A septic tank works by letting solids settle into sludge at the bottom and grease float as scum on top. Pump before those layers reach the outlet, and the drain field stays clear.
The model
How fast the layers build depends on two things: how much the tank holds, and how many people use it. A bigger tank has more room before it needs service; more people add solids faster. The published interval comes from the Penn State and Ohio State Extension tables (after Mancl).
The interval table
Years between pumping, with no garbage disposal:
Garbage disposals
A disposal adds solids, so it shortens the interval by roughly 25%. Skip the bacterial additives — university studies find they provide little benefit and some can harm the system.
Why it matters
Skipping pump-outs lets solids wash into the drain field and clog it. You trade a few hundred dollars of pumping for a $3,000–$15,000 field replacement.