Scum and sludge can build up and may be carried into the drainfield. They will clog the drainfield, causing it to fail and require replacement.
Scum and sludge in the tank reduces the amount of room available to hold wastewater.
How Often?
Many experts recommend pumping a tank every 2-3 years, however, it depends on wastewater quantity and quality.
Another method is to establish your own pumping interval.
After having your tank pumped, have a septic professional inspect the scum and sludge layers annually until they build up to a level that requires pumping.
Use that time period as your pumping interval, until your waste generation rates change (decrease due to someone leaving, or increase due to adding a garbage disposal, more people in the home or children becoming teens).
If / when your waste generation rates change, you will have to determine the new pumping interval.