the drain area. Like ashes in a fire, the solids accumulate and need to be removed. When the lid is removed, if there is a blanket of floating solids, it's time to pump the tank. Rule of thumb: pump every 3 years.
It is common to think that when the effluent is near the top of the tank it's full. But this is untrue. The normal operating level of a septic tank is close to the top of the tank, at the height of the outlet pipe. When 10 gallons of effluent enters the tank, the level rises slightly above the outlet pipe, until 10 gallons slowly leaches out into the drain area. The tank acts as a trap for the solids leaving the house.
After about 1 week of regular use following the tank being emptied, the level in the tank will be back near the top again. The tank will be mostly clear water, ready for solids to start building up again.
If the level inside the tank is to the brim or overflowing, the outlet filter (if present) needs to be cleaned OR the drain field is failing (saturated or damaged).
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